What is rustc?
Welcome to "The rustc book"! rustc
is the compiler for the Rust programming
language, provided by the project itself. Compilers take your source code and
produce binary code, either as a library or executable.
Most Rust programmers don't invoke rustc
directly, but instead do it through
Cargo. It's all in service of rustc
though! If you
want to see how Cargo calls rustc
, you can
$ cargo build --verbose
And it will print out each rustc
invocation. This book can help you
understand what each of these options does. Additionally, while most
Rustaceans use Cargo, not all do: sometimes they integrate rustc
into other
build systems. This book should provide a guide to all of the options you'd
need to do so.
Basic usage
Let's say you've got a little hello world program in a file hello.rs
:
fn main() {
println!("Hello, world!");
}
To turn this source code into an executable, you can use rustc
:
$ rustc hello.rs
$ ./hello # on a *NIX
$ .\hello.exe # on Windows
Note that we only ever pass rustc
the crate root, not every file we wish
to compile. For example, if we had a main.rs
that looked like this:
mod foo;
fn main() {
foo::hello();
}
And a foo.rs
that had this:
pub fn hello() {
println!("Hello, world!");
}
To compile this, we'd run this command:
$ rustc main.rs
No need to tell rustc
about foo.rs
; the mod
statements give it
everything that it needs. This is different than how you would use a C
compiler, where you invoke the compiler on each file, and then link
everything together. In other words, the crate is a translation unit, not a
particular module.
Command-line Arguments
Here's a list of command-line arguments to rustc
and what they do.
-h
/--help
: get help
This flag will print out help information for rustc
.
--cfg
: configure the compilation environment
This flag can turn on or off various #[cfg]
settings for conditional
compilation.
The value can either be a single identifier or two identifiers separated by =
.
For examples, --cfg 'verbose'
or --cfg 'feature="serde"'
. These correspond
to #[cfg(verbose)]
and #[cfg(feature = "serde")]
respectively.
--check-cfg
: configure compile-time checking of conditional compilation
This flag enables checking conditional configurations of the crate at compile-time,
specifically it helps configure the set of expected cfg names and values, in order
to check that every reachable #[cfg]
matches the expected config names and values.
This is different from the --cfg
flag above which activates some config but do
not expect them. This is useful to prevent stalled conditions, typos, ...
Refer to the Checking conditional configurations of this book for further details and explanation.
For examples, --check-cfg 'cfg(verbose)'
or --check-cfg 'cfg(feature, values("serde"))'
.
These correspond to #[cfg(verbose)]
and #[cfg(feature = "serde")]
respectively.
-L
: add a directory to the library search path
The -L
flag adds a path to search for external crates and libraries.
The kind of search path can optionally be specified with the form -L KIND=PATH
where KIND
may be one of:
dependency
— Only search for transitive dependencies in this directory.crate
— Only search for this crate's direct dependencies in this directory.native
— Only search for native libraries in this directory.framework
— Only search for macOS frameworks in this directory.all
— Search for all library kinds in this directory, except frameworks. This is the default ifKIND
is not specified.
-l
: link the generated crate to a native library
Syntax: -l [KIND[:MODIFIERS]=]NAME[:RENAME]
.
This flag allows you to specify linking to a specific native library when building a crate.
The kind of library can optionally be specified with the form -l KIND=lib
where KIND
may be one of:
dylib
— A native dynamic library.static
— A native static library (such as a.a
archive).framework
— A macOS framework.
If the kind is specified, then linking modifiers can be attached to it.
Modifiers are specified as a comma-delimited string with each modifier prefixed with
either a +
or -
to indicate that the modifier is enabled or disabled, respectively.
Specifying multiple modifiers
arguments in a single link
attribute,
or multiple identical modifiers in the same modifiers
argument is not currently supported.
Example: -l static:+whole-archive=mylib
.
The kind of library and the modifiers can also be specified in a #[link]
attribute. If the kind is not specified in the link
attribute or on the command-line, it will link a dynamic library by default,
except when building a static executable. If the kind is specified on the
command-line, it will override the kind specified in a link
attribute.
The name used in a link
attribute may be overridden using the form -l ATTR_NAME:LINK_NAME
where ATTR_NAME
is the name in the link
attribute,
and LINK_NAME
is the name of the actual library that will be linked.
Linking modifiers: whole-archive
This modifier is only compatible with the static
linking kind.
Using any other kind will result in a compiler error.
+whole-archive
means that the static library is linked as a whole archive
without throwing any object files away.
This modifier translates to --whole-archive
for ld
-like linkers,
to /WHOLEARCHIVE
for link.exe
, and to -force_load
for ld64
.
The modifier does nothing for linkers that don't support it.
The default for this modifier is -whole-archive
.
Linking modifiers: bundle
This modifier is only compatible with the static
linking kind.
Using any other kind will result in a compiler error.
When building a rlib or staticlib +bundle
means that the native static library
will be packed into the rlib or staticlib archive, and then retrieved from there
during linking of the final binary.
When building a rlib -bundle
means that the native static library is registered as a dependency
of that rlib "by name", and object files from it are included only during linking of the final
binary, the file search by that name is also performed during final linking.
When building a staticlib -bundle
means that the native static library is simply not included
into the archive and some higher level build system will need to add it later during linking of
the final binary.
This modifier has no effect when building other targets like executables or dynamic libraries.
The default for this modifier is +bundle
.
Linking modifiers: verbatim
This modifier is compatible with all linking kinds.
+verbatim
means that rustc itself won't add any target-specified library prefixes or suffixes
(like lib
or .a
) to the library name, and will try its best to ask for the same thing from the
linker.
For ld
-like linkers supporting GNU extensions rustc will use the -l:filename
syntax (note the
colon) when passing the library, so the linker won't add any prefixes or suffixes to it.
See -l namespec
in ld documentation for
more details.
For linkers not supporting any verbatim modifiers (e.g. link.exe
or ld64
) the library name will
be passed as is. So the most reliable cross-platform use scenarios for this option are when no
linker is involved, for example bundling native libraries into rlibs.
-verbatim
means that rustc will either add a target-specific prefix and suffix to the library
name before passing it to linker, or won't prevent linker from implicitly adding it.
In case of raw-dylib
kind in particular .dll
will be added to the library name on Windows.
The default for this modifier is -verbatim
.
NOTE: Even with +verbatim
and -l:filename
syntax ld
-like linkers do not typically support
passing absolute paths to libraries. Usually such paths need to be passed as input files without
using any options like -l
, e.g. ld /my/absolute/path
.
-Clink-arg=/my/absolute/path
can be used for doing this from stable rustc
.
--crate-type
: a list of types of crates for the compiler to emit
This instructs rustc
on which crate type to build. This flag accepts a
comma-separated list of values, and may be specified multiple times. The valid
crate types are:
lib
— Generates a library kind preferred by the compiler, currently defaults torlib
.rlib
— A Rust static library.staticlib
— A native static library.dylib
— A Rust dynamic library.cdylib
— A native dynamic library.bin
— A runnable executable program.proc-macro
— Generates a format suitable for a procedural macro library that may be loaded by the compiler.
The crate type may be specified with the crate_type
attribute.
The --crate-type
command-line value will override the crate_type
attribute.
More details may be found in the linkage chapter of the reference.
--crate-name
: specify the name of the crate being built
This informs rustc
of the name of your crate.
--edition
: specify the edition to use
This flag takes a value of 2015
, 2018
or 2021
. The default is 2015
. More
information about editions may be found in the edition guide.
--emit
: specifies the types of output files to generate
This flag controls the types of output files generated by the compiler. It accepts a comma-separated list of values, and may be specified multiple times. The valid emit kinds are:
asm
— Generates a file with the crate's assembly code. The default output filename isCRATE_NAME.s
.dep-info
— Generates a file with Makefile syntax that indicates all the source files that were loaded to generate the crate. The default output filename isCRATE_NAME.d
.link
— Generates the crates specified by--crate-type
. The default output filenames depend on the crate type and platform. This is the default if--emit
is not specified.llvm-bc
— Generates a binary file containing the LLVM bitcode. The default output filename isCRATE_NAME.bc
.llvm-ir
— Generates a file containing LLVM IR. The default output filename isCRATE_NAME.ll
.metadata
— Generates a file containing metadata about the crate. The default output filename islibCRATE_NAME.rmeta
.mir
— Generates a file containing rustc's mid-level intermediate representation. The default output filename isCRATE_NAME.mir
.obj
— Generates a native object file. The default output filename isCRATE_NAME.o
.
The output filename can be set with the -o
flag. A
suffix may be added to the filename with the
-C extra-filename
flag.
Output files are written to the current directory unless the
--out-dir
flag is used.
Custom paths for individual emit kinds
Each emit type can optionally be followed by =
to specify an explicit output
path that only applies to the output of that type. For example:
--emit=link,dep-info=/path/to/dep-info.d
- Emit the crate itself as normal, and also emit dependency info to the specified path.
--emit=llvm-ir=-,mir
- Emit MIR to the default filename (based on crate name), and emit LLVM IR to stdout.
Emitting to stdout
When using --emit
or -o
, output can be sent to stdout
by specifying -
as the path (e.g. -o -
).
Binary output types can only be written to stdout if it is not a tty.
Text output types (asm
, dep-info
, llvm-ir
and mir
) can be written to
stdout regardless of whether it is a tty or not.
Only one type of output can be written to stdout. Attempting to write multiple types to stdout at the same time will result in an error.
--print
: print compiler information
This flag prints out various information about the compiler. This flag may be
specified multiple times, and the information is printed in the order the
flags are specified. Specifying a --print
flag will usually disable the
--emit
step and will only print the requested information.
The valid types of print values are:
crate-name
— The name of the crate.file-names
— The names of the files created by thelink
emit kind.sysroot
— Path to the sysroot.target-libdir
- Path to the target libdir.cfg
— List of cfg values. See conditional compilation for more information about cfg values.target-list
— List of known targets. The target may be selected with the--target
flag.target-cpus
— List of available CPU values for the current target. The target CPU may be selected with the-C target-cpu=val
flag.target-features
— List of available target features for the current target. Target features may be enabled with the-C target-feature=val
flag. This flag is unsafe. See known issues for more details.relocation-models
— List of relocation models. Relocation models may be selected with the-C relocation-model=val
flag.code-models
— List of code models. Code models may be selected with the-C code-model=val
flag.tls-models
— List of Thread Local Storage models supported. The model may be selected with the-Z tls-model=val
flag.native-static-libs
— This may be used when creating astaticlib
crate type. If this is the only flag, it will perform a full compilation and include a diagnostic note that indicates the linker flags to use when linking the resulting static library. The note starts with the textnative-static-libs:
to make it easier to fetch the output.link-args
— This flag does not disable the--emit
step. When linking, this flag causesrustc
to print the full linker invocation in a human-readable form. This can be useful when debugging linker options. The exact format of this debugging output is not a stable guarantee, other than that it will include the linker executable and the text of each command-line argument passed to the linker.deployment-target
- The currently selected deployment target (or minimum OS version) for the selected Apple platform target. This value can be used or passed along to other components alongside a Rust build that need this information, such as C compilers. This returns rustc's minimum supported deployment target if no*_DEPLOYMENT_TARGET
variable is present in the environment, or otherwise returns the variable's parsed value.
A filepath may optionally be specified for each requested information kind, in
the format --print KIND=PATH
, just like for --emit
. When a path is
specified, information will be written there instead of to stdout.
-g
: include debug information
A synonym for -C debuginfo=2
.
-O
: optimize your code
A synonym for -C opt-level=2
.
-o
: filename of the output
This flag controls the output filename.
--out-dir
: directory to write the output in
The outputted crate will be written to this directory. This flag is ignored if
the -o
flag is used.
--explain
: provide a detailed explanation of an error message
Each error of rustc
's comes with an error code; this will print
out a longer explanation of a given error.
--test
: build a test harness
When compiling this crate, rustc
will ignore your main
function
and instead produce a test harness. See the Tests chapter
for more information about tests.
--target
: select a target triple to build
This controls which target to produce.
-W
: set lint warnings
This flag will set which lints should be set to the warn level.
Note: The order of these lint level arguments is taken into account, see lint level via compiler flag for more information.
--force-warn
: force a lint to warn
This flag sets the given lint to the forced warn level and the level cannot be overridden, even ignoring the lint caps.
-A
: set lint allowed
This flag will set which lints should be set to the allow level.
Note: The order of these lint level arguments is taken into account, see lint level via compiler flag for more information.
-D
: set lint denied
This flag will set which lints should be set to the deny level.
Note: The order of these lint level arguments is taken into account, see lint level via compiler flag for more information.
-F
: set lint forbidden
This flag will set which lints should be set to the forbid level.
Note: The order of these lint level arguments is taken into account, see lint level via compiler flag for more information.
-Z
: set unstable options
This flag will allow you to set unstable options of rustc. In order to set multiple options,
the -Z flag can be used multiple times. For example: rustc -Z verbose-internals -Z time-passes
.
Specifying options with -Z is only available on nightly. To view all available options
run: rustc -Z help
, or see The Unstable Book.
--cap-lints
: set the most restrictive lint level
This flag lets you 'cap' lints, for more, see here.
-C
/--codegen
: code generation options
This flag will allow you to set codegen options.
-V
/--version
: print a version
This flag will print out rustc
's version.
-v
/--verbose
: use verbose output
This flag, when combined with other flags, makes them produce extra output.
--extern
: specify where an external library is located
This flag allows you to pass the name and location for an external crate of a
direct dependency. Indirect dependencies (dependencies of dependencies) are
located using the -L
flag. The given crate name is
added to the extern prelude, similar to specifying extern crate
within the
root module. The given crate name does not need to match the name
the library was built with.
Specifying --extern
has one behavior difference from extern crate
:
--extern
merely makes the crate a candidate for being linked; it does not
actually link it unless it's actively used. In rare occasions you may wish
to ensure a crate is linked even if you don't actively use it from your
code: for example, if it changes the global allocator or if it contains
#[no_mangle]
symbols for use by other programming languages. In such
cases you'll need to use extern crate
.
This flag may be specified multiple times. This flag takes an argument with either of the following formats:
CRATENAME=PATH
— Indicates the given crate is found at the given path.CRATENAME
— Indicates the given crate may be found in the search path, such as within the sysroot or via the-L
flag.
The same crate name may be specified multiple times for different crate types.
If both an rlib
and dylib
are found, an internal algorithm is used to
decide which to use for linking. The -C prefer-dynamic
flag may be used to influence which is used.
If the same crate name is specified with and without a path, the one with the path is used and the pathless flag has no effect.
--sysroot
: Override the system root
The "sysroot" is where rustc
looks for the crates that come with the Rust
distribution; this flag allows that to be overridden.
--error-format
: control how errors are produced
This flag lets you control the format of messages. Messages are printed to stderr. The valid options are:
human
— Human-readable output. This is the default.json
— Structured JSON output. See the JSON chapter for more detail.short
— Short, one-line messages.
--color
: configure coloring of output
This flag lets you control color settings of the output. The valid options are:
auto
— Use colors if output goes to a tty. This is the default.always
— Always use colors.never
— Never colorize output.
--diagnostic-width
: specify the terminal width for diagnostics
This flag takes a number that specifies the width of the terminal in characters. Formatting of diagnostics will take the width into consideration to make them better fit on the screen.
--remap-path-prefix
: remap source names in output
Remap source path prefixes in all output, including compiler diagnostics,
debug information, macro expansions, etc. It takes a value of the form
FROM=TO
where a path prefix equal to FROM
is rewritten to the value TO
.
The FROM
may itself contain an =
symbol, but the TO
value may not. This
flag may be specified multiple times.
This is useful for normalizing build products, for example by removing the
current directory out of pathnames emitted into the object files. The
replacement is purely textual, with no consideration of the current system's
pathname syntax. For example --remap-path-prefix foo=bar
will match
foo/lib.rs
but not ./foo/lib.rs
.
When multiple remappings are given and several of them match, the last matching one is applied.
--json
: configure json messages printed by the compiler
When the --error-format=json
option is passed to
rustc then all of the compiler's diagnostic output will be emitted in the form
of JSON blobs. The --json
argument can be used in conjunction with
--error-format=json
to configure what the JSON blobs contain as well as
which ones are emitted.
With --error-format=json
the compiler will always emit any compiler errors as
a JSON blob, but the following options are also available to the --json
flag
to customize the output:
-
diagnostic-short
- json blobs for diagnostic messages should use the "short" rendering instead of the normal "human" default. This means that the output of--error-format=short
will be embedded into the JSON diagnostics instead of the default--error-format=human
. -
diagnostic-rendered-ansi
- by default JSON blobs in theirrendered
field will contain a plain text rendering of the diagnostic. This option instead indicates that the diagnostic should have embedded ANSI color codes intended to be used to colorize the message in the manner rustc typically already does for terminal outputs. Note that this is usefully combined with crates likefwdansi
to translate these ANSI codes on Windows to console commands orstrip-ansi-escapes
if you'd like to optionally remove the ansi colors afterwards. -
artifacts
- this instructs rustc to emit a JSON blob for each artifact that is emitted. An artifact corresponds to a request from the--emit
CLI argument, and as soon as the artifact is available on the filesystem a notification will be emitted. -
future-incompat
- includes a JSON message that contains a report if the crate contains any code that may fail to compile in the future.
Note that it is invalid to combine the --json
argument with the
--color
argument, and it is required to combine --json
with --error-format=json
.
See the JSON chapter for more detail.
@path
: load command-line flags from a path
If you specify @path
on the command-line, then it will open path
and read
command line options from it. These options are one per line; a blank line indicates
an empty option. The file can use Unix or Windows style line endings, and must be
encoded as UTF-8.
Codegen Options
All of these options are passed to rustc
via the -C
flag, short for "codegen." You can see
a version of this list for your exact compiler by running rustc -C help
.
ar
This option is deprecated and does nothing.
code-model
This option lets you choose which code model to use.
Code models put constraints on address ranges that the program and its symbols may use.
With smaller address ranges machine instructions
may be able to use more compact addressing modes.
The specific ranges depend on target architectures and addressing modes available to them.
For x86 more detailed description of its code models can be found in
System V Application Binary Interface
specification.
Supported values for this option are:
tiny
- Tiny code model.small
- Small code model. This is the default model for majority of supported targets.kernel
- Kernel code model.medium
- Medium code model.large
- Large code model.
Supported values can also be discovered by running rustc --print code-models
.
codegen-units
This flag controls the maximum number of code generation units the crate is split into. It takes an integer greater than 0.
When a crate is split into multiple codegen units, LLVM is able to process them in parallel. Increasing parallelism may speed up compile times, but may also produce slower code. Setting this to 1 may improve the performance of generated code, but may be slower to compile.
The default value, if not specified, is 16 for non-incremental builds. For incremental builds the default is 256 which allows caching to be more granular.
collapse-macro-debuginfo
This flag controls whether code locations from a macro definition are collapsed into a single location associated with that macro's call site, when generating debuginfo for this crate.
This option, if passed, overrides both default collapsing behavior and #[collapse_debuginfo]
attributes in code.
y
,yes
,on
,true
: collapse code locations in debuginfo.n
,no
,off
orfalse
: do not collapse code locations in debuginfo.external
: collapse code locations in debuginfo only if the macro comes from a different crate.
control-flow-guard
This flag controls whether LLVM enables the Windows Control Flow Guard platform security feature. This flag is currently ignored for non-Windows targets. It takes one of the following values:
y
,yes
,on
,true
,checks
, or no value: enable Control Flow Guard.nochecks
: emit Control Flow Guard metadata without runtime enforcement checks (this should only be used for testing purposes as it does not provide security enforcement).n
,no
,off
,false
: do not enable Control Flow Guard (the default).
debug-assertions
This flag lets you turn cfg(debug_assertions)
conditional
compilation on
or off. It takes one of the following values:
y
,yes
,on
,true
, or no value: enable debug-assertions.n
,no
,off
orfalse
: disable debug-assertions.
If not specified, debug assertions are automatically enabled only if the opt-level is 0.
debuginfo
This flag controls the generation of debug information. It takes one of the following values:
0
ornone
: no debug info at all (the default).line-directives-only
: line info directives only. For the nvptx* targets this enables profiling. For other use cases,line-tables-only
is the better, more compatible choice.line-tables-only
: line tables only. Generates the minimal amount of debug info for backtraces with filename/line number info, but not anything else, i.e. no variable or function parameter info.1
orlimited
: debug info without type or variable-level information.2
orfull
: full debug info.
Note: The -g
flag is an alias for -C debuginfo=2
.
default-linker-libraries
This flag controls whether or not the linker includes its default libraries. It takes one of the following values:
y
,yes
,on
,true
: include default libraries.n
,no
,off
orfalse
or no value: exclude default libraries (the default).
For example, for gcc flavor linkers, this issues the -nodefaultlibs
flag to
the linker.
dlltool
On windows-gnu
targets, this flag controls which dlltool rustc
invokes to
generate import libraries when using the raw-dylib
link kind.
It takes a path to the dlltool executable.
If this flag is not specified, a dlltool executable will be inferred based on
the host environment and target.
embed-bitcode
This flag controls whether or not the compiler embeds LLVM bitcode into object files. It takes one of the following values:
y
,yes
,on
,true
or no value: put bitcode in rlibs (the default).n
,no
,off
orfalse
: omit bitcode from rlibs.
LLVM bitcode is required when rustc is performing link-time optimization (LTO).
It is also required on some targets like iOS ones where vendors look for LLVM
bitcode. Embedded bitcode will appear in rustc-generated object files inside of
a section whose name is defined by the target platform. Most of the time this is
.llvmbc
.
The use of -C embed-bitcode=no
can significantly improve compile times and
reduce generated file sizes if your compilation does not actually need bitcode
(e.g. if you're not compiling for iOS or you're not performing LTO). For these
reasons, Cargo uses -C embed-bitcode=no
whenever possible. Likewise, if you
are building directly with rustc
we recommend using -C embed-bitcode=no
whenever you are not using LTO.
If combined with -C lto
, -C embed-bitcode=no
will cause rustc
to abort
at start-up, because the combination is invalid.
Note: if you're building Rust code with LTO then you probably don't even need the
embed-bitcode
option turned on. You'll likely want to use-Clinker-plugin-lto
instead which skips generating object files entirely and simply replaces object files with LLVM bitcode. The only purpose for-Cembed-bitcode
is when you're generating an rlib that is both being used with and without LTO. For example Rust's standard library ships with embedded bitcode since users link to it both with and without LTO.This also may make you wonder why the default is
yes
for this option. The reason for that is that it's how it was for rustc 1.44 and prior. In 1.45 this option was added to turn off what had always been the default.
extra-filename
This option allows you to put extra data in each output filename. It takes a
string to add as a suffix to the filename. See the --emit
flag for more information.
force-frame-pointers
This flag forces the use of frame pointers. It takes one of the following values:
y
,yes
,on
,true
or no value: force-enable frame pointers.n
,no
,off
orfalse
: do not force-enable frame pointers. This does not necessarily mean frame pointers will be removed.
The default behaviour, if frame pointers are not force-enabled, depends on the target.
force-unwind-tables
This flag forces the generation of unwind tables. It takes one of the following values:
y
,yes
,on
,true
or no value: Unwind tables are forced to be generated.n
,no
,off
orfalse
: Unwind tables are not forced to be generated. If unwind tables are required by the target an error will be emitted.
The default if not specified depends on the target.
incremental
This flag allows you to enable incremental compilation, which allows rustc
to save information after compiling a crate to be reused when recompiling the
crate, improving re-compile times. This takes a path to a directory where
incremental files will be stored.
inline-threshold
This option is deprecated and does nothing.
Consider using -Cllvm-args=--inline-threshold=...
.
instrument-coverage
This option enables instrumentation-based code coverage support. See the chapter on instrumentation-based code coverage for more information.
Note that while the -C instrument-coverage
option is stable, the profile data
format produced by the resulting instrumentation may change, and may not work
with coverage tools other than those built and shipped with the compiler.
link-arg
This flag lets you append a single extra argument to the linker invocation.
"Append" is significant; you can pass this flag multiple times to add multiple arguments.
link-args
This flag lets you append multiple extra arguments to the linker invocation. The options should be separated by spaces.
link-dead-code
This flag controls whether the linker will keep dead code. It takes one of the following values:
y
,yes
,on
,true
or no value: keep dead code.n
,no
,off
orfalse
: remove dead code (the default).
An example of when this flag might be useful is when trying to construct code coverage metrics.
link-self-contained
On windows-gnu
, linux-musl
, and wasi
targets, this flag controls whether the
linker will use libraries and objects shipped with Rust instead of those in the system.
It takes one of the following values:
- no value: rustc will use heuristic to disable self-contained mode if system has necessary tools.
y
,yes
,on
,true
: use only libraries/objects shipped with Rust.n
,no
,off
orfalse
: rely on the user or the linker to provide non-Rust libraries/objects.
This allows overriding cases when detection fails or user wants to use shipped libraries.
linker
This flag controls which linker rustc
invokes to link your code. It takes a
path to the linker executable. If this flag is not specified, the linker will
be inferred based on the target. See also the linker-flavor
flag for another way to specify the linker.
linker-flavor
This flag controls the linker flavor used by rustc
. If a linker is given with
the -C linker
flag, then the linker flavor is inferred from the
value provided. If no linker is given then the linker flavor is used to
determine the linker to use. Every rustc
target defaults to some linker
flavor. Valid options are:
em
: use Emscriptenemcc
.gcc
: use thecc
executable, which is typically gcc or clang on many systems.ld
: use theld
executable.msvc
: use thelink.exe
executable from Microsoft Visual Studio MSVC.wasm-ld
: use thewasm-ld
executable, a port of LLVMlld
for WebAssembly.ld64.lld
: use the LLVMlld
executable with the-flavor darwin
flag for Apple'sld
.ld.lld
: use the LLVMlld
executable with the-flavor gnu
flag for GNU binutils'ld
.lld-link
: use the LLVMlld
executable with the-flavor link
flag for Microsoft'slink.exe
.
linker-plugin-lto
This flag defers LTO optimizations to the linker. See linker-plugin-LTO for more details. It takes one of the following values:
y
,yes
,on
,true
or no value: enable linker plugin LTO.n
,no
,off
orfalse
: disable linker plugin LTO (the default).- A path to the linker plugin.
More specifically this flag will cause the compiler to replace its typical
object file output with LLVM bitcode files. For example an rlib produced with
-Clinker-plugin-lto
will still have *.o
files in it, but they'll all be LLVM
bitcode instead of actual machine code. It is expected that the native platform
linker is capable of loading these LLVM bitcode files and generating code at
link time (typically after performing optimizations).
Note that rustc can also read its own object files produced with
-Clinker-plugin-lto
. If an rlib is only ever going to get used later with a
-Clto
compilation then you can pass -Clinker-plugin-lto
to speed up
compilation and avoid generating object files that aren't used.
llvm-args
This flag can be used to pass a list of arguments directly to LLVM.
The list must be separated by spaces.
Pass --help
to see a list of options.
lto
This flag controls whether LLVM uses link time optimizations to produce better optimized code, using whole-program analysis, at the cost of longer linking time. It takes one of the following values:
y
,yes
,on
,true
,fat
, or no value: perform "fat" LTO which attempts to perform optimizations across all crates within the dependency graph.n
,no
,off
,false
: disables LTO.thin
: perform "thin" LTO. This is similar to "fat", but takes substantially less time to run while still achieving performance gains similar to "fat".
If -C lto
is not specified, then the compiler will attempt to perform "thin
local LTO" which performs "thin" LTO on the local crate only across its
codegen units. When -C lto
is not specified, LTO is
disabled if codegen units is 1 or optimizations are disabled (-C opt-level=0
). That is:
- When
-C lto
is not specified:codegen-units=1
: disable LTO.opt-level=0
: disable LTO.
- When
-C lto
is specified:lto
: 16 codegen units, perform fat LTO across crates.codegen-units=1
+lto
: 1 codegen unit, fat LTO across crates.
See also linker-plugin-lto for cross-language LTO.
metadata
This option allows you to control the metadata used for symbol mangling. This takes a space-separated list of strings. Mangled symbols will incorporate a hash of the metadata. This may be used, for example, to differentiate symbols between two different versions of the same crate being linked.
no-prepopulate-passes
This flag tells the pass manager to use an empty list of passes, instead of the usual pre-populated list of passes.
no-redzone
This flag allows you to disable the red zone. It takes one of the following values:
y
,yes
,on
,true
or no value: disable the red zone.n
,no
,off
orfalse
: enable the red zone.
The default behaviour, if the flag is not specified, depends on the target.
no-stack-check
This option is deprecated and does nothing.
no-vectorize-loops
This flag disables loop vectorization.
no-vectorize-slp
This flag disables vectorization using superword-level parallelism.
opt-level
This flag controls the optimization level.
0
: no optimizations, also turns oncfg(debug_assertions)
(the default).1
: basic optimizations.2
: some optimizations.3
: all optimizations.s
: optimize for binary size.z
: optimize for binary size, but also turn off loop vectorization.
Note: The -O
flag is an alias for -C opt-level=2
.
The default is 0
.
overflow-checks
This flag allows you to control the behavior of runtime integer overflow. When overflow-checks are enabled, a panic will occur on overflow. This flag takes one of the following values:
y
,yes
,on
,true
or no value: enable overflow checks.n
,no
,off
orfalse
: disable overflow checks.
If not specified, overflow checks are enabled if debug-assertions are enabled, disabled otherwise.
panic
This option lets you control what happens when the code panics.
abort
: terminate the process upon panicunwind
: unwind the stack upon panic
If not specified, the default depends on the target.
passes
This flag can be used to add extra LLVM passes to the compilation.
The list must be separated by spaces.
See also the no-prepopulate-passes
flag.
prefer-dynamic
By default, rustc
prefers to statically link dependencies. This option will
indicate that dynamic linking should be used if possible if both a static and
dynamic versions of a library are available.
There is an internal algorithm for determining whether or not it is possible to statically or dynamically link with a dependency.
This flag takes one of the following values:
y
,yes
,on
,true
or no value: prefer dynamic linking.n
,no
,off
orfalse
: prefer static linking (the default).
profile-generate
This flag allows for creating instrumented binaries that will collect profiling data for use with profile-guided optimization (PGO). The flag takes an optional argument which is the path to a directory into which the instrumented binary will emit the collected data. See the chapter on profile-guided optimization for more information.
profile-use
This flag specifies the profiling data file to be used for profile-guided
optimization (PGO). The flag takes a mandatory argument which is the path
to a valid .profdata
file. See the chapter on
profile-guided optimization for more information.
relocation-model
This option controls generation of position-independent code (PIC).
Supported values for this option are:
Primary relocation models
-
static
- non-relocatable code, machine instructions may use absolute addressing modes. -
pic
- fully relocatable position independent code, machine instructions need to use relative addressing modes.
Equivalent to the "uppercase"-fPIC
or-fPIE
options in other compilers, depending on the produced crate types.
This is the default model for majority of supported targets. -
pie
- position independent executable, relocatable code but without support for symbol interpositioning (replacing symbols by name usingLD_PRELOAD
and similar). Equivalent to the "uppercase"-fPIE
option in other compilers.pie
code cannot be linked into shared libraries (you'll get a linking error on attempt to do this).
Special relocation models
dynamic-no-pic
- relocatable external references, non-relocatable code.
Only makes sense on Darwin and is rarely used.
If StackOverflow tells you to use this as an opt-out of PIC or PIE, don't believe it, use-C relocation-model=static
instead.ropi
,rwpi
andropi-rwpi
- relocatable code and read-only data, relocatable read-write data, and combination of both, respectively.
Only makes sense for certain embedded ARM targets.default
- relocation model default to the current target.
Only makes sense as an override for some other explicitly specified relocation model previously set on the command line.
Supported values can also be discovered by running rustc --print relocation-models
.
Linking effects
In addition to codegen effects, relocation-model
has effects during linking.
If the relocation model is pic
and the current target supports position-independent executables
(PIE), the linker will be instructed (-pie
) to produce one.
If the target doesn't support both position-independent and statically linked executables,
then -C target-feature=+crt-static
"wins" over -C relocation-model=pic
,
and the linker is instructed (-static
) to produce a statically linked
but not position-independent executable.
relro-level
This flag controls what level of RELRO (Relocation Read-Only) is enabled. RELRO is an exploit mitigation which makes the Global Offset Table (GOT) read-only.
Supported values for this option are:
off
: Dynamically linked functions are resolved lazily and the GOT is writable.partial
: Dynamically linked functions are resolved lazily and written into the Procedure Linking Table (PLT) part of the GOT (.got.plt
). The non-PLT part of the GOT (.got
) is made read-only and both are moved to prevent writing from buffer overflows.full
: Dynamically linked functions are resolved at the start of program execution and the Global Offset Table (.got
/.got.plt
) is populated eagerly and then made read-only. The GOT is also moved to prevent writing from buffer overflows. Full RELRO uses more memory and increases process startup time.
This flag is ignored on platforms where RELRO is not supported (targets which do not use the ELF binary format), such as Windows or macOS. Each rustc target has its own default for RELRO. rustc enables Full RELRO by default on platforms where it is supported.
remark
This flag lets you print remarks for optimization passes.
The list of passes should be separated by spaces.
all
will remark on every pass.
rpath
This flag controls whether rpath
is
enabled. It takes one of the following values:
y
,yes
,on
,true
or no value: enable rpath.n
,no
,off
orfalse
: disable rpath (the default).
save-temps
This flag controls whether temporary files generated during compilation are deleted once compilation finishes. It takes one of the following values:
y
,yes
,on
,true
or no value: save temporary files.n
,no
,off
orfalse
: delete temporary files (the default).
soft-float
This option controls whether rustc
generates code that emulates floating
point instructions in software. It takes one of the following values:
y
,yes
,on
,true
or no value: use soft floats.n
,no
,off
orfalse
: use hardware floats (the default).
split-debuginfo
This option controls the emission of "split debuginfo" for debug information
that rustc
generates. The default behavior of this option is
platform-specific, and not all possible values for this option work on all
platforms. Possible values are:
-
off
- This is the default for platforms with ELF binaries and windows-gnu (not Windows MSVC and not macOS). This typically means that DWARF debug information can be found in the final artifact in sections of the executable. This option is not supported on Windows MSVC. On macOS this options prevents the final execution ofdsymutil
to generate debuginfo. -
packed
- This is the default for Windows MSVC and macOS. The term "packed" here means that all the debug information is packed into a separate file from the main executable. On Windows MSVC this is a*.pdb
file, on macOS this is a*.dSYM
folder, and on other platforms this is a*.dwp
file. -
unpacked
- This means that debug information will be found in separate files for each compilation unit (object file). This is not supported on Windows MSVC. On macOS this means the original object files will contain debug information. On other Unix platforms this means that*.dwo
files will contain debug information.
Note that all three options are supported on Linux and Apple platforms,
packed
is supported on Windows-MSVC, and all other platforms support off
.
Attempting to use an unsupported option requires using the nightly channel
with the -Z unstable-options
flag.
strip
The option -C strip=val
controls stripping of debuginfo and similar auxiliary
data from binaries during linking.
Supported values for this option are:
none
- debuginfo and symbols are not modified.debuginfo
- debuginfo sections and debuginfo symbols from the symbol table section are stripped at link time and are not copied to the produced binary. This should leave backtraces mostly-intact but may make using a debugger like gdb or lldb ineffectual. Prior to 1.79, this unintentionally disabled the generation of*.pdb
files on MSVC, resulting in the absence of symbols.symbols
- same asdebuginfo
, but the rest of the symbol table section is stripped as well, depending on platform support. On platforms which depend on this symbol table for backtraces, profiling, and similar, this can affect them so negatively as to make the trace incomprehensible. Programs which may be combined with others, such as CLI pipelines and developer tooling, or even anything which wants crash-reporting, should usually avoid-Cstrip=symbols
.
Note that, at any level, removing debuginfo only necessarily impacts "friendly"
introspection. -Cstrip
cannot be relied on as a meaningful security or
obfuscation measure, as disassemblers and decompilers can extract considerable
information even in the absence of symbols.
symbol-mangling-version
This option controls the name mangling format for encoding Rust item names for the purpose of generating object code and linking.
Supported values for this option are:
v0
— The "v0" mangling scheme.
The default, if not specified, will use a compiler-chosen default which may change in the future.
See the Symbol Mangling chapter for details on symbol mangling and the mangling format.
target-cpu
This instructs rustc
to generate code specifically for a particular processor.
You can run rustc --print target-cpus
to see the valid options to pass
and the default target CPU for the current build target.
Each target has a default base CPU. Special values include:
native
can be passed to use the processor of the host machine.generic
refers to an LLVM target with minimal features but modern tuning.
target-feature
Individual targets will support different features; this flag lets you control
enabling or disabling a feature. Each feature should be prefixed with a +
to
enable it or -
to disable it.
Features from multiple -C target-feature
options are combined.
Multiple features can be specified in a single option by separating them
with commas - -C target-feature=+x,-y
.
If some feature is specified more than once with both +
and -
,
then values passed later override values passed earlier.
For example, -C target-feature=+x,-y,+z -Ctarget-feature=-x,+y
is equivalent to -C target-feature=-x,+y,+z
.
To see the valid options and an example of use, run rustc --print target-features
.
Using this flag is unsafe and might result in undefined runtime behavior.
See also the target_feature
attribute
for controlling features per-function.
This also supports the feature +crt-static
and -crt-static
to control
static C runtime linkage.
Each target and target-cpu
has a default set of enabled
features.
tune-cpu
This instructs rustc
to schedule code specifically for a particular
processor. This does not affect the compatibility (instruction sets or ABI),
but should make your code slightly more efficient on the selected CPU.
The valid options are the same as those for target-cpu
.
The default is None
, which LLVM translates as the target-cpu
.
This is an unstable option. Use -Z tune-cpu=machine
to specify a value.
Due to limitations in LLVM (12.0.0-git9218f92), this option is currently effective only for x86 targets.
Jobserver
Internally, rustc
may take advantage of parallelism. rustc
will coordinate
with the build system calling it if a GNU Make jobserver is passed in the
MAKEFLAGS
environment variable. Other flags may have an effect as well, such
as CARGO_MAKEFLAGS
. If a jobserver is not passed, then rustc
will choose
the number of jobs to use.
Starting with Rust 1.76.0, rustc
will warn if a jobserver appears to be
available but is not accessible, e.g.:
$ echo 'fn main() {}' | MAKEFLAGS=--jobserver-auth=3,4 rustc -
warning: failed to connect to jobserver from environment variable `MAKEFLAGS="--jobserver-auth=3,4"`: cannot open file descriptor 3 from the jobserver environment variable value: Bad file descriptor (os error 9)
|
= note: the build environment is likely misconfigured
Integration with build systems
The following subsections contain recommendations on how to integrate rustc
with build systems so that the jobserver is handled appropriately.
GNU Make
When calling rustc
from GNU Make, it is recommended that all rustc
invocations are marked as recursive in the Makefile
(by prefixing the command
line with the +
indicator), so that GNU Make enables the jobserver for them.
For instance:
x:
+@echo 'fn main() {}' | rustc -
In particular, GNU Make 4.3 (a widely used version as of 2024) passes a simple
pipe jobserver in MAKEFLAGS
even when it was not made available for the child
process, which in turn means rustc
will warn about it. For instance, if the
+
indicator is removed from the example above and GNU Make is called with e.g.
make -j2
, then the aforementioned warning will trigger.
For calls to rustc
inside $(shell ...)
inside a recursive Make, one can
disable the jobserver manually by clearing the MAKEFLAGS
variable, e.g.:
S := $(shell MAKEFLAGS= rustc --print sysroot)
x:
@$(MAKE) y
y:
@echo $(S)
CMake
CMake 3.28 supports the JOB_SERVER_AWARE
option in its add_custom_target
command, e.g.:
cmake_minimum_required(VERSION 3.28)
project(x)
add_custom_target(x
JOB_SERVER_AWARE TRUE
COMMAND echo 'fn main() {}' | rustc -
)
For earlier versions, when using CMake with the Makefile generator, one
workaround is to have $(MAKE)
somewhere in the command so that GNU Make
treats it as a recursive Make call, e.g.:
cmake_minimum_required(VERSION 3.22)
project(x)
add_custom_target(x
COMMAND DUMMY_VARIABLE=$(MAKE) echo 'fn main() {}' | rustc -
)
Lints
In software, a "lint" is a tool used to help improve your source code. The Rust compiler contains a number of lints, and when it compiles your code, it will also run the lints. These lints may produce a warning, an error, or nothing at all, depending on how you've configured things.
Here's a small example:
$ cat main.rs
fn main() {
let x = 5;
}
$ rustc main.rs
warning: unused variable: `x`
--> main.rs:2:9
|
2 | let x = 5;
| ^
|
= note: `#[warn(unused_variables)]` on by default
= note: to avoid this warning, consider using `_x` instead
This is the unused_variables
lint, and it tells you that you've introduced
a variable that you don't use in your code. That's not wrong, so it's not
an error, but it might be a bug, so you get a warning.
Future-incompatible lints
Sometimes the compiler needs to be changed to fix an issue that can cause existing code to stop compiling. "Future-incompatible" lints are issued in these cases to give users of Rust a smooth transition to the new behavior. Initially, the compiler will continue to accept the problematic code and issue a warning. The warning has a description of the problem, a notice that this will become an error in the future, and a link to a tracking issue that provides detailed information and an opportunity for feedback. This gives users some time to fix the code to accommodate the change. After some time, the warning may become an error.
The following is an example of what a future-incompatible looks like:
warning: borrow of packed field is unsafe and requires unsafe function or block (error E0133)
--> lint_example.rs:11:13
|
11 | let y = &x.data.0;
| ^^^^^^^^^
|
= note: `#[warn(safe_packed_borrows)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #46043 <https://github.com/rust-lang/rust/issues/46043>
= note: fields of packed structs might be misaligned: dereferencing a misaligned pointer or even just creating a misaligned reference is undefined behavior
For more information about the process and policy of future-incompatible changes, see RFC 1589.
Lint Levels
In rustc
, lints are divided into six levels:
- allow
- expect
- warn
- force-warn
- deny
- forbid
Each lint has a default level (explained in the lint listing later in this chapter), and the compiler has a default warning level. First, let's explain what these levels mean, and then we'll talk about configuration.
allow
These lints exist, but by default, do nothing. For example, consider this source:
pub fn foo() {}
Compiling this file produces no warnings:
$ rustc lib.rs --crate-type=lib
$
But this code violates the missing_docs
lint.
These lints exist mostly to be manually turned on via configuration, as we'll talk about later in this section.
expect
Sometimes, it can be helpful to suppress lints, but at the same time ensure that
the code in question still emits them. The 'expect' level does exactly this. If
the lint in question is not emitted, the unfulfilled_lint_expectation
lint
triggers on the expect
attribute, notifying you that the expectation is no
longer fulfilled.
fn main() {
#[expect(unused_variables)]
let unused = "Everyone ignores me";
#[expect(unused_variables)] // `unused_variables` lint is not emitted
let used = "I'm useful"; // the expectation is therefore unfulfilled
println!("The `used` value is equal to: {:?}", used);
}
This will produce the following warning:
warning: this lint expectation is unfulfilled
--> src/main.rs:7:14
|
7 | #[expect(unused_variables)]
| ^^^^^^^^^^^^^^^^
|
= note: `#[warn(unfulfilled_lint_expectations)]` on by default
This level can only be defined via the #[expect]
attribute, there is no equivalent
flag. Lints with the special 'force-warn' level will still be emitted as usual.
warn
The 'warn' lint level will produce a warning if you violate the lint. For example,
this code runs afoul of the unused_variables
lint:
pub fn foo() {
let x = 5;
}
This will produce this warning:
$ rustc lib.rs --crate-type=lib
warning: unused variable: `x`
--> lib.rs:2:9
|
2 | let x = 5;
| ^
|
= note: `#[warn(unused_variables)]` on by default
= note: to avoid this warning, consider using `_x` instead
force-warn
'force-warn' is a special lint level. It's the same as 'warn' in that a lint at this level will produce a warning, but unlike the 'warn' level, the 'force-warn' level cannot be overridden. If a lint is set to 'force-warn', it is guaranteed to warn: no more, no less. This is true even if the overall lint level is capped via cap-lints.
deny
A 'deny' lint produces an error if you violate it. For example, this code
runs into the exceeding_bitshifts
lint.
fn main() {
100u8 << 10;
}
$ rustc main.rs
error: bitshift exceeds the type's number of bits
--> main.rs:2:13
|
2 | 100u8 << 10;
| ^^^^^^^^^^^
|
= note: `#[deny(exceeding_bitshifts)]` on by default
What's the difference between an error from a lint and a regular old error?
Lints are configurable via levels, so in a similar way to 'allow' lints,
warnings that are 'deny' by default let you allow them. Similarly, you may
wish to set up a lint that is warn
by default to produce an error instead.
This lint level gives you that.
forbid
'forbid' is a special lint level that fills the same role for 'deny' that
'force-warn' does for 'warn'. It's the same as 'deny' in that a lint at this
level will produce an error, but unlike the 'deny' level, the 'forbid' level
can not be overridden to be anything lower than an error. However, lint
levels may still be capped with --cap-lints
(see below) so rustc --cap-lints warn
will make lints set to 'forbid' just warn.
Configuring warning levels
Remember our missing_docs
example from the 'allow' lint level?
$ cat lib.rs
pub fn foo() {}
$ rustc lib.rs --crate-type=lib
$
We can configure this lint to operate at a higher level, both with compiler flags, as well as with an attribute in the source code.
You can also "cap" lints so that the compiler can choose to ignore certain lint levels. We'll talk about that last.
Via compiler flag
The -A
, -W
, --force-warn
-D
, and -F
flags let you turn one or more lints
into allowed, warning, force-warn, deny, or forbid levels, like this:
$ rustc lib.rs --crate-type=lib -W missing-docs
warning: missing documentation for crate
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
|
= note: requested on the command line with `-W missing-docs`
warning: missing documentation for a function
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
$ rustc lib.rs --crate-type=lib -D missing-docs
error: missing documentation for crate
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
|
= note: requested on the command line with `-D missing-docs`
error: missing documentation for a function
--> lib.rs:1:1
|
1 | pub fn foo() {}
| ^^^^^^^^^^^^
error: aborting due to 2 previous errors
You can also pass each flag more than once for changing multiple lints:
$ rustc lib.rs --crate-type=lib -D missing-docs -D unused-variables
And of course, you can mix these five flags together:
$ rustc lib.rs --crate-type=lib -D missing-docs -A unused-variables
The order of these command line arguments is taken into account. The following allows the unused-variables
lint, because it is the last argument for that lint:
$ rustc lib.rs --crate-type=lib -D unused-variables -A unused-variables
You can make use of this behavior by overriding the level of one specific lint out of a group of lints. The following example denies all the lints in the unused
group, but explicitly allows the unused-variables
lint in that group (forbid still trumps everything regardless of ordering):
$ rustc lib.rs --crate-type=lib -D unused -A unused-variables
Since force-warn
and forbid
cannot be overridden, setting
one of them will prevent any later level for the same lint from
taking effect.
Via an attribute
You can also modify the lint level with a crate-wide attribute:
$ cat lib.rs
#![warn(missing_docs)]
pub fn foo() {}
$ rustc lib.rs --crate-type=lib
warning: missing documentation for crate
--> lib.rs:1:1
|
1 | / #![warn(missing_docs)]
2 | |
3 | | pub fn foo() {}
| |_______________^
|
note: lint level defined here
--> lib.rs:1:9
|
1 | #![warn(missing_docs)]
| ^^^^^^^^^^^^
warning: missing documentation for a function
--> lib.rs:3:1
|
3 | pub fn foo() {}
| ^^^^^^^^^^^^
warn
, allow
, deny
, and forbid
all work this way. There is
no way to set a lint to force-warn
using an attribute.
You can also pass in multiple lints per attribute:
#![warn(missing_docs, unused_variables)]
pub fn foo() {}
And use multiple attributes together:
#![warn(missing_docs)]
#![deny(unused_variables)]
pub fn foo() {}
All lint attributes support an additional reason
parameter, to give context why
a certain attribute was added. This reason will be displayed as part of the lint
message, if the lint is emitted at the defined level.
use std::path::PathBuf;
pub fn get_path() -> PathBuf {
#[allow(unused_mut, reason = "this is only modified on some platforms")]
let mut file_name = PathBuf::from("git");
#[cfg(target_os = "windows")]
file_name.set_extension("exe");
file_name
}
Capping lints
rustc
supports a flag, --cap-lints LEVEL
that sets the "lint cap level."
This is the maximum level for all lints. So for example, if we take our
code sample from the "deny" lint level above:
fn main() {
100u8 << 10;
}
And we compile it, capping lints to warn:
$ rustc lib.rs --cap-lints warn
warning: bitshift exceeds the type's number of bits
--> lib.rs:2:5
|
2 | 100u8 << 10;
| ^^^^^^^^^^^
|
= note: `#[warn(exceeding_bitshifts)]` on by default
warning: this expression will panic at run-time
--> lib.rs:2:5
|
2 | 100u8 << 10;
| ^^^^^^^^^^^ attempt to shift left with overflow
It now only warns, rather than errors. We can go further and allow all lints:
$ rustc lib.rs --cap-lints allow
$
This feature is used heavily by Cargo; it will pass --cap-lints allow
when
compiling your dependencies, so that if they have any warnings, they do not
pollute the output of your build.
Lint Groups
rustc
has the concept of a "lint group", where you can toggle several warnings
through one name.
For example, the nonstandard-style
lint sets non-camel-case-types
,
non-snake-case
, and non-upper-case-globals
all at once. So these are
equivalent:
$ rustc -D nonstandard-style
$ rustc -D non-camel-case-types -D non-snake-case -D non-upper-case-globals
Here's a list of each lint group, and the lints that they are made up of:
Additionally, there's a bad-style
lint group that's a deprecated alias for nonstandard-style
.
Finally, you can also see the table above by invoking rustc -W help
. This will give you the exact values for the specific
compiler you have installed.
Lint Listing
This section lists out all of the lints, grouped by their default lint levels.
You can also see this list by running rustc -W help
.
Allowed-by-default Lints
These lints are all set to the 'allow' level by default. As such, they won't show up unless you set them to a higher lint level with a flag or attribute.
absolute_paths_not_starting_with_crate
ambiguous_negative_literals
async-idents
closure_returning_async_block
deprecated_safe_2024
disjoint-capture-migration
edition_2024_expr_fragment_specifier
elided-lifetime-in-path
elided_lifetimes_in_paths
explicit_outlives_requirements
ffi_unwind_calls
fuzzy_provenance_casts
impl_trait_overcaptures
keyword-idents
keyword_idents_2018
keyword_idents_2024
let_underscore_drop
lossy_provenance_casts
macro_use_extern_crate
meta_variable_misuse
missing_abi
missing_copy_implementations
missing_debug_implementations
missing_docs
missing_unsafe_on_extern
multiple_supertrait_upcastable
must_not_suspend
non_ascii_idents
non_exhaustive_omitted_patterns
non_local_definitions
or-patterns-back-compat
redundant_imports
redundant_lifetimes
rust_2021_incompatible_closure_captures
rust_2021_incompatible_or_patterns
rust_2021_prefixes_incompatible_syntax
rust_2021_prelude_collisions
rust_2024_incompatible_pat
rust_2024_prelude_collisions
single-use-lifetime
single_use_lifetimes
tail_expr_drop_order
trivial_casts
trivial_numeric_casts
unit_bindings
unnameable_types
unreachable_pub
unsafe_attr_outside_unsafe
unsafe_code
unsafe_op_in_unsafe_fn
unstable_features
unused_crate_dependencies
unused_extern_crates
unused_import_braces
unused_lifetimes
unused_macro_rules
unused_qualifications
unused_results
variant_size_differences
absolute-paths-not-starting-with-crate
The absolute_paths_not_starting_with_crate
lint detects fully
qualified paths that start with a module name instead of crate
,
self
, or an extern crate name
Example
#![deny(absolute_paths_not_starting_with_crate)]
mod foo {
pub fn bar() {}
}
fn main() {
::foo::bar();
}
This will produce:
error: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
--> lint_example.rs:8:5
|
8 | ::foo::bar();
| ^^^^^^^^^^ help: use `crate`: `crate::foo::bar`
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
= note: for more information, see issue #53130 <https://github.com/rust-lang/rust/issues/53130>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(absolute_paths_not_starting_with_crate)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Rust editions allow the language to evolve without breaking
backwards compatibility. This lint catches code that uses absolute
paths in the style of the 2015 edition. In the 2015 edition, absolute
paths (those starting with ::
) refer to either the crate root or an
external crate. In the 2018 edition it was changed so that they only
refer to external crates. The path prefix crate::
should be used
instead to reference items from the crate root.
If you switch the compiler from the 2015 to 2018 edition without
updating the code, then it will fail to compile if the old style paths
are used. You can manually change the paths to use the crate::
prefix to transition to the 2018 edition.
This lint solves the problem automatically. It is "allow" by default
because the code is perfectly valid in the 2015 edition. The cargo fix
tool with the --edition
flag will switch this lint to "warn"
and automatically apply the suggested fix from the compiler. This
provides a completely automated way to update old code to the 2018
edition.
ambiguous-negative-literals
The ambiguous_negative_literals
lint checks for cases that are
confusing between a negative literal and a negation that's not part
of the literal.
Example
#![deny(ambiguous_negative_literals)]
#![allow(unused)]
-1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1
This will produce:
error: `-` has lower precedence than method calls, which might be unexpected
--> lint_example.rs:4:1
|
4 | -1i32.abs(); // equals -1, while `(-1i32).abs()` equals 1
| ^^^^^^^^^^^
|
= note: e.g. `-4.abs()` equals `-4`; while `(-4).abs()` equals `4`
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(ambiguous_negative_literals)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: add parentheses around the `-` and the literal to call the method on a negative literal
|
4 | (-1i32).abs(); // equals -1, while `(-1i32).abs()` equals 1
| + +
help: add parentheses around the literal and the method call to keep the current behavior
|
4 | -(1i32.abs()); // equals -1, while `(-1i32).abs()` equals 1
| + +
Explanation
Method calls take precedence over unary precedence. Setting the precedence explicitly makes the code clearer and avoid potential bugs.
async-idents
The lint async-idents
has been renamed to keyword-idents
.
closure-returning-async-block
The closure_returning_async_block
lint detects cases where users
write a closure that returns an async block.
Example
#![warn(closure_returning_async_block)]
let c = |x: &str| async {};
This will produce:
warning: unknown lint: `closure_returning_async_block`
--> lint_example.rs:1:1
|
1 | #![warn(closure_returning_async_block)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: the `closure_returning_async_block` lint is unstable
= note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
= help: add `#![feature(async_closure)]` to the crate attributes to enable
= note: this compiler was built on 2024-09-06; consider upgrading it if it is out of date
= note: `#[warn(unknown_lints)]` on by default
Explanation
Using an async closure is preferable over a closure that returns an async block, since async closures are less restrictive in how its captures are allowed to be used.
For example, this code does not work with a closure returning an async block:
async fn callback(x: &str) {}
let captured_str = String::new();
let c = move || async {
callback(&captured_str).await;
};
But it does work with async closures:
#![feature(async_closure)]
async fn callback(x: &str) {}
let captured_str = String::new();
let c = async move || {
callback(&captured_str).await;
};
deprecated-safe-2024
The deprecated_safe_2024
lint detects unsafe functions being used as
safe functions.
Example
#![deny(deprecated_safe)]
// edition 2021
use std::env;
fn enable_backtrace() {
env::set_var("RUST_BACKTRACE", "1");
}
This will produce:
error: call to deprecated safe function `std::env::set_var` is unsafe and requires unsafe block
--> lint_example.rs:6:5
|
6 | env::set_var("RUST_BACKTRACE", "1");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
|
= warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
= note: for more information, see issue #27970 <https://github.com/rust-lang/rust/issues/27970>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(deprecated_safe)]
| ^^^^^^^^^^^^^^^
= note: `#[deny(deprecated_safe_2024)]` implied by `#[deny(deprecated_safe)]`
help: you can wrap the call in an `unsafe` block if you can guarantee that the environment access only happens in single-threaded code
|
6 + // TODO: Audit that the environment access only happens in single-threaded code.
7 ~ unsafe { env::set_var("RUST_BACKTRACE", "1") };
|
Explanation
Rust editions allow the language to evolve without breaking backward
compatibility. This lint catches code that uses unsafe
functions that
were declared as safe (non-unsafe
) in editions prior to Rust 2024. If
you switch the compiler to Rust 2024 without updating the code, then it
will fail to compile if you are using a function previously marked as
safe.
You can audit the code to see if it suffices the preconditions of the
unsafe
code, and if it does, you can wrap it in an unsafe
block. If
you can't fulfill the preconditions, you probably need to switch to a
different way of doing what you want to achieve.
This lint can automatically wrap the calls in unsafe
blocks, but this
obviously cannot verify that the preconditions of the unsafe
functions are fulfilled, so that is still up to the user.
The lint is currently "allow" by default, but that might change in the future.
disjoint-capture-migration
The lint disjoint-capture-migration
has been renamed to rust-2021-incompatible-closure-captures
.
edition-2024-expr-fragment-specifier
The edition_2024_expr_fragment_specifier
lint detects the use of
expr
fragments in macros during migration to the 2024 edition.
The expr
fragment specifier will accept more expressions in the 2024
edition. To maintain the behavior from the 2021 edition and earlier, use
the expr_2021
fragment specifier.
Example
#![deny(edition_2024_expr_fragment_specifier)]
macro_rules! m {
($e:expr) => {
$e
}
}
fn main() {
m!(1);
}
This will produce:
error: the `expr` fragment specifier will accept more expressions in the 2024 edition
--> lint_example.rs:3:7
|
3 | ($e:expr) => {
| ^^^^
|
= warning: this changes meaning in Rust 2024
= note: for more information, see Migration Guide <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/macro-fragment-specifiers.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(edition_2024_expr_fragment_specifier)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to keep the existing behavior, use the `expr_2021` fragment specifier
|
3 | ($e:expr_2021) => {
| ~~~~~~~~~
Explanation
Rust editions allow the language to evolve without breaking backwards compatibility. This lint catches code that uses macro matcher fragment specifiers that have changed meaning in the 2024 edition. If you switch to the new edition without updating the code, your macros may behave differently.
In the 2024 edition, the expr
fragment specifier expr
will also
match const { ... }
blocks. This means if a macro had a pattern that
matched $e:expr
and another that matches const { $e: expr }
, for
example, that under the 2024 edition the first pattern would match while
in the 2021 and earlier editions the second pattern would match. To keep
the old behavior, use the expr_2021
fragment specifier.
This lint detects macros whose behavior might change due to the changing
meaning of the expr
fragment specifier. It is "allow" by default
because the code is perfectly valid in older editions. The cargo fix
tool with the --edition
flag will switch this lint to "warn" and
automatically apply the suggested fix from the compiler. This provides a
completely automated way to update old code for a new edition.
Using cargo fix --edition
with this lint will ensure that your code
retains the same behavior. This may not be the desired, as macro authors
often will want their macros to use the latest grammar for matching
expressions. Be sure to carefully review changes introduced by this lint
to ensure the macros implement the desired behavior.
elided-lifetime-in-path
The lint elided-lifetime-in-path
has been renamed to elided-lifetimes-in-paths
.
elided-lifetimes-in-paths
The elided_lifetimes_in_paths
lint detects the use of hidden
lifetime parameters.
Example
#![deny(elided_lifetimes_in_paths)]
#![deny(warnings)]
struct Foo<'a> {
x: &'a u32
}
fn foo(x: &Foo) {
}
This will produce:
error: hidden lifetime parameters in types are deprecated
--> lint_example.rs:8:12
|
8 | fn foo(x: &Foo) {
| ^^^ expected lifetime parameter
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(elided_lifetimes_in_paths)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
help: indicate the anonymous lifetime
|
8 | fn foo(x: &Foo<'_>) {
| ++++
Explanation
Elided lifetime parameters can make it difficult to see at a glance
that borrowing is occurring. This lint ensures that lifetime
parameters are always explicitly stated, even if it is the '_
placeholder lifetime.
This lint is "allow" by default because it has some known issues, and may require a significant transition for old code.
explicit-outlives-requirements
The explicit_outlives_requirements
lint detects unnecessary
lifetime bounds that can be inferred.
Example
#![allow(unused)]
#![deny(explicit_outlives_requirements)]
#![deny(warnings)]
struct SharedRef<'a, T>
where
T: 'a,
{
data: &'a T,
}
This will produce:
error: outlives requirements can be inferred
--> lint_example.rs:6:24
|
6 | struct SharedRef<'a, T>
| ________________________^
7 | | where
8 | | T: 'a,
| |__________^ help: remove this bound
|
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![deny(explicit_outlives_requirements)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
If a struct
contains a reference, such as &'a T
, the compiler
requires that T
outlives the lifetime 'a
. This historically
required writing an explicit lifetime bound to indicate this
requirement. However, this can be overly explicit, causing clutter and
unnecessary complexity. The language was changed to automatically
infer the bound if it is not specified. Specifically, if the struct
contains a reference, directly or indirectly, to T
with lifetime
'x
, then it will infer that T: 'x
is a requirement.
This lint is "allow" by default because it can be noisy for existing code that already had these requirements. This is a stylistic choice, as it is still valid to explicitly state the bound. It also has some false positives that can cause confusion.
See RFC 2093 for more details.
ffi-unwind-calls
The ffi_unwind_calls
lint detects calls to foreign functions or function pointers with
C-unwind
or other FFI-unwind ABIs.
Example
#![warn(ffi_unwind_calls)]
extern "C-unwind" {
fn foo();
}
fn bar() {
unsafe { foo(); }
let ptr: unsafe extern "C-unwind" fn() = foo;
unsafe { ptr(); }
}
This will produce:
warning: call to foreign function with FFI-unwind ABI
--> lint_example.rs:9:14
|
9 | unsafe { foo(); }
| ^^^^^ call to foreign function with FFI-unwind ABI
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![warn(ffi_unwind_calls)]
| ^^^^^^^^^^^^^^^^
warning: call to function pointer with FFI-unwind ABI
--> lint_example.rs:11:14
|
11 | unsafe { ptr(); }
| ^^^^^ call to function pointer with FFI-unwind ABI
Explanation
For crates containing such calls, if they are compiled with -C panic=unwind
then the
produced library cannot be linked with crates compiled with -C panic=abort
. For crates
that desire this ability it is therefore necessary to avoid such calls.
fuzzy-provenance-casts
The fuzzy_provenance_casts
lint detects an as
cast between an integer
and a pointer.
Example
#![feature(strict_provenance)]
#![warn(fuzzy_provenance_casts)]
fn main() {
let _dangling = 16_usize as *const u8;
}
This will produce:
warning: strict provenance disallows casting integer `usize` to pointer `*const u8`
--> lint_example.rs:5:21
|
5 | let _dangling = 16_usize as *const u8;
| ^^^^^^^^^^^^^^^^^^^^^
|
= help: if you can't comply with strict provenance and don't have a pointer with the correct provenance you can use `std::ptr::with_exposed_provenance()` instead
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![warn(fuzzy_provenance_casts)]
| ^^^^^^^^^^^^^^^^^^^^^^
help: use `.with_addr()` to adjust a valid pointer in the same allocation, to this address
|
5 | let _dangling = (...).with_addr(16_usize);
| ++++++++++++++++ ~
Explanation
This lint is part of the strict provenance effort, see issue #95228.
Casting an integer to a pointer is considered bad style, as a pointer
contains, besides the address also a provenance, indicating what
memory the pointer is allowed to read/write. Casting an integer, which
doesn't have provenance, to a pointer requires the compiler to assign
(guess) provenance. The compiler assigns "all exposed valid" (see the
docs of ptr::with_exposed_provenance
for more information about this
"exposing"). This penalizes the optimiser and is not well suited for
dynamic analysis/dynamic program verification (e.g. Miri or CHERI
platforms).
It is much better to use ptr::with_addr
instead to specify the
provenance you want. If using this function is not possible because the
code relies on exposed provenance then there is as an escape hatch
ptr::with_exposed_provenance
.
impl-trait-overcaptures
The impl_trait_overcaptures
lint warns against cases where lifetime
capture behavior will differ in edition 2024.
In the 2024 edition, impl Trait
s will capture all lifetimes in scope,
rather than just the lifetimes that are mentioned in the bounds of the type.
Often these sets are equal, but if not, it means that the impl Trait
may
cause erroneous borrow-checker errors.
Example
#![deny(impl_trait_overcaptures)]
use std::fmt::Display;
let mut x = vec![];
x.push(1);
fn test(x: &Vec<i32>) -> impl Display {
x[0]
}
let element = test(&x);
x.push(2);
println!("{element}");
This will produce:
error: `impl std::fmt::Display` will capture more lifetimes than possibly intended in edition 2024
--> lint_example.rs:7:26
|
7 | fn test(x: &Vec<i32>) -> impl Display {
| ^^^^^^^^^^^^
|
= warning: this changes meaning in Rust 2024
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/rpit-lifetime-capture.html>
note: specifically, this lifetime is in scope but not mentioned in the type's bounds
--> lint_example.rs:7:12
|
7 | fn test(x: &Vec<i32>) -> impl Display {
| ^
= note: all lifetimes in scope will be captured by `impl Trait`s in edition 2024
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(impl_trait_overcaptures)]
| ^^^^^^^^^^^^^^^^^^^^^^^
help: use the precise capturing `use<...>` syntax to make the captures explicit
|
7 | fn test(x: &Vec<i32>) -> impl Display + use<> {
| +++++++
Explanation
In edition < 2024, the returned impl Display
doesn't capture the
lifetime from the &Vec<i32>
, so the vector can be mutably borrowed
while the impl Display
is live.
To fix this, we can explicitly state that the impl Display
doesn't
capture any lifetimes, using impl Display + use<>
.
keyword-idents
The lint keyword-idents
has been renamed to keyword-idents-2018
.
keyword-idents-2018
The keyword_idents_2018
lint detects edition keywords being used as an
identifier.
Example
#![deny(keyword_idents_2018)]
// edition 2015
fn dyn() {}
This will produce:
error: `dyn` is a keyword in the 2018 edition
--> lint_example.rs:4:4
|
4 | fn dyn() {}
| ^^^ help: you can use a raw identifier to stay compatible: `r#dyn`
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
= note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(keyword_idents_2018)]
| ^^^^^^^^^^^^^^^^^^^
Explanation
Rust editions allow the language to evolve without breaking backwards compatibility. This lint catches code that uses new keywords that are added to the language that are used as identifiers (such as a variable name, function name, etc.). If you switch the compiler to a new edition without updating the code, then it will fail to compile if you are using a new keyword as an identifier.
You can manually change the identifiers to a non-keyword, or use a
raw identifier, for example r#dyn
, to transition to a new edition.
This lint solves the problem automatically. It is "allow" by default
because the code is perfectly valid in older editions. The cargo fix
tool with the --edition
flag will switch this lint to "warn"
and automatically apply the suggested fix from the compiler (which is
to use a raw identifier). This provides a completely automated way to
update old code for a new edition.
keyword-idents-2024
The keyword_idents_2024
lint detects edition keywords being used as an
identifier.
Example
#![deny(keyword_idents_2024)]
// edition 2015
fn gen() {}
This will produce:
error: `gen` is a keyword in the 2024 edition
--> lint_example.rs:4:4
|
4 | fn gen() {}
| ^^^ help: you can use a raw identifier to stay compatible: `r#gen`
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2024!
= note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(keyword_idents_2024)]
| ^^^^^^^^^^^^^^^^^^^
Explanation
Rust editions allow the language to evolve without breaking backwards compatibility. This lint catches code that uses new keywords that are added to the language that are used as identifiers (such as a variable name, function name, etc.). If you switch the compiler to a new edition without updating the code, then it will fail to compile if you are using a new keyword as an identifier.
You can manually change the identifiers to a non-keyword, or use a
raw identifier, for example r#gen
, to transition to a new edition.
This lint solves the problem automatically. It is "allow" by default
because the code is perfectly valid in older editions. The cargo fix
tool with the --edition
flag will switch this lint to "warn"
and automatically apply the suggested fix from the compiler (which is
to use a raw identifier). This provides a completely automated way to
update old code for a new edition.
let-underscore-drop
The let_underscore_drop
lint checks for statements which don't bind
an expression which has a non-trivial Drop implementation to anything,
causing the expression to be dropped immediately instead of at end of
scope.
Example
struct SomeStruct;
impl Drop for SomeStruct {
fn drop(&mut self) {
println!("Dropping SomeStruct");
}
}
fn main() {
#[warn(let_underscore_drop)]
// SomeStruct is dropped immediately instead of at end of scope,
// so "Dropping SomeStruct" is printed before "end of main".
// The order of prints would be reversed if SomeStruct was bound to
// a name (such as "_foo").
let _ = SomeStruct;
println!("end of main");
}
This will produce:
warning: non-binding let on a type that implements `Drop`
--> lint_example.rs:14:5
|
14 | let _ = SomeStruct;
| ^^^^^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:9:11
|
9 | #[warn(let_underscore_drop)]
| ^^^^^^^^^^^^^^^^^^^
help: consider binding to an unused variable to avoid immediately dropping the value
|
14 | let _unused = SomeStruct;
| ~~~~~~~
help: consider immediately dropping the value
|
14 | drop(SomeStruct);
| ~~~~~ +
Explanation
Statements which assign an expression to an underscore causes the
expression to immediately drop instead of extending the expression's
lifetime to the end of the scope. This is usually unintended,
especially for types like MutexGuard
, which are typically used to
lock a mutex for the duration of an entire scope.
If you want to extend the expression's lifetime to the end of the scope,
assign an underscore-prefixed name (such as _foo
) to the expression.
If you do actually want to drop the expression immediately, then
calling std::mem::drop
on the expression is clearer and helps convey
intent.
lossy-provenance-casts
The lossy_provenance_casts
lint detects an as
cast between a pointer
and an integer.
Example
#![feature(strict_provenance)]
#![warn(lossy_provenance_casts)]
fn main() {
let x: u8 = 37;
let _addr: usize = &x as *const u8 as usize;
}
This will produce:
warning: under strict provenance it is considered bad style to cast pointer `*const u8` to integer `usize`
--> lint_example.rs:6:24
|
6 | let _addr: usize = &x as *const u8 as usize;
| ^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: if you can't comply with strict provenance and need to expose the pointer provenance you can use `.expose_provenance()` instead
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![warn(lossy_provenance_casts)]
| ^^^^^^^^^^^^^^^^^^^^^^
help: use `.addr()` to obtain the address of a pointer
|
6 | let _addr: usize = (&x as *const u8).addr();
| + ~~~~~~~~
Explanation
This lint is part of the strict provenance effort, see issue #95228. Casting a pointer to an integer is a lossy operation, because beyond just an address a pointer may be associated with a particular provenance. This information is used by the optimiser and for dynamic analysis/dynamic program verification (e.g. Miri or CHERI platforms).
Since this cast is lossy, it is considered good style to use the
ptr::addr
method instead, which has a similar effect, but doesn't
"expose" the pointer provenance. This improves optimisation potential.
See the docs of ptr::addr
and ptr::expose_provenance
for more information
about exposing pointer provenance.
If your code can't comply with strict provenance and needs to expose
the provenance, then there is ptr::expose_provenance
as an escape hatch,
which preserves the behaviour of as usize
casts while being explicit
about the semantics.
macro-use-extern-crate
The macro_use_extern_crate
lint detects the use of the macro_use
attribute.
Example
#![deny(macro_use_extern_crate)]
#[macro_use]
extern crate serde_json;
fn main() {
let _ = json!{{}};
}
This will produce:
error: applying the `#[macro_use]` attribute to an `extern crate` item is deprecated
--> src/main.rs:3:1
|
3 | #[macro_use]
| ^^^^^^^^^^^^
|
= help: remove it and import macros at use sites with a `use` item instead
note: the lint level is defined here
--> src/main.rs:1:9
|
1 | #![deny(macro_use_extern_crate)]
| ^^^^^^^^^^^^^^^^^^^^^^
Explanation
The macro_use
attribute on an extern crate
item causes
macros in that external crate to be brought into the prelude of the
crate, making the macros in scope everywhere. As part of the efforts
to simplify handling of dependencies in the 2018 edition, the use of
extern crate
is being phased out. To bring macros from extern crates
into scope, it is recommended to use a use
import.
This lint is "allow" by default because this is a stylistic choice that has not been settled, see issue #52043 for more information.
meta-variable-misuse
The meta_variable_misuse
lint detects possible meta-variable misuse
in macro definitions.
Example
#![deny(meta_variable_misuse)]
macro_rules! foo {
() => {};
($( $i:ident = $($j:ident),+ );*) => { $( $( $i = $k; )+ )* };
}
fn main() {
foo!();
}
This will produce:
error: unknown macro variable `k`
--> lint_example.rs:5:55
|
5 | ($( $i:ident = $($j:ident),+ );*) => { $( $( $i = $k; )+ )* };
| ^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(meta_variable_misuse)]
| ^^^^^^^^^^^^^^^^^^^^
Explanation
There are quite a few different ways a macro_rules
macro can be
improperly defined. Many of these errors were previously only detected
when the macro was expanded or not at all. This lint is an attempt to
catch some of these problems when the macro is defined.
This lint is "allow" by default because it may have false positives and other issues. See issue #61053 for more details.
missing-abi
The missing_abi
lint detects cases where the ABI is omitted from
extern declarations.
Example
#![deny(missing_abi)]
extern fn foo() {}
This will produce:
error: extern declarations without an explicit ABI are deprecated
--> lint_example.rs:4:1
|
4 | extern fn foo() {}
| ^^^^^^^^^^^^^^^ ABI should be specified here
|
= help: the default ABI is C
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(missing_abi)]
| ^^^^^^^^^^^
Explanation
Historically, Rust implicitly selected C as the ABI for extern
declarations. We expect to add new ABIs, like C-unwind
, in the future,
though this has not yet happened, and especially with their addition
seeing the ABI easily will make code review easier.
missing-copy-implementations
The missing_copy_implementations
lint detects potentially-forgotten
implementations of Copy
for public types.
Example
#![deny(missing_copy_implementations)]
pub struct Foo {
pub field: i32
}
fn main() {}
This will produce:
error: type could implement `Copy`; consider adding `impl Copy`
--> lint_example.rs:2:1
|
2 | / pub struct Foo {
3 | | pub field: i32
4 | | }
| |_^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(missing_copy_implementations)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Historically (before 1.0), types were automatically marked as Copy
if possible. This was changed so that it required an explicit opt-in
by implementing the Copy
trait. As part of this change, a lint was
added to alert if a copyable type was not marked Copy
.
This lint is "allow" by default because this code isn't bad; it is
common to write newtypes like this specifically so that a Copy
type
is no longer Copy
. Copy
types can result in unintended copies of
large data which can impact performance.
missing-debug-implementations
The missing_debug_implementations
lint detects missing
implementations of fmt::Debug
for public types.
Example
#![deny(missing_debug_implementations)]
pub struct Foo;
fn main() {}
This will produce:
error: type does not implement `Debug`; consider adding `#[derive(Debug)]` or a manual implementation
--> lint_example.rs:2:1
|
2 | pub struct Foo;
| ^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(missing_debug_implementations)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Having a Debug
implementation on all types can assist with
debugging, as it provides a convenient way to format and display a
value. Using the #[derive(Debug)]
attribute will automatically
generate a typical implementation, or a custom implementation can be
added by manually implementing the Debug
trait.
This lint is "allow" by default because adding Debug
to all types can
have a negative impact on compile time and code size. It also requires
boilerplate to be added to every type, which can be an impediment.
missing-docs
The missing_docs
lint detects missing documentation for public items.
Example
#![deny(missing_docs)]
pub fn foo() {}
This will produce:
error: missing documentation for the crate
--> lint_example.rs:1:1
|
1 | / #![deny(missing_docs)]
2 | | fn main() {
3 | | pub fn foo() {}
4 | | }
| |_^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(missing_docs)]
| ^^^^^^^^^^^^
Explanation
This lint is intended to ensure that a library is well-documented. Items without documentation can be difficult for users to understand how to use properly.
This lint is "allow" by default because it can be noisy, and not all projects may want to enforce everything to be documented.
missing-unsafe-on-extern
The missing_unsafe_on_extern
lint detects missing unsafe keyword on extern declarations.
Example
#![warn(missing_unsafe_on_extern)]
#![allow(dead_code)]
extern "C" {
fn foo(_: i32);
}
fn main() {}
This will produce:
warning: extern blocks should be unsafe
--> lint_example.rs:4:1
|
4 | extern "C" {
| ^
| |
| _help: needs `unsafe` before the extern keyword: `unsafe`
| |
5 | | fn foo(_: i32);
6 | | }
| |_^
|
= warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
= note: for more information, see issue #123743 <https://github.com/rust-lang/rust/issues/123743>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![warn(missing_unsafe_on_extern)]
| ^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Declaring extern items, even without ever using them, can cause Undefined Behavior. We should consider all sources of Undefined Behavior to be unsafe.
This is a future-incompatible lint to transition this to a hard error in the future.
multiple-supertrait-upcastable
The multiple_supertrait_upcastable
lint detects when an object-safe trait has multiple
supertraits.
Example
#![feature(multiple_supertrait_upcastable)]
trait A {}
trait B {}
#[warn(multiple_supertrait_upcastable)]
trait C: A + B {}
This will produce:
warning: `C` is object-safe and has multiple supertraits
--> lint_example.rs:7:1
|
7 | trait C: A + B {}
| ^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:6:8
|
6 | #[warn(multiple_supertrait_upcastable)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
To support upcasting with multiple supertraits, we need to store multiple vtables and this can result in extra space overhead, even if no code actually uses upcasting. This lint allows users to identify when such scenarios occur and to decide whether the additional overhead is justified.
must-not-suspend
The must_not_suspend
lint guards against values that shouldn't be held across suspend points
(.await
)
Example
#![feature(must_not_suspend)]
#![warn(must_not_suspend)]
#[must_not_suspend]
struct SyncThing {}
async fn yield_now() {}
pub async fn uhoh() {
let guard = SyncThing {};
yield_now().await;
let _guard = guard;
}
This will produce:
warning: `SyncThing` held across a suspend point, but should not be
--> lint_example.rs:11:9
|
11 | let guard = SyncThing {};
| ^^^^^
12 | yield_now().await;
| ----- the value is held across this suspend point
|
help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
--> lint_example.rs:11:9
|
11 | let guard = SyncThing {};
| ^^^^^
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![warn(must_not_suspend)]
| ^^^^^^^^^^^^^^^^
Explanation
The must_not_suspend
lint detects values that are marked with the #[must_not_suspend]
attribute being held across suspend points. A "suspend" point is usually a .await
in an async
function.
This attribute can be used to mark values that are semantically incorrect across suspends
(like certain types of timers), values that have async alternatives, and values that
regularly cause problems with the Send
-ness of async fn's returned futures (like
MutexGuard
's)
non-ascii-idents
The non_ascii_idents
lint detects non-ASCII identifiers.
Example
#![allow(unused)]
#![deny(non_ascii_idents)]
fn main() {
let föö = 1;
}
This will produce:
error: identifier contains non-ASCII characters
--> lint_example.rs:4:9
|
4 | let föö = 1;
| ^^^
|
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![deny(non_ascii_idents)]
| ^^^^^^^^^^^^^^^^
Explanation
This lint allows projects that wish to retain the limit of only using ASCII characters to switch this lint to "forbid" (for example to ease collaboration or for security reasons). See RFC 2457 for more details.
non-exhaustive-omitted-patterns
The non_exhaustive_omitted_patterns
lint aims to help consumers of a #[non_exhaustive]
struct or enum who want to match all of its fields/variants explicitly.
The #[non_exhaustive]
annotation forces matches to use wildcards, so exhaustiveness
checking cannot be used to ensure that all fields/variants are matched explicitly. To remedy
this, this allow-by-default lint warns the user when a match mentions some but not all of
the fields/variants of a #[non_exhaustive]
struct or enum.
Example
// crate A
#[non_exhaustive]
pub enum Bar {
A,
B, // added variant in non breaking change
}
// in crate B
#![feature(non_exhaustive_omitted_patterns_lint)]
#[warn(non_exhaustive_omitted_patterns)]
match Bar::A {
Bar::A => {},
_ => {},
}
This will produce:
warning: some variants are not matched explicitly
--> $DIR/reachable-patterns.rs:70:9
|
LL | match Bar::A {
| ^ pattern `Bar::B` not covered
|
note: the lint level is defined here
--> $DIR/reachable-patterns.rs:69:16
|
LL | #[warn(non_exhaustive_omitted_patterns)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: ensure that all variants are matched explicitly by adding the suggested match arms
= note: the matched value is of type `Bar` and the `non_exhaustive_omitted_patterns` attribute was found
Warning: setting this to deny
will make upstream non-breaking changes (adding fields or
variants to a #[non_exhaustive]
struct or enum) break your crate. This goes against
expected semver behavior.
Explanation
Structs and enums tagged with #[non_exhaustive]
force the user to add a (potentially
redundant) wildcard when pattern-matching, to allow for future addition of fields or
variants. The non_exhaustive_omitted_patterns
lint detects when such a wildcard happens to
actually catch some fields/variants. In other words, when the match without the wildcard
would not be exhaustive. This lets the user be informed if new fields/variants were added.
non-local-definitions
The non_local_definitions
lint checks for impl
blocks and #[macro_export]
macro inside bodies (functions, enum discriminant, ...).
Example
#![warn(non_local_definitions)]
trait MyTrait {}
struct MyStruct;
fn foo() {
impl MyTrait for MyStruct {}
}
This will produce:
warning: non-local `impl` definition, `impl` blocks should be written at the same level as their item
--> lint_example.rs:7:5
|
6 | fn foo() {
| -------- move the `impl` block outside of this function `foo` and up 2 bodies
7 | impl MyTrait for MyStruct {}
| ^^^^^-------^^^^^--------
| | |
| | `MyStruct` is not local
| `MyTrait` is not local
|
= note: `impl` may be usable in bounds, etc. from outside the expression, which might e.g. make something constructible that previously wasn't, because it's still on a publicly-visible type
= note: an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`
= note: this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <https://github.com/rust-lang/rust/issues/120363>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![warn(non_local_definitions)]
| ^^^^^^^^^^^^^^^^^^^^^
Explanation
Creating non-local definitions go against expectation and can create discrepancies in tooling. It should be avoided. It may become deny-by-default in edition 2024 and higher, see the tracking issue https://github.com/rust-lang/rust/issues/120363.
An impl
definition is non-local if it is nested inside an item and neither
the type nor the trait are at the same nesting level as the impl
block.
All nested bodies (functions, enum discriminant, array length, consts) (expect for
const _: Ty = { ... }
in top-level module, which is still undecided) are checked.
or-patterns-back-compat
The lint or-patterns-back-compat
has been renamed to rust-2021-incompatible-or-patterns
.
redundant-imports
The redundant_imports
lint detects imports that are redundant due to being
imported already; either through a previous import, or being present in
the prelude.
Example
#![deny(redundant_imports)]
use std::option::Option::None;
fn foo() -> Option<i32> { None }
This will produce:
error: the item `None` is imported redundantly
--> lint_example.rs:3:5
|
3 | use std::option::Option::None;
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
::: /home/ci/project/library/std/src/prelude/mod.rs:155:13
|
155 | pub use core::prelude::rust_2021::*;
| ------------------------ the item `None` is already defined here
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(redundant_imports)]
| ^^^^^^^^^^^^^^^^^
Explanation
Redundant imports are unnecessary and can be removed to simplify code.
If you intended to re-export the item to make it available outside of the
module, add a visibility modifier like pub
.
redundant-lifetimes
The redundant_lifetimes
lint detects lifetime parameters that are
redundant because they are equal to another named lifetime.
Example
#[deny(redundant_lifetimes)]
// `'a = 'static`, so all usages of `'a` can be replaced with `'static`
pub fn bar<'a: 'static>() {}
// `'a = 'b`, so all usages of `'b` can be replaced with `'a`
pub fn bar<'a: 'b, 'b: 'a>() {}
This will produce:
error: unnecessary lifetime parameter `'a`
--> lint_example.rs:5:12
|
5 | pub fn bar<'a: 'static>() {}
| ^^
|
= note: you can use the `'static` lifetime directly, in place of `'a`
note: the lint level is defined here
--> lint_example.rs:2:8
|
2 | #[deny(redundant_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^
Explanation
Unused lifetime parameters may signal a mistake or unfinished code. Consider removing the parameter.
rust-2021-incompatible-closure-captures
The rust_2021_incompatible_closure_captures
lint detects variables that aren't completely
captured in Rust 2021, such that the Drop
order of their fields may differ between
Rust 2018 and 2021.
It can also detect when a variable implements a trait like Send
, but one of its fields does not,
and the field is captured by a closure and used with the assumption that said field implements
the same trait as the root variable.
Example of drop reorder
#![deny(rust_2021_incompatible_closure_captures)]
#![allow(unused)]
struct FancyInteger(i32);
impl Drop for FancyInteger {
fn drop(&mut self) {
println!("Just dropped {}", self.0);
}
}
struct Point { x: FancyInteger, y: FancyInteger }
fn main() {
let p = Point { x: FancyInteger(10), y: FancyInteger(20) };
let c = || {
let x = p.x;
};
c();
// ... More code ...
}
This will produce:
error: changes to closure capture in Rust 2021 will affect drop order
--> lint_example.rs:17:11
|
17 | let c = || {
| ^^
18 | let x = p.x;
| --- in Rust 2018, this closure captures all of `p`, but in Rust 2021, it will only capture `p.x`
...
24 | }
| - in Rust 2018, `p` is dropped here, but in Rust 2021, only `p.x` will be dropped here as part of the closure
|
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(rust_2021_incompatible_closure_captures)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: add a dummy let to cause `p` to be fully captured
|
17 ~ let c = || {
18 + let _ = &p;
|
Explanation
In the above example, p.y
will be dropped at the end of f
instead of
with c
in Rust 2021.
Example of auto-trait
#![deny(rust_2021_incompatible_closure_captures)]
use std::thread;
struct Pointer(*mut i32);
unsafe impl Send for Pointer {}
fn main() {
let mut f = 10;
let fptr = Pointer(&mut f as *mut i32);
thread::spawn(move || unsafe {
*fptr.0 = 20;
});
}
This will produce:
error: changes to closure capture in Rust 2021 will affect which traits the closure implements
--> lint_example.rs:10:19
|
10 | thread::spawn(move || unsafe {
| ^^^^^^^ in Rust 2018, this closure implements `Send` as `fptr` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` because `fptr` is not fully captured and `fptr.0` does not implement `Send`
11 | *fptr.0 = 20;
| ------- in Rust 2018, this closure captures all of `fptr`, but in Rust 2021, it will only capture `fptr.0`
|
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(rust_2021_incompatible_closure_captures)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: add a dummy let to cause `fptr` to be fully captured
|
10 ~ thread::spawn(move || { let _ = &fptr; unsafe {
11 | *fptr.0 = 20;
12 ~ } });
|
Explanation
In the above example, only fptr.0
is captured in Rust 2021.
The field is of type *mut i32
, which doesn't implement Send
,
making the code invalid as the field cannot be sent between threads safely.
rust-2021-incompatible-or-patterns
The rust_2021_incompatible_or_patterns
lint detects usage of old versions of or-patterns.
Example
#![deny(rust_2021_incompatible_or_patterns)]
macro_rules! match_any {
( $expr:expr , $( $( $pat:pat )|+ => $expr_arm:expr ),+ ) => {
match $expr {
$(
$( $pat => $expr_arm, )+
)+
}
};
}
fn main() {
let result: Result<i64, i32> = Err(42);
let int: i64 = match_any!(result, Ok(i) | Err(i) => i.into());
assert_eq!(int, 42);
}
This will produce:
error: the meaning of the `pat` fragment specifier is changing in Rust 2021, which may affect this macro
--> lint_example.rs:4:26
|
4 | ( $expr:expr , $( $( $pat:pat )|+ => $expr_arm:expr ),+ ) => {
| ^^^^^^^^ help: use pat_param to preserve semantics: `$pat:pat_param`
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/or-patterns-macro-rules.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(rust_2021_incompatible_or_patterns)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
In Rust 2021, the pat
matcher will match additional patterns, which include the |
character.
rust-2021-prefixes-incompatible-syntax
The rust_2021_prefixes_incompatible_syntax
lint detects identifiers that will be parsed as a
prefix instead in Rust 2021.
Example
#![deny(rust_2021_prefixes_incompatible_syntax)]
macro_rules! m {
(z $x:expr) => ();
}
m!(z"hey");
This will produce:
error: prefix `z` is unknown
--> lint_example.rs:8:4
|
8 | m!(z"hey");
| ^ unknown prefix
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/reserving-syntax.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(rust_2021_prefixes_incompatible_syntax)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: insert whitespace here to avoid this being parsed as a prefix in Rust 2021
|
8 | m!(z "hey");
| +
Explanation
In Rust 2015 and 2018, z"hey"
is two tokens: the identifier z
followed by the string literal "hey"
. In Rust 2021, the z
is
considered a prefix for "hey"
.
This lint suggests to add whitespace between the z
and "hey"
tokens
to keep them separated in Rust 2021.
rust-2021-prelude-collisions
The rust_2021_prelude_collisions
lint detects the usage of trait methods which are ambiguous
with traits added to the prelude in future editions.
Example
#![deny(rust_2021_prelude_collisions)]
trait Foo {
fn try_into(self) -> Result<String, !>;
}
impl Foo for &str {
fn try_into(self) -> Result<String, !> {
Ok(String::from(self))
}
}
fn main() {
let x: String = "3".try_into().unwrap();
// ^^^^^^^^
// This call to try_into matches both Foo::try_into and TryInto::try_into as
// `TryInto` has been added to the Rust prelude in 2021 edition.
println!("{x}");
}
This will produce:
error: trait method `try_into` will become ambiguous in Rust 2021
--> lint_example.rs:14:21
|
14 | let x: String = "3".try_into().unwrap();
| ^^^^^^^^^^^^^^ help: disambiguate the associated function: `Foo::try_into(&*"3")`
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/prelude.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(rust_2021_prelude_collisions)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
In Rust 2021, one of the important introductions is the prelude changes, which add
TryFrom
, TryInto
, and FromIterator
into the standard library's prelude. Since this
results in an ambiguity as to which method/function to call when an existing try_into
method is called via dot-call syntax or a try_from
/from_iter
associated function
is called directly on a type.
rust-2024-incompatible-pat
The rust_2024_incompatible_pat
lint
detects patterns whose meaning will change in the Rust 2024 edition.
Example
#![feature(ref_pat_eat_one_layer_2024)]
#![warn(rust_2024_incompatible_pat)]
if let Some(&a) = &Some(&0u8) {
let _: u8 = a;
}
if let Some(mut _a) = &mut Some(0u8) {
_a = 7u8;
}
This will produce:
warning: the semantics of this pattern will change in edition 2024
--> lint_example.rs:5:8
|
5 | if let Some(&a) = &Some(&0u8) {
| -^^^^^^^
| |
| help: desugar the match ergonomics: `&`
|
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![warn(rust_2024_incompatible_pat)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: the semantics of this pattern will change in edition 2024
--> lint_example.rs:8:8
|
8 | if let Some(mut _a) = &mut Some(0u8) {
| -^^^^^^^^^^^
| |
| help: desugar the match ergonomics: `&mut`
Explanation
In Rust 2024 and above, the mut
keyword does not reset the pattern binding mode,
and nor do &
or &mut
patterns. The lint will suggest code that
has the same meaning in all editions.
rust-2024-prelude-collisions
The rust_2024_prelude_collisions
lint detects the usage of trait methods which are ambiguous
with traits added to the prelude in future editions.
Example
#![deny(rust_2024_prelude_collisions)]
trait Meow {
fn poll(&self) {}
}
impl<T> Meow for T {}
fn main() {
core::pin::pin!(async {}).poll();
// ^^^^^^
// This call to try_into matches both Future::poll and Meow::poll as
// `Future` has been added to the Rust prelude in 2024 edition.
}
This will produce:
error: trait method `poll` will become ambiguous in Rust 2024
--> lint_example.rs:8:5
|
8 | core::pin::pin!(async {}).poll();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: disambiguate the associated function: `Meow::poll(&core::pin::pin!(async {}))`
|
= warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/prelude.html>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(rust_2024_prelude_collisions)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Rust 2024, introduces two new additions to the standard library's prelude:
Future
and IntoFuture
. This results in an ambiguity as to which method/function
to call when an existing poll
/into_future
method is called via dot-call syntax or
a poll
/into_future
associated function is called directly on a type.
single-use-lifetime
The lint single-use-lifetime
has been renamed to single-use-lifetimes
.
single-use-lifetimes
The single_use_lifetimes
lint detects lifetimes that are only used
once.
Example
#![deny(single_use_lifetimes)]
fn foo<'a>(x: &'a u32) {}
This will produce:
error: lifetime parameter `'a` only used once
--> lint_example.rs:4:8
|
4 | fn foo<'a>(x: &'a u32) {}
| ^^ -- ...is used only here
| |
| this lifetime...
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(single_use_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^^
help: elide the single-use lifetime
|
4 - fn foo<'a>(x: &'a u32) {}
4 + fn foo(x: &u32) {}
|
Explanation
Specifying an explicit lifetime like 'a
in a function or impl
should only be used to link together two things. Otherwise, you should
just use '_
to indicate that the lifetime is not linked to anything,
or elide the lifetime altogether if possible.
This lint is "allow" by default because it was introduced at a time
when '_
and elided lifetimes were first being introduced, and this
lint would be too noisy. Also, there are some known false positives
that it produces. See RFC 2115 for historical context, and issue
#44752 for more details.
tail-expr-drop-order
The tail_expr_drop_order
lint looks for those values generated at the tail expression location, that of type
with a significant Drop
implementation, such as locks.
In case there are also local variables of type with significant Drop
implementation as well,
this lint warns you of a potential transposition in the drop order.
Your discretion on the new drop order introduced by Edition 2024 is required.
Example
#![feature(shorter_tail_lifetimes)]
#![warn(tail_expr_drop_order)]
struct Droppy(i32);
impl Droppy {
fn get(&self) -> i32 {
self.0
}
}
impl Drop for Droppy {
fn drop(&mut self) {
// This is a custom destructor and it induces side-effects that is observable
// especially when the drop order at a tail expression changes.
println!("loud drop {}", self.0);
}
}
fn edition_2024() -> i32 {
let another_droppy = Droppy(0);
Droppy(1).get()
}
fn main() {
edition_2024();
}
This will produce:
warning: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
--> lint_example.rs:18:5
|
17 | let another_droppy = Droppy(0);
| -------------- these values have significant drop implementation and will observe changes in drop order under Edition 2024
18 | Droppy(1).get()
| ^^^^^^^^^
|
= warning: this changes meaning in Rust 2024
= note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![warn(tail_expr_drop_order)]
| ^^^^^^^^^^^^^^^^^^^^
Explanation
In tail expression of blocks or function bodies,
values of type with significant Drop
implementation has an ill-specified drop order
before Edition 2024 so that they are dropped only after dropping local variables.
Edition 2024 introduces a new rule with drop orders for them,
so that they are dropped first before dropping local variables.
A significant Drop::drop
destructor here refers to an explicit, arbitrary
implementation of the Drop
trait on the type, with exceptions including Vec
,
Box
, Rc
, BTreeMap
and HashMap
that are marked by the compiler otherwise
so long that the generic types have no significant destructor recursively.
In other words, a type has a significant drop destructor when it has a Drop
implementation
or its destructor invokes a significant destructor on a type.
Since we cannot completely reason about the change by just inspecting the existence of
a significant destructor, this lint remains only a suggestion and is set to allow
by default.
This lint only points out the issue with Droppy
, which will be dropped before another_droppy
does in Edition 2024.
No fix will be proposed by this lint.
However, the most probable fix is to hoist Droppy
into its own local variable binding.
struct Droppy(i32);
impl Droppy {
fn get(&self) -> i32 {
self.0
}
}
fn edition_2024() -> i32 {
let value = Droppy(0);
let another_droppy = Droppy(1);
value.get()
}
trivial-casts
The trivial_casts
lint detects trivial casts which could be replaced
with coercion, which may require a temporary variable.
Example
#![deny(trivial_casts)]
let x: &u32 = &42;
let y = x as *const u32;
This will produce:
error: trivial cast: `&u32` as `*const u32`
--> lint_example.rs:4:9
|
4 | let y = x as *const u32;
| ^^^^^^^^^^^^^^^
|
= help: cast can be replaced by coercion; this might require a temporary variable
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(trivial_casts)]
| ^^^^^^^^^^^^^
Explanation
A trivial cast is a cast e as T
where e
has type U
and U
is a
subtype of T
. This type of cast is usually unnecessary, as it can be
usually be inferred.
This lint is "allow" by default because there are situations, such as with FFI interfaces or complex type aliases, where it triggers incorrectly, or in situations where it will be more difficult to clearly express the intent. It may be possible that this will become a warning in the future, possibly with an explicit syntax for coercions providing a convenient way to work around the current issues. See RFC 401 (coercions), RFC 803 (type ascription) and RFC 3307 (remove type ascription) for historical context.
trivial-numeric-casts
The trivial_numeric_casts
lint detects trivial numeric casts of types
which could be removed.
Example
#![deny(trivial_numeric_casts)]
let x = 42_i32 as i32;
This will produce:
error: trivial numeric cast: `i32` as `i32`
--> lint_example.rs:3:9
|
3 | let x = 42_i32 as i32;
| ^^^^^^^^^^^^^
|
= help: cast can be replaced by coercion; this might require a temporary variable
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(trivial_numeric_casts)]
| ^^^^^^^^^^^^^^^^^^^^^
Explanation
A trivial numeric cast is a cast of a numeric type to the same numeric type. This type of cast is usually unnecessary.
This lint is "allow" by default because there are situations, such as with FFI interfaces or complex type aliases, where it triggers incorrectly, or in situations where it will be more difficult to clearly express the intent. It may be possible that this will become a warning in the future, possibly with an explicit syntax for coercions providing a convenient way to work around the current issues. See RFC 401 (coercions), RFC 803 (type ascription) and RFC 3307 (remove type ascription) for historical context.
unit-bindings
The unit_bindings
lint detects cases where bindings are useless because they have
the unit type ()
as their inferred type. The lint is suppressed if the user explicitly
annotates the let binding with the unit type ()
, or if the let binding uses an underscore
wildcard pattern, i.e. let _ = expr
, or if the binding is produced from macro expansions.
Example
#![deny(unit_bindings)]
fn foo() {
println!("do work");
}
pub fn main() {
let x = foo(); // useless binding
}
This will produce:
error: binding has unit type `()`
--> lint_example.rs:8:5
|
8 | let x = foo(); // useless binding
| ^^^^-^^^^^^^^^
| |
| this pattern is inferred to be the unit type `()`
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unit_bindings)]
| ^^^^^^^^^^^^^
Explanation
Creating a local binding with the unit type ()
does not do much and can be a sign of a
user error, such as in this example:
fn main() {
let mut x = [1, 2, 3];
x[0] = 5;
let y = x.sort(); // useless binding as `sort` returns `()` and not the sorted array.
println!("{:?}", y); // prints "()"
}
unnameable-types
The unnameable_types
lint detects types for which you can get objects of that type,
but cannot name the type itself.
Example
#![allow(unused)]
#![deny(unnameable_types)]
mod m {
pub struct S;
}
pub fn get_unnameable() -> m::S { m::S }
fn main() {}
This will produce:
error: struct `S` is reachable but cannot be named
--> lint_example.rs:4:5
|
4 | pub struct S;
| ^^^^^^^^^^^^ reachable at visibility `pub`, but can only be named at visibility `pub(crate)`
|
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![deny(unnameable_types)]
| ^^^^^^^^^^^^^^^^
Explanation
It is often expected that if you can obtain an object of type T
, then
you can name the type T
as well, this lint attempts to enforce this rule.
The recommended action is to either reexport the type properly to make it nameable,
or document that users are not supposed to be able to name it for one reason or another.
Besides types, this lint applies to traits because traits can also leak through signatures,
and you may obtain objects of their dyn Trait
or impl Trait
types.
unreachable-pub
The unreachable_pub
lint triggers for pub
items not reachable from other crates - that
means neither directly accessible, nor reexported, nor leaked through things like return
types.
Example
#![deny(unreachable_pub)]
mod foo {
pub mod bar {
}
}
This will produce:
error: unreachable `pub` item
--> lint_example.rs:4:5
|
4 | pub mod bar {
| ---^^^^^^^^
| |
| help: consider restricting its visibility: `pub(crate)`
|
= help: or consider exporting it for use by other crates
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unreachable_pub)]
| ^^^^^^^^^^^^^^^
Explanation
The pub
keyword both expresses an intent for an item to be publicly available, and also
signals to the compiler to make the item publicly accessible. The intent can only be
satisfied, however, if all items which contain this item are also publicly accessible.
Thus, this lint serves to identify situations where the intent does not match the reality.
If you wish the item to be accessible elsewhere within the crate, but not outside it, the
pub(crate)
visibility is recommended to be used instead. This more clearly expresses the
intent that the item is only visible within its own crate.
This lint is "allow" by default because it will trigger for a large amount existing Rust code, and has some false-positives. Eventually it is desired for this to become warn-by-default.
unsafe-attr-outside-unsafe
The unsafe_attr_outside_unsafe
lint detects a missing unsafe keyword
on attributes considered unsafe.
Example
#![warn(unsafe_attr_outside_unsafe)]
#[no_mangle]
extern "C" fn foo() {}
fn main() {}
This will produce:
warning: unsafe attribute used without unsafe
--> lint_example.rs:3:3
|
3 | #[no_mangle]
| ^^^^^^^^^ usage of unsafe attribute
|
= warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
= note: for more information, see issue #123757 <https://github.com/rust-lang/rust/issues/123757>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![warn(unsafe_attr_outside_unsafe)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
help: wrap the attribute in `unsafe(...)`
|
3 | #[unsafe(no_mangle)]
| +++++++ +
Explanation
Some attributes (e.g. no_mangle
, export_name
, link_section
-- see
issue #82499 for a more complete list) are considered "unsafe" attributes.
An unsafe attribute must only be used inside unsafe(...).
This lint can automatically wrap the attributes in unsafe(...)
, but this
obviously cannot verify that the preconditions of the unsafe
attributes are fulfilled, so that is still up to the user.
The lint is currently "allow" by default, but that might change in the future.
unsafe-code
The unsafe_code
lint catches usage of unsafe
code and other
potentially unsound constructs like no_mangle
, export_name
,
and link_section
.
Example
#![deny(unsafe_code)]
fn main() {
unsafe {
}
}
#[no_mangle]
fn func_0() { }
#[export_name = "exported_symbol_name"]
pub fn name_in_rust() { }
#[no_mangle]
#[link_section = ".example_section"]
pub static VAR1: u32 = 1;
This will produce:
error: usage of an `unsafe` block
--> lint_example.rs:3:5
|
3 | / unsafe {
4 | |
5 | | }
| |_____^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unsafe_code)]
| ^^^^^^^^^^^
error: declaration of a `no_mangle` function
--> lint_example.rs:8:1
|
8 | #[no_mangle]
| ^^^^^^^^^^^^
|
= note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
error: declaration of a function with `export_name`
--> lint_example.rs:11:1
|
11 | #[export_name = "exported_symbol_name"]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
error: declaration of a `no_mangle` static
--> lint_example.rs:14:1
|
14 | #[no_mangle]
| ^^^^^^^^^^^^
|
= note: the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
error: declaration of a static with `link_section`
--> lint_example.rs:15:1
|
15 | #[link_section = ".example_section"]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: the program's behavior with overridden link sections on items is unpredictable and Rust cannot provide guarantees when you manually override them
Explanation
This lint is intended to restrict the usage of unsafe
blocks and other
constructs (including, but not limited to no_mangle
, link_section
and export_name
attributes) wrong usage of which causes undefined
behavior.
unsafe-op-in-unsafe-fn
The unsafe_op_in_unsafe_fn
lint detects unsafe operations in unsafe
functions without an explicit unsafe block.
Example
#![deny(unsafe_op_in_unsafe_fn)]
unsafe fn foo() {}
unsafe fn bar() {
foo();
}
fn main() {}
This will produce:
error[E0133]: call to unsafe function `foo` is unsafe and requires unsafe block
--> lint_example.rs:6:5
|
6 | foo();
| ^^^^^ call to unsafe function
|
= note: for more information, see issue #71668 <https://github.com/rust-lang/rust/issues/71668>
= note: consult the function's documentation for information on how to avoid undefined behavior
note: an unsafe function restricts its caller, but its body is safe by default
--> lint_example.rs:5:1
|
5 | unsafe fn bar() {
| ^^^^^^^^^^^^^^^
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unsafe_op_in_unsafe_fn)]
| ^^^^^^^^^^^^^^^^^^^^^^
Explanation
Currently, an unsafe fn
allows any unsafe operation within its
body. However, this can increase the surface area of code that needs
to be scrutinized for proper behavior. The unsafe
block provides a
convenient way to make it clear exactly which parts of the code are
performing unsafe operations. In the future, it is desired to change
it so that unsafe operations cannot be performed in an unsafe fn
without an unsafe
block.
The fix to this is to wrap the unsafe code in an unsafe
block.
This lint is "allow" by default on editions up to 2021, from 2024 it is "warn" by default; the plan for increasing severity further is still being considered. See RFC #2585 and issue #71668 for more details.
unstable-features
The unstable_features
lint detects uses of #![feature]
.
Example
#![deny(unstable_features)]
#![feature(test)]
This will produce:
error: use of an unstable feature
--> lint_example.rs:2:12
|
2 | #![feature(test)]
| ^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unstable_features)]
| ^^^^^^^^^^^^^^^^^
Explanation
In larger nightly-based projects which
- consist of a multitude of crates where a subset of crates has to compile on
stable either unconditionally or depending on a
cfg
flag to for example allow stable users to depend on them, - don't use nightly for experimental features but for, e.g., unstable options only,
this lint may come in handy to enforce policies of these kinds.
unused-crate-dependencies
The unused_crate_dependencies
lint detects crate dependencies that
are never used.
Example
#![deny(unused_crate_dependencies)]
This will produce:
error: extern crate `regex` is unused in crate `lint_example`
|
= help: remove the dependency or add `use regex as _;` to the crate root
note: the lint level is defined here
--> src/lib.rs:1:9
|
1 | #![deny(unused_crate_dependencies)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
After removing the code that uses a dependency, this usually also
requires removing the dependency from the build configuration.
However, sometimes that step can be missed, which leads to time wasted
building dependencies that are no longer used. This lint can be
enabled to detect dependencies that are never used (more specifically,
any dependency passed with the --extern
command-line flag that is
never referenced via use
, extern crate
, or in any path).
This lint is "allow" by default because it can provide false positives depending on how the build system is configured. For example, when using Cargo, a "package" consists of multiple crates (such as a library and a binary), but the dependencies are defined for the package as a whole. If there is a dependency that is only used in the binary, but not the library, then the lint will be incorrectly issued in the library.
unused-extern-crates
The unused_extern_crates
lint guards against extern crate
items
that are never used.
Example
#![deny(unused_extern_crates)]
#![deny(warnings)]
extern crate proc_macro;
This will produce:
error: unused extern crate
--> lint_example.rs:4:1
|
4 | extern crate proc_macro;
| ^^^^^^^^^^^^^^^^^^^^^^^^ help: remove it
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unused_extern_crates)]
| ^^^^^^^^^^^^^^^^^^^^
Explanation
extern crate
items that are unused have no effect and should be
removed. Note that there are some cases where specifying an extern crate
is desired for the side effect of ensuring the given crate is
linked, even though it is not otherwise directly referenced. The lint
can be silenced by aliasing the crate to an underscore, such as
extern crate foo as _
. Also note that it is no longer idiomatic to
use extern crate
in the 2018 edition, as extern crates are now
automatically added in scope.
This lint is "allow" by default because it can be noisy, and produce
false-positives. If a dependency is being removed from a project, it
is recommended to remove it from the build configuration (such as
Cargo.toml
) to ensure stale build entries aren't left behind.
unused-import-braces
The unused_import_braces
lint catches unnecessary braces around an
imported item.
Example
#![deny(unused_import_braces)]
use test::{A};
pub mod test {
pub struct A;
}
fn main() {}
This will produce:
error: braces around A is unnecessary
--> lint_example.rs:2:1
|
2 | use test::{A};
| ^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unused_import_braces)]
| ^^^^^^^^^^^^^^^^^^^^
Explanation
If there is only a single item, then remove the braces (use test::A;
for example).
This lint is "allow" by default because it is only enforcing a stylistic choice.
unused-lifetimes
The unused_lifetimes
lint detects lifetime parameters that are never
used.
Example
#[deny(unused_lifetimes)]
pub fn foo<'a>() {}
This will produce:
error: lifetime parameter `'a` never used
--> lint_example.rs:4:12
|
4 | pub fn foo<'a>() {}
| -^^- help: elide the unused lifetime
|
note: the lint level is defined here
--> lint_example.rs:2:8
|
2 | #[deny(unused_lifetimes)]
| ^^^^^^^^^^^^^^^^
Explanation
Unused lifetime parameters may signal a mistake or unfinished code. Consider removing the parameter.
unused-macro-rules
The unused_macro_rules
lint detects macro rules that were not used.
Note that the lint is distinct from the unused_macros
lint, which
fires if the entire macro is never called, while this lint fires for
single unused rules of the macro that is otherwise used.
unused_macro_rules
fires only if unused_macros
wouldn't fire.
Example
#[warn(unused_macro_rules)]
macro_rules! unused_empty {
(hello) => { println!("Hello, world!") }; // This rule is unused
() => { println!("empty") }; // This rule is used
}
fn main() {
unused_empty!(hello);
}
This will produce:
warning: rule #2 of macro `unused_empty` is never used
--> lint_example.rs:4:5
|
4 | () => { println!("empty") }; // This rule is used
| ^^
|
note: the lint level is defined here
--> lint_example.rs:1:8
|
1 | #[warn(unused_macro_rules)]
| ^^^^^^^^^^^^^^^^^^
Explanation
Unused macro rules may signal a mistake or unfinished code. Furthermore, they slow down compilation. Right now, silencing the warning is not supported on a single rule level, so you have to add an allow to the entire macro definition.
If you intended to export the macro to make it
available outside of the crate, use the macro_export
attribute.
unused-qualifications
The unused_qualifications
lint detects unnecessarily qualified
names.
Example
#![deny(unused_qualifications)]
mod foo {
pub fn bar() {}
}
fn main() {
use foo::bar;
foo::bar();
bar();
}
This will produce:
error: unnecessary qualification
--> lint_example.rs:8:5
|
8 | foo::bar();
| ^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unused_qualifications)]
| ^^^^^^^^^^^^^^^^^^^^^
help: remove the unnecessary path segments
|
8 - foo::bar();
8 + bar();
|
Explanation
If an item from another module is already brought into scope, then
there is no need to qualify it in this case. You can call bar()
directly, without the foo::
.
This lint is "allow" by default because it is somewhat pedantic, and doesn't indicate an actual problem, but rather a stylistic choice, and can be noisy when refactoring or moving around code.
unused-results
The unused_results
lint checks for the unused result of an
expression in a statement.
Example
#![deny(unused_results)]
fn foo<T>() -> T { panic!() }
fn main() {
foo::<usize>();
}
This will produce:
error: unused result of type `usize`
--> lint_example.rs:5:5
|
5 | foo::<usize>();
| ^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(unused_results)]
| ^^^^^^^^^^^^^^
Explanation
Ignoring the return value of a function may indicate a mistake. In
cases were it is almost certain that the result should be used, it is
recommended to annotate the function with the must_use
attribute.
Failure to use such a return value will trigger the unused_must_use
lint which is warn-by-default. The unused_results
lint is
essentially the same, but triggers for all return values.
This lint is "allow" by default because it can be noisy, and may not be
an actual problem. For example, calling the remove
method of a Vec
or HashMap
returns the previous value, which you may not care about.
Using this lint would require explicitly ignoring or discarding such
values.
variant-size-differences
The variant_size_differences
lint detects enums with widely varying
variant sizes.
Example
#![deny(variant_size_differences)]
enum En {
V0(u8),
VBig([u8; 1024]),
}
This will produce:
error: enum variant is more than three times larger (1024 bytes) than the next largest
--> lint_example.rs:5:5
|
5 | VBig([u8; 1024]),
| ^^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(variant_size_differences)]
| ^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
It can be a mistake to add a variant to an enum that is much larger than the other variants, bloating the overall size required for all variants. This can impact performance and memory usage. This is triggered if one variant is more than 3 times larger than the second-largest variant.
Consider placing the large variant's contents on the heap (for example
via Box
) to keep the overall size of the enum itself down.
This lint is "allow" by default because it can be noisy, and may not be an actual problem. Decisions about this should be guided with profiling and benchmarking.
Warn-by-default Lints
These lints are all set to the 'warn' level by default.
ambiguous_glob_imports
ambiguous_glob_reexports
ambiguous_wide_pointer_comparisons
anonymous_parameters
array_into_iter
asm_sub_register
async_fn_in_trait
bad_asm_style
bare-trait-object
bare_trait_objects
boxed_slice_into_iter
break_with_label_and_loop
clashing_extern_declarations
coherence_leak_check
confusable_idents
const_eval_mutable_ptr_in_final_value
const_evaluatable_unchecked
const_item_mutation
dead_code
dependency_on_unit_never_type_fallback
deprecated
deprecated_where_clause_location
deref_into_dyn_supertrait
deref_nullptr
drop_bounds
dropping_copy_types
dropping_references
duplicate_macro_attributes
dyn_drop
elided_named_lifetimes
ellipsis_inclusive_range_patterns
exported_private_dependencies
for_loops_over_fallibles
forbidden_lint_groups
forgetting_copy_types
forgetting_references
function_item_references
hidden_glob_reexports
impl_trait_redundant_captures
improper_ctypes
improper_ctypes_definitions
incomplete_features
inline_no_sanitize
internal_features
invalid_from_utf8
invalid_macro_export_arguments
invalid_nan_comparisons
invalid_value
irrefutable_let_patterns
large_assignments
late_bound_lifetime_arguments
legacy_derive_helpers
map_unit_fn
mixed_script_confusables
named_arguments_used_positionally
never_type_fallback_flowing_into_unsafe
no_mangle_generic_items
non-fmt-panic
non_camel_case_types
non_contiguous_range_endpoints
non_fmt_panics
non_shorthand_field_patterns
non_snake_case
non_upper_case_globals
noop_method_call
opaque_hidden_inferred_bound
out_of_scope_macro_calls
overlapping-patterns
overlapping_range_endpoints
path_statements
private_bounds
private_interfaces
private_macro_use
ptr_cast_add_auto_to_object
redundant-semicolon
redundant_semicolons
refining_impl_trait_internal
refining_impl_trait_reachable
renamed_and_removed_lints
repr_transparent_external_private_fields
self_constructor_from_outer_item
semicolon_in_expressions_from_macros
special_module_name
stable_features
static-mut-ref
static_mut_refs
suspicious_double_ref_op
temporary_cstring_as_ptr
trivial_bounds
type_alias_bounds
tyvar_behind_raw_pointer
uncommon_codepoints
unconditional_recursion
uncovered_param_in_projection
undefined_naked_function_abi
unexpected_cfgs
unfulfilled_lint_expectations
ungated_async_fn_track_caller
uninhabited_static
unknown_lints
unknown_or_malformed_diagnostic_attributes
unnameable_test_items
unreachable_code
unreachable_patterns
unstable-name-collision
unstable_name_collisions
unstable_syntax_pre_expansion
unsupported_calling_conventions
unused-doc-comment
unused-tuple-struct-fields
unused_allocation
unused_assignments
unused_associated_type_bounds
unused_attributes
unused_braces
unused_comparisons
unused_doc_comments
unused_features
unused_imports
unused_labels
unused_macros
unused_must_use
unused_mut
unused_parens
unused_unsafe
unused_variables
useless_ptr_null_checks
warnings
while_true
ambiguous-glob-imports
The ambiguous_glob_imports
lint detects glob imports that should report ambiguity
errors, but previously didn't do that due to rustc bugs.
Example
#![deny(ambiguous_glob_imports)]
pub fn foo() -> u32 {
use sub::*;
C
}
mod sub {
mod mod1 { pub const C: u32 = 1; }
mod mod2 { pub const C: u32 = 2; }
pub use mod1::*;
pub use mod2::*;
}
This will produce:
error: `C` is ambiguous
--> lint_example.rs:5:5
|
5 | C
| ^ ambiguous name
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #114095 <https://github.com/rust-lang/rust/issues/114095>
= note: ambiguous because of multiple glob imports of a name in the same module
note: `C` could refer to the constant imported here
--> lint_example.rs:12:13
|
12 | pub use mod1::*;
| ^^^^^^^
= help: consider adding an explicit import of `C` to disambiguate
note: `C` could also refer to the constant imported here
--> lint_example.rs:13:13
|
13 | pub use mod2::*;
| ^^^^^^^
= help: consider adding an explicit import of `C` to disambiguate
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(ambiguous_glob_imports)]
| ^^^^^^^^^^^^^^^^^^^^^^
Explanation
Previous versions of Rust compile it successfully because it
had lost the ambiguity error when resolve use sub::mod2::*
.
This is a future-incompatible lint to transition this to a hard error in the future.
ambiguous-glob-reexports
The ambiguous_glob_reexports
lint detects cases where names re-exported via globs
collide. Downstream users trying to use the same name re-exported from multiple globs
will receive a warning pointing out redefinition of the same name.
Example
#![deny(ambiguous_glob_reexports)]
pub mod foo {
pub type X = u8;
}
pub mod bar {
pub type Y = u8;
pub type X = u8;
}
pub use foo::*;
pub use bar::*;
pub fn main() {}
This will produce:
error: ambiguous glob re-exports
--> lint_example.rs:11:9
|
11 | pub use foo::*;
| ^^^^^^ the name `X` in the type namespace is first re-exported here
12 | pub use bar::*;
| ------ but the name `X` in the type namespace is also re-exported here
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(ambiguous_glob_reexports)]
| ^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
This was previously accepted but it could silently break a crate's downstream users code.
For example, if foo::*
and bar::*
were re-exported before bar::X
was added to the
re-exports, down stream users could use this_crate::X
without problems. However, adding
bar::X
would cause compilation errors in downstream crates because X
is defined
multiple times in the same namespace of this_crate
.
ambiguous-wide-pointer-comparisons
The ambiguous_wide_pointer_comparisons
lint checks comparison
of *const/*mut ?Sized
as the operands.
Example
struct A;
struct B;
trait T {}
impl T for A {}
impl T for B {}
let ab = (A, B);
let a = &ab.0 as *const dyn T;
let b = &ab.1 as *const dyn T;
let _ = a == b;
This will produce:
warning: ambiguous wide pointer comparison, the comparison includes metadata which may not be expected
--> lint_example.rs:13:9
|
13 | let _ = a == b;
| ^^^^^^
|
= note: `#[warn(ambiguous_wide_pointer_comparisons)]` on by default
help: use `std::ptr::addr_eq` or untyped pointers to only compare their addresses
|
13 | let _ = std::ptr::addr_eq(a, b);
| ++++++++++++++++++ ~ +
Explanation
The comparison includes metadata which may not be expected.
anonymous-parameters
The anonymous_parameters
lint detects anonymous parameters in trait
definitions.
Example
#![deny(anonymous_parameters)]
// edition 2015
pub trait Foo {
fn foo(usize);
}
fn main() {}
This will produce:
error: anonymous parameters are deprecated and will be removed in the next edition
--> lint_example.rs:4:12
|
4 | fn foo(usize);
| ^^^^^ help: try naming the parameter or explicitly ignoring it: `_: usize`
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
= note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(anonymous_parameters)]
| ^^^^^^^^^^^^^^^^^^^^
Explanation
This syntax is mostly a historical accident, and can be worked around
quite easily by adding an _
pattern or a descriptive identifier:
trait Foo {
fn foo(_: usize);
}
This syntax is now a hard error in the 2018 edition. In the 2015
edition, this lint is "warn" by default. This lint
enables the cargo fix
tool with the --edition
flag to
automatically transition old code from the 2015 edition to 2018. The
tool will run this lint and automatically apply the
suggested fix from the compiler (which is to add _
to each
parameter). This provides a completely automated way to update old
code for a new edition. See issue #41686 for more details.
array-into-iter
The array_into_iter
lint detects calling into_iter
on arrays.
Example
#![allow(unused)]
[1, 2, 3].into_iter().for_each(|n| { *n; });
This will produce:
warning: this method call resolves to `<&[T; N] as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to `<[T; N] as IntoIterator>::into_iter` in Rust 2021
--> lint_example.rs:3:11
|
3 | [1, 2, 3].into_iter().for_each(|n| { *n; });
| ^^^^^^^^^
|
= warning: this changes meaning in Rust 2021
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/IntoIterator-for-arrays.html>
= note: `#[warn(array_into_iter)]` on by default
help: use `.iter()` instead of `.into_iter()` to avoid ambiguity
|
3 | [1, 2, 3].iter().for_each(|n| { *n; });
| ~~~~
help: or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
|
3 | IntoIterator::into_iter([1, 2, 3]).for_each(|n| { *n; });
| ++++++++++++++++++++++++ ~
Explanation
Since Rust 1.53, arrays implement IntoIterator
. However, to avoid
breakage, array.into_iter()
in Rust 2015 and 2018 code will still
behave as (&array).into_iter()
, returning an iterator over
references, just like in Rust 1.52 and earlier.
This only applies to the method call syntax array.into_iter()
, not to
any other syntax such as for _ in array
or IntoIterator::into_iter(array)
.
asm-sub-register
The asm_sub_register
lint detects using only a subset of a register
for inline asm inputs.
Example
#[cfg(target_arch="x86_64")]
use std::arch::asm;
fn main() {
#[cfg(target_arch="x86_64")]
unsafe {
asm!("mov {0}, {0}", in(reg) 0i16);
}
}
This will produce:
warning: formatting may not be suitable for sub-register argument
--> src/main.rs:7:19
|
7 | asm!("mov {0}, {0}", in(reg) 0i16);
| ^^^ ^^^ ---- for this argument
|
= note: `#[warn(asm_sub_register)]` on by default
= help: use the `x` modifier to have the register formatted as `ax`
= help: or use the `r` modifier to keep the default formatting of `rax`
Explanation
Registers on some architectures can use different names to refer to a subset of the register. By default, the compiler will use the name for the full register size. To explicitly use a subset of the register, you can override the default by using a modifier on the template string operand to specify when subregister to use. This lint is issued if you pass in a value with a smaller data type than the default register size, to alert you of possibly using the incorrect width. To fix this, add the suggested modifier to the template, or cast the value to the correct size.
See register template modifiers in the reference for more details.
async-fn-in-trait
The async_fn_in_trait
lint detects use of async fn
in the
definition of a publicly-reachable trait.
Example
pub trait Trait {
async fn method(&self);
}
fn main() {}
This will produce:
warning: use of `async fn` in public traits is discouraged as auto trait bounds cannot be specified
--> lint_example.rs:2:5
|
2 | async fn method(&self);
| ^^^^^
|
= note: you can suppress this lint if you plan to use the trait only in your own code, or do not care about auto traits like `Send` on the `Future`
= note: `#[warn(async_fn_in_trait)]` on by default
help: you can alternatively desugar to a normal `fn` that returns `impl Future` and add any desired bounds such as `Send`, but these cannot be relaxed without a breaking API change
|
2 - async fn method(&self);
2 + fn method(&self) -> impl std::future::Future<Output = ()> + Send;
|
Explanation
When async fn
is used in a trait definition, the trait does not
promise that the opaque Future
returned by the associated function
or method will implement any auto traits such as Send
. This may
be surprising and may make the associated functions or methods on the
trait less useful than intended. On traits exposed publicly from a
crate, this may affect downstream crates whose authors cannot alter
the trait definition.
For example, this code is invalid:
pub trait Trait {
async fn method(&self) {}
}
fn test<T: Trait>(x: T) {
fn spawn<T: Send>(_: T) {}
spawn(x.method()); // Not OK.
}
This lint exists to warn authors of publicly-reachable traits that
they may want to consider desugaring the async fn
to a normal fn
that returns an opaque impl Future<..> + Send
type.
For example, instead of:
pub trait Trait {
async fn method(&self) {}
}
The author of the trait may want to write:
use core::future::Future;
pub trait Trait {
fn method(&self) -> impl Future<Output = ()> + Send { async {} }
}
This still allows the use of async fn
within impls of the trait.
However, it also means that the trait will never be compatible with
impls where the returned Future
of the method does not implement
Send
.
Conversely, if the trait is used only locally, if it is never used in
generic functions, or if it is only used in single-threaded contexts
that do not care whether the returned Future
implements Send
,
then the lint may be suppressed.
bad-asm-style
The bad_asm_style
lint detects the use of the .intel_syntax
and
.att_syntax
directives.
Example
#[cfg(target_arch="x86_64")]
use std::arch::asm;
fn main() {
#[cfg(target_arch="x86_64")]
unsafe {
asm!(
".att_syntax",
"movq %{0}, %{0}", in(reg) 0usize
);
}
}
This will produce:
warning: avoid using `.att_syntax`, prefer using `options(att_syntax)` instead
--> src/main.rs:8:14
|
8 | ".att_syntax",
| ^^^^^^^^^^^
|
= note: `#[warn(bad_asm_style)]` on by default
Explanation
On x86, asm!
uses the intel assembly syntax by default. While this
can be switched using assembler directives like .att_syntax
, using the
att_syntax
option is recommended instead because it will also properly
prefix register placeholders with %
as required by AT&T syntax.
bare-trait-object
The lint bare-trait-object
has been renamed to bare-trait-objects
.
bare-trait-objects
The bare_trait_objects
lint suggests using dyn Trait
for trait
objects.
Example
trait Trait { }
fn takes_trait_object(_: Box<Trait>) {
}
This will produce:
warning: trait objects without an explicit `dyn` are deprecated
--> lint_example.rs:4:30
|
4 | fn takes_trait_object(_: Box<Trait>) {
| ^^^^^
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
= note: `#[warn(bare_trait_objects)]` on by default
help: if this is an object-safe trait, use `dyn`
|
4 | fn takes_trait_object(_: Box<dyn Trait>) {
| +++
Explanation
Without the dyn
indicator, it can be ambiguous or confusing when
reading code as to whether or not you are looking at a trait object.
The dyn
keyword makes it explicit, and adds a symmetry to contrast
with impl Trait
.
boxed-slice-into-iter
The boxed_slice_into_iter
lint detects calling into_iter
on boxed slices.
Example
#![allow(unused)]
vec![1, 2, 3].into_boxed_slice().into_iter().for_each(|n| { *n; });
This will produce:
warning: this method call resolves to `<&Box<[T]> as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to `<Box<[T]> as IntoIterator>::into_iter` in Rust 2024
--> lint_example.rs:3:34
|
3 | vec![1, 2, 3].into_boxed_slice().into_iter().for_each(|n| { *n; });
| ^^^^^^^^^
|
= warning: this changes meaning in Rust 2024
= note: `#[warn(boxed_slice_into_iter)]` on by default
help: use `.iter()` instead of `.into_iter()` to avoid ambiguity
|
3 | vec![1, 2, 3].into_boxed_slice().iter().for_each(|n| { *n; });
| ~~~~
help: or use `IntoIterator::into_iter(..)` instead of `.into_iter()` to explicitly iterate by value
|
3 | IntoIterator::into_iter(vec![1, 2, 3].into_boxed_slice()).for_each(|n| { *n; });
| ++++++++++++++++++++++++ ~
Explanation
Since Rust 1.80.0, boxed slices implement IntoIterator
. However, to avoid
breakage, boxed_slice.into_iter()
in Rust 2015, 2018, and 2021 code will still
behave as (&boxed_slice).into_iter()
, returning an iterator over
references, just like in Rust 1.79.0 and earlier.
This only applies to the method call syntax boxed_slice.into_iter()
, not to
any other syntax such as for _ in boxed_slice
or IntoIterator::into_iter(boxed_slice)
.
break-with-label-and-loop
The break_with_label_and_loop
lint detects labeled break
expressions with
an unlabeled loop as their value expression.
Example
'label: loop {
break 'label loop { break 42; };
};
This will produce:
warning: this labeled break expression is easy to confuse with an unlabeled break with a labeled value expression
--> lint_example.rs:3:5
|
3 | break 'label loop { break 42; };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(break_with_label_and_loop)]` on by default
help: wrap this expression in parentheses
|
3 | break 'label (loop { break 42; });
| + +
Explanation
In Rust, loops can have a label, and break
expressions can refer to that label to
break out of specific loops (and not necessarily the innermost one). break
expressions
can also carry a value expression, which can be another loop. A labeled break
with an
unlabeled loop as its value expression is easy to confuse with an unlabeled break with
a labeled loop and is thus discouraged (but allowed for compatibility); use parentheses
around the loop expression to silence this warning. Unlabeled break
expressions with
labeled loops yield a hard error, which can also be silenced by wrapping the expression
in parentheses.
clashing-extern-declarations
The clashing_extern_declarations
lint detects when an extern fn
has been declared with the same name but different types.
Example
mod m {
extern "C" {
fn foo();
}
}
extern "C" {
fn foo(_: u32);
}
This will produce:
warning: `foo` redeclared with a different signature
--> lint_example.rs:9:5
|
4 | fn foo();
| --------- `foo` previously declared here
...
9 | fn foo(_: u32);
| ^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
|
= note: expected `unsafe extern "C" fn()`
found `unsafe extern "C" fn(u32)`
= note: `#[warn(clashing_extern_declarations)]` on by default
Explanation
Because two symbols of the same name cannot be resolved to two
different functions at link time, and one function cannot possibly
have two types, a clashing extern declaration is almost certainly a
mistake. Check to make sure that the extern
definitions are correct
and equivalent, and possibly consider unifying them in one location.
This lint does not run between crates because a project may have
dependencies which both rely on the same extern function, but declare
it in a different (but valid) way. For example, they may both declare
an opaque type for one or more of the arguments (which would end up
distinct types), or use types that are valid conversions in the
language the extern fn
is defined in. In these cases, the compiler
can't say that the clashing declaration is incorrect.
coherence-leak-check
The coherence_leak_check
lint detects conflicting implementations of
a trait that are only distinguished by the old leak-check code.
Example
trait SomeTrait { }
impl SomeTrait for for<'a> fn(&'a u8) { }
impl<'a> SomeTrait for fn(&'a u8) { }
This will produce:
warning: conflicting implementations of trait `SomeTrait` for type `for<'a> fn(&'a u8)`
--> lint_example.rs:4:1
|
3 | impl SomeTrait for for<'a> fn(&'a u8) { }
| ------------------------------------- first implementation here
4 | impl<'a> SomeTrait for fn(&'a u8) { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `for<'a> fn(&'a u8)`
|
= warning: the behavior may change in a future release
= note: for more information, see issue #56105 <https://github.com/rust-lang/rust/issues/56105>
= note: this behavior recently changed as a result of a bug fix; see rust-lang/rust#56105 for details
= note: `#[warn(coherence_leak_check)]` on by default
Explanation
In the past, the compiler would accept trait implementations for identical functions that differed only in where the lifetime binder appeared. Due to a change in the borrow checker implementation to fix several bugs, this is no longer allowed. However, since this affects existing code, this is a future-incompatible lint to transition this to a hard error in the future.
Code relying on this pattern should introduce "newtypes",
like struct Foo(for<'a> fn(&'a u8))
.
See issue #56105 for more details.
confusable-idents
The confusable_idents
lint detects visually confusable pairs between
identifiers.
Example
// Latin Capital Letter E With Caron
pub const Ě: i32 = 1;
// Latin Capital Letter E With Breve
pub const Ĕ: i32 = 2;
This will produce:
warning: found both `Ě` and `Ĕ` as identifiers, which look alike
--> lint_example.rs:5:11
|
3 | pub const Ě: i32 = 1;
| - other identifier used here
4 | // Latin Capital Letter E With Breve
5 | pub const Ĕ: i32 = 2;
| ^ this identifier can be confused with `Ě`
|
= note: `#[warn(confusable_idents)]` on by default
Explanation
This lint warns when different identifiers may appear visually similar, which can cause confusion.
The confusable detection algorithm is based on Unicode® Technical
Standard #39 Unicode Security Mechanisms Section 4 Confusable
Detection. For every distinct identifier X execute
the function skeleton(X)
. If there exist two distinct identifiers X
and Y in the same crate where skeleton(X) = skeleton(Y)
report it.
The compiler uses the same mechanism to check if an identifier is too
similar to a keyword.
Note that the set of confusable characters may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.
const-eval-mutable-ptr-in-final-value
The const_eval_mutable_ptr_in_final_value
lint detects if a mutable pointer
has leaked into the final value of a const expression.
Example
pub enum JsValue {
Undefined,
Object(std::cell::Cell<bool>),
}
impl ::std::ops::Drop for JsValue {
fn drop(&mut self) {}
}
const UNDEFINED: &JsValue = &JsValue::Undefined;
fn main() {
}
This will produce:
warning: encountered mutable pointer in final value of constant
--> lint_example.rs:10:1
|
10 | const UNDEFINED: &JsValue = &JsValue::Undefined;
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #122153 <https://github.com/rust-lang/rust/issues/122153>
= note: `#[warn(const_eval_mutable_ptr_in_final_value)]` on by default
Explanation
In the 1.77 release, the const evaluation machinery adopted some stricter rules to reject expressions with values that could end up holding mutable references to state stored in static memory (which is inherently immutable).
This is a future-incompatible lint to ease the transition to an error. See issue #122153 for more details.
const-evaluatable-unchecked
The const_evaluatable_unchecked
lint detects a generic constant used
in a type.
Example
const fn foo<T>() -> usize {
if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
4
} else {
8
}
}
fn test<T>() {
let _ = [0; foo::<T>()];
}
This will produce:
warning: cannot use constants which depend on generic parameters in types
--> lint_example.rs:11:17
|
11 | let _ = [0; foo::<T>()];
| ^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>
= note: `#[warn(const_evaluatable_unchecked)]` on by default
Explanation
In the 1.43 release, some uses of generic parameters in array repeat expressions were accidentally allowed. This is a future-incompatible lint to transition this to a hard error in the future. See issue #76200 for a more detailed description and possible fixes.
const-item-mutation
The const_item_mutation
lint detects attempts to mutate a const
item.
Example
const FOO: [i32; 1] = [0];
fn main() {
FOO[0] = 1;
// This will print "[0]".
println!("{:?}", FOO);
}
This will produce:
warning: attempting to modify a `const` item
--> lint_example.rs:4:5
|
4 | FOO[0] = 1;
| ^^^^^^^^^^
|
= note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
--> lint_example.rs:1:1
|
1 | const FOO: [i32; 1] = [0];
| ^^^^^^^^^^^^^^^^^^^
= note: `#[warn(const_item_mutation)]` on by default
Explanation
Trying to directly mutate a const
item is almost always a mistake.
What is happening in the example above is that a temporary copy of the
const
is mutated, but the original const
is not. Each time you
refer to the const
by name (such as FOO
in the example above), a
separate copy of the value is inlined at that location.
This lint checks for writing directly to a field (FOO.field = some_value
) or array entry (FOO[0] = val
), or taking a mutable
reference to the const item (&mut FOO
), including through an
autoderef (FOO.some_mut_self_method()
).
There are various alternatives depending on what you are trying to accomplish:
- First, always reconsider using mutable globals, as they can be difficult to use correctly, and can make the code more difficult to use or understand.
- If you are trying to perform a one-time initialization of a global:
- If the value can be computed at compile-time, consider using const-compatible values (see Constant Evaluation).
- For more complex single-initialization cases, consider using
std::sync::LazyLock
.
- If you truly need a mutable global, consider using a
static
, which has a variety of options:- Simple data types can be directly defined and mutated with an
atomic
type. - More complex types can be placed in a synchronization primitive
like a
Mutex
, which can be initialized with one of the options listed above. - A mutable
static
is a low-level primitive, requiring unsafe. Typically This should be avoided in preference of something higher-level like one of the above.
- Simple data types can be directly defined and mutated with an
dead-code
The dead_code
lint detects unused, unexported items.
Example
fn foo() {}
This will produce:
warning: function `foo` is never used
--> lint_example.rs:2:4
|
2 | fn foo() {}
| ^^^
|
= note: `#[warn(dead_code)]` on by default
Explanation
Dead code may signal a mistake or unfinished code. To silence the
warning for individual items, prefix the name with an underscore such
as _foo
. If it was intended to expose the item outside of the crate,
consider adding a visibility modifier like pub
.
To preserve the numbering of tuple structs with unused fields,
change the unused fields to have unit type or use
PhantomData
.
Otherwise consider removing the unused code.
Limitations
Removing fields that are only used for side-effects and never read will result in behavioral changes. Examples of this include:
- If a field's value performs an action when it is dropped.
- If a field's type does not implement an auto trait
(e.g.
Send
,Sync
,Unpin
).
For side-effects from dropping field values, this lint should
be allowed on those fields. For side-effects from containing
field types, PhantomData
should be used.
dependency-on-unit-never-type-fallback
The dependency_on_unit_never_type_fallback
lint detects cases where code compiles with
never type fallback being ()
, but will stop compiling with fallback being !
.
Example
#![deny(dependency_on_unit_never_type_fallback)]
fn main() {
if true {
// return has type `!` which, is some cases, causes never type fallback
return
} else {
// the type produced by this call is not specified explicitly,
// so it will be inferred from the previous branch
Default::default()
};
// depending on the fallback, this may compile (because `()` implements `Default`),
// or it may not (because `!` does not implement `Default`)
}
This will produce:
error: this function depends on never type fallback being `()`
--> lint_example.rs:2:1
|
2 | fn main() {
| ^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
= help: specify the types explicitly
note: in edition 2024, the requirement `!: Default` will fail
--> lint_example.rs:9:9
|
9 | Default::default()
| ^^^^^^^^^^^^^^^^^^
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(dependency_on_unit_never_type_fallback)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Due to historic reasons never type fallback was ()
, meaning that !
got spontaneously
coerced to ()
. There are plans to change that, but they may make the code such as above
not compile. Instead of depending on the fallback, you should specify the type explicitly:
if true {
return
} else {
// type is explicitly specified, fallback can't hurt us no more
<() as Default>::default()
};
See Tracking Issue for making !
fall back to !
.
deprecated
The deprecated
lint detects use of deprecated items.
Example
#[deprecated]
fn foo() {}
fn bar() {
foo();
}
This will produce:
warning: use of deprecated function `main::foo`
--> lint_example.rs:6:5
|
6 | foo();
| ^^^
|
= note: `#[warn(deprecated)]` on by default
Explanation
Items may be marked "deprecated" with the deprecated
attribute to
indicate that they should no longer be used. Usually the attribute
should include a note on what to use instead, or check the
documentation.
deprecated-where-clause-location
The deprecated_where_clause_location
lint detects when a where clause in front of the equals
in an associated type.
Example
trait Trait {
type Assoc<'a> where Self: 'a;
}
impl Trait for () {
type Assoc<'a> where Self: 'a = ();
}
This will produce:
warning: where clause not allowed here
--> lint_example.rs:7:18
|
7 | type Assoc<'a> where Self: 'a = ();
| ^^^^^^^^^^^^^^
|
= note: see issue #89122 <https://github.com/rust-lang/rust/issues/89122> for more information
= note: `#[warn(deprecated_where_clause_location)]` on by default
help: move it to the end of the type declaration
|
7 - type Assoc<'a> where Self: 'a = ();
7 + type Assoc<'a> = () where Self: 'a;
|
Explanation
The preferred location for where clauses on associated types is after the type. However, for most of generic associated types development, it was only accepted before the equals. To provide a transition period and further evaluate this change, both are currently accepted. At some point in the future, this may be disallowed at an edition boundary; but, that is undecided currently.
deref-into-dyn-supertrait
The deref_into_dyn_supertrait
lint is output whenever there is a use of the
Deref
implementation with a dyn SuperTrait
type as Output
.
These implementations will become shadowed when the trait_upcasting
feature is stabilized.
The deref
functions will no longer be called implicitly, so there might be behavior change.
Example
#![deny(deref_into_dyn_supertrait)]
#![allow(dead_code)]
use core::ops::Deref;
trait A {}
trait B: A {}
impl<'a> Deref for dyn 'a + B {
type Target = dyn A;
fn deref(&self) -> &Self::Target {
todo!()
}
}
fn take_a(_: &dyn A) { }
fn take_b(b: &dyn B) {
take_a(b);
}
This will produce:
error: this `Deref` implementation is covered by an implicit supertrait coercion
--> lint_example.rs:9:1
|
9 | impl<'a> Deref for dyn 'a + B {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn B` implements `Deref<Target = dyn A>` which conflicts with supertrait `A`
10 | type Target = dyn A;
| -------------------- target type is a supertrait of `dyn B`
|
= warning: this will change its meaning in a future release!
= note: for more information, see issue #89460 <https://github.com/rust-lang/rust/issues/89460>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(deref_into_dyn_supertrait)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
The dyn upcasting coercion feature adds new coercion rules, taking priority over certain other coercion rules, which will cause some behavior change.
deref-nullptr
The deref_nullptr
lint detects when an null pointer is dereferenced,
which causes undefined behavior.
Example
#![allow(unused)]
use std::ptr;
unsafe {
let x = &*ptr::null::<i32>();
let x = ptr::addr_of!(*ptr::null::<i32>());
let x = *(0 as *const i32);
}
This will produce:
warning: dereferencing a null pointer
--> lint_example.rs:5:14
|
5 | let x = &*ptr::null::<i32>();
| ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
|
= note: `#[warn(deref_nullptr)]` on by default
warning: dereferencing a null pointer
--> lint_example.rs:6:27
|
6 | let x = ptr::addr_of!(*ptr::null::<i32>());
| ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
warning: dereferencing a null pointer
--> lint_example.rs:7:13
|
7 | let x = *(0 as *const i32);
| ^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
Explanation
Dereferencing a null pointer causes undefined behavior even as a place expression,
like &*(0 as *const i32)
or addr_of!(*(0 as *const i32))
.
drop-bounds
The drop_bounds
lint checks for generics with std::ops::Drop
as
bounds.
Example
fn foo<T: Drop>() {}
This will produce:
warning: bounds on `T: Drop` are most likely incorrect, consider instead using `std::mem::needs_drop` to detect whether a type can be trivially dropped
--> lint_example.rs:2:11
|
2 | fn foo<T: Drop>() {}
| ^^^^
|
= note: `#[warn(drop_bounds)]` on by default
Explanation
A generic trait bound of the form T: Drop
is most likely misleading
and not what the programmer intended (they probably should have used
std::mem::needs_drop
instead).
Drop
bounds do not actually indicate whether a type can be trivially
dropped or not, because a composite type containing Drop
types does
not necessarily implement Drop
itself. Naïvely, one might be tempted
to write an implementation that assumes that a type can be trivially
dropped while also supplying a specialization for T: Drop
that
actually calls the destructor. However, this breaks down e.g. when T
is String
, which does not implement Drop
itself but contains a
Vec
, which does implement Drop
, so assuming T
can be trivially
dropped would lead to a memory leak here.
Furthermore, the Drop
trait only contains one method, Drop::drop
,
which may not be called explicitly in user code (E0040
), so there is
really no use case for using Drop
in trait bounds, save perhaps for
some obscure corner cases, which can use #[allow(drop_bounds)]
.
dropping-copy-types
The dropping_copy_types
lint checks for calls to std::mem::drop
with a value
that derives the Copy trait.
Example
let x: i32 = 42; // i32 implements Copy
std::mem::drop(x); // A copy of x is passed to the function, leaving the
// original unaffected
This will produce:
warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
--> lint_example.rs:3:1
|
3 | std::mem::drop(x); // A copy of x is passed to the function, leaving the
| ^^^^^^^^^^^^^^^-^
| |
| argument has type `i32`
|
= note: `#[warn(dropping_copy_types)]` on by default
help: use `let _ = ...` to ignore the expression or result
|
3 - std::mem::drop(x); // A copy of x is passed to the function, leaving the
3 + let _ = x; // A copy of x is passed to the function, leaving the
|
Explanation
Calling std::mem::drop
does nothing for types that
implement Copy, since the
value will be copied and moved into the function on invocation.
dropping-references
The dropping_references
lint checks for calls to std::mem::drop
with a reference
instead of an owned value.
Example
fn operation_that_requires_mutex_to_be_unlocked() {} // just to make it compile
let mutex = std::sync::Mutex::new(1); // just to make it compile
let mut lock_guard = mutex.lock();
std::mem::drop(&lock_guard); // Should have been drop(lock_guard), mutex
// still locked
operation_that_requires_mutex_to_be_unlocked();
This will produce:
warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
--> lint_example.rs:5:1
|
5 | std::mem::drop(&lock_guard); // Should have been drop(lock_guard), mutex
| ^^^^^^^^^^^^^^^-----------^
| |
| argument has type `&Result<MutexGuard<'_, i32>, PoisonError<MutexGuard<'_, i32>>>`
|
= note: `#[warn(dropping_references)]` on by default
help: use `let _ = ...` to ignore the expression or result
|
5 - std::mem::drop(&lock_guard); // Should have been drop(lock_guard), mutex
5 + let _ = &lock_guard; // Should have been drop(lock_guard), mutex
|
Explanation
Calling drop
on a reference will only drop the
reference itself, which is a no-op. It will not call the drop
method (from
the Drop
trait implementation) on the underlying referenced value, which
is likely what was intended.
duplicate-macro-attributes
The duplicate_macro_attributes
lint detects when a #[test]
-like built-in macro
attribute is duplicated on an item. This lint may trigger on bench
, cfg_eval
, test
and test_case
.
Example
#[test]
#[test]
fn foo() {}
This will produce:
warning: duplicated attribute
--> src/lib.rs:2:1
|
2 | #[test]
| ^^^^^^^
|
= note: `#[warn(duplicate_macro_attributes)]` on by default
Explanation
A duplicated attribute may erroneously originate from a copy-paste and the effect of it being duplicated may not be obvious or desirable.
For instance, doubling the #[test]
attributes registers the test to be run twice with no
change to its environment.
dyn-drop
The dyn_drop
lint checks for trait objects with std::ops::Drop
.
Example
fn foo(_x: Box<dyn Drop>) {}
This will produce:
warning: types that do not implement `Drop` can still have drop glue, consider instead using `std::mem::needs_drop` to detect whether a type is trivially dropped
--> lint_example.rs:2:20
|
2 | fn foo(_x: Box<dyn Drop>) {}
| ^^^^
|
= note: `#[warn(dyn_drop)]` on by default
Explanation
A trait object bound of the form dyn Drop
is most likely misleading
and not what the programmer intended.
Drop
bounds do not actually indicate whether a type can be trivially
dropped or not, because a composite type containing Drop
types does
not necessarily implement Drop
itself. Naïvely, one might be tempted
to write a deferred drop system, to pull cleaning up memory out of a
latency-sensitive code path, using dyn Drop
trait objects. However,
this breaks down e.g. when T
is String
, which does not implement
Drop
, but should probably be accepted.
To write a trait object bound that accepts anything, use a placeholder trait with a blanket implementation.
trait Placeholder {}
impl<T> Placeholder for T {}
fn foo(_x: Box<dyn Placeholder>) {}
elided-named-lifetimes
The elided_named_lifetimes
lint detects when an elided
lifetime ends up being a named lifetime, such as 'static
or some lifetime parameter 'a
.
Example
#![deny(elided_named_lifetimes)]
struct Foo;
impl Foo {
pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
unsafe { &mut *(x as *mut _) }
}
}
This will produce:
error: elided lifetime has a name
--> lint_example.rs:5:50
|
5 | pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
| ^ this elided lifetime gets resolved as `'static`
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(elided_named_lifetimes)]
| ^^^^^^^^^^^^^^^^^^^^^^
Explanation
Lifetime elision is quite useful, because it frees you from having
to give each lifetime its own name, but sometimes it can produce
somewhat surprising resolutions. In safe code, it is mostly okay,
because the borrow checker prevents any unsoundness, so the worst
case scenario is you get a confusing error message in some other place.
But with unsafe
code, such unexpected resolutions may lead to unsound code.
ellipsis-inclusive-range-patterns
The ellipsis_inclusive_range_patterns
lint detects the ...
range
pattern, which is deprecated.
Example
let x = 123;
match x {
0...100 => {}
_ => {}
}
This will produce:
warning: `...` range patterns are deprecated
--> lint_example.rs:4:6
|
4 | 0...100 => {}
| ^^^ help: use `..=` for an inclusive range
|
= warning: this is accepted in the current edition (Rust 2018) but is a hard error in Rust 2021!
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
= note: `#[warn(ellipsis_inclusive_range_patterns)]` on by default
Explanation
The ...
range pattern syntax was changed to ..=
to avoid potential
confusion with the ..
range expression. Use the new form instead.
exported-private-dependencies
The exported_private_dependencies
lint detects private dependencies
that are exposed in a public interface.
Example
pub fn foo() -> Option<some_private_dependency::Thing> {
None
}
This will produce:
warning: type `bar::Thing` from private dependency 'bar' in public interface
--> src/lib.rs:3:1
|
3 | pub fn foo() -> Option<bar::Thing> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(exported_private_dependencies)]` on by default
Explanation
Dependencies can be marked as "private" to indicate that they are not exposed in the public interface of a crate. This can be used by Cargo to independently resolve those dependencies because it can assume it does not need to unify them with other packages using that same dependency. This lint is an indication of a violation of that contract.
To fix this, avoid exposing the dependency in your public interface. Or, switch the dependency to a public dependency.
Note that support for this is only available on the nightly channel. See RFC 1977 for more details, as well as the Cargo documentation.
for-loops-over-fallibles
The for_loops_over_fallibles
lint checks for for
loops over Option
or Result
values.
Example
let opt = Some(1);
for x in opt { /* ... */}
This will produce:
warning: for loop over an `Option`. This is more readably written as an `if let` statement
--> lint_example.rs:3:10
|
3 | for x in opt { /* ... */}
| ^^^
|
= note: `#[warn(for_loops_over_fallibles)]` on by default
help: to check pattern in a loop use `while let`
|
3 | while let Some(x) = opt { /* ... */}
| ~~~~~~~~~~~~~~~ ~~~
help: consider using `if let` to clear intent
|
3 | if let Some(x) = opt { /* ... */}
| ~~~~~~~~~~~~ ~~~
Explanation
Both Option
and Result
implement IntoIterator
trait, which allows using them in a for
loop.
for
loop over Option
or Result
will iterate either 0 (if the value is None
/Err(_)
)
or 1 time (if the value is Some(_)
/Ok(_)
). This is not very useful and is more clearly expressed
via if let
.
for
loop can also be accidentally written with the intention to call a function multiple times,
while the function returns Some(_)
, in these cases while let
loop should be used instead.
The "intended" use of IntoIterator
implementations for Option
and Result
is passing them to
generic code that expects something implementing IntoIterator
. For example using .chain(option)
to optionally add a value to an iterator.
forbidden-lint-groups
The forbidden_lint_groups
lint detects violations of
forbid
applied to a lint group. Due to a bug in the compiler,
these used to be overlooked entirely. They now generate a warning.
Example
#![forbid(warnings)]
#![deny(bad_style)]
fn main() {}
This will produce:
warning: deny(bad_style) incompatible with previous forbid
--> lint_example.rs:2:9
|
1 | #![forbid(warnings)]
| -------- `forbid` level set here
2 | #![deny(bad_style)]
| ^^^^^^^^^ overruled by previous forbid
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
= note: `#[warn(forbidden_lint_groups)]` on by default
Recommended fix
If your crate is using #![forbid(warnings)]
,
we recommend that you change to #![deny(warnings)]
.
Explanation
Due to a compiler bug, applying forbid
to lint groups
previously had no effect. The bug is now fixed but instead of
enforcing forbid
we issue this future-compatibility warning
to avoid breaking existing crates.
forgetting-copy-types
The forgetting_copy_types
lint checks for calls to std::mem::forget
with a value
that derives the Copy trait.
Example
let x: i32 = 42; // i32 implements Copy
std::mem::forget(x); // A copy of x is passed to the function, leaving the
// original unaffected
This will produce:
warning: calls to `std::mem::forget` with a value that implements `Copy` does nothing
--> lint_example.rs:3:1
|
3 | std::mem::forget(x); // A copy of x is passed to the function, leaving the
| ^^^^^^^^^^^^^^^^^-^
| |
| argument has type `i32`
|
= note: `#[warn(forgetting_copy_types)]` on by default
help: use `let _ = ...` to ignore the expression or result
|
3 - std::mem::forget(x); // A copy of x is passed to the function, leaving the
3 + let _ = x; // A copy of x is passed to the function, leaving the
|
Explanation
Calling std::mem::forget
does nothing for types that
implement Copy since the
value will be copied and moved into the function on invocation.
An alternative, but also valid, explanation is that Copy types do not
implement the Drop trait, which means they have no destructors. Without a
destructor, there is nothing for std::mem::forget
to ignore.
forgetting-references
The forgetting_references
lint checks for calls to std::mem::forget
with a reference
instead of an owned value.
Example
let x = Box::new(1);
std::mem::forget(&x); // Should have been forget(x), x will still be dropped
This will produce:
warning: calls to `std::mem::forget` with a reference instead of an owned value does nothing
--> lint_example.rs:3:1
|
3 | std::mem::forget(&x); // Should have been forget(x), x will still be dropped
| ^^^^^^^^^^^^^^^^^--^
| |
| argument has type `&Box<i32>`
|
= note: `#[warn(forgetting_references)]` on by default
help: use `let _ = ...` to ignore the expression or result
|
3 - std::mem::forget(&x); // Should have been forget(x), x will still be dropped
3 + let _ = &x; // Should have been forget(x), x will still be dropped
|
Explanation
Calling forget
on a reference will only forget the
reference itself, which is a no-op. It will not forget the underlying
referenced value, which is likely what was intended.
function-item-references
The function_item_references
lint detects function references that are
formatted with fmt::Pointer
or transmuted.
Example
fn foo() { }
fn main() {
println!("{:p}", &foo);
}
This will produce:
warning: taking a reference to a function item does not give a function pointer
--> lint_example.rs:4:22
|
4 | println!("{:p}", &foo);
| ^^^^ help: cast `foo` to obtain a function pointer: `foo as fn()`
|
= note: `#[warn(function_item_references)]` on by default
Explanation
Taking a reference to a function may be mistaken as a way to obtain a
pointer to that function. This can give unexpected results when
formatting the reference as a pointer or transmuting it. This lint is
issued when function references are formatted as pointers, passed as
arguments bound by fmt::Pointer
or transmuted.
hidden-glob-reexports
The hidden_glob_reexports
lint detects cases where glob re-export items are shadowed by
private items.
Example
#![deny(hidden_glob_reexports)]
pub mod upstream {
mod inner { pub struct Foo {}; pub struct Bar {}; }
pub use self::inner::*;
struct Foo {} // private item shadows `inner::Foo`
}
// mod downstream {
// fn test() {
// let _ = crate::upstream::Foo; // inaccessible
// }
// }
pub fn main() {}
This will produce:
error: private item shadows public glob re-export
--> lint_example.rs:6:5
|
6 | struct Foo {} // private item shadows `inner::Foo`
| ^^^^^^^^^^^^^
|
note: the name `Foo` in the type namespace is supposed to be publicly re-exported here
--> lint_example.rs:5:13
|
5 | pub use self::inner::*;
| ^^^^^^^^^^^^^^
note: but the private item here shadows it
--> lint_example.rs:6:5
|
6 | struct Foo {} // private item shadows `inner::Foo`
| ^^^^^^^^^^^^^
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(hidden_glob_reexports)]
| ^^^^^^^^^^^^^^^^^^^^^
Explanation
This was previously accepted without any errors or warnings but it could silently break a
crate's downstream user code. If the struct Foo
was added, dep::inner::Foo
would
silently become inaccessible and trigger a "struct
Foo is private
" visibility error at
the downstream use site.
impl-trait-redundant-captures
The impl_trait_redundant_captures
lint warns against cases where use of the
precise capturing use<...>
syntax is not needed.
In the 2024 edition, impl Trait
s will capture all lifetimes in scope.
If precise-capturing use<...>
syntax is used, and the set of parameters
that are captures are equal to the set of parameters in scope, then
the syntax is redundant, and can be removed.
Example
#![feature(lifetime_capture_rules_2024)]
#![deny(impl_trait_redundant_captures)]
fn test<'a>(x: &'a i32) -> impl Sized + use<'a> { x }
This will produce:
error: all possible in-scope parameters are already captured, so `use<...>` syntax is redundant
--> lint_example.rs:4:28
|
4 | fn test<'a>(x: &'a i32) -> impl Sized + use<'a> { x }
| ^^^^^^^^^^^^^-------
| |
| help: remove the `use<...>` syntax
|
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![deny(impl_trait_redundant_captures)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
To fix this, remove the use<'a>
, since the lifetime is already captured
since it is in scope.
improper-ctypes
The improper_ctypes
lint detects incorrect use of types in foreign
modules.
Example
extern "C" {
static STATIC: String;
}
This will produce:
warning: `extern` block uses type `String`, which is not FFI-safe
--> lint_example.rs:3:20
|
3 | static STATIC: String;
| ^^^^^^ not FFI-safe
|
= help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
= note: this struct has unspecified layout
= note: `#[warn(improper_ctypes)]` on by default
Explanation
The compiler has several checks to verify that types used in extern
blocks are safe and follow certain rules to ensure proper
compatibility with the foreign interfaces. This lint is issued when it
detects a probable mistake in a definition. The lint usually should
provide a description of the issue, along with possibly a hint on how
to resolve it.
improper-ctypes-definitions
The improper_ctypes_definitions
lint detects incorrect use of
extern
function definitions.
Example
#![allow(unused)]
pub extern "C" fn str_type(p: &str) { }
This will produce:
warning: `extern` fn uses type `str`, which is not FFI-safe
--> lint_example.rs:3:31
|
3 | pub extern "C" fn str_type(p: &str) { }
| ^^^^ not FFI-safe
|
= help: consider using `*const u8` and a length instead
= note: string slices have no C equivalent
= note: `#[warn(improper_ctypes_definitions)]` on by default
Explanation
There are many parameter and return types that may be specified in an
extern
function that are not compatible with the given ABI. This
lint is an alert that these types should not be used. The lint usually
should provide a description of the issue, along with possibly a hint
on how to resolve it.
incomplete-features
The incomplete_features
lint detects unstable features enabled with
the feature
attribute that may function improperly in some or all
cases.
Example
#![feature(generic_const_exprs)]
This will produce:
warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
--> lint_example.rs:1:12
|
1 | #![feature(generic_const_exprs)]
| ^^^^^^^^^^^^^^^^^^^
|
= note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
= note: `#[warn(incomplete_features)]` on by default
Explanation
Although it is encouraged for people to experiment with unstable features, some of them are known to be incomplete or faulty. This lint is a signal that the feature has not yet been finished, and you may experience problems with it.
inline-no-sanitize
The inline_no_sanitize
lint detects incompatible use of
#[inline(always)]
and #[no_sanitize(...)]
.
Example
#![feature(no_sanitize)]
#[inline(always)]
#[no_sanitize(address)]
fn x() {}
fn main() {
x()
}
This will produce:
warning: `no_sanitize` will have no effect after inlining
--> lint_example.rs:4:1
|
4 | #[no_sanitize(address)]
| ^^^^^^^^^^^^^^^^^^^^^^^
|
note: inlining requested here
--> lint_example.rs:3:1
|
3 | #[inline(always)]
| ^^^^^^^^^^^^^^^^^
= note: `#[warn(inline_no_sanitize)]` on by default
Explanation
The use of the #[inline(always)]
attribute prevents the
the #[no_sanitize(...)]
attribute from working.
Consider temporarily removing inline
attribute.
internal-features
The internal_features
lint detects unstable features enabled with
the feature
attribute that are internal to the compiler or standard
library.
Example
#![feature(rustc_attrs)]
This will produce:
warning: the feature `rustc_attrs` is internal to the compiler or standard library
--> lint_example.rs:1:12
|
1 | #![feature(rustc_attrs)]
| ^^^^^^^^^^^
|
= note: using it is strongly discouraged
= note: `#[warn(internal_features)]` on by default
Explanation
These features are an implementation detail of the compiler and standard library and are not supposed to be used in user code.
invalid-from-utf8
The invalid_from_utf8
lint checks for calls to
std::str::from_utf8
and std::str::from_utf8_mut
with a known invalid UTF-8 value.
Example
#[allow(unused)]
std::str::from_utf8(b"Ru\x82st");
This will produce:
warning: calls to `std::str::from_utf8` with a invalid literal always return an error
--> lint_example.rs:3:1
|
3 | std::str::from_utf8(b"Ru\x82st");
| ^^^^^^^^^^^^^^^^^^^^-----------^
| |
| the literal was valid UTF-8 up to the 2 bytes
|
= note: `#[warn(invalid_from_utf8)]` on by default
Explanation
Trying to create such a str
would always return an error as per documentation
for std::str::from_utf8
and std::str::from_utf8_mut
.
invalid-macro-export-arguments
The invalid_macro_export_arguments
lint detects cases where #[macro_export]
is being used with invalid arguments.
Example
#![deny(invalid_macro_export_arguments)]
#[macro_export(invalid_parameter)]
macro_rules! myMacro {
() => {
// [...]
}
}
#[macro_export(too, many, items)]
This will produce:
error: `invalid_parameter` isn't a valid `#[macro_export]` argument
--> lint_example.rs:4:16
|
4 | #[macro_export(invalid_parameter)]
| ^^^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(invalid_macro_export_arguments)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
The only valid argument is #[macro_export(local_inner_macros)]
or no argument (#[macro_export]
).
You can't have multiple arguments in a #[macro_export(..)]
, or mention arguments other than local_inner_macros
.
invalid-nan-comparisons
The invalid_nan_comparisons
lint checks comparison with f32::NAN
or f64::NAN
as one of the operand.
Example
let a = 2.3f32;
if a == f32::NAN {}
This will produce:
warning: incorrect NaN comparison, NaN cannot be directly compared to itself
--> lint_example.rs:3:4
|
3 | if a == f32::NAN {}
| ^^^^^^^^^^^^^
|
= note: `#[warn(invalid_nan_comparisons)]` on by default
help: use `f32::is_nan()` or `f64::is_nan()` instead
|
3 - if a == f32::NAN {}
3 + if a.is_nan() {}
|
Explanation
NaN does not compare meaningfully to anything – not even itself – so those comparisons are always false.
invalid-value
The invalid_value
lint detects creating a value that is not valid,
such as a null reference.
Example
#![allow(unused)]
unsafe {
let x: &'static i32 = std::mem::zeroed();
}
This will produce:
warning: the type `&i32` does not permit zero-initialization
--> lint_example.rs:4:27
|
4 | let x: &'static i32 = std::mem::zeroed();
| ^^^^^^^^^^^^^^^^^^
| |
| this code causes undefined behavior when executed
| help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
|
= note: references must be non-null
= note: `#[warn(invalid_value)]` on by default
Explanation
In some situations the compiler can detect that the code is creating an invalid value, which should be avoided.
In particular, this lint will check for improper use of
mem::zeroed
, mem::uninitialized
, mem::transmute
, and
MaybeUninit::assume_init
that can cause undefined behavior. The
lint should provide extra information to indicate what the problem is
and a possible solution.
irrefutable-let-patterns
The irrefutable_let_patterns
lint detects irrefutable patterns
in if let
s, while let
s, and if let
guards.
Example
if let _ = 123 {
println!("always runs!");
}
This will produce:
warning: irrefutable `if let` pattern
--> lint_example.rs:2:4
|
2 | if let _ = 123 {
| ^^^^^^^^^^^
|
= note: this pattern will always match, so the `if let` is useless
= help: consider replacing the `if let` with a `let`
= note: `#[warn(irrefutable_let_patterns)]` on by default
Explanation
There usually isn't a reason to have an irrefutable pattern in an
if let
or while let
statement, because the pattern will always match
successfully. A let
or loop
statement will suffice. However,
when generating code with a macro, forbidding irrefutable patterns
would require awkward workarounds in situations where the macro
doesn't know if the pattern is refutable or not. This lint allows
macros to accept this form, while alerting for a possibly incorrect
use in normal code.
See RFC 2086 for more details.
large-assignments
The large_assignments
lint detects when objects of large
types are being moved around.
Example
let x = [0; 50000];
let y = x;
produces:
warning: moving a large value
--> $DIR/move-large.rs:1:3
let y = x;
- Copied large value here
Explanation
When using a large type in a plain assignment or in a function argument, idiomatic code can be inefficient. Ideally appropriate optimizations would resolve this, but such optimizations are only done in a best-effort manner. This lint will trigger on all sites of large moves and thus allow the user to resolve them in code.
late-bound-lifetime-arguments
The late_bound_lifetime_arguments
lint detects generic lifetime
arguments in path segments with late bound lifetime parameters.
Example
struct S;
impl S {
fn late(self, _: &u8, _: &u8) {}
}
fn main() {
S.late::<'static>(&0, &0);
}
This will produce:
warning: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
--> lint_example.rs:8:14
|
4 | fn late(self, _: &u8, _: &u8) {}
| - the late bound lifetime parameter is introduced here
...
8 | S.late::<'static>(&0, &0);
| ^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #42868 <https://github.com/rust-lang/rust/issues/42868>
= note: `#[warn(late_bound_lifetime_arguments)]` on by default
Explanation
It is not clear how to provide arguments for early-bound lifetime parameters if they are intermixed with late-bound parameters in the same list. For now, providing any explicit arguments will trigger this lint if late-bound parameters are present, so in the future a solution can be adopted without hitting backward compatibility issues. This is a future-incompatible lint to transition this to a hard error in the future. See issue #42868 for more details, along with a description of the difference between early and late-bound parameters.
legacy-derive-helpers
The legacy_derive_helpers
lint detects derive helper attributes
that are used before they are introduced.
Example
#[serde(rename_all = "camelCase")]
#[derive(Deserialize)]
struct S { /* fields */ }
produces:
warning: derive helper attribute is used before it is introduced
--> $DIR/legacy-derive-helpers.rs:1:3
|
1 | #[serde(rename_all = "camelCase")]
| ^^^^^
...
2 | #[derive(Deserialize)]
| ----------- the attribute is introduced here
Explanation
Attributes like this work for historical reasons, but attribute expansion works in
left-to-right order in general, so, to resolve #[serde]
, compiler has to try to "look
into the future" at not yet expanded part of the item , but such attempts are not always
reliable.
To fix the warning place the helper attribute after its corresponding derive.
#[derive(Deserialize)]
#[serde(rename_all = "camelCase")]
struct S { /* fields */ }
map-unit-fn
The map_unit_fn
lint checks for Iterator::map
receive
a callable that returns ()
.
Example
fn foo(items: &mut Vec<u8>) {
items.sort();
}
fn main() {
let mut x: Vec<Vec<u8>> = vec![
vec![0, 2, 1],
vec![5, 4, 3],
];
x.iter_mut().map(foo);
}
This will produce:
warning: `Iterator::map` call that discard the iterator's values
--> lint_example.rs:10:18
|
1 | fn foo(items: &mut Vec<u8>) {
| --------------------------- this function returns `()`, which is likely not what you wanted
...
10 | x.iter_mut().map(foo);
| ^^^^---^
| | |
| | called `Iterator::map` with callable that returns `()`
| after this call to map, the resulting iterator is `impl Iterator<Item = ()>`, which means the only information carried by the iterator is the number of items
|
= note: `Iterator::map`, like many of the methods on `Iterator`, gets executed lazily, meaning that its effects won't be visible until it is iterated
= note: `#[warn(map_unit_fn)]` on by default
help: you might have meant to use `Iterator::for_each`
|
10 | x.iter_mut().for_each(foo);
| ~~~~~~~~
Explanation
Mapping to ()
is almost always a mistake.
mixed-script-confusables
The mixed_script_confusables
lint detects visually confusable
characters in identifiers between different scripts.
Example
// The Japanese katakana character エ can be confused with the Han character 工.
const エ: &'static str = "アイウ";
This will produce:
warning: the usage of Script Group `Japanese, Katakana` in this crate consists solely of mixed script confusables
--> lint_example.rs:3:7
|
3 | const エ: &'static str = "アイウ";
| ^^
|
= note: the usage includes 'エ' (U+30A8)
= note: please recheck to make sure their usages are indeed what you want
= note: `#[warn(mixed_script_confusables)]` on by default
Explanation
This lint warns when characters between different scripts may appear visually similar, which can cause confusion.
If the crate contains other identifiers in the same script that have
non-confusable characters, then this lint will not be issued. For
example, if the example given above has another identifier with
katakana characters (such as let カタカナ = 123;
), then this indicates
that you are intentionally using katakana, and it will not warn about
it.
Note that the set of confusable characters may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.
named-arguments-used-positionally
The named_arguments_used_positionally
lint detects cases where named arguments are only
used positionally in format strings. This usage is valid but potentially very confusing.
Example
#![deny(named_arguments_used_positionally)]
fn main() {
let _x = 5;
println!("{}", _x = 1); // Prints 1, will trigger lint
println!("{}", _x); // Prints 5, no lint emitted
println!("{_x}", _x = _x); // Prints 5, no lint emitted
}
This will produce:
error: named argument `_x` is not used by name
--> lint_example.rs:4:20
|
4 | println!("{}", _x = 1); // Prints 1, will trigger lint
| -- ^^ this named argument is referred to by position in formatting string
| |
| this formatting argument uses named argument `_x` by position
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(named_arguments_used_positionally)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use the named argument by name to avoid ambiguity
|
4 | println!("{_x}", _x = 1); // Prints 1, will trigger lint
| ++
Explanation
Rust formatting strings can refer to named arguments by their position, but this usage is potentially confusing. In particular, readers can incorrectly assume that the declaration of named arguments is an assignment (which would produce the unit type). For backwards compatibility, this is not a hard error.
never-type-fallback-flowing-into-unsafe
The never_type_fallback_flowing_into_unsafe
lint detects cases where never type fallback
affects unsafe function calls.
Never type fallback
When the compiler sees a value of type !
it implicitly inserts a coercion (if possible),
to allow type check to infer any type:
// this
let x: u8 = panic!();
// is (essentially) turned by the compiler into
let x: u8 = absurd(panic!());
// where absurd is a function with the following signature
// (it's sound, because `!` always marks unreachable code):
fn absurd<T>(never: !) -> T { ... }
While it's convenient to be able to use non-diverging code in one of the branches (like
if a { b } else { return }
) this could lead to compilation errors:
// this
{ panic!() };
// gets turned into this
{ absurd(panic!()) }; // error: can't infer the type of `absurd`
To prevent such errors, compiler remembers where it inserted absurd
calls, and if it
can't infer their type, it sets the type to fallback. { absurd::<Fallback>(panic!()) };
.
This is what is known as "never type fallback".
Example
#![deny(never_type_fallback_flowing_into_unsafe)]
fn main() {
if true {
// return has type `!` which, is some cases, causes never type fallback
return
} else {
// `zeroed` is an unsafe function, which returns an unbounded type
unsafe { std::mem::zeroed() }
};
// depending on the fallback, `zeroed` may create `()` (which is completely sound),
// or `!` (which is instant undefined behavior)
}
This will produce:
error: never type fallback affects this call to an `unsafe` function
--> lint_example.rs:8:18
|
8 | unsafe { std::mem::zeroed() }
| ^^^^^^^^^^^^^^^^^^
|
= warning: this will change its meaning in a future release!
= note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
= help: specify the type explicitly
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(never_type_fallback_flowing_into_unsafe)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
Due to historic reasons never type fallback was ()
, meaning that !
got spontaneously
coerced to ()
. There are plans to change that, but they may make the code such as above
unsound. Instead of depending on the fallback, you should specify the type explicitly:
if true {
return
} else {
// type is explicitly specified, fallback can't hurt us no more
unsafe { std::mem::zeroed::<()>() }
};
See Tracking Issue for making !
fall back to !
.
no-mangle-generic-items
The no_mangle_generic_items
lint detects generic items that must be
mangled.
Example
#[no_mangle]
fn foo<T>(t: T) {
}
This will produce:
warning: functions generic over types or consts must be mangled
--> lint_example.rs:3:1
|
2 | #[no_mangle]
| ------------ help: remove this attribute
3 | / fn foo<T>(t: T) {
4 | |
5 | | }
| |_^
|
= note: `#[warn(no_mangle_generic_items)]` on by default
Explanation
A function with generics must have its symbol mangled to accommodate
the generic parameter. The no_mangle
attribute has no effect in
this situation, and should be removed.
non-fmt-panic
The lint non-fmt-panic
has been renamed to non-fmt-panics
.
non-camel-case-types
The non_camel_case_types
lint detects types, variants, traits and
type parameters that don't have camel case names.
Example
struct my_struct;
This will produce:
warning: type `my_struct` should have an upper camel case name
--> lint_example.rs:2:8
|
2 | struct my_struct;
| ^^^^^^^^^ help: convert the identifier to upper camel case: `MyStruct`
|
= note: `#[warn(non_camel_case_types)]` on by default
Explanation
The preferred style for these identifiers is to use "camel case", such
as MyStruct
, where the first letter should not be lowercase, and
should not use underscores between letters. Underscores are allowed at
the beginning and end of the identifier, as well as between
non-letters (such as X86_64
).
non-contiguous-range-endpoints
The non_contiguous_range_endpoints
lint detects likely off-by-one errors when using
exclusive range patterns.
Example
let x = 123u32;
match x {
0..100 => { println!("small"); }
101..1000 => { println!("large"); }
_ => { println!("larger"); }
}
This will produce:
warning: multiple ranges are one apart
--> lint_example.rs:4:5
|
4 | 0..100 => { println!("small"); }
| ^^^^^^
| |
| this range doesn't match `100_u32` because `..` is an exclusive range
| help: use an inclusive range instead: `0_u32..=100_u32`
5 | 101..1000 => { println!("large"); }
| --------- this could appear to continue range `0_u32..100_u32`, but `100_u32` isn't matched by either of them
|
= note: `#[warn(non_contiguous_range_endpoints)]` on by default
Explanation
It is likely a mistake to have range patterns in a match expression that miss out a single
number. Check that the beginning and end values are what you expect, and keep in mind that
with ..=
the right bound is inclusive, and with ..
it is exclusive.
non-fmt-panics
The non_fmt_panics
lint detects panic!(..)
invocations where the first
argument is not a formatting string.
Example
panic!("{}");
panic!(123);
This will produce:
warning: panic message contains an unused formatting placeholder
--> lint_example.rs:2:9
|
2 | panic!("{}");
| ^^
|
= note: this message is not used as a format string when given without arguments, but will be in Rust 2021
= note: `#[warn(non_fmt_panics)]` on by default
help: add the missing argument
|
2 | panic!("{}", ...);
| +++++
help: or add a "{}" format string to use the message literally
|
2 | panic!("{}", "{}");
| +++++
warning: panic message is not a string literal
--> lint_example.rs:3:8
|
3 | panic!(123);
| ^^^
|
= note: this usage of `panic!()` is deprecated; it will be a hard error in Rust 2021
= note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html>
help: add a "{}" format string to `Display` the message
|
3 | panic!("{}", 123);
| +++++
help: or use std::panic::panic_any instead
|
3 | std::panic::panic_any(123);
| ~~~~~~~~~~~~~~~~~~~~~
Explanation
In Rust 2018 and earlier, panic!(x)
directly uses x
as the message.
That means that panic!("{}")
panics with the message "{}"
instead
of using it as a formatting string, and panic!(123)
will panic with
an i32
as message.
Rust 2021 always interprets the first argument as format string.
non-shorthand-field-patterns
The non_shorthand_field_patterns
lint detects using Struct { x: x }
instead of Struct { x }
in a pattern.
Example
struct Point {
x: i32,
y: i32,
}
fn main() {
let p = Point {
x: 5,
y: 5,
};
match p {
Point { x: x, y: y } => (),
}
}
This will produce:
warning: the `x:` in this pattern is redundant
--> lint_example.rs:14:17
|
14 | Point { x: x, y: y } => (),
| ^^^^ help: use shorthand field pattern: `x`
|
= note: `#[warn(non_shorthand_field_patterns)]` on by default
warning: the `y:` in this pattern is redundant
--> lint_example.rs:14:23
|
14 | Point { x: x, y: y } => (),
| ^^^^ help: use shorthand field pattern: `y`
Explanation
The preferred style is to avoid the repetition of specifying both the field name and the binding name if both identifiers are the same.
non-snake-case
The non_snake_case
lint detects variables, methods, functions,
lifetime parameters and modules that don't have snake case names.
Example
let MY_VALUE = 5;
This will produce:
warning: variable `MY_VALUE` should have a snake case name
--> lint_example.rs:2:5
|
2 | let MY_VALUE = 5;
| ^^^^^^^^ help: convert the identifier to snake case: `my_value`
|
= note: `#[warn(non_snake_case)]` on by default
Explanation
The preferred style for these identifiers is to use "snake case",
where all the characters are in lowercase, with words separated with a
single underscore, such as my_value
.
non-upper-case-globals
The non_upper_case_globals
lint detects static items that don't have
uppercase identifiers.
Example
static max_points: i32 = 5;
This will produce:
warning: static variable `max_points` should have an upper case name
--> lint_example.rs:2:8
|
2 | static max_points: i32 = 5;
| ^^^^^^^^^^ help: convert the identifier to upper case: `MAX_POINTS`
|
= note: `#[warn(non_upper_case_globals)]` on by default
Explanation
The preferred style is for static item names to use all uppercase
letters such as MAX_POINTS
.
noop-method-call
The noop_method_call
lint detects specific calls to noop methods
such as a calling <&T as Clone>::clone
where T: !Clone
.
Example
#![allow(unused)]
struct Foo;
let foo = &Foo;
let clone: &Foo = foo.clone();
This will produce:
warning: call to `.clone()` on a reference in this situation does nothing
--> lint_example.rs:5:22
|
5 | let clone: &Foo = foo.clone();
| ^^^^^^^^
|
= note: the type `Foo` does not implement `Clone`, so calling `clone` on `&Foo` copies the reference, which does not do anything and can be removed
= note: `#[warn(noop_method_call)]` on by default
help: remove this redundant call
|
5 - let clone: &Foo = foo.clone();
5 + let clone: &Foo = foo;
|
help: if you meant to clone `Foo`, implement `Clone` for it
|
3 + #[derive(Clone)]
4 | struct Foo;
|
Explanation
Some method calls are noops meaning that they do nothing. Usually such methods
are the result of blanket implementations that happen to create some method invocations
that end up not doing anything. For instance, Clone
is implemented on all &T
, but
calling clone
on a &T
where T
does not implement clone, actually doesn't do anything
as references are copy. This lint detects these calls and warns the user about them.
opaque-hidden-inferred-bound
The opaque_hidden_inferred_bound
lint detects cases in which nested
impl Trait
in associated type bounds are not written generally enough
to satisfy the bounds of the associated type.
Explanation
This functionality was removed in #97346, but then rolled back in #99860 because it caused regressions.
We plan on reintroducing this as a hard error, but in the meantime, this lint serves to warn and suggest fixes for any use-cases which rely on this behavior.
Example
#![feature(type_alias_impl_trait)]
trait Duh {}
impl Duh for i32 {}
trait Trait {
type Assoc: Duh;
}
impl<F: Duh> Trait for F {
type Assoc = F;
}
type Tait = impl Sized;
fn test() -> impl Trait<Assoc = Tait> {
42
}
fn main() {}
This will produce:
warning: opaque type `impl Trait<Assoc = Tait>` does not satisfy its associated type bounds
--> lint_example.rs:17:25
|
8 | type Assoc: Duh;
| --- this associated type bound is unsatisfied for `Tait`
...
17 | fn test() -> impl Trait<Assoc = Tait> {
| ^^^^^^^^^^^^
|
= note: `#[warn(opaque_hidden_inferred_bound)]` on by default
In this example, test
declares that the associated type Assoc
for
impl Trait
is impl Sized
, which does not satisfy the bound Duh
on the associated type.
Although the hidden type, i32
does satisfy this bound, we do not
consider the return type to be well-formed with this lint. It can be
fixed by changing Tait = impl Sized
into Tait = impl Sized + Duh
.
out-of-scope-macro-calls
The out_of_scope_macro_calls
lint detects macro_rules
called when they are not in scope,
above their definition, which may happen in key-value attributes.
Example
#![doc = in_root!()]
macro_rules! in_root { () => { "" } }
fn main() {}
This will produce:
warning: cannot find macro `in_root` in this scope
--> lint_example.rs:1:10
|
1 | #![doc = in_root!()]
| ^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #124535 <https://github.com/rust-lang/rust/issues/124535>
= help: import `macro_rules` with `use` to make it callable above its definition
= note: `#[warn(out_of_scope_macro_calls)]` on by default
Explanation
The scope in which a macro_rules
item is visible starts at that item and continues
below it. This is more similar to let
than to other items, which are in scope both above
and below their definition.
Due to a bug macro_rules
were accidentally in scope inside some key-value attributes
above their definition. The lint catches such cases.
To address the issue turn the macro_rules
into a regularly scoped item by importing it
with use
.
This is a future-incompatible lint to transition this to a hard error in the future.
overlapping-patterns
The lint overlapping-patterns
has been renamed to overlapping-range-endpoints
.
overlapping-range-endpoints
The overlapping_range_endpoints
lint detects match
arms that have range patterns that
overlap on their endpoints.
Example
let x = 123u8;
match x {
0..=100 => { println!("small"); }
100..=255 => { println!("large"); }
}
This will produce:
warning: multiple patterns overlap on their endpoints
--> lint_example.rs:5:5
|
4 | 0..=100 => { println!("small"); }
| ------- this range overlaps on `100_u8`...
5 | 100..=255 => { println!("large"); }
| ^^^^^^^^^ ... with this range
|
= note: you likely meant to write mutually exclusive ranges
= note: `#[warn(overlapping_range_endpoints)]` on by default
Explanation
It is likely a mistake to have range patterns in a match expression that overlap in this
way. Check that the beginning and end values are what you expect, and keep in mind that
with ..=
the left and right bounds are inclusive.
path-statements
The path_statements
lint detects path statements with no effect.
Example
let x = 42;
x;
This will produce:
warning: path statement with no effect
--> lint_example.rs:4:1
|
4 | x;
| ^^
|
= note: `#[warn(path_statements)]` on by default
Explanation
It is usually a mistake to have a statement that has no effect.
private-bounds
The private_bounds
lint detects types in a secondary interface of an item,
that are more private than the item itself. Secondary interface of an item consists of
bounds on generic parameters and where clauses, including supertraits for trait items.
Example
#![allow(unused)]
#![deny(private_bounds)]
struct PrivTy;
pub struct S
where PrivTy:
{}
fn main() {}
This will produce:
error: type `PrivTy` is more private than the item `S`
--> lint_example.rs:5:1
|
5 | pub struct S
| ^^^^^^^^^^^^ struct `S` is reachable at visibility `pub`
|
note: but type `PrivTy` is only usable at visibility `pub(crate)`
--> lint_example.rs:4:1
|
4 | struct PrivTy;
| ^^^^^^^^^^^^^
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![deny(private_bounds)]
| ^^^^^^^^^^^^^^
Explanation
Having private types or traits in item bounds makes it less clear what interface the item actually provides.
private-interfaces
The private_interfaces
lint detects types in a primary interface of an item,
that are more private than the item itself. Primary interface of an item is all
its interface except for bounds on generic parameters and where clauses.
Example
#![allow(unused)]
#![deny(private_interfaces)]
struct SemiPriv;
mod m1 {
struct Priv;
impl crate::SemiPriv {
pub fn f(_: Priv) {}
}
}
fn main() {}
This will produce:
error: type `Priv` is more private than the item `m1::<impl SemiPriv>::f`
--> lint_example.rs:8:9
|
8 | pub fn f(_: Priv) {}
| ^^^^^^^^^^^^^^^^^ associated function `m1::<impl SemiPriv>::f` is reachable at visibility `pub(crate)`
|
note: but type `Priv` is only usable at visibility `pub(self)`
--> lint_example.rs:6:5
|
6 | struct Priv;
| ^^^^^^^^^^^
note: the lint level is defined here
--> lint_example.rs:2:9
|
2 | #![deny(private_interfaces)]
| ^^^^^^^^^^^^^^^^^^
Explanation
Having something private in primary interface guarantees that the item will be unusable from outer modules due to type privacy.
private-macro-use
The private_macro_use
lint detects private macros that are imported
with #[macro_use]
.
Example
// extern_macro.rs
macro_rules! foo_ { () => {}; }
use foo_ as foo;
// code.rs
#![deny(private_macro_use)]
#[macro_use]
extern crate extern_macro;
fn main() {
foo!();
}
This will produce:
error: cannot find macro `foo` in this scope
Explanation
This lint arises from overlooking visibility checks for macros in an external crate.
This is a future-incompatible lint to transition this to a hard error in the future.
ptr-cast-add-auto-to-object
The ptr_cast_add_auto_to_object
lint detects casts of raw pointers to trait
objects, which add auto traits.
Example
let ptr: *const dyn core::any::Any = &();
_ = ptr as *const dyn core::any::Any + Send;
This will produce:
warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
--> lint_example.rs:3:5
|
3 | _ = ptr as *const dyn core::any::Any + Send;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #127323 <https://github.com/rust-lang/rust/issues/127323>
= note: `#[warn(ptr_cast_add_auto_to_object)]` on by default
Explanation
Adding an auto trait can make the vtable invalid, potentially causing UB in safe code afterwards. For example:
#![feature(arbitrary_self_types)]
trait Trait {
fn f(self: *const Self)
where
Self: Send;
}
impl Trait for *const () {
fn f(self: *const Self) {
unreachable!()
}
}
fn main() {
let unsend: *const () = &();
let unsend: *const dyn Trait = &unsend;
let send_bad: *const (dyn Trait + Send) = unsend as _;
send_bad.f(); // this crashes, since vtable for `*const ()` does not have an entry for `f`
}
Generally you must ensure that vtable is right for the pointer's type, before passing the pointer to safe code.
redundant-semicolon
The lint redundant-semicolon
has been renamed to redundant-semicolons
.
redundant-semicolons
The redundant_semicolons
lint detects unnecessary trailing
semicolons.
Example
let _ = 123;;
This will produce:
warning: unnecessary trailing semicolon
--> lint_example.rs:2:13
|
2 | let _ = 123;;
| ^ help: remove this semicolon
|
= note: `#[warn(redundant_semicolons)]` on by default
Explanation
Extra semicolons are not needed, and may be removed to avoid confusion and visual clutter.
refining-impl-trait-internal
The refining_impl_trait_internal
lint detects impl Trait
return
types in method signatures that are refined by a trait implementation,
meaning the implementation adds information about the return type that
is not present in the trait.
Example
#![deny(refining_impl_trait)]
use std::fmt::Display;
trait AsDisplay {
fn as_display(&self) -> impl Display;
}
impl<'s> AsDisplay for &'s str {
fn as_display(&self) -> Self {
*self
}
}
fn main() {
// users can observe that the return type of
// `<&str as AsDisplay>::as_display()` is `&str`.
let _x: &str = "".as_display();
}
This will produce:
error: impl trait in impl method signature does not match trait method signature
--> lint_example.rs:10:29
|
6 | fn as_display(&self) -> impl Display;
| ------------ return type from trait method defined here
...
10 | fn as_display(&self) -> Self {
| ^^^^
|
= note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
= note: we are soliciting feedback, see issue #121718 <https://github.com/rust-lang/rust/issues/121718> for more information
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(refining_impl_trait)]
| ^^^^^^^^^^^^^^^^^^^
= note: `#[deny(refining_impl_trait_internal)]` implied by `#[deny(refining_impl_trait)]`
help: replace the return type so that it matches the trait
|
10 | fn as_display(&self) -> impl std::fmt::Display {
| ~~~~~~~~~~~~~~~~~~~~~~
Explanation
Callers of methods for types where the implementation is known are able to observe the types written in the impl signature. This may be intended behavior, but may also lead to implementation details being revealed unintentionally. In particular, it may pose a semver hazard for authors of libraries who do not wish to make stronger guarantees about the types than what is written in the trait signature.
refining_impl_trait
is a lint group composed of two lints:
refining_impl_trait_reachable
, for refinements that are publically reachable outside a crate, andrefining_impl_trait_internal
, for refinements that are only visible within a crate.
We are seeking feedback on each of these lints; see issue #121718 for more information.
refining-impl-trait-reachable
The refining_impl_trait_reachable
lint detects impl Trait
return
types in method signatures that are refined by a publically reachable
trait implementation, meaning the implementation adds information about
the return type that is not present in the trait.
Example
#![deny(refining_impl_trait)]
use std::fmt::Display;
pub trait AsDisplay {
fn as_display(&self) -> impl Display;
}
impl<'s> AsDisplay for &'s str {
fn as_display(&self) -> Self {
*self
}
}
fn main() {
// users can observe that the return type of
// `<&str as AsDisplay>::as_display()` is `&str`.
let _x: &str = "".as_display();
}
This will produce:
error: impl trait in impl method signature does not match trait method signature
--> lint_example.rs:10:29
|
6 | fn as_display(&self) -> impl Display;
| ------------ return type from trait method defined here
...
10 | fn as_display(&self) -> Self {
| ^^^^
|
= note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
= note: we are soliciting feedback, see issue #121718 <https://github.com/rust-lang/rust/issues/121718> for more information
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(refining_impl_trait)]
| ^^^^^^^^^^^^^^^^^^^
= note: `#[deny(refining_impl_trait_reachable)]` implied by `#[deny(refining_impl_trait)]`
help: replace the return type so that it matches the trait
|
10 | fn as_display(&self) -> impl std::fmt::Display {
| ~~~~~~~~~~~~~~~~~~~~~~
Explanation
Callers of methods for types where the implementation is known are able to observe the types written in the impl signature. This may be intended behavior, but may also lead to implementation details being revealed unintentionally. In particular, it may pose a semver hazard for authors of libraries who do not wish to make stronger guarantees about the types than what is written in the trait signature.
refining_impl_trait
is a lint group composed of two lints:
refining_impl_trait_reachable
, for refinements that are publically reachable outside a crate, andrefining_impl_trait_internal
, for refinements that are only visible within a crate.
We are seeking feedback on each of these lints; see issue #121718 for more information.
renamed-and-removed-lints
The renamed_and_removed_lints
lint detects lints that have been
renamed or removed.
Example
#![deny(raw_pointer_derive)]
This will produce:
warning: lint `raw_pointer_derive` has been removed: using derive with raw pointers is ok
--> lint_example.rs:1:9
|
1 | #![deny(raw_pointer_derive)]
| ^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(renamed_and_removed_lints)]` on by default
Explanation
To fix this, either remove the lint or use the new name. This can help avoid confusion about lints that are no longer valid, and help maintain consistency for renamed lints.
repr-transparent-external-private-fields
The repr_transparent_external_private_fields
lint
detects types marked #[repr(transparent)]
that (transitively)
contain an external ZST type marked #[non_exhaustive]
or containing
private fields
Example
#![deny(repr_transparent_external_private_fields)]
use foo::NonExhaustiveZst;
#[repr(transparent)]
struct Bar(u32, ([u32; 0], NonExhaustiveZst));
This will produce:
error: zero-sized fields in repr(transparent) cannot contain external non-exhaustive types
--> src/main.rs:5:28
|
5 | struct Bar(u32, ([u32; 0], NonExhaustiveZst));
| ^^^^^^^^^^^^^^^^
|
note: the lint level is defined here
--> src/main.rs:1:9
|
1 | #![deny(repr_transparent_external_private_fields)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
= note: this struct contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, and makes it not a breaking change to become non-zero-sized in the future.
Explanation
Previous, Rust accepted fields that contain external private zero-sized types, even though it should not be a breaking change to add a non-zero-sized field to that private type.
This is a future-incompatible lint to transition this to a hard error in the future. See issue #78586 for more details.
self-constructor-from-outer-item
The self_constructor_from_outer_item
lint detects cases where the Self
constructor
was silently allowed due to a bug in the resolver, and which may produce surprising
and unintended behavior.
Using a Self
type alias from an outer item was never intended, but was silently allowed.
This is deprecated -- and is a hard error when the Self
type alias references generics
that are not in scope.
Example
#![deny(self_constructor_from_outer_item)]
struct S0(usize);
impl S0 {
fn foo() {
const C: S0 = Self(0);
fn bar() -> S0 {
Self(0)
}
}
}
This will produce:
error: can't reference `Self` constructor from outer item
--> lint_example.rs:8:23
|
6 | impl S0 {
| ------- the inner item doesn't inherit generics from this impl, so `Self` is invalid to reference
7 | fn foo() {
8 | const C: S0 = Self(0);
| ^^^^ help: replace `Self` with the actual type: `S0`
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #124186 <https://github.com/rust-lang/rust/issues/124186>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(self_constructor_from_outer_item)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: can't reference `Self` constructor from outer item
--> lint_example.rs:10:13
|
6 | impl S0 {
| ------- the inner item doesn't inherit generics from this impl, so `Self` is invalid to reference
...
10 | Self(0)
| ^^^^ help: replace `Self` with the actual type: `S0`
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #124186 <https://github.com/rust-lang/rust/issues/124186>
Explanation
The Self
type alias should not be reachable because nested items are not associated with
the scope of the parameters from the parent item.
semicolon-in-expressions-from-macros
The semicolon_in_expressions_from_macros
lint detects trailing semicolons
in macro bodies when the macro is invoked in expression position.
This was previous accepted, but is being phased out.
Example
#![deny(semicolon_in_expressions_from_macros)]
macro_rules! foo {
() => { true; }
}
fn main() {
let val = match true {
true => false,
_ => foo!()
};
}
This will produce:
error: trailing semicolon in macro used in expression position
--> lint_example.rs:3:17
|
3 | () => { true; }
| ^
...
9 | _ => foo!()
| ------ in this macro invocation
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(semicolon_in_expressions_from_macros)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: this error originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
Explanation
Previous, Rust ignored trailing semicolon in a macro body when a macro was invoked in expression position. However, this makes the treatment of semicolons in the language inconsistent, and could lead to unexpected runtime behavior in some circumstances (e.g. if the macro author expects a value to be dropped).
This is a future-incompatible lint to transition this to a hard error in the future. See issue #79813 for more details.
special-module-name
The special_module_name
lint detects module
declarations for files that have a special meaning.
Example
mod lib;
fn main() {
lib::run();
}
This will produce:
warning: found module declaration for lib.rs
--> lint_example.rs:1:1
|
1 | mod lib;
| ^^^^^^^^
|
= note: lib.rs is the root of this crate's library target
= help: to refer to it from other targets, use the library's name as the path
= note: `#[warn(special_module_name)]` on by default
Explanation
Cargo recognizes lib.rs
and main.rs
as the root of a
library or binary crate, so declaring them as modules
will lead to miscompilation of the crate unless configured
explicitly.
To access a library from a binary target within the same crate,
use your_crate_name::
as the path instead of lib::
:
// bar/src/lib.rs
fn run() {
// ...
}
// bar/src/main.rs
fn main() {
bar::run();
}
Binary targets cannot be used as libraries and so declaring one as a module is not allowed.
stable-features
The stable_features
lint detects a feature
attribute that
has since been made stable.
Example
#![feature(test_accepted_feature)]
fn main() {}
This will produce:
warning: the feature `test_accepted_feature` has been stable since 1.0.0 and no longer requires an attribute to enable
--> lint_example.rs:1:12
|
1 | #![feature(test_accepted_feature)]
| ^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(stable_features)]` on by default
Explanation
When a feature is stabilized, it is no longer necessary to include a
#![feature]
attribute for it. To fix, simply remove the
#![feature]
attribute.
static-mut-ref
The lint static-mut-ref
has been renamed to static-mut-refs
.
static-mut-refs
The static_mut_refs
lint checks for shared or mutable references
of mutable static inside unsafe
blocks and unsafe
functions.
Example
fn main() {
static mut X: i32 = 23;
static mut Y: i32 = 24;
unsafe {
let y = &X;
let ref x = X;
let (x, y) = (&X, &Y);
foo(&X);
}
}
unsafe fn _foo() {
static mut X: i32 = 23;
static mut Y: i32 = 24;
let y = &X;
let ref x = X;
let (x, y) = (&X, &Y);
foo(&X);
}
fn foo<'a>(_x: &'a i32) {}
This will produce:
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:6:17
|
6 | let y = &X;
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
= note: `#[warn(static_mut_refs)]` on by default
help: use `addr_of!` instead to create a raw pointer
|
6 | let y = addr_of!(X);
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:7:21
|
7 | let ref x = X;
| ^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
7 | let ref x = addr_of!(X);
| +++++++++ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:8:23
|
8 | let (x, y) = (&X, &Y);
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
8 | let (x, y) = (addr_of!(X), &Y);
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:8:27
|
8 | let (x, y) = (&X, &Y);
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
8 | let (x, y) = (&X, addr_of!(Y));
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:9:13
|
9 | foo(&X);
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
9 | foo(addr_of!(X));
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:17:13
|
17 | let y = &X;
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
17 | let y = addr_of!(X);
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:18:17
|
18 | let ref x = X;
| ^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
18 | let ref x = addr_of!(X);
| +++++++++ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:19:19
|
19 | let (x, y) = (&X, &Y);
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
19 | let (x, y) = (addr_of!(X), &Y);
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:19:23
|
19 | let (x, y) = (&X, &Y);
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
19 | let (x, y) = (&X, addr_of!(Y));
| ~~~~~~~~~ +
warning: creating a shared reference to mutable static is discouraged
--> lint_example.rs:20:9
|
20 | foo(&X);
| ^^ shared reference to mutable static
|
= note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
= note: this will be a hard error in the 2024 edition
= note: this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
help: use `addr_of!` instead to create a raw pointer
|
20 | foo(addr_of!(X));
| ~~~~~~~~~ +
Explanation
Shared or mutable references of mutable static are almost always a mistake and can lead to undefined behavior and various other problems in your code.
This lint is "warn" by default on editions up to 2021, in 2024 there is a hard error instead.
suspicious-double-ref-op
The suspicious_double_ref_op
lint checks for usage of .clone()
/.borrow()
/.deref()
on an &&T
when T: !Deref/Borrow/Clone
, which means the call will return the inner &T
,
instead of performing the operation on the underlying T
and can be confusing.
Example
#![allow(unused)]
struct Foo;
let foo = &&Foo;
let clone: &Foo = foo.clone();
This will produce:
warning: using `.clone()` on a double reference, which returns `&Foo` instead of cloning the inner type
--> lint_example.rs:5:22
|
5 | let clone: &Foo = foo.clone();
| ^^^^^^^^
|
= note: `#[warn(suspicious_double_ref_op)]` on by default
Explanation
Since Foo
doesn't implement Clone
, running .clone()
only dereferences the double
reference, instead of cloning the inner type which should be what was intended.
temporary-cstring-as-ptr
The temporary_cstring_as_ptr
lint detects getting the inner pointer of
a temporary CString
.
Example
#![allow(unused)]
use std::ffi::CString;
let c_str = CString::new("foo").unwrap().as_ptr();
This will produce:
warning: getting the inner pointer of a temporary `CString`
--> lint_example.rs:4:42
|
4 | let c_str = CString::new("foo").unwrap().as_ptr();
| ---------------------------- ^^^^^^ this pointer will be invalid
| |
| this `CString` is deallocated at the end of the statement, bind it to a variable to extend its lifetime
|
= note: pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
= help: for more information, see https://doc.rust-lang.org/reference/destructors.html
= note: `#[warn(temporary_cstring_as_ptr)]` on by default
Explanation
The inner pointer of a CString
lives only as long as the CString
it
points to. Getting the inner pointer of a temporary CString
allows the CString
to be dropped at the end of the statement, as it is not being referenced as far as the
typesystem is concerned. This means outside of the statement the pointer will point to
freed memory, which causes undefined behavior if the pointer is later dereferenced.
trivial-bounds
The trivial_bounds
lint detects trait bounds that don't depend on
any type parameters.
Example
#![feature(trivial_bounds)]
pub struct A where i32: Copy;
This will produce:
warning: trait bound i32: Copy does not depend on any type or lifetime parameters
--> lint_example.rs:3:25
|
3 | pub struct A where i32: Copy;
| ^^^^
|
= note: `#[warn(trivial_bounds)]` on by default
Explanation
Usually you would not write a trait bound that you know is always
true, or never true. However, when using macros, the macro may not
know whether or not the constraint would hold or not at the time when
generating the code. Currently, the compiler does not alert you if the
constraint is always true, and generates an error if it is never true.
The trivial_bounds
feature changes this to be a warning in both
cases, giving macros more freedom and flexibility to generate code,
while still providing a signal when writing non-macro code that
something is amiss.
See RFC 2056 for more details. This feature is currently only available on the nightly channel, see tracking issue #48214.
type-alias-bounds
The type_alias_bounds
lint detects bounds in type aliases.
Example
type SendVec<T: Send> = Vec<T>;
This will produce:
warning: bounds on generic parameters in type aliases are not enforced
--> lint_example.rs:2:17
|
2 | type SendVec<T: Send> = Vec<T>;
| --^^^^
| | |
| | will not be checked at usage sites of the type alias
| help: remove this bound
|
= note: this is a known limitation of the type checker that may be lifted in a future edition.
see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
= help: add `#![feature(lazy_type_alias)]` to the crate attributes to enable the desired semantics
= note: `#[warn(type_alias_bounds)]` on by default
Explanation
Trait and lifetime bounds on generic parameters and in where clauses of type aliases are not checked at usage sites of the type alias. Moreover, they are not thoroughly checked for correctness at their definition site either similar to the aliased type.
This is a known limitation of the type checker that may be lifted in a future edition. Permitting such bounds in light of this was unintentional.
While these bounds may have secondary effects such as enabling the use of "shorthand" associated type paths1 and affecting the default trait object lifetime2 of trait object types passed to the type alias, this should not have been allowed until the aforementioned restrictions of the type checker have been lifted.
Using such bounds is highly discouraged as they are actively misleading.
I.e., paths of the form T::Assoc
where T
is a type parameter
bounded by trait Trait
which defines an associated type called Assoc
as opposed to a fully qualified path of the form <T as Trait>::Assoc
.
tyvar-behind-raw-pointer
The tyvar_behind_raw_pointer
lint detects raw pointer to an
inference variable.
Example
// edition 2015
let data = std::ptr::null();
let _ = &data as *const *const ();
if data.is_null() {}
This will produce:
warning: type annotations needed
--> lint_example.rs:6:9
|
6 | if data.is_null() {}
| ^^^^^^^
|
= warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
= note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
= note: `#[warn(tyvar_behind_raw_pointer)]` on by default
Explanation
This kind of inference was previously allowed, but with the future arrival of arbitrary self types, this can introduce ambiguity. To resolve this, use an explicit type instead of relying on type inference.
This is a future-incompatible lint to transition this to a hard error in the 2018 edition. See issue #46906 for more details. This is currently a hard-error on the 2018 edition, and is "warn" by default in the 2015 edition.
uncommon-codepoints
The uncommon_codepoints
lint detects uncommon Unicode codepoints in
identifiers.
Example
#![allow(unused)]
const µ: f64 = 0.000001;
This will produce:
warning: identifier contains a non normalized (NFKC) character: 'µ'
--> lint_example.rs:3:7
|
3 | const µ: f64 = 0.000001;
| ^
|
= note: this character is included in the Not_NFKC Unicode general security profile
= note: `#[warn(uncommon_codepoints)]` on by default
Explanation
This lint warns about using characters which are not commonly used, and may cause visual confusion.
This lint is triggered by identifiers that contain a codepoint that is not part of the set of "Allowed" codepoints as described by Unicode® Technical Standard #39 Unicode Security Mechanisms Section 3.1 General Security Profile for Identifiers.
Note that the set of uncommon codepoints may change over time. Beware that if you "forbid" this lint that existing code may fail in the future.
unconditional-recursion
The unconditional_recursion
lint detects functions that cannot
return without calling themselves.
Example
fn foo() {
foo();
}
This will produce:
warning: function cannot return without recursing
--> lint_example.rs:2:1
|
2 | fn foo() {
| ^^^^^^^^ cannot return without recursing
3 | foo();
| ----- recursive call site
|
= help: a `loop` may express intention better if this is on purpose
= note: `#[warn(unconditional_recursion)]` on by default
Explanation
It is usually a mistake to have a recursive call that does not have
some condition to cause it to terminate. If you really intend to have
an infinite loop, using a loop
expression is recommended.
uncovered-param-in-projection
The uncovered_param_in_projection
lint detects a violation of one of Rust's orphan rules for
foreign trait implementations that concerns the use of type parameters inside trait associated
type paths ("projections") whose output may not be a local type that is mistakenly considered
to "cover" said parameters which is unsound and which may be rejected by a future version
of the compiler.
Originally reported in #99554.
Example
// dependency.rs
#![crate_type = "lib"]
pub trait Trait<T, U> {}
// dependent.rs
trait Identity {
type Output;
}
impl<T> Identity for T {
type Output = T;
}
struct Local;
impl<T> dependency::Trait<Local, T> for <T as Identity>::Output {}
fn main() {}
This will produce:
warning[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
--> dependent.rs:11:6
|
11 | impl<T> dependency::Trait<Local, T> for <T as Identity>::Output {}
| ^ type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #124559 <https://github.com/rust-lang/rust/issues/124559>
= note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
= note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
= note: `#[warn(uncovered_param_in_projection)]` on by default
Explanation
FIXME(fmease): Write explainer.
undefined-naked-function-abi
The undefined_naked_function_abi
lint detects naked function definitions that
either do not specify an ABI or specify the Rust ABI.
Example
#![feature(asm_experimental_arch, naked_functions)]
use std::arch::asm;
#[naked]
pub fn default_abi() -> u32 {
unsafe { asm!("", options(noreturn)); }
}
#[naked]
pub extern "Rust" fn rust_abi() -> u32 {
unsafe { asm!("", options(noreturn)); }
}
This will produce:
warning: Rust ABI is unsupported in naked functions
--> lint_example.rs:7:1
|
7 | pub fn default_abi() -> u32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(undefined_naked_function_abi)]` on by default
warning: Rust ABI is unsupported in naked functions
--> lint_example.rs:12:1
|
12 | pub extern "Rust" fn rust_abi() -> u32 {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
The Rust ABI is currently undefined. Therefore, naked functions should specify a non-Rust ABI.
unexpected-cfgs
The unexpected_cfgs
lint detects unexpected conditional compilation conditions.
Example
rustc --check-cfg 'cfg()'
#[cfg(widnows)]
fn foo() {}
This will produce:
warning: unexpected `cfg` condition name: `widnows`
--> lint_example.rs:1:7
|
1 | #[cfg(widnows)]
| ^^^^^^^
|
= note: `#[warn(unexpected_cfgs)]` on by default
Explanation
This lint is only active when --check-cfg
arguments are being
passed to the compiler and triggers whenever an unexpected condition name or value is
used.
See the Checking Conditional Configurations section for more details.
See the Cargo Specifics section for configuring this lint in
Cargo.toml
.
unfulfilled-lint-expectations
The unfulfilled_lint_expectations
lint detects when a lint expectation is
unfulfilled.
Example
#[expect(unused_variables)]
let x = 10;
println!("{}", x);
This will produce:
warning: this lint expectation is unfulfilled
--> lint_example.rs:2:10
|
2 | #[expect(unused_variables)]
| ^^^^^^^^^^^^^^^^
|
= note: `#[warn(unfulfilled_lint_expectations)]` on by default
Explanation
The #[expect]
attribute can be used to create a lint expectation. The
expectation is fulfilled, if a #[warn]
attribute at the same location
would result in a lint emission. If the expectation is unfulfilled,
because no lint was emitted, this lint will be emitted on the attribute.
ungated-async-fn-track-caller
The ungated_async_fn_track_caller
lint warns when the
#[track_caller]
attribute is used on an async function
without enabling the corresponding unstable feature flag.
Example
#[track_caller]
async fn foo() {}
This will produce:
warning: `#[track_caller]` on async functions is a no-op
--> lint_example.rs:2:1
|
2 | #[track_caller]
| ^^^^^^^^^^^^^^^
3 | async fn foo() {}
| ----------------- this function will not propagate the caller location
|
= note: see issue #110011 <https://github.com/rust-lang/rust/issues/110011> for more information
= help: add `#![feature(async_fn_track_caller)]` to the crate attributes to enable
= note: this compiler was built on 2024-09-06; consider upgrading it if it is out of date
= note: `#[warn(ungated_async_fn_track_caller)]` on by default
Explanation
The attribute must be used in conjunction with the
async_fn_track_caller
feature flag. Otherwise, the #[track_caller]
annotation will function as a no-op.
uninhabited-static
The uninhabited_static
lint detects uninhabited statics.
Example
enum Void {}
extern {
static EXTERN: Void;
}
This will produce:
warning: static of uninhabited type
--> lint_example.rs:4:5
|
4 | static EXTERN: Void;
| ^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
= note: uninhabited statics cannot be initialized, and any access would be an immediate error
= note: `#[warn(uninhabited_static)]` on by default
Explanation
Statics with an uninhabited type can never be initialized, so they are impossible to define.
However, this can be side-stepped with an extern static
, leading to problems later in the
compiler which assumes that there are no initialized uninhabited places (such as locals or
statics). This was accidentally allowed, but is being phased out.
unknown-lints
The unknown_lints
lint detects unrecognized lint attributes.
Example
#![allow(not_a_real_lint)]
This will produce:
warning: unknown lint: `not_a_real_lint`
--> lint_example.rs:1:10
|
1 | #![allow(not_a_real_lint)]
| ^^^^^^^^^^^^^^^
|
= note: `#[warn(unknown_lints)]` on by default
Explanation
It is usually a mistake to specify a lint that does not exist. Check the spelling, and check the lint listing for the correct name. Also consider if you are using an old version of the compiler, and the lint is only available in a newer version.
unknown-or-malformed-diagnostic-attributes
The unknown_or_malformed_diagnostic_attributes
lint detects unrecognized or otherwise malformed
diagnostic attributes.
Example
#![feature(diagnostic_namespace)]
#[diagnostic::does_not_exist]
struct Foo;
This will produce:
warning: unknown diagnostic attribute
--> lint_example.rs:3:15
|
3 | #[diagnostic::does_not_exist]
| ^^^^^^^^^^^^^^
|
= note: `#[warn(unknown_or_malformed_diagnostic_attributes)]` on by default
Explanation
It is usually a mistake to specify a diagnostic attribute that does not exist. Check the spelling, and check the diagnostic attribute listing for the correct name. Also consider if you are using an old version of the compiler, and the attribute is only available in a newer version.
unnameable-test-items
The unnameable_test_items
lint detects #[test]
functions
that are not able to be run by the test harness because they are in a
position where they are not nameable.
Example
fn main() {
#[test]
fn foo() {
// This test will not fail because it does not run.
assert_eq!(1, 2);
}
}
This will produce:
warning: cannot test inner items
--> lint_example.rs:2:5
|
2 | #[test]
| ^^^^^^^
|
= note: `#[warn(unnameable_test_items)]` on by default
= note: this warning originates in the attribute macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
Explanation
In order for the test harness to run a test, the test function must be located in a position where it can be accessed from the crate root. This generally means it must be defined in a module, and not anywhere else such as inside another function. The compiler previously allowed this without an error, so a lint was added as an alert that a test is not being used. Whether or not this should be allowed has not yet been decided, see RFC 2471 and issue #36629.
unreachable-code
The unreachable_code
lint detects unreachable code paths.
Example
panic!("we never go past here!");
let x = 5;
This will produce:
warning: unreachable statement
--> lint_example.rs:4:1
|
2 | panic!("we never go past here!");
| -------------------------------- any code following this expression is unreachable
3 |
4 | let x = 5;
| ^^^^^^^^^^ unreachable statement
|
= note: `#[warn(unreachable_code)]` on by default
Explanation
Unreachable code may signal a mistake or unfinished code. If the code is no longer in use, consider removing it.
unreachable-patterns
The unreachable_patterns
lint detects unreachable patterns.
Example
let x = 5;
match x {
y => (),
5 => (),
}
This will produce:
warning: unreachable pattern
--> lint_example.rs:5:5
|
4 | y => (),
| - matches any value
5 | 5 => (),
| ^ no value can reach this
|
= note: `#[warn(unreachable_patterns)]` on by default
Explanation
This usually indicates a mistake in how the patterns are specified or
ordered. In this example, the y
pattern will always match, so the
five is impossible to reach. Remember, match arms match in order, you
probably wanted to put the 5
case above the y
case.
unstable-name-collision
The lint unstable-name-collision
has been renamed to unstable-name-collisions
.
unstable-name-collisions
The unstable_name_collisions
lint detects that you have used a name
that the standard library plans to add in the future.
Example
trait MyIterator : Iterator {
// is_partitioned is an unstable method that already exists on the Iterator trait
fn is_partitioned<P>(self, predicate: P) -> bool
where
Self: Sized,
P: FnMut(Self::Item) -> bool,
{true}
}
impl<T: ?Sized> MyIterator for T where T: Iterator { }
let x = vec![1, 2, 3];
let _ = x.iter().is_partitioned(|_| true);
This will produce:
warning: a method with this name may be added to the standard library in the future
--> lint_example.rs:14:18
|
14 | let _ = x.iter().is_partitioned(|_| true);
| ^^^^^^^^^^^^^^
|
= warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
= note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
= help: call with fully qualified syntax `MyIterator::is_partitioned(...)` to keep using the current method
= note: `#[warn(unstable_name_collisions)]` on by default
help: add `#![feature(iter_is_partitioned)]` to the crate attributes to enable `is_partitioned`
|
1 + #![feature(iter_is_partitioned)]
|
Explanation
When new methods are added to traits in the standard library, they are
usually added in an "unstable" form which is only available on the
nightly channel with a feature
attribute. If there is any
preexisting code which extends a trait to have a method with the same
name, then the names will collide. In the future, when the method is
stabilized, this will cause an error due to the ambiguity. This lint
is an early-warning to let you know that there may be a collision in
the future. This can be avoided by adding type annotations to
disambiguate which trait method you intend to call, such as
MyIterator::is_partitioned(my_iter, my_predicate)
or renaming or removing the method.
unstable-syntax-pre-expansion
The unstable_syntax_pre_expansion
lint detects the use of unstable
syntax that is discarded during attribute expansion.
Example
#[cfg(FALSE)]
macro foo() {}
This will produce:
warning: `macro` is experimental
--> lint_example.rs:3:1
|
3 | macro foo() {}
| ^^^^^^^^^^^^^^
|
= note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
= help: add `#![feature(decl_macro)]` to the crate attributes to enable
= note: this compiler was built on 2024-09-06; consider upgrading it if it is out of date
= warning: unstable syntax can change at any point in the future, causing a hard error!
= note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
Explanation
The input to active attributes such as #[cfg]
or procedural macro
attributes is required to be valid syntax. Previously, the compiler only
gated the use of unstable syntax features after resolving #[cfg]
gates
and expanding procedural macros.
To avoid relying on unstable syntax, move the use of unstable syntax into a position where the compiler does not parse the syntax, such as a functionlike macro.
#![deny(unstable_syntax_pre_expansion)]
macro_rules! identity {
( $($tokens:tt)* ) => { $($tokens)* }
}
#[cfg(FALSE)]
identity! {
macro foo() {}
}
This is a future-incompatible lint to transition this to a hard error in the future. See issue #65860 for more details.
unsupported-calling-conventions
The unsupported_calling_conventions
lint is output whenever there is a use of the
stdcall
, fastcall
, thiscall
, vectorcall
calling conventions (or their unwind
variants) on targets that cannot meaningfully be supported for the requested target.
For example stdcall
does not make much sense for a x86_64 or, more apparently, powerpc
code, because this calling convention was never specified for those targets.
Historically MSVC toolchains have fallen back to the regular C calling convention for targets other than x86, but Rust doesn't really see a similar need to introduce a similar hack across many more targets.
Example
extern "stdcall" fn stdcall() {}
This will produce:
warning: use of calling convention not supported on this target
--> $DIR/unsupported.rs:39:1
|
LL | extern "stdcall" fn stdcall() {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unsupported_calling_conventions)]` on by default
= warning: this was previously accepted by the compiler but is being phased out;
it will become a hard error in a future release!
= note: for more information, see issue ...
Explanation
On most of the targets the behaviour of stdcall
and similar calling conventions is not
defined at all, but was previously accepted due to a bug in the implementation of the
compiler.
unused-doc-comment
The lint unused-doc-comment
has been renamed to unused-doc-comments
.
unused-tuple-struct-fields
The lint unused-tuple-struct-fields
has been renamed to dead-code
.
unused-allocation
The unused_allocation
lint detects unnecessary allocations that can
be eliminated.
Example
fn main() {
let a = Box::new([1, 2, 3]).len();
}
This will produce:
warning: unnecessary allocation, use `&` instead
--> lint_example.rs:2:13
|
2 | let a = Box::new([1, 2, 3]).len();
| ^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_allocation)]` on by default
Explanation
When a box
expression is immediately coerced to a reference, then
the allocation is unnecessary, and a reference (using &
or &mut
)
should be used instead to avoid the allocation.
unused-assignments
The unused_assignments
lint detects assignments that will never be read.
Example
let mut x = 5;
x = 6;
This will produce:
warning: value assigned to `x` is never read
--> lint_example.rs:3:1
|
3 | x = 6;
| ^
|
= help: maybe it is overwritten before being read?
= note: `#[warn(unused_assignments)]` on by default
Explanation
Unused assignments may signal a mistake or unfinished code. If the
variable is never used after being assigned, then the assignment can
be removed. Variables with an underscore prefix such as _x
will not
trigger this lint.
unused-associated-type-bounds
The unused_associated_type_bounds
lint is emitted when an
associated type bound is added to a trait object, but the associated
type has a where Self: Sized
bound, and is thus unavailable on the
trait object anyway.
Example
trait Foo {
type Bar where Self: Sized;
}
type Mop = dyn Foo<Bar = ()>;
This will produce:
warning: unnecessary associated type bound for not object safe associated type
--> lint_example.rs:5:20
|
5 | type Mop = dyn Foo<Bar = ()>;
| ^^^^^^^^ help: remove this bound
|
= note: this associated type has a `where Self: Sized` bound, and while the associated type can be specified, it cannot be used because trait objects are never `Sized`
= note: `#[warn(unused_associated_type_bounds)]` on by default
Explanation
Just like methods with Self: Sized
bounds are unavailable on trait
objects, associated types can be removed from the trait object.
unused-attributes
The unused_attributes
lint detects attributes that were not used by
the compiler.
Example
#![ignore]
This will produce:
warning: `#[ignore]` only has an effect on functions
--> lint_example.rs:1:1
|
1 | #![ignore]
| ^^^^^^^^^^
|
= note: `#[warn(unused_attributes)]` on by default
Explanation
Unused attributes may indicate the attribute is placed in the wrong
position. Consider removing it, or placing it in the correct position.
Also consider if you intended to use an inner attribute (with a !
such as #![allow(unused)]
) which applies to the item the attribute
is within, or an outer attribute (without a !
such as
#[allow(unused)]
) which applies to the item following the
attribute.
unused-braces
The unused_braces
lint detects unnecessary braces around an
expression.
Example
if { true } {
// ...
}
This will produce:
warning: unnecessary braces around `if` condition
--> lint_example.rs:2:4
|
2 | if { true } {
| ^^ ^^
|
= note: `#[warn(unused_braces)]` on by default
help: remove these braces
|
2 - if { true } {
2 + if true {
|
Explanation
The braces are not needed, and should be removed. This is the preferred style for writing these expressions.
unused-comparisons
The unused_comparisons
lint detects comparisons made useless by
limits of the types involved.
Example
fn foo(x: u8) {
x >= 0;
}
This will produce:
warning: comparison is useless due to type limits
--> lint_example.rs:3:5
|
3 | x >= 0;
| ^^^^^^
|
= note: `#[warn(unused_comparisons)]` on by default
Explanation
A useless comparison may indicate a mistake, and should be fixed or removed.
unused-doc-comments
The unused_doc_comments
lint detects doc comments that aren't used
by rustdoc
.
Example
/// docs for x
let x = 12;
This will produce:
warning: unused doc comment
--> lint_example.rs:2:1
|
2 | /// docs for x
| ^^^^^^^^^^^^^^
3 | let x = 12;
| ----------- rustdoc does not generate documentation for statements
|
= help: use `//` for a plain comment
= note: `#[warn(unused_doc_comments)]` on by default
Explanation
rustdoc
does not use doc comments in all positions, and so the doc
comment will be ignored. Try changing it to a normal comment with //
to avoid the warning.
unused-features
The unused_features
lint detects unused or unknown features found in
crate-level feature
attributes.
Note: This lint is currently not functional, see issue #44232 for more details.
unused-imports
The unused_imports
lint detects imports that are never used.
Example
use std::collections::HashMap;
This will produce:
warning: unused import: `std::collections::HashMap`
--> lint_example.rs:2:5
|
2 | use std::collections::HashMap;
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default
Explanation
Unused imports may signal a mistake or unfinished code, and clutter
the code, and should be removed. If you intended to re-export the item
to make it available outside of the module, add a visibility modifier
like pub
.
unused-labels
The unused_labels
lint detects labels that are never used.
Example
'unused_label: loop {}
This will produce:
warning: unused label
--> lint_example.rs:2:1
|
2 | 'unused_label: loop {}
| ^^^^^^^^^^^^^
|
= note: `#[warn(unused_labels)]` on by default
Explanation
Unused labels may signal a mistake or unfinished code. To silence the
warning for the individual label, prefix it with an underscore such as
'_my_label:
.
unused-macros
The unused_macros
lint detects macros that were not used.
Note that this lint is distinct from the unused_macro_rules
lint,
which checks for single rules that never match of an otherwise used
macro, and thus never expand.
Example
macro_rules! unused {
() => {};
}
fn main() {
}
This will produce:
warning: unused macro definition: `unused`
--> lint_example.rs:1:14
|
1 | macro_rules! unused {
| ^^^^^^
|
= note: `#[warn(unused_macros)]` on by default
Explanation
Unused macros may signal a mistake or unfinished code. To silence the
warning for the individual macro, prefix the name with an underscore
such as _my_macro
. If you intended to export the macro to make it
available outside of the crate, use the macro_export
attribute.
unused-must-use
The unused_must_use
lint detects unused result of a type flagged as
#[must_use]
.
Example
fn returns_result() -> Result<(), ()> {
Ok(())
}
fn main() {
returns_result();
}
This will produce:
warning: unused `Result` that must be used
--> lint_example.rs:6:5
|
6 | returns_result();
| ^^^^^^^^^^^^^^^^
|
= note: this `Result` may be an `Err` variant, which should be handled
= note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
|
6 | let _ = returns_result();
| +++++++
Explanation
The #[must_use]
attribute is an indicator that it is a mistake to
ignore the value. See the reference for more details.
unused-mut
The unused_mut
lint detects mut variables which don't need to be
mutable.
Example
let mut x = 5;
This will produce:
warning: variable does not need to be mutable
--> lint_example.rs:2:5
|
2 | let mut x = 5;
| ----^
| |
| help: remove this `mut`
|
= note: `#[warn(unused_mut)]` on by default
Explanation
The preferred style is to only mark variables as mut
if it is
required.
unused-parens
The unused_parens
lint detects if
, match
, while
and return
with parentheses; they do not need them.
Examples
if(true) {}
This will produce:
warning: unnecessary parentheses around `if` condition
--> lint_example.rs:2:3
|
2 | if(true) {}
| ^ ^
|
= note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
|
2 - if(true) {}
2 + if true {}
|
Explanation
The parentheses are not needed, and should be removed. This is the preferred style for writing these expressions.
unused-unsafe
The unused_unsafe
lint detects unnecessary use of an unsafe
block.
Example
unsafe {}
This will produce:
warning: unnecessary `unsafe` block
--> lint_example.rs:2:1
|
2 | unsafe {}
| ^^^^^^ unnecessary `unsafe` block
|
= note: `#[warn(unused_unsafe)]` on by default
Explanation
If nothing within the block requires unsafe
, then remove the
unsafe
marker because it is not required and may cause confusion.
unused-variables
The unused_variables
lint detects variables which are not used in
any way.
Example
let x = 5;
This will produce:
warning: unused variable: `x`
--> lint_example.rs:2:5
|
2 | let x = 5;
| ^ help: if this is intentional, prefix it with an underscore: `_x`
|
= note: `#[warn(unused_variables)]` on by default
Explanation
Unused variables may signal a mistake or unfinished code. To silence
the warning for the individual variable, prefix it with an underscore
such as _x
.
useless-ptr-null-checks
The useless_ptr_null_checks
lint checks for useless null checks against pointers
obtained from non-null types.
Example
fn test() {}
let fn_ptr: fn() = /* somehow obtained nullable function pointer */
test;
if (fn_ptr as *const ()).is_null() { /* ... */ }
This will produce:
warning: function pointers are not nullable, so checking them for null will always return false
--> lint_example.rs:6:4
|
6 | if (fn_ptr as *const ()).is_null() { /* ... */ }
| ^------^^^^^^^^^^^^^^^^^^^^^^^^
| |
| expression has type `fn()`
|
= help: wrap the function pointer inside an `Option` and use `Option::is_none` to check for null pointer value
= note: `#[warn(useless_ptr_null_checks)]` on by default
Explanation
Function pointers and references are assumed to be non-null, checking them for null will always return false.
warnings
The warnings
lint allows you to change the level of other
lints which produce warnings.
Example
#![deny(warnings)]
fn foo() {}
This will produce:
error: function `foo` is never used
--> lint_example.rs:3:4
|
3 | fn foo() {}
| ^^^
|
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(warnings)]
| ^^^^^^^^
= note: `#[deny(dead_code)]` implied by `#[deny(warnings)]`
Explanation
The warnings
lint is a bit special; by changing its level, you
change every other warning that would produce a warning to whatever
value you'd like. As such, you won't ever trigger this lint in your
code directly.
while-true
The while_true
lint detects while true { }
.
Example
while true {
}
This will produce:
warning: denote infinite loops with `loop { ... }`
--> lint_example.rs:2:1
|
2 | while true {
| ^^^^^^^^^^ help: use `loop`
|
= note: `#[warn(while_true)]` on by default
Explanation
while true
should be replaced with loop
. A loop
expression is
the preferred way to write an infinite loop because it more directly
expresses the intent of the loop.
Deny-by-default Lints
These lints are all set to the 'deny' level by default.
ambiguous_associated_items
arithmetic_overflow
binary_asm_labels
bindings_with_variant_name
cenum_impl_drop_cast
conflicting_repr_hints
deprecated_cfg_attr_crate_type_name
elided_lifetimes_in_associated_constant
enum_intrinsics_non_enums
exceeding-bitshifts
explicit_builtin_cfgs_in_flags
ill_formed_attribute_input
incomplete_include
ineffective_unstable_trait_impl
invalid_atomic_ordering
invalid_doc_attributes
invalid_from_utf8_unchecked
invalid_reference_casting
invalid_type_param_default
let_underscore_lock
long_running_const_eval
macro_expanded_macro_exports_accessed_by_absolute_paths
missing_fragment_specifier
mutable_transmutes
named_asm_labels
no_mangle_const_items
order_dependent_trait_objects
overflowing_literals
patterns_in_fns_without_body
proc_macro_derive_resolution_fallback
pub_use_of_private_extern_crate
soft_unstable
test_unstable_lint
text_direction_codepoint_in_comment
text_direction_codepoint_in_literal
unconditional_panic
undropped_manually_drops
unknown_crate_types
useless_deprecated
wasm_c_abi
ambiguous-associated-items
The ambiguous_associated_items
lint detects ambiguity between
associated items and enum variants.
Example
enum E {
V
}
trait Tr {
type V;
fn foo() -> Self::V;
}
impl Tr for E {
type V = u8;
// `Self::V` is ambiguous because it may refer to the associated type or
// the enum variant.
fn foo() -> Self::V { 0 }
}
This will produce:
error: ambiguous associated item
--> lint_example.rs:15:17
|
15 | fn foo() -> Self::V { 0 }
| ^^^^^^^ help: use fully-qualified syntax: `<E as Tr>::V`
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #57644 <https://github.com/rust-lang/rust/issues/57644>
note: `V` could refer to the variant defined here
--> lint_example.rs:3:5
|
3 | V
| ^
note: `V` could also refer to the associated type defined here
--> lint_example.rs:7:5
|
7 | type V;
| ^^^^^^
= note: `#[deny(ambiguous_associated_items)]` on by default
Explanation
Previous versions of Rust did not allow accessing enum variants through type aliases. When this ability was added (see RFC 2338), this introduced some situations where it can be ambiguous what a type was referring to.
To fix this ambiguity, you should use a qualified path to explicitly
state which type to use. For example, in the above example the
function can be written as fn f() -> <Self as Tr>::V { 0 }
to
specifically refer to the associated type.
This is a future-incompatible lint to transition this to a hard error in the future. See issue #57644 for more details.
arithmetic-overflow
The arithmetic_overflow
lint detects that an arithmetic operation
will overflow.
Example
1_i32 << 32;
This will produce:
error: this arithmetic operation will overflow
--> lint_example.rs:2:1
|
2 | 1_i32 << 32;
| ^^^^^^^^^^^ attempt to shift left by `32_i32`, which would overflow
|
= note: `#[deny(arithmetic_overflow)]` on by default
Explanation
It is very likely a mistake to perform an arithmetic operation that overflows its value. If the compiler is able to detect these kinds of overflows at compile-time, it will trigger this lint. Consider adjusting the expression to avoid overflow, or use a data type that will not overflow.
binary-asm-labels
The binary_asm_labels
lint detects the use of numeric labels containing only binary
digits in the inline asm!
macro.
Example
#![cfg(target_arch = "x86_64")]
use std::arch::asm;
fn main() {
unsafe {
asm!("0: jmp 0b");
}
}
This will produce:
error: avoid using labels containing only the digits `0` and `1` in inline assembly
--> <source>:7:15
|
7 | asm!("0: jmp 0b");
| ^ use a different label that doesn't start with `0` or `1`
|
= help: start numbering with `2` instead
= note: an LLVM bug makes these labels ambiguous with a binary literal number on x86
= note: see <https://github.com/llvm/llvm-project/issues/99547> for more information
= note: `#[deny(binary_asm_labels)]` on by default
Explanation
An LLVM bug causes this code to fail to compile because it interprets the 0b
as a binary
literal instead of a reference to the previous local label 0
. To work around this bug,
don't use labels that could be confused with a binary literal.
This behavior is platform-specific to x86 and x86-64.
See the explanation in Rust By Example for more details.
bindings-with-variant-name
The bindings_with_variant_name
lint detects pattern bindings with
the same name as one of the matched variants.
Example
pub enum Enum {
Foo,
Bar,
}
pub fn foo(x: Enum) {
match x {
Foo => {}
Bar => {}
}
}
This will produce:
error[E0170]: pattern binding `Foo` is named the same as one of the variants of the type `main::Enum`
--> lint_example.rs:9:9
|
9 | Foo => {}
| ^^^ help: to match on the variant, qualify the path: `main::Enum::Foo`
|
= note: `#[deny(bindings_with_variant_name)]` on by default
Explanation
It is usually a mistake to specify an enum variant name as an
identifier pattern. In the example above, the match
arms are
specifying a variable name to bind the value of x
to. The second arm
is ignored because the first one matches all values. The likely
intent is that the arm was intended to match on the enum variant.
Two possible solutions are:
- Specify the enum variant using a path pattern, such as
Enum::Foo
. - Bring the enum variants into local scope, such as adding
use Enum::*;
to the beginning of thefoo
function in the example above.
cenum-impl-drop-cast
The cenum_impl_drop_cast
lint detects an as
cast of a field-less
enum
that implements Drop
.
Example
#![allow(unused)]
enum E {
A,
}
impl Drop for E {
fn drop(&mut self) {
println!("Drop");
}
}
fn main() {
let e = E::A;
let i = e as u32;
}
This will produce:
error: cannot cast enum `E` into integer `u32` because it implements `Drop`
--> lint_example.rs:14:13
|
14 | let i = e as u32;
| ^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #73333 <https://github.com/rust-lang/rust/issues/73333>
= note: `#[deny(cenum_impl_drop_cast)]` on by default
Explanation
Casting a field-less enum
that does not implement Copy
to an
integer moves the value without calling drop
. This can result in
surprising behavior if it was expected that drop
should be called.
Calling drop
automatically would be inconsistent with other move
operations. Since neither behavior is clear or consistent, it was
decided that a cast of this nature will no longer be allowed.
This is a future-incompatible lint to transition this to a hard error in the future. See issue #73333 for more details.
conflicting-repr-hints
The conflicting_repr_hints
lint detects repr
attributes with
conflicting hints.
Example
#[repr(u32, u64)]
enum Foo {
Variant1,
}
This will produce:
error[E0566]: conflicting representation hints
--> lint_example.rs:2:8
|
2 | #[repr(u32, u64)]
| ^^^ ^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #68585 <https://github.com/rust-lang/rust/issues/68585>
= note: `#[deny(conflicting_repr_hints)]` on by default
Explanation
The compiler incorrectly accepted these conflicting representations in the past. This is a future-incompatible lint to transition this to a hard error in the future. See issue #68585 for more details.
To correct the issue, remove one of the conflicting hints.
deprecated-cfg-attr-crate-type-name
The deprecated_cfg_attr_crate_type_name
lint detects uses of the
#![cfg_attr(..., crate_type = "...")]
and
#![cfg_attr(..., crate_name = "...")]
attributes to conditionally
specify the crate type and name in the source code.
Example
#![cfg_attr(debug_assertions, crate_type = "lib")]
This will produce:
error: `crate_type` within an `#![cfg_attr]` attribute is deprecated
--> lint_example.rs:1:31
|
1 | #![cfg_attr(debug_assertions, crate_type = "lib")]
| ^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #91632 <https://github.com/rust-lang/rust/issues/91632>
= note: `#[deny(deprecated_cfg_attr_crate_type_name)]` on by default
Explanation
The #![crate_type]
and #![crate_name]
attributes require a hack in
the compiler to be able to change the used crate type and crate name
after macros have been expanded. Neither attribute works in combination
with Cargo as it explicitly passes --crate-type
and --crate-name
on
the commandline. These values must match the value used in the source
code to prevent an error.
To fix the warning use --crate-type
on the commandline when running
rustc instead of #![cfg_attr(..., crate_type = "...")]
and
--crate-name
instead of #![cfg_attr(..., crate_name = "...")]
.
elided-lifetimes-in-associated-constant
The elided_lifetimes_in_associated_constant
lint detects elided lifetimes
in associated constants when there are other lifetimes in scope. This was
accidentally supported, and this lint was later relaxed to allow eliding
lifetimes to 'static
when there are no lifetimes in scope.
Example
#![deny(elided_lifetimes_in_associated_constant)]
struct Foo<'a>(&'a ());
impl<'a> Foo<'a> {
const STR: &str = "hello, world";
}
This will produce:
error: `&` without an explicit lifetime name cannot be used here
--> lint_example.rs:7:16
|
7 | const STR: &str = "hello, world";
| ^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #115010 <https://github.com/rust-lang/rust/issues/115010>
note: cannot automatically infer `'static` because of other lifetimes in scope
--> lint_example.rs:6:6
|
6 | impl<'a> Foo<'a> {
| ^^
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(elided_lifetimes_in_associated_constant)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: use the `'static` lifetime
|
7 | const STR: &'static str = "hello, world";
| +++++++
Explanation
Previous version of Rust
Implicit static-in-const behavior was decided against for associated constants because of ambiguity. This, however, regressed and the compiler erroneously treats elided lifetimes in associated constants as lifetime parameters on the impl.
This is a future-incompatible lint to transition this to a hard error in the future.
enum-intrinsics-non-enums
The enum_intrinsics_non_enums
lint detects calls to
intrinsic functions that require an enum (core::mem::discriminant
,
core::mem::variant_count
), but are called with a non-enum type.
Example
#![deny(enum_intrinsics_non_enums)]
core::mem::discriminant::<i32>(&123);
This will produce:
error: the return value of `mem::discriminant` is unspecified when called with a non-enum type
--> lint_example.rs:3:1
|
3 | core::mem::discriminant::<i32>(&123);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: the argument to `discriminant` should be a reference to an enum, but it was passed a reference to a `i32`, which is not an enum
--> lint_example.rs:3:32
|
3 | core::mem::discriminant::<i32>(&123);
| ^^^^
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(enum_intrinsics_non_enums)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^
Explanation
In order to accept any enum, the mem::discriminant
and
mem::variant_count
functions are generic over a type T
.
This makes it technically possible for T
to be a non-enum,
in which case the return value is unspecified.
This lint prevents such incorrect usage of these functions.
exceeding-bitshifts
The lint exceeding-bitshifts
has been renamed to arithmetic-overflow
.
explicit-builtin-cfgs-in-flags
The explicit_builtin_cfgs_in_flags
lint detects builtin cfgs set via the --cfg
flag.
Example
rustc --cfg unix
fn main() {}
This will produce:
error: unexpected `--cfg unix` flag
|
= note: config `unix` is only supposed to be controlled by `--target`
= note: manually setting a built-in cfg can and does create incoherent behaviors
= note: `#[deny(explicit_builtin_cfgs_in_flags)]` on by default
Explanation
Setting builtin cfgs can and does produce incoherent behavior, it's better to the use
the appropriate rustc
flag that controls the config. For example setting the windows
cfg but on Linux based target.
ill-formed-attribute-input
The ill_formed_attribute_input
lint detects ill-formed attribute
inputs that were previously accepted and used in practice.
Example
#[inline = "this is not valid"]
fn foo() {}
This will produce:
error: valid forms for the attribute are `#[inline]` and `#[inline(always|never)]`
--> lint_example.rs:2:1
|
2 | #[inline = "this is not valid"]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #57571 <https://github.com/rust-lang/rust/issues/57571>
= note: `#[deny(ill_formed_attribute_input)]` on by default
Explanation
Previously, inputs for many built-in attributes weren't validated and nonsensical attribute inputs were accepted. After validation was added, it was determined that some existing projects made use of these invalid forms. This is a future-incompatible lint to transition this to a hard error in the future. See issue #57571 for more details.
Check the attribute reference for details on the valid inputs for attributes.
incomplete-include
The incomplete_include
lint detects the use of the include!
macro with a file that contains more than one expression.
Example
fn main() {
include!("foo.txt");
}
where the file foo.txt
contains:
println!("hi!");
produces:
error: include macro expected single expression in source
--> foo.txt:1:14
|
1 | println!("1");
| ^
|
= note: `#[deny(incomplete_include)]` on by default
Explanation
The include!
macro is currently only intended to be used to
include a single expression or multiple items. Historically it
would ignore any contents after the first expression, but that can be
confusing. In the example above, the println!
expression ends just
before the semicolon, making the semicolon "extra" information that is
ignored. Perhaps even more surprising, if the included file had
multiple print statements, the subsequent ones would be ignored!
One workaround is to place the contents in braces to create a block expression. Also consider alternatives, like using functions to encapsulate the expressions, or use proc-macros.
This is a lint instead of a hard error because existing projects were
found to hit this error. To be cautious, it is a lint for now. The
future semantics of the include!
macro are also uncertain, see
issue #35560.
ineffective-unstable-trait-impl
The ineffective_unstable_trait_impl
lint detects #[unstable]
attributes which are not used.
Example
#![feature(staged_api)]
#[derive(Clone)]
#[stable(feature = "x", since = "1")]
struct S {}
#[unstable(feature = "y", issue = "none")]
impl Copy for S {}
This will produce:
error: an `#[unstable]` annotation here has no effect
--> lint_example.rs:8:1
|
8 | #[unstable(feature = "y", issue = "none")]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #55436 <https://github.com/rust-lang/rust/issues/55436> for more information
= note: `#[deny(ineffective_unstable_trait_impl)]` on by default
Explanation
staged_api
does not currently support using a stability attribute on impl
blocks.
impl
s are always stable if both the type and trait are stable, and always unstable otherwise.
invalid-atomic-ordering
The invalid_atomic_ordering
lint detects passing an Ordering
to an atomic operation that does not support that ordering.
Example
use core::sync::atomic::{AtomicU8, Ordering};
let atom = AtomicU8::new(0);
let value = atom.load(Ordering::Release);
let _ = value;
This will produce:
error: atomic loads cannot have `Release` or `AcqRel` ordering
--> lint_example.rs:4:23
|
4 | let value = atom.load(Ordering::Release);
| ^^^^^^^^^^^^^^^^^
|
= help: consider using ordering modes `Acquire`, `SeqCst` or `Relaxed`
= note: `#[deny(invalid_atomic_ordering)]` on by default
Explanation
Some atomic operations are only supported for a subset of the
atomic::Ordering
variants. Passing an unsupported variant will cause
an unconditional panic at runtime, which is detected by this lint.
This lint will trigger in the following cases: (where AtomicType
is an
atomic type from core::sync::atomic
, such as AtomicBool
,
AtomicPtr
, AtomicUsize
, or any of the other integer atomics).
-
Passing
Ordering::Acquire
orOrdering::AcqRel
toAtomicType::store
. -
Passing
Ordering::Release
orOrdering::AcqRel
toAtomicType::load
. -
Passing
Ordering::Relaxed
tocore::sync::atomic::fence
orcore::sync::atomic::compiler_fence
. -
Passing
Ordering::Release
orOrdering::AcqRel
as the failure ordering for any ofAtomicType::compare_exchange
,AtomicType::compare_exchange_weak
, orAtomicType::fetch_update
.
invalid-doc-attributes
The invalid_doc_attributes
lint detects when the #[doc(...)]
is
misused.
Example
#![deny(warnings)]
pub mod submodule {
#![doc(test(no_crate_inject))]
}
This will produce:
error: this attribute can only be applied at the crate level
--> lint_example.rs:5:12
|
5 | #![doc(test(no_crate_inject))]
| ^^^^^^^^^^^^^^^^^^^^^
|
= note: read <https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html#at-the-crate-level> for more information
= note: `#[deny(invalid_doc_attributes)]` on by default
Explanation
Previously, incorrect usage of the #[doc(..)]
attribute was not
being validated. Usually these should be rejected as a hard error,
but this lint was introduced to avoid breaking any existing
crates which included them.
invalid-from-utf8-unchecked
The invalid_from_utf8_unchecked
lint checks for calls to
std::str::from_utf8_unchecked
and std::str::from_utf8_unchecked_mut
with a known invalid UTF-8 value.
Example
#[allow(unused)]
unsafe {
std::str::from_utf8_unchecked(b"Ru\x82st");
}
This will produce:
error: calls to `std::str::from_utf8_unchecked` with a invalid literal are undefined behavior
--> lint_example.rs:4:5
|
4 | std::str::from_utf8_unchecked(b"Ru\x82st");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-----------^
| |
| the literal was valid UTF-8 up to the 2 bytes
|
= note: `#[deny(invalid_from_utf8_unchecked)]` on by default
Explanation
Creating such a str
would result in undefined behavior as per documentation
for std::str::from_utf8_unchecked
and std::str::from_utf8_unchecked_mut
.
invalid-reference-casting
The invalid_reference_casting
lint checks for casts of &T
to &mut T
without using interior mutability.
Example
fn x(r: &i32) {
unsafe {
*(r as *const i32 as *mut i32) += 1;
}
}
This will produce:
error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
--> lint_example.rs:4:9
|
4 | *(r as *const i32 as *mut i32) += 1;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
= note: `#[deny(invalid_reference_casting)]` on by default
Explanation
Casting &T
to &mut T
without using interior mutability is undefined behavior,
as it's a violation of Rust reference aliasing requirements.
UnsafeCell
is the only way to obtain aliasable data that is considered
mutable.
invalid-type-param-default
The invalid_type_param_default
lint detects type parameter defaults
erroneously allowed in an invalid location.
Example
fn foo<T=i32>(t: T) {}
This will produce:
error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
--> lint_example.rs:2:8
|
2 | fn foo<T=i32>(t: T) {}
| ^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
= note: `#[deny(invalid_type_param_default)]` on by default
Explanation
Default type parameters were only intended to be allowed in certain situations, but historically the compiler allowed them everywhere. This is a future-incompatible lint to transition this to a hard error in the future. See issue #36887 for more details.
let-underscore-lock
The let_underscore_lock
lint checks for statements which don't bind
a mutex to anything, causing the lock to be released immediately instead
of at end of scope, which is typically incorrect.
Example
use std::sync::{Arc, Mutex};
use std::thread;
let data = Arc::new(Mutex::new(0));
thread::spawn(move || {
// The lock is immediately released instead of at the end of the
// scope, which is probably not intended.
let _ = data.lock().unwrap();
println!("doing some work");
let mut lock = data.lock().unwrap();
*lock += 1;
});
This will produce:
error: non-binding let on a synchronization lock
--> lint_example.rs:9:9
|
9 | let _ = data.lock().unwrap();
| ^ this lock is not assigned to a binding and is immediately dropped
|
= note: `#[deny(let_underscore_lock)]` on by default
help: consider binding to an unused variable to avoid immediately dropping the value
|
9 | let _unused = data.lock().unwrap();
| ~~~~~~~
help: consider immediately dropping the value
|
9 | drop(data.lock().unwrap());
| ~~~~~ +
Explanation
Statements which assign an expression to an underscore causes the
expression to immediately drop instead of extending the expression's
lifetime to the end of the scope. This is usually unintended,
especially for types like MutexGuard
, which are typically used to
lock a mutex for the duration of an entire scope.
If you want to extend the expression's lifetime to the end of the scope,
assign an underscore-prefixed name (such as _foo
) to the expression.
If you do actually want to drop the expression immediately, then
calling std::mem::drop
on the expression is clearer and helps convey
intent.
long-running-const-eval
The long_running_const_eval
lint is emitted when const
eval is running for a long time to ensure rustc terminates
even if you accidentally wrote an infinite loop.
Example
const FOO: () = loop {};
This will produce:
error: constant evaluation is taking a long time
--> lint_example.rs:2:17
|
2 | const FOO: () = loop {};
| ^^^^^^^
|
= note: this lint makes sure the compiler doesn't get stuck due to infinite loops in const eval.
If your compilation actually takes a long time, you can safely allow the lint.
help: the constant being evaluated
--> lint_example.rs:2:1
|
2 | const FOO: () = loop {};
| ^^^^^^^^^^^^^
= note: `#[deny(long_running_const_eval)]` on by default
Explanation
Loops allow const evaluation to compute arbitrary code, but may also cause infinite loops or just very long running computations. Users can enable long running computations by allowing the lint on individual constants or for entire crates.
Unconditional warnings
Note that regardless of whether the lint is allowed or set to warn, the compiler will issue warnings if constant evaluation runs significantly longer than this lint's limit. These warnings are also shown to downstream users from crates.io or similar registries. If you are above the lint's limit, both you and downstream users might be exposed to these warnings. They might also appear on compiler updates, as the compiler makes minor changes about how complexity is measured: staying below the limit ensures that there is enough room, and given that the lint is disabled for people who use your dependency it means you will be the only one to get the warning and can put out an update in your own time.
macro-expanded-macro-exports-accessed-by-absolute-paths
The macro_expanded_macro_exports_accessed_by_absolute_paths
lint
detects macro-expanded macro_export
macros from the current crate
that cannot be referred to by absolute paths.
Example
macro_rules! define_exported {
() => {
#[macro_export]
macro_rules! exported {
() => {};
}
};
}
define_exported!();
fn main() {
crate::exported!();
}
This will produce:
error: macro-expanded `macro_export` macros from the current crate cannot be referred to by absolute paths
--> lint_example.rs:13:5
|
13 | crate::exported!();
| ^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #52234 <https://github.com/rust-lang/rust/issues/52234>
note: the macro is defined here
--> lint_example.rs:4:9
|
4 | / macro_rules! exported {
5 | | () => {};
6 | | }
| |_________^
...
10 | define_exported!();
| ------------------ in this macro invocation
= note: `#[deny(macro_expanded_macro_exports_accessed_by_absolute_paths)]` on by default
= note: this error originates in the macro `define_exported` (in Nightly builds, run with -Z macro-backtrace for more info)
Explanation
The intent is that all macros marked with the #[macro_export]
attribute are made available in the root of the crate. However, when a
macro_rules!
definition is generated by another macro, the macro
expansion is unable to uphold this rule. This is a
future-incompatible lint to transition this to a hard error in the
future. See issue #53495 for more details.
missing-fragment-specifier
The missing_fragment_specifier
lint is issued when an unused pattern in a
macro_rules!
macro definition has a meta-variable (e.g. $e
) that is not
followed by a fragment specifier (e.g. :expr
).
This warning can always be fixed by removing the unused pattern in the
macro_rules!
macro definition.
Example
macro_rules! foo {
() => {};
($name) => { };
}
fn main() {
foo!();
}
This will produce:
error: missing fragment specifier
--> lint_example.rs:3:5
|
3 | ($name) => { };
| ^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #40107 <https://github.com/rust-lang/rust/issues/40107>
= note: `#[deny(missing_fragment_specifier)]` on by default
Explanation
To fix this, remove the unused pattern from the macro_rules!
macro definition:
macro_rules! foo {
() => {};
}
fn main() {
foo!();
}
mutable-transmutes
The mutable_transmutes
lint catches transmuting from &T
to &mut T
because it is undefined behavior.
Example
unsafe {
let y = std::mem::transmute::<&i32, &mut i32>(&5);
}
This will produce:
error: transmuting &T to &mut T is undefined behavior, even if the reference is unused, consider instead using an UnsafeCell
--> lint_example.rs:3:13
|
3 | let y = std::mem::transmute::<&i32, &mut i32>(&5);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[deny(mutable_transmutes)]` on by default
Explanation
Certain assumptions are made about aliasing of data, and this transmute
violates those assumptions. Consider using UnsafeCell
instead.
named-asm-labels
The named_asm_labels
lint detects the use of named labels in the
inline asm!
macro.
Example
#![feature(asm_experimental_arch)]
use std::arch::asm;
fn main() {
unsafe {
asm!("foo: bar");
}
}
This will produce:
error: avoid using named labels in inline assembly
--> lint_example.rs:6:15
|
6 | asm!("foo: bar");
| ^^^
|
= help: only local labels of the form `<number>:` should be used in inline asm
= note: see the asm section of Rust By Example <https://doc.rust-lang.org/nightly/rust-by-example/unsafe/asm.html#labels> for more information
= note: `#[deny(named_asm_labels)]` on by default
Explanation
LLVM is allowed to duplicate inline assembly blocks for any reason, for example when it is in a function that gets inlined. Because of this, GNU assembler local labels must be used instead of labels with a name. Using named labels might cause assembler or linker errors.
See the explanation in Rust By Example for more details.
no-mangle-const-items
The no_mangle_const_items
lint detects any const
items with the
no_mangle
attribute.
Example
#[no_mangle]
const FOO: i32 = 5;
This will produce:
error: const items should never be `#[no_mangle]`
--> lint_example.rs:3:1
|
3 | const FOO: i32 = 5;
| -----^^^^^^^^^^^^^^
| |
| help: try a static value: `pub static`
|
= note: `#[deny(no_mangle_const_items)]` on by default
Explanation
Constants do not have their symbols exported, and therefore, this
probably means you meant to use a static
, not a const
.
order-dependent-trait-objects
The order_dependent_trait_objects
lint detects a trait coherency
violation that would allow creating two trait impls for the same
dynamic trait object involving marker traits.
Example
pub trait Trait {}
impl Trait for dyn Send + Sync { }
impl Trait for dyn Sync + Send { }
This will produce:
error: conflicting implementations of trait `Trait` for type `(dyn Send + Sync + 'static)`: (E0119)
--> lint_example.rs:5:1
|
4 | impl Trait for dyn Send + Sync { }
| ------------------------------ first implementation here
5 | impl Trait for dyn Sync + Send { }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn Send + Sync + 'static)`
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #56484 <https://github.com/rust-lang/rust/issues/56484>
= note: `#[deny(order_dependent_trait_objects)]` on by default
Explanation
A previous bug caused the compiler to interpret traits with different
orders (such as Send + Sync
and Sync + Send
) as distinct types
when they were intended to be treated the same. This allowed code to
define separate trait implementations when there should be a coherence
error. This is a future-incompatible lint to transition this to a
hard error in the future. See issue #56484 for more details.
overflowing-literals
The overflowing_literals
lint detects literal out of range for its
type.
Example
let x: u8 = 1000;
This will produce:
error: literal out of range for `u8`
--> lint_example.rs:2:13
|
2 | let x: u8 = 1000;
| ^^^^
|
= note: the literal `1000` does not fit into the type `u8` whose range is `0..=255`
= note: `#[deny(overflowing_literals)]` on by default
Explanation
It is usually a mistake to use a literal that overflows the type where it is used. Either use a literal that is within range, or change the type to be within the range of the literal.
patterns-in-fns-without-body
The patterns_in_fns_without_body
lint detects mut
identifier
patterns as a parameter in functions without a body.
Example
trait Trait {
fn foo(mut arg: u8);
}
This will produce:
error: patterns aren't allowed in functions without bodies
--> lint_example.rs:3:12
|
3 | fn foo(mut arg: u8);
| ^^^^^^^ help: remove `mut` from the parameter: `arg`
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #35203 <https://github.com/rust-lang/rust/issues/35203>
= note: `#[deny(patterns_in_fns_without_body)]` on by default
Explanation
To fix this, remove mut
from the parameter in the trait definition;
it can be used in the implementation. That is, the following is OK:
trait Trait {
fn foo(arg: u8); // Removed `mut` here
}
impl Trait for i32 {
fn foo(mut arg: u8) { // `mut` here is OK
}
}
Trait definitions can define functions without a body to specify a
function that implementors must define. The parameter names in the
body-less functions are only allowed to be _
or an identifier for
documentation purposes (only the type is relevant). Previous versions
of the compiler erroneously allowed identifier patterns with the
mut
keyword, but this was not intended to be allowed. This is a
future-incompatible lint to transition this to a hard error in the
future. See issue #35203 for more details.
proc-macro-derive-resolution-fallback
The proc_macro_derive_resolution_fallback
lint detects proc macro
derives using inaccessible names from parent modules.
Example
// foo.rs
#![crate_type = "proc-macro"]
extern crate proc_macro;
use proc_macro::*;
#[proc_macro_derive(Foo)]
pub fn foo1(a: TokenStream) -> TokenStream {
drop(a);
"mod __bar { static mut BAR: Option<Something> = None; }".parse().unwrap()
}
// bar.rs
#[macro_use]
extern crate foo;
struct Something;
#[derive(Foo)]
struct Another;
fn main() {}
This will produce:
warning: cannot find type `Something` in this scope
--> src/main.rs:8:10
|
8 | #[derive(Foo)]
| ^^^ names from parent modules are not accessible without an explicit import
|
= note: `#[warn(proc_macro_derive_resolution_fallback)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #50504 <https://github.com/rust-lang/rust/issues/50504>
Explanation
If a proc-macro generates a module, the compiler unintentionally allowed items in that module to refer to items in the crate root without importing them. This is a future-incompatible lint to transition this to a hard error in the future. See issue #50504 for more details.
pub-use-of-private-extern-crate
The pub_use_of_private_extern_crate
lint detects a specific
situation of re-exporting a private extern crate
.
Example
extern crate core;
pub use core as reexported_core;
This will produce:
error[E0365]: extern crate `core` is private and cannot be re-exported
--> lint_example.rs:3:9
|
3 | pub use core as reexported_core;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #127909 <https://github.com/rust-lang/rust/issues/127909>
= note: `#[deny(pub_use_of_private_extern_crate)]` on by default
help: consider making the `extern crate` item publicly accessible
|
2 | pub extern crate core;
| +++
Explanation
A public use
declaration should not be used to publicly re-export a
private extern crate
. pub extern crate
should be used instead.
This was historically allowed, but is not the intended behavior according to the visibility rules. This is a future-incompatible lint to transition this to a hard error in the future. See issue #127909 for more details.
soft-unstable
The soft_unstable
lint detects unstable features that were
unintentionally allowed on stable.
Example
#[cfg(test)]
extern crate test;
#[bench]
fn name(b: &mut test::Bencher) {
b.iter(|| 123)
}
This will produce:
error: use of unstable library feature 'test': `bench` is a part of custom test frameworks which are unstable
--> lint_example.rs:5:3
|
5 | #[bench]
| ^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
= note: `#[deny(soft_unstable)]` on by default
Explanation
The bench
attribute was accidentally allowed to be specified on
the stable release channel. Turning this to a hard error would have
broken some projects. This lint allows those projects to continue to
build correctly when --cap-lints
is used, but otherwise signal an
error that #[bench]
should not be used on the stable channel. This
is a future-incompatible lint to transition this to a hard error in
the future. See issue #64266 for more details.
test-unstable-lint
The test_unstable_lint
lint tests unstable lints and is perma-unstable.
Example
#![allow(test_unstable_lint)]
This will produce:
warning: unknown lint: `test_unstable_lint`
--> lint_example.rs:1:1
|
1 | #![allow(test_unstable_lint)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: the `test_unstable_lint` lint is unstable
= help: add `#![feature(test_unstable_lint)]` to the crate attributes to enable
= note: this compiler was built on 2024-09-06; consider upgrading it if it is out of date
= note: `#[warn(unknown_lints)]` on by default
Explanation
In order to test the behavior of unstable lints, a permanently-unstable lint is required. This lint can be used to trigger warnings and errors from the compiler related to unstable lints.
text-direction-codepoint-in-comment
The text_direction_codepoint_in_comment
lint detects Unicode codepoints in comments that
change the visual representation of text on screen in a way that does not correspond to
their on memory representation.
Example
#![deny(text_direction_codepoint_in_comment)]
fn main() {
println!("{:?}"); // '');
}
This will produce:
error: unicode codepoint changing visible direction of text present in comment
--> lint_example.rs:3:23
|
3 | println!("{:?}"); // '�');
| ^^^^-^^^
| | |
| | '\u{202e}'
| this comment contains an invisible unicode text flow control codepoint
|
= note: these kind of unicode codepoints change the way text flows on applications that support them, but can cause confusion because they change the order of characters on the screen
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(text_direction_codepoint_in_comment)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: if their presence wasn't intentional, you can remove them
Explanation
Unicode allows changing the visual flow of text on screen in order to support scripts that are written right-to-left, but a specially crafted comment can make code that will be compiled appear to be part of a comment, depending on the software used to read the code. To avoid potential problems or confusion, such as in CVE-2021-42574, by default we deny their use.
text-direction-codepoint-in-literal
The text_direction_codepoint_in_literal
lint detects Unicode codepoints that change the
visual representation of text on screen in a way that does not correspond to their on
memory representation.
Explanation
The unicode characters \u{202A}
, \u{202B}
, \u{202D}
, \u{202E}
, \u{2066}
,
\u{2067}
, \u{2068}
, \u{202C}
and \u{2069}
make the flow of text on screen change
its direction on software that supports these codepoints. This makes the text "abc" display
as "cba" on screen. By leveraging software that supports these, people can write specially
crafted literals that make the surrounding code seem like it's performing one action, when
in reality it is performing another. Because of this, we proactively lint against their
presence to avoid surprises.
Example
#![deny(text_direction_codepoint_in_literal)]
fn main() {
println!("{:?}", '');
}
This will produce:
error: unicode codepoint changing visible direction of text present in literal
--> lint_example.rs:3:22
|
3 | println!("{:?}", '�');
| ^-^
| ||
| |'\u{202e}'
| this literal contains an invisible unicode text flow control codepoint
|
= note: these kind of unicode codepoints change the way text flows on applications that support them, but can cause confusion because they change the order of characters on the screen
note: the lint level is defined here
--> lint_example.rs:1:9
|
1 | #![deny(text_direction_codepoint_in_literal)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= help: if their presence wasn't intentional, you can remove them
help: if you want to keep them but make them visible in your source code, you can escape them
|
3 | println!("{:?}", '\u{202e}');
| ~~~~~~~~
unconditional-panic
The unconditional_panic
lint detects an operation that will cause a
panic at runtime.
Example
#![allow(unused)]
let x = 1 / 0;
This will produce:
error: this operation will panic at runtime
--> lint_example.rs:3:9
|
3 | let x = 1 / 0;
| ^^^^^ attempt to divide `1_i32` by zero
|
= note: `#[deny(unconditional_panic)]` on by default
Explanation
This lint detects code that is very likely incorrect because it will
always panic, such as division by zero and out-of-bounds array
accesses. Consider adjusting your code if this is a bug, or using the
panic!
or unreachable!
macro instead in case the panic is intended.
undropped-manually-drops
The undropped_manually_drops
lint check for calls to std::mem::drop
with
a value of std::mem::ManuallyDrop
which doesn't drop.
Example
struct S;
drop(std::mem::ManuallyDrop::new(S));
This will produce:
error: calls to `std::mem::drop` with `std::mem::ManuallyDrop` instead of the inner value does nothing
--> lint_example.rs:3:1
|
3 | drop(std::mem::ManuallyDrop::new(S));
| ^^^^^------------------------------^
| |
| argument has type `ManuallyDrop<S>`
|
= note: `#[deny(undropped_manually_drops)]` on by default
help: use `std::mem::ManuallyDrop::into_inner` to get the inner value
|
3 | drop(std::mem::ManuallyDrop::into_inner(std::mem::ManuallyDrop::new(S)));
| +++++++++++++++++++++++++++++++++++ +
Explanation
ManuallyDrop
does not drop it's inner value so calling std::mem::drop
will
not drop the inner value of the ManuallyDrop
either.
unknown-crate-types
The unknown_crate_types
lint detects an unknown crate type found in
a crate_type
attribute.
Example
#![crate_type="lol"]
fn main() {}
This will produce:
error: invalid `crate_type` value
--> lint_example.rs:1:15
|
1 | #![crate_type="lol"]
| ^^^^^
|
= note: `#[deny(unknown_crate_types)]` on by default
Explanation
An unknown value give to the crate_type
attribute is almost
certainly a mistake.
useless-deprecated
The useless_deprecated
lint detects deprecation attributes with no effect.
Example
struct X;
#[deprecated = "message"]
impl Default for X {
fn default() -> Self {
X
}
}
This will produce:
error: this `#[deprecated]` annotation has no effect
--> lint_example.rs:4:1
|
4 | #[deprecated = "message"]
| ^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove the unnecessary deprecation attribute
|
= note: `#[deny(useless_deprecated)]` on by default
Explanation
Deprecation attributes have no effect on trait implementations.
wasm-c-abi
The wasm_c_abi
lint detects crate dependencies that are incompatible
with future versions of Rust that will emit spec-compliant C ABI.
Example
#![deny(wasm_c_abi)]
This will produce:
error: the following packages contain code that will be rejected by a future version of Rust: wasm-bindgen v0.2.87
|
note: the lint level is defined here
--> src/lib.rs:1:9
|
1 | #![deny(wasm_c_abi)]
| ^^^^^^^^^^
Explanation
Rust has historically emitted non-spec-compliant C ABI. This has caused incompatibilities between other compilers and Wasm targets. In a future version of Rust this will be fixed and therefore dependencies relying on the non-spec-compliant C ABI will stop functioning.
JSON Output
This chapter documents the JSON structures emitted by rustc
. JSON may be
enabled with the --error-format=json
flag. Additional
options may be specified with the --json
flag which can
change which messages are generated, and the format of the messages.
JSON messages are emitted one per line to stderr.
If parsing the output with Rust, the
cargo_metadata
crate provides
some support for parsing the messages.
Each type of message has a $message_type
field which can be used to
distinguish the different formats. When parsing, care should be taken
to be forwards-compatible with future changes to the format. Optional
values may be null
. New fields may be added. Enumerated fields like
"level" or "suggestion_applicability" may add new values.
Diagnostics
Diagnostic messages provide errors or possible concerns generated during
compilation. rustc
provides detailed information about where the diagnostic
originates, along with hints and suggestions.
Diagnostics are arranged in a parent/child relationship where the parent diagnostic value is the core of the diagnostic, and the attached children provide additional context, help, and information.
Diagnostics have the following format:
{
/* Type of this message */
"$message_type": "diagnostic",
/* The primary message. */
"message": "unused variable: `x`",
/* The diagnostic code.
Some messages may set this value to null.
*/
"code": {
/* A unique string identifying which diagnostic triggered. */
"code": "unused_variables",
/* An optional string explaining more detail about the diagnostic code. */
"explanation": null
},
/* The severity of the diagnostic.
Values may be:
- "error": A fatal error that prevents compilation.
- "warning": A possible error or concern.
- "note": Additional information or context about the diagnostic.
- "help": A suggestion on how to resolve the diagnostic.
- "failure-note": A note attached to the message for further information.
- "error: internal compiler error": Indicates a bug within the compiler.
*/
"level": "warning",
/* An array of source code locations to point out specific details about
where the diagnostic originates from. This may be empty, for example
for some global messages, or child messages attached to a parent.
Character offsets are offsets of Unicode Scalar Values.
*/
"spans": [
{
/* The file where the span is located.
Note that this path may not exist. For example, if the path
points to the standard library, and the rust src is not
available in the sysroot, then it may point to a nonexistent
file. Beware that this may also point to the source of an
external crate.
*/
"file_name": "lib.rs",
/* The byte offset where the span starts (0-based, inclusive). */
"byte_start": 21,
/* The byte offset where the span ends (0-based, exclusive). */
"byte_end": 22,
/* The first line number of the span (1-based, inclusive). */
"line_start": 2,
/* The last line number of the span (1-based, inclusive). */
"line_end": 2,
/* The first character offset of the line_start (1-based, inclusive). */
"column_start": 9,
/* The last character offset of the line_end (1-based, exclusive). */
"column_end": 10,
/* Whether or not this is the "primary" span.
This indicates that this span is the focal point of the
diagnostic.
There are rare cases where multiple spans may be marked as
primary. For example, "immutable borrow occurs here" and
"mutable borrow ends here" can be two separate primary spans.
The top (parent) message should always have at least one
primary span, unless it has zero spans. Child messages may have
zero or more primary spans.
*/
"is_primary": true,
/* An array of objects showing the original source code for this
span. This shows the entire lines of text where the span is
located. A span across multiple lines will have a separate
value for each line.
*/
"text": [
{
/* The entire line of the original source code. */
"text": " let x = 123;",
/* The first character offset of the line of
where the span covers this line (1-based, inclusive). */
"highlight_start": 9,
/* The last character offset of the line of
where the span covers this line (1-based, exclusive). */
"highlight_end": 10
}
],
/* An optional message to display at this span location.
This is typically null for primary spans.
*/
"label": null,
/* An optional string of a suggested replacement for this span to
solve the issue. Tools may try to replace the contents of the
span with this text.
*/
"suggested_replacement": null,
/* An optional string that indicates the confidence of the
"suggested_replacement". Tools may use this value to determine
whether or not suggestions should be automatically applied.
Possible values may be:
- "MachineApplicable": The suggestion is definitely what the
user intended. This suggestion should be automatically
applied.
- "MaybeIncorrect": The suggestion may be what the user
intended, but it is uncertain. The suggestion should result
in valid Rust code if it is applied.
- "HasPlaceholders": The suggestion contains placeholders like
`(...)`. The suggestion cannot be applied automatically
because it will not result in valid Rust code. The user will
need to fill in the placeholders.
- "Unspecified": The applicability of the suggestion is unknown.
*/
"suggestion_applicability": null,
/* An optional object indicating the expansion of a macro within
this span.
If a message occurs within a macro invocation, this object will
provide details of where within the macro expansion the message
is located.
*/
"expansion": {
/* The span of the macro invocation.
Uses the same span definition as the "spans" array.
*/
"span": {/*...*/}
/* Name of the macro, such as "foo!" or "#[derive(Eq)]". */
"macro_decl_name": "some_macro!",
/* Optional span where the relevant part of the macro is
defined. */
"def_site_span": {/*...*/},
}
}
],
/* Array of attached diagnostic messages.
This is an array of objects using the same format as the parent
message. Children are not nested (children do not themselves
contain "children" definitions).
*/
"children": [
{
"message": "`#[warn(unused_variables)]` on by default",
"code": null,
"level": "note",
"spans": [],
"children": [],
"rendered": null
},
{
"message": "if this is intentional, prefix it with an underscore",
"code": null,
"level": "help",
"spans": [
{
"file_name": "lib.rs",
"byte_start": 21,
"byte_end": 22,
"line_start": 2,
"line_end": 2,
"column_start": 9,
"column_end": 10,
"is_primary": true,
"text": [
{
"text": " let x = 123;",
"highlight_start": 9,
"highlight_end": 10
}
],
"label": null,
"suggested_replacement": "_x",
"suggestion_applicability": "MachineApplicable",
"expansion": null
}
],
"children": [],
"rendered": null
}
],
/* Optional string of the rendered version of the diagnostic as displayed
by rustc. Note that this may be influenced by the `--json` flag.
*/
"rendered": "warning: unused variable: `x`\n --> lib.rs:2:9\n |\n2 | let x = 123;\n | ^ help: if this is intentional, prefix it with an underscore: `_x`\n |\n = note: `#[warn(unused_variables)]` on by default\n\n"
}
Artifact notifications
Artifact notifications are emitted when the --json=artifacts
flag is used. They indicate that a file artifact has been saved
to disk. More information about emit kinds may be found in the --emit
flag documentation. Notifications can contain more than one file
for each type, for example when using multiple codegen units.
{
/* Type of this message */
"$message_type": "artifact",
/* The filename that was generated. */
"artifact": "libfoo.rlib",
/* The kind of artifact that was generated. Possible values:
- "link": The generated crate as specified by the crate-type.
- "dep-info": The `.d` file with dependency information in a Makefile-like syntax.
- "metadata": The Rust `.rmeta` file containing metadata about the crate.
- "asm": The `.s` file with generated assembly
- "llvm-ir": The `.ll` file with generated textual LLVM IR
- "llvm-bc": The `.bc` file with generated LLVM bitcode
- "mir": The `.mir` file with rustc's mid-level intermediate representation.
- "obj": The `.o` file with generated native object code
*/
"emit": "link"
}
Future-incompatible reports
If the --json=future-incompat
flag is used, then a separate
JSON structure will be emitted if the crate may stop compiling in the future.
This contains diagnostic information about the particular warnings that may be
turned into a hard error in the future. This will include the diagnostic
information, even if the diagnostics have been suppressed (such as with an
#[allow]
attribute or the --cap-lints
option).
{
/* Type of this message */
"$message_type": "future_incompat",
/* An array of objects describing a warning that will become a hard error
in the future.
*/
"future_incompat_report":
[
{
/* A diagnostic structure as defined in
https://doc.rust-lang.org/rustc/json.html#diagnostics
*/
"diagnostic": {...},
}
]
}
Unused Dependency Notifications
The options --json=unused-externs
and --json=unused-externs-silent
in
conjunction with the unused-crate-dependencies
lint will emit JSON structures
reporting any crate dependencies (specified with --extern
) which never had any
symbols referenced. These are intended to be consumed by the build system which
can then emit diagnostics telling the user to remove the unused dependencies
from Cargo.toml
(or whatever build-system file defines dependencies).
The JSON structure is:
{
"lint_level": "deny", /* Level of the warning */
"unused_names": [
"foo" /* Names of unused crates, as specified with --extern foo=libfoo.rlib */
],
}
The warn/deny/forbid lint level (as defined either on the command line or in the
source) dictates the lint_level
in the JSON. With unused-externs
, a
deny
or forbid
level diagnostic will also cause rustc
to exit with a
failure exit code.
unused-externs-silent
will report the diagnostic the same way, but will not
cause rustc
to exit with failure - it's up to the consumer to flag failure
appropriately. (This is needed by Cargo which shares the same dependencies
across multiple build targets, so it should only report an unused dependency if
its not used by any of the targets.)
Tests
rustc
has a built-in facility for building and running tests for a crate.
More information about writing and running tests may be found in the Testing
Chapter of the Rust Programming Language book.
Tests are written as free functions with the #[test]
attribute. For example:
#[test]
fn it_works() {
assert_eq!(2 + 2, 4);
}
Tests "pass" if they return without an error. They "fail" if they panic, or
return a type such as Result
that implements the Termination
trait
with a non-zero value.
By passing the --test
option to rustc
, the compiler will build the crate
in a special mode to construct an executable that will run the tests in the
crate. The --test
flag will make the following changes:
- The crate will be built as a
bin
crate type, forcing it to be an executable. - Links the executable with
libtest
, the test harness that is part of the standard library, which handles running the tests. - Synthesizes a
main
function which will process command-line arguments and run the tests. This newmain
function will replace any existingmain
function as the entry point of the executable, though the existingmain
will still be compiled. - Enables the
test
cfg option, which allows your code to use conditional compilation to detect if it is being built as a test. - Enables building of functions annotated with the
test
andbench
attributes, which will be run by the test harness.
After the executable is created, you can run it to execute the tests and
receive a report on what passes and fails. If you are using Cargo to manage
your project, it has a built-in cargo test
command which handles all of
this automatically. An example of the output looks like this:
running 4 tests
test it_works ... ok
test check_valid_args ... ok
test invalid_characters ... ok
test walks_the_dog ... ok
test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
Note: Tests must be built with the
unwind
panic strategy. This is because all tests run in the same process, and they are intended to catch panics, which is not possible with theabort
strategy. See the unstable-Z panic-abort-tests
option for experimental support of theabort
strategy by spawning tests in separate processes.
Test attributes
Tests are indicated using attributes on free functions. The following attributes are used for testing, see the linked documentation for more details:
#[test]
— Indicates a function is a test to be run.#[bench]
— Indicates a function is a benchmark to be run. Benchmarks are currently unstable and only available in the nightly channel, see the unstable docs for more details.#[should_panic]
— Indicates that the test function will only pass if the function panics.#[ignore]
— Indicates that the test function will be compiled, but not run by default. See the--ignored
and--include-ignored
options to run these tests.
CLI arguments
The libtest harness has several command-line arguments to control its behavior.
Note: When running with
cargo test
, the libtest CLI arguments must be passed after the--
argument to differentiate between flags for Cargo and those for the harness. For example:cargo test -- --nocapture
Filters
Positional arguments (those without a -
prefix) are treated as filters which
will only run tests whose name matches one of those strings. The filter will
match any substring found in the full path of the test function. For example,
if the test function it_works
is located in the module
utils::paths::tests
, then any of the filters works
, path
, utils::
, or
utils::paths::tests::it_works
will match that test.
See Selection options for more options to control which tests are run.
Action options
The following options perform different actions other than running tests.
--list
Prints a list of all tests and benchmarks. Does not run any of the tests. Filters can be used to list only matching tests.
-h
, --help
Displays usage information and command-line options.
Selection options
The following options change how tests are selected.
--test
This is the default mode where all tests will be run as well as running all
benchmarks with only a single iteration (to ensure the benchmark works,
without taking the time to actually perform benchmarking). This can be
combined with the --bench
flag to run both tests and perform full
benchmarking.
--bench
This runs in a mode where tests are ignored, and only runs benchmarks. This
can be combined with --test
to run both benchmarks and tests.
--exact
This forces filters to match the full path of the test exactly.
For example, if the test it_works
is in the module utils::paths::tests
,
then only the string utils::paths::tests::it_works
will match that test.
--skip
FILTER
Skips any tests whose name contains the given FILTER string. This flag may be passed multiple times.
--ignored
Runs only tests that are marked with the ignore
attribute.
--include-ignored
Runs both ignored and non-ignored tests.
--exclude-should-panic
Excludes tests marked with the should_panic
attribute.
⚠️ 🚧 This option is unstable, and requires the -Z unstable-options
flag. See tracking issue
#82348 for more information.
Execution options
The following options affect how tests are executed.
--test-threads
NUM_THREADS
Sets the number of threads to use for running tests in parallel. By default,
uses the amount of concurrency available on the hardware as indicated by
available_parallelism
.
This can also be specified with the RUST_TEST_THREADS
environment variable.
--force-run-in-process
Forces the tests to run in a single process when using the abort
panic
strategy.
⚠️ 🚧 This only works with the unstable -Z panic-abort-tests
option, and
requires the -Z unstable-options
flag. See tracking issue
#67650 for more information.
--ensure-time
⚠️ 🚧 This option is unstable, and requires the -Z unstable-options
flag. See tracking issue
#64888 and the unstable
docs for more information.
--shuffle
Runs the tests in random order, as opposed to the default alphabetical order.
This may also be specified by setting the RUST_TEST_SHUFFLE
environment
variable to anything but 0
.
The random number generator seed that is output can be passed to
--shuffle-seed
to run the tests in the same order
again.
Note that --shuffle
does not affect whether the tests are run in parallel. To
run the tests in random order sequentially, use --shuffle --test-threads 1
.
⚠️ 🚧 This option is unstable, and requires the -Z unstable-options
flag. See tracking issue
#89583 for more information.
--shuffle-seed
SEED
Like --shuffle
, but seeds the random number generator with
SEED. Thus, calling the test harness with --shuffle-seed
SEED twice runs
the tests in the same order both times.
SEED is any 64-bit unsigned integer, for example, one produced by
--shuffle
.
This can also be specified with the RUST_TEST_SHUFFLE_SEED
environment
variable.
⚠️ 🚧 This option is unstable, and requires the -Z unstable-options
flag. See tracking issue
#89583 for more information.
Output options
The following options affect the output behavior.
-q
, --quiet
Displays one character per test instead of one line per test. This is an alias
for --format=terse
.
--nocapture
Does not capture the stdout and stderr of the test, and allows tests to print to the console. Usually the output is captured, and only displayed if the test fails.
This may also be specified by setting the RUST_TEST_NOCAPTURE
environment
variable to anything but 0
.
--show-output
Displays the stdout and stderr of successful tests after all tests have run.
Contrast this with --nocapture
which allows tests to print
while they are running, which can cause interleaved output if there are
multiple tests running in parallel, --show-output
ensures the output is
contiguous, but requires waiting for all tests to finish.
--color
COLOR
Control when colored terminal output is used. Valid options:
auto
: Colorize if stdout is a tty and--nocapture
is not used. This is the default.always
: Always colorize the output.never
: Never colorize the output.
--format
FORMAT
Controls the format of the output. Valid options:
pretty
: This is the default format, with one line per test.terse
: Displays only a single character per test.--quiet
is an alias for this option.json
: Emits JSON objects, one per line. ⚠️ 🚧 This option is unstable, and requires the-Z unstable-options
flag. See tracking issue #49359 for more information.
--logfile
PATH
Writes the results of the tests to the given file.
--report-time
⚠️ 🚧 This option is unstable, and requires the -Z unstable-options
flag. See tracking issue
#64888 and the unstable
docs for more information.
Unstable options
Some CLI options are added in an "unstable" state, where they are intended for
experimentation and testing to determine if the option works correctly, has
the right design, and is useful. The option may not work correctly, break, or
change at any time. To signal that you acknowledge that you are using an
unstable option, they require passing the -Z unstable-options
command-line
flag.
Benchmarks
The libtest harness supports running benchmarks for functions annotated with
the #[bench]
attribute. Benchmarks are currently unstable, and only
available on the nightly channel. More information may be found in the
unstable book.
Custom test frameworks
Experimental support for using custom test harnesses is available on the nightly channel. See tracking issue #50297 and the custom_test_frameworks documentation for more information.
Platform Support
Support for different platforms ("targets") are organized into three tiers, each with a different set of guarantees. For more information on the policies for targets at each tier, see the Target Tier Policy.
Targets are identified by their "target triple" which is the string to inform the compiler what kind of output should be produced.
Component availability is tracked here.
Tier 1 with Host Tools
Tier 1 targets can be thought of as "guaranteed to work". The Rust project builds official binary releases for each tier 1 target, and automated testing ensures that each tier 1 target builds and passes tests after each change.
Tier 1 targets with host tools additionally support running tools like rustc
and cargo
natively on the target, and automated testing ensures that tests
pass for the host tools as well. This allows the target to be used as a
development platform, not just a compilation target. For the full requirements,
see Tier 1 with Host Tools in
the Target Tier Policy.
All tier 1 targets with host tools support the full standard library.
target | notes |
---|---|
aarch64-unknown-linux-gnu | ARM64 Linux (kernel 4.1, glibc 2.17+) |
aarch64-apple-darwin | ARM64 macOS (11.0+, Big Sur+) |
i686-pc-windows-gnu | 32-bit MinGW (Windows 10+, Windows Server 2016+) 1 |
i686-pc-windows-msvc | 32-bit MSVC (Windows 10+, Windows Server 2016+) 1 |
i686-unknown-linux-gnu | 32-bit Linux (kernel 3.2+, glibc 2.17+) 1 |
x86_64-apple-darwin | 64-bit macOS (10.12+, Sierra+) |
x86_64-pc-windows-gnu | 64-bit MinGW (Windows 10+, Windows Server 2016+) |
x86_64-pc-windows-msvc | 64-bit MSVC (Windows 10+, Windows Server 2016+) |
x86_64-unknown-linux-gnu | 64-bit Linux (kernel 3.2+, glibc 2.17+) |
Due to limitations of the C ABI, floating-point support on i686
targets is non-compliant: floating-point return values are passed via an x87 register, so NaN payload bits can be lost. Functions with the default Rust ABI are not affected. See issue #115567.
Tier 1
Tier 1 targets can be thought of as "guaranteed to work". The Rust project builds official binary releases for each tier 1 target, and automated testing ensures that each tier 1 target builds and passes tests after each change. For the full requirements, see Tier 1 target policy in the Target Tier Policy.
At this time, all Tier 1 targets are Tier 1 with Host Tools.
Tier 2 with Host Tools
Tier 2 targets can be thought of as "guaranteed to build". The Rust project
builds official binary releases of the standard library (or, in some cases,
only the core
library) for each tier 2 target, and automated builds
ensure that each tier 2 target can be used as build target after each change. Automated tests are
not always run so it's not guaranteed to produce a working build, but tier 2
targets often work to quite a good degree and patches are always welcome!
Tier 2 target-specific code is not closely scrutinized by Rust team(s) when modifications are made. Bugs are possible in all code, but the level of quality control for these targets is likely to be lower. See library team policy for details on the review practices for standard library code.
Tier 2 targets with host tools additionally support running tools like rustc
and cargo
natively on the target, and automated builds ensure that the host
tools build as well. This allows the target to be used as a development
platform, not just a compilation target. For the full requirements, see Tier 2
with Host Tools in the Target
Tier Policy.
All tier 2 targets with host tools support the full standard library.
NOTE: The rust-docs
component is not usually built for tier 2 targets,
so Rustup may install the documentation for a similar tier 1 target instead.
target | notes |
---|---|
aarch64-pc-windows-msvc | ARM64 Windows MSVC |
aarch64-unknown-linux-musl | ARM64 Linux with musl 1.2.3 |
arm-unknown-linux-gnueabi | Armv6 Linux (kernel 3.2, glibc 2.17) |
arm-unknown-linux-gnueabihf | Armv6 Linux, hardfloat (kernel 3.2, glibc 2.17) |
armv7-unknown-linux-gnueabihf | Armv7-A Linux, hardfloat (kernel 3.2, glibc 2.17) |
loongarch64-unknown-linux-gnu | LoongArch64 Linux, LP64D ABI (kernel 5.19, glibc 2.36) |
loongarch64-unknown-linux-musl | LoongArch64 Linux, LP64D ABI (kernel 5.19, musl 1.2.5) |
powerpc-unknown-linux-gnu | PowerPC Linux (kernel 3.2, glibc 2.17) |
powerpc64-unknown-linux-gnu | PPC64 Linux (kernel 3.2, glibc 2.17) |
powerpc64le-unknown-linux-gnu | PPC64LE Linux (kernel 3.10, glibc 2.17) |
riscv64gc-unknown-linux-gnu | RISC-V Linux (kernel 4.20, glibc 2.29) |
riscv64gc-unknown-linux-musl | RISC-V Linux (kernel 4.20, musl 1.2.3) |
s390x-unknown-linux-gnu | S390x Linux (kernel 3.2, glibc 2.17) |
x86_64-unknown-freebsd | 64-bit FreeBSD |
x86_64-unknown-illumos | illumos |
x86_64-unknown-linux-musl | 64-bit Linux with musl 1.2.3 |
x86_64-unknown-netbsd | NetBSD/amd64 |
Tier 2 without Host Tools
Tier 2 targets can be thought of as "guaranteed to build". The Rust project
builds official binary releases of the standard library (or, in some cases,
only the core
library) for each tier 2 target, and automated builds
ensure that each tier 2 target can be used as build target after each change. Automated tests are
not always run so it's not guaranteed to produce a working build, but tier 2
targets often work to quite a good degree and patches are always welcome! For
the full requirements, see Tier 2 target
policy in the Target Tier Policy.
The std
column in the table below has the following meanings:
- ✓ indicates the full standard library is available.
- * indicates the target only supports
no_std
development. - ? indicates the standard library support is a work-in-progress.
Tier 2 target-specific code is not closely scrutinized by Rust team(s) when modifications are made. Bugs are possible in all code, but the level of quality control for these targets is likely to be lower. See library team policy for details on the review practices for standard library code.
NOTE: The rust-docs
component is not usually built for tier 2 targets,
so Rustup may install the documentation for a similar tier 1 target instead.
target | std | notes |
---|---|---|
aarch64-apple-ios | ✓ | ARM64 iOS |
aarch64-apple-ios-macabi | ✓ | Mac Catalyst on ARM64 |
aarch64-apple-ios-sim | ✓ | Apple iOS Simulator on ARM64 |
aarch64-fuchsia | ✓ | Alias for aarch64-unknown-fuchsia |
aarch64-unknown-fuchsia | ✓ | ARM64 Fuchsia |
aarch64-linux-android | ✓ | ARM64 Android |
aarch64-pc-windows-gnullvm | ✓ | ARM64 MinGW (Windows 10+), LLVM ABI |
aarch64-unknown-linux-ohos | ✓ | ARM64 OpenHarmony |
aarch64-unknown-none-softfloat | * | Bare ARM64, softfloat |
aarch64-unknown-none | * | Bare ARM64, hardfloat |
aarch64-unknown-uefi | ? | ARM64 UEFI |
arm-linux-androideabi | ✓ | Armv6 Android |
arm-unknown-linux-musleabi | ✓ | Armv6 Linux with musl 1.2.3 |
arm-unknown-linux-musleabihf | ✓ | Armv6 Linux with musl 1.2.3, hardfloat |
arm64ec-pc-windows-msvc | ✓ | Arm64EC Windows MSVC |
armebv7r-none-eabi | * | Bare Armv7-R, Big Endian |
armebv7r-none-eabihf | * | Bare Armv7-R, Big Endian, hardfloat |
armv5te-unknown-linux-gnueabi | ✓ | Armv5TE Linux (kernel 4.4, glibc 2.23) |
armv5te-unknown-linux-musleabi | ✓ | Armv5TE Linux with musl 1.2.3 |
armv7-linux-androideabi | ✓ | Armv7-A Android |
armv7-unknown-linux-gnueabi | ✓ | Armv7-A Linux (kernel 4.15, glibc 2.27) |
armv7-unknown-linux-musleabi | ✓ | Armv7-A Linux with musl 1.2.3 |
armv7-unknown-linux-musleabihf | ✓ | Armv7-A Linux with musl 1.2.3, hardfloat |
armv7-unknown-linux-ohos | ✓ | Armv7-A OpenHarmony |
armv7a-none-eabi | * | Bare Armv7-A |
armv7r-none-eabi | * | Bare Armv7-R |
armv7r-none-eabihf | * | Bare Armv7-R, hardfloat |
i586-pc-windows-msvc | * | 32-bit Windows w/o SSE 2 |
i586-unknown-linux-gnu | ✓ | 32-bit Linux w/o SSE (kernel 3.2, glibc 2.17) 2 |
i586-unknown-linux-musl | ✓ | 32-bit Linux w/o SSE, musl 1.2.3 2 |
i686-linux-android | ✓ | 32-bit x86 Android 1 |
i686-pc-windows-gnullvm | ✓ | 32-bit x86 MinGW (Windows 10+), LLVM ABI 1 |
i686-unknown-freebsd | ✓ | 32-bit FreeBSD 1 |
i686-unknown-linux-musl | ✓ | 32-bit Linux with musl 1.2.3 1 |
i686-unknown-uefi | ? | 32-bit UEFI |
loongarch64-unknown-none | * | LoongArch64 Bare-metal (LP64D ABI) |
loongarch64-unknown-none-softfloat | * | LoongArch64 Bare-metal (LP64S ABI) |
nvptx64-nvidia-cuda | * | --emit=asm generates PTX code that runs on NVIDIA GPUs |
riscv32imac-unknown-none-elf | * | Bare RISC-V (RV32IMAC ISA) |
riscv32i-unknown-none-elf | * | Bare RISC-V (RV32I ISA) |
riscv32im-unknown-none-elf | * | Bare RISC-V (RV32IM ISA) |
riscv32imc-unknown-none-elf | * | Bare RISC-V (RV32IMC ISA) |
riscv32imafc-unknown-none-elf | * | Bare RISC-V (RV32IMAFC ISA) |
riscv64gc-unknown-none-elf | * | Bare RISC-V (RV64IMAFDC ISA) |
riscv64imac-unknown-none-elf | * | Bare RISC-V (RV64IMAC ISA) |
sparc64-unknown-linux-gnu | ✓ | SPARC Linux (kernel 4.4, glibc 2.23) |
sparcv9-sun-solaris | ✓ | SPARC Solaris 11, illumos |
thumbv6m-none-eabi | * | Bare Armv6-M |
thumbv7em-none-eabi | * | Bare Armv7E-M |
thumbv7em-none-eabihf | * | Bare Armv7E-M, hardfloat |
thumbv7m-none-eabi | * | Bare Armv7-M |
thumbv7neon-linux-androideabi | ✓ | Thumb2-mode Armv7-A Android with NEON |
thumbv7neon-unknown-linux-gnueabihf | ✓ | Thumb2-mode Armv7-A Linux with NEON (kernel 4.4, glibc 2.23) |
thumbv8m.base-none-eabi | * | Bare Armv8-M Baseline |
thumbv8m.main-none-eabi | * | Bare Armv8-M Mainline |
thumbv8m.main-none-eabihf | * | Bare Armv8-M Mainline, hardfloat |
wasm32-unknown-emscripten | ✓ | WebAssembly via Emscripten |
wasm32-unknown-unknown | ✓ | WebAssembly |
wasm32-wasi | ✓ | WebAssembly with WASI (undergoing a rename to wasm32-wasip1 ) |
wasm32-wasip1 | ✓ | WebAssembly with WASI |
wasm32-wasip1-threads | ✓ | WebAssembly with WASI Preview 1 and threads |
x86_64-apple-ios | ✓ | 64-bit x86 iOS |
x86_64-apple-ios-macabi | ✓ | Mac Catalyst on x86_64 |
x86_64-fortanix-unknown-sgx | ✓ | Fortanix ABI for 64-bit Intel SGX |
x86_64-fuchsia | ✓ | Alias for x86_64-unknown-fuchsia |
x86_64-unknown-fuchsia | ✓ | 64-bit x86 Fuchsia |
x86_64-linux-android | ✓ | 64-bit x86 Android |
x86_64-pc-solaris | ✓ | 64-bit Solaris 11, illumos |
x86_64-pc-windows-gnullvm | ✓ | 64-bit x86 MinGW (Windows 10+), LLVM ABI |
x86_64-unknown-linux-gnux32 | ✓ | 64-bit Linux (x32 ABI) (kernel 4.15, glibc 2.27) |
x86_64-unknown-linux-ohos | ✓ | x86_64 OpenHarmony |
x86_64-unknown-none | * | Freestanding/bare-metal x86_64, softfloat |
x86_64-unknown-redox | ✓ | Redox OS |
x86_64-unknown-uefi | ? | 64-bit UEFI |
Floating-point support on i586
targets is non-compliant: the x87
registers and instructions used for these targets do not provide IEEE-754-compliant behavior, in particular when it comes to rounding and NaN payload bits. See issue #114479.
Tier 3
Tier 3 targets are those which the Rust codebase has support for, but which the Rust project does not build or test automatically, so they may or may not work. Official builds are not available. For the full requirements, see Tier 3 target policy in the Target Tier Policy.
The std
column in the table below has the following meanings:
- ✓ indicates the full standard library is available.
- * indicates the target only supports
no_std
development. - ? indicates the standard library support is unknown or a work-in-progress.
Tier 3 target-specific code is not closely scrutinized by Rust team(s) when modifications are made. Bugs are possible in all code, but the level of quality control for these targets is likely to be lower. See library team policy for details on the review practices for standard library code.
The host
column indicates whether the codebase includes support for building
host tools.
target | std | host | notes |
---|---|---|---|
arm64e-apple-ios | ✓ | ARM64e Apple iOS | |
arm64e-apple-darwin | ✓ | ✓ | ARM64e Apple Darwin |
aarch64-apple-tvos | ✓ | ARM64 tvOS | |
aarch64-apple-tvos-sim | ✓ | ARM64 tvOS Simulator | |
aarch64-apple-watchos | ✓ | ARM64 Apple WatchOS | |
aarch64-apple-watchos-sim | ✓ | ARM64 Apple WatchOS Simulator | |
aarch64-apple-visionos | ✓ | ARM64 Apple visionOS | |
aarch64-apple-visionos-sim | ✓ | ARM64 Apple visionOS Simulator | |
aarch64-kmc-solid_asp3 | ✓ | ARM64 SOLID with TOPPERS/ASP3 | |
aarch64-nintendo-switch-freestanding | * | ARM64 Nintendo Switch, Horizon | |
aarch64-unknown-teeos | ? | ARM64 TEEOS | |
aarch64-unknown-nto-qnx700 | ? | ARM64 QNX Neutrino 7.0 RTOS | |
aarch64-unknown-nto-qnx710 | ✓ | ARM64 QNX Neutrino 7.1 RTOS | |
aarch64-unknown-freebsd | ✓ | ✓ | ARM64 FreeBSD |
aarch64-unknown-hermit | ✓ | ARM64 Hermit | |
aarch64-unknown-illumos | ✓ | ✓ | ARM64 illumos |
aarch64-unknown-linux-gnu_ilp32 | ✓ | ✓ | ARM64 Linux (ILP32 ABI) |
aarch64-unknown-netbsd | ✓ | ✓ | ARM64 NetBSD |
aarch64-unknown-openbsd | ✓ | ✓ | ARM64 OpenBSD |
aarch64-unknown-redox | ✓ | ARM64 Redox OS | |
aarch64-unknown-trusty | ? | ||
aarch64-uwp-windows-msvc | ✓ | ||
aarch64-wrs-vxworks | ✓ | ARM64 VxWorks OS | |
aarch64_be-unknown-linux-gnu_ilp32 | ✓ | ✓ | ARM64 Linux (big-endian, ILP32 ABI) |
aarch64_be-unknown-linux-gnu | ✓ | ✓ | ARM64 Linux (big-endian) |
aarch64_be-unknown-netbsd | ✓ | ✓ | ARM64 NetBSD (big-endian) |
arm64_32-apple-watchos | ✓ | Arm Apple WatchOS 64-bit with 32-bit pointers | |
armeb-unknown-linux-gnueabi | ✓ | ? | Arm BE8 the default Arm big-endian architecture since Armv6. |
armv4t-none-eabi | * | Bare Armv4T | |
armv4t-unknown-linux-gnueabi | ? | Armv4T Linux | |
armv5te-none-eabi | * | Bare Armv5TE | |
armv5te-unknown-linux-uclibceabi | ? | Armv5TE Linux with uClibc | |
armv6-unknown-freebsd | ✓ | ✓ | Armv6 FreeBSD |
armv6-unknown-netbsd-eabihf | ✓ | ✓ | Armv6 NetBSD w/hard-float |
armv6k-nintendo-3ds | ? | Armv6k Nintendo 3DS, Horizon (Requires devkitARM toolchain) | |
armv7-sony-vita-newlibeabihf | ✓ | Armv7-A Cortex-A9 Sony PlayStation Vita (requires VITASDK toolchain) | |
armv7-unknown-linux-uclibceabi | ✓ | ✓ | Armv7-A Linux with uClibc, softfloat |
armv7-unknown-linux-uclibceabihf | ✓ | ? | Armv7-A Linux with uClibc, hardfloat |
armv7-unknown-freebsd | ✓ | ✓ | Armv7-A FreeBSD |
armv7-unknown-netbsd-eabihf | ✓ | ✓ | Armv7-A NetBSD w/hard-float |
armv7-unknown-trusty | ? | ||
armv7-wrs-vxworks-eabihf | ✓ | Armv7-A for VxWorks | |
armv7a-kmc-solid_asp3-eabi | ✓ | ARM SOLID with TOPPERS/ASP3 | |
armv7a-kmc-solid_asp3-eabihf | ✓ | ARM SOLID with TOPPERS/ASP3, hardfloat | |
armv7a-none-eabihf | * | Bare Armv7-A, hardfloat | |
armv7k-apple-watchos | ✓ | Armv7-A Apple WatchOS | |
armv7s-apple-ios | ✓ | Armv7-A Apple-A6 Apple iOS | |
armv8r-none-eabihf | * | Bare Armv8-R, hardfloat | |
avr-unknown-gnu-atmega328 | * | AVR. Requires -Z build-std=core | |
bpfeb-unknown-none | * | BPF (big endian) | |
bpfel-unknown-none | * | BPF (little endian) | |
csky-unknown-linux-gnuabiv2 | ✓ | C-SKY abiv2 Linux (little endian) | |
csky-unknown-linux-gnuabiv2hf | ✓ | C-SKY abiv2 Linux, hardfloat (little endian) | |
hexagon-unknown-none-elf | * | Bare Hexagon (v60+, HVX) | |
hexagon-unknown-linux-musl | ✓ | Hexagon Linux with musl 1.2.3 | |
i386-apple-ios | ✓ | 32-bit x86 iOS 1 | |
i386-lynx-lynxos178 | * | 32-bit LynxOS-178 (2022.07) | |
i586-pc-nto-qnx700 | * | 32-bit x86 QNX Neutrino 7.0 RTOS 1 | |
i586-unknown-netbsd | ✓ | 32-bit x86, restricted to Pentium | |
i686-apple-darwin | ✓ | ✓ | 32-bit macOS (10.12+, Sierra+) 1 |
i686-unknown-haiku | ✓ | ✓ | 32-bit Haiku 1 |
i686-unknown-hurd-gnu | ✓ | ✓ | 32-bit GNU/Hurd 1 |
i686-unknown-netbsd | ✓ | ✓ | NetBSD/i386 with SSE2 1 |
i686-unknown-openbsd | ✓ | ✓ | 32-bit OpenBSD 1 |
i686-unknown-redox | ✓ | i686 Redox OS | |
i686-uwp-windows-gnu | ✓ | 1 | |
i686-uwp-windows-msvc | ✓ | 1 | |
i686-win7-windows-msvc | ✓ | 32-bit Windows 7 support 1 | |
i686-wrs-vxworks | ✓ | 1 | |
m68k-unknown-linux-gnu | ? | Motorola 680x0 Linux | |
mips-unknown-linux-gnu | ✓ | ✓ | MIPS Linux (kernel 4.4, glibc 2.23) |
mips-unknown-linux-musl | ✓ | MIPS Linux with musl 1.2.3 | |
mips-unknown-linux-uclibc | ✓ | MIPS Linux with uClibc | |
mips64-openwrt-linux-musl | ? | MIPS64 for OpenWrt Linux musl 1.2.3 | |
mips64-unknown-linux-gnuabi64 | ✓ | ✓ | MIPS64 Linux, N64 ABI (kernel 4.4, glibc 2.23) |
mips64-unknown-linux-muslabi64 | ✓ | MIPS64 Linux, N64 ABI, musl 1.2.3 | |
mips64el-unknown-linux-gnuabi64 | ✓ | ✓ | MIPS64 (little endian) Linux, N64 ABI (kernel 4.4, glibc 2.23) |
mips64el-unknown-linux-muslabi64 | ✓ | MIPS64 (little endian) Linux, N64 ABI, musl 1.2.3 | |
mipsel-unknown-linux-gnu | ✓ | ✓ | MIPS (little endian) Linux (kernel 4.4, glibc 2.23) |
mipsel-unknown-linux-musl | ✓ | MIPS (little endian) Linux with musl 1.2.3 | |
mipsel-unknown-netbsd | ✓ | ✓ | 32-bit MIPS (LE), requires mips32 cpu support |
mipsel-sony-psp | * | MIPS (LE) Sony PlayStation Portable (PSP) | |
mipsel-sony-psx | * | MIPS (LE) Sony PlayStation 1 (PSX) | |
mipsel-unknown-linux-uclibc | ✓ | MIPS (LE) Linux with uClibc | |
mipsel-unknown-none | * | Bare MIPS (LE) softfloat | |
mipsisa32r6-unknown-linux-gnu | ? | 32-bit MIPS Release 6 Big Endian | |
mipsisa32r6el-unknown-linux-gnu | ? | 32-bit MIPS Release 6 Little Endian | |
mipsisa64r6-unknown-linux-gnuabi64 | ? | 64-bit MIPS Release 6 Big Endian | |
mipsisa64r6el-unknown-linux-gnuabi64 | ✓ | ✓ | 64-bit MIPS Release 6 Little Endian |
msp430-none-elf | * | 16-bit MSP430 microcontrollers | |
powerpc-unknown-linux-gnuspe | ✓ | PowerPC SPE Linux | |
powerpc-unknown-linux-musl | ? | PowerPC Linux with musl 1.2.3 | |
powerpc-unknown-linux-muslspe | ? | PowerPC SPE Linux | |
powerpc-unknown-netbsd | ✓ | ✓ | NetBSD 32-bit powerpc systems |
powerpc-unknown-openbsd | * | ||
powerpc-wrs-vxworks-spe | ✓ | ||
powerpc-wrs-vxworks | ✓ | ||
powerpc64-unknown-freebsd | ✓ | ✓ | PPC64 FreeBSD (ELFv1 and ELFv2) |
powerpc64le-unknown-freebsd | PPC64LE FreeBSD | ||
powerpc-unknown-freebsd | PowerPC FreeBSD | ||
powerpc64-unknown-linux-musl | ? | 64-bit PowerPC Linux with musl 1.2.3 | |
powerpc64-wrs-vxworks | ✓ | ||
powerpc64le-unknown-linux-musl | ? | 64-bit PowerPC Linux with musl 1.2.3, Little Endian | |
powerpc64-unknown-openbsd | ✓ | ✓ | OpenBSD/powerpc64 |
powerpc64-ibm-aix | ? | 64-bit AIX (7.2 and newer) | |
riscv32gc-unknown-linux-gnu | RISC-V Linux (kernel 5.4, glibc 2.33) | ||
riscv32gc-unknown-linux-musl | RISC-V Linux (kernel 5.4, musl 1.2.3 + RISCV32 support patches) | ||
riscv32im-risc0-zkvm-elf | ? | RISC Zero's zero-knowledge Virtual Machine (RV32IM ISA) | |
riscv32ima-unknown-none-elf | * | Bare RISC-V (RV32IMA ISA) | |
riscv32imac-unknown-xous-elf | ? | RISC-V Xous (RV32IMAC ISA) | |
riscv32imc-esp-espidf | ✓ | RISC-V ESP-IDF | |
riscv32imac-esp-espidf | ✓ | RISC-V ESP-IDF | |
riscv32imafc-esp-espidf | ✓ | RISC-V ESP-IDF | |
riscv64gc-unknown-hermit | ✓ | RISC-V Hermit | |
riscv64gc-unknown-freebsd | RISC-V FreeBSD | ||
riscv64gc-unknown-fuchsia | RISC-V Fuchsia | ||
riscv64gc-unknown-netbsd | ✓ | ✓ | RISC-V NetBSD |
riscv64gc-unknown-openbsd | ✓ | ✓ | OpenBSD/riscv64 |
riscv64-linux-android | RISC-V 64-bit Android | ||
s390x-unknown-linux-musl | S390x Linux (kernel 3.2, musl 1.2.3) | ||
sparc-unknown-linux-gnu | ✓ | 32-bit SPARC Linux | |
sparc-unknown-none-elf | * | Bare 32-bit SPARC V7+ | |
sparc64-unknown-netbsd | ✓ | ✓ | NetBSD/sparc64 |
sparc64-unknown-openbsd | ✓ | ✓ | OpenBSD/sparc64 |
thumbv4t-none-eabi | * | Thumb-mode Bare Armv4T | |
thumbv5te-none-eabi | * | Thumb-mode Bare Armv5TE | |
thumbv7a-pc-windows-msvc | ✓ | ||
thumbv7a-uwp-windows-msvc | ✓ | ||
thumbv7neon-unknown-linux-musleabihf | ? | Thumb2-mode Armv7-A Linux with NEON, musl 1.2.3 | |
wasm32-wasip2 | ✓ | WebAssembly | |
wasm64-unknown-unknown | ? | WebAssembly | |
x86_64-apple-tvos | ✓ | x86 64-bit tvOS | |
x86_64-apple-watchos-sim | ✓ | x86 64-bit Apple WatchOS simulator | |
x86_64-pc-nto-qnx710 | ✓ | x86 64-bit QNX Neutrino 7.1 RTOS | |
x86_64-unikraft-linux-musl | ✓ | 64-bit Unikraft with musl 1.2.3 | |
x86_64-unknown-dragonfly | ✓ | ✓ | 64-bit DragonFlyBSD |
x86_64-unknown-haiku | ✓ | ✓ | 64-bit Haiku |
x86_64-unknown-hermit | ✓ | x86_64 Hermit | |
x86_64-unknown-l4re-uclibc | ? | ||
x86_64-unknown-openbsd | ✓ | ✓ | 64-bit OpenBSD |
x86_64-uwp-windows-gnu | ✓ | ||
x86_64-uwp-windows-msvc | ✓ | ||
x86_64-win7-windows-msvc | ✓ | 64-bit Windows 7 support | |
x86_64-wrs-vxworks | ✓ | ||
x86_64h-apple-darwin | ✓ | ✓ | macOS with late-gen Intel (at least Haswell) |
x86_64-unknown-linux-none | * | 64-bit Linux with no libc | |
xtensa-esp32-none-elf | * | Xtensa ESP32 | |
xtensa-esp32-espidf | ✓ | Xtensa ESP32 | |
xtensa-esp32s2-none-elf | * | Xtensa ESP32-S2 | |
xtensa-esp32s2-espidf | ✓ | Xtensa ESP32-S2 | |
xtensa-esp32s3-none-elf | * | Xtensa ESP32-S3 | |
xtensa-esp32s3-espidf | ✓ | Xtensa ESP32-S3 | |
thumbv6m-nuttx-eabi | * | ARMv6M with NuttX | |
thumbv7m-nuttx-eabi | * | ARMv7M with NuttX | |
thumbv7em-nuttx-eabi | * | ARMv7EM with NuttX | |
thumbv7em-nuttx-eabihf | * | ARMv7EM with NuttX, hardfloat | |
thumbv8m.base-nuttx-eabi | * | ARMv8M Baseline with NuttX | |
thumbv8m.main-nuttx-eabi | * | ARMv8M Mainline with NuttX | |
thumbv8m.main-nuttx-eabihf | * | ARMv8M Mainline with NuttX, hardfloat | |
riscv32imc-unknown-nuttx-elf | * | RISC-V 32bit with NuttX | |
riscv32imac-unknown-nuttx-elf | * | RISC-V 32bit with NuttX | |
riscv32imafc-unknown-nuttx-elf | * | RISC-V 32bit with NuttX | |
riscv64imac-unknown-nuttx-elf | * | RISC-V 64bit with NuttX | |
riscv64gc-unknown-nuttx-elf | * | RISC-V 64bit with NuttX | |
aarch64-unknown-ferrocenecoretest | ✓ | ✓ | Internal target for tests |
Target Tier Policy
Table of Contents
General
Rust provides three tiers of target support:
- Rust provides no guarantees about tier 3 targets; they exist in the codebase, but may or may not build.
- Rust's continuous integration checks that tier 2 targets will always build, but they may or may not pass tests.
- Rust's continuous integration checks that tier 1 targets will always build and pass tests.
Adding a new tier 3 target imposes minimal requirements; we focus primarily on avoiding disruption to other ongoing Rust development.
Tier 2 and tier 1 targets place work on Rust project developers as a whole, to avoid breaking the target. The broader Rust community may also feel more inclined to support higher-tier targets in their crates (though they are not obligated to do so). Thus, these tiers require commensurate and ongoing efforts from the maintainers of the target, to demonstrate value and to minimize any disruptions to ongoing Rust development.
This policy defines the requirements for accepting a proposed target at a given level of support.
Each tier builds on all the requirements from the previous tier, unless
overridden by a stronger requirement. Targets at tier 2 and tier 1 may also
provide host tools (such as rustc
and cargo
); each of those tiers
includes a set of supplementary requirements that must be met if supplying host
tools for the target. A target at tier 2 or tier 1 is not required to supply
host tools, but if it does, it must meet the corresponding additional
requirements for host tools.
The policy for each tier also documents the Rust governance teams that must approve the addition of any target at that tier. Those teams are responsible for reviewing and evaluating the target, based on these requirements and their own judgment. Those teams may apply additional requirements, including subjective requirements, such as to deal with issues not foreseen by this policy. (Such requirements may subsequently motivate additions to this policy.)
While these criteria attempt to document the policy, that policy still involves human judgment. Targets must fulfill the spirit of the requirements as well, as determined by the judgment of the approving teams. Reviewers and team members evaluating targets and target-specific patches should always use their own best judgment regarding the quality of work, and the suitability of a target for the Rust project. Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party.
Before filing an issue or pull request (PR) to introduce or promote a target, the target should already meet the corresponding tier requirements. This does not preclude an existing target's maintainers using issues (on the Rust repository or otherwise) to track requirements that have not yet been met, as appropriate; however, before officially proposing the introduction or promotion of a target, it should meet all of the necessary requirements. A target proposal must quote the corresponding requirements verbatim and respond to them as part of explaining how the target meets those requirements. (For the requirements that simply state that the target or the target developers must not do something, it suffices to acknowledge the requirement.)
For a list of all supported targets and their corresponding tiers ("tier 3", "tier 2", "tier 2 with host tools", "tier 1", or "tier 1 with host tools"), see platform support.
Several parts of this policy require providing target-specific documentation. Such documentation should typically appear in a subdirectory of the platform-support section of this rustc manual, with a link from the target's entry in platform support. Use TEMPLATE.md as a base, and see other documentation in that directory for examples.
Note that a target must have already received approval for the next lower tier, and spent a reasonable amount of time at that tier, before making a proposal for promotion to the next higher tier; this is true even if a target meets the requirements for several tiers at once. This policy leaves the precise interpretation of "reasonable amount of time" up to the approving teams; those teams may scale the amount of time required based on their confidence in the target and its demonstrated track record at its current tier. At a minimum, multiple stable releases of Rust should typically occur between promotions of a target.
The availability or tier of a target in stable Rust is not a hard stability guarantee about the future availability or tier of that target. Higher-level target tiers are an increasing commitment to the support of a target, and we will take that commitment and potential disruptions into account when evaluating the potential demotion or removal of a target that has been part of a stable release. The promotion or demotion of a target will not generally affect existing stable releases, only current development and future releases.
In this policy, the words "must" and "must not" specify absolute requirements that a target must meet to qualify for a tier. The words "should" and "should not" specify requirements that apply in almost all cases, but for which the approving teams may grant an exception for good reason. The word "may" indicates something entirely optional, and does not indicate guidance or recommendations. This language is based on IETF RFC 2119.
Adding a new target
New targets typically start as Tier 3 and then can be promoted later.
To propose addition of a new target, open a pull request on rust-lang/rust
:
- Copy the Tier 3 target policy to the description and fill it out, see example.
- Add a new description for the target in
src/doc/rustc/src/platform-support
using the template. - Add the target to the SUMMARY.md (allows wildcards) and platform-support.md (must name all targets verbatim). Link to the created description page.
- Ensure the pull request is assigned to a member of the Rust compiler team by commenting:
r? compiler-team
Tier 3 target policy
At this tier, the Rust project provides no official support for a target, so we place minimal requirements on the introduction of targets.
A proposed new tier 3 target must be reviewed and approved by a member of the compiler team based on these requirements. The reviewer may choose to gauge broader compiler team consensus via a Major Change Proposal (MCP).
A proposed target or target-specific patch that substantially changes code shared with other targets (not just target-specific code) must be reviewed and approved by the appropriate team for that shared code before acceptance.
- A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)
- Targets must use naming consistent with any existing targets; for instance, a
target for the same CPU or OS as an existing Rust target should use the same
name for that CPU or OS. Targets should normally use the same names and
naming conventions as used elsewhere in the broader ecosystem beyond Rust
(such as in other toolchains), unless they have a very good reason to
diverge. Changing the name of a target can be highly disruptive, especially
once the target reaches a higher tier, so getting the name right is important
even for a tier 3 target.
- Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.
- If possible, use only letters, numbers, dashes and underscores for the name.
Periods (
.
) are known to cause issues in Cargo.
- Tier 3 targets may have unusual requirements to build or use, but must not
create legal issues or impose onerous legal terms for the Rust project or for
Rust developers or users.
- The target must not introduce license incompatibilities.
- Anything added to the Rust repository must be under the standard Rust
license (
MIT OR Apache-2.0
). - The target must not cause the Rust tools or libraries built for any other
host (even when supporting cross-compilation to the target) to depend
on any new dependency less permissive than the Rust licensing policy. This
applies whether the dependency is a Rust crate that would require adding
new license exceptions (as specified by the
tidy
tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements. - Compiling, linking, and emitting functional binaries, libraries, or other
code for the target (whether hosted on the target itself or cross-compiling
from another target) must not depend on proprietary (non-FOSS) libraries.
Host tools built for the target itself may depend on the ordinary runtime
libraries supplied by the platform and commonly used by other applications
built for the target, but those libraries must not be required for code
generation for the target; cross-compilation to the target must not require
such libraries at all. For instance,
rustc
built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3. - "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.
- Neither this policy nor any decisions made regarding targets shall create any
binding agreement or estoppel by any party. If any member of an approving
Rust team serves as one of the maintainers of a target, or has any legal or
employment requirement (explicit or implicit) that might affect their
decisions regarding a target, they must recuse themselves from any approval
decisions regarding the target's tier status, though they may otherwise
participate in discussions.
- This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.
- Tier 3 targets should attempt to implement as much of the standard libraries
as possible and appropriate (
core
for most targets,alloc
for targets that can support dynamic memory allocation,std
for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions. - The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.
- Tier 3 targets must not impose burden on the authors of pull requests, or
other developers in the community, to maintain the target. In particular,
do not post comments (automated or manual) on a PR that derail or suggest a
block on the PR based on a tier 3 target. Do not send automated messages or
notifications (via any medium, including via
@
) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.- Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.
- Patches adding or updating tier 3 targets must not break any existing tier 2
or tier 1 target, and must not knowingly break another tier 3 target without
approval of either the compiler team or the maintainers of the other tier 3
target.
- In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.
- Tier 3 targets must be able to produce assembly using at least one of rustc's supported backends from any host target. (Having support in a fork of the backend is not sufficient, it must be upstream.)
If a tier 3 target stops meeting these requirements, or the target maintainers no longer have interest or time, or the target shows no signs of activity and has not built for some time, or removing the target would improve the quality of the Rust codebase, we may post a PR to remove it; any such PR will be CCed to the target maintainers (and potentially other people who have previously worked on the target), to check potential interest in improving the situation.
Tier 2 target policy
At this tier, the Rust project guarantees that a target builds, and will reject patches that fail to build on a target. Thus, we place requirements that ensure the target will not block forward progress of the Rust project.
A proposed new tier 2 target must be reviewed and approved by the compiler team based on these requirements. Such review and approval may occur via a Major Change Proposal (MCP).
In addition, the infrastructure team must approve the integration of the target into Continuous Integration (CI), and the tier 2 CI-related requirements. This review and approval may take place in a PR adding the target to CI, or simply by an infrastructure team member reporting the outcome of a team discussion.
- A tier 2 target must have value to people other than its maintainers. (It may still be a niche target, but it must not be exclusively useful for an inherently closed group.)
- A tier 2 target must have a designated team of developers (the "target
maintainers") available to consult on target-specific build-breaking issues,
or if necessary to develop target-specific language or library implementation
details. This team must have at least 2 developers.
- The target maintainers should not only fix target-specific issues, but should use any such issue as an opportunity to educate the Rust community about portability to their target, and enhance documentation of the target.
- The target must not place undue burden on Rust developers not specifically concerned with that target. Rust developers are expected to not gratuitously break a tier 2 target, but are not expected to become experts in every tier 2 target, and are not expected to provide target-specific implementations for every tier 2 target.
- The target must provide documentation for the Rust community explaining how to build for the target using cross-compilation, and explaining how to run tests for the target. If at all possible, this documentation should show how to run Rust programs and tests for the target using emulation, to allow anyone to do so. If the target cannot be feasibly emulated, the documentation should explain how to obtain and work with physical hardware, cloud systems, or equivalent.
- The target must document its baseline expectations for the features or versions of CPUs, operating systems, libraries, runtime environments, and similar.
- If introducing a new tier 2 or higher target that is identical to an existing
Rust target except for the baseline expectations for the features or versions
of CPUs, operating systems, libraries, runtime environments, and similar,
then the proposed target must document to the satisfaction of the approving
teams why the specific difference in baseline expectations provides
sufficient value to justify a separate target.
- Note that in some cases, based on the usage of existing targets within the
Rust community, Rust developers or a target's maintainers may wish to
modify the baseline expectations of a target, or split an existing target
into multiple targets with different baseline expectations. A proposal to
do so will be treated similarly to the analogous promotion, demotion, or
removal of a target, according to this policy, with the same team approvals
required.
- For instance, if an OS version has become obsolete and unsupported, a target for that OS may raise its baseline expectations for OS version (treated as though removing a target corresponding to the older versions), or a target for that OS may split out support for older OS versions into a lower-tier target (treated as though demoting a target corresponding to the older versions, and requiring justification for a new target at a lower tier for the older OS versions).
- Note that in some cases, based on the usage of existing targets within the
Rust community, Rust developers or a target's maintainers may wish to
modify the baseline expectations of a target, or split an existing target
into multiple targets with different baseline expectations. A proposal to
do so will be treated similarly to the analogous promotion, demotion, or
removal of a target, according to this policy, with the same team approvals
required.
- Tier 2 targets must not leave any significant portions of
core
or the standard library unimplemented or stubbed out, unless they cannot possibly be supported on the target.- The right approach to handling a missing feature from a target may depend on whether the target seems likely to develop the feature in the future. In some cases, a target may be co-developed along with Rust support, and Rust may gain new features on the target as that target gains the capabilities to support those features.
- As an exception, a target identical to an existing tier 1 target except for
lower baseline expectations for the OS, CPU, or similar, may propose to
qualify as tier 2 (but not higher) without support for
std
if the target will primarily be used inno_std
applications, to reduce the support burden for the standard library. In this case, evaluation of the proposed target's value will take this limitation into account.
- The code generation backend for the target should not have deficiencies that
invalidate Rust safety properties, as evaluated by the Rust compiler team.
(This requirement does not apply to arbitrary security enhancements or
mitigations provided by code generation backends, only to those properties
needed to ensure safe Rust code cannot cause undefined behavior or other
unsoundness.) If this requirement does not hold, the target must clearly and
prominently document any such limitations as part of the target's entry in
the target tier list, and ideally also via a failing test in the testsuite.
The Rust compiler team must be satisfied with the balance between these
limitations and the difficulty of implementing the necessary features.
- For example, if Rust relies on a specific code generation feature to ensure that safe code cannot overflow the stack, the code generation for the target should support that feature.
- If the Rust compiler introduces new safety properties (such as via new capabilities of a compiler backend), the Rust compiler team will determine if they consider those new safety properties a best-effort improvement for specific targets, or a required property for all Rust targets. In the latter case, the compiler team may require the maintainers of existing targets to either implement and confirm support for the property or update the target tier list with documentation of the missing property.
- If the target supports C code, and the target has an interoperable calling
convention for C code, the Rust target must support that C calling convention
for the platform via
extern "C"
. The C calling convention does not need to be the default Rust calling convention for the target, however. - The target must build reliably in CI, for all components that Rust's CI considers mandatory.
- The approving teams may additionally require that a subset of tests pass in
CI, such as enough to build a functional "hello world" program,
./x.py test --no-run
, or equivalent "smoke tests". In particular, this requirement may apply if the target builds host tools, or if the tests in question provide substantial value via early detection of critical problems. - Building the target in CI must not take substantially longer than the current slowest target in CI, and should not substantially raise the maintenance burden of the CI infrastructure. This requirement is subjective, to be evaluated by the infrastructure team, and will take the community importance of the target into account.
- Tier 2 targets should, if at all possible, support cross-compiling. Tier 2 targets should not require using the target as the host for builds, even if the target supports host tools.
- In addition to the legal requirements for all targets (specified in the tier
3 requirements), because a tier 2 target typically involves the Rust project
building and supplying various compiled binaries, incorporating the target
and redistributing any resulting compiled binaries (e.g. built libraries,
host tools if any) must not impose any onerous license requirements on any
members of the Rust project, including infrastructure team members and those
operating CI systems. This is a subjective requirement, to be evaluated by
the approving teams.
- As an exception to this, if the target's primary purpose is to build components for a Free and Open Source Software (FOSS) project licensed under "copyleft" terms (terms which require licensing other code under compatible FOSS terms), such as kernel modules or plugins, then the standard libraries for the target may potentially be subject to copyleft terms, as long as such terms are satisfied by Rust's existing practices of providing full corresponding source code. Note that anything added to the Rust repository itself must still use Rust's standard license terms.
- Tier 2 targets must not impose burden on the authors of pull requests, or
other developers in the community, to ensure that tests pass for the target.
In particular, do not post comments (automated or manual) on a PR that derail
or suggest a block on the PR based on tests failing for the target. Do not
send automated messages or notifications (via any medium, including via
@
) to a PR author or others involved with a PR regarding the PR breaking tests on a tier 2 target, unless they have opted into such messages.- Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.
- The target maintainers should regularly run the testsuite for the target, and should fix any test failures in a reasonably timely fashion.
- All requirements for tier 3 apply.
A tier 2 target may be demoted or removed if it no longer meets these requirements. Any proposal for demotion or removal will be CCed to the target maintainers, and will be communicated widely to the Rust community before being dropped from a stable release. (The amount of time between such communication and the next stable release may depend on the nature and severity of the failed requirement, the timing of its discovery, whether the target has been part of a stable release yet, and whether the demotion or removal can be a planned and scheduled action.)
In some circumstances, especially if the target maintainers do not respond in a
timely fashion, Rust teams may land pull requests that temporarily disable some
targets in the nightly compiler, in order to implement a feature not yet
supported by those targets. (As an example, this happened when introducing the
128-bit types u128
and i128
.) Such a pull request will include notification
and coordination with the maintainers of such targets, and will ideally happen
towards the beginning of a new development cycle to give maintainers time to
update their targets. The maintainers of such targets will then be expected to
implement the corresponding target-specific support in order to re-enable the
target. If the maintainers of such targets cannot provide such support in time
for the next stable release, this may result in demoting or removing the
targets.
Tier 2 with host tools
Some tier 2 targets may additionally have binaries built to run on them as a
host (such as rustc
and cargo
). This allows the target to be used as a
development platform, not just a compilation target.
A proposed new tier 2 target with host tools must be reviewed and approved by the compiler team based on these requirements. Such review and approval may occur via a Major Change Proposal (MCP).
In addition, the infrastructure team must approve the integration of the target's host tools into Continuous Integration (CI), and the CI-related requirements for host tools. This review and approval may take place in a PR adding the target's host tools to CI, or simply by an infrastructure team member reporting the outcome of a team discussion.
- Depending on the target, its capabilities, its performance, and the
likelihood of use for any given tool, the host tools provided for a tier 2
target may include only
rustc
andcargo
, or may include additional tools such asclippy
andrustfmt
. - Approval of host tools will take into account the additional time required to build the host tools, and the substantial additional storage required for the host tools.
- The host tools must have direct value to people other than the target's
maintainers. (It may still be a niche target, but the host tools must not be
exclusively useful for an inherently closed group.) This requirement will be
evaluated independently from the corresponding tier 2 requirement.
- The requirement to provide "direct value" means that it does not suffice to argue that having host tools will help the target's maintainers more easily provide the target to others. The tools themselves must provide value to others.
- There must be a reasonable expectation that the host tools will be used, for purposes other than to prove that they can be used.
- The host tools must build and run reliably in CI (for all components that Rust's CI considers mandatory), though they may or may not pass tests.
- Building host tools for the target must not take substantially longer than building host tools for other targets, and should not substantially raise the maintenance burden of the CI infrastructure.
- The host tools must provide a substantively similar experience as on other
targets, subject to reasonable target limitations.
- Adding a substantively different interface to an existing tool, or a
target-specific interface to the functionality of an existing tool,
requires design and implementation approval (e.g. RFC/MCP) from the
appropriate approving teams for that tool.
- Such an interface should have a design that could potentially work for other targets with similar properties.
- This should happen separately from the review and approval of the target, to simplify the target review and approval processes, and to simplify the review and approval processes for the proposed new interface.
- By way of example, a target that runs within a sandbox may need to modify the handling of files, tool invocation, and similar to meet the expectations and conventions of the sandbox, but must not introduce a separate "sandboxed compilation" interface separate from the CLI interface without going through the normal approval process for such an interface. Such an interface should take into account potential other targets with similar sandboxes.
- Adding a substantively different interface to an existing tool, or a
target-specific interface to the functionality of an existing tool,
requires design and implementation approval (e.g. RFC/MCP) from the
appropriate approving teams for that tool.
- If the host tools for the platform would normally be expected to be signed or
equivalent (e.g. if running unsigned binaries or similar involves a
"developer mode" or an additional prompt), it must be possible for the Rust
project's automated builds to apply the appropriate signature process,
without any manual intervention by either Rust developers, target
maintainers, or a third party. This process must meet the approval of the
infrastructure team.
- This process may require one-time or semi-regular manual steps by the infrastructure team, such as registration or renewal of a signing key. Any such manual process must meet the approval of the infrastructure team.
- This process may require the execution of a legal agreement with the signature provider. Such a legal agreement may be revocable, and may potentially require a nominal fee, but must not be otherwise onerous. Any such legal agreement must meet the approval of the infrastructure team. (The infrastructure team is not expected or required to sign binding legal agreements on behalf of the Rust project; this review and approval exists to ensure no terms are onerous or cause problems for infrastructure, especially if such terms may impose requirements or obligations on people who have access to target-specific infrastructure.)
- Changes to this process, or to any legal agreements involved, may cause a target to stop meeting this requirement.
- This process involved must be available under substantially similar non-onerous terms to the general public. Making it available exclusively to the Rust project does not suffice.
- This requirement exists to ensure that Rust builds, including nightly builds, can meet the necessary requirements to allow users to smoothly run the host tools.
- Providing host tools does not exempt a target from requirements to support cross-compilation if at all possible.
- All requirements for tier 2 apply.
A target may be promoted directly from tier 3 to tier 2 with host tools if it meets all the necessary requirements, but doing so may introduce substantial additional complexity. If in doubt, the target should qualify for tier 2 without host tools first.
Tier 1 target policy
At this tier, the Rust project guarantees that a target builds and passes all tests, and will reject patches that fail to build or pass the testsuite on a target. We hold tier 1 targets to our highest standard of requirements.
A proposed new tier 1 target must be reviewed and approved by the compiler team based on these requirements. In addition, the release team must approve the viability and value of supporting the target. For a tier 1 target, this will typically take place via a full RFC proposing the target, to be jointly reviewed and approved by the compiler team and release team.
In addition, the infrastructure team must approve the integration of the target into Continuous Integration (CI), and the tier 1 CI-related requirements. This review and approval may take place in a PR adding the target to CI, by an infrastructure team member reporting the outcome of a team discussion, or by including the infrastructure team in the RFC proposing the target.
- Tier 1 targets must have substantial, widespread interest within the developer community, and must serve the ongoing needs of multiple production users of Rust across multiple organizations or projects. These requirements are subjective, and determined by consensus of the approving teams. A tier 1 target may be demoted or removed if it becomes obsolete or no longer meets this requirement.
- The target maintainer team must include at least 3 developers.
- The target must build and pass tests reliably in CI, for all components that
Rust's CI considers mandatory.
- The target must not disable an excessive number of tests or pieces of tests in the testsuite in order to do so. This is a subjective requirement.
- If the target does not have host tools support, or if the target has low performance, the infrastructure team may choose to have CI cross-compile the testsuite from another platform, and then run the compiled tests either natively or via accurate emulation. However, the approving teams may take such performance considerations into account when determining the viability of the target or of its host tools.
- The target must provide as much of the Rust standard library as is feasible
and appropriate to provide. For instance, if the target can support dynamic
memory allocation, it must provide an implementation of
alloc
and the associated data structures. - Building the target and running the testsuite for the target must not take
substantially longer than other targets, and should not substantially raise
the maintenance burden of the CI infrastructure.
- In particular, if building the target takes a reasonable amount of time, but the target cannot run the testsuite in a timely fashion due to low performance of either native code or accurate emulation, that alone may prevent the target from qualifying as tier 1.
- If running the testsuite requires additional infrastructure (such as physical
systems running the target), the target maintainers must arrange to provide
such resources to the Rust project, to the satisfaction and approval of the
Rust infrastructure team.
- Such resources may be provided via cloud systems, via emulation, or via physical hardware.
- If the target requires the use of emulation to meet any of the tier requirements, the approving teams for those requirements must have high confidence in the accuracy of the emulation, such that discrepancies between emulation and native operation that affect test results will constitute a high-priority bug in either the emulation or the implementation of the target.
- If it is not possible to run the target via emulation, these resources must additionally be sufficient for the Rust infrastructure team to make them available for access by Rust team members, for the purposes of development and testing. (Note that the responsibility for doing target-specific development to keep the target well maintained remains with the target maintainers. This requirement ensures that it is possible for other Rust developers to test the target, but does not obligate other Rust developers to make target-specific fixes.)
- Resources provided for CI and similar infrastructure must be available for continuous exclusive use by the Rust project. Resources provided for access by Rust team members for development and testing must be available on an exclusive basis when in use, but need not be available on a continuous basis when not in use.
- Tier 1 targets must not have a hard requirement for signed, verified, or
otherwise "approved" binaries. Developers must be able to build, run, and
test binaries for the target on systems they control, or provide such
binaries for others to run. (Doing so may require enabling some appropriate
"developer mode" on such systems, but must not require the payment of any
additional fee or other consideration, or agreement to any onerous legal
agreements.)
- The Rust project may decide to supply appropriately signed binaries if doing so provides a smoother experience for developers using the target, and a tier 2 target with host tools already requires providing appropriate mechanisms that enable our infrastructure to provide such signed binaries. However, this additional tier 1 requirement ensures that Rust developers can develop and test Rust software for the target (including Rust itself), and that development or testing for the target is not limited.
- All requirements for tier 2 apply.
A tier 1 target may be demoted if it no longer meets these requirements but still meets the requirements for a lower tier. Any proposal for demotion of a tier 1 target requires a full RFC process, with approval by the compiler and release teams. Any such proposal will be communicated widely to the Rust community, both when initially proposed and before being dropped from a stable release. A tier 1 target is highly unlikely to be directly removed without first being demoted to tier 2 or tier 3. (The amount of time between such communication and the next stable release may depend on the nature and severity of the failed requirement, the timing of its discovery, whether the target has been part of a stable release yet, and whether the demotion or removal can be a planned and scheduled action.)
Raising the baseline expectations of a tier 1 target (such as the minimum CPU features or OS version required) requires the approval of the compiler and release teams, and should be widely communicated as well, but does not necessarily require a full RFC.
Tier 1 with host tools
Some tier 1 targets may additionally have binaries built to run on them as a
host (such as rustc
and cargo
). This allows the target to be used as a
development platform, not just a compilation target.
A proposed new tier 1 target with host tools must be reviewed and approved by the compiler team based on these requirements. In addition, the release team must approve the viability and value of supporting host tools for the target. For a tier 1 target, this will typically take place via a full RFC proposing the target, to be jointly reviewed and approved by the compiler team and release team.
In addition, the infrastructure team must approve the integration of the target's host tools into Continuous Integration (CI), and the CI-related requirements for host tools. This review and approval may take place in a PR adding the target's host tools to CI, by an infrastructure team member reporting the outcome of a team discussion, or by including the infrastructure team in the RFC proposing the target.
- Tier 1 targets with host tools should typically include all of the additional
tools such as
clippy
andrustfmt
, unless there is a target-specific reason why a tool cannot possibly make sense for the target.- Unlike with tier 2, for tier 1 we will not exclude specific tools on the sole basis of them being less likely to be used; rather, we'll take that into account when considering whether the target should be at tier 1 with host tools. In general, on any tier 1 target with host tools, people should be able to expect to find and install all the same components that they would for any other tier 1 target with host tools.
- Approval of host tools will take into account the additional time required to build the host tools, and the substantial additional storage required for the host tools.
- Host tools for the target must have substantial, widespread interest within the developer community, and must serve the ongoing needs of multiple production users of Rust across multiple organizations or projects. These requirements are subjective, and determined by consensus of the approving teams. This requirement will be evaluated independently from the corresponding tier 1 requirement; it is possible for a target to have sufficient interest for cross-compilation, but not have sufficient interest for native compilation. The host tools may be dropped if they no longer meet this requirement, even if the target otherwise qualifies as tier 1.
- The host tools must build, run, and pass tests reliably in CI, for all
components that Rust's CI considers mandatory.
- The target must not disable an excessive number of tests or pieces of tests in the testsuite in order to do so. This is a subjective requirement.
- Building the host tools and running the testsuite for the host tools must not
take substantially longer than other targets, and should not substantially raise
the maintenance burden of the CI infrastructure.
- In particular, if building the target's host tools takes a reasonable amount of time, but the target cannot run the testsuite in a timely fashion due to low performance of either native code or accurate emulation, that alone may prevent the target from qualifying as tier 1 with host tools.
- Providing host tools does not exempt a target from requirements to support cross-compilation if at all possible.
- All requirements for tier 2 targets with host tools apply.
- All requirements for tier 1 apply.
A target seeking promotion to tier 1 with host tools should typically either be tier 2 with host tools or tier 1 without host tools, to reduce the number of requirements to simultaneously review and approve.
In addition to the general process for demoting a tier 1 target, a tier 1 target with host tools may be demoted (including having its host tools dropped, or being demoted to tier 2 with host tools) if it no longer meets these requirements but still meets the requirements for a lower tier. Any proposal for demotion of a tier 1 target (with or without host tools) requires a full RFC process, with approval by the compiler and release teams. Any such proposal will be communicated widely to the Rust community, both when initially proposed and before being dropped from a stable release.
target-name-here
Tier: 3
One-sentence description of the target (e.g. CPU, OS)
Target maintainers
- Some Person, https://github.com/...
Requirements
Does the target support host tools, or only cross-compilation? Does the target support std, or alloc (either with a default allocator, or if the user supplies an allocator)?
Document the expectations of binaries built for the target. Do they assume specific minimum features beyond the baseline of the CPU/environment/etc? What version of the OS or environment do they expect?
Are there notable #[target_feature(...)]
or -C target-feature=
values that
programs may wish to use?
What calling convention does extern "C"
use on the target?
What format do binaries use by default? ELF, PE, something else?
Building the target
If Rust doesn't build the target by default, how can users build it? Can users
just add it to the target
list in config.toml
?
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
Testing
Does the target support running binaries, or do binaries have varying expectations that prevent having a standard way to run them? If users can run binaries, can they do so in some common emulator, or do they need native hardware? Does the target support running the Rust testsuite?
Cross-compilation toolchains and C code
Does the target support C code? If so, what toolchain target should users use to build compatible C code? (This may match the target triple, or it may be a toolchain for a different target triple, potentially with specific options or caveats.)
arm64ec-pc-windows-msvc
Tier: 2
Arm64EC ("Emulation Compatible") for mixed architecture (AArch64 and x86_64) applications on AArch64 Windows 11. See https://learn.microsoft.com/en-us/windows/arm/arm64ec.
Target maintainers
Requirements
Builds Arm64EC static and dynamic libraries and executables which can be run on AArch64 Windows 11 devices. Arm64EC static libraries can also be linked into Arm64X dynamic libraries and executables.
Only supported backend is LLVM 18 or above:
- 18.1.0 added initial support for Arm64EC.
- 18.1.2 fixed import library generation (required for
raw-dylib
support). - 18.1.4 fixed linking issue for some intrinsics implemented in
compiler_builtins
.
Visual Studio 2022 (or above) with the "ARM64/ARM64EC built tools" component and the Windows 11 SDK are required.
Reusing code from other architectures - x86_64 or AArch64?
Arm64EC uses arm64ec
as its target_arch
, but it is possible to reuse
existing architecture-specific code in most cases. The best mental model for
deciding which architecture to reuse is to is to think of Arm64EC as an x86_64
process that happens to use the AArch64 instruction set (with some caveats) and
has a completely custom ABI.
To put this in practice:
- Arm64EC interacts with the operating system, other processes and other DLLs as
x86_64.
- For example, in
backtrace
we use the x86_64 versions ofCONTEXT
andRtlVirtualUnwind
. - If you are configuring a search path to find DLLs (e.g., to load plugins or addons into your application), you should use the same path as the x86_64 version of your application, not the AArch64 path (since Arm64EC (i.e., x86_64) processes cannot load native AArch64 DLLs).
- For example, in
- Arm64EC uses AArch64 intrinsics.
- Assembly for AArch64 might be reusable for Arm64EC, but there are many
caveats. For full details see Microsoft's documentation on the Arm64EC ABI
but in brief:
- Arm64EC uses a subset of AArch64 registers.
- Arm64EC uses a different name mangling scheme than AArch64.
- Arm64EC requires entry and exit thunks be generated for some functions.
- Indirect calls must be done via a call checker.
- Control Flow Guard and stack checks use different functions than AArch64.
Building the target
You can build Rust with support for the targets by adding it to the target
list in config.toml
:
[build]
target = [ "arm64ec-pc-windows-msvc" ]
Building Rust programs
These targets are distributed through rustup
, and otherwise require no
special configuration.
Testing
Tests can be run on AArch64 Windows 11 devices.
Cross-compilation toolchains and C code
C code can be built using the Arm64-targeting MSVC or Clang toolchain.
To compile:
cl /arm64EC /c ...
To link:
link /MACHINE:ARM64EC ...
Further reading: https://learn.microsoft.com/en-us/windows/arm/arm64ec-build
*-apple-darwin
Apple macOS targets.
Tier: 1
x86_64-apple-darwin
: macOS on 64-bit x86.aarch64-apple-darwin
: macOS on ARM64 (M1-family or later Apple Silicon CPUs).
Target maintainers
Requirements
OS version
The minimum supported version is macOS 10.12 Sierra on x86, and macOS 11.0 Big Sur on ARM64.
This version can be raised per-binary by changing the deployment target,
which might yield more performance optimizations. rustc
respects the common
environment variables used by Xcode to do so, in this case
MACOSX_DEPLOYMENT_TARGET
.
The current default deployment target for rustc
can be retrieved with
rustc --print=deployment-target
.
Binary format
The default binary format is Mach-O, the executable format used on Apple's platforms.
Building
These targets are distributed through rustup
, and otherwise require no
special configuration.
Testing
There are no special requirements for testing and running this target.
x86 binaries can be run on Apple Silicon by using Rosetta.
Cross-compilation toolchains and C code
Cross-compilation of these targets are supported using Clang, but may require
Xcode or the macOS SDK (MacOSX.sdk
) to be available to compile C code and
to link.
The path to the SDK can be passed to rustc
using the common SDKROOT
environment variable.
i686-apple-darwin
Apple macOS on 32-bit x86.
Target maintainers
Requirements
See the docs on *-apple-darwin
for general macOS requirements.
Building the target
You'll need the macOS 10.13 SDK shipped with Xcode 9. The location of the SDK
can be passed to rustc
using the common SDKROOT
environment variable.
Once you have that, you can build Rust with support for the target by adding
it to the target
list in config.toml
:
[build]
target = ["i686-apple-darwin"]
Using the unstable -Zbuild-std
with a nightly Cargo may also work.
Building Rust programs
Rust no longer ships pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy using build-std
or
similar.
Testing
Running this target requires an Intel Macbook running macOS 10.14 or earlier, as later versions removed support for running 32-bit binaries.
x86_64h-apple-darwin
Tier: 3
Target for macOS on late-generation x86_64
Apple chips, usable as the
x86_64h
entry in universal binaries, and equivalent to LLVM's
x86_64h-apple-macosx*
targets.
Target maintainers
- Thom Chiovoloni
thom@shift.click
https://github.com/thomcc
Requirements
This target is an x86_64
target that only supports Apple's late-gen
(Haswell-compatible) Intel chips. It enables a set of target features available
on these chips (AVX2 and similar), and MachO binaries built with this target may
be used as the x86_64h
entry in universal binaries ("fat" MachO binaries), and
will fail to load on machines that do not support this.
It should support the full standard library (std
and alloc
either with
default or user-defined allocators). This target is probably most useful when
targeted via cross-compilation (including from x86_64-apple-darwin
), but if
built manually, the host tools work.
It is similar to x86_64-apple-darwin
in nearly all
respects.
Building the target
Users on Apple targets can build this by adding it to the target
list in
config.toml
, or with -Zbuild-std
.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
Testing
Code built with this target can be run on the set of Intel macOS machines that
support running x86_64h
binaries (relatively recent Intel macs). The Rust test
suite seems to work.
Cross-compilation toolchains and C code
Cross-compilation to this target from Apple hosts should generally work without
much configuration, so long as XCode and the CommandLineTools are installed.
Targeting it from non-Apple hosts is difficult, but no more so than targeting
x86_64-apple-darwin
.
When compiling C code for this target, either the "x86_64h-apple-macosx*
" LLVM
targets should be used, or an argument like -arch x86_64h
should be passed to
the C compiler.
arm64e-apple-darwin
Tier: 3 (with Host Tools)
ARM64e macOS (11.0+, Big Sur+)
Target maintainers
- Artyom Tetyukhin (@arttet)
Requirements
Target for macOS
on late-generation M
series Apple chips.
See the docs on *-apple-darwin
for general macOS requirements.
Building the target
You can build Rust with support for the targets by adding it to the target
list in config.toml
:
[build]
target = [ "arm64e-apple-darwin" ]
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for this target, you will need to build Rust with the target enabled (see Building the target above).
Testing
The target does support running binaries on macOS platforms with arm64e
architecture.
Cross-compilation toolchains and C code
The targets do support C
code.
To build compatible C
code, you have to use XCode with the same compiler and flags.
*-apple-ios
Apple iOS / iPadOS targets.
Tier: 2 (without Host Tools)
aarch64-apple-ios
: Apple iOS on ARM64.aarch64-apple-ios-sim
: Apple iOS Simulator on ARM64.x86_64-apple-ios
: Apple iOS Simulator on 64-bit x86.
Tier: 3
armv7s-apple-ios
: Apple iOS on Armv7-A.i386-apple-ios
: Apple iOS Simulator on 32-bit x86.
Target maintainers
Requirements
These targets are cross-compiled, and require the corresponding iOS SDK
(iPhoneOS.sdk
or iPhoneSimulator.sdk
), as provided by Xcode. To build the
ARM64 targets, Xcode 12 or higher is required.
The path to the SDK can be passed to rustc
using the common SDKROOT
environment variable.
OS version
The minimum supported version is iOS 10.0.
This can be raised per-binary by changing the deployment target. rustc
respects the common environment variables used by Xcode to do so, in this
case IPHONEOS_DEPLOYMENT_TARGET
.
Building the target
The tier 2 targets are distributed through rustup
, and can be installed
using one of:
$ rustup target add aarch64-apple-ios
$ rustup target add aarch64-apple-ios-sim
$ rustup target add x86_64-apple-ios
The tier 3 targets can be built by enabling them for a rustc
build in
config.toml
, by adding, for example:
[build]
target = ["armv7s-apple-ios", "i386-apple-ios"]
Using the unstable -Zbuild-std
with a nightly Cargo may also work.
Building Rust programs
Rust programs can be built for these targets by specifying --target
, if
rustc
has been built with support for them. For example:
$ rustc --target aarch64-apple-ios your-code.rs
Testing
There is no support for running the Rust or standard library testsuite at the moment. Testing has mostly been done manually with builds of static libraries embedded into applications called from Xcode or a simulator.
It hopefully will be possible to improve this in the future.
*-apple-ios-macabi
Apple Mac Catalyst targets.
Tier: 2 (without Host Tools)
aarch64-apple-ios-macabi
: Mac Catalyst on ARM64.x86_64-apple-ios-macabi
: Mac Catalyst on 64-bit x86.
Target maintainers
Requirements
These targets are cross-compiled, and require the corresponding macOS SDK
(MacOSX.sdk
) which contain ./System/iOSSupport
headers to allow linking to
iOS-specific headers, as provided by Xcode 11 or higher.
The path to the SDK can be passed to rustc
using the common SDKROOT
environment variable.
OS version
The minimum supported version is iOS 13.1.
This can be raised per-binary by changing the deployment target. rustc
respects the common environment variables used by Xcode to do so, in this
case IPHONEOS_DEPLOYMENT_TARGET
.
Building the target
The targets are distributed through rustup
, and can be installed using one
of:
$ rustup target add aarch64-apple-ios-macabi
$ rustup target add x86_64-apple-ios-macabi
Sanitizers
Due to CMake having poor support for Mac Catalyst, sanitizer runtimes are not currently available, see #129069.
Building Rust programs
Rust programs can be built for these targets by specifying --target
, if
rustc
has been built with support for them. For example:
$ rustc --target aarch64-apple-ios-macabi your-code.rs
Testing
Mac Catalyst binaries can be run directly on macOS 10.15 Catalina or newer.
x86 binaries can be run on Apple Silicon by using Rosetta.
Note that using certain UIKit functionality requires the binary to be bundled.
arm64e-apple-ios
Tier: 3
ARM64e iOS (12.0+)
Target maintainers
- Artyom Tetyukhin (@arttet)
Requirements
See the docs on *-apple-ios
for general iOS requirements.
Building the target
You can build Rust with support for the targets by adding it to the target
list in config.toml
:
[build]
target = [ "arm64e-apple-ios" ]
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for this target, you will need to build Rust with the target enabled (see Building the target above).
Testing
The target does support running binaries on iOS platforms with arm64e
architecture.
Cross-compilation toolchains and C code
The targets do support C
code.
To build compatible C
code, you have to use XCode with the same compiler and flags.
*-apple-tvos
Apple tvOS targets.
Tier: 3
aarch64-apple-tvos
: Apple tvOS on ARM64.aarch64-apple-tvos-sim
: Apple tvOS Simulator on ARM64.x86_64-apple-tvos
: Apple tvOS Simulator on x86_64.
Target maintainers
Requirements
These targets are cross-compiled, and require the corresponding tvOS SDK
(AppleTVOS.sdk
or AppleTVSimulator.sdk
), as provided by Xcode. To build the
ARM64 targets, Xcode 12 or higher is required.
The path to the SDK can be passed to rustc
using the common SDKROOT
environment variable.
OS version
The minimum supported version is tvOS 10.0, although the actual minimum version you can target may be newer than this, for example due to the versions of Xcode and your SDKs.
The version can be raised per-binary by changing the deployment target. rustc
respects the common environment variables used by Xcode to do so, in this
case TVOS_DEPLOYMENT_TARGET
.
Incompletely supported library functionality
The targets support most of the standard library including the allocator to the best of my knowledge, however they are very new, not yet well-tested, and it is possible that there are various bugs.
The following APIs are currently known to have missing or incomplete support:
std::process::Command
's API will return an error if it is configured in a manner which cannot be performed usingposix_spawn
-- this is because the more flexiblefork
/exec
-based approach is prohibited on these platforms in favor ofposix_spawn{,p}
(which still probably will get you rejected from app stores, so is likely sideloading-only). A concrete set of cases where this will occur is difficult to enumerate (and would quickly become stale), but in some cases it may be worked around by tweaking the manner in whichCommand
is invoked.
Building the target
The targets can be built by enabling them for a rustc
build in
config.toml
, by adding, for example:
[build]
build-stage = 1
target = ["aarch64-apple-tvos", "aarch64-apple-tvos-sim"]
Using the unstable -Zbuild-std
with a nightly Cargo may also work.
Building Rust programs
Rust programs can be built for these targets by specifying --target
, if
rustc
has been built with support for them. For example:
$ rustc --target aarch64-apple-tvos your-code.rs
Testing
There is no support for running the Rust or standard library testsuite at the moment. Testing has mostly been done manually with builds of static libraries embedded into applications called from Xcode or a simulator.
It hopefully will be possible to improve this in the future.
*-apple-watchos
Apple watchOS targets.
Tier: 3
aarch64-apple-watchos
: Apple WatchOS on ARM64.aarch64-apple-watchos-sim
: Apple WatchOS Simulator on ARM64.x86_64-apple-watchos-sim
: Apple WatchOS Simulator on 64-bit x86.arm64_32-apple-watchos
: Apple WatchOS on Arm 64_32.armv7k-apple-watchos
: Apple WatchOS on Armv7k.
Target maintainers
Requirements
These targets are cross-compiled, and require the corresponding watchOS SDK
(WatchOS.sdk
or WatchSimulator.sdk
), as provided by Xcode. To build the
ARM64 targets, Xcode 12 or higher is required.
The path to the SDK can be passed to rustc
using the common SDKROOT
environment variable.
OS version
The minimum supported version is watchOS 5.0.
This can be raised per-binary by changing the deployment target. rustc
respects the common environment variables used by Xcode to do so, in this
case WATCHOS_DEPLOYMENT_TARGET
.
Building the target
The targets can be built by enabling them for a rustc
build in
config.toml
, by adding, for example:
[build]
build-stage = 1
target = ["aarch64-apple-watchos", "aarch64-apple-watchos-sim"]
Using the unstable -Zbuild-std
with a nightly Cargo may also work.
Building Rust programs
Rust programs can be built for these targets by specifying --target
, if
rustc
has been built with support for them. For example:
$ rustc --target aarch64-apple-watchos-sim your-code.rs
Testing
There is no support for running the Rust or standard library testsuite at the moment. Testing has mostly been done manually with builds of static libraries embedded into applications called from Xcode or a simulator.
It hopefully will be possible to improve this in the future.
*-apple-visionos
Apple visionOS / xrOS targets.
Tier: 3
aarch64-apple-visionos
: Apple visionOS on arm64.aarch64-apple-visionos-sim
: Apple visionOS Simulator on arm64.
Target maintainers
Requirements
These targets are cross-compiled, and require the corresponding visionOS SDK
(XROS.sdk
or XRSimulator.sdk
), as provided by Xcode 15 or newer.
The path to the SDK can be passed to rustc
using the common SDKROOT
environment variable.
OS version
The minimum supported version is visionOS 1.0.
This can be raised per-binary by changing the deployment target. rustc
respects the common environment variables used by Xcode to do so, in this
case XROS_DEPLOYMENT_TARGET
.
Building the target
The targets can be built by enabling them for a rustc
build in
config.toml
, by adding, for example:
[build]
target = ["aarch64-apple-visionos", "aarch64-apple-visionos-sim"]
Using the unstable -Zbuild-std
with a nightly Cargo may also work.
Note: Currently, a newer version of libc
and cc
may be required, this will
be fixed in #124560.
Building Rust programs
Rust programs can be built for these targets by specifying --target
, if
rustc
has been built with support for them. For example:
$ rustc --target aarch64-apple-visionos-sim your-code.rs
Testing
There is no support for running the Rust or standard library testsuite at the moment. Testing has mostly been done manually with builds of static libraries embedded into applications called from Xcode or a simulator.
It hopefully will be possible to improve this in the future.
Cross-compilation toolchains and C code
The Clang target is suffixed with -xros
for historical reasons.
LLVM 18 or newer is required to build this target.
aarch64-nintendo-switch-freestanding
Tier: 3
Nintendo Switch with pure-Rust toolchain.
Designated Developers
Requirements
This target is cross-compiled. It has no special requirements for the host.
Building
The target can be built by enabling it for a rustc
build:
[build]
build-stage = 1
target = ["aarch64-nintendo-switch-freestanding"]
Cross-compilation
This target can be cross-compiled from any host.
Testing
Currently there is no support to run the rustc test suite for this target.
Building Rust programs
If rustc
has support for that target and the library artifacts are available,
then Rust programs can be built for that target:
rustc --target aarch64-nintendo-switch-freestanding your-code.rs
To generate binaries in the NRO format that can be easily run on-device, you can use cargo-nx:
cargo nx --triple=aarch64-nintendo-switch-freestanding
armeb-unknown-linux-gnueabi
Tier: 3
Target for cross-compiling Linux user-mode applications targeting the Arm BE8 architecture.
Overview
BE8 architecture retains the same little-endian ordered code-stream used by conventional little endian Arm systems, however the data accesses are in big-endian. BE8 is used primarily in high-performance networking applications where the ability to read packets in their native "Network Byte Order" is important (many network protocols transmit data in big-endian byte order for their wire formats).
History
BE8 architecture is the default big-endian architecture for Arm since Armv6. It's predecessor, used for Armv4 and Armv5 devices was BE32. On Armv6 architecture, endianness can be configured via system registers. However, BE32 was withdrawn for Armv7 onwards.
Target Maintainers
Requirements
The target is cross-compiled. This target supports std
in the normal way (indeed only nominal changes are required from the standard Arm configuration).
Target definition
The target definition can be seen here. In particular, it should be noted that the features
specify that this target is built for the Armv8 core. Though this can likely be modified as required.
Building the target
Because it is Tier 3, rust does not yet ship pre-compiled artifacts for this target.
Therefore, you can build Rust with support for the target by adding it to the target list in config.toml, a sample configuration is shown below. It is expected that the user already have a working GNU compiler toolchain and update the paths accordingly.
[llvm]
download-ci-llvm = false
optimize = true
ninja = true
targets = "ARM;X86"
clang = false
[build]
target = ["x86_64-unknown-linux-gnu", "armeb-unknown-linux-gnueabi"]
docs = false
docs-minification = false
compiler-docs = false
[install]
prefix = "/home/user/x-tools/rust/"
[rust]
debug-logging=true
backtrace = true
incremental = true
[target.x86_64-unknown-linux-gnu]
[dist]
[target.armeb-unknown-linux-gnueabi]
cc = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-gcc"
cxx = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-g++"
ar = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-ar"
ranlib = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-ranlib"
linker = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-gcc"
llvm-config = "/home/user/x-tools/clang/bin/llvm-config"
llvm-filecheck = "/home/user/x-tools/clang/bin/FileCheck"
Building Rust programs
The following .cargo/config
is needed inside any project directory to build for the BE8 target:
[build]
target = "armeb-unknown-linux-gnueabi"
[target.armeb-unknown-linux-gnueabi]
linker = "armeb-unknown-linux-gnueabi-gcc"
Note that it is expected that the user has a suitable linker from the GNU toolchain.
{arm,thumb}*-none-eabi(hf)?
Common Target Details
This documentation covers details that apply to a range of bare-metal targets
for 32-bit Arm CPUs. The arm-none-eabi
flavor of the GNU compiler toolchain is
often used to assist compilation to these targets.
Details that apply only to only a specific target in this group are covered in their own document.
Tier 2 Target List
- Arm A-Profile Architectures
armv7a-none-eabi
- Arm R-Profile Architectures
- Arm M-Profile Architectures
- Legacy Arm Architectures
- None
Tier 3 Target List
- Arm A-Profile Architectures
armv7a-none-eabihf
- Arm R-Profile Architectures
- Arm M-Profile Architectures
- None
- Legacy Arm Architectures
Instruction Sets
There are two 32-bit instruction set architectures (ISAs) defined by Arm:
- The A32 ISA, with fixed-width 32-bit instructions. Previously known as the Arm ISA, this originated with the original Arm1 of 1985 and has been updated by various revisions to the architecture specifications ever since.
- The T32 ISA, with a mix of 16-bit and 32-bit width instructions. Note that this term includes both the original 16-bit width Thumb ISA introduced with the Armv4T architecture in 1994, and the later 16/32-bit sized Thumb-2 ISA introduced with the Armv6T2 architecture in 2003.
Again, these ISAs have been revised by subsequent revisions to the relevant Arm architecture specifications.
There is also a 64-bit ISA with fixed-width 32-bit instructions called the A64
ISA, but targets which implement that instruction set generally start with
aarch64*
and are discussed elsewhere.
Rust targets starting with arm*
generate Arm (A32) code by default, whilst
targets named thumb*
generate Thumb (T32) code by default. Most Arm chips
support both Thumb mode and Arm mode, with the notable exception that M-profile
processors (thumbv*m*-none-eabi*
targets) only support Thumb-mode.
Rust targets ending with eabi
use the so-called soft-float ABI: functions
which take f32
or f64
as arguments will have those values packed into
integer registers. This means that an FPU is not required from an ABI
perspective, but within a function floating-point instructions may still be used
if the code is compiled with a target-cpu
or target-feature
option that
enables FPU support.
Rust targets ending in eabihf
use the so-called hard-float ABI: functions
which take f32
or f64
as arguments will have them passed via FPU registers.
These targets therefore require the availability of an FPU and will assume some
baseline level of floating-point support is available (which can vary depending
on the target). More advanced floating-point instructions may be generated if
the code is compiled with a target-cpu
or target-feature
option that enables
such additional FPU support. For example, if a given hard-float target has
baseline single-precision (f32
) support in hardware, there may be
target-cpu
or target-feature
options that tell LLVM to assume your processor
in fact also has double-precision (f64
) support.
You may of course use the f32
and f64
types in your code, regardless of the
ABI being used, or the level of support your processor has for performing such
operations in hardware. Any floating-point operations that LLVM assumes your
processor cannot support will be lowered to library calls (like __aeabi_dadd
)
which perform the floating-point operation in software using integer
instructions.
Target CPU and Target Feature options
It is possible to tell Rust (or LLVM) that you have a specific model of Arm
processor, using the -C target-cpu
option. You can also control
whether Rust (or LLVM) will include instructions that target optional hardware
features, e.g. hardware floating-point, or Advanced SIMD operations, using -C target-feature
.
It is important to note that selecting a target-cpu will typically enable
all the optional features available from Arm on that model of CPU and your
particular implementation of that CPU may not have those features available. In
that case, you can use -C target-feature=-option
to turn off the specific CPU
features you do not have available, leaving you with the optimized instruction
scheduling and support for the features you do have. More details are available
in the detailed target-specific documentation.
Note: Many target-features are currently unstable and subject to change, and if you use them you should disassemble the compiler output and manually inspect it to ensure only appropriate instructions for your CPU have been generated.
If you wish to use the target-cpu and target-feature options, you can add
them to your .cargo/config.toml
file alongside any other flags your project
uses (likely linker related ones):
rustflags = [
# Usual Arm bare-metal linker setup
"-Clink-arg=-Tlink.x",
"-Clink-arg=--nmagic",
# tell Rust we have a Cortex-M55
"-Ctarget-cpu=cortex-m55",
# tell Rust our Cortex-M55 doesn't have Floating-Point M-Profile Vector
# Extensions (but it does have everything else a Cortex-M55 could have).
"-Ctarget-feature=-mve.fp"
]
[build]
target = "thumbv8m.main-none-eabihf"
Requirements
These targets are cross-compiled and use static linking.
By default, the lld
linker included with Rust will be used; however, you may
want to use the GNU linker instead. This can be obtained for Windows/Mac/Linux
from the Arm Developer Website, or possibly from your OS's
package manager. To use it, add the following to your .cargo/config.toml
:
[target.<your-target>]
linker = "arm-none-eabi-ld"
The GNU linker can also be used by specifying arm-none-eabi-gcc
as the
linker. This is needed when using GCC's link time optimization.
These targets don't provide a linker script, so you'll need to bring your own
according to the specific device you are using. Pass
-Clink-arg=-Tyour_script.ld
as a rustc argument to make the linker use
your_script.ld
during linking.
For the arm*
targets, Thumb-mode code generation can be enabled by using -C target-feature=+thumb-mode
. Using the unstable
#![feature(arm_target_feature)]
, the attribute #[target_feature(enable = "thumb-mode")]
can be applied to individual unsafe
functions to cause those
functions to be compiled to Thumb-mode code.
Building Rust Programs
For the Tier 3 targets in this family, rust does not ship pre-compiled artifacts.
Just use the build-std
nightly cargo feature to build the core
library. You
can pass this as a command line argument to cargo, or your .cargo/config.toml
file might include the following lines:
[unstable]
build-std = ["core"]
Most of core
should work as expected, with the following notes:
- Floating-point operations are emulated in software unless LLVM is told to
enable FPU support (either by using an
eabihf
target, specifying atarget-cpu
with FPU support, or using atarget-feature
to support for a specific kind of FPU) - Integer division is also emulated in software on some targets, depending on
the target,
target-cpu
andtarget-feature
s. - Older Arm architectures (e.g. Armv4, Armv5TE and Armv6-M) are limited to basic
load
andstore
operations, and not more advanced operations likefetch_add
orcompare_exchange
.
alloc
is also supported, as long as you provide your own global allocator.
Rust programs are output as ELF files.
Testing
This is a cross-compiled target that you will need to emulate during testing.
The exact emulator that you'll need depends on the specific device you want to run your code on.
Cross-compilation toolchains and C code
The target supports C code compiled with the arm-none-eabi
target triple and
a suitable -march
or -mcpu
flag.
gcc
or clang
can be used, but note that gcc
uses -fshort-enums
by
default for arm-none*
targets, while clang
does not. rustc
matches the
gcc
behavior, i.e., the size of a #[repr(C)] enum
in Rust can be as little
as 1 byte, rather than 4, as they are on arm-linux
targets.
armv4t-none-eabi / thumbv4t-none-eabi
Tier 3
These two targets are part of the arm-none-eabi
target
group, and all the information there applies.
Both of these targets can be used on the Game Boy Advance (GBA), among other
things. On the GBA, one should usually use the thumb
target to get the best
overall performance.
Target Maintainers
Testing
This is a cross-compiled target that you will need to emulate during testing.
Because this is a device-agnostic target, and the exact emulator that you'll need depends on the specific device you want to run your code on.
- When building for the GBA, mgba-test-runner
can be used to make a normal set of rust tests be run within the
mgba
emulator.
armv5te-none-eabi
Tier: 3
Bare-metal target for any cpu in the Armv5TE architecture family, supporting
ARM/Thumb code interworking (aka A32
/T32
), with A32
code as the default code
generation.
The thumbv5te-none-eabi
target is the same as this one, but the instruction set defaults to T32
.
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets.
Target Maintainers
Testing
This is a cross-compiled target that you will need to emulate during testing.
Because this is a device-agnostic target, and the exact emulator that you'll need depends on the specific device you want to run your code on.
For example, when programming for the DS, you can use one of the several available DS emulators, such as melonDS.
arm(eb)?v7r-none-eabi(hf)?
Tier: 2
Bare-metal target for CPUs in the Armv7-R architecture family, supporting dual ARM/Thumb mode, with ARM mode as the default.
Processors in this family include the Arm Cortex-R4, 5, 7, and 8.
The eb
versions of this target generate code for big-endian processors.
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets.
Target maintainers
- Chris Copeland,
chris@chrisnc.net
Requirements
When using the big-endian version of this target, note that some variants of
the Cortex-R have both big-endian instructions and data. This configuration is
known as BE-32, while data-only big-endianness is known as BE-8. To build
programs for BE-32 processors, the GNU linker must be used with the -mbe32
option. See ARM Cortex-R Series Programmer's Guide: Endianness
for more details about different endian modes.
When using the hardfloat targets, the minimum floating-point features assumed
are those of the vfpv3-d16
, which includes single- and double-precision, with
16 double-precision registers. This floating-point unit appears in Cortex-R4F
and Cortex-R5F processors. See VFP in the Cortex-R processors
for more details on the possible FPU variants.
If your processor supports a different set of floating-point features than the
default expectations of vfpv3-d16
, then these should also be enabled or
disabled as needed with -C target-feature=(+/-)
.
Cross-compilation toolchains and C code
This target supports C code compiled with the arm-none-eabi
target triple and
-march=armv7-r
or a suitable -mcpu
flag.
armv8r-none-eabihf
Tier: 3
Bare-metal target for CPUs in the Armv8-R architecture family, supporting dual ARM/Thumb mode, with ARM mode as the default.
Processors in this family include the Arm Cortex-R52 and Cortex-R52+.
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets.
Target maintainers
- Chris Copeland,
chris@chrisnc.net
Requirements
The Cortex-R52 family always includes a floating-point unit, so there is no
non-hf
version of this target. The floating-point features assumed by this
target are those of the single-precision-only config of the Cortex-R52, which
has 16 double-precision registers, accessible as 32 single-precision registers.
The other variant of Cortex-R52 includes double-precision, 32 double-precision
registers, and Advanced SIMD (Neon).
The manual refers to this as the "Full Advanced SIMD config". To compile code
for this variant, use: -C target-feature=+fp64,+d32,+neon
. See the Advanced
SIMD and floating-point support section of the Cortex-R52 Processor
Technical Reference Manual for more details.
Cross-compilation toolchains and C code
This target supports C code compiled with the arm-none-eabi
target triple and
-march=armv8-r
or a suitable -mcpu
flag.
thumbv6m-none-eabi
Tier: 2
Bare-metal target for CPUs in the Armv6-M architecture family, supporting a subset of the T32 ISA.
Processors in this family include the:
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets.
This target uses the soft-float ABI: functions which take f32
or f64
as
arguments will have those values packed into integer registers. This is the
only option because there is no FPU support in Armv6-M.
Target maintainers
- Rust Embedded Devices Working Group Cortex-M
Team,
cortex-m@teams.rust-embedded.org
Target CPU and Target Feature options
See the bare-metal Arm docs for details on how to use these flags.
Table of supported CPUs
CPU | FPU | Target CPU | Target Features |
---|---|---|---|
Cortex-M0 | No | cortex-m0 | None |
Cortex-M0+ | No | cortex-m0plus | None |
Cortex-M1 | No | cortex-m1 | None |
Arm Cortex-M0
The target CPU option is cortex-m0
.
There are no relevant feature flags, and the FPU is not available.
Arm Cortex-M0+
The target CPU option is cortex-m0plus
.
There are no relevant feature flags, and the FPU is not available.
Arm Cortex-M1
The target CPU option is cortex-m1
.
There are no relevant feature flags, and the FPU is not available.
thumbv7em-none-eabi
and thumbv7em-none-eabihf
Tier: 2
Bare-metal target for CPUs in the Armv7E-M architecture family, supporting a subset of the T32 ISA.
Processors in this family include the:
- Arm Cortex-M4 and Arm Cortex-M4F
- Arm Cortex-M7 and Arm Cortex-M7F
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets, in particular the difference between the eabi
and
eabihf
ABI.
Target maintainers
- Rust Embedded Devices Working Group Cortex-M
Team,
cortex-m@teams.rust-embedded.org
Target CPU and Target Feature options
See the bare-metal Arm docs for details on how to use these flags.
Table of supported CPUs for thumbv7em-none-eabi
CPU | FPU | DSP | Target CPU | Target Features |
---|---|---|---|---|
Any | No | Yes | None | None |
Cortex-M4 | No | Yes | cortex-m4 | +soft-float |
Cortex-M4F | SP | Yes | cortex-m4 | None |
Cortex-M7 | No | Yes | cortex-m7 | +soft-float |
Cortex-M7F | SP | Yes | cortex-m7 | -fp64 |
Cortex-M7F | DP | Yes | cortex-m7 | None |
Table of supported CPUs for thumbv7em-none-eabihf
CPU | FPU | DSP | Target CPU | Target Features |
---|---|---|---|---|
Any | SP | Yes | None | None |
Cortex-M4F | SP | Yes | cortex-m4 | None |
Cortex-M7F | SP | Yes | cortex-m7 | -fp64 |
Cortex-M7F | DP | Yes | cortex-m7 | None |
Arm Cortex-M4 and Arm Cortex-M4F
The target CPU is cortex-m4
.
- All Cortex-M4 have DSP extensions
- support is controlled by the
dsp
target-feature - enabled by default with this target
- support is controlled by the
- Cortex-M4F has a single precision FPU
- support is enabled by default with this target-cpu
- disable support using the
+soft-float
feature (eabi
only)
Arm Cortex-M7 and Arm Cortex-M7F
The target CPU is cortex-m7
.
- All Cortex-M7 have DSP extensions
- support is controlled by the
dsp
target-feature - enabled by default with this target
- support is controlled by the
- Cortex-M7F have either a single-precision or double-precision FPU
- double-precision support is enabled by default with this target-cpu
- opt-out by using the
-f64
target-feature
- opt-out by using the
- disable support entirely using the
+soft-float
feature (eabi
only)
- double-precision support is enabled by default with this target-cpu
thumbv7m-none-eabi
Tier: 2
Bare-metal target for CPUs in the Armv7-M architecture family, supporting a subset of the T32 ISA.
Processors in this family include the:
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets.
This target uses the soft-float ABI: functions which take f32
or f64
as
arguments will have those values packed into integer registers. This is the
only option because there is no FPU support in Armv7-M.
Target maintainers
- Rust Embedded Devices Working Group Cortex-M
Team,
cortex-m@teams.rust-embedded.org
Target CPU and Target Feature options
See the bare-metal Arm docs for details on how to use these flags.
Table of supported CPUs
CPU | FPU | Target CPU | Target Features |
---|---|---|---|
Cortex-M3 | No | cortex-m3 | None |
Arm Cortex-M3
The target CPU option is cortex-m3
.
There are no relevant feature flags, and the FPU is not available.
thumbv8m.base-none-eabi
Tier: 2
Bare-metal target for CPUs in the Baseline Armv8-M architecture family, supporting a subset of the T32 ISA.
Processors in this family include the:
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets.
This target uses the soft-float ABI: functions which take f32
or f64
as
arguments will have those values packed into integer registers. This is the
only option because there is no FPU support in Armv8-M Baseline.
Target maintainers
- Rust Embedded Devices Working Group Cortex-M
Team,
cortex-m@teams.rust-embedded.org
Target CPU and Target Feature options
See the bare-metal Arm docs for details on how to use these flags.
Table of supported CPUs
CPU | FPU | Target CPU | Target Features |
---|---|---|---|
Cortex-M23 | No | cortex-m23 | None |
Arm Cortex-M23
The target CPU option is cortex-m23
.
There are no relevant feature flags, and the FPU is not available.
thumbv8m.main-none-eabi
and thumbv8m.main-none-eabihf
Tier: 2
Bare-metal target for CPUs in the Mainline Armv8-M architecture family, supporting a subset of the T32 ISA.
Processors in this family include the:
See arm-none-eabi
for information applicable to all
arm-none-eabi
targets, in particular the difference between the eabi
and
eabihf
ABI.
Target maintainers
- Rust Embedded Devices Working Group Cortex-M
Team,
cortex-m@teams.rust-embedded.org
Target CPU and Target Feature options
See the bare-metal Arm docs for details on how to use these flags.
Table of supported CPUs for thumbv8m.main-none-eabi
CPU | FPU | DSP | MVE | Target CPU | Target Features |
---|---|---|---|---|---|
Unspecified | No | No | No | None | None |
Cortex-M33 | No | No | No | cortex-m33 | +soft-float,-dsp |
Cortex-M33 | No | Yes | No | cortex-m33 | +soft-float |
Cortex-M33 | SP | No | No | cortex-m33 | -dsp |
Cortex-M33 | SP | Yes | No | cortex-m33 | None |
Cortex-M35P | No | No | No | cortex-m35p | +soft-float,-dsp |
Cortex-M35P | No | Yes | No | cortex-m35p | +soft-float |
Cortex-M35P | SP | No | No | cortex-m35p | -dsp |
Cortex-M35P | SP | Yes | No | cortex-m35p | None |
Cortex-M55 | No | Yes | No | cortex-m55 | +soft-float,-mve |
Cortex-M55 | DP | Yes | No | cortex-m55 | -mve |
Cortex-M55 | No | Yes | Int | cortex-m55 | +soft-float,-mve.fp |
Cortex-M55 | DP | Yes | Int | cortex-m55 | -mve.fp |
Cortex-M55 | DP | Yes | Int+Float | cortex-m55 | None |
Cortex-M85 | No | Yes | No | cortex-m85 | +soft-float,-mve |
Cortex-M85 | DP | Yes | No | cortex-m85 | -mve |
Cortex-M85 | No | Yes | Int | cortex-m85 | +soft-float,-mve.fp |
Cortex-M85 | DP | Yes | Int | cortex-m85 | -mve.fp |
Cortex-M85 | DP | Yes | Int+Float | cortex-m85 | None |
Table of supported CPUs for thumbv8m.main-none-eabihf
CPU | FPU | DSP | MVE | Target CPU | Target Features |
---|---|---|---|---|---|
Unspecified | SP | No | No | None | None |
Cortex-M33 | SP | No | No | cortex-m33 | -dsp |
Cortex-M33 | SP | Yes | No | cortex-m33 | None |
Cortex-M33P | SP | No | No | cortex-m35p | -dsp |
Cortex-M33P | SP | Yes | No | cortex-m35p | None |
Cortex-M55 | DP | Yes | No | cortex-m55 | -mve |
Cortex-M55 | DP | Yes | Int | cortex-m55 | -mve.fp |
Cortex-M55 | DP | Yes | Int+Float | cortex-m55 | None |
Cortex-M85 | DP | Yes | No | cortex-m85 | -mve |
Cortex-M85 | DP | Yes | Int | cortex-m85 | -mve.fp |
Cortex-M85 | DP | Yes | Int+Float | cortex-m85 | None |
Arm Cortex-M33
The target CPU is cortex-m33
.
- Has optional DSP extensions
- support is controlled by the
dsp
target-feature - enabled by default with this target-cpu
- support is controlled by the
- Has an optional single precision FPU
- support is enabled by default with this target-cpu
- disable support using the
+soft-float
feature (eabi
only)
Arm Cortex-M35P
The target CPU is cortex-m35p
.
- Has optional DSP extensions
- support is controlled by the
dsp
target-feature - enabled by default with this target-cpu
- support is controlled by the
- Has an optional single precision FPU
- support is enabled by default with this target-cpu
- disable support using the
+soft-float
feature (eabi
only)
Arm Cortex-M55
The target CPU is cortex-m55
.
- Has DSP extensions
- support is controlled by the
dsp
target-feature - enabled by default with this target-cpu
- support is controlled by the
- Has an optional double-precision FPU that also supports half-precision FP16
values
- support is enabled by default with this target-cpu
- disable support using the
+soft-float
feature (eabi
only)
- Has optional support for M-Profile Vector Extensions
- Also known as Helium Technology
- Available with only integer support, or both integer/float support
- The appropriate feature for the MVE is either
mve
(integer) ormve.fp
(float) mve.fp
is enabled by default on this target CPU- disable using
-mve.fp
(disable float MVE) or-mve
(disable all MVE)
Arm Cortex-M85
The target CPU is cortex-m85
.
- Has DSP extensions
- support is controlled by the
dsp
target-feature - enabled by default with this target-cpu
- support is controlled by the
- Has an optional double-precision FPU that also supports half-precision FP16
values
- support is enabled by default with this target-cpu
- disable support using the
+soft-float
feature (eabi
only)
- Has optional support for M-Profile Vector Extensions
- Also known as Helium Technology
- Available with only integer support, or both integer/float support
- The appropriate feature for the MVE is either
mve
(integer) ormve.fp
(float) mve.fp
is enabled by default on this target CPU- disable using
-mve.fp
(disable float MVE) or-mve
(disable all MVE)
armv6k-nintendo-3ds
Tier: 3
The Nintendo 3DS platform, which has an Armv6k processor, and its associated
operating system (horizon
).
Rust support for this target is not affiliated with Nintendo, and is not derived from nor used with any official Nintendo SDK.
Target maintainers
This target is maintained by members of the @rust3ds organization:
Requirements
This target is cross-compiled. Dynamic linking is not supported.
#![no_std]
crates can be built using build-std
to build core
and optionally
alloc
, and either panic_abort
or panic_unwind
.
std
is partially supported, but mostly works. Some APIs are unimplemented
and will simply return an error, such as std::process
. An allocator is provided
by default.
In order to support some APIs, binaries must be linked against libc
written
for the target, using a linker for the target. These are provided by the
devkitARM toolchain. See
Cross-compilation toolchains and C code
for more details.
Additionally, some helper crates provide implementations of some libc
functions
use by std
that may otherwise be missing. These, or an alternate implementation
of the relevant functions, are required to use std
:
pthread-3ds
provides pthread APIs forstd::thread
.shim-3ds
fulfills some other missing libc APIs (e.g.getrandom
).
Binaries built for this target should be compatible with all variants of the 3DS (and 2DS) hardware and firmware, but testing is limited and some versions may not work correctly.
This target generates binaries in the ELF format.
Building the target
You can build Rust with support for the target by adding it to the target
list in config.toml
and providing paths to the devkitARM toolchain.
[build]
build-stage = 1
target = ["armv6k-nintendo-3ds"]
[target.armv6k-nintendo-3ds]
cc = "/opt/devkitpro/devkitARM/bin/arm-none-eabi-gcc"
cxx = "/opt/devkitpro/devkitARM/bin/arm-none-eabi-g++"
ar = "/opt/devkitpro/devkitARM/bin/arm-none-eabi-ar"
ranlib = "/opt/devkitpro/devkitARM/bin/arm-none-eabi-ranlib"
linker = "/opt/devkitpro/devkitARM/bin/arm-none-eabi-gcc"
Also, to build compiler_builtins
for the target, export these flags before
building the Rust toolchain:
export CFLAGS_armv6k_nintendo_3ds="-mfloat-abi=hard -mtune=mpcore -mtp=soft -march=armv6k"
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target.
The recommended way to build binaries is by using the
cargo-3ds tool, which uses build-std
and provides commands that work like the usual cargo run
, cargo build
, etc.
The cargo 3ds new
will automatically set up a new project with the dependencies
needed to build a simple binary.
You can also build Rust with the target enabled (see Building the target above).
As mentioned in Requirements, programs that use std
must link
against both the devkitARM toolchain and libraries providing the libc
APIs used
in std
. There is a general-purpose utility crate for working with nonstandard
APIs provided by the OS: ctru-rs
.
Add it to Cargo.toml to use it in your program:
[dependencies]
ctru-rs = { git = "https://github.com/rust3ds/ctru-rs.git" }
Depending on ctru-rs
ensures that all the necessary symbols are available at
link time.
Testing
Binaries built for this target can be run in an emulator (most commonly
Citra), or sent to a device through
the use of a tool like devkitARM's 3dslink
. They may also simply be copied
to an SD card to be inserted in the device.
The cargo-3ds
tool mentioned in Building Rust programs
supports the use of 3dslink
with cargo 3ds run
. The default Rust test runner
is not supported, but
custom test frameworks
can be used with cargo 3ds test
to run unit tests on a device.
The Rust test suite for library/std
is not yet supported.
Cross-compilation toolchains and C code
C code can be built for this target using the
devkitARM toolchain.
This toolchain provides arm-none-eabi-gcc
as the linker used to link Rust
programs as well.
The toolchain also provides a libc
implementation, which is required by std
for many of its APIs, and a helper library libctru
which is used by several
of the helper crates listed in Requirements.
This toolchain does not, however, include all of the APIs expected by std
,
and the remaining APIs are implemented by pthread-3ds
and linker-fix-3ds
.
armv7-sony-vita-newlibeabihf
Tier: 3
This tier supports the ARM Cortex A9 processor running on a PlayStation Vita console.
Rust support for this target is not affiliated with Sony, and is not derived from nor used with any official Sony SDK.
Target maintainers
Requirements
This target is cross-compiled, and requires installing VITASDK toolchain on your system. Dynamic linking is not supported.
#![no_std]
crates can be built using build-std
to build core
, and optionally
alloc
, and panic_abort
.
std
is partially supported, but mostly works. Some APIs are unimplemented
and will simply return an error, such as std::process
.
This target generates binaries in the ELF format with thumb ISA by default.
Binaries are linked with arm-vita-eabi-gcc
provided by VITASDK toolchain.
Building the target
Rust does not ship pre-compiled artifacts for this target. You can use build-std
flag to build ELF binaries with std
:
cargo build -Z build-std=std,panic_abort --target=armv7-sony-vita-newlibeabihf --release
Building Rust programs
The recommended way to build artifacts that can be installed and run on PlayStation Vita is by using the cargo-vita tool. This tool uses build-std
and VITASDK toolchain to build artifacts runnable on Vita.
To install the tool run:
cargo install cargo-vita
VITASDK toolchain must be installed, and the VITASDK
environment variable must be set to its location, e.g.:
export VITASDK=/opt/vitasdk
Add the following section to your project's Cargo.toml
:
[package.metadata.vita]
# A unique 9 character alphanumeric identifier of the app.
title_id = "RUSTAPP01"
# A title that will be used for the app. Optional, name will be used if not defined
title_name = "My application"
To build a VPK with ELF in the release profile, run:
cargo vita build vpk --release
After building a *.vpk file it can be uploaded to a PlayStation Vita and installed, or used with a Vita3K emulator.
Testing
The default Rust test runner is supported, and tests can be compiled to an elf and packed to a *.vpk file using cargo-vita
tool. Filtering tests is not currently supported since passing command-line arguments to the executable is not supported on Vita, so the runner will always execute all tests.
The Rust test suite for library/std
is not yet supported.
Cross-compilation
This target can be cross-compiled from x86_64
on Windows, MacOS or Linux systems. Other hosts are not supported for cross-compilation.
armv7-unknown-linux-uclibceabi
Tier: 3
This target supports Armv7-A softfloat CPUs and uses the uclibc-ng standard library. This is a common configuration on many consumer routers (e.g., Netgear R7000, Asus RT-AC68U).
Target maintainers
Requirements
This target is cross compiled, and requires a cross toolchain.
This target supports host tools and std.
Building the target
You will need to download or build a 'C'
cross toolchain that targets Armv7-A softfloat and that uses the uclibc-ng standard library. If your target hardware is something like a router or an embedded device, keep in mind that manufacturer supplied SDKs for this class of CPU could be outdated and potentially unsuitable for bootstrapping rust.
Here is a sample toolchain that is built using buildroot. It uses modern toolchain components, older thus universal kernel headers (2.6.36.4), and is used for a project called Tomatoware. This toolchain is patched so that its sysroot is located at /mmc (e.g., /mmc/bin, /mmc/lib, /mmc/include). This is useful in scenarios where the root filesystem is read-only but you are able attach external storage loaded with user applications. Tomatoware is an example of this that even allows you to run various compilers and developer tools natively on the target device.
Utilizing the Tomatoware toolchain this target can be built for cross compilation and native compilation (host tools) with project
rust-bootstrap-armv7-unknown-linux-uclibceabi.
Here is a sample config if using your own toolchain.
[build]
build-stage = 2
target = ["armv7-unknown-linux-uclibceabi"]
[target.armv7-unknown-linux-uclibceabi]
cc = "/path/to/arm-unknown-linux-uclibcgnueabi-gcc"
cxx = "/path/to/arm-unknown-linux-uclibcgnueabi-g++"
ar = "path/to/arm-unknown-linux-uclibcgnueabi-ar"
ranlib = "path/to/arm-unknown-linux-uclibcgnueabi-ranlib"
linker = "/path/to/arm-unknown-linux-uclibcgnueabi-gcc"
Building Rust programs
The following assumes you are using the Tomatoware toolchain and environment. Adapt if you are using your own toolchain.
Native compilation
Since this target supports host tools, you can natively build rust applications directly on your target device. This can be convenient because it removes the complexities of cross compiling and you can immediately test and deploy your binaries. One downside is that compiling on your Armv7-A CPU will probably be much slower than cross compilation on your x86 machine.
To setup native compilation:
- Download Tomatoware to your device using the latest nightly release found here.
- Extract
tar zxvf arm-soft-mmc.tgz -C /mmc
- Add
/mmc/bin:/mmc:sbin/
to your PATH, orsource /mmc/etc/profile
apt update && apt install rust
If you bootstrap rust on your own using the project above, it will create a .deb file that you then can install with
dpkg -i rust_1.xx.x-x_arm.deb
After completing these steps you can use rust normally in a native environment.
Cross Compilation
To cross compile, you'll need to:
-
Build the rust cross toolchain using rust-bootstrap-armv7-unknown-linux-uclibceabi or your own built toolchain.
-
Link your built toolchain with
rustup toolchain link stage2 \ ${HOME}/rust-bootstrap-armv7-unknown-linux-uclibceabi/src/rust/rust/build/x86_64-unknown-linux-gnu/stage2
-
Build with:
CC_armv7_unknown_linux_uclibceabi=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-gcc \ CXX_armv7_unknown_linux_uclibceabi=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-g++ \ AR_armv7_unknown_linux_uclibceabi=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-ar \ CFLAGS_armv7_unknown_linux_uclibceabi="-march=armv7-a -mtune=cortex-a9" \ CXXFLAGS_armv7_unknown_linux_uclibceabi="-march=armv7-a -mtune=cortex-a9" \ CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABI_LINKER=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-gcc \ CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABI_RUSTFLAGS='-Clink-arg=-s -Clink-arg=-Wl,--dynamic-linker=/mmc/lib/ld-uClibc.so.1 -Clink-arg=-Wl,-rpath,/mmc/lib' \ cargo +stage2 \ build \ --target armv7-unknown-linux-uclibceabi \ --release
-
Copy the binary to your target device and run.
We specify CC
, CXX
, AR
, CFLAGS
, and CXXFLAGS
environment variables because sometimes a project or a subproject requires the use of your 'C'
cross toolchain. Since Tomatoware has a modified sysroot we also pass via RUSTFLAGS the location of the dynamic-linker and rpath.
Test with QEMU
To test a cross-compiled binary on your build system follow the instructions for Cross Compilation
, install qemu-arm-static
, and run with the following.
CC_armv7_unknown_linux_uclibceabi=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-gcc \
CXX_armv7_unknown_linux_uclibceabi=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-g++ \
AR_armv7_unknown_linux_uclibceabi=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-ar \
CFLAGS_armv7_unknown_linux_uclibceabi="-march=armv7-a -mtune=cortex-a9" \
CXXFLAGS_armv7_unknown_linux_uclibceabi="-march=armv7-a -mtune=cortex-a9" \
CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABI_LINKER=/opt/tomatoware/arm-soft-mmc/bin/arm-linux-gcc \
CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABI_RUNNER="qemu-arm-static -L /opt/tomatoware/arm-soft-mmc/arm-tomatoware-linux-uclibcgnueabi/sysroot/" \
cargo +stage2 \
run \
--target armv7-unknown-linux-uclibceabi \
--release
Run in a chroot
It's also possible to build in a chroot environment. This is a convenient way to work without needing to access the target hardware.
To build the chroot:
sudo debootstrap --arch armel bullseye $HOME/debian
sudo chroot $HOME/debian/ /bin/bash
mount proc /proc -t proc
mount -t sysfs /sys sys/
export PATH=/mmc/bin:/mmc/sbin:$PATH
From here you can setup your environment (e.g., add user, install wget).
- Download Tomatoware to the chroot environment using the latest nightly release found here.
- Extract
tar zxvf arm-soft-mmc.tgz -C /mmc
- Add
/mmc/bin:/mmc:sbin/
to your PATH, orsource /mmc/etc/profile
sudo /mmc/bin/apt update && sudo /mmc/bin/apt install rust
After completing these steps you can use rust normally in a chroot environment.
Remember when using sudo
the root user's PATH could differ from your user's PATH.
armv7-unknown-linux-uclibceabihf
Tier: 3
This tier supports the Armv7-A processor running a Linux kernel and uClibc-ng standard library. It provides full support for rust and the rust standard library.
Designated Developers
Requirements
This target is cross compiled, and requires a cross toolchain. You can find suitable pre-built toolchains at bootlin or build one yourself via buildroot.
Building
Get a C toolchain
Compiling rust for this target has been tested on x86_64
linux hosts. Other host types have not been tested, but may work, if you can find a suitable cross compilation toolchain for them.
If you don't already have a suitable toolchain, download one here, and unpack it into a directory.
Configure rust
The target can be built by enabling it for a rustc
build, by placing the following in config.toml
:
[build]
target = ["armv7-unknown-linux-uclibceabihf"]
stage = 2
[target.armv7-unknown-linux-uclibceabihf]
# ADJUST THIS PATH TO POINT AT YOUR TOOLCHAIN
cc = "/TOOLCHAIN_PATH/bin/arm-buildroot-linux-uclibcgnueabihf-gcc"
Build
# in rust dir
./x.py build --stage 2
Building and Running Rust Programs
To test cross-compiled binaries on a x86_64
system, you can use the qemu-arm
userspace emulation program. This avoids having a full emulated ARM system by doing dynamic binary translation and dynamic system call translation. It lets you run ARM programs directly on your x86_64
kernel. It's very convenient!
To use:
- Install
qemu-arm
according to your distro. - Link your built toolchain via:
rustup toolchain link stage2 ${RUST}/build/x86_64-unknown-linux-gnu/stage2
- Create a test program
cargo new hello_world
cd hello_world
- Build and run
CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABIHF_RUNNER="qemu-arm -L ${TOOLCHAIN}/arm-buildroot-linux-uclibcgnueabihf/sysroot/" \
CARGO_TARGET_ARMV7_UNKNOWN_LINUX_UCLIBCEABIHF_LINKER=${TOOLCHAIN}/bin/arm-buildroot-linux-uclibcgnueabihf-gcc \
cargo +stage2 run --target armv7-unknown-linux-uclibceabihf
*-linux-android and *-linux-androideabi
Tier: 2
Android is a mobile operating system built on top of the Linux kernel.
Target maintainers
- Chris Wailes (@chriswailes)
- Matthew Maurer (@maurer)
- Martin Geisler (@mgeisler)
Requirements
This target is cross-compiled from a host environment. Development may be done from the source tree or using the Android NDK.
Android targets support std. Generated binaries use the ELF file format.
NDK/API Update Policy
Rust will support the most recent Long Term Support (LTS) Android Native Development Kit (NDK). By default Rust will support all API levels supported by the NDK, but a higher minimum API level may be required if deemed necessary.
Building the target
To build Rust binaries for Android you'll need a copy of the most recent LTS edition of the Android NDK. Supported Android targets are:
- aarch64-linux-android
- arm-linux-androideabi
- armv7-linux-androideabi
- i686-linux-android
- thumbv7neon-linux-androideabi
- x86_64-linux-android
The riscv64-linux-android target is supported as a Tier 3 target.
A list of all supported targets can be found here
Architecture Notes
riscv64-linux-android
Currently the riscv64-linux-android
target requires the following architecture features/extensions:
a
(atomics)d
(double-precision floating-point)c
(compressed instruction set)f
(single-precision floating-point)m
(multiplication and division)v
(vector)Zba
(address calculation instructions)Zbb
(base instructions)Zbs
(single-bit instructions)
aarch64-linux-android on Nightly compilers
As soon as -Zfixed-x18
compiler flag is supplied, the ShadowCallStack
sanitizer
instrumentation is also made avaiable by supplying the second compiler flag -Zsanitizer=shadow-call-stack
.
*-unknown-linux-ohos
Tier: 2
Targets for the OpenHarmony operating system.
Target maintainers
- Amanieu d'Antras (@Amanieu)
- Lu Binglun (@lubinglun)
Setup
The OpenHarmony SDK doesn't currently support Rust compilation directly, so some setup is required.
First, you must obtain the OpenHarmony SDK from this page. Select the version of OpenHarmony you are developing for and download the "Public SDK package for the standard system".
Create the following shell scripts that wrap Clang from the OpenHarmony SDK:
aarch64-unknown-linux-ohos-clang.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang \
-target aarch64-linux-ohos \
--sysroot=/path/to/ohos-sdk/linux/native/sysroot \
-D__MUSL__ \
"$@"
aarch64-unknown-linux-ohos-clang++.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang++ \
-target aarch64-linux-ohos \
--sysroot=/path/to/ohos-sdk/linux/native/sysroot \
-D__MUSL__ \
"$@"
armv7-unknown-linux-ohos-clang.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang \
-target arm-linux-ohos \
--sysroot=/path/to/ohos-sdk/linux/native/sysroot \
-D__MUSL__ \
-march=armv7-a \
-mfloat-abi=softfp \
-mtune=generic-armv7-a \
-mthumb \
"$@"
armv7-unknown-linux-ohos-clang++.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang++ \
-target arm-linux-ohos \
--sysroot=/path/to/ohos-sdk/linux/native/sysroot \
-D__MUSL__ \
-march=armv7-a \
-mfloat-abi=softfp \
-mtune=generic-armv7-a \
-mthumb \
"$@"
x86_64-unknown-linux-ohos-clang.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang \
-target x86_64-linux-ohos \
--sysroot=/path/to/ohos-sdk/linux/native/sysroot \
-D__MUSL__ \
"$@"
x86_64-unknown-linux-ohos-clang++.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang++ \
-target x86_64-linux-ohos \
--sysroot=/path/to/ohos-sdk/linux/native/sysroot \
-D__MUSL__ \
"$@"
Future versions of the OpenHarmony SDK will avoid the need for this process.
Building Rust programs
Rustup ships pre-compiled artifacts for this target, which you can install with:
rustup target add aarch64-unknown-linux-ohos
rustup target add armv7-unknown-linux-ohos
rustup target add x86_64-unknown-linux-ohos
You will need to configure the linker to use in ~/.cargo/config.toml
:
[target.aarch64-unknown-linux-ohos]
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
linker = "/path/to/aarch64-unknown-linux-ohos-clang.sh"
[target.armv7-unknown-linux-ohos]
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
linker = "/path/to/armv7-unknown-linux-ohos-clang.sh"
[target.x86_64-unknown-linux-ohos]
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
linker = "/path/to/x86_64-unknown-linux-ohos-clang.sh"
Building the target from source
Instead of using rustup
, you can instead build a rust toolchain from source.
Create a config.toml
with the following contents:
profile = "compiler"
change-id = 115898
[build]
sanitizers = true
profiler = true
[target.aarch64-unknown-linux-ohos]
cc = "/path/to/aarch64-unknown-linux-ohos-clang.sh"
cxx = "/path/to/aarch64-unknown-linux-ohos-clang++.sh"
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
ranlib = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ranlib"
linker = "/path/to/aarch64-unknown-linux-ohos-clang.sh"
[target.armv7-unknown-linux-ohos]
cc = "/path/to/armv7-unknown-linux-ohos-clang.sh"
cxx = "/path/to/armv7-unknown-linux-ohos-clang++.sh"
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
ranlib = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ranlib"
linker = "/path/to/armv7-unknown-linux-ohos-clang.sh"
[target.x86_64-unknown-linux-ohos]
cc = "/path/to/x86_64-unknown-linux-ohos-clang.sh"
cxx = "/path/to/x86_64-unknown-linux-ohos-clang++.sh"
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
ranlib = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ranlib"
linker = "/path/to/x86_64-unknown-linux-ohos-clang.sh"
Testing
Running the Rust testsuite is possible, but currently difficult due to the way the OpenHarmony emulator is set up (no networking).
Cross-compilation toolchains and C code
You can use the shell scripts above to compile C code for the target.
i686-unknown-hurd-gnu
Tier: 3
[GNU/Hurd] is the GNU Hurd is the GNU project's replacement for the Unix kernel.
Target maintainers
- Samuel Thibault,
samuel.thibault@ens-lyon.org
, https://github.com/sthibaul/
Requirements
The target supports host tools.
The GNU/Hurd target supports std
and uses the standard ELF file format.
Building the target
This target can be built by adding i686-unknown-hurd-gnu
as target in the rustc list.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
Testing
Tests can be run in the same way as a regular binary.
Cross-compilation toolchains and C code
The target supports C code, the GNU toolchain calls the target
i686-unknown-gnu
.
aarch64-unknown-teeos
Tier: 3
Target for the TEEOS operating system.
TEEOS is a mini os run in TrustZone, for trusted/security apps. The kernel of TEEOS is HongMeng/ChCore micro kernel. The libc for TEEOS is a part of musl. It's very small that there is no RwLock, no network, no stdin, and no file system for apps in TEEOS.
Some abbreviation:
Abbreviation | The full text | Description |
---|---|---|
TEE | Trusted Execution Environment | ARM TrustZone divides the system into two worlds/modes -- the secure world/mode and the normal world/mode. TEE is in the secure world/mode. |
REE | Rich Execution Environment | The normal world. for example, Linux for Android phone is in REE side. |
TA | Trusted Application | The app run in TEE side system. |
CA | Client Application | The progress run in REE side system. |
TEEOS is open source in progress. MORE about
Target maintainers
- Petrochenkov Vadim
- Sword-Destiny
Setup
We use OpenHarmony SDK for TEEOS.
The OpenHarmony SDK doesn't currently support Rust compilation directly, so some setup is required.
First, you must obtain the OpenHarmony SDK from this page. Select the version of OpenHarmony you are developing for and download the "Public SDK package for the standard system".
Create the following shell scripts that wrap Clang from the OpenHarmony SDK:
aarch64-unknown-teeos-clang.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang \
-target aarch64-linux-gnu \
"$@"
aarch64-unknown-teeos-clang++.sh
#!/bin/sh
exec /path/to/ohos-sdk/linux/native/llvm/bin/clang++ \
-target aarch64-linux-gnu \
"$@"
Building the target
To build a rust toolchain, create a config.toml
with the following contents:
profile = "compiler"
change-id = 115898
[build]
sanitizers = true
profiler = true
target = ["x86_64-unknown-linux-gnu", "aarch64-unknown-teeos"]
submodules = false
compiler-docs = false
extended = true
[install]
bindir = "bin"
libdir = "lib"
[target.aarch64-unknown-teeos]
cc = "/path/to/scripts/aarch64-unknown-teeos-clang.sh"
cxx = "/path/to/scripts/aarch64-unknown-teeos-clang.sh"
linker = "/path/to/scripts/aarch64-unknown-teeos-clang.sh"
ar = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ar"
ranlib = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-ranlib"
llvm-config = "/path/to/ohos-sdk/linux/native/llvm/bin/llvm-config"
note: You need to insert "/usr/include/x86_64-linux-gnu/" into environment variable: $C_INCLUDE_PATH
if some header files like bits/xxx.h not found.
note: You can install gcc-aarch64-linux-gnu,g++-aarch64-linux-gnu if some files like crti.o not found.
note: You may need to install libc6-dev-i386 libc6-dev if "gnu/stubs-32.h" not found.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
You will need to configure the linker to use in ~/.cargo/config
:
[target.aarch64-unknown-teeos]
linker = "/path/to/aarch64-unknown-teeos-clang.sh" # or aarch64-linux-gnu-ld
Testing
Running the Rust testsuite is not possible now.
More information about how to test CA/TA. See here
*-espidf
Tier: 3
Targets for the ESP-IDF development framework running on RISC-V and Xtensa CPUs.
Target maintainers
- Ivan Markov @ivmarkov
- Scott Mabin @MabezDev
- Sergio Gasquez @SergioGasquez
Requirements
The target names follow this format: $ARCH-esp-espidf
, where $ARCH
specifies the target processor architecture. The following targets are currently defined:
Target name | Target CPU(s) | Minimum ESP-IDF version |
---|---|---|
riscv32imc-esp-espidf | ESP32-C2 | v5.0 |
riscv32imc-esp-espidf | ESP32-C3 | v4.4 |
riscv32imac-esp-espidf | ESP32-C6 | v5.1 |
riscv32imac-esp-espidf | ESP32-H2 | v5.1 |
riscv32imafc-esp-espidf | ESP32-P4 | v5.2 |
xtensa-esp32-espidf | ESP32 | v4.4 |
xtensa-esp32s2-espidf | ESP32-S2 | v4.4 |
xtensa-esp32s3-espidf | ESP32-S3 | v4.4 |
It is recommended to use the latest ESP-IDF stable release if possible.
Building the target
The target can be built by enabling it for a rustc
build. The build-std
feature is required to build the standard library for ESP-IDF. ldproxy
is also required for linking, it can be installed from crates.io.
[build]
target = ["$ARCH-esp-espidf"]
[target.$ARCH-esp-espidf]
linker = "ldproxy"
[unstable]
build-std = ["std", "panic_abort"]
The esp-idf-sys
crate will handle the compilation of ESP-IDF, including downloading the relevant toolchains for the build.
Cross-compilation toolchains and C code
esp-idf-sys
exposes the toolchain used in the compilation of ESP-IDF, see the crate documentation for build output propagation for more information.
aarch64-unknown-fuchsia
and x86_64-unknown-fuchsia
Tier: 2
Fuchsia is a modern open source operating system that's simple, secure, updatable, and performant.
Target maintainers
The Fuchsia team:
- Tyler Mandry (@tmandry)
- David Koloski (@djkoloski)
- Julia Ryan (@P1n3appl3)
- Erick Tryzelaar (@erickt)
As the team evolves over time, the specific members listed here may differ from
the members reported by the API. The API should be considered to be
authoritative if this occurs. Instead of pinging individual members, use
@rustbot ping fuchsia
to contact the team on GitHub.
Table of contents
- Requirements
- Walkthrough structure
- Compiling a Rust binary targeting Fuchsia
- Creating a Fuchsia package
- Publishing a Fuchsia package
- Running a Fuchsia component on an emulator
.gitignore
extensions- Testing
- Debugging
Requirements
This target is cross-compiled from a host environment. You will need a recent copy of the Fuchsia SDK, which provides the tools, libraries, and binaries required to build and link programs for Fuchsia.
Development may also be done from the source tree.
Fuchsia targets support std
and follow the sysv64
calling convention on
x86_64. Fuchsia binaries use the ELF file format.
Walkthrough structure
This walkthrough will cover:
- Compiling a Rust binary targeting Fuchsia.
- Building a Fuchsia package.
- Publishing and running a Fuchsia package to a Fuchsia emulator.
For the purposes of this walkthrough, we will only target x86_64-unknown-fuchsia
.
Compiling a Rust binary targeting Fuchsia
Today, there are two main ways to build a Rust binary targeting Fuchsia using the Fuchsia SDK:
- Allow rustup to handle the installation of Fuchsia targets for you.
- Build a toolchain locally that can target Fuchsia.
Targeting Fuchsia with rustup and cargo
The easiest way to build a Rust binary targeting Fuchsia is by allowing rustup to handle the installation of Fuchsia targets for you. This can be done by issuing the following commands:
rustup target add x86_64-unknown-fuchsia
rustup target add aarch64-unknown-fuchsia
After installing our Fuchsia targets, we can now compile a Rust binary that targets Fuchsia.
To create our Rust project, we can use cargo
as follows:
From base working directory
cargo new hello_fuchsia
The rest of this walkthrough will take place from hello_fuchsia
, so we can
change into that directory now:
cd hello_fuchsia
Note: From this point onwards, all commands will be issued from the hello_fuchsia/
directory, and all hello_fuchsia/
prefixes will be removed from references for sake of brevity.
We can edit our src/main.rs
to include a test as follows:
src/main.rs
fn main() {
println!("Hello Fuchsia!");
}
#[test]
fn it_works() {
assert_eq!(2 + 2, 4);
}
In addition to the standard workspace created, we will want to create a
.cargo/config.toml
file to link necessary libraries
during compilation:
.cargo/config.toml
[target.x86_64-unknown-fuchsia]
rustflags = [
"-Lnative=<SDK_PATH>/arch/x64/lib",
"-Lnative=<SDK_PATH>/arch/x64/sysroot/lib"
]
Note: Make sure to fill out <SDK_PATH>
with the path to the downloaded Fuchsia SDK.
These options configure the following:
-Lnative=${SDK_PATH}/arch/${ARCH}/lib
: Link against Fuchsia libraries from the SDK-Lnative=${SDK_PATH}/arch/${ARCH}/sysroot/lib
: Link against Fuchsia sysroot libraries from the SDK
In total, our new project will look like:
Current directory structure
hello_fuchsia/
┣━ src/
┃ ┗━ main.rs
┣━ Cargo.toml
┗━ .cargo/
┗━ config.toml
Finally, we can build our rust binary as:
cargo build --target x86_64-unknown-fuchsia
Now we have a Rust binary at target/x86_64-unknown-fuchsia/debug/hello_fuchsia
,
targeting our desired Fuchsia target.
Current directory structure
hello_fuchsia/
┣━ src/
┃ ┗━ main.rs
┣━ target/
┃ ┗━ x86_64-unknown-fuchsia/
┃ ┗━ debug/
┃ ┗━ hello_fuchsia
┣━ Cargo.toml
┗━ .cargo/
┗━ config.toml
Targeting Fuchsia with a compiler built from source
An alternative to the first workflow is to target Fuchsia by using
rustc
built from source.
Before building Rust for Fuchsia, you'll need a clang toolchain that supports Fuchsia as well. A recent version (14+) of clang should be sufficient to compile Rust for Fuchsia.
x86-64 and AArch64 Fuchsia targets can be enabled using the following
configuration in config.toml
:
[build]
target = ["<host_platform>", "aarch64-unknown-fuchsia", "x86_64-unknown-fuchsia"]
[rust]
lld = true
[llvm]
download-ci-llvm = false
[target.x86_64-unknown-fuchsia]
cc = "clang"
cxx = "clang++"
[target.aarch64-unknown-fuchsia]
cc = "clang"
cxx = "clang++"
Though not strictly required, you may also want to use clang
for your host
target as well:
[target.<host_platform>]
cc = "clang"
cxx = "clang++"
By default, the Rust compiler installs itself to /usr/local
on most UNIX
systems. You may want to install it to another location (e.g. a local install
directory) by setting a custom prefix in config.toml
:
[install]
# Make sure to use the absolute path to your install directory
prefix = "<RUST_SRC_PATH>/install"
Next, the following environment variables must be configured. For example, using
a script we name config-env.sh
:
# Configure this environment variable to be the path to the downloaded SDK
export SDK_PATH="<SDK path goes here>"
export CFLAGS_aarch64_unknown_fuchsia="--target=aarch64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/arm64/sysroot -I${SDK_PATH}/pkg/fdio/include"
export CXXFLAGS_aarch64_unknown_fuchsia="--target=aarch64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/arm64/sysroot -I${SDK_PATH}/pkg/fdio/include"
export LDFLAGS_aarch64_unknown_fuchsia="--target=aarch64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/arm64/sysroot -L${SDK_PATH}/arch/arm64/lib"
export CARGO_TARGET_AARCH64_UNKNOWN_FUCHSIA_RUSTFLAGS="-C link-arg=--sysroot=${SDK_PATH}/arch/arm64/sysroot -Lnative=${SDK_PATH}/arch/arm64/sysroot/lib -Lnative=${SDK_PATH}/arch/arm64/lib"
export CFLAGS_x86_64_unknown_fuchsia="--target=x86_64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/x64/sysroot -I${SDK_PATH}/pkg/fdio/include"
export CXXFLAGS_x86_64_unknown_fuchsia="--target=x86_64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/x64/sysroot -I${SDK_PATH}/pkg/fdio/include"
export LDFLAGS_x86_64_unknown_fuchsia="--target=x86_64-unknown-fuchsia --sysroot=${SDK_PATH}/arch/x64/sysroot -L${SDK_PATH}/arch/x64/lib"
export CARGO_TARGET_X86_64_UNKNOWN_FUCHSIA_RUSTFLAGS="-C link-arg=--sysroot=${SDK_PATH}/arch/x64/sysroot -Lnative=${SDK_PATH}/arch/x64/sysroot/lib -Lnative=${SDK_PATH}/arch/x64/lib"
Finally, the Rust compiler can be built and installed:
(source config-env.sh && ./x.py install)
Once rustc
is installed, we can create a new working directory to work from,
hello_fuchsia
along with hello_fuchsia/src
:
mkdir hello_fuchsia
cd hello_fuchsia
mkdir src
Note: From this point onwards, all commands will be issued from the hello_fuchsia/
directory, and all hello_fuchsia/
prefixes will be removed from references for sake of brevity.
There, we can create a new file named src/hello_fuchsia.rs
:
src/hello_fuchsia.rs
fn main() {
println!("Hello Fuchsia!");
}
#[test]
fn it_works() {
assert_eq!(2 + 2, 4);
}
Current directory structure
hello_fuchsia/
┗━ src/
┗━ hello_fuchsia.rs
Using your freshly installed rustc
, you can compile a binary for Fuchsia using
the following options:
--target x86_64-unknown-fuchsia
/--target aarch64-unknown-fuchsia
: Targets the Fuchsia platform of your choice-Lnative ${SDK_PATH}/arch/${ARCH}/lib
: Link against Fuchsia libraries from the SDK-Lnative ${SDK_PATH}/arch/${ARCH}/sysroot/lib
: Link against Fuchsia sysroot libraries from the SDK
Putting it all together:
# Configure these for the Fuchsia target of your choice
TARGET_ARCH="<x86_64-unknown-fuchsia|aarch64-unknown-fuchsia>"
ARCH="<x64|aarch64>"
rustc \
--target ${TARGET_ARCH} \
-Lnative=${SDK_PATH}/arch/${ARCH}/lib \
-Lnative=${SDK_PATH}/arch/${ARCH}/sysroot/lib \
--out-dir bin src/hello_fuchsia.rs
Current directory structure
hello_fuchsia/
┣━ src/
┃ ┗━ hello_fuchsia.rs
┗━ bin/
┗━ hello_fuchsia
Creating a Fuchsia package
Before moving on, double check your directory structure:
Current directory structure
hello_fuchsia/
┣━ src/ (if using rustc)
┃ ┗━ hello_fuchsia.rs ...
┣━ bin/ ...
┃ ┗━ hello_fuchsia ...
┣━ src/ (if using cargo)
┃ ┗━ main.rs ...
┗━ target/ ...
┗━ x86_64-unknown-fuchsia/ ...
┗━ debug/ ...
┗━ hello_fuchsia ...
With our Rust binary built, we can move to creating a Fuchsia package. On Fuchsia, a package is the unit of distribution for software. We'll need to create a new package directory where we will place files like our finished binary and any data it may need.
To start, make the pkg
, and pkg/meta
directories:
mkdir pkg
mkdir pkg/meta
Current directory structure
hello_fuchsia/
┗━ pkg/
┗━ meta/
Now, create the following files inside:
pkg/meta/package
{
"name": "hello_fuchsia",
"version": "0"
}
The package
file describes our package's name and version number. Every
package must contain one.
pkg/hello_fuchsia.manifest
if using cargo
bin/hello_fuchsia=target/x86_64-unknown-fuchsia/debug/hello_fuchsia
lib/ld.so.1=<SDK_PATH>/arch/x64/sysroot/dist/lib/ld.so.1
lib/libfdio.so=<SDK_PATH>/arch/x64/dist/libfdio.so
meta/package=pkg/meta/package
meta/hello_fuchsia.cm=pkg/meta/hello_fuchsia.cm
pkg/hello_fuchsia.manifest
if using rustc
bin/hello_fuchsia=bin/hello_fuchsia
lib/ld.so.1=<SDK_PATH>/arch/x64/sysroot/dist/lib/ld.so.1
lib/libfdio.so=<SDK_PATH>/arch/x64/dist/libfdio.so
meta/package=pkg/meta/package
meta/hello_fuchsia.cm=pkg/meta/hello_fuchsia.cm
Note: Relative manifest paths are resolved starting from the working directory
of ffx
. Make sure to fill out <SDK_PATH>
with the path to the downloaded
SDK.
The .manifest
file will be used to describe the contents of the package by
relating their location when installed to their location on the file system. The
bin/hello_fuchsia=
entry will be different depending on how your Rust binary
was built, so choose accordingly.
Current directory structure
hello_fuchsia/
┗━ pkg/
┣━ meta/
┃ ┗━ package
┗━ hello_fuchsia.manifest
Creating a Fuchsia component
On Fuchsia, components require a component manifest written in Fuchsia's markup language called CML. The Fuchsia devsite contains an overview of CML and a reference for the file format. Here's a basic one that can run our single binary:
pkg/hello_fuchsia.cml
{
include: [ "syslog/client.shard.cml" ],
program: {
runner: "elf",
binary: "bin/hello_fuchsia",
},
}
Current directory structure
hello_fuchsia/
┗━ pkg/
┣━ meta/
┃ ┗━ package
┣━ hello_fuchsia.manifest
┗━ hello_fuchsia.cml
Now we can compile that CML into a component manifest:
${SDK_PATH}/tools/${ARCH}/cmc compile \
pkg/hello_fuchsia.cml \
--includepath ${SDK_PATH}/pkg \
-o pkg/meta/hello_fuchsia.cm
Note: --includepath
tells the compiler where to look for include
s from our CML.
In our case, we're only using syslog/client.shard.cml
.
Current directory structure
hello_fuchsia/
┗━ pkg/
┣━ meta/
┃ ┣━ package
┃ ┗━ hello_fuchsia.cm
┣━ hello_fuchsia.manifest
┗━ hello_fuchsia.cml
Building a Fuchsia package
Next, we'll build a package manifest as defined by our manifest:
${SDK_PATH}/tools/${ARCH}/ffx package build \
--api-level $(${SDK_PATH}/tools/${ARCH}/ffx --machine json version | jq .tool_version.api_level) \
--out pkg/hello_fuchsia_manifest \
pkg/hello_fuchsia.manifest
This will produce pkg/hello_fuchsia_manifest/
which is a package manifest we can
publish directly to a repository.
Current directory structure
hello_fuchsia/
┗━ pkg/
┣━ meta/
┃ ┣━ package
┃ ┗━ hello_fuchsia.cm
┣━ hello_fuchsia_manifest/
┃ ┗━ ...
┣━ hello_fuchsia.manifest
┣━ hello_fuchsia.cml
┗━ hello_fuchsia_package_manifest
We are now ready to publish the package.
Publishing a Fuchsia package
With our package and component manifests setup, we can now publish our package. The first step will be to create a Fuchsia package repository to publish to.
Creating a Fuchsia package repository
We can set up our repository with:
${SDK_PATH}/tools/${ARCH}/ffx repository create pkg/repo
Current directory structure
hello_fuchsia/
┗━ pkg/
┣━ meta/
┃ ┣━ package
┃ ┗━ hello_fuchsia.cm
┣━ hello_fuchsia_manifest/
┃ ┗━ ...
┣━ repo/
┃ ┗━ ...
┣━ hello_fuchsia.manifest
┣━ hello_fuchsia.cml
┗━ hello_fuchsia_package_manifest
Publishing Fuchsia package to repository
We can publish our new package to that repository with:
${SDK_PATH}/tools/${ARCH}/ffx repository publish \
--package pkg/hello_fuchsia_package_manifest \
pkg/repo
Then we can add the repository to ffx
's package server as hello-fuchsia
using:
${SDK_PATH}/tools/${ARCH}/ffx repository add-from-pm \
--repository hello-fuchsia \
pkg/repo
Running a Fuchsia component on an emulator
At this point, we are ready to run our Fuchsia component. For reference, our final directory structure will look like:
Final directory structure
hello_fuchsia/
┣━ src/ (if using rustc)
┃ ┗━ hello_fuchsia.rs ...
┣━ bin/ ...
┃ ┗━ hello_fuchsia ...
┣━ src/ (if using cargo)
┃ ┗━ main.rs ...
┣━ target/ ...
┃ ┗━ x86_64-unknown-fuchsia/ ...
┃ ┗━ debug/ ...
┃ ┗━ hello_fuchsia ...
┗━ pkg/
┣━ meta/
┃ ┣━ package
┃ ┗━ hello_fuchsia.cm
┣━ hello_fuchsia_manifest/
┃ ┗━ ...
┣━ repo/
┃ ┗━ ...
┣━ hello_fuchsia.manifest
┣━ hello_fuchsia.cml
┗━ hello_fuchsia_package_manifest
Starting the Fuchsia emulator
Start a Fuchsia emulator in a new terminal using:
${SDK_PATH}/tools/${ARCH}/ffx product-bundle get workstation_eng.qemu-${ARCH}
${SDK_PATH}/tools/${ARCH}/ffx emu start workstation_eng.qemu-${ARCH} --headless
Watching emulator logs
Once the emulator is running, open a separate terminal to watch the emulator logs:
In separate terminal
${SDK_PATH}/tools/${ARCH}/ffx log \
--since now
Serving a Fuchsia package
Now, start a package repository server to serve our package to the emulator:
${SDK_PATH}/tools/${ARCH}/ffx repository server start
Once the repository server is up and running, register it with the target Fuchsia system running in the emulator:
${SDK_PATH}/tools/${ARCH}/ffx target repository register \
--repository hello-fuchsia
Running a Fuchsia component
Finally, run the component:
${SDK_PATH}/tools/${ARCH}/ffx component run \
/core/ffx-laboratory:hello_fuchsia \
fuchsia-pkg://hello-fuchsia/hello_fuchsia_manifest#meta/hello_fuchsia.cm
On reruns of the component, the --recreate
argument may also need to be
passed.
${SDK_PATH}/tools/${ARCH}/ffx component run \
--recreate \
/core/ffx-laboratory:hello_fuchsia \
fuchsia-pkg://hello-fuchsia/hello_fuchsia_manifest#meta/hello_fuchsia.cm
.gitignore
extensions
Optionally, we can create/extend our .gitignore
file to ignore files and
directories that are not helpful to track:
pkg/repo
pkg/meta/hello_fuchsia.cm
pkg/hello_fuchsia_manifest
pkg/hello_fuchsia_package_manifest
Testing
Running unit tests
Tests can be run in the same way as a regular binary.
-
If using
cargo
, you can simply passtest --no-run
to thecargo
invocation and then repackage and rerun the Fuchsia package. From our previous example, this would look likecargo test --target x86_64-unknown-fuchsia --no-run
, and moving the executable binary path found from the lineExecutable unittests src/main.rs (target/x86_64-unknown-fuchsia/debug/deps/hello_fuchsia-<HASH>)
intopkg/hello_fuchsia.manifest
. -
If using the compiled
rustc
, you can simply pass--test
to therustc
invocation and then repackage and rerun the Fuchsia package.
The test harness will run the applicable unit tests.
Often when testing, you may want to pass additional command line arguments to your binary. Additional arguments can be set in the component manifest:
pkg/hello_fuchsia.cml
{
include: [ "syslog/client.shard.cml" ],
program: {
runner: "elf",
binary: "bin/hello_fuchsia",
args: ["it_works"],
},
}
This will pass the argument it_works
to the binary, filtering the tests to
only those tests that match the pattern. There are many more configuration
options available in CML including environment variables. More documentation is
available on the Fuchsia devsite.
Running the compiler test suite
The commands in this section assume that they are being run from inside your local Rust source checkout:
cd ${RUST_SRC_PATH}
To run the Rust test suite on an emulated Fuchsia device, you'll also need to download a copy of the Fuchsia SDK. The current minimum supported SDK version is 20.20240412.3.1.
Fuchsia's test runner interacts with the Fuchsia emulator and is located at
src/ci/docker/scripts/fuchsia-test-runner.py
. First, add the following
variables to your existing config-env.sh
:
# TEST_TOOLCHAIN_TMP_DIR can point anywhere, but it:
# - must be less than 108 characters, otherwise qemu can't handle the path
# - must be consistent across calls to this file (don't use `mktemp -d` here)
export TEST_TOOLCHAIN_TMP_DIR="/tmp/rust-tmp"
# Keep existing contents of `config-env.sh` from earlier, including SDK_PATH
We can then use the script to start our test environment with:
( \
source config-env.sh && \
src/ci/docker/scripts/fuchsia-test-runner.py start \
--rust-build ${RUST_SRC_PATH}/build \
--sdk ${SDK_PATH} \
--target {x86_64-unknown-fuchsia|aarch64-unknown-fuchsia} \
--verbose \
)
Where ${RUST_SRC_PATH}/build
is the build-dir
set in config.toml
.
Once our environment is started, we can run our tests using x.py
as usual. The
test runner script will run the compiled tests on an emulated Fuchsia device. To
run the full tests/ui
test suite:
( \
source config-env.sh && \
./x.py \
--config config.toml \
--stage=2 \
test tests/ui \
--target x86_64-unknown-fuchsia \
--run=always \
--test-args --target-rustcflags \
--test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/sysroot/lib \
--test-args --target-rustcflags \
--test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/lib \
--test-args --target-rustcflags \
--test-args -Clink-arg=--undefined-version \
--test-args --remote-test-client \
--test-args src/ci/docker/scripts/fuchsia-test-runner.py \
)
By default, x.py
compiles test binaries with panic=unwind
. If you built your
Rust toolchain with -Cpanic=abort
, you need to tell x.py
to compile test
binaries with panic=abort
as well:
--test-args --target-rustcflags \
--test-args -Cpanic=abort \
--test-args --target-rustcflags \
--test-args -Zpanic_abort_tests \
When finished testing, the test runner can be used to stop the test environment:
src/ci/docker/scripts/fuchsia-test-runner.py stop
Debugging
zxdb
Debugging components running on a Fuchsia emulator can be done using the
console-mode debugger: zxdb. We will demonstrate attaching necessary symbol
paths to debug our hello-fuchsia
component.
Attaching zxdb
In a separate terminal, issue the following command from our hello_fuchsia
directory to launch zxdb
:
In separate terminal
${SDK_PATH}/tools/${ARCH}/ffx debug connect -- \
--symbol-path target/x86_64-unknown-fuchsia/debug
--symbol-path
gets required symbol paths, which are necessary for stepping through your program.
The "displaying source code in zxdb
"
section describes how you can display Rust and/or Fuchsia source code in your
debugging session.
Using zxdb
Once launched, you will be presented with the window:
Connecting (use "disconnect" to cancel)...
Connected successfully.
👉 To get started, try "status" or "help".
[zxdb]
To attach to our program, we can run:
[zxdb] attach hello_fuchsia
Expected output
Waiting for process matching "hello_fuchsia".
Type "filter" to see the current filters.
Next, we can create a breakpoint at main using "b main":
[zxdb] b main
Expected output
Created Breakpoint 1 @ main
Finally, we can re-run the "hello_fuchsia" component from our original terminal:
${SDK_PATH}/tools/${ARCH}/ffx component run \
--recreate \
fuchsia-pkg://hello-fuchsia/hello_fuchsia_manifest#meta/hello_fuchsia.cm
Once our component is running, our zxdb
window will stop execution
in our main as desired:
Expected output
Breakpoint 1 now matching 1 addrs for main
🛑 on bp 1 hello_fuchsia::main() • main.rs:2
1 fn main() {
▶ 2 println!("Hello Fuchsia!");
3 }
4
[zxdb]
zxdb
has similar commands to other debuggers like gdb.
To list the available commands, run "help" in the
zxdb
window or visit the zxdb documentation.
[zxdb] help
Expected output
Help!
Type "help <command>" for command-specific help.
Other help topics (see "help <topic>")
...
Displaying source code in zxdb
By default, the debugger will not be able to display
source code while debugging. For our user code, we displayed
source code by pointing our debugger to our debug binary via
the --symbol-path
arg. To display library source code in
the debugger, you must provide paths to the source using
--build-dir
. For example, to display the Rust and Fuchsia
source code:
${SDK_PATH}/tools/${ARCH}/ffx debug connect -- \
--symbol-path target/x86_64-unknown-fuchsia/debug \
--build-dir ${RUST_SRC_PATH}/rust \
--build-dir ${FUCHSIA_SRC_PATH}/fuchsia/out/default
--build-dir
links against source code paths, which are not strictly necessary for debugging, but is a nice-to-have for displaying source code inzxdb
.
Linking to a Fuchsia checkout can help with debugging Fuchsia libraries, such as fdio.
Debugging the compiler test suite
Debugging the compiler test suite requires some special configuration:
First, we have to properly configure zxdb so it will be able to find debug symbols and source information for our test. The test runner can do this for us with:
src/ci/docker/scripts/fuchsia-test-runner.py debug \
--rust-src ${RUST_SRC_PATH} \
--fuchsia-src ${FUCHSIA_SRC_PATH} \
--test ${TEST}
where ${TEST}
is relative to Rust's tests
directory (e.g. ui/abi/...
).
This will start a zxdb session that is properly configured for the specific test
being run. All three arguments are optional, so you can omit --fuchsia-src
if
you don't have it downloaded. Now is a good time to set any desired breakpoints,
like b main
.
Next, we have to tell x.py
not to optimize or strip debug symbols from our
test suite binaries. We can do this by passing some new arguments to rustc
through our x.py
invocation. The full invocation is:
( \
source config-env.sh && \
./x.py \
--config config.toml \
--stage=2 \
test tests/${TEST} \
--target x86_64-unknown-fuchsia \
--run=always \
--test-args --target-rustcflags \
--test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/sysroot/lib \
--test-args --target-rustcflags \
--test-args -Lnative=${SDK_PATH}/arch/{x64|arm64}/lib \
--test-args --target-rustcflags \
--test-args -Clink-arg=--undefined-version \
--test-args --target-rustcflags \
--test-args -Cdebuginfo=2 \
--test-args --target-rustcflags \
--test-args -Copt-level=0 \
--test-args --target-rustcflags \
--test-args -Cstrip=none \
--test-args --remote-test-client \
--test-args src/ci/docker/scripts/fuchsia-test-runner.py \
)
If you built your Rust toolchain with panic=abort
, make sure to include the
previous flags so your test binaries are also compiled with panic=abort
.
Upon running this command, the test suite binary will be run and zxdb will attach and load any relevant debug symbols.
aarch64-unknown-trusty
and armv7-unknown-trusty
Tier: 3
Trusty is a secure Operating System that provides a Trusted Execution Environment (TEE) for Android.
Target maintainers
- Nicole LeGare (@randomPoison)
- Stephen Crane (@rinon)
- As a fallback trusty-dev-team@google.com can be contacted
Requirements
These targets are cross-compiled. They have no special requirements for the host.
Support for the standard library is work-in-progress. It is expected that they will support alloc with the default allocator, and partially support std.
Trusty uses the ELF file format.
Building the target
The targets can be built by enabling them for a rustc
build, for example:
[build]
build-stage = 1
target = ["aarch64-unknown-trusty", "armv7-unknown-trusty"]
Building Rust programs
There is currently no supported way to build a Trusty app with Cargo. You can follow the Trusty build instructions to build the Trusty kernel along with any Rust apps that are setup in the project.
Testing
See the Trusty build instructions for information on how to build Rust code within the main Trusty project. The main project also includes infrastructure for testing Rust applications within a QEMU emulator.
Cross-compilation toolchains and C code
See the Trusty build instructions for information on how C code is built within Trusty.
*-kmc-solid_*
Tier: 3
SOLID embedded development platform by Kyoto Microcomputer Co., Ltd.
The target names follow this format: $ARCH-kmc-solid_$KERNEL-$ABI
, where $ARCH
specifies the target processor architecture, $KERNEL
the base kernel, and $ABI
the target ABI (optional). The following targets are currently defined:
Target name | target_arch | target_vendor | target_os |
---|---|---|---|
aarch64-kmc-solid_asp3 | aarch64 | kmc | solid_asp3 |
armv7a-kmc-solid_asp3-eabi | arm | kmc | solid_asp3 |
armv7a-kmc-solid_asp3-eabihf | arm | kmc | solid_asp3 |
Designated Developers
Requirements
This target is cross-compiled. A platform-provided C compiler toolchain is required, though it can be substituted by GNU Arm Embedded Toolchain for the purpose of building Rust and functional binaries.
Building
The target can be built by enabling it for a rustc
build.
[build]
target = ["aarch64-kmc-solid_asp3"]
Make sure aarch64-kmc-elf-gcc
is included in $PATH
. Alternatively, you can use GNU Arm Embedded Toolchain by adding the following to config.toml
:
[target.aarch64-kmc-solid_asp3]
cc = "arm-none-eabi-gcc"
Cross-compilation
This target can be cross-compiled from any hosts.
Testing
Currently there is no support to run the rustc test suite for this target.
Building Rust programs
Building executables is not supported yet.
If rustc
has support for that target and the library artifacts are available, then Rust static libraries can be built for that target:
$ rustc --target aarch64-kmc-solid_asp3 your-code.rs --crate-type staticlib
$ ls libyour_code.a
On Rust Nightly it's possible to build without the target artifacts available:
cargo build -Z build-std --target aarch64-kmc-solid_asp3
csky-unknown-linux-gnuabiv2
Tier: 3
This target supports C-SKY CPUs with abi
v2 and glibc
.
target | std | host | notes |
---|---|---|---|
csky-unknown-linux-gnuabiv2 | ✓ | C-SKY abiv2 Linux (little endian) | |
csky-unknown-linux-gnuabiv2hf | ✓ | C-SKY abiv2 Linux, hardfloat (little endian) |
Reference:
other links:
- https://c-sky.github.io/
- https://gitlab.com/c-sky/
Target maintainers
Requirements
Building the target
Get a C toolchain
Compiling rust for this target has been tested on x86_64
linux hosts. Other host types have not been tested, but may work, if you can find a suitable cross compilation toolchain for them.
If you don't already have a suitable toolchain, you can download from here, and unpack it into a directory.
Configure rust
The target can be built by enabling it for a rustc
build, by placing the following in config.toml
:
[build]
target = ["x86_64-unknown-linux-gnu", "csky-unknown-linux-gnuabiv2", "csky-unknown-linux-gnuabiv2hf"]
stage = 2
[target.csky-unknown-linux-gnuabiv2]
# ADJUST THIS PATH TO POINT AT YOUR TOOLCHAIN
cc = "${TOOLCHAIN_PATH}/bin/csky-linux-gnuabiv2-gcc"
[target.csky-unknown-linux-gnuabiv2hf]
# ADJUST THIS PATH TO POINT AT YOUR TOOLCHAIN
cc = "${TOOLCHAIN_PATH}/bin/csky-linux-gnuabiv2-gcc"
Build
# in rust dir
./x.py build --stage 2
Building and Running Rust programs
To test cross-compiled binaries on a x86_64
system, you can use the qemu-cskyv2
. This avoids having a full emulated ARM system by doing dynamic binary translation and dynamic system call translation. It lets you run CSKY programs directly on your x86_64
kernel. It's very convenient!
To use:
- Install
qemu-cskyv2
(If you don't already have a qemu, you can download from here, and unpack it into a directory.) - Link your built toolchain via:
rustup toolchain link stage2 ${RUST}/build/x86_64-unknown-linux-gnu/stage2
- Create a test program
cargo new hello_world
cd hello_world
- Build and run
CARGO_TARGET_CSKY_UNKNOWN_LINUX_GNUABIV2_RUNNER=${QEMU_PATH}/bin/qemu-cskyv2 -L ${TOOLCHAIN_PATH}/csky-linux-gnuabiv2/libc \
CARGO_TARGET_CSKY_UNKNOWN_LINUX_GNUABIV2_LINKER=${TOOLCHAIN_PATH}/bin/csky-linux-gnuabiv2-gcc \
RUSTFLAGS="-C target-features=+crt-static" \
cargo +stage2 run --target csky-unknown-linux-gnuabiv2
Attention: The dynamic-linked program may nor be run by qemu-cskyv2
but can be run on the target.
hexagon-unknown-linux-musl
Tier: 3
Target for cross-compiling Linux user-mode applications targeting the Hexagon DSP architecture.
Target | Descriptions |
---|---|
hexagon-unknown-linux-musl | Hexagon 32-bit Linux |
Target maintainers
- Brian Cain,
bcain@quicinc.com
Requirements
The target is cross-compiled. This target supports std
. By default, code
generated with this target should run on Hexagon DSP hardware.
-Ctarget-cpu=hexagonv73
adds support for instructions defined up to Hexagon V73.
Binaries can be run using QEMU user emulation. On Debian-based systems, it should be
sufficient to install the package qemu-user-static
to be able to run simple static
binaries:
# apt install qemu-user-static
# qemu-hexagon-static ./hello
In order to build linux programs with Rust, you will require a linker capable
of targeting hexagon. You can use clang
/lld
from the hexagon toolchain
using exclusively public open source repos.
Also included in that toolchain is the C library that can be used when creating dynamically linked executables.
# /opt/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/bin/qemu-hexagon -L /opt/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr/ ./hello
Building the target
Because it is Tier 3, rust does not yet ship pre-compiled artifacts for this target.
Therefore, you can build Rust with support for the target by adding it to the
target list in config.toml
, a sample configuration is shown below.
[build]
target = [ "hexagon-unknown-linux-musl"]
[target.hexagon-unknown-linux-musl]
cc = "hexagon-unknown-linux-musl-clang"
cxx = "hexagon-unknown-linux-musl-clang++"
linker = "hexagon-unknown-linux-musl-clang"
ar = "hexagon-unknown-linux-musl-ar"
ranlib = "hexagon-unknown-linux-musl-ranlib"
musl-root = "/opt/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr"
llvm-libunwind = 'in-tree'
qemu-rootfs = "/opt/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr"
Testing
Currently there is no support to run the rustc test suite for this target.
Building Rust programs
Download and install the hexagon open source toolchain from https://github.com/quic/toolchain_for_hexagon/releases
The following .cargo/config
is needed inside any project directory to build
for the Hexagon Linux target:
[build]
target = "hexagon-unknown-linux-musl"
[target.hexagon-unknown-linux-musl]
linker = "hexagon-unknown-linux-musl-clang"
ar = "hexagon-unknown-linux-musl-ar"
runner = "qemu-hexagon -L /opt/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr"
Edit the "runner" in .cargo/config
to point to the path to your toolchain's
C library.
...
runner = "qemu-hexagon -L /path/to/my/inst/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/target/hexagon-unknown-linux-musl/usr"
...
Build/run your rust program with qemu-hexagon
in your PATH
:
export PATH=/path/to/my/inst/clang+llvm-18.1.0-cross-hexagon-unknown-linux-musl/x86_64-linux-gnu/bin/:$PATH
cargo run -Zbuild-std -Zbuild-std-features=llvm-libunwind
hexagon-unknown-none-elf
Tier: 3
Rust for baremetal Hexagon DSPs.
Target | Descriptions |
---|---|
hexagon-unknown-none-elf | Hexagon 32-bit (freestanding, hardfloat) |
Target maintainers
- Brian Cain,
bcain@quicinc.com
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
By default, code generated with this target should run on Hexagon DSP hardware.
-Ctarget-cpu=hexagonv73
adds support for instructions defined up to Hexagon V73.
Functions marked extern "C"
use the Hexagon architecture calling convention.
This target generates PIC ELF binaries.
Building the target
You can build Rust with support for the target by adding it to the target
list in config.toml
:
[build]
build-stage = 1
host = ["<target for your host>"]
target = ["<target for your host>", "hexagon-unknown-none-elf"]
[target.hexagon-unknown-none-elf]
cc = "hexagon-unknown-none-elf-clang"
cxx = "hexagon-unknown-none-elf-clang++"
linker = "hexagon-unknown-none-elf-clang"
ranlib = "hexagon-unknown-none-elf-ranlib"
ar = "hexagon-unknown-none-elf-ar"
llvm-libunwind = 'in-tree'
Replace <target for your host>
with x86_64-unknown-linux-gnu
or whatever
else is appropriate for your host machine.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
Testing
Since hexagon-unknown-none-elf
supports a variety of different environments and
does not support std
, this target does not support running the Rust test suite.
Cross-compilation toolchains and C code
This target has been tested using qemu-system-hexagon
.
A common use case for hexagon-unknown-none-elf
is building libraries that
link against C code and can be used in emulation or on a device with a
Hexagon DSP.
The Hexagon SDK has libraries which are useful to link against when running on a device.
Standalone OS
The script below will build an executable against "hexagon standalone OS" which is suitable for emulation or bare-metal on-device testing.
First, run cargo new --bin demo1_hexagon
then add the source below as
src/main.rs
. This program demonstrates the console output via semihosting.
#![no_std]
#![no_main]
extern "C" {
fn putchar(ch: i32);
fn _exit(code: i32) -> !;
}
#[no_mangle]
extern "C" fn main() -> i32 {
let message = "Hello, this is Rust!";
for b in message.bytes() {
unsafe {
putchar(b as i32);
}
}
0
}
#[panic_handler]
fn panic(_panic: &core::panic::PanicInfo) -> ! {
unsafe {
_exit(1);
}
}
Next, save the script below as build.sh
and edit it to suit your
environment.
hex_toolchain
below refers to the hexagon toolchain using exclusively public open source repos.cc
below refers to clang. You can useclang
from your distribution, as long as it's at leastclang-17
. Or you can usehexagon-unknown-none-elf-clang
from one of the hexagon open source toolchain releases.
# Hexagon SDK, required for target libraries:
hex_sdk_root=/local/mnt/workspace/Qualcomm/Hexagon_SDK/5.3.0.0
hex_sdk_toolchain=${hex_sdk_root}/tools/HEXAGON_Tools/8.6.06
sdk_libs=${hex_sdk_toolchain}/Tools/target/hexagon/lib
q6_arch=v65
g0_lib_path=${sdk_libs}/${q6_arch}/G0
pic_lib_path=${sdk_libs}/${q6_arch}/G0/pic
build_cfg=release
cargo build --target=hexagon-unknown-none-elf -Zbuild-std --release
# Builds an executable against "hexagon standalone OS" suitable for emulation:
${cc} --target=hexagon-unknown-none-elf -o testit \
-fuse-ld=lld \
-m${q6_arch} \
-nodefaultlibs \
-nostartfiles \
${g0_lib_path}/crt0_standalone.o \
${g0_lib_path}/crt0.o \
${g0_lib_path}/init.o \
-L${sdk_libs}/${q6_arch}/ \
-L${sdk_libs}/ \
wrap.c \
target/hexagon-unknown-none-elf/${build_cfg}/libdemo1_hexagon.rlib \
target/hexagon-unknown-none-elf/${build_cfg}/deps/libcore-*.rlib \
target/hexagon-unknown-none-elf/${build_cfg}/deps/libcompiler_builtins-*.rlib \
-Wl,--start-group \
-Wl,--defsym,_SDA_BASE_=0,--defsym,__sbss_start=0,--defsym,__sbss_end=0 \
${g0_lib_path}/libstandalone.a \
${g0_lib_path}/libc.a \
-lgcc \
-lc_eh \
-Wl,--end-group \
${g0_lib_path}/fini.o \
${hex_toolchain}/x86_64-linux-gnu/bin/qemu-system-hexagon -monitor none -display none -kernel ./testit
QuRT OS
First, run cargo new --lib demo2_hexagon
then add the source below as
src/lib.rs
. This program demonstrates inline assembly and console output
via semihosting.
#![no_std]
#![no_main]
#![feature(lang_items)]
#![feature(asm_experimental_arch)]
use core::arch::asm;
extern "C" {
fn putchar(ch: i32);
fn _exit(code: i32) -> !;
}
fn hexagon_specific() {
let mut buffer = [0_u8; 128];
unsafe {
let mut x = &buffer;
asm!(
"{{\n\t",
" v0=vmem({addr}+#0)\n\t",
" {tmp} = and({tmp}, #1)\n\t",
"}}\n\t",
addr = in(reg) x,
tmp = out(reg) _,
);
}
}
#[no_mangle]
extern "C" fn hello() -> i32 {
let message = "Hello, this is Rust!\n";
for b in message.bytes() {
unsafe {
putchar(b as i32);
}
}
hexagon_specific();
0
}
#[panic_handler]
fn panic(_panic: &core::panic::PanicInfo) -> ! {
unsafe {
_exit(1);
}
}
#[lang = "eh_personality"]
fn rust_eh_personality() {}
Next, create a C program as an entry point, save the content below as
wrap.c
:
int hello();
int main() {
hello();
}
Then, save the script below as build.sh
and edit it to suit your
environment. The script below will build a shared object against the QuRT
RTOS which is suitable for emulation or on-device testing when loaded via
the fastrpc-shell.
# Hexagon SDK, required for target libraries:
hex_sdk_root=/local/mnt/workspace/Qualcomm/Hexagon_SDK/5.3.0.0
hex_sdk_toolchain=${hex_sdk_root}/tools/HEXAGON_Tools/8.6.06
sdk_libs=${hex_sdk_toolchain}/Tools/target/hexagon/lib
q6_arch=v65
g0_lib_path=${sdk_libs}/${q6_arch}/G0
pic_lib_path=${sdk_libs}/${q6_arch}/G0/pic
runelf=${hex_sdk_root}/rtos/qurt/computev65/sdksim_bin/runelf.pbn
rmohs=${hex_sdk_root}/libs/run_main_on_hexagon/ship/hexagon_toolv86_${q6_arch}/run_main_on_hexagon_sim
# Builds a library suitable for loading into "run_main_on_hexagon_sim" for
# emulation or frpc shell on real target:
${cc} --target=hexagon-unknown-none-elf -o testit.so \
-fuse-ld=lld \
-fPIC -shared \
-nostdlib \
-Wl,-Bsymbolic \
-Wl,--wrap=malloc \
-Wl,--wrap=calloc \
-Wl,--wrap=free \
-Wl,--wrap=realloc \
-Wl,--wrap=memalign \
-m${q6_arch} \
wrap.c \
target/hexagon-unknown-none-elf/${build_cfg}/libdemo2_hexagon.rlib \
target/hexagon-unknown-none-elf/${build_cfg}/deps/libcore-*.rlib \
target/hexagon-unknown-none-elf/${build_cfg}/deps/libcompiler_builtins-*.rlib \
-Wl,-soname=testit \
${pic_lib_path}/libc.so
# -Bsymbolic above for memory alloc funcs is necessary to access the heap on
# target, but otherwise not required.
# multi-stage loader: runelf => run_main_on_hexagon_sim => testit.so{`main`}
${hex_toolchain}/x86_64-linux-gnu/bin/qemu-system-hexagon \
-monitor none \
-display none \
-kernel ${runelf} \
-append "${rmohs} -- ./testit.so"
loongarch*-unknown-linux-*
Tier: 2 (with Host Tools)
LoongArch Linux targets. LoongArch is a RISC ISA developed by Loongson Technology Corporation Limited.
Target | Description |
---|---|
loongarch64-unknown-linux-gnu | LoongArch64 Linux, LP64D ABI (kernel 5.19, glibc 2.36) |
loongarch64-unknown-linux-musl | LoongArch64 Linux, LP64D ABI (kernel 5.19, musl 1.2.5) |
These support both native and cross builds, and have full support for std
.
Reference material:
Target maintainers
- WANG Rui
wangrui@loongson.cn
- ZHAI Xiang
zhaixiang@loongson.cn
- ZHAI Xiaojuan
zhaixiaojuan@loongson.cn
- WANG Xuerui
git@xen0n.name
Requirements
OS Version
The minimum supported Linux version is 5.19.
Some Linux distributions, mostly commercial ones, may provide forked Linux
kernels that has a version number less than 5.19 for their LoongArch ports.
Such kernels may still get patched to be compatible with the upstream Linux
5.19 UAPI, therefore supporting the targets described in this document, but
this is not always the case. The rustup
installer contains a check for this,
and will abort if incompatibility is detected.
Host toolchain
The targets require a reasonably up-to-date LoongArch toolchain on the host. Currently the following components are used by the Rust CI to build the target, and the versions can be seen as the minimum requirement:
- GNU Binutils 2.40
- GCC 13.x
- glibc 2.36
- linux-headers 5.19
Of these, glibc and linux-headers are at their respective earliest versions with mainline LoongArch support, so it is impossible to use older versions of these. Older versions of Binutils and GCC will not work either, due to lack of support for newer LoongArch ELF relocation types, among other features.
Recent LLVM/Clang toolchains may be able to build the targets, but are not currently being actively tested.
Building
These targets are distributed through rustup
, and otherwise require no
special configuration.
If you need to build your own Rust for some reason though, the targets can be
simply enabled in config.toml
. For example:
[build]
target = ["loongarch64-unknown-linux-gnu"]
Make sure the LoongArch toolchain binaries are reachable from $PATH
.
Alternatively, you can explicitly configure the paths in config.toml
:
[target.loongarch64-unknown-linux-gnu]
# Adjust the paths to point at your toolchain
# Suppose the toolchain is placed at /TOOLCHAIN_PATH, and the cross prefix is
# "loongarch64-unknown-linux-gnu-":
cc = "/TOOLCHAIN_PATH/bin/loongarch64-unknown-linux-gnu-gcc"
cxx = "/TOOLCHAIN_PATH/bin/loongarch64-unknown-linux-gnu-g++"
ar = "/TOOLCHAIN_PATH/bin/loongarch64-unknown-linux-gnu-ar"
ranlib = "/TOOLCHAIN_PATH/bin/loongarch64-unknown-linux-gnu-ranlib"
linker = "/TOOLCHAIN_PATH/bin/loongarch64-unknown-linux-gnu-gcc"
Cross-compilation
This target can be cross-compiled on a x86_64-unknown-linux-gnu
host.
Other hosts are also likely to work, but not actively tested.
You can test the cross build directly on the host, thanks to QEMU linux-user emulation. An example is given below:
# Suppose the cross toolchain is placed at $TOOLCHAIN_PATH, with a cross prefix
# of "loongarch64-unknown-linux-gnu-".
export CC_loongarch64_unknown_linux_gnu="$TOOLCHAIN_PATH"/bin/loongarch64-unknown-linux-gnu-gcc
export CXX_loongarch64_unknown_linux_gnu="$TOOLCHAIN_PATH"/bin/loongarch64-unknown-linux-gnu-g++
export AR_loongarch64_unknown_linux_gnu="$TOOLCHAIN_PATH"/bin/loongarch64-unknown-linux-gnu-gcc-ar
export CARGO_TARGET_LOONGARCH64_UNKNOWN_LINUX_GNU_LINKER="$TOOLCHAIN_PATH"/bin/loongarch64-unknown-linux-gnu-gcc
# Point qemu-loongarch64 to the LoongArch sysroot.
# Suppose the sysroot is located at "sysroot" below the toolchain root:
export CARGO_TARGET_LOONGARCH64_UNKNOWN_LINUX_GNU_RUNNER="qemu-loongarch64 -L $TOOLCHAIN_PATH/sysroot"
# Or alternatively, if binfmt_misc is set up for running LoongArch binaries
# transparently:
export QEMU_LD_PREFIX="$TOOLCHAIN_PATH"/sysroot
cargo run --target loongarch64-unknown-linux-gnu --release
Testing
There are no special requirements for testing and running the targets. For testing cross builds on the host, please refer to the "Cross-compilation" section above.
Building Rust programs
As the targets are available through rustup
, it is very easy to build Rust
programs for these targets: same as with other architectures.
Note that you will need a LoongArch C/C++ toolchain for linking, or if you want
to compile C code along with Rust (such as for Rust crates with C dependencies).
rustup target add loongarch64-unknown-linux-gnu
cargo build --target loongarch64-unknown-linux-gnu
Availability of pre-built artifacts through rustup
are as follows:
loongarch64-unknown-linux-gnu
: since Rust 1.71;loongarch64-unknown-linux-musl
: since Rust 1.81.
loongarch*-unknown-none*
Tier: 2
Freestanding/bare-metal LoongArch64 binaries in ELF format: firmware, kernels, etc.
Target | Description |
---|---|
loongarch64-unknown-none | LoongArch 64-bit, LP64D ABI (freestanding, hard-float) |
loongarch64-unknown-none-softfloat | LoongArch 64-bit, LP64S ABI (freestanding, soft-float) |
Target maintainers
- WANG Rui
wangrui@loongson.cn
- WANG Xuerui
git@xen0n.name
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
The *-softfloat
target does not assume existence of FPU or any other LoongArch
ISA extension, and does not make use of any non-GPR register.
This allows the generated code to run in environments, such as kernels, which
may need to avoid the use of such registers or which may have special considerations
about the use of such registers (e.g. saving and restoring them to avoid breaking
userspace code using the same registers). You can change code generation to use
additional CPU features via the -C target-feature=
codegen options to rustc, or
via the #[target_feature]
mechanism within Rust code.
By default, code generated with the soft-float target should run on any LoongArch64 hardware, with the hard-float target additionally requiring an FPU; enabling additional target features may raise this baseline.
Code generated with the targets will use the small
code model by default.
You can change this using the -C code-model=
option to rustc.
On loongarch64-unknown-none*
, extern "C"
uses the architecture's standard calling convention.
The targets generate binaries in the ELF format. Any alternate formats or special considerations for binary layout will require linker options or linker scripts.
Building the target
You can build Rust with support for the targets by adding them to the target
list in config.toml
:
[build]
build-stage = 1
target = [
"loongarch64-unknown-none",
"loongarch64-unknown-none-softfloat",
]
Testing
As the targets support a variety of different environments and do not support
std
, they do not support running the Rust test suite.
Building Rust programs
Starting with Rust 1.74, precompiled artifacts are provided via rustup
:
# install cross-compile toolchain
rustup target add loongarch64-unknown-none
# target flag may be used with any cargo or rustc command
cargo build --target loongarch64-unknown-none
Cross-compilation toolchains and C code
For cross builds, you will need an appropriate LoongArch C/C++ toolchain for linking, or if you want to compile C code along with Rust (such as for Rust crates with C dependencies).
Rust may be able to use an loongarch64-unknown-linux-gnu-
toolchain with
appropriate standalone flags to build for this toolchain (depending on the assumptions
of that toolchain, see below), or you may wish to use a separate
loongarch64-unknown-none
toolchain.
On some LoongArch hosts that use ELF binaries, you may be able to use the host C toolchain, if it does not introduce assumptions about the host environment that don't match the expectations of a standalone environment. Otherwise, you may need a separate toolchain for standalone/freestanding development, just as when cross-compiling from a non-LoongArch platform.
m68k-unknown-linux-gnu
Tier: 3
Motorola 680x0 Linux
Designated Developers
Requirements
This target requires a Linux/m68k build environment for cross-compilation which is available on Debian and Debian-based systems, openSUSE and other distributions.
On Debian, it should be sufficient to install a g++ cross-compiler for the m68k architecture which will automatically pull in additional dependencies such as the glibc cross development package:
# apt install g++-m68k-linux-gnu
Binaries can be run using QEMU user emulation. On Debian-based systems, it should be
sufficient to install the package qemu-user-static
to be able to run simple static
binaries:
# apt install qemu-user-static
To run more complex programs, it will be necessary to set up a Debian/m68k chroot with
the help of the command debootstrap
:
# apt install debootstrap debian-ports-archive-keyring
# debootstrap --keyring=/usr/share/keyrings/debian-ports-archive-keyring.gpg --arch=m68k unstable debian-68k http://ftp.ports.debian.org/debian-ports
This chroot can then seamlessly entered using the normal chroot
command thanks to
QEMU user emulation:
# chroot /path/to/debian-68k
To get started with native builds, which are currently untested, a native Debian/m68k system can be installed either on real hardware such as 68k-based Commodore Amiga or Atari systems or emulated environments such as QEMU version 4.2 or newer or ARAnyM.
ISO images for installation are provided by the Debian Ports team and can be obtained from the Debian CD image server available at:
https://cdimage.debian.org/cdimage/ports/current
Documentation for Debian/m68k is available on the Debian Wiki at:
Support is available either through the debian-68k
mailing list:
https://lists.debian.org/debian-68k/
or the #debian-68k
IRC channel on OFTC network.
Building
The codegen for this target should be built by default. However, core and std are currently missing but are being worked on and should become available in the near future.
Cross-compilation
This target can be cross-compiled from a standard Debian or Debian-based, openSUSE or any other distribution which has a basic m68k cross-toolchain available.
Testing
Currently there is no support to run the rustc test suite for this target.
Building Rust programs
Rust programs can be built for that target:
rustc --target m68k-unknown-linux-gnu your-code.rs
Very simple programs can be run using the qemu-m68k-static
program:
$ qemu-m68k-static your-code
For more complex applications, a chroot or native (emulated) Debian/m68k system are required for testing.
mips64-openwrt-linux-musl
Tier: 3
Target maintainers
- Donald Hoskins
grommish@gmail.com
, https://github.com/Itus-Shield
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
By default, Rust code generated for this target uses -msoft-float
and is
dynamically linked.
This target generated binaries in the ELF format.
Building the target
This target is built exclusively within the OpenWrt
build system via
the rust-lang
HOST package
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for this target, you will either need to build Rust with the target enabled (see "Building the target" above).
Testing
As mips64-openwrt-linux-musl
supports a variety of different environments and does
not support std
, this target does not support running the Rust testsuite at this
time.
mipsel-sony-psx
Tier: 3
Sony PlayStation 1 (psx)
Designated Developer
Requirements
This target is cross-compiled. It has no special requirements for the host.
Building
The target can be built by enabling it for a rustc
build:
[build]
build-stage = 1
target = ["mipsel-sony-psx"]
Cross-compilation
This target can be cross-compiled from any host.
Testing
Currently there is no support to run the rustc test suite for this target.
Building Rust programs
Since it is Tier 3, rust doesn't ship pre-compiled artifacts for this target.
Just use the build-std
nightly cargo feature to build the core
and alloc
libraries:
cargo build -Zbuild-std=core,alloc --target mipsel-sony-psx
The command above generates an ELF. To generate binaries in the PSEXE format that emulators run, you can use cargo-psx:
cargo psx build
or use -Clink-arg=--oformat=binary
to produce a flat binary.
mipsisa*r6*-unknown-linux-gnu*
Tier: 3
MIPS Release 6, or simply MIPS R6, is the latest iteration of the MIPS instruction set architecture (ISA).
MIPS R6 is experimental in nature, as there is not yet real hardware. However, Qemu emulation is available and we have two Linux distros maintained for development and evaluation purposes. This documentation describes the Rust support for MIPS R6 targets under mipsisa*r6*-unknown-linux-gnu*
.
The target name follow this format: <machine>-<vendor>-<os><abi_suffix>
, where <machine>
specifies the CPU family/model, <vendor>
specifies the vendor and <os>
the operating system name. The <abi_suffix>
denotes the base ABI (32/n32/64/o64).
ABI suffix | Description |
---|---|
abi64 | Uses the 64-bit (64) ABI |
abin32 | Uses the n32 ABI |
N/A | Uses the (assumed) 32-bit (32) ABI |
Target Maintainers
- Xuan Chen henry.chen@oss.cipunited.com
- Walter Ji walter.ji@oss.cipunited.com
- Xinhui Yang cyan@oss.cipunited.com
- Lain Yang lain.yang@oss.cipunited.com
Requirements
C/C++ Toolchain
A GNU toolchain for one of the MIPS R6 target is required. AOSC OS provides working native and cross-compiling build environments. You may also supply your own a toolchain consisting of recent versions of GCC and Binutils.
Target libraries
A minimum set of libraries is required to perform dynamic linking:
- GNU glibc
- OpenSSL
- Zlib
- Linux API Headers
This set of libraries should be installed to make up minimal target sysroot.
For AOSC OS, You may install such a sysroot with the following commands:
cd /tmp
# linux+api, glibc, and file system structure are included in the toolchain.
sudo apt install gcc+cross-mips64r6el binutils+cross-mips64r6el
# Download and extract required libraries.
wget https://repo.aosc.io/debs/pool/stable/main/z/zlib_1.2.13-0_mips64r6el.deb -O zlib.deb
wget https://repo.aosc.io/debs/pool/stable/main/o/openssl_1.1.1q-1_mips64r6el.deb -O openssl.deb
# Extract them to your desired location.
for i in zlib openssl ; do
sudo dpkg-deb -vx $i.deb /var/ab/cross-root/mips64r6el
done
# Workaround a possible ld bug when using -Wl,-Bdynamic.
sudo sed -i 's|/usr|=/usr|g' /var/ab/cross-root/mips64r6el/usr/lib/libc.so
For other distros, you may build them manually.
Building
The following procedure outlines the build process for the MIPS64 R6 target with 64-bit (64) ABI (mipsisa64r6el-unknown-linux-gnuabi64
).
Prerequisite: Disable debuginfo
An LLVM bug makes rustc crash if debug or debug info generation is enabled. You need to edit config.toml
to disable this:
[rust]
debug = false
debug-info-level = 0
Prerequisite: Enable rustix's libc backend
The crate rustix
may try to link itself against MIPS R2 assembly, resulting in linkage error. To avoid this, you may force rustix
to use its fallback libc
backend by setting relevant RUSTFLAGS
:
export RUSTFLAGS="--cfg rustix_use_libc"
This will trigger warnings during build, as -D warnings
is enabled by default. Disable -D warnings
by editing config.toml
to append the following:
[rust]
deny-warnings = false
Prerequisite: Supplying OpenSSL
As a Tier 3 target, openssl_sys
lacks the vendored OpenSSL library for this target. You will need to provide a prebuilt OpenSSL library to link cargo
. Since we have a pre-configured sysroot, we can point to it directly:
export MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_OPENSSL_NO_VENDOR=y
export MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_OPENSSL_DIR="/var/ab/cross-root/mips64r6el/usr"
On Debian, you may need to provide library path and include path separately:
export MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_OPENSSL_NO_VENDOR=y
export MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_OPENSSL_LIB_DIR="/usr/lib/mipsisa64r6el-linux-gnuabi64/"
export MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_OPENSSL_INCLUDE_DIR="/usr/include"
Launching x.py
[build]
target = ["mipsisa64r6el-unknown-linux-gnuabi64"]
Make sure that mipsisa64r6el-unknown-linux-gnuabi64-gcc
is available from your executable search path ($PATH
).
Alternatively, you can specify the directories to all necessary toolchain executables in config.toml
:
[target.mipsisa64r6el-unknown-linux-gnuabi64]
# Adjust the paths below to point to your toolchain installation prefix.
cc = "/toolchain_prefix/bin/mipsisa64r6el-unknown-linux-gnuabi64-gcc"
cxx = "/toolchain_prefix/bin/mipsisa64r6el-unknown-linux-gnuabi64-g++"
ar = "/toolchain_prefix/bin/mipsisa64r6el-unknown-linux-gnuabi64-gcc-ar"
ranlib = "/toolchain_prefix/bin/mipsisa64r6el-unknown-linux-gnuabi64-ranlib"
linker = "/toolchain_prefix/bin/mipsisa64r6el-unknown-linux-gnuabi64-gcc"
Or, you can specify your cross compiler toolchain with an environment variable:
export CROSS_COMPILE="/opt/abcross/mips64r6el/bin/mipsisa64r6el-aosc-linux-gnuabi64-"
Finally, launch the build script:
./x.py build
Tips
- Avoid setting
cargo-native-static
tofalse
, as this will result in a redundant artifact error while building clippy:duplicate artifacts found when compiling a tool, this typically means that something was recompiled because a transitive dependency has different features activated than in a previous build: the following dependencies have different features: syn 2.0.8 (registry+https://github.com/rust-lang/crates.io-index) `clippy-driver` additionally enabled features {"full"} at ... `cargo` additionally enabled features {} at ... to fix this you will probably want to edit the local src/tools/rustc-workspace-hack/Cargo.toml crate, as that will update the dependency graph to ensure that these crates all share the same feature set thread 'main' panicked at 'tools should not compile multiple copies of the same crate', tool.rs:250:13 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Building Rust programs
To build Rust programs for MIPS R6 targets, for instance, the mipsisa64r6el-unknown-linux-gnuabi64
target:
cargo build --target mipsisa64r6el-unknown-linux-gnuabi64
Testing
To test a cross-compiled binary on your build system, install the Qemu user emulator that support the MIPS R6 architecture (qemu-user-mipsel
or qemu-user-mips64el
). GCC runtime libraries (libgcc_s
) for the target architecture should be present in target sysroot to run the program.
env \
CARGO_TARGET_MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_LINKER="/opt/abcross/mips64r6el/bin/mipsisa64r6el-aosc-linux-gnuabi64-gcc" \
CARGO_TARGET_MIPSISA64R6EL_UNKNOWN_LINUX_GNUABI64_RUNNER="qemu-mips64el-static -L /var/ab/cross-root/mips64r6el" \
cargo run --release \
--target mipsisa64r6el-unknown-linux-gnuabi64
Tips for building Rust programs for MIPS R6
- Until we finalize a fix, please make sure the aforementioned workarounds for
rustix
crate and LLVM are always applied. This can be achieved by setting the relevant environment variables, and editingCargo.toml
before building.
nvptx64-nvidia-cuda
Tier: 2
This is the target meant for deploying code for Nvidia® accelerators based on their CUDA platform.
Target maintainers
- Riccardo D'Ambrosio, https://github.com/RDambrosio016
- Kjetil Kjeka, https://github.com/kjetilkjeka
Designated maintainers
powerpc-unknown-openbsd
is not maintained by OpenBSD developers and there are currently no active rustc maintainers.
powerpc-unknown-linux-muslspe
Tier: 3
This target is very similar to already existing ones like powerpc_unknown_linux_musl
and powerpc_unknown_linux_gnuspe
.
This one has PowerPC SPE support for musl. Unfortunately, the last supported gcc version with PowerPC SPE is 8.4.0.
Target maintainers
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
This target generated binaries in the ELF format.
Building the target
This target was tested and used within the OpenWrt
build system for CZ.NIC Turris 1.x routers using Freescale P2020.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
Testing
This is a cross-compiled target and there is no support to run rustc test suite.
powerpc64-ibm-aix
Tier: 3
Rust for AIX operating system, currently only 64-bit PowerPC is supported.
Target maintainers
- QIU Chaofan
qiucofan@cn.ibm.com
, https://github.com/ecnelises - Kai LUO,
lkail@cn.ibm.com
, https://github.com/bzEq
Requirements
This target supports host tools, std and alloc. This target cannot be cross-compiled as for now, mainly because of the unavailability of system linker on other platforms.
Binary built for this target is expected to run on Power7 or newer CPU, and AIX 7.2 or newer version.
Binary format of this platform is XCOFF. Archive file format is 'AIX big format'.
Testing
This target supports running test suites natively, but it's not available to cross-compile and execute in emulator.
Interoperability with C code
This target supports C code. C code compiled by XL, Open XL and Clang are compatible with Rust. Typical triple of AIX on 64-bit PowerPC of these compilers are also powerpc64-ibm-aix
.
riscv32im-risc0-zkvm-elf
Tier: 3
RISC Zero's Zero Knowledge Virtual Machine (zkVM) implementing the RV32IM instruction set.
Target maintainers
- Frank Laub,
frank@risczero.com
, https://github.com/flaub - Jeremy Bruestle,
jeremy@risczero.com
, https://github.com/jbruestle - Erik Kaneda,
erik@risczero.com
, https://github.com/SchmErik
Background
This target is an execution environment to produce a proof of execution of a RISC-V ELF binary and any output that the developer of the binary wishes to display publicly. In order to do this, the target will execute the ELF to generate a receipt containing the output of the computation along with a cryptographic seal. This receipt can be verified to ensure the integrity of the computation and its result. This target is implemented as software only; it has no hardware implementation.
We have a cargo extension called cargo-risczero that allow users to generate project templates, install tools for improved user experience, build the binary using a docker environment and test programs.
Requirements
The target only supports cross compilation and no host tools. The target
supports alloc
with a default allocator and has experimental support for
std
. The target expects the binaries to be in ELF.
The target's execution environment is single threaded, non-preemptive, and does
not support any privileged instructions, nor unaligned accesses. At the time of
writing the VM has 192 MB of memory and text/data, heap, and stack need to be
with in the address range 0x400
- 0x0C000000
. The binaries themselves expect
no operating system and can be thought of as running on bare-metal. The target
does not use #[target_feature(...)]
or -C target-feature=
values.
Calling extern "C"
on the target uses the C calling convention outlined in the
RISC-V specification.
Building for the zkVM
Programs for the zkVM could be built by adding it to the target
list in
config.toml
. However, we recommend building programs in our starter template
generated by the cargo-risczero utility and the risc0-build crate. This
crate calls rustc
with -C "link-arg=-Ttext=
so that it maps the text in the
appropriate location as well as generating variables that represent the ELF and
a unique ID associated with the ELF. The starter template provides developers
with system calls that are useful to zero knowledge computing such as writing to
the public output, hashing using sha256, and multiply big integers.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above). We do not recommend using build-std
as we have
run into issues building core in the past on our starter template. An alternate
solution is to download the risc0 tool chain by running cargo risczero install
.
Testing
Note: the target is implemented as a software emulator called the zkVM and there is no hardware implementation of the target.
The most practical way to test the target program is to use our starter template
that can be generated by using the cargo risczero new
command. The template
generates a sample "host" and "guest" code. The guest code compiled to the
target (which is RV32IM) whereas the "host" code is compiled to run on the
programmer's machine running either a Linux distribution or macOS. The host
program is responsible for running the guest binary on the zkVM and retrieving
its public output.
The target currently does not support running the Rust test suite.
Cross-compilation toolchains and C code
Compatible C code can be built for this target on any compiler that has a RV32IM
target. On clang and ld.lld linker, it can be generated using the
-march=rv32im
, -mabi=ilp32
with llvm features flag features=+m
and llvm
target riscv32-unknown-none
.
riscv32imac-unknown-xous-elf
Tier: 3
Xous microkernel, message-based operating system that powers devices such as Precursor and Betrusted. The operating system is written entirely in Rust, so no additional software is required to compile programs for Xous.
Target maintainers
Requirements
Building the target itself requires a RISC-V compiler that is supported by cc-rs
. For example, you can use the prebuilt xPack toolchain.
Cross-compiling programs does not require any additional software beyond the toolchain. Prebuilt versions of the toolchain are available from Betrusted.
Building the target
The target can be built by enabling it for a rustc
build.
[build]
target = ["riscv32imac-unknown-xous-elf"]
Make sure your C compiler is included in $PATH
, then add it to the config.toml
:
[target.riscv32imac-unknown-xous-elf]
cc = "riscv-none-elf-gcc"
ar = "riscv-none-elf-ar"
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for this target, you will need to do one of the following:
- Build Rust with the target enabled (see "Building the target" above)
- Build your own copy of
core
by usingbuild-std
or similar - Download a prebuilt toolchain from Betrusted
Cross-compilation
This target can be cross-compiled from any host.
Testing
Currently there is no support to run the rustc test suite for this target.
riscv32{i,im,ima,imc,imac,imafc}-unknown-none-elf
Tier: 2
Bare-metal target for RISC-V CPUs with the RV32I, RV32IM, RV32IMC, RV32IMAFC and RV32IMAC ISAs.
Tier: 3
Bare-metal target for RISC-V CPUs with the RV32IMA ISA.
Target maintainers
- Rust Embedded Working Group, RISC-V team
Requirements
The target is cross-compiled, and uses static linking. No external toolchain
is required and the default rust-lld
linker works, but you must specify
a linker script. The riscv-rt
crate provides a suitable one. The
riscv-rust-quickstart
repository gives an example of an RV32 project.
Building the target
This target is included in Rust and can be installed via rustup
.
Testing
This is a cross-compiled no-std
target, which must be run either in a simulator
or by programming them onto suitable hardware. It is not possible to run the
Rust test-suite on this target.
Cross-compilation toolchains and C code
This target supports C code. If interlinking with C or C++, you may need to use
riscv64-unknown-elf-gcc
as a linker instead of rust-lld
.
riscv64gc-unknown-linux-gnu
Tier: 2 (with Host Tools)
RISC-V targets using the RV64I base instruction set with the G collection of extensions, as well as the C extension.
Target maintainers
- Kito Cheng, kito.cheng@gmail.com, @kito-cheng
- Michael Maitland, michaeltmaitland@gmail.com, @michaelmaitland
- Robin Randhawa, robin.randhawa@sifive.com, @robin-randhawa-sifive
- Craig Topper, craig.topper@sifive.com, @topperc
Requirements
This target requires:
- Linux Kernel version 4.20 or later
- glibc 2.17 or later
Building the target
These targets are distributed through rustup
, and otherwise require no
special configuration.
If you need to build your own Rust for some reason though, the targets can be
enabled in config.toml
. For example:
[build]
target = ["riscv64gc-unknown-linux-gnu"]
Building Rust programs
On a RISC-V host, the riscv64gc-unknown-linux-gnu
target should be automatically
installed and used by default.
On a non-RISC-V host, add the target:
rustup target add riscv64gc-unknown-linux-gnu
Then cross compile crates with:
cargo build --target riscv64gc-unknown-linux-gnu
Testing
There are no special requirements for testing and running the targets. For testing cross builds on the host, please refer to the "Cross-compilation toolchains and C code" section below.
Cross-compilation toolchains and C code
A RISC-V toolchain can be obtained for Windows/Mac/Linux from the
riscv-gnu-toolchain
repostory. Binaries are available via
embecosm,
and may also be available from your OS's package manager.
On Ubuntu, a RISC-V toolchain can be installed with:
apt install gcc-riscv64-linux-gnu g++-riscv64-linux-gnu libc6-dev-riscv64-cross
Depending on your system, you may need to configure the target to use the GNU
GCC linker. To use it, add the following to your .cargo/config.toml
:
[target.riscv64gc-unknown-linux-gnu]
linker = "riscv64-linux-gnu-gcc"
If your riscv64-linux-gnu-*
toolchain is not in your PATH
you may need to
configure additional settings:
[target.riscv64gc-unknown-linux-gnu]
# Adjust the paths to point at your toolchain
cc = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-gcc"
cxx = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-g++"
ar = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-ar"
ranlib = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-ranlib"
linker = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-gcc"
To test cross compiled binaries on a non-RISCV-V host, you can use
qemu
.
On Ubuntu, a RISC-V emulator can be obtained with:
apt install qemu-system-riscv64
Then, in .cargo/config.toml
set the runner
:
[target.riscv64gc-unknown-linux-gnu]
runner = "qemu-riscv64-static -L /usr/riscv64-linux-gnu -cpu rv64"
On Mac and Linux, it's also possible to use
lima
to emulate RISC-V in a similar way to
how WSL2 works on Windows:
limactl start template://riscv
limactl shell riscv
Using Docker (with BuildKit) the
riscv64/ubuntu
image can be used
to buiild or run riscv64gc-unknown-linux-gnu
binaries.
docker run --platform linux/riscv64 -ti --rm --mount "type=bind,src=$(pwd),dst=/checkout" riscv64/ubuntu bash
riscv64gc-unknown-linux-musl
Tier: 2
Target for RISC-V Linux programs using musl libc.
Target maintainers
Requirements
Building the target itself requires a RISC-V compiler that is supported by cc-rs
.
Building the target
The target can be built by enabling it for a rustc
build.
[build]
target = ["riscv64gc-unknown-linux-musl"]
Make sure your C compiler is included in $PATH
, then add it to the config.toml
:
[target.riscv64gc-unknown-linux-musl]
cc = "riscv64-linux-gnu-gcc"
cxx = "riscv64-linux-gnu-g++"
ar = "riscv64-linux-gnu-ar"
linker = "riscv64-linux-gnu-gcc"
Building Rust programs
This target are distributed through rustup
, and otherwise require no
special configuration.
Cross-compilation
This target can be cross-compiled from any host.
Testing
This target can be tested as normal with x.py
on a RISC-V host or via QEMU
emulation.
sparc-unknown-none-elf
Tier: 3
Rust for bare-metal 32-bit SPARC V7 and V8 systems, e.g. the Gaisler LEON3.
Target | Descriptions |
---|---|
sparc-unknown-none-elf | SPARC V7 32-bit (freestanding, hardfloat) |
Target maintainers
- Jonathan Pallant, jonathan.pallant@ferrous-systems.com, https://ferrous-systems.com
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
By default, code generated with this target should run on any SPARC
hardware;
enabling additional target features may raise this baseline.
-
-Ctarget-cpu=v8
adds the extra SPARC V8 instructions. -
-Ctarget-cpu=leon3
adds the SPARC V8 instructions and sets up scheduling to suit the Gaisler Leon3.
Functions marked extern "C"
use the standard SPARC architecture calling
convention.
This target generates ELF binaries. Any alternate formats or special considerations for binary layout will require linker options or linker scripts.
Building the target
You can build Rust with support for the target by adding it to the target
list in config.toml
:
[build]
build-stage = 1
host = ["<target for your host>"]
target = ["<target for your host>", "sparc-unknown-none-elf"]
Replace <target for your host>
with x86_64-unknown-linux-gnu
or whatever
else is appropriate for your host machine.
Building Rust programs
To build with this target, pass it to the --target
argument, like:
cargo build --target sparc-unknown-none-elf
This target uses GCC as a linker, and so you will need an appropriate GCC
compatible sparc-unknown-none
toolchain. The default linker binary is
sparc-elf-gcc
, but you can override this in your project configuration, as
follows:
.cargo/config.toml
:
[target.sparc-unknown-none-elf]
linker = "sparc-custom-elf-gcc"
Testing
As sparc-unknown-none-elf
supports a variety of different environments and does
not support std
, this target does not support running the Rust test suite.
Cross-compilation toolchains and C code
This target was initially tested using BCC2 from Gaisler, along with the TSIM Leon3 processor simulator. Both BCC2 GCC and BCC2 Clang have been shown to work. To work with these tools, your project configuration should contain something like:
.cargo/config.toml
:
[target.sparc-unknown-none-elf]
linker = "sparc-gaisler-elf-gcc"
runner = "tsim-leon3"
[build]
target = ["sparc-unknown-none-elf"]
rustflags = "-Ctarget-cpu=leon3"
With this configuration, running cargo run
will compile your code for the
SPARC V8 compatible Gaisler Leon3 processor and then start the tsim-leon3
simulator. The libcore
was pre-compiled as part of the rustc
compilation
process using the SPARC V7 baseline, but if you are using a nightly toolchain
you can use the
-Z build-std=core
option to rebuild libcore
from source. This may be useful if you want to
compile it for SPARC V8 and take advantage of the extra instructions.
.cargo/config.toml
:
[target.sparc-unknown-none-elf]
linker = "sparc-gaisler-elf-gcc"
runner = "tsim-leon3"
[build]
target = ["sparc-unknown-none-elf"]
rustflags = "-Ctarget-cpu=leon3"
[unstable]
build-std = ["core"]
Either way, once the simulator is running, simply enter the command run
to
start the code executing in the simulator.
The default C toolchain libraries are linked in, so with the Gaisler BCC2
toolchain, and using its default Leon3 BSP, you can use call the C putchar
function and friends to output to the simulator console. The default linker
script is also appropriate for the Leon3 simulator, so no linker script is
required.
Here's a complete example using the above config file:
#![no_std]
#![no_main]
extern "C" {
fn putchar(ch: i32);
fn _exit(code: i32) -> !;
}
#[no_mangle]
extern "C" fn main() -> i32 {
let message = "Hello, this is Rust!";
for b in message.bytes() {
unsafe {
putchar(b as i32);
}
}
0
}
#[panic_handler]
fn panic(_panic: &core::panic::PanicInfo) -> ! {
unsafe {
_exit(1);
}
}
$ cargo run --target=sparc-unknown-none-elf
Compiling sparc-demo-rust v0.1.0 (/work/sparc-demo-rust)
Finished dev [unoptimized + debuginfo] target(s) in 3.44s
Running `tsim-leon3 target/sparc-unknown-none-elf/debug/sparc-demo-rust`
TSIM3 LEON3 SPARC simulator, version 3.1.9 (evaluation version)
Copyright (C) 2023, Frontgrade Gaisler - all rights reserved.
This software may only be used with a valid license.
For latest updates, go to https://www.gaisler.com/
Comments or bug-reports to support@gaisler.com
This TSIM evaluation version will expire 2023-11-28
Number of CPUs: 2
system frequency: 50.000 MHz
icache: 1 * 4 KiB, 16 bytes/line (4 KiB total)
dcache: 1 * 4 KiB, 16 bytes/line (4 KiB total)
Allocated 8192 KiB SRAM memory, in 1 bank at 0x40000000
Allocated 32 MiB SDRAM memory, in 1 bank at 0x60000000
Allocated 8192 KiB ROM memory at 0x00000000
section: .text, addr: 0x40000000, size: 20528 bytes
section: .rodata, addr: 0x40005030, size: 128 bytes
section: .data, addr: 0x400050b0, size: 1176 bytes
read 347 symbols
tsim> run
Initializing and starting from 0x40000000
Hello, this is Rust!
Program exited normally on CPU 0.
tsim>
*-pc-windows-gnullvm
Tier: 2 (without host tools)
Windows targets similar to *-pc-windows-gnu
but using UCRT as the runtime and various LLVM tools/libraries instead of GCC/Binutils.
Target triples available so far:
aarch64-pc-windows-gnullvm
i686-pc-windows-gnullvm
x86_64-pc-windows-gnullvm
Target maintainers
Requirements
The easiest way to obtain these targets is cross-compilation, but native build from x86_64-pc-windows-gnu
is possible with few hacks which I don't recommend.
Std support is expected to be on pair with *-pc-windows-gnu
.
Binaries for this target should be at least on pair with *-pc-windows-gnu
in terms of requirements and functionality.
Those targets follow Windows calling convention for extern "C"
.
Like with any other Windows target, created binaries are in PE format.
Building the target
These targets can be easily cross-compiled
using llvm-mingw toolchain or MSYS2 CLANG* environments.
Just fill [target.*]
sections for both build and resulting compiler and set installation prefix in config.toml
.
Then run ./x.py install
.
In my case I had ran ./x.py install --host x86_64-pc-windows-gnullvm --target x86_64-pc-windows-gnullvm
inside MSYS2 MINGW64 shell
so x86_64-pc-windows-gnu
was my build toolchain.
Native bootstrapping is doable in two ways:
- cross-compile gnullvm host toolchain and use it as build toolchain for the next build,
- copy libunwind libraries and rename them to mimic libgcc like here: https://github.com/msys2/MINGW-packages/blob/68e640756df2df6df6afa60f025e3f936e7b977c/mingw-w64-rust/PKGBUILD#L108-L109, stage0 compiler will be mostly broken but good enough to build the next stage.
The second option might stop working anytime, so it's not recommended.
Building Rust programs
Rust does ship a pre-compiled std library for those targets. That means one can easily cross-compile for those targets from other hosts if C proper toolchain is installed.
Alternatively full toolchain can be built as described in the previous section.
Testing
Created binaries work fine on Windows or Wine using native hardware. Testing AArch64 on x86_64 is problematic though and requires spending some time with QEMU.
Most of x86_64 testsuite does pass when cross-compiling,
with exception for rustdoc
and ui-fulldeps
that fail with and error regarding a missing library,
they do pass in native builds though.
The only failing test is std's process::tests::test_proc_thread_attributes
for unknown reason.
Cross-compilation toolchains and C code
Compatible C code can be built with Clang's aarch64-pc-windows-gnu
, i686-pc-windows-gnullvm
and x86_64-pc-windows-gnu
targets as long as LLVM-based C toolchains are used.
Those include:
nto-qnx
Tier: 3
QNX® Neutrino (nto) Real-time operating system. The support has been implemented jointly by Elektrobit Automotive GmbH and Blackberry QNX.
Target maintainers
- Florian Bartels,
Florian.Bartels@elektrobit.com
, https://github.com/flba-eb - Tristan Roach,
TRoach@blackberry.com
, https://github.com/gh-tr - Jonathan Pallant
Jonathan.Pallant@ferrous-systems.com
, https://github.com/jonathanpallant - Jorge Aparicio
Jorge.Aparicio@ferrous-systems.com
, https://github.com/japaric
Requirements
Currently, the following QNX Neutrino versions and compilation targets are supported:
QNX Neutrino Version | Target Architecture | Full support | no_std support |
---|---|---|---|
7.1 | AArch64 | ✓ | ✓ |
7.1 | x86_64 | ✓ | ✓ |
7.0 | AArch64 | ? | ✓ |
7.0 | x86 | ✓ |
Adding other architectures that are supported by QNX Neutrino is possible.
In the table above, 'full support' indicates support for building Rust applications with the full standard library.
'no_std
support' indicates that only core
and alloc
are available.
For building or using the Rust toolchain for QNX Neutrino, the
QNX Software Development Platform (SDP)
must be installed and initialized.
Initialization is usually done by sourcing qnxsdp-env.sh
(this will be installed as part of the SDP, see also installation instruction provided with the SDP).
Afterwards qcc
(QNX C/C++ compiler)
should be available (in the $PATH
variable).
qcc
will be called e.g. for linking executables.
When linking no_std
applications, they must link against libc.so
(see example). This is
required because applications always link against the crt
library and crt
depends on libc.so
.
This is done automatically when using the standard library.
Disabling RELocation Read-Only (RELRO)
While not recommended by default, some QNX kernel setups may require the RELRO
to be disabled with -C relro_level=off
, e.g. by adding it to the .cargo/config.toml
file:
[target.aarch64-unknown-nto-qnx700]
rustflags = ["-C", "relro_level=off"]
If your QNX kernel does not allow it, and relro
is not disabled, running compiled binary would fail with syntax error: ... unexpected
or similar. This is due to kernel trying to interpret compiled binary with /bin/sh
, and obviously failing. To verify that this is really the case, run your binary with the DL_DEBUG=all
env var, and look for this output. If you see it, you should disable relro
as described above.
Resolution scope for Executable->/bin/sh:
Executable->/bin/sh
libc.so.4->/usr/lib/ldqnx-64.so.2
Small example application
Small no_std
example is shown below. Applications using the standard library work as well.
#![no_std]
#![no_main]
#![feature(lang_items)]
// We must always link against libc, even if no external functions are used
// "extern C" - Block can be empty but must be present
#[link(name = "c")]
extern "C" {
pub fn printf(format: *const core::ffi::c_char, ...) -> core::ffi::c_int;
}
#[no_mangle]
pub extern "C" fn main(_argc: isize, _argv: *const *const u8) -> isize {
const HELLO: &'static str = "Hello World, the answer is %d\n\0";
unsafe {
printf(HELLO.as_ptr() as *const _, 42);
}
0
}
use core::panic::PanicInfo;
#[panic_handler]
fn panic(_panic: &PanicInfo<'_>) -> ! {
loop {}
}
#[lang = "eh_personality"]
#[no_mangle]
pub extern "C" fn rust_eh_personality() {}
The QNX Neutrino support of Rust has been tested with QNX Neutrino 7.0 and 7.1.
There are no further known requirements.
Conditional compilation
For conditional compilation, following QNX Neutrino specific attributes are defined:
target_os
="nto"
target_env
="nto71"
(for QNX Neutrino 7.1)target_env
="nto70"
(for QNX Neutrino 7.0)
Building the target
- Create a
config.toml
Example content:
profile = "compiler"
change-id = 115898
- Compile the Rust toolchain for an
x86_64-unknown-linux-gnu
host (for bothaarch64
andx86_64
targets)
Compiling the Rust toolchain requires the same environment variables used for compiling C binaries. Refer to the QNX developer manual.
To compile for QNX Neutrino (aarch64 and x86_64) and Linux (x86_64):
export build_env='
CC_aarch64-unknown-nto-qnx710=qcc
CFLAGS_aarch64-unknown-nto-qnx710=-Vgcc_ntoaarch64le_cxx
CXX_aarch64-unknown-nto-qnx710=qcc
AR_aarch64_unknown_nto_qnx710=ntoaarch64-ar
CC_x86_64-pc-nto-qnx710=qcc
CFLAGS_x86_64-pc-nto-qnx710=-Vgcc_ntox86_64_cxx
CXX_x86_64-pc-nto-qnx710=qcc
AR_x86_64_pc_nto_qnx710=ntox86_64-ar'
env $build_env \
./x.py build \
--target aarch64-unknown-nto-qnx710,x86_64-pc-nto-qnx710,x86_64-unknown-linux-gnu \
rustc library/core library/alloc library/std
Running the Rust test suite
The test suites of the Rust compiler and standard library can be executed much like other Rust targets.
The environment for testing should match the one used during compiler compilation (refer to build_env
and qcc
/PATH
above) with the
addition of the TEST_DEVICE_ADDR environment variable.
The TEST_DEVICE_ADDR variable controls the remote runner and should point to the target, despite localhost being shown in the following example.
Note that some tests are failing which is why they are currently excluded by the target maintainers which can be seen in the following example.
To run all tests on a x86_64 QNX Neutrino target:
export TEST_DEVICE_ADDR="localhost:12345" # must address the test target, can be a SSH tunnel
export build_env='
CC_aarch64-unknown-nto-qnx710=qcc
CFLAGS_aarch64-unknown-nto-qnx710=-Vgcc_ntoaarch64le_cxx
CXX_aarch64-unknown-nto-qnx710=qcc
AR_aarch64_unknown_nto_qnx710=ntoaarch64-ar
CC_x86_64-pc-nto-qnx710=qcc
CFLAGS_x86_64-pc-nto-qnx710=-Vgcc_ntox86_64_cxx
CXX_x86_64-pc-nto-qnx710=qcc
AR_x86_64_pc_nto_qnx710=ntox86_64-ar'
# Disable tests that only work on the host or don't make sense for this target.
# See also:
# - src/ci/docker/host-x86_64/i686-gnu/Dockerfile
# - https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Running.20tests.20on.20remote.20target
# - .github/workflows/ci.yml
export exclude_tests='
--exclude src/bootstrap
--exclude src/tools/error_index_generator
--exclude src/tools/linkchecker
--exclude tests/ui-fulldeps
--exclude rustc
--exclude rustdoc'
env $build_env \
./x.py test \
$exclude_tests \
--stage 1 \
--target x86_64-pc-nto-qnx710
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target.
To compile for this target, you must either build Rust with the target enabled (see "Building the target" above),
or build your own copy of core
by using build-std
or similar.
Testing
Compiled executables can run directly on QNX Neutrino.
Rust std library test suite
The target needs sufficient resources to execute all tests. The commands below assume that a QEMU image is used.
-
Ensure that the temporary directory used by
remote-test-server
has enough free space and inodes. 5GB of free space and 40000 inodes are known to be sufficient (the test will create more than 32k files). To create a QEMU image in an empty directory, run this command inside the directory:mkqnximage --type=qemu --ssh-ident=$HOME/.ssh/id_ed25519.pub --data-size=5000 --data-inodes=40000
/data
should have enough free resources. Set theTMPDIR
environment variable accordingly when runningremote-test-server
, e.g.:TMPDIR=/data/tmp/rust remote-test-server --bind 0.0.0.0:12345
-
Ensure the TCP stack can handle enough parallel connections (default is 200, should be 300 or higher). After creating an image (see above), edit the file
output/build/startup.sh
:- Search for
io-pkt-v6-hc
- Add the parameter
-ptcpip threads_max=300
, e.g.:io-pkt-v6-hc -U 33:33 -d e1000 -ptcpip threads_max=300
- Update the image by running
mkqnximage
again with the same parameters as above for creating it.
- Search for
-
Running and stopping the virtual machine
To start the virtual machine, run inside the directory of the VM:
mkqnximage --run=-h
To stop the virtual machine, run inside the directory of the VM:
mkqnximage --stop
-
Ensure local networking
Ensure that 'localhost' is getting resolved to 127.0.0.1. If you can't ping the localhost, some tests may fail. Ensure it's appended to /etc/hosts (if first
ping
command fails). Commands have to be executed inside the virtual machine!$ ping localhost ping: Cannot resolve "localhost" (Host name lookup failure) $ echo "127.0.0.1 localhost" >> /etc/hosts $ ping localhost PING localhost (127.0.0.1): 56 data bytes 64 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=1 ms
Cross-compilation toolchains and C code
Compiling C code requires the same environment variables to be set as compiling the Rust toolchain (see above),
to ensure qcc
is used with proper arguments.
To ensure compatibility, do not specify any further arguments that for example change calling conventions or memory layout.
*-unikraft-linux-musl
Tier: 3
Targets for the Unikraft Unikernel Development Kit (with musl).
Target triplets available so far:
x86_64-unikraft-linux-musl
Target maintainers
- Martin Kröning (@mkroening)
Requirements
These targets only support cross-compilation. The targets do support std.
Unikraft pretends to behave exactly like Linux.
How much of that functionality is available depends on the individual unikernel configuration.
For example, the basic Unikraft + musl config does not support poll
or networking out of the box.
That functionality requires enabling LIBPOSIX_EVENT
or lwIP respectively.
The Unikraft targets follow Linux's extern "C"
calling convention.
For these targets, rustc
does not perform the final linking step.
Instead, the Unikraft build system will produce the final Unikernel image for the selected platform (e.g., KVM, Linux user space, and Xen).
Building the targets
You can build Rust with support for the targets by adding it to the target
list in config.toml
:
[build]
build-stage = 1
target = [ "x86_64-unikraft-linux-musl" ]
Building Rust programs
Rust does not yet ship pre-compiled artifacts for these targets.
To compile for these targets, you will either need to build Rust with the targets enabled
(see “Building the targets” above), or build your own copy of core
by using build-std
or similar.
Linking requires a KraftKit shim. See unikraft/kraftkit#612 for more information.
Testing
The targets do support running binaries in the form of unikernel images. How the unikernel image is run depends on the specific platform (e.g., KVM, Linux user space, and Xen). The targets do not support running the Rust test suite.
Cross-compilation toolchains and C code
The targets do support C code. To build compatible C code, you have to use the same compiler and flags as does the Unikraft build system for your specific configuration. The easiest way to achieve that, is to build the C code with the Unikraft build system when building your unikernel image.
*-unknown-hermit
Tier: 3
The Hermit unikernel target allows compiling your applications into self-contained, specialized unikernel images that can be run in small virtual machines.
Target triplets available so far:
x86_64-unknown-hermit
aarch64-unknown-hermit
riscv64gc-unknown-hermit
Target maintainers
- Stefan Lankes (@stlankes)
- Martin Kröning (@mkroening)
Requirements
These targets only support cross-compilation. The targets do support std.
When building binaries for this target, the Hermit unikernel is built from scratch. The application developer themselves specializes the target and sets corresponding expectations.
The Hermit targets follow Linux's extern "C"
calling convention.
Hermit binaries have the ELF format.
Building the target
You can build Rust with support for the targets by adding it to the target
list in config.toml
.
To run the Hermit build scripts, you also have to enable your host target.
The build scripts rely on llvm-tools
and binaries are linked using rust-lld
, so those have to be enabled as well.
[build]
build-stage = 1
target = [
"<HOST_TARGET>",
"x86_64-unknown-hermit",
"aarch64-unknown-hermit",
"riscv64gc-unknown-hermit",
]
[rust]
lld = true
llvm-tools = true
Building Rust programs
Rust does not yet ship pre-compiled artifacts for these targets.
To compile for these targets, you will either need to build Rust with the targets enabled
(see “Building the targets” above), or build your own copy of core
by using build-std
or similar.
As all Hermit programs are unikernels, building a Rust program also requires including the operating system code. A guide for doing so is provided in our starter hermit-rs-template.
Testing
The targets support running binaries in the form of self-contained unikernel images. These images can be chainloaded by Hermit's loader or hypervisor (Uhyve). QEMU can be used to boot Hermit binaries using the loader on any architecture. The targets do not support running the Rust test suite.
Cross-compilation toolchains and C code
The targets do not yet support C code and Rust code at the same time.
*-unknown-netbsd
Tier: 3
NetBSD multi-platform 4.4BSD-based UNIX-like operating system.
The target names follow this format: $ARCH-unknown-netbsd{-$SUFFIX}
,
where $ARCH
specifies the target processor architecture and
-$SUFFIX
(optional) might indicate the ABI. The following targets
are currently defined running NetBSD:
Target name | NetBSD Platform |
---|---|
x86_64-unknown-netbsd | amd64 / x86_64 systems |
armv7-unknown-netbsd-eabihf | 32-bit ARMv7 systems with hard-float |
armv6-unknown-netbsd-eabihf | 32-bit ARMv6 systems with hard-float |
aarch64-unknown-netbsd | 64-bit ARM systems, little-endian |
aarch64_be-unknown-netbsd | 64-bit ARM systems, big-endian |
i586-unknown-netbsd | 32-bit i386, restricted to Pentium |
i686-unknown-netbsd | 32-bit i386 with SSE |
mipsel-unknown-netbsd | 32-bit mips, requires mips32 cpu support |
powerpc-unknown-netbsd | Various 32-bit PowerPC systems, e.g. MacPPC |
riscv64gc-unknown-netbsd | 64-bit RISC-V |
sparc64-unknown-netbsd | Sun UltraSPARC systems |
All use the "native" stdc++
library which goes along with the natively
supplied GNU C++ compiler for the given OS version. Many of the bootstraps
are built for NetBSD 9.x, although some exceptions exist (some
are built for NetBSD 8.x but also work on newer OS versions).
Designated Developers
- @he32,
he@NetBSD.org
- NetBSD/pkgsrc-wip's rust maintainer (see MAINTAINER variable). This package is part of "pkgsrc work-in-progress" and is used for deployment and testing of new versions of rust
- NetBSD's pkgsrc lang/rust for the "proper" package in pkgsrc.
- NetBSD's pkgsrc lang/rust-bin which re-uses the bootstrap kit as a binary distribution and therefore avoids the rather protracted native build time of rust itself
Fallback to pkgsrc-users@NetBSD.org, or fault reporting via NetBSD's bug reporting system.
Requirements
The x86_64-unknown-netbsd
artifacts is being distributed by the
rust project.
The other targets are built by the designated developers (see above), and the targets are initially cross-compiled, but many if not most of them are also built natively as part of testing.
Building
The default build mode for the packages is a native build.
Cross-compilation
These targets can be cross-compiled, and we do that via the pkgsrc package(s).
Cross-compilation typically requires the "tools" and "dest" trees
resulting from a normal cross-build of NetBSD itself, ref. our main
build script, build.sh
.
See e.g. do-cross.mk
Makefile
for the Makefile used to cross-build all the above NetBSD targets
(except for the amd64
target).
The major option for the rust build is whether to build rust with
the LLVM rust carries in its distribution, or use the LLVM package
installed from pkgsrc. The PKG_OPTIONS.rust
option is
rust-internal-llvm
, ref. the rust package's options.mk make
fragment.
It defaults to being set for a few of the above platforms, for
various reasons (see comments), but is otherwise unset and therefore
indicates use of the pkgsrc LLVM.
Testing
The Rust testsuite could presumably be run natively.
For the systems where the maintainer can build natively, the rust compiler itself is re-built natively. This involves the rust compiler being re-built with the newly self-built rust compiler, so exercises the result quite extensively.
Additionally, for some systems we build librsvg
, and for the more
capable systems we build and test firefox
(amd64, i386, aarch64).
Building Rust programs
Rust ships pre-compiled artifacts for the x86_64-unknown-netbsd
target.
For the other systems mentioned above, using the pkgsrc
route is
probably the easiest, possibly via the rust-bin
package to save
time, see the RUST_TYPE
variable from the rust.mk
Makefile
fragment.
The pkgsrc rust package has a few files to assist with building
pkgsrc packages written in rust, ref. the rust.mk
and cargo.mk
Makefile fragments in the lang/rust
package.
*-unknown-openbsd
Tier: 3
OpenBSD multi-platform 4.4BSD-based UNIX-like operating system.
The target names follow this format: $ARCH-unknown-openbsd
, where $ARCH
specifies the target processor architecture. The following targets are currently defined:
Target name | C++ library | OpenBSD Platform |
---|---|---|
aarch64-unknown-openbsd | libc++ | 64-bit ARM systems |
i686-unknown-openbsd | libc++ | Standard PC and clones based on the Intel i386 architecture and compatible processors |
powerpc64-unknown-openbsd | libc++ | IBM POWER-based PowerNV systems |
riscv64gc-unknown-openbsd | libc++ | 64-bit RISC-V systems |
sparc64-unknown-openbsd | estdc++ | Sun UltraSPARC and Fujitsu SPARC64 systems |
x86_64-unknown-openbsd | libc++ | AMD64-based systems |
Note that all OS versions are major even if using X.Y notation (6.8
and 6.9
are different major versions) and could be binary incompatibles (with breaking changes).
Designated Developers
Fallback to ports@openbsd.org, OpenBSD third parties public mailing-list (with openbsd developers readers)
Requirements
These targets are natively compiled and could be cross-compiled. C compiler toolchain is required for the purpose of building Rust and functional binaries.
Building
The target can be built by enabling it for a rustc
build.
[build]
target = ["$ARCH-unknown-openbsd"]
[target.$ARCH-unknown-openbsd]
cc = "$ARCH-openbsd-cc"
Cross-compilation
These targets can be cross-compiled, but LLVM might not build out-of-box.
Testing
The Rust testsuite could be run natively.
Building Rust programs
Rust does not yet ship pre-compiled artifacts for these targets.
*-unknown-redox
Tier: 2/3
Targets for the Redox OS operating system.
Target triplets available so far:
x86_64-unknown-redox
(tier 2)aarch64-unknown-redox
(tier 3)i686-unknown-redox
(tier 3)
Target maintainers
- Jeremy Soller (@jackpot51)
Requirements
These targets are natively compiled and can be cross-compiled. Std is fully supported.
The targets are only expected to work with the latest version of Redox OS as the ABI is not yet stable.
extern "C"
uses the official calling convention of the respective architectures.
Redox OS binaries use ELF as file format.
Building the target
You can build Rust with support for the targets by adding it to the target
list in config.toml
. In addition a copy of relibc needs to be present in the linker search path.
[build]
build-stage = 1
target = [
"<HOST_TARGET>",
"x86_64-unknown-redox",
"aarch64-unknown-redox",
"i686-unknown-redox",
]
Building Rust programs and testing
Rust does not yet ship pre-compiled artifacts for Redox OS except for x86_64-unknown-redox.
The easiest way to build and test programs for Redox OS is using redoxer which sets up the required compiler toolchain for building as well as runs programs inside a Redox OS VM using QEMU.
Cross-compilation toolchains and C code
The target supports C code. Pre-compiled C toolchains can be found at https://static.redox-os.org/toolchain/.
*-unknown-uefi
Tier: 2
Unified Extensible Firmware Interface (UEFI) targets for application, driver, and core UEFI binaries.
Available targets:
aarch64-unknown-uefi
i686-unknown-uefi
x86_64-unknown-uefi
Target maintainers
- David Rheinsberg (@dvdhrm)
- Nicholas Bishop (@nicholasbishop)
Requirements
All UEFI targets can be used as no-std
environments via cross-compilation.
Support for std
is present, but incomplete and extremely new. alloc
is supported if
an allocator is provided by the user or if using std. No host tools are supported.
The UEFI environment resembles the environment for Microsoft Windows, with some
minor differences. Therefore, cross-compiling for UEFI works with the same
tools as cross-compiling for Windows. The target binaries are PE32+ encoded,
the calling convention is different for each architecture, but matches what
Windows uses (if the architecture is supported by Windows). The special
efiapi
Rust calling-convention chooses the right ABI for the target platform
(extern "C"
is incorrect on Intel targets at least). The specification has an
elaborate section on the different supported calling-conventions, if more
details are desired.
MMX, SSE, and other FP-units are disabled by default, to allow for compilation of core UEFI code that runs before they are set up. This can be overridden for individual compilations via rustc command-line flags. Not all firmwares correctly configure those units, though, so careful inspection is required.
As native to PE32+, binaries are position-dependent, but can be relocated at runtime if their desired location is unavailable. The code must be statically linked. Dynamic linking is not supported. Code is shared via UEFI interfaces, rather than dynamic linking. Additionally, UEFI forbids running code on anything but the boot CPU/thread, nor is interrupt-usage allowed (apart from the timer interrupt). Device drivers are required to use polling methods.
UEFI uses a single address-space to run all code in. Multiple applications can be loaded simultaneously and are dispatched via cooperative multitasking on a single stack.
By default, the UEFI targets use the link
-flavor of the LLVM linker lld
to
link binaries into the final PE32+ file suffixed with *.efi
. The PE subsystem
is set to EFI_APPLICATION
, but can be modified by passing /subsystem:<...>
to the linker. Similarly, the entry-point is set to efi_main
but can be
changed via /entry:<...>
. The panic-strategy is set to abort
,
The UEFI specification is available online for free: UEFI Specification Directory
Building rust for UEFI targets
Rust can be built for the UEFI targets by enabling them in the rustc
build
configuration. Note that you can only build the standard libraries. The
compiler and host tools currently cannot be compiled for UEFI targets. A sample
configuration would be:
[build]
build-stage = 1
target = ["x86_64-unknown-uefi"]
Building Rust programs
Starting with Rust 1.67, precompiled artifacts are provided via
rustup
. For example, to use x86_64-unknown-uefi
:
# install cross-compile toolchain
rustup target add x86_64-unknown-uefi
# target flag may be used with any cargo or rustc command
cargo build --target x86_64-unknown-uefi
Building a driver
There are three types of UEFI executables: application, boot service
driver, and runtime driver. All of Rust's UEFI targets default to
producing applications. To build a driver instead, pass a
subsystem
linker flag with a value of
efi_boot_service_driver
or efi_runtime_driver
.
Example:
# In .cargo/config.toml:
[build]
rustflags = ["-C", "link-args=/subsystem:efi_runtime_driver"]
Testing
UEFI applications can be copied into the ESP on any UEFI system and executed via the firmware boot menu. The qemu suite allows emulating UEFI systems and executing UEFI applications as well. See its documentation for details.
The uefi-run rust tool is a simple
wrapper around qemu
that can spawn UEFI applications in qemu. You can install
it via cargo install uefi-run
and execute qemu applications as
uefi-run ./application.efi
.
Cross-compilation toolchains and C code
There are 3 common ways to compile native C code for UEFI targets:
- Use the official SDK by Intel: Tianocore/EDK2. This supports a multitude of platforms, comes with the full specification transposed into C, lots of examples and build-system integrations. This is also the only officially supported platform by Intel, and is used by many major firmware implementations. Any code compiled via the SDK is compatible to rust binaries compiled for the UEFI targets. You can link them directly into your rust binaries, or call into each other via UEFI protocols.
- Use the GNU-EFI suite. This approach is used by many UEFI applications in the Linux/OSS ecosystem. The GCC compiler is used to compile ELF binaries, and linked with a pre-loader that converts the ELF binary to PE32+ at runtime. You can combine such binaries with the rust UEFI targets only via UEFI protocols. Linking both into the same executable will fail, since one is an ELF executable, and one a PE32+. If linking to GNU-EFI executables is desired, you must compile your rust code natively for the same GNU target as GNU-EFI and use their pre-loader. This requires careful consideration about which calling-convention to use when calling into native UEFI protocols, or calling into linked GNU-EFI code (similar to how these differences need to be accounted for when writing GNU-EFI C code).
- Use native Windows targets. This means compiling your C code for the Windows platform as if it was the UEFI platform. This works for static libraries, but needs adjustments when linking into an UEFI executable. You can, however, link such static libraries seamlessly into rust code compiled for UEFI targets. Be wary of any includes that are not specifically suitable for UEFI targets (especially the C standard library includes are not always compatible). Freestanding compilations are recommended to avoid incompatibilities.
Ecosystem
The rust language has a long history of supporting UEFI targets. Many crates have been developed to provide access to UEFI protocols and make UEFI programming more ergonomic in rust. The following list is a short overview (in alphabetical ordering):
- efi: Ergonomic Rust bindings for writing UEFI applications. Provides rustified access to UEFI protocols, implements allocators and a safe environment to write UEFI applications.
- r-efi: UEFI Reference Specification Protocol Constants and Definitions. A pure transpose of the UEFI specification into rust. This provides the raw definitions from the specification, without any extended helpers or rustification. It serves as baseline to implement any more elaborate rust UEFI layers.
- uefi-rs: Safe and easy-to-use wrapper for building UEFI apps. An elaborate library providing safe abstractions for UEFI protocols and features. It implements allocators and provides an execution environment to UEFI applications written in rust.
- uefi-run: Run UEFI applications. A small wrapper around qemu to spawn
UEFI applications in an emulated
x86_64
machine.
Example: Freestanding
The following code is a valid UEFI application returning immediately upon execution with an exit code of 0. A panic handler is provided. This is executed by rust on panic. For simplicity, we simply end up in an infinite loop.
This example can be compiled as binary crate via cargo
:
cargo build --target x86_64-unknown-uefi
#![no_main]
#![no_std]
#[panic_handler]
fn panic_handler(_info: &core::panic::PanicInfo) -> ! {
loop {}
}
#[export_name = "efi_main"]
pub extern "C" fn main(_h: *mut core::ffi::c_void, _st: *mut core::ffi::c_void) -> usize {
0
}
Example: Hello World
This is an example UEFI application that prints "Hello World!", then waits for
key input before it exits. It serves as base example how to write UEFI
applications without any helper modules other than the standalone UEFI protocol
definitions provided by the r-efi
crate.
This extends the "Freestanding" example and builds upon its setup. See there for instruction how to compile this as binary crate.
Note that UEFI uses UTF-16 strings. Since rust literals are UTF-8, we have to
use an open-coded, zero-terminated, UTF-16 array as argument to
output_string()
. Similarly to the panic handler, real applications should
rather use UTF-16 modules.
#![no_main]
#![no_std]
use r_efi::efi;
#[panic_handler]
fn panic_handler(_info: &core::panic::PanicInfo) -> ! {
loop {}
}
#[export_name = "efi_main"]
pub extern "C" fn main(_h: efi::Handle, st: *mut efi::SystemTable) -> efi::Status {
let s = [
0x0048u16, 0x0065u16, 0x006cu16, 0x006cu16, 0x006fu16, // "Hello"
0x0020u16, // " "
0x0057u16, 0x006fu16, 0x0072u16, 0x006cu16, 0x0064u16, // "World"
0x0021u16, // "!"
0x000au16, // "\n"
0x0000u16, // NUL
];
// Print "Hello World!".
let r =
unsafe { ((*(*st).con_out).output_string)((*st).con_out, s.as_ptr() as *mut efi::Char16) };
if r.is_error() {
return r;
}
// Wait for key input, by waiting on the `wait_for_key` event hook.
let r = unsafe {
let mut x: usize = 0;
((*(*st).boot_services).wait_for_event)(1, &mut (*(*st).con_in).wait_for_key, &mut x)
};
if r.is_error() {
return r;
}
efi::Status::SUCCESS
}
Rust std for UEFI
This section contains information on how to use std on UEFI.
Build std
The building std part is pretty much the same as the official docs.
The linker that should be used is rust-lld
. Here is a sample config.toml
:
[rust]
lld = true
Then just build using x.py
:
./x.py build --target x86_64-unknown-uefi --stage 1
Alternatively, it is possible to use the build-std
feature. However, you must use a toolchain which has the UEFI std patches.
Then just build the project using the following command:
cargo build --target x86_64-unknown-uefi -Zbuild-std=std,panic_abort
Implemented features
alloc
- Implemented using
EFI_BOOT_SERVICES.AllocatePool()
andEFI_BOOT_SERVICES.FreePool()
. - Passes all the tests.
- Currently uses
EfiLoaderData
as theEFI_ALLOCATE_POOL->PoolType
.
cmath
- Provided by compiler-builtins.
env
- Just some global constants.
locks
- The provided locks should work on all standard single-threaded UEFI implementations.
os_str
- While the strings in UEFI should be valid UCS-2, in practice, many implementations just do not care and use UTF-16 strings.
- Thus, the current implementation supports full UTF-16 strings.
stdio
- Uses
Simple Text Input Protocol
andSimple Text Output Protocol
. - Note: UEFI uses CRLF for new line. This means Enter key is registered as CR instead of LF.
args
- Uses
EFI_LOADED_IMAGE_PROTOCOL->LoadOptions
Example: Hello World With std
The following code features a valid UEFI application, including stdio
and alloc
(OsString
and Vec
):
This example can be compiled as binary crate via cargo
using the toolchain
compiled from the above source (named custom):
cargo +custom build --target x86_64-unknown-uefi
#![feature(uefi_std)]
use r_efi::{efi, protocols::simple_text_output};
use std::{
ffi::OsString,
os::uefi::{env, ffi::OsStrExt}
};
pub fn main() {
println!("Starting Rust Application...");
// Use System Table Directly
let st = env::system_table().as_ptr() as *mut efi::SystemTable;
let mut s: Vec<u16> = OsString::from("Hello World!\n").encode_wide().collect();
s.push(0);
let r =
unsafe {
let con_out: *mut simple_text_output::Protocol = (*st).con_out;
let output_string: extern "efiapi" fn(_: *mut simple_text_output::Protocol, *mut u16) -> efi::Status = (*con_out).output_string;
output_string(con_out, s.as_ptr() as *mut efi::Char16)
};
assert!(!r.is_error())
}
BootServices
The current implementation of std makes BootServices
unavailable once ExitBootServices
is called. Refer to Runtime Drivers for more information regarding how to handle switching from using physical addresses to using virtual addresses.
Note: It should be noted that it is up to the user to drop all allocated memory before ExitBootServices
is called.
*-wrs-vxworks
Tier: 3
Targets for the VxWorks operating system.
Target triplets available:
x86_64-wrs-vxworks
aarch64-wrs-vxworks
i686-wrs-vxworks
armv7-wrs-vxworks-eabihf
powerpc-wrs-vxworks
powerpc64-wrs-vxworks
powerpc-wrs-vxworks-spe
Target maintainers
- B I Mohammed Abbas (@biabbas)
Requirements
OS version
The minimum supported version is VxWorks 7.
Building
Rust for each target can be cross-compiled with its specific target vsb configuration. Std support is added but not yet fully tested.
Building the target
You can build Rust with support for the targets by adding it to the target
list in config.toml
. In addition the workbench and wr-cc have to configured and activated.
[build]
build-stage = 1
target = [
"<HOST_TARGET>",
"x86_64-wrs-vxworks",
"aarch64-wrs-vxworks",
"i686-wrs-vxworks",
"armv7-wrs-vxworks-eabihf",
"powerpc-wrs-vxworks",
"powerpc64-wrs-vxworks",
"powerpc-wrs-vxworks-spe",
]
Building Rust programs
Rust does not yet ship pre-compiled artifacts for VxWorks.
The easiest way to build and test programs for VxWorks is to use the shipped rustc and cargo in VxWorks workbench, following the official windriver guidelines.
Cross-compilation toolchains and C code
The target supports C code. Pre-compiled C toolchains can be found in provided VxWorks workbench.
wasm32-wasip1
Tier: 2
The wasm32-wasip1
target is a WebAssembly compilation target which
assumes that the [WASIp1] (aka "WASI preview1") set of "syscalls" are available
for use in the standard library. Historically this target in the Rust compiler
was one of the first for WebAssembly where Rust and C code are explicitly
intended to interoperate as well.
There's a bit of history to the target and current development which is also
worth explaining before going much further. Historically this target was
originally called wasm32-wasi
in both rustc and Clang. This was first added
to Rust in 2019. In the intervening years leading up to 2024 the WASI standard
continued to be developed and was eventually "rebased" on top of the Component
Model. This was a large change to the WASI specification and was released as
0.2.0 ("WASIp2" colloquially) in January 2024. The previous target's name in
rustc, wasm32-wasi
, was then renamed to wasm32-wasip1
, to avoid
confusion with this new target to be added to rustc as wasm32-wasip2
.
Some more context can be found in these MCPs:
At this point the wasm32-wasip1
target is intended for historical
compatibility with the first version of the WASI standard. As of now (January
2024) the 0.2.0 target of WASI ("WASIp2") is relatively new. The state of
WASI will likely change in few years after which point this documentation will
probably receive another update.
Today the wasm32-wasip1
target will generate core WebAssembly modules
which will import functions from the wasi_snapshot_preview1
module for
OS-related functionality (e.g. printing).
Target maintainers
When this target was added to the compiler platform-specific documentation here was not maintained at that time. This means that the list below is not exhaustive and there are more interested parties in this target. That being said since when this document was last updated those interested in maintaining this target are:
- Alex Crichton, https://github.com/alexcrichton
Requirements
This target is cross-compiled. The target includes support for std
itself,
but not all of the standard library works. For example spawning a thread will
always return an error (see the wasm32-wasip1-threads
target for
example). Another example is that spawning a process will always return an
error. Operations such as opening a file, however, will be implemented by
calling WASI-defined APIs.
The WASI targets for Rust are explicitly intended to interoperate with other languages compiled to WebAssembly, for example C/C++. Any ABI differences or mismatches are considered bugs that need to be fixed.
By default the WASI targets in Rust ship in rustup with a precompiled copy of
wasi-libc
meaning that a WebAssembly-targeting-Clang is not required to
use the WASI targets from Rust. If there is no actual interoperation with C
then rustup target add wasm32-wasip1
is all that's needed to get
started with WASI.
Note that this behavior can be controlled with -Clinker
and
-Clink-self-contained
, however. By specifying clang
as a linker and
disabling the link-self-contained
option an external version of libc.a
can
be used instead.
Building the target
To build this target first acquire a copy of
wasi-sdk
. At this time version 22
is the minimum needed.
Next configure the WASI_SDK_PATH
environment variable to point to where this
is installed. For example:
export WASI_SDK_PATH=/path/to/wasi-sdk-22.0
Next be sure to enable LLD when building Rust from source as LLVM's wasm-ld
driver for LLD is required when linking WebAssembly code together. Rust's build
system will automatically pick up any necessary binaries and programs from
WASI_SDK_PATH
.
Building Rust programs
The wasm32-wasip1
target is shipped with rustup so users can install
the target with:
rustup target add wasm32-wasip1
Note: the
wasm32-wasip1
target is new and may only be available on nightly by the time you're reading this. Ifwasm32-wasip1
isn't available on stable Rust thenwasm32-wasi
should be available instead.
Rust programs can be built for that target:
rustc --target wasm32-wasip1 your-code.rs
Cross-compilation
This target can be cross-compiled from any hosts.
Testing
This target is tested in rust-lang/rust CI on all merges. A subset of tests are
run in the test-various
builder such as the UI tests and libcore tests. This
can be tested locally, for example, with:
./x.py test --target wasm32-wasip1 tests/ui
Conditionally compiling code
It's recommended to conditionally compile code for this target with:
#[cfg(all(target_os = "wasi", target_env = "p1"))]
Note that the target_env = "p1"
condition first appeared in Rust 1.80. Prior
to Rust 1.80 the target_env
condition was not set.
Enabled WebAssembly features
The default set of WebAssembly features enabled for compilation is currently the
same as wasm32-unknown-unknown
. See the
documentation there for more information.
wasm32-wasip1-threads
Tier: 2
The wasm32-wasip1-threads
target is a new and still (as of July 2023) an
experimental target. This target is an extension to wasm32-wasip1
target,
originally known as wasm32-wasi
. It extends the original target with a
standardized set of syscalls that are intended to empower WebAssembly binaries
with native multi threading capabilities.
Note: Prior to March 2024 this target was known as
wasm32-wasi-preview1-threads
, and even longer before that it was known aswasm32-wasi-threads
.
Target maintainers
- Georgii Rylov, https://github.com/g0djan
- Alex Crichton, https://github.com/alexcrichton
- Andrew Brown, https://github.com/abrown
- Marcin Kolny, https://github.com/loganek
Requirements
This target is cross-compiled. The target supports std
fully.
The Rust target definition here is interesting in a few ways. We want to serve two use cases here with this target:
- First, we want Rust usage of the target to be as hassle-free as possible, ideally avoiding the need to configure and install a local wasm32-wasip1-threads toolchain.
- Second, one of the primary use cases of LLVM's new wasm backend and the
wasm support in LLD is that any compiled language can interoperate with
any other. The
wasm32-wasip1-threads
target is the first with a viable C standard library and sysroot common definition, so we want Rust and C/C++ code to interoperate when compiled towasm32-unknown-unknown
.
You'll note, however, that the two goals above are somewhat at odds with one
another. To attempt to solve both use cases in one go we define a target
that (ab)uses the crt-static
target feature to indicate which one you're
in.
No interop with C required
By default the crt-static
target feature is enabled, and when enabled
this means that the bundled version of libc.a
found in liblibc.rlib
is used. This isn't intended really for interoperation with a C because it
may be the case that Rust's bundled C library is incompatible with a
foreign-compiled C library. In this use case, though, we use rust-lld
and
some copied crt startup object files to ensure that you can download the
wasi target for Rust and you're off to the races, no further configuration
necessary.
All in all, by default, no external dependencies are required. You can
compile wasm32-wasip1-threads
binaries straight out of the box. You can't, however,
reliably interoperate with C code in this mode (yet).
Interop with C required
For the second goal we repurpose the target-feature
flag, meaning that
you'll need to do a few things to have C/Rust code interoperate.
- All Rust code needs to be compiled with
-C target-feature=-crt-static
, indicating that the bundled C standard library in the Rust sysroot will not be used. - If you're using rustc to build a linked artifact then you'll need to
specify
-C linker
to aclang
binary that supportswasm32-wasip1-threads
and is configured with thewasm32-wasip1-threads
sysroot. This will cause Rust code to be linked against the libc.a that the specifiedclang
provides. - If you're building a staticlib and integrating Rust code elsewhere, then
compiling with
-C target-feature=-crt-static
is all you need to do.
All in all, by default, no external dependencies are required. You can
compile wasm32-wasip1-threads
binaries straight out of the box. You can't, however,
reliably interoperate with C code in this mode (yet).
Also note that at this time the wasm32-wasip1-threads
target assumes the
presence of other merged wasm proposals such as (with their LLVM feature flags):
- Bulk memory -
+bulk-memory
- Mutable imported globals -
+mutable-globals
- Atomics -
+atomics
LLVM 16 is required for this target. The reason is related to linker flags: prior to LLVM 16, --import-memory and --export-memory were not allowed together. The reason both are needed is an artifact of how WASI currently does things; see https://github.com/WebAssembly/WASI/issues/502 for more details.
The target intends to match the corresponding Clang target for its "C"
ABI.
Note: due to the relatively early-days nature of this target when working with this target you may encounter LLVM bugs. If an assertion hit or a bug is found it's recommended to open an issue either with rust-lang/rust or ideally with LLVM itself.
Platform requirements
The runtime should support the same set of APIs as any other supported wasi target for interacting with the host environment through the WASI standard. The runtime also should have implementation of wasi-threads proposal.
This target is not a stable target. This means that there are a few engines
which implement the wasi-threads
feature and if they do they're likely behind a
flag, for example:
- Wasmtime -
--wasm-features=threads --wasi-modules=experimental-wasi-threads
- WAMR - needs to be built with WAMR_BUILD_LIB_WASI_THREADS=1
Building the target
Users need to install or built wasi-sdk since release 20.0
https://github.com/WebAssembly/wasi-sdk/releases/tag/wasi-sdk-20
and specify path to wasi-root config.toml
[target.wasm32-wasip1-threads]
wasi-root = ".../wasi-libc/sysroot"
After that users can build this by adding it to the target
list in
config.toml
, or with -Zbuild-std
.
Building Rust programs
From Rust Nightly 1.71.1 (2023-08-03) on the artifacts are shipped pre-compiled:
rustup target add wasm32-wasip1-threads --toolchain nightly
Rust programs can be built for that target:
rustc --target wasm32-wasip1-threads your-code.rs
Cross-compilation
This target can be cross-compiled from any hosts.
Testing
Currently testing is not well supported for wasm32-wasip1-threads
and the
Rust project doesn't run any tests for this target. However the UI testsuite can be run
manually following this instructions:
- Ensure wamr, wasmtime
or another engine that supports
wasi-threads
is installed and can be found in the$PATH
env variable. - Clone master branch.
- Apply such a change with an engine from the step 1.
- Run
./x.py test --target wasm32-wasip1-threads tests/ui
and save the list of failed tests. - Checkout branch with your changes.
- Apply such a change with an engine from the step 1.
- Run
./x.py test --target wasm32-wasip1-threads tests/ui
and save the list of failed tests. - For both lists of failed tests run
cat list | sort > sorted_list
and compare it withdiff sorted_list1 sorted_list2
.
Conditionally compiling code
It's recommended to conditionally compile code for this target with:
#[cfg(all(target_os = "wasi", target_env = "p1", target_feature = "atomics"))]
Prior to Rust 1.80 the target_env = "p1"
key was not set. Currently the
target_feature = "atomics"
is Nightly-only. Note that the precise #[cfg]
necessary to detect this target may change as the target becomes more stable.
Enabled WebAssembly features
The default set of WebAssembly features enabled for compilation includes two
more features in addition to that which
wasm32-unknown-unknown
enables:
bulk-memory
atomics
For more information about features see the documentation for
wasm32-unknown-unknown
, but note that the
mvp
CPU in LLVM does not support this target as it's required that
bulk-memory
, atomics
, and mutable-globals
are all enabled.
wasm32-wasip2
Tier: 3
The wasm32-wasip2
target is a new and still (as of January 2024) an
experimental target. This target is an extension to wasm32-wasip1
target,
originally known as wasm32-wasi
. It is the next evolution in the development of
wasi (the WebAssembly System Interface) that uses the WebAssembly
component model to allow for a standardized set of syscalls that are intended to empower
WebAssembly binaries with native host capabilities.
Target maintainers
- Alex Crichton, https://github.com/alexcrichton
- Ryan Levick, https://github.com/rylev
Requirements
This target is cross-compiled. The target supports std
fully.
Platform requirements
The WebAssembly runtime should support the wasi preview 2 API set. Runtimes also are required to support components since this target outputs a component as opposed to a core wasm module. As of the time of this writing Wasmtime 17 and above is able to run this target natively with no extra flags.
Building the target
To build this target first acquire a copy of
wasi-sdk
. At this time version 22
is the minimum needed.
Next configure the WASI_SDK_PATH
environment variable to point to where this
is installed. For example:
export WASI_SDK_PATH=/path/to/wasi-sdk-22.0
Next be sure to enable LLD when building Rust from source as LLVM's wasm-ld
driver for LLD is required when linking WebAssembly code together. Rust's build
system will automatically pick up any necessary binaries and programs from
WASI_SDK_PATH
.
Testing
This target is not tested in CI at this time. Locally it can be tested with a
wasmtime
binary in PATH
like so:
./x.py test --target wasm32-wasip2 tests/ui
Conditionally compiling code
It's recommended to conditionally compile code for this target with:
#[cfg(all(target_os = "wasi", target_env = "p2"))]
Enabled WebAssembly features
The default set of WebAssembly features enabled for compilation is currently the
same as wasm32-unknown-unknown
. See the
documentation there for more information.
wasm32-unknown-unknown
Tier: 2
The wasm32-unknown-unknown
target is a WebAssembly compilation target which
does not import any functions from the host for the standard library. This is
the "minimal" WebAssembly in the sense of making the fewest assumptions about
the host environment. This target is often used when compiling to the web or
JavaScript environments as there is no standard for what functions can be
imported on the web. This target can also be useful for creating minimal or
bare-bones WebAssembly binaries.
The wasm32-unknown-unknown
target has support for the Rust standard library
but many parts of the standard library do not work and return errors. For
example println!
does nothing, std::fs
always return errors, and
std::thread::spawn
will panic. There is no means by which this can be
overridden. For a WebAssembly target that more fully supports the standard
library see the wasm32-wasip1
or
wasm32-wasip2
targets.
The wasm32-unknown-unknown
target has full support for the core
and alloc
crates. It additionally supports the HashMap
type in the std
crate, although
hash maps are not randomized like they are on other platforms.
One existing user of this target (please feel free to edit and expand this list
too) is the wasm-bindgen
project
which facilitates Rust code interoperating with JavaScript code. Note, though,
that not all uses of wasm32-unknown-unknown
are using JavaScript and the web.
Target maintainers
When this target was added to the compiler, platform-specific documentation here was not maintained at that time. This means that the list below is not exhaustive, and there are more interested parties in this target. That being said, those interested in maintaining this target are:
- Alex Crichton, https://github.com/alexcrichton
Requirements
This target is cross-compiled. The target includes support for std
itself,
but as mentioned above many pieces of functionality that require an operating
system do not work and will return errors.
This target currently has no equivalent in C/C++. There is no C/C++ toolchain for this target. While interop is theoretically possible it's recommended to instead use one of:
wasm32-unknown-emscripten
- for web-based use cases the Emscripten toolchain is typically chosen for running C/C++.wasm32-wasip1
- the wasi-sdk toolchain is used to compile C/C++ on this target and can interop with Rust code. WASI works on the web so far as there's no blocker, but an implementation of WASI APIs must be either chosen or reimplemented.
This target has no build requirements beyond what's in-tree in the Rust
repository. Linking binaries requires LLD to be enabled for the wasm-ld
driver. This target uses the dlmalloc
crate as the default global allocator.
Building the target
Building this target can be done by:
- Configure the
wasm32-unknown-unknown
target to get built. - Configure LLD to be built.
- Ensure the
WebAssembly
target backend is not disabled in LLVM.
These are all controlled through config.toml
options. It should be possible
to build this target on any platform.
Building Rust programs
Rust programs can be compiled by adding this target via rustup:
$ rustup target add wasm32-unknown-unknown
and then compiling with the target:
$ rustc foo.rs --target wasm32-unknown-unknown
$ file foo.wasm
Cross-compilation
This target can be cross-compiled from any host.
Testing
This target is not tested in CI for the rust-lang/rust repository. Many tests
must be disabled to run on this target and failures are non-obvious because
println!
doesn't work in the standard library. It's recommended to test the
wasm32-wasip1
target instead for WebAssembly compatibility.
Conditionally compiling code
It's recommended to conditionally compile code for this target with:
#[cfg(all(target_family = "wasm", target_os = "unknown"))]
Note that there is no way to tell via #[cfg]
whether code will be running on
the web or not.
Enabled WebAssembly features
WebAssembly is an evolving standard which adds new features such as new
instructions over time. This target's default set of supported WebAssembly
features will additionally change over time. The wasm32-unknown-unknown
target
inherits the default settings of LLVM which typically matches the default
settings of Emscripten as well.
Changes to WebAssembly go through a proposals process but reaching the final stage (stage 5) does not automatically mean that the feature will be enabled in LLVM and Rust by default. At this time the general guidance is that features must be present in most engines for a "good chunk of time" before they're enabled in LLVM by default. There is currently no exact number of months or engines that are required to enable features by default.
As of the time of this writing the proposals that are enabled by default (the
generic
CPU in LLVM terminology) are:
multivalue
mutable-globals
reference-types
sign-ext
If you're compiling WebAssembly code for an engine that does not support a feature in LLVM's default feature set then the feature must be disabled at compile time. Note, though, that enabled features may be used in the standard library or precompiled libraries shipped via rustup. This means that not only does your own code need to be compiled with the correct set of flags but the Rust standard library additionally must be recompiled.
Compiling all code for the initial release of WebAssembly looks like:
$ export RUSTFLAGS=-Ctarget-cpu=mvp
$ cargo +nightly build -Zbuild-std=panic_abort,std --target wasm32-unknown-unknown
Here the mvp
"cpu" is a placeholder in LLVM for disabling all supported
features by default. Cargo's -Zbuild-std
feature, a Nightly Rust feature, is
then used to recompile the standard library in addition to your own code. This
will produce a binary that uses only the original WebAssembly features by
default and no proposals since its inception.
To enable individual features it can be done with -Ctarget-feature=+foo
.
Available features for Rust code itself are documented in the reference and
can also be found through:
$ rustc -Ctarget-feature=help --target wasm32-unknown-unknown
You'll need to consult your WebAssembly engine's documentation to learn more about the supported WebAssembly features the engine has.
Note that it is still possible for Rust crates and libraries to enable WebAssembly features on a per-function level. This means that the build command above may not be sufficient to disable all WebAssembly features. If the final binary still has SIMD instructions, for example, the function in question will need to be found and the crate in question will likely contain something like:
#[target_feature(enable = "simd128")]
fn foo() {
// ...
}
In this situation there is no compiler flag to disable emission of SIMD
instructions and the crate must instead be modified to not include this function
at compile time either by default or through a Cargo feature. For crate authors
it's recommended to avoid #[target_feature(enable = "...")]
except where
necessary and instead use:
#[cfg(target_feature = "simd128")]
fn foo() {
// ...
}
That is to say instead of enabling target features it's recommended to conditionally compile code instead. This is notably different to the way native platforms such as x86_64 work, and this is due to the fact that WebAssembly binaries must only contain code the engine understands. Native binaries work so long as the CPU doesn't execute unknown code dynamically at runtime.
wasm64-unknown-unknown
Tier: 3
WebAssembly target which uses 64-bit memories, relying on the memory64 WebAssembly proposal.
Target maintainers
- Alex Crichton, https://github.com/alexcrichton
Requirements
This target is cross-compiled. The target supports std
in the same manner as
the wasm32-unknown-unknown
target which is to say that it comes with the
standard library but many I/O functions such as std::fs
and std::net
will
simply return error. Additionally I/O operations like println!
don't actually
do anything and the prints aren't routed anywhere. This is the same as the
wasm32-unknown-unknown
target. This target comes by default with an allocator,
currently dlmalloc which is ported to rust.
The difference of this target with wasm32-unknown-unknown
is that it's
compiled for 64-bit memories instead of 32-bit memories. This means that usize
is 8-bytes large as well as pointers. The tradeoff, though, is that the maximum
memory size is now the full 64-bit address space instead of the 4GB as limited
by the 32-bit address space for wasm32-unknown-unknown
.
This target is not a stable target. The memory64 WebAssembly proposal is still
in-progress and not standardized. This means that there are not many engines
which implement the memory64
feature and if they do they're likely behind a
flag, for example:
- Nodejs -
--experimental-wasm-memory64
- Wasmtime -
--wasm-features memory64
Also note that at this time the wasm64-unknown-unknown
target assumes the
presence of other merged wasm proposals such as (with their LLVM feature flags):
- Bulk memory -
+bulk-memory
- Mutable imported globals -
+mutable-globals
- Sign-extending operations -
+sign-ext
- Non-trapping fp-to-int operations -
+nontrapping-fptoint
The wasm64-unknown-unknown
target intends to match the default Clang targets
for its "C"
ABI, which is likely to be the same as Clang's
wasm32-unknown-unknown
largely.
Note: due to the relatively early-days nature of this target when working with this target you may encounter LLVM bugs. If an assertion hit or a bug is found it's recommended to open an issue either with rust-lang/rust or ideally with LLVM itself.
This target does not support panic=unwind
at this time.
Building the target
You can build Rust with support for the target by adding it to the target
list in config.toml
, and the target also requires lld
to be built to work.
[build]
target = ["wasm64-unknown-unknown"]
[rust]
lld = true
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of std
by using
build-std
or similar.
Note that the following cfg
directives are set for wasm64-unknown-unknown
:
cfg(target_arch = "wasm64")
cfg(target_family = "wasm")
Testing
Currently testing is not well supported for wasm64-unknown-unknown
and the
Rust project doesn't run any tests for this target. Testing support sort of
works but without println!
it's not the most exciting tests to run.
Cross-compilation toolchains and C code
Compiling Rust code with C code for wasm64-unknown-unknown
is theoretically
possible, but there are no known toolchains to do this at this time. At the time
of this writing there is no known "libc" for wasm that works with
wasm64-unknown-unknown
, which means that mixing C & Rust with this target
effectively cannot be done.
*-win7-windows-msvc
Tier: 3
Windows targets continuing support of windows7.
Target maintainers
- @roblabla
Requirements
This target supports all of core, alloc, std and test. This is automatically tested every night on private infrastructure hosted by the maintainer. Host tools may also work, though those are not currently tested.
Those targets follow Windows calling convention for extern "C".
Like any other Windows target, the created binaries are in PE format.
Building the target
You can build Rust with support for the targets by adding it to the target list in config.toml:
[build]
build-stage = 1
target = [ "x86_64-win7-windows-msvc" ]
Building Rust programs
Rust does not ship pre-compiled artifacts for this target. To compile for this
target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy by using build-std
or
similar.
Testing
Created binaries work fine on Windows or Wine using native hardware. Remote
testing is possible using the remote-test-server
described here.
Cross-compilation toolchains and C code
Compatible C code can be built with either MSVC's cl.exe
or LLVM's clang-cl.
Cross-compilation is possible using clang-cl/lld-link. It also requires the
Windows SDK, which can be acquired using xwin
.
-
Install
clang-cl
andlld-link
on your machine, and make sure they are in your $PATH. -
Install
xwin
:cargo install xwin
-
Use
xwin
to install the Windows SDK:xwin splat --output winsdk
-
Create an
xwin-lld-link
script with the following content:#!/usr/bin/env bash set -e XWIN=/path/to/winsdk lld-link "$@" /libpath:$XWIN/crt/lib/x86_64 /libpath:$XWIN/sdk/lib/um/x86_64 /libpath:$XWIN/sdk/lib/ucrt/x86_64
-
Create an
xwin-clang-cl
script with the following content:#!/usr/bin/env bash set -e XWIN=/path/to/winsdk clang-cl /imsvc "$XWIN/crt/include" /imsvc "$XWIN/sdk/include/ucrt" /imsvc "$XWIN/sdk/include/um" /imsvc "$XWIN/sdk/include/shared" --target="x86_64-pc-windows-msvc" "$@"
-
In your config.toml, add the following lines:
[target.x86_64-win7-windows-msvc] linker = "path/to/xwin-lld-link" cc = "path/to/xwin-clang-cl"
You should now be able to cross-compile the Rust std, and any rust program.
x86_64-fortanix-unknown-sgx
Tier: 2
Secure enclaves using Intel Software Guard Extensions (SGX) based on the ABI defined by Fortanix for the Enclave Development Platform (EDP).
Target maintainers
The EDP team at Fortanix.
- Jethro Beekman @jethrogb
- Raoul Strackx @raoulstrackx
- Mohsen Zohrevandi @mzohreva
Requirements
The target supports std
with a default allocator. Only cross compilation is
supported.
Binaries support all CPUs that include Intel SGX. Only 64-bit mode is supported.
Not all std
features are supported, see Using Rust's
std for details.
The extern "C"
calling convention is the System V AMD64 ABI.
The supported ABI is the fortanix-sgx-abi.
The compiler output is ELF, but the native format for the platform is the SGX stream (SGXS) format. A converter like ftxsgx-elf2sgxs is needed.
Programs in SGXS format adhering to the Fortanix SGX ABI can be run with any compatible runner, such as ftxsgx-runner.
See the EDP installation guide for recommendations on how to setup a development and runtime environment.
Building the target
As a tier 2 target, the target is built by the Rust project.
You can configure bootstrap like so:
[build]
build-stage = 1
target = ["x86_64-fortanix-unknown-sgx"]
Building Rust programs
Standard build flows using cargo
or rustc
should work.
Testing
The Rust test suite as well as custom unit and integration tests will run on hardware that has Intel SGX enabled if a cargo runner is configured correctly, see the requirements section.
Cross-compilation toolchains and C code
C code is not generally supported, as there is no libc. C code compiled for x86-64 in freestanding mode using the System V AMD64 ABI may work. The rs-libc crate contains a subset of libc that's known to work with this target.
x86_64-unknown-linux-none
Tier: 3
Freestanding x86-64 linux binary with no dependency on libc.
Target maintainers
Requirements
This target is cross compiled and can be built from any host.
This target has no support for host tools, std, or alloc.
Building the target
The target can be built by enabling it for a rustc
build:
[build]
build-stage = 1
target = ["x86_64-unknown-linux-none"]
Building Rust programs
Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of core
by using
build-std
or similar.
Testing
Created binaries will run on linux without any external requirements
Cross-compilation toolchains and C code
Support for C code is currently untested
x86_64-unknown-none
Tier: 2
Freestanding/bare-metal x86-64 binaries in ELF format: firmware, kernels, etc.
Target maintainers
- Harald Hoyer
harald@profian.com
, https://github.com/haraldh - Mike Leany, https://github.com/mikeleany
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
By default, Rust code generated for this target does not use any vector or
floating-point registers (e.g. SSE, AVX). This allows the generated code to run
in environments, such as kernels, which may need to avoid the use of such
registers or which may have special considerations about the use of such
registers (e.g. saving and restoring them to avoid breaking userspace code
using the same registers). You can change code generation to use additional CPU
features via the -C target-feature=
codegen options to rustc, or via the
#[target_feature]
mechanism within Rust code.
By default, code generated with this target should run on any x86_64
hardware; enabling additional target features may raise this baseline.
Code generated with this target will use the kernel
code model by default.
You can change this using the -C code-model=
option to rustc.
On x86_64-unknown-none
, extern "C"
uses the standard System V calling
convention, without red zones.
This target generates binaries in the ELF format. Any alternate formats or special considerations for binary layout will require linker options or linker scripts.
Building the target
You can build Rust with support for the target by adding it to the target
list in config.toml
:
[build]
build-stage = 1
target = ["x86_64-unknown-none"]
Building Rust programs
Starting with Rust 1.62, precompiled artifacts are provided via rustup
:
# install cross-compile toolchain
rustup target add x86_64-unknown-none
# target flag may be used with any cargo or rustc command
cargo build --target x86_64-unknown-none
Testing
As x86_64-unknown-none
supports a variety of different environments and does
not support std
, this target does not support running the Rust test suite.
Cross-compilation toolchains and C code
If you want to compile C code along with Rust (such as for Rust crates with C
dependencies), you will need an appropriate x86_64
toolchain.
Rust may be able to use an x86_64-linux-gnu-
toolchain with appropriate
standalone flags to build for this toolchain (depending on the assumptions of
that toolchain, see below), or you may wish to use a separate
x86_64-unknown-none
(or x86_64-elf-
) toolchain.
On some x86_64
hosts that use ELF binaries, you may be able to use the host
C toolchain, if it does not introduce assumptions about the host environment
that don't match the expectations of a standalone environment. Otherwise, you
may need a separate toolchain for standalone/freestanding development, just as
when cross-compiling from a non-x86_64
platform.
xtensa-*-none-elf
Tier: 3
Targets for Xtensa CPUs.
Target maintainers
- Scott Mabin @MabezDev
- Sergio Gasquez @SergioGasquez
Requirements
The target names follow this format: xtensa-$CPU
, where $CPU
specifies the target chip. The following targets are currently defined:
Target name | Target CPU(s) |
---|---|
xtensa-esp32-none-elf | ESP32 |
xtensa-esp32s2-none-elf | ESP32-S2 |
xtensa-esp32s3-none-elf | ESP32-S3 |
Xtensa targets that support std
are documented in the ESP-IDF platform support document
Building the targets
The targets can be built by installing the Xtensa enabled Rust channel. See instructions in the RISC-V and Xtensa Targets section of the The Rust on ESP Book.
*-nuttx-elf
Tier: 3
Targets for the Apache NuttX.
Apache NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. It is scalable from 8-bit to 64-bit microcontroller environments. The primary governing standards in NuttX are POSIX and ANSI standards.
NuttX adopts additional standard APIs from Unix and other common RTOSs, such as VxWorks. These APIs are used for functionality not available under the POSIX and ANSI standards. However, some APIs, like fork(), are not appropriate for deeply-embedded environments and are not implemented in NuttX.
For brevity, many parts of the documentation will refer to Apache NuttX as simply NuttX.
Target maintainers
- Qi Huang @no1wudi
Requirements
The target name follow this format: ARCH[-VENDOR]-nuttx-ABI
, where ARCH
is the target architecture, VENDOR
is the vendor name, and ABI
is the ABI used.
The following target names are defined:
thumbv6m-nuttx-eal
thumbv7m-nuttx-eal
thumbv7em-nuttx-eabi
thumbv7em-nuttx-eabihf
thumbv8m.base-nuttx-eabi
thumbv8m.main-nuttx-eabi
thumbv8m.main-nuttx-eabihf
riscv32imc-unknown-nuttx-elf
riscv32imac-unknown-nuttx-elf
riscv32imafc-unknown-nuttx-elf
riscv64imac-unknown-nuttx-elf
riscv64gc-unknown-nuttx-elf
Building the target
The target can be built by enabled in the rustc
build:
[build]
target = "riscv32imc-unknown-nuttx-elf"
[target.'riscv32imc-unknown-nuttx-elf']
linker = "riscv-none-elf-gcc"
The toolchain for the target can be found in NuttX's quick start guide.
Testing
This is a cross-compiled no-std
target, which must be run either in a simulator
or by programming them onto suitable hardware. It is not possible to run the
Rust test-suite on this target.
Cross-compilation toolchains and C code
This target supports C code. If interlinking with C or C++, you may need to use
riscv-none-elf-gcc
or arm-none-eabi-gcc
as a linker instead of rust-lld
.
i386-lynx-lynxos178
Tier: 3
Lynx LynxOS-178 POSIX Real-time operating system.
This support has been implemented by Ferrous Systems as part of the Ferrocene Compiler.
Note that LynxOS-178 is a different RTOS to LynxOS. There is no support for LynxOS in this target, only LynxOS-178.
Target maintainers
- Jonathan Pallant
jonathan.pallant@ferrous-systems.com
, https://ferrous-systems.com
Requirements
This target is cross-compiled. There is no support for std
. There is no
default allocator, but it's possible to use alloc
by supplying an allocator.
This target generates binaries in the ELF format. Any alternate formats or
special considerations for binary layout will require linker options or linker
scripts. The target requires you to have the i386-lynx-lynxos178-gcc-7.1.0
linker from Lynx mosa-ic 2022.07 or similar.
Building Rust programs
Precompiled arfefacts are only supplied with the Ferrocene compiler. Only
libcore
is available, not libstd
.
# install cross-compile toolchain
criticalup target add i386-lynx-lynxos178
# target flag may be used with any cargo or rustc command
cargo build --target i386-lynx-lynxos178
Small example application
#![no_std]
#![no_main]
use core::fmt::Write;
/// Represents our standard output stream.
pub struct STDOUT;
impl core::fmt::Write for &STDOUT {
fn write_str(&mut self, s: &str) -> core::fmt::Result {
unsafe {
write(1, s.as_ptr(), s.len());
}
Ok(())
}
}
#[link(name = "c")]
extern "C" {
pub fn write(fd: i32, ptr: *const u8, len: usize) -> i32;
pub fn _exit(code: i32);
}
#[no_mangle]
pub extern "C" fn main() -> isize {
writeln!(&STDOUT, "Hello, LynxOS-178").unwrap();
0
}
/// The default panic handler - prints the panic to stdout.
#[panic_handler]
fn panic(info: &core::panic::PanicInfo) -> ! {
let _ = writeln!(&STDOUT, "Panic!: {:#?}", info);
unsafe {
_exit(1);
}
loop {}
}
This example will need to be linked against both lib/libc.a
and
lib/libpthread.a
. You can use a build.rs
file to do this:
fn main() {
let bsp_path = std::env::var("BSP_ROOT").expect("Set BSP_ROOT");
println!("cargo:rustc-link-lib=static=c");
println!("cargo:rustc-link-lib=static=pthread");
println!("cargo:rustc-link-search=native={}/lib", bsp_path);
}
Testing
As i386-lynx-lynxos178
does not support std
, this target does not support
running the Rust test suite.
Targets
rustc
is a cross-compiler by default. This means that you can use any compiler to build for any
architecture. The list of targets are the possible architectures that you can build for. See
the Platform Support page for a detailed list of targets, or
Built-in Targets for instructions on how to view what is available for your version
of rustc
.
To see all the options that you can set with a target, see the docs here.
To compile to a particular target, use the --target
flag:
$ rustc src/main.rs --target=wasm32-unknown-unknown
Target Features
x86
, and ARMv8
are two popular CPU architectures. Their instruction sets form a common baseline across most CPUs. However, some CPUs extend these with custom instruction sets, e.g. vector (AVX
), bitwise manipulation (BMI
) or cryptographic (AES
).
Developers, who know on which CPUs their compiled code is going to run can choose to add (or remove) CPU specific instruction sets via the -C target-feature=val
flag.
Please note, that this flag is generally considered as unsafe. More details can be found in this section.
Built-in Targets
rustc
ships with the ability to compile to many targets automatically, we
call these "built-in" targets, and they generally correspond to targets that
the team is supporting directly. To see the list of built-in targets, you can
run rustc --print target-list
.
Typically, a target needs a compiled copy of the Rust standard library to work. If using rustup, then check out the documentation on Cross-compilation on how to download a pre-built standard library built by the official Rust distributions. Most targets will need a system linker, and possibly other things.
Custom Targets
If you'd like to build for a target that is not yet supported by rustc
, you can use a
"custom target specification" to define a target. These target specification files
are JSON. To see the JSON for the host target, you can run:
rustc +nightly -Z unstable-options --print target-spec-json
To see it for a different target, add the --target
flag:
rustc +nightly -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json
To use a custom target, see the (unstable) build-std
feature of cargo
.
Custom Target Lookup Path
When rustc
is given an option --target=TARGET
(where TARGET
is any string), it uses the following logic:
- if
TARGET
is the name of a built-in target, use that - if
TARGET
is a path to a file, read that file as a json target - otherwise, search the colon-seperated list of directories found
in the
RUST_TARGET_PATH
environment variable from left to right for a file namedTARGET.json
.
These steps are tried in order, so if there are multple potentially valid interpretations for a target, whichever is found first will take priority. If none of these methods find a target, an error is thrown.
Known Issues
This section informs you about known "gotchas". Keep in mind, that this section is (and always will be) incomplete. For suggestions and amendments, feel free to contribute to this guide.
Target Features
Most target-feature problems arise, when mixing code that have the target-feature enabled with code that have it disabled. If you want to avoid undefined behavior, it is recommended to build all code (including the standard library and imported crates) with a common set of target-features.
By default, compiling your code with the -C target-feature
flag will not recompile the entire standard library and/or imported crates with matching target features. Therefore, target features are generally considered as unsafe. Using #[target_feature]
on individual functions makes the function unsafe.
Examples:
Target-Feature | Issue | Seen on | Description | Details |
---|---|---|---|---|
+soft-float and -sse | Segfaults and ABI mismatches | x86 and x86-64 | The x86 and x86_64 architecture uses SSE registers (aka xmm ) for floating point operations. Using software emulated floats ("soft-floats") disables usage of xmm registers, but parts of Rust's core libraries (e.g. std::f32 or std::f64 ) are compiled without soft-floats and expect parameters to be passed in xmm registers. This leads to ABI mismatches. Attempting to compile with disabled SSE causes the same error, too. | #63466 |
Profile-guided Optimization
rustc
supports doing profile-guided optimization (PGO).
This chapter describes what PGO is, what it is good for, and how it can be used.
What Is Profiled-Guided Optimization?
The basic concept of PGO is to collect data about the typical execution of a program (e.g. which branches it is likely to take) and then use this data to inform optimizations such as inlining, machine-code layout, register allocation, etc.
There are different ways of collecting data about a program's execution.
One is to run the program inside a profiler (such as perf
) and another
is to create an instrumented binary, that is, a binary that has data
collection built into it, and run that.
The latter usually provides more accurate data and it is also what is
supported by rustc
.
Usage
Generating a PGO-optimized program involves following a workflow with four steps:
- Compile the program with instrumentation enabled
(e.g.
rustc -Cprofile-generate=/tmp/pgo-data main.rs
) - Run the instrumented program (e.g.
./main
) which generates adefault_<id>.profraw
file - Convert the
.profraw
file into a.profdata
file using LLVM'sllvm-profdata
tool - Compile the program again, this time making use of the profiling data
(for example
rustc -Cprofile-use=merged.profdata main.rs
)
An instrumented program will create one or more .profraw
files, one for each
instrumented binary. E.g. an instrumented executable that loads two instrumented
dynamic libraries at runtime will generate three .profraw
files. Running an
instrumented binary multiple times, on the other hand, will re-use the
respective .profraw
files, updating them in place.
These .profraw
files have to be post-processed before they can be fed back
into the compiler. This is done by the llvm-profdata
tool. This tool
is most easily installed via
rustup component add llvm-tools-preview
Note that installing the llvm-tools-preview
component won't add
llvm-profdata
to the PATH
. Rather, the tool can be found in:
~/.rustup/toolchains/<toolchain>/lib/rustlib/<target-triple>/bin/
Alternatively, an llvm-profdata
coming with a recent LLVM or Clang
version usually works too.
The llvm-profdata
tool merges multiple .profraw
files into a single
.profdata
file that can then be fed back into the compiler via
-Cprofile-use
:
# STEP 1: Compile the binary with instrumentation
rustc -Cprofile-generate=/tmp/pgo-data -O ./main.rs
# STEP 2: Run the binary a few times, maybe with common sets of args.
# Each run will create or update `.profraw` files in /tmp/pgo-data
./main mydata1.csv
./main mydata2.csv
./main mydata3.csv
# STEP 3: Merge and post-process all the `.profraw` files in /tmp/pgo-data
llvm-profdata merge -o ./merged.profdata /tmp/pgo-data
# STEP 4: Use the merged `.profdata` file during optimization. All `rustc`
# flags have to be the same.
rustc -Cprofile-use=./merged.profdata -O ./main.rs
A Complete Cargo Workflow
Using this feature with Cargo works very similar to using it with rustc
directly. Again, we generate an instrumented binary, run it to produce data,
merge the data, and feed it back into the compiler. Some things of note:
-
We use the
RUSTFLAGS
environment variable in order to pass the PGO compiler flags to the compilation of all crates in the program. -
We pass the
--target
flag to Cargo, which prevents theRUSTFLAGS
arguments to be passed to Cargo build scripts. We don't want the build scripts to generate a bunch of.profraw
files. -
We pass
--release
to Cargo because that's where PGO makes the most sense. In theory, PGO can also be done on debug builds but there is little reason to do so. -
It is recommended to use absolute paths for the argument of
-Cprofile-generate
and-Cprofile-use
. Cargo can invokerustc
with varying working directories, meaning thatrustc
will not be able to find the supplied.profdata
file. With absolute paths this is not an issue. -
It is good practice to make sure that there is no left-over profiling data from previous compilation sessions. Just deleting the directory is a simple way of doing so (see
STEP 0
below).
This is what the entire workflow looks like:
# STEP 0: Make sure there is no left-over profiling data from previous runs
rm -rf /tmp/pgo-data
# STEP 1: Build the instrumented binaries
RUSTFLAGS="-Cprofile-generate=/tmp/pgo-data" \
cargo build --release --target=x86_64-unknown-linux-gnu
# STEP 2: Run the instrumented binaries with some typical data
./target/x86_64-unknown-linux-gnu/release/myprogram mydata1.csv
./target/x86_64-unknown-linux-gnu/release/myprogram mydata2.csv
./target/x86_64-unknown-linux-gnu/release/myprogram mydata3.csv
# STEP 3: Merge the `.profraw` files into a `.profdata` file
llvm-profdata merge -o /tmp/pgo-data/merged.profdata /tmp/pgo-data
# STEP 4: Use the `.profdata` file for guiding optimizations
RUSTFLAGS="-Cprofile-use=/tmp/pgo-data/merged.profdata" \
cargo build --release --target=x86_64-unknown-linux-gnu
Troubleshooting
-
It is recommended to pass
-Cllvm-args=-pgo-warn-missing-function
during the-Cprofile-use
phase. LLVM by default does not warn if it cannot find profiling data for a given function. Enabling this warning will make it easier to spot errors in your setup. -
There is a known issue in Cargo prior to version 1.39 that will prevent PGO from working correctly. Be sure to use Cargo 1.39 or newer when doing PGO.
Further Reading
rustc
's PGO support relies entirely on LLVM's implementation of the feature
and is equivalent to what Clang offers via the -fprofile-generate
/
-fprofile-use
flags. The Profile Guided Optimization section
in Clang's documentation is therefore an interesting read for anyone who wants
to use PGO with Rust.
Community Maintained Tools
As an alternative to directly using the compiler for Profile-Guided Optimization,
you may choose to go with cargo-pgo
, which has an intuitive command-line API
and saves you the trouble of doing all the manual work. You can read more about
it in their repository accessible from this link: https://github.com/Kobzol/cargo-pgo
For the sake of completeness, here are the corresponding steps using cargo-pgo
:
# Install if you haven't already
cargo install cargo-pgo
cargo pgo build
cargo pgo optimize
These steps will do the following just as before:
- Build an instrumented binary from the source code.
- Run the instrumented binary to gather PGO profiles.
- Use the gathered PGO profiles from the last step to build an optimized binary.
Instrumentation-based Code Coverage
Introduction
The Rust compiler includes two code coverage implementations:
- A GCC-compatible, gcov-based coverage implementation, enabled with
-Z profile
, which derives coverage data based on DebugInfo. - A source-based code coverage implementation, enabled with
-C instrument-coverage
, which uses LLVM's native, efficient coverage instrumentation to generate very precise coverage data.
This document describes how to enable and use the LLVM instrumentation-based coverage, via the -C instrument-coverage
compiler flag.
How it works
When -C instrument-coverage
is enabled, the Rust compiler enhances rust-based libraries and binaries by:
- Automatically injecting calls to an LLVM intrinsic (
llvm.instrprof.increment
), at functions and branches in compiled code, to increment counters when conditional sections of code are executed. - Embedding additional information in the data section of each library and binary (using the LLVM Code Coverage Mapping Format Version 5, if compiling with LLVM 12, or Version 6, if compiling with LLVM 13 or higher), to define the code regions (start and end positions in the source code) being counted.
When running a coverage-instrumented program, the counter values are written to a profraw
file at program termination. LLVM bundles tools that read the counter results, combine those results with the coverage map (embedded in the program binary), and generate coverage reports in multiple formats.
Note:
-C instrument-coverage
also automatically enables-C symbol-mangling-version=v0
(tracking issue #60705). Thev0
symbol mangler is strongly recommended. Thev0
demangler can be overridden by explicitly adding-Z unstable-options -C symbol-mangling-version=legacy
.
Enable coverage profiling in the Rust compiler
Rust's source-based code coverage requires the Rust "profiler runtime". Without it, compiling with -C instrument-coverage
generates an error that the profiler runtime is missing.
The Rust nightly
distribution channel includes the profiler runtime, by default.
Important: If you are building the Rust compiler from the source distribution, the profiler runtime is not enabled in the default
config.example.toml
. Edit yourconfig.toml
file and ensure theprofiler
feature is set it totrue
(either under the[build]
section, or under the settings for an individual[target.<triple>]
):# Build the profiler runtime (required when compiling with options that depend # on this runtime, such as `-C profile-generate` or `-C instrument-coverage`). profiler = true
Building the demangler
LLVM coverage reporting tools generate results that can include function names and other symbol references, and the raw coverage results report symbols using the compiler's "mangled" version of the symbol names, which can be difficult to interpret. To work around this issue, LLVM coverage tools also support a user-specified symbol name demangler.
One option for a Rust demangler is rustfilt
, which can be installed with:
cargo install rustfilt
Compiling with coverage enabled
Set the -C instrument-coverage
compiler flag in order to enable LLVM source-based code coverage profiling.
The default option generates coverage for all functions, including unused (never called) functions and generics. The compiler flag supports an optional value to tailor this behavior. (See -C instrument-coverage=<options>
, below.)
With cargo
, you can instrument your program binary and dependencies at the same time.
For example (if your project's Cargo.toml builds a binary by default):
$ cd your-project
$ cargo clean
$ RUSTFLAGS="-C instrument-coverage" cargo build
If cargo
is not configured to use your profiler
-enabled version of rustc
, set the path explicitly via the RUSTC
environment variable. Here is another example, using a stage1
build of rustc
to compile an example
binary (from the json5format
crate):
$ RUSTC=$HOME/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc \
RUSTFLAGS="-C instrument-coverage" \
cargo build --example formatjson5
Note: that some compiler options, combined with
-C instrument-coverage
, can produce LLVM IR and/or linked binaries that are incompatible with LLVM coverage maps. For example, coverage requires references to actual functions in LLVM IR. If any covered function is optimized out, the coverage tools may not be able to process the coverage results. If you need to pass additional options, with coverage enabled, test them early, to confirm you will get the coverage results you expect.
Running the instrumented binary to generate raw coverage profiling data
In the previous example, cargo
generated the coverage-instrumented binary formatjson5
:
$ echo "{some: 'thing'}" | target/debug/examples/formatjson5 -
{
some: "thing",
}
After running this program, a new file named like default_11699812450447639123_0_20944
should be in the current working directory.
A new, unique file name will be generated each time the program is run to avoid overwriting previous data.
$ echo "{some: 'thing'}" | target/debug/examples/formatjson5 -
...
$ ls default_*.profraw
default_11699812450447639123_0_20944.profraw
You can also set a specific file name or path for the generated .profraw
files by using the environment variable LLVM_PROFILE_FILE
:
$ echo "{some: 'thing'}" \
| LLVM_PROFILE_FILE="formatjson5.profraw" target/debug/examples/formatjson5 -
...
$ ls formatjson5.profraw
formatjson5.profraw
If LLVM_PROFILE_FILE
contains a path to a nonexistent directory, the missing directory structure will be created. Additionally, the following special pattern strings are rewritten:
%p
- The process ID.%h
- The hostname of the machine running the program.%t
- The value of the TMPDIR environment variable.%Nm
- the instrumented binary’s signature: The runtime creates a pool of N raw profiles, used for on-line profile merging. The runtime takes care of selecting a raw profile from the pool, locking it, and updating it before the program exits.N
must be between1
and9
, and defaults to1
if omitted (with simply%m
).%c
- Does not add anything to the filename, but enables a mode (on some platforms, including Darwin) in which profile counter updates are continuously synced to a file. This means that if the instrumented program crashes, or is killed by a signal, perfect coverage information can still be recovered.
In the first example above, the value 11699812450447639123_0
in the generated filename is the instrumented binary's signature,
which replaced the %m
pattern and the value 20944
is the process ID of the binary being executed.
Installing LLVM coverage tools
LLVM's supplies two tools—llvm-profdata
and llvm-cov
—that process coverage data and generate reports. There are several ways to find and/or install these tools, but note that the coverage mapping data generated by the Rust compiler requires LLVM version 12 or higher, and processing the raw data may require exactly the LLVM version used by the compiler. (llvm-cov --version
typically shows the tool's LLVM version number, and rustc --verbose --version
shows the version of LLVM used by the Rust compiler.)
- You can install compatible versions of these tools via the
rustup
componentllvm-tools-preview
. This component is the recommended path, though the specific tools available and their interface is not currently subject to Rust's usual stability guarantees. In this case, you may also findcargo-binutils
useful as a wrapper around these tools. - You can install a compatible version of LLVM tools from your operating system distribution, or from your distribution of LLVM.
- If you are building the Rust compiler from source, you can optionally use the bundled LLVM tools, built from source. Those tool binaries can typically be found in your build platform directory at something like:
rust/build/x86_64-unknown-linux-gnu/llvm/bin/llvm-*
.
The examples in this document show how to use the llvm tools directly.
Creating coverage reports
Raw profiles have to be indexed before they can be used to generate coverage reports. This is done using llvm-profdata merge
, which can combine multiple raw profiles and index them at the same time:
$ llvm-profdata merge -sparse formatjson5.profraw -o formatjson5.profdata
Finally, the .profdata
file is used, in combination with the coverage map (from the program binary) to generate coverage reports using llvm-cov report
, for a coverage summaries; and llvm-cov show
, to see detailed coverage of lines and regions (character ranges) overlaid on the original source code.
These commands have several display and filtering options. For example:
$ llvm-cov show -Xdemangler=rustfilt target/debug/examples/formatjson5 \
-instr-profile=formatjson5.profdata \
-show-line-counts-or-regions \
-show-instantiations \
-name=add_quoted_string
![Screenshot of sample `llvm-cov show` result, for function add_quoted_string](images/llvm-cov-show-01.png)
Some of the more notable options in this example include:
--Xdemangler=rustfilt
- the command name or path used to demangle Rust symbols (rustfilt
in the example)target/debug/examples/formatjson5
- the instrumented binary (from which to extract the coverage map)--instr-profile=<path-to-file>.profdata
- the location of the.profdata
file created byllvm-profdata merge
(from the.profraw
file generated by the instrumented binary)--name=<exact-function-name>
- to show coverage for a specific function (or, consider using another filter option, such as--name-regex=<pattern>
)
Note: Coverage can also be disabled on an individual function by annotating the function with the [
coverage(off)
attribute] (which requires the feature flag#![feature(coverage)]
).
Interpreting reports
There are four statistics tracked in a coverage summary:
- Function coverage is the percentage of functions that have been executed at least once. A function is considered to be executed if any of its instantiations are executed.
- Instantiation coverage is the percentage of function instantiations that have been executed at least once. Generic functions and functions generated from macros are two kinds of functions that may have multiple instantiations.
- Line coverage is the percentage of code lines that have been executed at least once. Only executable lines within function bodies are considered to be code lines.
- Region coverage is the percentage of code regions that have been executed at least once. A code region may span multiple lines: for example, in a large function body with no control flow. In other cases, a single line can contain multiple code regions:
return x || (y && z)
has countable code regions forx
(which may resolve the expression, ifx
istrue
),|| (y && z)
(executed only ifx
wasfalse
), andreturn
(executed in either situation).
Of these four statistics, function coverage is usually the least granular while region coverage is the most granular. The project-wide totals for each statistic are listed in the summary.
Test coverage
A typical use case for coverage analysis is test coverage. Rust's source-based coverage tools can both measure your tests' code coverage as percentage, and pinpoint functions and branches not tested.
The following example (using the json5format
crate, for demonstration purposes) show how to generate and analyze coverage results for all tests in a crate.
Since cargo test
both builds and runs the tests, we set the additional RUSTFLAGS
, to add the -C instrument-coverage
flag.
$ RUSTFLAGS="-C instrument-coverage" \
cargo test --tests
Note: The default for
LLVM_PROFILE_FILE
isdefault_%m_%p.profraw
. Versions prior to 1.65 had a default ofdefault.profraw
, so if using those earlier versions, it is recommended to explicitly setLLVM_PROFILE_FILE="default_%m_%p.profraw"
to avoid having multiple tests overwrite the.profraw
files.
Make note of the test binary file paths, displayed after the word "Running
" in the test output:
...
Compiling json5format v0.1.3 ($HOME/json5format)
Finished test [unoptimized + debuginfo] target(s) in 14.60s
Running target/debug/deps/json5format-fececd4653271682
running 25 tests
...
test result: ok. 25 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
Running target/debug/deps/lib-30768f9c53506dc5
running 31 tests
...
test result: ok. 31 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
You should have one or more .profraw
files now, one for each test binary. Run the profdata
tool to merge them:
$ llvm-profdata merge -sparse default_*.profraw -o json5format.profdata
Then run the cov
tool, with the profdata
file and all test binaries:
$ llvm-cov report \
--use-color --ignore-filename-regex='/.cargo/registry' \
--instr-profile=json5format.profdata \
--object target/debug/deps/lib-30768f9c53506dc5 \
--object target/debug/deps/json5format-fececd4653271682
$ llvm-cov show \
--use-color --ignore-filename-regex='/.cargo/registry' \
--instr-profile=json5format.profdata \
--object target/debug/deps/lib-30768f9c53506dc5 \
--object target/debug/deps/json5format-fececd4653271682 \
--show-instantiations --show-line-counts-or-regions \
--Xdemangler=rustfilt | less -R
Note: If overriding the default
profraw
file name via theLLVM_PROFILE_FILE
environment variable, it's highly recommended to use the%m
and%p
special pattern strings to generate unique file names in the case of more than a single test binary being executed.
Note: The command line option
--ignore-filename-regex=/.cargo/registry
, which excludes the sources for dependencies from the coverage results._
Tips for listing the binaries automatically
For bash
users, one suggested way to automatically complete the cov
command with the list of binaries is with a command like:
$ llvm-cov report \
$( \
for file in \
$( \
RUSTFLAGS="-C instrument-coverage" \
cargo test --tests --no-run --message-format=json \
| jq -r "select(.profile.test == true) | .filenames[]" \
| grep -v dSYM - \
); \
do \
printf "%s %s " -object $file; \
done \
) \
--instr-profile=json5format.profdata --summary-only # and/or other options
Adding --no-run --message-format=json
to the same cargo test
command used to run
the tests (including the same environment variables and flags) generates output in a JSON
format that jq
can easily query.
The printf
command takes this list and generates the --object <binary>
arguments
for each listed test binary.
Including doc tests
The previous examples run cargo test
with --tests
, which excludes doc tests.1
To include doc tests in the coverage results, drop the --tests
flag, and apply the
-C instrument-coverage
flag, and some doc-test-specific options in the
RUSTDOCFLAGS
environment variable. (The llvm-profdata
command does not change.)
$ RUSTFLAGS="-C instrument-coverage" \
RUSTDOCFLAGS="-C instrument-coverage -Z unstable-options --persist-doctests target/debug/doctestbins" \
cargo test
$ llvm-profdata merge -sparse default_*.profraw -o json5format.profdata
The -Z unstable-options --persist-doctests
flag is required, to save the test binaries
(with their coverage maps) for llvm-cov
.
$ llvm-cov report \
$( \
for file in \
$( \
RUSTFLAGS="-C instrument-coverage" \
RUSTDOCFLAGS="-C instrument-coverage -Z unstable-options --persist-doctests target/debug/doctestbins" \
cargo test --no-run --message-format=json \
| jq -r "select(.profile.test == true) | .filenames[]" \
| grep -v dSYM - \
) \
target/debug/doctestbins/*/rust_out; \
do \
[[ -x $file ]] && printf "%s %s " -object $file; \
done \
) \
--instr-profile=json5format.profdata --summary-only # and/or other options
Note: The differences in this
llvm-cov
invocation, compared with the version without doc tests, include:
- The
cargo test ... --no-run
command is updated with the same environment variables and flags used to build the tests, including the doc tests. - The file glob pattern
target/debug/doctestbins/*/rust_out
adds therust_out
binaries generated for doc tests (note, however, that somerust_out
files may not be executable binaries). [[ -x $file ]] &&
filters the files passed on to theprintf
, to include only executable binaries.
There is ongoing work to resolve a known issue
(#79417) that doc test coverage
generates incorrect source line numbers in llvm-cov show
results.
-C instrument-coverage=<options>
-C instrument-coverage=no
(orn
/off
/false
): Don't enable coverage instrumentation. No functions will be instrumented for coverage.- This is the same as not using the
-C instrument-coverage
flag at all.
- This is the same as not using the
-C instrument-coverage=yes
(ory
/on
/true
): Enable coverage instrumentation with the default behaviour. Currently this instruments all functions, including unused functions and unused generics.- This is the same as
-C instrument-coverage
with no value.
- This is the same as
Other values
-C instrument-coverage=all
: Currently an alias foryes
, but may behave differently in the future if more fine-grained coverage options are added. Using this value is currently not recommended.
-Z coverage-options=<options>
This unstable option is described in the Unstable Book.
Other references
Rust's implementation and workflow for source-based code coverage is based on the same library and tools used to implement source-based code coverage in Clang. (This document is partially based on the Clang guide.)
Linker-plugin-based LTO
The -C linker-plugin-lto
flag allows for deferring the LTO optimization
to the actual linking step, which in turn allows for performing
interprocedural optimizations across programming language boundaries if
all the object files being linked were created by LLVM based toolchains.
The prime example here would be linking Rust code together with
Clang-compiled C/C++ code.
Usage
There are two main cases how linker plugin based LTO can be used:
- compiling a Rust
staticlib
that is used as a C ABI dependency - compiling a Rust binary where
rustc
invokes the linker
In both cases the Rust code has to be compiled with -C linker-plugin-lto
and
the C/C++ code with -flto
or -flto=thin
so that object files are emitted
as LLVM bitcode.
Rust staticlib
as dependency in C/C++ program
In this case the Rust compiler just has to make sure that the object files in
the staticlib
are in the right format. For linking, a linker with the
LLVM plugin must be used (e.g. LLD).
Using rustc
directly:
# Compile the Rust staticlib
rustc --crate-type=staticlib -Clinker-plugin-lto -Copt-level=2 ./lib.rs
# Compile the C code with `-flto=thin`
clang -c -O2 -flto=thin -o cmain.o ./cmain.c
# Link everything, making sure that we use an appropriate linker
clang -flto=thin -fuse-ld=lld -L . -l"name-of-your-rust-lib" -o main -O2 ./cmain.o
Using cargo
:
# Compile the Rust staticlib
RUSTFLAGS="-Clinker-plugin-lto" cargo build --release
# Compile the C code with `-flto=thin`
clang -c -O2 -flto=thin -o cmain.o ./cmain.c
# Link everything, making sure that we use an appropriate linker
clang -flto=thin -fuse-ld=lld -L . -l"name-of-your-rust-lib" -o main -O2 ./cmain.o
C/C++ code as a dependency in Rust
In this case the linker will be invoked by rustc
. We again have to make sure
that an appropriate linker is used.
Using rustc
directly:
# Compile C code with `-flto`
clang ./clib.c -flto=thin -c -o ./clib.o -O2
# Create a static library from the C code
ar crus ./libxyz.a ./clib.o
# Invoke `rustc` with the additional arguments
rustc -Clinker-plugin-lto -L. -Copt-level=2 -Clinker=clang -Clink-arg=-fuse-ld=lld ./main.rs
Using cargo
directly:
# Compile C code with `-flto`
clang ./clib.c -flto=thin -c -o ./clib.o -O2
# Create a static library from the C code
ar crus ./libxyz.a ./clib.o
# Set the linking arguments via RUSTFLAGS
RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build --release
Explicitly specifying the linker plugin to be used by rustc
If one wants to use a linker other than LLD, the LLVM linker plugin has to be
specified explicitly. Otherwise the linker cannot read the object files. The
path to the plugin is passed as an argument to the -Clinker-plugin-lto
option:
rustc -Clinker-plugin-lto="/path/to/LLVMgold.so" -L. -Copt-level=2 ./main.rs
Usage with clang-cl and x86_64-pc-windows-msvc
Cross language LTO can be used with the x86_64-pc-windows-msvc target, but this requires using the clang-cl compiler instead of the MSVC cl.exe included with Visual Studio Build Tools, and linking with lld-link. Both clang-cl and lld-link can be downloaded from LLVM's download page. Note that most crates in the ecosystem are likely to assume you are using cl.exe if using this target and that some things, like for example vcpkg, don't work very well with clang-cl.
You will want to make sure your rust major LLVM version matches your installed LLVM tooling version, otherwise it is likely you will get linker errors:
rustc -V --verbose
clang-cl --version
If you are compiling any proc-macros, you will get this error:
error: Linker plugin based LTO is not supported together with `-C prefer-dynamic` when
targeting Windows-like targets
This is fixed if you explicitly set the target, for example
cargo build --target x86_64-pc-windows-msvc
Without an explicit --target the flags will be passed to all compiler invocations (including build
scripts and proc macros), see cargo docs on rustflags
If you have dependencies using the cc
crate, you will need to set these
environment variables:
set CC=clang-cl
set CXX=clang-cl
set CFLAGS=/clang:-flto=thin /clang:-fuse-ld=lld-link
set CXXFLAGS=/clang:-flto=thin /clang:-fuse-ld=lld-link
REM Needed because msvc's lib.exe crashes on LLVM LTO .obj files
set AR=llvm-lib
If you are specifying lld-link as your linker by setting linker = "lld-link.exe"
in your cargo config,
you may run into issues with some crates that compile code with separate cargo invocations. You should be
able to get around this problem by setting -Clinker=lld-link
in RUSTFLAGS
Toolchain Compatibility
In order for this kind of LTO to work, the LLVM linker plugin must be able to
handle the LLVM bitcode produced by both rustc
and clang
.
Best results are achieved by using a rustc
and clang
that are based on the
exact same version of LLVM. One can use rustc -vV
in order to view the LLVM
used by a given rustc
version. Note that the version number given
here is only an approximation as Rust sometimes uses unstable revisions of
LLVM. However, the approximation is usually reliable.
The following table shows known good combinations of toolchain versions.
Rust Version | Clang Version |
---|---|
1.34 - 1.37 | 8 |
1.38 - 1.44 | 9 |
1.45 - 1.46 | 10 |
1.47 - 1.51 | 11 |
1.52 - 1.55 | 12 |
1.56 - 1.59 | 13 |
1.60 - 1.64 | 14 |
1.65 - 1.69 | 15 |
1.70 - 1.72 | 16 |
1.73 - 1.77 | 17 |
1.78 | 18 |
Note that the compatibility policy for this feature might change in the future.
Checking conditional configurations
rustc
supports checking that every reachable1 #[cfg]
matches a list of the
expected config names and values.
This can help with verifying that the crate is correctly handling conditional compilation for different target platforms or features. It ensures that the cfg settings are consistent between what is intended and what is used, helping to catch potential bugs or errors early in the development process.
In order to accomplish that goal, rustc
accepts the --check-cfg
flag, which specifies
whether to check conditions and how to check them.
Note: For interacting with this through Cargo, see Cargo Specifics page.
rustc
promises to at least check reachable #[cfg]
, and while non-reachable
#[cfg]
are not currently checked, they may well be checked in the future without it being a
breaking change.
Specifying expected names and values
To specify expected names and values, the check cfg specification provides the cfg(...)
option which enables specifying for an expected config name and it's expected values.
Note: No implicit expectation is added when using
--cfg
. Users are expected to pass all expected names and values using the check cfg specification.
It has this basic form:
rustc --check-cfg 'cfg(name, values("value1", "value2", ... "valueN"))'
where name
is a bare identifier (has no quotes) and each "value"
term is a quoted literal
string. name
specifies the name of the condition, such as feature
or my_cfg
.
"value"
specify one of the value of that condition name.
When the cfg(...)
option is specified, rustc
will check every1:
#[cfg(name = "value")]
attribute#[cfg_attr(name = "value")]
attribute#[link(name = "a", cfg(name = "value"))]
attributecfg!(name = "value")
macro call
The command line
--cfg
arguments are currently NOT checked but may very well be checked in the future.
rustc
will check that the "value"
specified is present in the list of expected values.
If "value"
is not in it, then rustc
will report an unexpected_cfgs
lint diagnostic.
The default diagnostic level for this lint is Warn
.
To check for the none value (ie #[cfg(foo)]
) one can use the none()
predicate inside
values()
: values(none())
. It can be followed or preceded by any number of "value"
.
To enable checking of values, but to provide an none/empty set of expected values
(ie. expect #[cfg(name)]
), use these forms:
rustc --check-cfg 'cfg(name)'
rustc --check-cfg 'cfg(name, values(none()))'
To enable checking of name but not values, use one of these forms:
-
No expected values (will lint on every value of
name
):rustc --check-cfg 'cfg(name, values())'
-
Unknown expected values (will never lint on value of
name
):rustc --check-cfg 'cfg(name, values(any()))'
To avoid repeating the same set of values, use this form:
rustc --check-cfg 'cfg(name1, ..., nameN, values("value1", "value2", ... "valueN"))'
To enable checking without specifying any names or values, use this form:
rustc --check-cfg 'cfg()'
The --check-cfg cfg(...)
option can be repeated, both for the same condition name and for
different names. If it is repeated for the same condition name, then the sets of values for that
condition are merged together (precedence is given to values(any())
).
To help out an equivalence table between
--cfg
arguments and--check-cfg
is available down below.
Well known names and values
rustc
maintains a list of well-known names and their corresponding values in order to avoid
the need to specify them manually.
Well known names and values are implicitly added as long as at least one --check-cfg
argument
is present.
As of 2024-08-20T
, the list of known names is as follows:
clippy
debug_assertions
doc
doctest
fmt_debug
miri
overflow_checks
panic
proc_macro
relocation_model
rustfmt
sanitize
sanitizer_cfi_generalize_pointers
sanitizer_cfi_normalize_integers
target_abi
target_arch
target_endian
target_env
target_family
target_feature
target_has_atomic
target_has_atomic_equal_alignment
target_has_atomic_load_store
target_os
target_pointer_width
target_thread_local
target_vendor
test
ub_checks
unix
windows
Like with values(any())
, well known names checking can be disabled by passing cfg(any())
as argument to --check-cfg
.
Equivalence table with --cfg
This table describe the equivalence between a --cfg
argument to a --check-cfg
argument.
--cfg | --check-cfg |
---|---|
nothing | nothing or --check-cfg=cfg() (to enable the checking) |
--cfg foo | --check-cfg=cfg(foo) or --check-cfg=cfg(foo, values(none())) |
--cfg foo="" | --check-cfg=cfg(foo, values("")) |
--cfg foo="bar" | --check-cfg=cfg(foo, values("bar")) |
--cfg foo="1" --cfg foo="2" | --check-cfg=cfg(foo, values("1", "2")) |
--cfg foo="1" --cfg bar="2" | --check-cfg=cfg(foo, values("1")) --check-cfg=cfg(bar, values("2")) |
--cfg foo --cfg foo="bar" | --check-cfg=cfg(foo, values(none(), "bar")) |
Examples
Example: Cargo-like feature
example
Consider this command line:
rustc --check-cfg 'cfg(feature, values("lion", "zebra"))' \
--cfg 'feature="lion"' example.rs
This command line indicates that this crate has two features:
lion
andzebra
. Thelion
feature is enabled, while thezebra
feature is disabled.
#[cfg(feature = "lion")] // This condition is expected, as "lion" is an
// expected value of `feature`
fn tame_lion(lion: Lion) {}
#[cfg(feature = "zebra")] // This condition is expected, as "zebra" is an expected
// value of `feature` but the condition will evaluate
// to false since only --cfg feature="lion" was passed
fn ride_zebra(z: Zebra) {}
#[cfg(feature = "platypus")] // This condition is UNEXPECTED, as "platypus" is NOT
// an expected value of `feature` and will cause a
// the compiler to emit the `unexpected_cfgs` lint
fn poke_platypus() {}
#[cfg(feechure = "lion")] // This condition is UNEXPECTED, as 'feechure' is NOT
// a expected condition name, no `cfg(feechure, ...)`
// was passed in `--check-cfg`
fn tame_lion() {}
#[cfg(windows = "unix")] // This condition is UNEXPECTED, as the well known
// 'windows' cfg doesn't expect any values
fn tame_windows() {}
Example: Multiple names and values
rustc --check-cfg 'cfg(is_embedded, has_feathers)' \
--check-cfg 'cfg(feature, values("zapping", "lasers"))' \
--cfg has_feathers --cfg 'feature="zapping"'
#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was
// provided in --check-cfg and doesn't take any value
fn do_embedded() {}
#[cfg(has_feathers)] // This condition is expected, as 'has_feathers' was
// provided in --check-cfg and doesn't take any value
fn do_features() {}
#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz'
// was NEVER provided in any --check-cfg arguments
fn do_mumble_frotz() {}
#[cfg(feature = "lasers")] // This condition is expected, as "lasers" is an
// expected value of `feature`
fn shoot_lasers() {}
#[cfg(feature = "monkeys")] // This condition is UNEXPECTED, as "monkeys" is NOT
// an expected value of `feature`
fn write_shakespeare() {}
Example: Condition names without values
rustc --check-cfg 'cfg(is_embedded, has_feathers, values(any()))' \
--cfg has_feathers
#[cfg(is_embedded)] // This condition is expected, as 'is_embedded' was
// provided in --check-cfg as condition name
fn do_embedded() {}
#[cfg(has_feathers)] // This condition is expected, as "has_feathers" was
// provided in --check-cfg as condition name
fn do_features() {}
#[cfg(has_feathers = "zapping")] // This condition is expected, as "has_feathers"
// was provided and because *any* values is
// expected for 'has_feathers' no
// warning is emitted for the value "zapping"
fn do_zapping() {}
#[cfg(has_mumble_frotz)] // This condition is UNEXPECTED, as 'has_mumble_frotz'
// was not provided in any --check-cfg arguments
fn do_mumble_frotz() {}
Cargo Specifics - Checking Conditional Configurations
This document is intended to summarize the principal ways Cargo interacts with
the unexpected_cfgs
lint and --check-cfg
flag. It is not intended to provide
individual details, for that refer to the --check-cfg
documentation and
to the Cargo book.
Cargo feature
See the [features]
section in the Cargo book for more details.
With the [features]
table, Cargo provides a mechanism to express conditional compilation and
optional dependencies. Cargo automatically declares corresponding cfgs for every feature as
expected.
Cargo.toml
:
[features]
serde = ["dep:serde"]
my_feature = []
check-cfg
in [lints.rust]
table
See the [lints]
section in the Cargo book for more details.
When using a statically known custom config (i.e., not dependent on a build-script), Cargo provides
the custom lint config check-cfg
under [lints.rust.unexpected_cfgs]
.
It can be used to set custom static --check-cfg
args, it is mainly useful when
the list of expected cfgs is known in advance.
Cargo.toml
:
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(has_foo)'] }
cargo::rustc-check-cfg
for build.rs
/build-script
See the cargo::rustc-check-cfg
section in the Cargo book for more details.
When setting a custom config with cargo::rustc-cfg
, Cargo provides the
corollary instruction: cargo::rustc-check-cfg
to expect custom configs.
build.rs
:
fn main() {
println!("cargo::rustc-check-cfg=cfg(has_foo)");
// ^^^^^^^^^^^^^^^^^^^^^^ new with Cargo 1.80
if has_foo() {
println!("cargo::rustc-cfg=has_foo");
}
}
Exploit Mitigations
This chapter documents the exploit mitigations supported by the Rust compiler, and is by no means an extensive survey of the Rust programming language’s security features.
This chapter is for software engineers working with the Rust programming language, and assumes prior knowledge of the Rust programming language and its toolchain.
Introduction
The Rust programming language provides memory[1] and thread[2] safety guarantees via its ownership[3], references and borrowing[4], and slice types[5] features. However, Unsafe Rust[6] introduces unsafe blocks, unsafe functions and methods, unsafe traits, and new types that are not subject to the borrowing rules.
Parts of the Rust standard library are implemented as safe abstractions over unsafe code (and historically have been vulnerable to memory corruption[7]). Furthermore, the Rust code and documentation encourage creating safe abstractions over unsafe code. This can cause a false sense of security if unsafe code is not properly reviewed and tested.
Unsafe Rust introduces features that do not provide the same memory and thread safety guarantees. This causes programs or libraries to be susceptible to memory corruption (CWE-119)[8] and concurrency issues (CWE-557)[9]. Modern C and C++ compilers provide exploit mitigations to increase the difficulty to exploit vulnerabilities resulting from these issues. Therefore, the Rust compiler must also support these exploit mitigations in order to mitigate vulnerabilities resulting from the use of Unsafe Rust. This chapter documents these exploit mitigations and how they apply to Rust.
This chapter does not discuss the effectiveness of these exploit mitigations as they vary greatly depending on several factors besides their design and implementation, but rather describe what they do, so their effectiveness can be understood within a given context.
Exploit mitigations
This section documents the exploit mitigations applicable to the Rust compiler when building programs for the Linux operating system on the AMD64 architecture and equivalent.1 All examples in this section were built using nightly builds of the Rust compiler on Debian testing.
The Rust Programming Language currently has no specification. The Rust compiler (i.e., rustc) is the language reference implementation. All references to “the Rust compiler” in this chapter refer to the language reference implementation.
Table I
Summary of exploit mitigations supported by the Rust compiler when building
programs for the Linux operating system on the AMD64 architecture and
equivalent.
Exploit mitigation | Supported and enabled by default | Since |
---|---|---|
Position-independent executable | Yes | 0.12.0 (2014-10-09) |
Integer overflow checks | Yes (enabled when debug assertions are enabled, and disabled when debug assertions are disabled) | 1.1.0 (2015-06-25) |
Non-executable memory regions | Yes | 1.8.0 (2016-04-14) |
Stack clashing protection | Yes | 1.20.0 (2017-08-31) |
Read-only relocations and immediate binding | Yes | 1.21.0 (2017-10-12) |
Heap corruption protection | Yes | 1.32.0 (2019-01-17) (via operating system default or specified allocator) |
Stack smashing protection | Yes | Nightly |
Forward-edge control flow protection | Yes | Nightly |
Backward-edge control flow protection (e.g., shadow and safe stack) | Yes | Nightly |
1. See https://github.com/rust-lang/rust/tree/master/compiler/rustc_target/src/spec for a list of targets and their default options. ↩
Position-independent executable
Position-independent executable increases the difficulty of the use of code reuse exploitation techniques, such as return-oriented programming (ROP) and variants, by generating position-independent code for the executable, and instructing the dynamic linker to load it similarly to a shared object at a random load address, thus also benefiting from address-space layout randomization (ASLR). This is also referred to as “full ASLR”.
The Rust compiler supports position-independent executable, and enables it by default since version 0.12.0 (2014-10-09)[10]–[13].
$ readelf -h target/release/hello-rust | grep Type:
Type: DYN (Shared object file)
Fig. 1. Checking if an executable is a position-independent executable.
An executable with an object type of ET_DYN
(i.e., shared object) and not
ET_EXEC
(i.e., executable) is a position-independent executable (see Fig. 1).
Integer overflow checks
Integer overflow checks protects programs from undefined and unintended behavior (which may cause vulnerabilities) by checking for results of signed and unsigned integer computations that cannot be represented in their type, resulting in an overflow or wraparound.
The Rust compiler supports integer overflow checks, and enables it when debug assertions are enabled since version 1.0.0 (2015-05-15)[14]–[17], but support for it was not completed until version 1.1.0 (2015-06-25)[16]. An option to control integer overflow checks was later stabilized in version 1.17.0 (2017-04-27)[18]–[20].
fn main() {
let u: u8 = 255;
println!("u: {}", u + 1);
}
Fig. 2. hello-rust-integer program.
$ cargo run
Compiling hello-rust-integer v0.1.0 (/home/rcvalle/hello-rust-integer)
Finished dev [unoptimized + debuginfo] target(s) in 0.23s
Running `target/debug/hello-rust-integer`
thread 'main' panicked at 'attempt to add with overflow', src/main.rs:3:23
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
Fig. 3. Build and execution of hello-rust-integer with debug assertions enabled.
$ cargo run --release
Compiling hello-rust-integer v0.1.0 (/home/rcvalle/hello-rust-integer)
Finished release [optimized] target(s) in 0.23s
Running `target/release/hello-rust-integer`
u: 0
Fig. 4. Build and execution of hello-rust-integer with debug assertions disabled.
Integer overflow checks are enabled when debug assertions are enabled (see Fig.
3), and disabled when debug assertions are disabled (see Fig. 4). To enable
integer overflow checks independently, use the option to control integer
overflow checks, scoped attributes, or explicit checking methods such as
checked_add
2.
It is recommended that explicit wrapping methods such as wrapping_add
be used
when wrapping semantics are intended, and that explicit checking and wrapping
methods always be used when using Unsafe Rust.
2. See the u32
docs for more
information on the checked, overflowing, saturating, and wrapping methods
(using u32 as an example). ↩
Non-executable memory regions
Non-executable memory regions increase the difficulty of exploitation by limiting the memory regions that can be used to execute arbitrary code. Most modern processors provide support for the operating system to mark memory regions as non executable, but it was previously emulated by software, such as in grsecurity/PaX’s PAGEEXEC and SEGMEXEC, on processors that did not provide support for it. This is also known as “No Execute (NX) Bit”, “Execute Disable (XD) Bit”, “Execute Never (XN) Bit”, and others.
The Rust compiler supports non-executable memory regions, and enables it by default since its initial release, version 0.1 (2012-01-20)[21], [22], but has regressed since then[23]–[25], and enforced by default since version 1.8.0 (2016-04-14)[25].
$ readelf -l target/release/hello-rust | grep -A 1 GNU_STACK
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 0x10
Fig. 5. Checking if non-executable memory regions are enabled for a given binary.
The presence of an element of type PT_GNU_STACK
in the program header table
with the PF_X
(i.e., executable) flag unset indicates non-executable memory
regions3 are enabled for a given binary (see Fig. 5).
Conversely, the presence of an element of type PT_GNU_STACK
in the program
header table with the PF_X
flag set or the absence of an element of type
PT_GNU_STACK
in the program header table indicates non-executable memory
regions are not enabled for a given binary.
3. See the Appendix section for more information on why it affects other memory regions besides the stack. ↩
Stack clashing protection
Stack clashing protection protects the stack from overlapping with another memory region—allowing arbitrary data in both to be overwritten using each other—by reading from the stack pages as the stack grows to cause a page fault when attempting to read from the guard page/region. This is also referred to as “stack probes” or “stack probing”.
The Rust compiler supports stack clashing protection via stack probing, and enables it by default since version 1.20.0 (2017-08-31)[26]–[29].
fn main() {
let v: [u8; 16384] = [1; 16384];
let first = &v[0];
println!("The first element is: {first}");
}
Fig. 6. hello-rust-stack-probe-1 program.
Fig. 7. The "unrolled loop" stack probe variant in modified hello-rust.
fn main() {
let v: [u8; 65536] = [1; 65536];
let first = &v[0];
println!("The first element is: {first}");
}
Fig. 8. hello-rust-stack-probe-2 program.
Fig. 9. The "standard loop" stack probe variant in modified hello-rust.
To check if stack clashing protection is enabled for a given binary, look for any of the two stack probe variants in the prologue of functions whose stack size is larger than a page size (see Figs. 6–9).
Read-only relocations and immediate binding
Read-only relocations protect segments containing relocations and
relocation information (i.e., .init_array
, .fini_array
, .dynamic
, and
.got
) from being overwritten by marking these segments read only. This is
also referred to as “partial RELRO”.
The Rust compiler supports read-only relocations, and enables it by default since version 1.21.0 (2017-10-12)[30], [31].
$ readelf -l target/release/hello-rust | grep GNU_RELRO
GNU_RELRO 0x000000000002ee00 0x000000000002fe00 0x000000000002fe00
Fig. 9. Checking if read-only relocations is enabled for a given binary.
The presence of an element of type PT_GNU_RELRO
in the program header table
indicates read-only relocations are enabled for a given binary (see Fig. 9).
Conversely, the absence of an element of type PT_GNU_RELRO
in the program
header table indicates read-only relocations are not enabled for a given
binary.
Immediate binding protects additional segments containing relocations
(i.e., .got.plt
) from being overwritten by instructing the dynamic linker to
perform all relocations before transferring control to the program during
startup, so all segments containing relocations can be marked read only (when
combined with read-only relocations). This is also referred to as “full RELRO”.
The Rust compiler supports immediate binding, and enables it by default since version 1.21.0 (2017-10-12)[30], [31].
$ readelf -d target/release/hello-rust | grep BIND_NOW
0x000000000000001e (FLAGS) BIND_NOW
Fig. 10. Checking if immediate binding is enabled for a given binary.
The presence of an element with the DT_BIND_NOW
tag and the DF_BIND_NOW
flag4 in the dynamic section indicates immediate binding
is enabled for a given binary (see Fig. 10). Conversely, the absence of an
element with the DT_BIND_NOW
tag and the DF_BIND_NOW
flag in the dynamic
section indicates immediate binding is not enabled for a given binary.
The presence of both an element of type PT_GNU_RELRO
in the program header
table and of an element with the DT_BIND_NOW
tag and the DF_BIND_NOW
flag
in the dynamic section indicates full RELRO is enabled for a given binary (see
Figs. 9–10).
4. And the DF_1_NOW
flag for some link editors. ↩
Heap corruption protection
Heap corruption protection protects memory allocated dynamically by performing several checks, such as checks for corrupted links between list elements, invalid pointers, invalid sizes, double/multiple “frees” of the same memory allocated, and many corner cases of these. These checks are implementation specific, and vary per allocator.
ARM Memory Tagging Extension (MTE), when available, will provide hardware assistance for a probabilistic mitigation to detect memory safety violations by tagging memory allocations, and automatically checking that the correct tag is used on every memory access.
Rust’s default allocator has historically been jemalloc, and it has long been the cause of issues and the subject of much discussion[32]–[38]. Consequently, it has been removed as the default allocator in favor of the operating system’s standard C library default allocator5 since version 1.32.0 (2019-01-17)[39].
fn main() {
let mut x = Box::new([0; 1024]);
for i in 0..1026 {
unsafe {
let elem = x.get_unchecked_mut(i);
*elem = 0x4141414141414141u64;
}
}
}
Fig. 11. hello-rust-heap program.
$ cargo run
Compiling hello-rust-heap v0.1.0 (/home/rcvalle/hello-rust-heap)
Finished dev [unoptimized + debuginfo] target(s) in 0.25s
Running `target/debug/hello-rust-heap`
free(): invalid next size (normal)
Aborted
Fig. 12. Build and execution of hello-rust-heap with debug assertions enabled.
$ cargo run --release
Compiling hello-rust-heap v0.1.0 (/home/rcvalle/hello-rust-heap)
Finished release [optimized] target(s) in 0.25s
Running `target/release/hello-rust-heap`
free(): invalid next size (normal)
Aborted
Fig. 13. Build and execution of hello-rust-heap with debug assertions disabled.
Heap corruption checks are performed when using the default allocator (i.e., the GNU Allocator) (see Figs. 12–13).
5. Linux's standard C library default allocator is the GNU Allocator, which is derived from ptmalloc (pthreads malloc) by Wolfram Gloger, which in turn is derived from dlmalloc (Doug Lea malloc) by Doug Lea. ↩
Stack smashing protection
Stack smashing protection protects programs from stack-based buffer overflows by inserting a random guard value between local variables and the saved return instruction pointer, and checking if this value has changed when returning from a function. This is also known as “Stack Protector” or “Stack Smashing Protector (SSP)”.
The Rust compiler supports stack smashing protection on nightly builds[40].
Fig. 14. IDA Pro listing cross references to
__stack_chk_fail
in hello-rust.
To check if stack smashing protection is enabled for a given binary, search for
cross references to __stack_chk_fail
(see Fig. 14).
Forward-edge control flow protection
Forward-edge control flow protection protects programs from having its control flow changed/hijacked by performing checks to ensure that destinations of indirect branches are one of their valid destinations in the control flow graph. The comprehensiveness of these checks vary per implementation. This is also known as “forward-edge control flow integrity (CFI)”.
Newer processors provide hardware assistance for forward-edge control flow protection, such as ARM Branch Target Identification (BTI), ARM Pointer Authentication, and Intel Indirect Branch Tracking (IBT) as part of Intel Control-flow Enforcement Technology (CET). However, ARM BTI and Intel IBT -based implementations are less comprehensive than software-based implementations such as LLVM ControlFlowIntegrity (CFI), and the commercially available grsecurity/PaX Reuse Attack Protector (RAP).
The Rust compiler supports forward-edge control flow protection on nightly builds[41]-[42] 6.
$ readelf -s -W target/release/hello-rust | grep "\.cfi"
5: 0000000000006480 657 FUNC LOCAL DEFAULT 15 _ZN10hello_rust4main17h4e359f1dcd627c83E.cfi
Fig. 15. Checking if LLVM CFI is enabled for a given binary.
The presence of symbols suffixed with ".cfi" or the __cfi_init
symbol (and
references to __cfi_check
) indicates that LLVM CFI (i.e., forward-edge
control flow protection) is enabled for a given binary. Conversely, the absence
of symbols suffixed with ".cfi" or the __cfi_init
symbol (and references to
__cfi_check
) indicates that LLVM CFI is not enabled for a given binary (see
Fig. 15).
6. It also supports Control Flow Guard (CFG) on Windows (see https://github.com/rust-lang/rust/issues/68793). ↩
Backward-edge control flow protection
Shadow stack protects saved return instruction pointers from being overwritten by storing a copy of them on a separate (shadow) stack, and using these copies as authoritative values when returning from functions. This is also known as “ShadowCallStack” and “Return Flow Guard”, and is considered an implementation of backward-edge control flow protection (or “backward-edge CFI”).
Safe stack protects not only the saved return instruction pointers, but also register spills and some local variables from being overwritten by storing unsafe variables, such as large arrays, on a separate (unsafe) stack, and using these unsafe variables on the separate stack instead. This is also known as “SafeStack”, and is also considered an implementation of backward-edge control flow protection.
Both shadow and safe stack are intended to be a more comprehensive alternatives to stack smashing protection as they protect the saved return instruction pointers (and other data in the case of safe stack) from arbitrary writes and non-linear out-of-bounds writes.
Newer processors provide hardware assistance for backward-edge control flow protection, such as ARM Pointer Authentication, and Intel Shadow Stack as part of Intel CET.
The Rust compiler supports shadow stack for the AArch64 architecture7on nightly builds[43]-[44], and also supports safe stack on nightly builds[45]-[46].
$ readelf -s target/release/hello-rust | grep __safestack_init
678: 0000000000008c80 426 FUNC GLOBAL DEFAULT 15 __safestack_init
Fig. 16. Checking if LLVM SafeStack is enabled for a given binary.
The presence of the __safestack_init
symbol indicates that LLVM SafeStack is
enabled for a given binary. Conversely, the absence of the __safestack_init
symbol indicates that LLVM SafeStack is not enabled for a given binary (see
Fig. 16).
7. The shadow stack implementation for the AMD64 architecture and equivalent in LLVM was removed due to performance and security issues. ↩
Appendix
As of the latest version of the Linux Standard Base (LSB) Core
Specification,
the PT_GNU_STACK
program header indicates whether the stack should be
executable, and the absence of this header indicates that the stack should be
executable. However, the Linux kernel currently sets the READ_IMPLIES_EXEC
personality upon loading any executable with the PT_GNU_STACK
program header
and the PF_X
flag set or with the absence of this header, resulting in not
only the stack, but also all readable virtual memory mappings being executable.
An attempt to fix this was made in
2012,
and another was made in
2020.
The former never landed, and the latter partially fixed it, but introduced
other issues—the absence of the PT_GNU_STACK
program header still causes not
only the stack, but also all readable virtual memory mappings to be executable
in some architectures, such as IA-32 and equivalent (or causes the stack to be
non-executable in some architectures, such as AMD64 and equivalent,
contradicting the LSB).
The READ_IMPLIES_EXEC
personality needs to be completely separated from the
PT_GNU_STACK
program header by having a separate option for it (or setarch -X
could just be used whenever READ_IMPLIES_EXEC
is needed), and the absence of
the PT_GNU_STACK
program header needs to have more secure defaults (unrelated
to READ_IMPLIES_EXEC
).
References
-
D. Hosfelt. “Fearless security: memory safety.” Mozilla Hacks. https://hacks.mozilla.org/2019/01/fearless-security-memory-safety/.
-
D. Hosfelt. “Fearless security: thread safety.” Mozilla Hacks. https://hacks.mozilla.org/2019/02/fearless-security-thread-safety/.
-
S. Klabnik and C. Nichols. “What Is Ownership?.” The Rust Programming Language. https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html.
-
S. Klabnik and C. Nichols. “References and Borrowing.” The Rust Programming Language. https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html.
-
S. Klabnik and C. Nichols. “The Slice Type.” The Rust Programming Language. https://doc.rust-lang.org/book/ch04-03-slices.html.
-
S. Klabnik and C. Nichols. “Unsafe Rust.” The Rust Programming Language. https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html.
-
S. Davidoff. “How Rust’s standard library was vulnerable for years and nobody noticed.” Medium. https://medium.com/@shnatsel/how-rusts-standard-library-was-vulnerable-for-years-and-nobody-noticed-aebf0503c3d6.
-
“Improper restriction of operations within the bounds of a memory buffer (CWE-119).” MITRE CWE List. https://cwe.mitre.org/data/definitions/119.html.
-
“Concurrency issues (CWE-557).” MITRE CWE List. https://cwe.mitre.org/data/definitions/557.html.
-
K. McAllister. “Memory exploit mitigations #15179.” GitHub. https://github.com/rust-lang/rust/issues/15179.
-
K. McAllister. “RFC: Memory exploit mitigation #145.” GitHub. https://github.com/rust-lang/rfcs/pull/145.
-
K. McAllister. “RFC: Memory exploit mitigation.” GitHub. https://github.com/kmcallister/rfcs/blob/hardening/active/0000-memory-exploit-mitigation.md.
-
D. Micay. “Enable PIE by default on Linux for full ASLR #16340.” GitHub. https://github.com/rust-lang/rust/pull/16340.
-
N. Matsakis. “Integer overflow #560.” GitHub. https://github.com/rust-lang/rfcs/pull/560.
-
G. Lehel and N. Matsakis. “Integer overflow.” GitHub. https://rust-lang.github.io/rfcs/0560-integer-overflow.html.
-
A. Turon. “Tracking issue for integer overflow (RFC 560) #22020.” GitHub. https://github.com/rust-lang/rust/issues/22020.
-
H. Wilson. “Myths and legends about integer overflow in Rust.” Huon on the Internet. http://huonw.github.io/blog/2016/04/myths-and-legends-about-integer-overflow-in-rust/.
-
B. Anderson. “Stabilize -C overflow-checks #1535.” GitHub. https://github.com/rust-lang/rfcs/pull/1535.
-
B. Anderson. “Stable overflow checks.” GitHub. https://github.com/brson/rfcs/blob/overflow/text/0000-stable-overflow-checks.md.
-
N. Froyd. “Add -C overflow-checks option #40037.” GitHub. https://github.com/rust-lang/rust/pull/40037.
-
R. Á. de Espíndola. “rustc requires executable stack #798.” GitHub. https://github.com/rust-lang/rust/issues/798.
-
A. Seipp. “Make sure librustrt.so is linked with a non-executable stack. #1066.” GitHub. https://github.com/rust-lang/rust/pull/1066.
-
D. Micay. “Rust binaries should not have an executable stack #5643.” GitHub. https://github.com/rust-lang/rust/issues/5643.
-
D. Micay. “Mark the assembly object stacks as non-executable #5647.” GitHub. https://github.com/rust-lang/rust/pull/5647.
-
A. Clark. “Explicitly disable stack execution on linux and bsd #30859.” GitHub. https://github.com/rust-lang/rust/pull/30859.
-
Zoxc. “Replace stack overflow checking with stack probes #16012.” GitHub. https://github.com/rust-lang/rust/issues/16012.
-
A. Crichton. “rustc: Implement stack probes for x86 #42816.” GitHub. https://github.com/rust-lang/rust/pull/42816.
-
A. Crichton. “Add __rust_probestack intrinsic #175.” GitHub. https://github.com/rust-lang/compiler-builtins/pull/175.
-
S. Guelton, S. Ledru, J. Stone. “Bringing Stack Clash Protection to Clang / X86 — the Open Source Way.” The LLVM Project Blog. https://blog.llvm.org/posts/2021-01-05-stack-clash-protection/.
-
B. Anderson. “Consider applying -Wl,-z,relro or -Wl,-z,relro,-z,now by default #29877.” GitHub. https://github.com/rust-lang/rust/issues/29877.
-
J. Löthberg. “Add support for full RELRO #43170.” GitHub. https://github.com/rust-lang/rust/pull/43170.
-
N. Matsakis. “Allocators in Rust.” Baby Steps. http://smallcultfollowing.com/babysteps/blog/2014/11/14/allocators-in-rust/.
-
A. Crichton. “RFC: Allow changing the default allocator #1183.” GitHub. https://github.com/rust-lang/rfcs/pull/1183.
-
A. Crichton. “RFC: Swap out jemalloc.” GitHub. https://rust-lang.github.io/rfcs/1183-swap-out-jemalloc.html.
-
A. Crichton. “Tracking issue for changing the global, default allocator (RFC 1974) #27389.” GitHub. https://github.com/rust-lang/rust/issues/27389.
-
S. Fackler. “Prepare global allocators for stabilization #1974.” GitHub. https://github.com/rust-lang/rfcs/pull/1974.
-
A. Crichton. “RFC: Global allocators.” GitHub. https://rust-lang.github.io/rfcs/1974-global-allocators.html.
-
B. Anderson. “Switch the default global allocator to System, remove alloc_jemalloc, use jemallocator in rustc #36963.” GitHub. https://github.com/rust-lang/rust/issues/36963.
-
A. Crichton. “Remove the alloc_jemalloc crate #55238.” GitHub. https://github.com/rust-lang/rust/pull/55238.
-
bbjornse. “Add codegen option for using LLVM stack smash protection #84197.” GitHub. https://github.com/rust-lang/rust/pull/84197
-
R. de C. Valle. “Tracking Issue for LLVM Control Flow Integrity (CFI) Support for Rust #89653.” GitHub. https://github.com/rust-lang/rust/issues/89653.
-
“ControlFlowIntegrity.” The Rust Unstable Book. https://doc.rust-lang.org/unstable-book/compiler-flags/sanitizer.html#controlflowintegrity.
-
I. Lozano. “Add support for LLVM ShadowCallStack #98208.” GitHub. https://github.com/rust-lang/rust/pull/98208.
-
“ShadowCallStack.” The Rust Unstable Book. https://doc.rust-lang.org/unstable-book/compiler-flags/sanitizer.html#shadowcallstack.
-
W. Wiser. “Add support for LLVM SafeStack #112000” GitHub. https://github.com/rust-lang/rust/pull/112000
-
“SafeStack.” The Rust Unstable Book. https://doc.rust-lang/org/unstable-book/compiler-flags/sanitizer.html#safestack.
Symbol Mangling
Symbol name mangling is used by rustc
to encode a unique name for symbols that are used during code generation.
The encoded names are used by the linker to associate the name with the thing it refers to.
The method for mangling the names can be controlled with the -C symbol-mangling-version
option.
Per-item control
The #[no_mangle]
attribute can be used on items to disable name mangling on that item.
The #[export_name]
attribute can be used to specify the exact name that will be used for a function or static.
Items listed in an extern
block use the identifier of the item without mangling to refer to the item.
The #[link_name]
attribute can be used to change that name.
Decoding
The encoded names may need to be decoded in some situations.
For example, debuggers and other tooling may need to demangle the name so that it is more readable to the user.
Recent versions of gdb
and lldb
have built-in support for demangling Rust identifiers.
In situations where you need to do your own demangling, the rustc-demangle
crate can be used to programmatically demangle names.
rustfilt
is a CLI tool which can demangle names.
An example of running rustfilt:
$ rustfilt _RNvCskwGfYPst2Cb_3foo16example_function
foo::example_function
Mangling versions
rustc
supports different mangling versions which encode the names in different ways.
The legacy version (which is currently the default) is not described here.
The "v0" mangling scheme addresses several limitations of the legacy format,
and is described in the v0 Symbol Format chapter.
v0 Symbol Format
The v0 mangling format was introduced in RFC 2603. It has the following properties:
- It provides an unambiguous string encoding for everything that can end up in a binary's symbol table.
- It encodes information about generic parameters in a reversible way.
- The mangled symbols are decodable such that the demangled form should be easily identifiable as some concrete instance of e.g. a polymorphic function.
- It has a consistent definition that does not rely on pretty-printing certain language constructs.
- Symbols can be restricted to only consist of the characters
A-Z
,a-z
,0-9
, and_
. This helps ensure that it is platform-independent, where other characters might have special meaning in some context (e.g..
for MSVCDEF
files). Unicode symbols are optionally supported. - It tries to stay efficient, avoiding unnecessarily long names, and avoiding computationally expensive operations to demangle.
The v0 format is not intended to be compatible with other mangling schemes (such as C++).
The v0 format is not presented as a stable ABI for Rust. This format is currently intended to be well-defined enough that a demangler can produce a reasonable human-readable form of the symbol. There are several implementation-defined portions that result in it not being possible to entirely predict how a given Rust entity will be encoded.
The sections below define the encoding of a v0 symbol. There is no standardized demangled form of the symbols, though suggestions are provided for how to demangle a symbol. Implementers may choose to demangle in different ways.
Extensions
This format may be extended in the future to add new tags as Rust is extended with new language items. To be forward compatible, demanglers should gracefully handle symbols that have encodings where it encounters a tag character not described in this document. For example, they may fall back to displaying the mangled symbol. The format may be extended anywhere there is a tag character, such as the type rule. The meaning of existing tags and encodings will not be changed.
Grammar notation
The format of an encoded symbol is illustrated as a context free grammar in an extended BNF-like syntax. A consolidated summary can be found in the Symbol grammar summary.
Name | Syntax | Example | Description |
---|---|---|---|
Rule | → | A production. | |
Concatenation | whitespace | Individual elements in sequence left-to-right. | |
Alternative | | | Matches either one or the other. | |
Grouping | () | Groups multiple elements as one. | |
Repetition | {} | Repeats the enclosed zero or more times. | |
Option | opt | An optional element. | |
Literal | monospace | G | A terminal matching the exact characters case-sensitive. |
Symbol name
symbol-name →
_R
decimal-numberopt path instantiating-crateopt vendor-specific-suffixopt
A mangled symbol starts with the two characters _R
which is a prefix to identify the symbol as a Rust symbol.
The prefix can optionally be followed by a decimal-number which specifies the encoding version.
This number is currently not used, and is never present in the current encoding.
Following that is a path which encodes the path to an entity.
The path is followed by an optional instantiating-crate which helps to disambiguate entities which may be instantiated multiple times in separate crates.
The final part is an optional vendor-specific-suffix.
Recommended Demangling
A symbol-name should be displayed as the path. The instantiating-crate and the vendor-specific-suffix usually need not be displayed.
Example:
std::path::PathBuf::new();
The symbol for
PathBuf::new
in cratemycrate
is:_RNvMsr_NtCs3ssYzQotkvD_3std4pathNtB5_7PathBuf3newCs15kBYyAo9fc_7mycrate ├┘└───────────────────────┬──────────────────────┘└──────────┬─────────┘ │ │ │ │ │ └── instantiating-crate path "mycrate" │ └───────────────────────────────────── path to std::path::PathBuf::new └─────────────────────────────────────────────────────────────── `_R` symbol prefix
Recommended demangling:
<std::path::PathBuf>::new
Symbol path
path →
crate-root
| inherent-impl
| trait-impl
| trait-definition
| nested-path
| generic-args
| backref
A path represents a variant of a Rust path to some entity.
In addition to typical Rust path segments using identifiers,
it uses extra elements to represent unnameable entities (like an impl
) or generic arguments for monomorphized items.
The initial tag character can be used to determine which kind of path it represents:
Tag | Rule | Description |
---|---|---|
C | crate-root | The root of a crate path. |
M | inherent-impl | An inherent implementation. |
X | trait-impl | A trait implementation. |
Y | trait-definition | A trait definition. |
N | nested-path | A nested path. |
I | generic-args | Generic arguments. |
B | backref | A back reference. |
Path: Crate root
crate-root →
C
identifier
A crate-root indicates a path referring to the root of a crate's module tree.
It consists of the character C
followed by the crate name as an identifier.
The crate name is the name as seen from the defining crate. Since Rust supports linking multiple crates with the same name, the disambiguator is used to make the name unique across the crate graph.
Recommended Demangling
A crate-root can be displayed as the identifier such as
mycrate
.Usually the disambiguator in the identifier need not be displayed, but as an alternate form the disambiguator can be shown in hex such as
mycrate[ca63f166dbe9294]
.
Example:
fn example() {}
The symbol for
example
in cratemycrate
is:_RNvCs15kBYyAo9fc_7mycrate7example │└────┬─────┘││└──┬──┘ │ │ ││ │ │ │ ││ └── crate-root identifier "mycrate" │ │ │└────── length 7 of "mycrate" │ │ └─────── end of base-62-number │ └────────────── disambiguator for crate-root "mycrate" 0xca63f166dbe9293 + 1 └──────────────────── crate-root
Recommended demangling:
mycrate::example
Note: The compiler may re-use the crate-root form to express arbitrary unscoped, undisambiguated identifiers, such as for new basic types that have not been added to the grammar yet. To achieve that, it will emit a crate-root without an explicit disambiguator, relying on the fact that such an undisambiguated crate name cannot occur in practice. For example, the basic type
f128
would be encode asC4f128
. For this to have the desired effect, demanglers are expected to never render zero disambiguators of crate roots. I.e.C4f128
is expected to be displayed asf128
and notf128[0]
.
Path: Inherent impl
An inherent-impl indicates a path to an inherent implementation.
It consists of the character M
followed by an impl-path, which uniquely identifies the impl block the item is defined in.
Following that is a type representing the Self
type of the impl.
Recommended Demangling
An inherent-impl can be displayed as a qualified path segment to the type within angled brackets. The impl-path usually need not be displayed.
Example:
struct Example; impl Example { fn foo() {} }
The symbol for
foo
in the impl forExample
is:_RNvMs_Cs4Cv8Wi1oAIB_7mycrateNtB4_7Example3foo │├┘└─────────┬──────────┘└────┬──────┘ ││ │ │ ││ │ └── Self type "Example" ││ └─────────────────── path to the impl's parent "mycrate" │└─────────────────────────────── disambiguator 1 └──────────────────────────────── inherent-impl
Recommended demangling:
<mycrate::Example>::foo
Path: Trait impl
A trait-impl indicates a path to a trait implementation.
It consists of the character X
followed by an impl-path to the impl's parent followed by the type representing the Self
type of the impl followed by a path to the trait.
Recommended Demangling
A trait-impl can be displayed as a qualified path segment using the
<
typeas
path>
syntax. The impl-path usually need not be displayed.
Example:
struct Example; trait Trait { fn foo(); } impl Trait for Example { fn foo() {} }
The symbol for
foo
in the trait impl forExample
is:_RNvXCs15kBYyAo9fc_7mycrateNtB2_7ExampleNtB2_5Trait3foo │└─────────┬──────────┘└─────┬─────┘└────┬────┘ │ │ │ │ │ │ │ └── path to the trait "Trait" │ │ └────────────── Self type "Example" │ └──────────────────────────────── path to the impl's parent "mycrate" └─────────────────────────────────────────── trait-impl
Recommended demangling:
<mycrate::Example as mycrate::Trait>::foo
Path: Impl
impl-path → disambiguatoropt path
An impl-path is a path used for inherent-impl and trait-impl to indicate the path to parent of an implementation. It consists of an optional disambiguator followed by a path. The path is the path to the parent that contains the impl. The disambiguator can be used to distinguish between multiple impls within the same parent.
Recommended Demangling
An impl-path usually need not be displayed (unless the location of the impl is desired).
Example:
struct Example; impl Example { fn foo() {} } impl Example { fn bar() {} }
The symbol for
foo
in the impl forExample
is:_RNvMCs7qp2U7fqm6G_7mycrateNtB2_7Example3foo └─────────┬──────────┘ │ └── path to the impl's parent crate-root "mycrate"
The symbol for
bar
is similar, though it has a disambiguator to indicate it is in a different impl block._RNvMs_Cs7qp2U7fqm6G_7mycrateNtB4_7Example3bar ├┘└─────────┬──────────┘ │ │ │ └── path to the impl's parent crate-root "mycrate" └────────────── disambiguator 1
Recommended demangling:
foo
:<mycrate::Example>::foo
bar
:<mycrate::Example>::bar
Path: Trait definition
A trait-definition is a path to a trait definition.
It consists of the character Y
followed by the type which is the Self
type of the referrer, followed by the path to the trait definition.
Recommended Demangling
A trait-definition can be displayed as a qualified path segment using the
<
typeas
path>
syntax.
Example:
trait Trait { fn example() {} } struct Example; impl Trait for Example {}
The symbol for
example
in the traitTrait
implemented forExample
is:_RNvYNtCs15kBYyAo9fc_7mycrate7ExampleNtB4_5Trait7exampleB4_ │└──────────────┬───────────────┘└────┬────┘ │ │ │ │ │ └── path to the trait "Trait" │ └──────────────────────── path to the implementing type "mycrate::Example" └──────────────────────────────────────── trait-definition
Recommended demangling:
<mycrate::Example as mycrate::Trait>::example
Path: Nested path
nested-path →
N
namespace path identifier
A nested-path is a path representing an optionally named entity.
It consists of the character N
followed by a namespace indicating the namespace of the entity,
followed by a path which is a path representing the parent of the entity,
followed by an identifier of the entity.
The identifier of the entity may have a length of 0 when the entity is not named. For example, entities like closures, tuple-like struct constructors, and anonymous constants may not have a name. The identifier may still have a disambiguator unless the disambiguator is 0.
Recommended Demangling
A nested-path can be displayed by first displaying the path followed by a
::
separator followed by the identifier. If the identifier is empty, then the separating::
should not be displayed.If a namespace is specified, then extra context may be added such as:
path::{
namespace (:
identifier)opt#
disambiguatoras base-10 number}
Here the namespace
C
may be printed asclosure
andS
asshim
. Others may be printed by their character tag. The:
name portion may be skipped if the name is empty.The disambiguator in the identifier may be displayed if a namespace is specified. In other situations, it is usually not necessary to display the disambiguator. If it is displayed, it is recommended to place it in brackets, for example
[284a76a8b41a7fd3]
. If the disambiguator is not present, then its value is 0 and it can always be omitted from display.
Example:
fn main() { let x = || {}; let y = || {}; x(); y(); }
The symbol for the closure
x
in cratemycrate
is:_RNCNvCsgStHSCytQ6I_7mycrate4main0B3_ ││└─────────────┬─────────────┘│ ││ │ │ ││ │ └── identifier with length 0 ││ └───────────────── path to "mycrate::main" │└──────────────────────────────── closure namespace └───────────────────────────────── nested-path
The symbol for the closure
y
is similar, with a disambiguator:_RNCNvCsgStHSCytQ6I_7mycrate4mains_0B3_ ││ │└── base-62-number 0 └─── disambiguator 1 (base-62-number+1)
Recommended demangling:
x
:mycrate::main::{closure#0}
y
:mycrate::main::{closure#1}
Path: Generic arguments
generic-args →
I
path {generic-arg}E
A generic-args is a path representing a list of generic arguments.
It consists of the character I
followed by a path to the defining entity, followed by zero or more generic-args terminated by the character E
.
Each generic-arg is either a lifetime (starting with the character L
), a type, or the character K
followed by a const representing a const argument.
Recommended Demangling
A generic-args may be printed as: path
::
opt<
comma-separated list of args>
The::
separator may be elided for type paths (similar to Rust's rules).
Example:
fn main() { example([123]); } fn example<T, const N: usize>(x: [T; N]) {}
The symbol for the function
example
is:_RINvCsgStHSCytQ6I_7mycrate7examplelKj1_EB2_ │└──────────────┬───────────────┘││││││ │ │ │││││└── end of generic-args │ │ ││││└─── end of const-data │ │ │││└──── const value `1` │ │ ││└───── const type `usize` │ │ │└────── const generic │ │ └─────── generic type i32 │ └──────────────────────── path to "mycrate::example" └──────────────────────────────────────── generic-args
Recommended demangling:
mycrate::example::<i32, 1>
Namespace
A namespace is used to segregate names into separate logical groups, allowing identical names to otherwise avoid collisions. It consists of a single character of an upper or lowercase ASCII letter. Lowercase letters are reserved for implementation-internal disambiguation categories (and demanglers should never show them). Uppercase letters are used for special namespaces which demanglers may display in a special way.
Uppercase namespaces are:
C
— A closure.S
— A shim. Shims are added by the compiler in some situations where an intermediate is needed. For example, afn()
pointer to a function with the#[track_caller]
attribute needs a shim to deal with the implicit caller location.
Recommended Demangling
See nested-path for recommended demangling.
Identifier
identifier → disambiguatoropt undisambiguated-identifier
undisambiguated-identifier →
u
opt decimal-number_
opt bytesbytes → {UTF-8 bytes}
An identifier is a named label used in a path to refer to an entity. It consists of an optional disambiguator followed by an undisambiguated-identifier.
The disambiguator is used to disambiguate identical identifiers that should not otherwise be considered the same. For example, closures have no name, so the disambiguator is the only differentiating element between two different closures in the same parent path.
The undisambiguated-identifier starts with an optional u
character,
which indicates that the identifier is encoded in Punycode.
The next part is a decimal-number which indicates the length of the bytes.
Following the identifier size is an optional _
character which is used to separate the length value from the identifier itself.
The _
is mandatory if the bytes starts with a decimal digit or _
in order to keep it unambiguous where the decimal-number ends and the bytes starts.
bytes is the identifier itself encoded in UTF-8.
Recommended Demangling
The display of an identifier can depend on its context. If it is Punycode-encoded, then it may first be decoded before being displayed.
The disambiguator may or may not be displayed; see recommendations for rules that use identifier.
Punycode identifiers
Because some environments are restricted to ASCII alphanumerics and _
,
Rust's Unicode identifiers may be encoded using a modified version of Punycode.
For example, the function:
mod gödel {
mod escher {
fn bach() {}
}
}
would be mangled as:
_RNvNtNtCsgOH4LzxkuMq_7mycrateu8gdel_5qa6escher4bach
││└───┬──┘
││ │
││ └── gdel_5qa translates to gödel
│└─────── 8 is the length
└──────── `u` indicates it is a Unicode identifier
Standard Punycode generates strings of the form ([[:ascii:]]+-)?[[:alnum:]]+
.
This is problematic because the -
character
(which is used to separate the ASCII part from the base-36 encoding)
is not in the supported character set for symbols.
For this reason, -
characters in the Punycode encoding are replaced with _
.
Here are some examples:
Original | Punycode | Punycode + Encoding |
---|---|---|
føø | f-5gaa | f_5gaa |
α_ω | _-ylb7e | __ylb7e |
铁锈 | n84amf | n84amf |
🤦 | fq9h | fq9h |
ρυστ | 2xaedc | 2xaedc |
Note: It is up to the compiler to decide whether or not to encode identifiers using Punycode or not. Some platforms may have native support for UTF-8 symbols, and the compiler may decide to use the UTF-8 encoding directly. Demanglers should be prepared to support either form.
Disambiguator
disambiguator →
s
base-62-number
A disambiguator is used in various parts of a symbol path to uniquely identify path elements that would otherwise be identical but should not be considered the same.
It starts with the character s
and is followed by a base-62-number.
If the disambiguator is not specified, then its value can be assumed to be zero.
Otherwise, when demangling, the value 1 should be added to the base-62-number
(thus a base-62-number of zero encoded as _
has a value of 1).
This allows disambiguators that are encoded sequentially to use minimal bytes.
Recommended Demangling
The disambiguator may or may not be displayed; see recommendations for rules that use disambiguator. Generally, it is recommended that zero disambiguators are never displayed unless their accompanying identifier is empty (like is the case for unnamed items such as closures). When rendering a disambiguator, it can be shortened to a length reasonable for the context, similar to how git commit hashes are rarely displayed in full.
Lifetime
lifetime →
L
base-62-number
A lifetime is used to encode an anonymous (numbered) lifetime, either erased or higher-ranked.
It starts with the character L
and is followed by a base-62-number.
Index 0 is always erased.
Indices starting from 1 refer (as de Bruijn indices) to a higher-ranked lifetime bound by one of the enclosing binders.
Recommended Demangling
A lifetime may be displayed like a Rust lifetime using a single quote.
Index 0 should be displayed as
'_
. Index 0 should not be displayed for lifetimes in a ref-type, mut-ref-type, or dyn-trait-type.A lifetime can be displayed by converting the De Bruijn index to a De Bruijn level (level = number of bound lifetimes - index) and selecting a unique name for each level. For example, starting with single lowercase letters such as
'a
for level 0. Levels over 25 may consider printing the numeric lifetime as in'_123
. See binder for more on lifetime indexes and ordering.
Example:
fn main() { example::<fn(&u8, &u16)>(); } pub fn example<T>() {}
The symbol for the function
example
is:_RINvCs7qp2U7fqm6G_7mycrate7exampleFG0_RL1_hRL0_tEuEB2_ │└┬┘│└┬┘││└┬┘││ │ │ │ │ ││ │ │└── end of input types │ │ │ │ ││ │ └─── type u16 │ │ │ │ ││ └───── lifetime #1 'b │ │ │ │ │└─────── reference type │ │ │ │ └──────── type u8 │ │ │ └────────── lifetime #2 'a │ │ └──────────── reference type │ └────────────── binder with 2 lifetimes └──────────────── function type
Recommended demangling:
mycrate::example::<for<'a, 'b> fn(&'a u8, &'b u16)>
Const
const →
type const-data
|p
| backrefconst-data →
n
opt {hex-digit}_
A const is used to encode a const value used in generics and types. It has the following forms:
- A constant value encoded as a type which represents the type of the constant and const-data which is the constant value, followed by
_
to terminate the const. - The character
p
which represents a placeholder. - A backref to a previously encoded const of the same value.
The encoding of the const-data depends on the type:
bool
— The valuefalse
is encoded as0_
, the value true is encoded as1_
.char
— The Unicode scalar value of the character is encoded in hexadecimal.- Unsigned integers — The value is encoded in hexadecimal.
- Signed integers — The character
n
is a prefix to indicate that it is negative, followed by the absolute value encoded in hexadecimal.
Recommended Demangling
A const may be displayed by the const value depending on the type.
The
p
placeholder should be displayed as the_
character.For specific types:
b
(bool) — Display astrue
orfalse
.c
(char) — Display the character in as a Rust character (such as'A'
or'\n'
).- integers — Display the integer (either in decimal or hex).
Example:
fn main() { example::<0x12345678>(); } pub fn example<const N: u64>() {}
The symbol for function
example
is:_RINvCs7qp2U7fqm6G_7mycrate7exampleKy12345678_EB2_ ││└───┬───┘ ││ │ ││ └── const-data 0x12345678 │└─────── const type u64 └──────── const generic arg
Recommended demangling:
mycrate::example::<305419896>
Placeholders
A placeholder may occur in circumstances where a type or const value is not relevant.
Example:
pub struct Example<T, const N: usize>([T; N]); impl<T, const N: usize> Example<T, N> { pub fn foo() -> &'static () { static EXAMPLE_STATIC: () = (); &EXAMPLE_STATIC } }
In this example, the static
EXAMPLE_STATIC
would not be monomorphized by the type or const parametersT
andN
. Those will use the placeholder for those generic arguments. Its symbol is:_RNvNvMCsd9PVOYlP1UU_7mycrateINtB4_7ExamplepKpE3foo14EXAMPLE_STATIC │ │││ │ ││└── const placeholder │ │└─── const generic argument │ └──── type placeholder └────────────────── generic-args
Recommended demangling:
<mycrate::Example<_, _>>::foo::EXAMPLE_STATIC
Type
type →
basic-type
| array-type
| slice-type
| tuple-type
| ref-type
| mut-ref-type
| const-ptr-type
| mut-ptr-type
| fn-type
| dyn-trait-type
| path
| backref
A type represents a Rust type. The initial character can be used to distinguish which type is encoded. The type encodings based on the initial tag character are:
- A basic-type is encoded as a single character:
a
—i8
b
—bool
c
—char
d
—f64
e
—str
f
—f32
h
—u8
i
—isize
j
—usize
l
—i32
m
—u32
n
—i128
o
—u128
s
—i16
t
—u16
u
— unit()
v
— variadic...
x
—i64
y
—u64
z
—!
p
— placeholder_
Remaining primitives are encoded as a crate production, e.g. C4f128
.
-
A
— An array[T; N]
.The tag
A
is followed by the type of the array followed by a const for the array size. -
S
— A slice[T]
.slice-type →
S
typeThe tag
S
is followed by the type of the slice. -
T
— A tuple(T1, T2, T3, ...)
.tuple-type →
T
{type}E
The tag
T
is followed by one or more types indicating the type of each field, followed by a terminatingE
character.Note that a zero-length tuple (unit) is encoded with the
u
basic-type. -
R
— A reference&T
.The tag
R
is followed by an optional lifetime followed by the type of the reference. The lifetime is not included if it has been erased. -
Q
— A mutable reference&mut T
.The tag
Q
is followed by an optional lifetime followed by the type of the mutable reference. The lifetime is not included if it has been erased. -
P
— A constant raw pointer*const T
.The tag
P
is followed by the type of the pointer.const-ptr-type →
P
type -
O
— A mutable raw pointer*mut T
.mut-ptr-type →
O
typeThe tag
O
is followed by the type of the pointer. -
F
— A function pointerfn(…) -> …
.fn-type →
F
fn-sigfn-sig → binderopt
U
opt (K
abi)opt {type}E
typeabi →
C
| undisambiguated-identifierThe tag
F
is followed by a fn-sig of the function signature. A fn-sig is the signature for a function pointer.It starts with an optional binder which represents the higher-ranked trait bounds (
for<…>
).Following that is an optional
U
character which is present for anunsafe
function.Following that is an optional
K
character which indicates that an abi is specified. If the ABI is not specified, it is assumed to be the"Rust"
ABI.The abi can be the letter
C
to indicate it is the"C"
ABI. Otherwise it is an undisambiguated-identifier of the ABI string with dashes converted to underscores.Following that is zero or more types which indicate the input parameters of the function.
Following that is the character
E
and then the type of the return value.
-
D
— A trait objectdyn Trait<Assoc=X> + Send + 'a
.dyn-trait-type →
D
dyn-bounds lifetimedyn-bounds → binderopt {dyn-trait}
E
dyn-trait → path {dyn-trait-assoc-binding}
dyn-trait-assoc-binding →
p
undisambiguated-identifier typeThe tag
D
is followed by a dyn-bounds which encodes the trait bounds, followed by a lifetime of the trait object lifetime bound.A dyn-bounds starts with an optional binder which represents the higher-ranked trait bounds (
for<…>
). Following that is a sequence of dyn-trait terminated by the characterE
.Each dyn-trait represents a trait bound, which consists of a path to the trait followed by zero or more dyn-trait-assoc-binding which list the associated types.
Each dyn-trait-assoc-binding consists of a character
p
followed a undisambiguated-identifier representing the associated binding name, and finally a type.
Recommended Demangling
A type may be displayed as the type it represents, using typical Rust syntax to represent the type.
Example:
fn main() { example::<[u16; 8]>(); } pub fn example<T>() {}
The symbol for function
example
is:_RINvCs7qp2U7fqm6G_7mycrate7exampleAtj8_EB2_ │││├┘│ ││││ └─── end of generic args │││└───── const data 8 ││└────── const type usize │└─────── array element type u16 └──────── array type
Recommended demangling:
mycrate::example::<[u16; 8]>
Binder
binder →
G
base-62-number
A binder represents the number of higher-ranked trait bound lifetimes to bind.
It consists of the character G
followed by a base-62-number.
The value 1 should be added to the base-62-number when decoding
(such that the base-62-number encoding of _
is interpreted as having 1 binder).
A lifetime rule can then refer to these numbered lifetimes. The lowest indices represent the innermost lifetimes. The number of bound lifetimes is the value of base-62-number plus one.
For example, in for<'a, 'b> fn(for<'c> fn (...))
, any lifetimes in ...
(but not inside more binders) will observe the indices 1, 2, and 3 to refer to 'c
, 'b
, and 'a
, respectively.
Recommended Demangling
A binder may be printed using
for<…>
syntax listing the lifetimes as recommended in lifetime. See lifetime for an example.
Backref
backref →
B
base-62-number
A backref is used to refer to a previous part of the mangled symbol. This provides a simple form of compression to reduce the length of the mangled symbol. This can help reduce the amount of work and resources needed by the compiler, linker, and loader.
It consists of the character B
followed by a base-62-number.
The number indicates the 0-based offset in bytes starting from just after the _R
prefix of the symbol.
The backref represents the corresponding element starting at that position.
backrefs always refer to a position before the backref itself.
The backref compression relies on the fact that all substitutable symbol elements have a self-terminating mangled form. Given the start position of the encoded node, the grammar guarantees that it is always unambiguous where the node ends. This is ensured by not allowing optional or repeating elements at the end of substitutable productions.
Recommended Demangling
A backref should be demangled by rendering the element that it points to. Care should be considered when handling deeply nested backrefs to avoid using too much stack.
Example:
fn main() { example::<Example, Example>(); } struct Example; pub fn example<T, U>() {}
The symbol for function
example
is:_RINvCs7qp2U7fqm6G_7mycrate7exampleNtB2_7ExampleBw_EB2_ │├┘ │├┘ │├┘ ││ ││ ││ ││ ││ │└── backref to offset 3 (crate-root) ││ ││ └─── backref for instantiating-crate path ││ │└────── backref to offset 33 (path to Example) ││ └─────── backref for second generic-arg │└───────────────── backref to offset 3 (crate-root) └────────────────── backref for first generic-arg (first segment of Example path)
Recommended demangling:
mycrate::example::<mycrate::Example, mycrate::Example>
Instantiating crate
instantiating-crate → path
The instantiating-crate is an optional element of the symbol-name which can be used to indicate which crate is instantiating the symbol. It consists of a single path.
This helps differentiate symbols that would otherwise be identical, for example the monomorphization of a function from an external crate may result in a duplicate if another crate is also instantiating the same generic function with the same types.
In practice, the instantiating crate is also often the crate where the symbol is defined, so it is usually encoded as a backref to the crate-root encoded elsewhere in the symbol.
Recommended Demangling
The instantiating-crate usually need not be displayed.
Example:
std::path::Path::new("example");
The symbol for
Path::new::<str>
instantiated from themycrate
crate is:_RINvMsY_NtCseXNvpPnDBDp_3std4pathNtB6_4Path3neweECs7qp2U7fqm6G_7mycrate └──┬───┘ │ └── instantiating crate identifier `mycrate`
Recommended demangling:
<std::path::Path>::new::<str>
Vendor-specific suffix
vendor-specific-suffix → (
.
|$
) suffixsuffix → {byte}
The vendor-specific-suffix is an optional element at the end of the symbol-name.
It consists of either a .
or $
character followed by zero or more bytes.
There are no restrictions on the characters following the period or dollar sign.
This suffix is added as needed by the implementation.
One example where this can happen is when locally unique names need to become globally unique.
LLVM can append a .llvm.<numbers>
suffix during LTO to ensure a unique name,
and $
can be used for thread-local data on Mach-O.
In these situations it's generally fine to ignore the suffix;
the suffixed name has the same semantics as the original.
Recommended Demangling
The vendor-specific-suffix usually need not be displayed.
Example:
use std::cell::RefCell; thread_local! { pub static EXAMPLE: RefCell<u32> = RefCell::new(1); }
The symbol for
EXAMPLE
on macOS may have the following for thread-local data:_RNvNvNvCs7qp2U7fqm6G_7mycrate7EXAMPLE7___getit5___KEY$tlv$init └───┬───┘ │ └── vendor-specific-suffix
Recommended demangling:
mycrate::EXAMPLE::__getit::__KEY
Common rules
decimal-number →
0
| non-zero-digit {digit}non-zero-digit →
1
|2
|3
|4
|5
|6
|7
|8
|9
digit →0
| non-zero-digitlower →
a
|b
|c
|d
|e
|f
|g
|h
|i
|j
|k
|l
|m
|n
|o
|p
|q
|r
|s
|t
|u
|v
|w
|x
|y
|z
upper →
A
|B
|C
|D
|E
|F
|G
|H
|I
|J
|K
|L
|M
|N
|O
|P
|Q
|R
|S
|T
|U
|V
|W
|X
|Y
|Z
A decimal-number is encoded as one or more digits indicating a numeric value in decimal.
The value zero is encoded as a single byte 0
.
Beware that there are situations where 0
may be followed by another digit that should not be decoded as part of the decimal-number.
For example, a zero-length identifier within a nested-path which is in turn inside another nested-path will result in two identifiers in a row, where the first one only has the encoding of 0
.
A digit is an ASCII number.
A lower and upper is an ASCII lower and uppercase letter respectively.
base-62-number
base-62-number → { digit | lower | upper }
_
A base-62-number is an encoding of a numeric value.
It uses ASCII numbers and lowercase and uppercase letters.
The value is terminated with the _
character.
If the value is 0, then the encoding is the _
character without any digits.
Otherwise, one is subtracted from the value, and it is encoded with the mapping:
0
-9
maps to 0-9a
-z
maps to 10 to 35A
-Z
maps to 36 to 61
The number is repeatedly divided by 62 (with integer division round towards zero) to choose the next character in the sequence. The remainder of each division is used in the mapping to choose the next character. This is repeated until the number is 0. The final sequence of characters is then reversed.
Decoding is a similar process in reverse.
Examples:
Value | Encoding |
---|---|
0 | _ |
1 | 0_ |
11 | a_ |
62 | Z_ |
63 | 10_ |
1000 | g7_ |
Symbol grammar summary
The following is a summary of all of the productions of the symbol grammar.
symbol-name →
_R
decimal-numberopt path instantiating-crateopt vendor-specific-suffixoptpath →
crate-root
| inherent-impl
| trait-impl
| trait-definition
| nested-path
| generic-args
| backrefcrate-root →
C
identifier
inherent-impl →M
impl-path type
trait-impl →X
impl-path type path
trait-definition →Y
type path
nested-path →N
namespace path identifier
generic-args →I
path {generic-arg}E
identifier → disambiguatoropt undisambiguated-identifier
undisambiguated-identifier →u
opt decimal-number_
opt bytes
bytes → {UTF-8 bytes}disambiguator →
s
base-62-numberimpl-path → disambiguatoropt path
type →
basic-type
| array-type
| slice-type
| tuple-type
| ref-type
| mut-ref-type
| const-ptr-type
| mut-ptr-type
| fn-type
| dyn-trait-type
| path
| backrefbasic-type → lower
array-type →A
type const
slice-type →S
type
tuple-type →T
{type}E
ref-type →R
lifetimeopt type
mut-ref-type →Q
lifetimeopt type
const-ptr-type →P
type
mut-ptr-type →O
type
fn-type →F
fn-sig
dyn-trait-type →D
dyn-bounds lifetimegeneric-arg →
lifetime
| type
|K
constconst →
type const-data
|p
| backrefconst-data →
n
opt {hex-digit}_
hex-digit → digit |
a
|b
|c
|d
|e
|f
fn-sig → binderopt
U
opt (K
abi)opt {type}E
typeabi →
C
| undisambiguated-identifierdyn-bounds → binderopt {dyn-trait}
E
dyn-trait → path {dyn-trait-assoc-binding}
dyn-trait-assoc-binding →p
undisambiguated-identifier typebinder →
G
base-62-numbervendor-specific-suffix → (
.
|$
) suffix
suffix → {byte}decimal-number →
0
| non-zero-digit {digit}base-62-number → { digit | lower | upper }
_
non-zero-digit →
1
|2
|3
|4
|5
|6
|7
|8
|9
digit →0
| non-zero-digit
lower →a
|b
|c
|d
|e
|f
|g
|h
|i
|j
|k
|l
|m
|n
|o
|p
|q
|r
|s
|t
|u
|v
|w
|x
|y
|z
upper →A
|B
|C
|D
|E
|F
|G
|H
|I
|J
|K
|L
|M
|N
|O
|P
|Q
|R
|S
|T
|U
|V
|W
|X
|Y
|Z
Encoding of Rust entities
The following are guidelines for how Rust entities are encoded in a symbol. The compiler has some latitude in how an entity is encoded as long as the symbol is unambiguous.
-
Named functions, methods, and statics shall be represented by a path production.
-
Paths should be rooted at the inner-most entity that can act as a path root. Roots can be crate-ids, inherent impls, trait impls, and (for items within default methods) trait definitions.
-
The compiler is free to choose disambiguation indices and namespace tags from the reserved ranges as long as it ascertains identifier unambiguity.
-
Generic arguments that are equal to the default should not be encoded in order to save space.
Contributing to rustc
We'd love to have your help improving rustc
! To that end, we've written a
whole book on its
internals, how it works, and how to get started working on it. To learn
more, you'll want to check that out.
If you would like to contribute to this book, you can find its source in the rustc source at src/doc/rustc.