C-string literals
Summary
- Literals of the form
c"foo"
orcr"foo"
represent a string of type&core::ffi::CStr
.
Details
Starting with Rust 1.77, C-strings can be written using C-string literal syntax with the c
or cr
prefix.
Previously, it was challenging to properly produce a valid string literal that could interoperate with C APIs which terminate with a NUL byte.
The cstr
crate was a popular solution, but that required compiling a proc-macro which was quite expensive.
Now, C-strings can be written directly using literal syntax notation, which will generate a value of type &core::ffi::CStr
which is automatically terminated with a NUL byte.
C-strings do not allow interior NUL bytes (such as with a \0
escape).
Similar to regular strings, C-strings also support "raw" syntax with the cr
prefix.
These raw C-strings do not process backslash escapes which can make it easier to write strings that contain backslashes.
Double-quotes can be included by surrounding the quotes with the #
character.
Multiple #
characters can be used to avoid ambiguity with internal "#
sequences.
See The Reference for more details.
Migration
Migration is only necessary for macros which may have been assuming a sequence of tokens that looks similar to c"…"
or cr"…"
, which previous to the 2021 edition would tokenize as two separate tokens, but in 2021 appears as a single token.
As part of the syntax reservation for the 2021 edition, any macro input which may run into this issue should issue a warning from the rust_2021_prefixes_incompatible_syntax
migration lint.
See that chapter for more detail.