Extern crate declarations

Syntax:
ExternCrate :
   extern crate CrateRef AsClause? ;

CrateRef :
   IDENTIFIER | self

AsClause :
   as ( IDENTIFIER | _ )

An extern crate declaration specifies a dependency on an external crate.

The external crate is then bound into the declaring scope as the given identifier in the type namespace.

Additionally, if the extern crate appears in the crate root, then the crate name is also added to the extern prelude, making it automatically in scope in all modules.

The as clause can be used to bind the imported crate to a different name.

The external crate is resolved to a specific soname at compile time, and a runtime linkage requirement to that soname is passed to the linker for loading at runtime. The soname is resolved at compile time by scanning the compiler’s library path and matching the optional crate_name provided against the crate_name attributes that were declared on the external crate when it was compiled. If no crate_name is provided, a default name attribute is assumed, equal to the identifier given in the extern crate declaration.

The self crate may be imported which creates a binding to the current crate. In this case the as clause must be used to specify the name to bind it to.

Three examples of extern crate declarations:

extern crate pcre;

extern crate std; // equivalent to: extern crate std as std;

extern crate std as ruststd; // linking to 'std' under another name

When naming Rust crates, hyphens are disallowed. However, Cargo packages may make use of them. In such case, when Cargo.toml doesn’t specify a crate name, Cargo will transparently replace - with _ (Refer to RFC 940 for more details).

Here is an example:

// Importing the Cargo package hello-world
extern crate hello_world; // hyphen replaced with an underscore

Underscore Imports

An external crate dependency can be declared without binding its name in scope by using an underscore with the form extern crate foo as _. This may be useful for crates that only need to be linked, but are never referenced, and will avoid being reported as unused.

The macro_use attribute works as usual and imports the macro names into the macro_use prelude.

The no_link attribute may be specified on an extern crate item to prevent linking the crate into the output. This is commonly used to load a crate to access only its macros.