For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

Extern crate declarations

ExternCrate :
   extern crate IDENTIFIER (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 identifier provided in the extern crate declaration.

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 crateid provided against the crateid attributes that were declared on the external crate when it was compiled. If no crateid is provided, a default name attribute is assumed, equal to the identifier given in the extern crate declaration.

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

Extern Prelude

External crates provided to the compiler are added to the "extern prelude" which exposes the crate names into lexical scope of every module without the need for specifying extern crate.

Edition Differences: In the 2015 edition, crates in the extern prelude cannot be referenced via use declarations, so it is generally standard practice to include extern crate declarations to bring them into scope.

Beginning in the 2018 edition, use declarations can reference crates in the extern prelude, so it is considered unidiomatic to use extern crate.

Note: Additional crates that ship with rustc, such as proc_macro, alloc, and test, currently aren't available in the extern prelude and must be brought into scope with an extern crate declaration, even in the 2018 edition. use paths must reference the extern crate item (such as using crate:: or self:: path prefixes).

# #![allow(unused_variables)]
#fn main() {
extern crate proc_macro;
// Cannot reference `proc_macro` directly because it is not in the extern prelude.
// use proc_macro::TokenStream;
// Instead, you must reference the item in scope from the `extern crate`
// declaration.
use self::proc_macro::TokenStream;