Extern crates - The Rust Reference

# Extern crate declarations

> Syntax:
> 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;
>     #}