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 imported with `extern crate` in the root module or provided to the compiler (as with the `--extern` flag with `rustc`) are added to the "extern prelude". Crates in the extern prelude are in scope in the entire crate, including inner modules. If imported with `extern crate orig_name as new_name`, then the symbol `new_name` is instead added to the prelude.

The `core` crate is always added to the extern prelude. The `std` crate is added as long as the `no_std` attribute is not specified in the crate root.

The `no_implicit_prelude` attribute can be used on a module to disable prelude lookups within that module.

> 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`, are not automatically included with the `--extern` flag when using Cargo. They must be brought into scope with an `extern crate` declaration, even in the 2018 edition.

>     # #![allow(unused_variables)]
>     #fn main() {
>     extern crate proc_macro;
>     use proc_macro::TokenStream;
>     #}

## 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 will work as usual and import the macro names into the macro-use prelude.