Use declarations

Syntax:
UseDeclaration :
   use UseTree ;

UseTree :
      (SimplePath? ::)? *
   | (SimplePath? ::)? { (UseTree ( , UseTree )* ,?)? }
   | SimplePath ( as ( IDENTIFIER | _ ) )?

A use declaration creates one or more local name bindings synonymous with some other path. Usually a use declaration is used to shorten the path required to refer to a module item. These declarations may appear in modules and blocks, usually at the top.

Use declarations support a number of convenient shortcuts:

  • Simultaneously binding a list of paths with a common prefix, using the glob-like brace syntax use a::b::{c, d, e::f, g::h::i};
  • Simultaneously binding a list of paths with a common prefix and their common parent module, using the self keyword, such as use a::b::{self, c, d::e};
  • Rebinding the target name as a new local name, using the syntax use p::q::r as x;. This can also be used with the last two features: use a::b::{self as ab, c as abc}.
  • Binding all paths matching a given prefix, using the asterisk wildcard syntax use a::b::*;.
  • Nesting groups of the previous features multiple times, such as use a::b::{self as ab, c, d::{*, e::f}};

An example of use declarations:

use std::collections::hash_map::{self, HashMap};

fn foo<T>(_: T){}
fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}

fn main() {
    // use declarations can also exist inside of functions
    use std::option::Option::{Some, None};

    // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
    // std::option::Option::None]);'
    foo(vec![Some(1.0f64), None]);

    // Both `hash_map` and `HashMap` are in scope.
    let map1 = HashMap::new();
    let map2 = hash_map::HashMap::new();
    bar(map1, map2);
}

use Visibility

Like items, use declarations are private to the containing module, by default. Also like items, a use declaration can be public, if qualified by the pub keyword. Such a use declaration serves to re-export a name. A public use declaration can therefore redirect some public name to a different target definition: even a definition with a private canonical path, inside a different module. If a sequence of such redirections form a cycle or cannot be resolved unambiguously, they represent a compile-time error.

An example of re-exporting:

mod quux {
    pub use self::foo::{bar, baz};
    pub mod foo {
        pub fn bar() {}
        pub fn baz() {}
    }
}

fn main() {
    quux::bar();
    quux::baz();
}

In this example, the module quux re-exports two public names defined in foo.

use Paths

Note: This section is incomplete.

Some examples of what will and will not work for use items:

#![allow(unused_imports)]
use std::path::{self, Path, PathBuf};  // good: std is a crate name
use crate::foo::baz::foobaz;    // good: foo is at the root of the crate

mod foo {

    pub mod example {
        pub mod iter {}
    }

    use crate::foo::example::iter; // good: foo is at crate root
//  use example::iter;      // bad in 2015 edition: relative paths are not allowed without `self`; good in 2018 edition
    use self::baz::foobaz;  // good: self refers to module 'foo'
    use crate::foo::bar::foobar;   // good: foo is at crate root

    pub mod bar {
        pub fn foobar() { }
    }

    pub mod baz {
        use super::bar::foobar; // good: super refers to module 'foo'
        pub fn foobaz() { }
    }
}

fn main() {}

Edition Differences: In the 2015 edition, use paths also allow accessing items in the crate root. Using the example above, the following use paths work in 2015 but not 2018:

mod foo {
    pub mod example { pub mod iter {} }
    pub mod baz { pub fn foobaz() {} }
}
use foo::example::iter;
use ::foo::baz::foobaz;
fn main() {}

The 2015 edition does not allow use declarations to reference the extern prelude. Thus extern crate declarations are still required in 2015 to reference an external crate in a use declaration. Beginning with the 2018 edition, use declarations can specify an external crate dependency the same way extern crate can.

In the 2018 edition, if an in-scope item has the same name as an external crate, then use of that crate name requires a leading :: to unambiguously select the crate name. This is to retain compatibility with potential future changes. 

// use std::fs; // Error, this is ambiguous.
use ::std::fs;  // Imports from the `std` crate, not the module below.
use self::std::fs as self_fs;  // Imports the module below.

mod std {
    pub mod fs {}
}
fn main() {}

Underscore Imports

Items can be imported without binding to a name by using an underscore with the form use path as _. This is particularly useful to import a trait so that its methods may be used without importing the trait's symbol, for example if the trait's symbol may conflict with another symbol. Another example is to link an external crate without importing its name.

Asterisk glob imports will import items imported with _ in their unnameable form.

mod foo {
    pub trait Zoo {
        fn zoo(&self) {}
    }

    impl<T> Zoo for T {}
}

use self::foo::Zoo as _;
struct Zoo;  // Underscore import avoids name conflict with this item.

fn main() {
    let z = Zoo;
    z.zoo();
}

The unique, unnameable symbols are created after macro expansion so that macros may safely emit multiple references to _ imports. For example, the following should not produce an error:


#![allow(unused)]
fn main() {
macro_rules! m {
    ($item: item) => { $item $item }
}

m!(use std as _;);
// This expands to:
// use std as _;
// use std as _;
}