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. A use declaration is also sometimes called an import, or, if it is public, a re-export.

Use declarations support a number of convenient shortcuts:

  • Simultaneously binding a list of paths with a common prefix, using the 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

The paths that are allowed in a use item follow the SimplePath grammar and are similar to the paths that may be used in an expression. They may create bindings for:

They cannot import associated items, generic parameters, local variables, paths with Self, or tool attributes. More restrictions are described below.

use will create bindings for all namespaces from the imported entities, with the exception that a self import will only import from the type namespace (as described below). For example, the following illustrates creating bindings for the same name in two namespaces:

#![allow(unused)]
fn main() {
mod stuff {
    pub struct Foo(pub i32);
}

// Imports the `Foo` type and the `Foo` constructor.
use stuff::Foo;

fn example() {
    let ctor = Foo; // Uses `Foo` from the value namespace.
    let x: Foo = ctor(123); // Uses `Foo` From the type namespace.
}
}

Edition differences: In the 2015 edition, use paths are relative to the crate root. For example:

mod foo {
    pub mod example { pub mod iter {} }
    pub mod baz { pub fn foobaz() {} }
}
mod bar {
    // Resolves `foo` from the crate root.
    use foo::example::iter;
    // The `::` prefix explicitly resolves `foo`
    // from the crate root.
    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.

as renames

The as keyword can be used to change the name of an imported entity. For example:

#![allow(unused)]
fn main() {
// Creates a non-public alias `bar` for the function `foo`.
use inner::foo as bar;

mod inner {
    pub fn foo() {}
}
}

Brace syntax

Braces can be used in the last segment of the path to import multiple entities from the previous segment, or, if there are no previous segments, from the current scope. Braces can be nested, creating a tree of paths, where each grouping of segments is logically combined with its parent to create a full path.

#![allow(unused)]
fn main() {
// Creates bindings to:
// - `std::collections::BTreeSet`
// - `std::collections::hash_map`
// - `std::collections::hash_map::HashMap`
use std::collections::{BTreeSet, hash_map::{self, HashMap}};
}

An empty brace does not import anything, though the leading path is validated that it is accessible.

Edition differences: In the 2015 edition, paths are relative to the crate root, so an import such as use {foo, bar}; will import the names foo and bar from the crate root, whereas starting in 2018, those names are relative to the current scope.

self imports

The keyword self may be used within brace syntax to create a binding of the parent entity under its own name.

mod stuff {
    pub fn foo() {}
    pub fn bar() {}
}
mod example {
    // Creates a binding for `stuff` and `foo`.
    use crate::stuff::{self, foo};
    pub fn baz() {
        foo();
        stuff::bar();
    }
}
fn main() {}

self only creates a binding from the type namespace of the parent entity. For example, in the following, only the foo mod is imported:

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

// This only imports the module `foo`. The function `foo` lives in
// the value namespace and is not imported.
use bar::foo::{self};

fn main() {
    foo(); //~ ERROR `foo` is a module
}

Note: self may also be used as the first segment of a path. The usage of self as the first segment and inside a use brace is logically the same; it means the current module of the parent segment, or the current module if there is no parent segment. See self in the paths chapter for more information on the meaning of a leading self.

Glob imports

The * character may be used as the last segment of a use path to import all importable entities from the entity of the preceding segment. For example:

#![allow(unused)]
fn main() {
// Creates a non-public alias to `bar`.
use foo::*;

mod foo {
    fn i_am_private() {}
    enum Example {
        V1,
        V2,
    }
    pub fn bar() {
        // Creates local aliases to `V1` and `V2`
        // of the `Example` enum.
        use Example::*;
        let x = V1;
    }
}
}

Items and named imports are allowed to shadow names from glob imports in the same namespace. That is, if there is a name already defined by another item in the same namespace, the glob import will be shadowed. For example:

#![allow(unused)]
fn main() {
// This creates a binding to the `clashing::Foo` tuple struct
// constructor, but does not import its type because that would
// conflict with the `Foo` struct defined here.
//
// Note that the order of definition here is unimportant.
use clashing::*;
struct Foo {
    field: f32,
}

fn do_stuff() {
    // Uses the constructor from `clashing::Foo`.
    let f1 = Foo(123);
    // The struct expression uses the type from
    // the `Foo` struct defined above.
    let f2 = Foo { field: 1.0 };
    // `Bar` is also in scope due to the glob import.
    let z = Bar {};
}

mod clashing {
    pub struct Foo(pub i32);
    pub struct Bar {}
}
}

* cannot be used as the first or intermediate segments. * cannot be used to import a module’s contents into itself (such as use self::*;).

Edition differences: In the 2015 edition, paths are relative to the crate root, so an import such as use *; is valid, and it means to import everything from the crate root. This cannot be used in the crate root itself.

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 _;
}

Restrictions

The following are restrictions for valid use declarations:

  • use crate; must use as to define the name to which to bind the crate root.
  • use {self}; is an error; there must be a leading segment when using self.
  • As with any item definition, use imports cannot create duplicate bindings of the same name in the same namespace in a module or block.
  • use paths with $crate are not allowed in a macro_rules expansion.
  • use paths cannot refer to enum variants through a type alias. For example: 
    #![allow(unused)]
fn main() {
enum MyEnum {
    MyVariant
}
type TypeAlias = MyEnum;

use MyEnum::MyVariant; //~ OK
use TypeAlias::MyVariant; //~ ERROR
}

Ambiguities

Note: This section is incomplete.

Some situations are an error when there is an ambiguity as to which name a use declaration refers. This happens when there are two name candidates that do not resolve to the same entity.

Glob imports are allowed to import conflicting names in the same namespace as long as the name is not used. For example:

mod foo {
    pub struct Qux;
}

mod bar {
    pub struct Qux;
}

use foo::*;
use bar::*; //~ OK, no name conflict.

fn main() {
    // This would be an error, due to the ambiguity.
    //let x = Qux;
}

Multiple glob imports are allowed to import the same name, and that name is allowed to be used, if the imports are of the same item (following re-exports). The visibility of the name is the maximum visibility of the imports. For example:

mod foo {
    pub struct Qux;
}

mod bar {
    pub use super::foo::Qux;
}

// These both import the same `Qux`. The visibility of `Qux`
// is `pub` because that is the maximum visibility between
// these two `use` declarations.
pub use bar::*;
use foo::*;

fn main() {
    let _: Qux = Qux;
}