Use declarations

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:

• Nameable items
• Enum variants
• Built-in types
• Attributes
• Derive macros
• macro_rules

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:

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.
}

as renames

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

// 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.

// 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.

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 may appear as the last segment of a use path, preceded by ::. A path of the form P::self is equivalent to P::{self}, and P::self as name is equivalent to P::{self as name}.

mod m {
    pub enum E { V1, V2 }
}
use m::self as _; // Equivalent to `use m::{self as _};`.
use m::E::self; // Equivalent to `use m::E::{self};`.
# fn main() {}

When self is used within brace syntax, the path preceding the brace group must resolve to a module, enumeration, or trait.

mod m {
    pub enum E { V1, V2 }
    pub trait Tr { fn f(&self); }
}
use m::{self as _}; // OK: Modules can be parents of `self`.
use m::E::{self, V1}; // OK: Enums can be parents of `self`.
use m::Tr::{self}; // OK: Traits can be parents of `self`.
# fn main() {}

struct S {}
use S::{self as _}; // ERROR: Structs cannot be parents of `self`.
# 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
}

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:

// 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:

// 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::*;).

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.

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();
}

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

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:

macro_rules! m {
    ($item: item) => { $item $item }
}

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

Restrictions

The following rules are restrictions for valid use declarations.

When using crate to import the current crate, you must use as to define the binding name.

When using $crate in a macro transcriber to import the current crate, you must use as to define the binding name.

When using self to import the current module, you must use as to define the binding name.

When using super to import a parent module, you must use as to define the binding name.

:: as the extern prelude cannot be imported.

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 cannot refer to enum variants through a type alias.