src/items/use-declarations.md

# Use declarations

r[items.use]

r[items.use.syntax]
> **<sup>Syntax:</sup>**\
> _UseDeclaration_ :\
> &nbsp;&nbsp; `use` _UseTree_ `;`
>
> _UseTree_ :\
> &nbsp;&nbsp; &nbsp;&nbsp; ([_SimplePath_]<sup>?</sup> `::`)<sup>?</sup> `*`\
> &nbsp;&nbsp; | ([_SimplePath_]<sup>?</sup> `::`)<sup>?</sup> `{` (_UseTree_ ( `,`  _UseTree_ )<sup>\*</sup> `,`<sup>?</sup>)<sup>?</sup> `}`\
> &nbsp;&nbsp; | [_SimplePath_]&nbsp;( `as` ( [IDENTIFIER] | `_` ) )<sup>?</sup>

r[items.use.intro]
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_.

[path]: ../paths.md
[modules]: modules.md
[blocks]: ../expressions/block-expr.md

r[items.use.forms]
Use declarations support a number of convenient shortcuts:

r[items.use.forms.multiple]
* Simultaneously binding a list of paths with a common prefix, using the
  brace syntax `use a::b::{c, d, e::f, g::h::i};`

r[items.use.forms.self]
* 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};`

r[items.use.forms.as]
* 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}`.

r[items.use.forms.glob]
* Binding all paths matching a given prefix, using the asterisk wildcard syntax
  `use a::b::*;`.

r[items.use.forms.nesting]
* 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:

```rust
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

r[items.use.visibility]

r[items.use.visibility.intro]
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.

r[items.use.visibility.unambiguous]
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:

```rust
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

r[items.use.path]

r[items.use.path.intro]
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]

r[items.use.path.disallowed]
They cannot import [associated items], [generic parameters], [local variables], paths with [`Self`], or [tool attributes]. More restrictions are described below.

r[items.use.path.namespace]
`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:

```rust
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.
}
```

r[items.use.path.edition2015]
> **Edition differences**: In the 2015 edition, `use` paths are relative to the crate root.
> For example:
>
> ```rust,edition2015
> 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

r[items.use.as]

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

```rust
// Creates a non-public alias `bar` for the function `foo`.
use inner::foo as bar;

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

## Brace syntax

r[items.use.multiple-syntax]

r[items.use.multiple-syntax.intro]
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.

```rust
// Creates bindings to:
// - `std::collections::BTreeSet`
// - `std::collections::hash_map`
// - `std::collections::hash_map::HashMap`
use std::collections::{BTreeSet, hash_map::{self, HashMap}};
```

r[items.use.multiple-syntax.empty]
An empty brace does not import anything, though the leading path is validated that it is accessible.
<!-- This is slightly wrong, see: https://github.com/rust-lang/rust/issues/61826 -->

r[items.use.multiple-syntax.edition2015]
> **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

r[items.use.self]

r[items.use.self.intro]
The keyword `self` may be used within [brace syntax](#brace-syntax) to create a binding of the parent entity under its own name.

```rust
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() {}
```

r[items.use.self.namespace]
`self` only creates a binding from the [type namespace] of the parent entity.
For example, in the following, only the `foo` mod is imported:

```rust,compile_fail
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

r[items.use.glob]

r[items.use.glob.intro]
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:

```rust
// 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;
    }
}
```

r[items.use.glob.shadowing]
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:

```rust
// 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 {}
}
```

r[items.use.glob.last-segment-only]
`*` cannot be used as the first or intermediate segments.

r[items.use.glob.self-import]
`*` cannot be used to import a module's contents into itself (such as `use self::*;`).

r[items.use.glob.edition2015]
> **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

r[items.use.as-underscore]

r[items.use.as-underscore.intro]
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.

r[items.use.as-underscore.glob]
Asterisk glob imports will import items imported with `_` in their unnameable
form.

```rust
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();
}
```

r[items.use.as-underscore.macro]
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:

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

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

## Restrictions

r[items.use.restrictions]

The following are restrictions for valid `use` declarations:

r[items.use.restrictions.crate]
* `use crate;` must use `as` to define the name to which to bind the crate root.

r[items.use.restrictions.self]
* `use {self};` is an error; there must be a leading segment when using `self`.

r[items.use.restrictions.duplicate-name]
* As with any item definition, `use` imports cannot create duplicate bindings of the same name in the same namespace in a module or block.

r[items.use.restrictions.macro-crate]
* `use` paths with `$crate` are not allowed in a [`macro_rules`] expansion.

r[items.use.restrictions.variant]
* `use` paths cannot refer to enum variants through a [type alias]. For example:
  ```rust,compile_fail
  enum MyEnum {
      MyVariant
  }
  type TypeAlias = MyEnum;

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

## Ambiguities

r[items.use.ambiguities]

> **Note**: This section is incomplete.

r[items.use.ambiguities.intro]
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.

r[items.use.ambiguities.glob]
Glob imports are allowed to import conflicting names in the same namespace as long as the name is not used.
For example:

```rust
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:

```rust
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;
}
```

[_SimplePath_]: ../paths.md#simple-paths
[`extern crate`]: extern-crates.md
[`macro_rules`]: ../macros-by-example.md
[`self`]: ../paths.md#self
[associated items]: associated-items.md
[Attributes]: ../attributes.md
[Built-in types]: ../types.md
[Derive macros]: ../procedural-macros.md#derive-macros
[Enum variants]: enumerations.md
[extern prelude]: ../names/preludes.md#extern-prelude
[generic parameters]: generics.md
[IDENTIFIER]: ../identifiers.md
[items]: ../items.md
[local variables]: ../variables.md
[namespace]: ../names/namespaces.md
[namespaces]: ../names/namespaces.md
[paths]: ../paths.md
[tool attributes]: ../attributes.md#tool-attributes
[type alias]: type-aliases.md
[type namespace]: ../names/namespaces.md