Paths

A path is a sequence of one or more path segments logically separated by a namespace qualifier (::). If a path consists of only one segment, it refers to either an item or a variable in a local control scope. If a path has multiple segments, it always refers to an item.

Two examples of simple paths consisting of only identifier segments:

x;
x::y::z;

Types of paths

Simple Paths

^Syntax
SimplePath :
::^? SimplePathSegment (:: SimplePathSegment)^*

SimplePathSegment :
IDENTIFIER | super | self | crate | $crate

Simple paths are used in visibility markers, attributes, macros, and use items. Examples:

#![allow(unused)]
fn main() {
use std::io::{self, Write};
mod m {
    #[clippy::cyclomatic_complexity = "0"]
    pub (in super) fn f1() {}
}
}

Paths in expressions

^Syntax
PathInExpression :
   ::^? PathExprSegment (:: PathExprSegment)^*

PathExprSegment :
   PathIdentSegment (:: GenericArgs)^?

PathIdentSegment :
   IDENTIFIER | super | self | Self | crate | $crate

GenericArgs :
      < >
   | < ( GenericArg , )^* GenericArg ,^? >

GenericArg :
   Lifetime | Type | GenericArgsConst | GenericArgsBinding

GenericArgsConst :
      BlockExpression
   | LiteralExpression
   | - LiteralExpression
   | SimplePathSegment

GenericArgsBinding :
   IDENTIFIER = Type

Paths in expressions allow for paths with generic arguments to be specified. They are used in various places in expressions and patterns.

The :: token is required before the opening < for generic arguments to avoid ambiguity with the less-than operator. This is colloquially known as "turbofish" syntax.

#![allow(unused)]
fn main() {
(0..10).collect::<Vec<_>>();
Vec::<u8>::with_capacity(1024);
}

The order of generic arguments is restricted to lifetime arguments, then type arguments, then const arguments, then equality constraints.

Const arguments must be surrounded by braces unless they are a literal or a single segment path.

The synthetic type parameters corresponding to impl Trait types are implicit, and these cannot be explicitly specified.

Qualified paths

^Syntax
QualifiedPathInExpression :
   QualifiedPathType (:: PathExprSegment)⁺

QualifiedPathType :
   < Type (as TypePath)^? >

QualifiedPathInType :
   QualifiedPathType (:: TypePathSegment)⁺

Fully qualified paths allow for disambiguating the path for trait implementations and for specifying canonical paths. When used in a type specification, it supports using the type syntax specified below.

#![allow(unused)]
fn main() {
struct S;
impl S {
    fn f() { println!("S"); }
}
trait T1 {
    fn f() { println!("T1 f"); }
}
impl T1 for S {}
trait T2 {
    fn f() { println!("T2 f"); }
}
impl T2 for S {}
S::f();  // Calls the inherent impl.
<S as T1>::f();  // Calls the T1 trait function.
<S as T2>::f();  // Calls the T2 trait function.
}

Paths in types

^Syntax
TypePath :
::^? TypePathSegment (:: TypePathSegment)^*

TypePathSegment :
PathIdentSegment ::^? (GenericArgs | TypePathFn)^?

TypePathFn :
( TypePathFnInputs^? ) (-> Type)^?

TypePathFnInputs :
Type (, Type)^* ,^?

Type paths are used within type definitions, trait bounds, type parameter bounds, and qualified paths.

Although the :: token is allowed before the generics arguments, it is not required because there is no ambiguity like there is in PathInExpression.

#![allow(unused)]
fn main() {
mod ops {
    pub struct Range<T> {f1: T}
    pub trait Index<T> {}
    pub struct Example<'a> {f1: &'a i32}
}
struct S;
impl ops::Index<ops::Range<usize>> for S { /*...*/ }
fn i<'a>() -> impl Iterator<Item = ops::Example<'a>> {
    // ...
   const EXAMPLE: Vec<ops::Example<'static>> = Vec::new();
   EXAMPLE.into_iter()
}
type G = std::boxed::Box<dyn std::ops::FnOnce(isize) -> isize>;
}

Path qualifiers

Paths can be denoted with various leading qualifiers to change the meaning of how it is resolved.

`::`

Paths starting with :: are considered to be global paths where the segments of the path start being resolved from a place which differs based on edition. Each identifier in the path must resolve to an item.

Edition Differences: In the 2015 Edition, identifiers resolve from the "crate root" (crate:: in the 2018 edition), which contains a variety of different items, including external crates, default crates such as std or core, and items in the top level of the crate (including use imports).

Beginning with the 2018 Edition, paths starting with :: resolve from crates in the extern prelude. That is, they must be followed by the name of a crate.

#![allow(unused)]
fn main() {
pub fn foo() {
    // In the 2018 edition, this accesses `std` via the extern prelude.
    // In the 2015 edition, this accesses `std` via the crate root.
    let now = ::std::time::Instant::now();
    println!("{:?}", now);
}
}

// 2015 Edition
mod a {
    pub fn foo() {}
}
mod b {
    pub fn foo() {
        ::a::foo(); // call `a`'s foo function
        // In Rust 2018, `::a` would be interpreted as the crate `a`.
    }
}
fn main() {}

`self`

self resolves the path relative to the current module. self can only be used as the first segment, without a preceding ::.

fn foo() {}
fn bar() {
    self::foo();
}
fn main() {}

`Self`

Self, with a capital "S", is used to refer to the implementing type within traits and implementations.

Self can only be used as the first segment, without a preceding ::.

#![allow(unused)]
fn main() {
trait T {
    type Item;
    const C: i32;
    // `Self` will be whatever type that implements `T`.
    fn new() -> Self;
    // `Self::Item` will be the type alias in the implementation.
    fn f(&self) -> Self::Item;
}
struct S;
impl T for S {
    type Item = i32;
    const C: i32 = 9;
    fn new() -> Self {           // `Self` is the type `S`.
        S
    }
    fn f(&self) -> Self::Item {  // `Self::Item` is the type `i32`.
        Self::C                  // `Self::C` is the constant value `9`.
    }
}
}

`super`

super in a path resolves to the parent module. It may only be used in leading segments of the path, possibly after an initial self segment.

mod a {
    pub fn foo() {}
}
mod b {
    pub fn foo() {
        super::a::foo(); // call a's foo function
    }
}
fn main() {}

super may be repeated several times after the first super or self to refer to ancestor modules.

mod a {
    fn foo() {}

    mod b {
        mod c {
            fn foo() {
                super::super::foo(); // call a's foo function
                self::super::super::foo(); // call a's foo function
            }
        }
    }
}
fn main() {}

`crate`

crate resolves the path relative to the current crate. crate can only be used as the first segment, without a preceding ::.

fn foo() {}
mod a {
    fn bar() {
        crate::foo();
    }
}
fn main() {}

`$crate`

$crate is only used within macro transcribers, and can only be used as the first segment, without a preceding ::. $crate will expand to a path to access items from the top level of the crate where the macro is defined, regardless of which crate the macro is invoked.

pub fn increment(x: u32) -> u32 {
    x + 1
}

#[macro_export]
macro_rules! inc {
    ($x:expr) => ( $crate::increment($x) )
}
fn main() { }

Canonical paths

Items defined in a module or implementation have a canonical path that corresponds to where within its crate it is defined. All other paths to these items are aliases. The canonical path is defined as a path prefix appended by the path segment the item itself defines.

Implementations and use declarations do not have canonical paths, although the items that implementations define do have them. Items defined in block expressions do not have canonical paths. Items defined in a module that does not have a canonical path do not have a canonical path. Associated items defined in an implementation that refers to an item without a canonical path, e.g. as the implementing type, the trait being implemented, a type parameter or bound on a type parameter, do not have canonical paths.

The path prefix for modules is the canonical path to that module. For bare implementations, it is the canonical path of the item being implemented surrounded by angle (<>) brackets. For trait implementations, it is the canonical path of the item being implemented followed by as followed by the canonical path to the trait all surrounded in angle (<>) brackets.

The canonical path is only meaningful within a given crate. There is no global namespace across crates; an item's canonical path merely identifies it within the crate.

// Comments show the canonical path of the item.

mod a { // crate::a
    pub struct Struct; // crate::a::Struct

    pub trait Trait { // crate::a::Trait
        fn f(&self); // crate::a::Trait::f
    }

    impl Trait for Struct {
        fn f(&self) {} // <crate::a::Struct as crate::a::Trait>::f
    }

    impl Struct {
        fn g(&self) {} // <crate::a::Struct>::g
    }
}

mod without { // crate::without
    fn canonicals() { // crate::without::canonicals
        struct OtherStruct; // None

        trait OtherTrait { // None
            fn g(&self); // None
        }

        impl OtherTrait for OtherStruct {
            fn g(&self) {} // None
        }

        impl OtherTrait for crate::a::Struct {
            fn g(&self) {} // None
        }

        impl crate::a::Trait for OtherStruct {
            fn f(&self) {} // None
        }
    }
}

fn main() {}

The Rust Reference