Type coercions

Type coercions are implicit operations that change the type of a value. They happen automatically at specific locations and are highly restricted in what types actually coerce.

Any conversions allowed by coercion can also be explicitly performed by the type cast operator, as.

Coercions are originally defined in RFC 401 and expanded upon in RFC 1558.

Coercion sites

A coercion can only occur at certain coercion sites in a program; these are typically places where the desired type is explicit or can be derived by propagation from explicit types (without type inference). Possible coercion sites are:

let statements where an explicit type is given.

For example, &mut 42 is coerced to have type &i8 in the following:

#![allow(unused)]
fn main() {
let _: &i8 = &mut 42;
}

static and const item declarations (similar to let statements).
Arguments for function calls

The value being coerced is the actual parameter, and it is coerced to the type of the formal parameter.

For example, &mut 42 is coerced to have type &i8 in the following:
```
fn bar(_: &i8) { }

fn main() {
    bar(&mut 42);
}
```
For method calls, the receiver (self parameter) type is coerced differently, see the documentation on method-call expressions for details.

Instantiations of struct, union, or enum variant fields

For example, &mut 42 is coerced to have type &i8 in the following:

struct Foo<'a> { x: &'a i8 }

fn main() {
    Foo { x: &mut 42 };
}

Function results—either the final line of a block if it is not semicolon-terminated or any expression in a return statement

For example, x is coerced to have type &dyn Display in the following:

#![allow(unused)]
fn main() {
use std::fmt::Display;
fn foo(x: &u32) -> &dyn Display {
    x
}
}

If the expression in one of these coercion sites is a coercion-propagating expression, then the relevant sub-expressions in that expression are also coercion sites. Propagation recurses from these new coercion sites. Propagating expressions and their relevant sub-expressions are:

Array literals, where the array has type [U; n]. Each sub-expression in the array literal is a coercion site for coercion to type U.
Array literals with repeating syntax, where the array has type [U; n]. The repeated sub-expression is a coercion site for coercion to type U.
Tuples, where a tuple is a coercion site to type (U_0, U_1, ..., U_n). Each sub-expression is a coercion site to the respective type, e.g. the zeroth sub-expression is a coercion site to type U_0.
Parenthesized sub-expressions ((e)): if the expression has type U, then the sub-expression is a coercion site to U.
Blocks: if a block has type U, then the last expression in the block (if it is not semicolon-terminated) is a coercion site to U. This includes blocks which are part of control flow statements, such as if/else, if the block has a known type.

Coercion types

Coercion is allowed between the following types:

T to U if T is a subtype of U (reflexive case)
T_1 to T_3 where T_1 coerces to T_2 and T_2 coerces to T_3 (transitive case)

Note that this is not fully supported yet.
&mut T to &T
*mut T to *const T
&T to *const T
&mut T to *mut T

&T or &mut T to &U if T implements Deref<Target = U>. For example:

use std::ops::Deref;

struct CharContainer {
    value: char,
}

impl Deref for CharContainer {
    type Target = char;

    fn deref<'a>(&'a self) -> &'a char {
        &self.value
    }
}

fn foo(arg: &char) {}

fn main() {
    let x = &mut CharContainer { value: 'y' };
    foo(x); //&mut CharContainer is coerced to &char.
}

&mut T to &mut U if T implements DerefMut<Target = U>.
TyCtor(T) to TyCtor(U), where TyCtor(T) is one of
- &T
- &mut T
- *const T
- *mut T
- Box<T>
and where U can be obtained from T by unsized coercion.
Function item types to fn pointers
Non capturing closures to fn pointers
! to any T

Unsized Coercions

The following coercions are called unsized coercions, since they relate to converting sized types to unsized types, and are permitted in a few cases where other coercions are not, as described above. They can still happen anywhere else a coercion can occur.

Two traits, Unsize and CoerceUnsized, are used to assist in this process and expose it for library use. The following coercions are built-ins and, if T can be coerced to U with one of them, then an implementation of Unsize for T will be provided:

[T; n] to [T].
T to dyn U, when T implements U + Sized, and U is object safe.
Foo<..., T, ...> to Foo<..., U, ...>, when:
- Foo is a struct.
- T implements Unsize.
- The last field of Foo has a type involving T.
- If that field has type Bar<T>, then Bar<T> implements Unsized<Bar>.
- T is not part of the type of any other fields.

Additionally, a type Foo<T> can implement CoerceUnsized<Foo> when T implements Unsize or CoerceUnsized<Foo>. This allows it to provide a unsized coercion to Foo.

Note: While the definition of the unsized coercions and their implementation has been stabilized, the traits themselves are not yet stable and therefore can't be used directly in stable Rust.

Least upper bound coercions

In some contexts, the compiler must coerce together multiple types to try and find the most general type. This is called a "Least Upper Bound" coercion. LUB coercion is used and only used in the following situations:

To find the common type for a series of if branches.
To find the common type for a series of match arms.
To find the common type for array elements.
To find the type for the return type of a closure with multiple return statements.
To check the type for the return type of a function with multiple return statements.

In each such case, there are a set of types T0..Tn to be mutually coerced to some target type T_t, which is unknown to start. Computing the LUB coercion is done iteratively. The target type T_t begins as the type T0. For each new type Ti, we consider whether

If Ti can be coerced to the current target type T_t, then no change is made.
Otherwise, check whether T_t can be coerced to Ti; if so, the T_t is changed to Ti. (This check is also conditioned on whether all of the source expressions considered thus far have implicit coercions.)
If not, try to compute a mutual supertype of T_t and Ti, which will become the new target type.

Examples:

#![allow(unused)]
fn main() {
let (a, b, c) = (0, 1, 2);
// For if branches
let bar = if true {
    a
} else if false {
    b
} else {
    c
};

// For match arms
let baw = match 42 {
    0 => a,
    1 => b,
    _ => c,
};

// For array elements
let bax = [a, b, c];

// For closure with multiple return statements
let clo = || {
    if true {
        a
    } else if false {
        b
    } else {
        c
    }
};
let baz = clo();

// For type checking of function with multiple return statements
fn foo() -> i32 {
    let (a, b, c) = (0, 1, 2);
    match 42 {
        0 => a,
        1 => b,
        _ => c,
    }
}
}

In these examples, types of the ba* are found by LUB coercion. And the compiler checks whether LUB coercion result of a, b, c is i32 in the processing of the function foo.

Caveat

This description is obviously informal. Making it more precise is expected to proceed as part of a general effort to specify the Rust type checker more precisely.

The Rust Reference