mirror of https://github.com/rust-lang/nomicon
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
71 lines
2.3 KiB
71 lines
2.3 KiB
% Coercions
|
|
|
|
Types can implicitly be coerced to change in certain contexts. These changes are
|
|
generally just *weakening* of types, largely focused around pointers and
|
|
lifetimes. They mostly exist to make Rust "just work" in more cases, and are
|
|
largely harmless.
|
|
|
|
Here's all the kinds of coercion:
|
|
|
|
Coercion is allowed between the following types:
|
|
|
|
* Transitivity: `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to
|
|
`T_3`
|
|
* Pointer Weakening:
|
|
* `&mut T` to `&T`
|
|
* `*mut T` to `*const T`
|
|
* `&T` to `*const T`
|
|
* `&mut T` to `*mut T`
|
|
* Unsizing: `T` to `U` if `T` implements `CoerceUnsized<U>`
|
|
|
|
`CoerceUnsized<Pointer<U>> for Pointer<T> where T: Unsize<U>` is implemented
|
|
for all pointer types (including smart pointers like Box and Rc). Unsize is
|
|
only implemented automatically, and enables the following transformations:
|
|
|
|
* `[T; n]` => `[T]`
|
|
* `T` => `Trait` where `T: Trait`
|
|
* `Foo<..., T, ...>` => `Foo<..., U, ...>` where:
|
|
* `T: Unsize<U>`
|
|
* `Foo` is a struct
|
|
* Only the last field of `Foo` has type `T`
|
|
* `T` is not part of the type of any other fields
|
|
|
|
Coercions occur at a *coercion site*. Any location that is explicitly typed
|
|
will cause a coercion to its type. If inference is necessary, the coercion will
|
|
not be performed. Exhaustively, the coercion sites for an expression `e` to
|
|
type `U` are:
|
|
|
|
* let statements, statics, and consts: `let x: U = e`
|
|
* Arguments to functions: `takes_a_U(e)`
|
|
* Any expression that will be returned: `fn foo() -> U { e }`
|
|
* Struct literals: `Foo { some_u: e }`
|
|
* Array literals: `let x: [U; 10] = [e, ..]`
|
|
* Tuple literals: `let x: (U, ..) = (e, ..)`
|
|
* The last expression in a block: `let x: U = { ..; e }`
|
|
|
|
Note that we do not perform coercions when matching traits (except for
|
|
receivers, see below). If there is an impl for some type `U` and `T` coerces to
|
|
`U`, that does not constitute an implementation for `T`. For example, the
|
|
following will not type check, even though it is OK to coerce `t` to `&T` and
|
|
there is an impl for `&T`:
|
|
|
|
```rust,ignore
|
|
trait Trait {}
|
|
|
|
fn foo<X: Trait>(t: X) {}
|
|
|
|
impl<'a> Trait for &'a i32 {}
|
|
|
|
|
|
fn main() {
|
|
let t: &mut i32 = &mut 0;
|
|
foo(t);
|
|
}
|
|
```
|
|
|
|
```text
|
|
<anon>:10:5: 10:8 error: the trait `Trait` is not implemented for the type `&mut i32` [E0277]
|
|
<anon>:10 foo(t);
|
|
^~~
|
|
```
|