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1 % Coercions
2
3 Types can implicitly be coerced to change in certain contexts. These changes are
4 generally just *weakening* of types, largely focused around pointers and
5 lifetimes. They mostly exist to make Rust "just work" in more cases, and are
6 largely harmless.
7
8 Here's all the kinds of coercion:
9
10 Coercion is allowed between the following types:
11
12 * Transitivity: `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to
13 `T_3`
14 * Pointer Weakening:
15 * `&mut T` to `&T`
16 * `*mut T` to `*const T`
17 * `&T` to `*const T`
18 * `&mut T` to `*mut T`
19 * Unsizing: `T` to `U` if `T` implements `CoerceUnsized<U>`
20
21 `CoerceUnsized<Pointer<U>> for Pointer<T> where T: Unsize<U>` is implemented
22 for all pointer types (including smart pointers like Box and Rc). Unsize is
23 only implemented automatically, and enables the following transformations:
24
25 * `[T; n]` => `[T]`
26 * `T` => `Trait` where `T: Trait`
27 * `Foo<..., T, ...>` => `Foo<..., U, ...>` where:
28 * `T: Unsize<U>`
29 * `Foo` is a struct
30 * Only the last field of `Foo` has type `T`
31 * `T` is not part of the type of any other fields
32
33 Coercions occur at a *coercion site*. Any location that is explicitly typed
34 will cause a coercion to its type. If inference is necessary, the coercion will
35 not be performed. Exhaustively, the coercion sites for an expression `e` to
36 type `U` are:
37
38 * let statements, statics, and consts: `let x: U = e`
39 * Arguments to functions: `takes_a_U(e)`
40 * Any expression that will be returned: `fn foo() -> U { e }`
41 * Struct literals: `Foo { some_u: e }`
42 * Array literals: `let x: [U; 10] = [e, ..]`
43 * Tuple literals: `let x: (U, ..) = (e, ..)`
44 * The last expression in a block: `let x: U = { ..; e }`
45
46 Note that we do not perform coercions when matching traits (except for
47 receivers, see below). If there is an impl for some type `U` and `T` coerces to
48 `U`, that does not constitute an implementation for `T`. For example, the
49 following will not type check, even though it is OK to coerce `t` to `&T` and
50 there is an impl for `&T`:
51
52 ```rust,ignore
53 trait Trait {}
54
55 fn foo<X: Trait>(t: X) {}
56
57 impl<'a> Trait for &'a i32 {}
58
59
60 fn main() {
61 let t: &mut i32 = &mut 0;
62 foo(t);
63 }
64 ```
65
66 ```text
67 <anon>:10:5: 10:8 error: the trait bound `&mut i32 : Trait` is not satisfied [E0277]
68 <anon>:10 foo(t);
69 ^~~
70 ```