blob: ff0cc75f7b64dd81cd47cfb8c27a0eff65c4c2f4 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
|
# Type aliases
> **<sup>Syntax</sup>**\
> _TypeAlias_ :\
> `type` [IDENTIFIER] [_GenericParams_]<sup>?</sup>
> ( `:` [_TypeParamBounds_] )<sup>?</sup>
> [_WhereClause_]<sup>?</sup> ( `=` [_Type_] )<sup>?</sup> `;`
A _type alias_ defines a new name for an existing [type]. Type aliases are
declared with the keyword `type`. Every value has a single, specific type, but
may implement several different traits, or be compatible with several different
type constraints.
For example, the following defines the type `Point` as a synonym for the type
`(u8, u8)`, the type of pairs of unsigned 8 bit integers:
```rust
type Point = (u8, u8);
let p: Point = (41, 68);
```
A type alias to a tuple-struct or unit-struct cannot be used to qualify that type's constructor:
```rust,compile_fail
struct MyStruct(u32);
use MyStruct as UseAlias;
type TypeAlias = MyStruct;
let _ = UseAlias(5); // OK
let _ = TypeAlias(5); // Doesn't work
```
A type alias without the [_Type_] specification may only appear as an
[associated type] in a [trait].
A type alias with [_TypeParamBounds_] may only specified when used as
an [associated type] in a [trait].
[IDENTIFIER]: ../identifiers.md
[_GenericParams_]: generics.md
[_TypeParamBounds_]: ../trait-bounds.md
[_WhereClause_]: generics.md#where-clauses
[_Type_]: ../types.md#type-expressions
[associated type]: associated-items.md#associated-types
[trait]: traits.md
[type]: ../types.md
|
