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type-aliases.md

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Type aliases

Syntax
TypeAlias :
   type IDENTIFIER GenericParams? ( : TypeParamBounds )? WhereClause? ( = Type WhereClause?)? ;

A type alias defines a new name for an existing type in the type namespace of the module or block where it is located. Type aliases are declared with the keyword type. Every value has a single, specific type, but may implement several different traits, and may 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:

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:

struct MyStruct(u32);

use MyStruct as UseAlias;
type TypeAlias = MyStruct;

let _ = UseAlias(5); // OK
let _ = TypeAlias(5); // Doesn't work

A type alias, when not used as an associated type, must include a Type and may not include TypeParamBounds.

A type alias, when used as an associated type in a trait, must not include a Type specification but may include TypeParamBounds.

A type alias, when used as an associated type in a trait impl, must include a Type specification and may not include TypeParamBounds.

Where clauses before the equals sign on a type alias in a trait impl (like type TypeAlias<T> where T: Foo = Bar<T>) are deprecated. Where clauses after the equals sign (like type TypeAlias<T> = Bar<T> where T: Foo) are preferred.