Type aliases - The Rust Reference

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[items.type]

# Type aliases

[items.type.syntax]

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

[items.type.intro]

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:

    #![allow(unused)]
    fn main() {
    type Point = (u8, u8);
    let p: Point = (41, 68);
    }

[items.type.constructor-alias]

A type alias to a tuple-struct or unit-struct cannot be used to qualify that type’s constructor:

    #![allow(unused)]
    fn main() {
    struct MyStruct(u32);

    use MyStruct as UseAlias;
    type TypeAlias = MyStruct;

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

[items.type.associated-type]

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

[items.type.associated-trait]

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

[items.type.associated-impl]

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

[items.type.deprecated]

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.