Keyword type

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Define an alias for an existing type.

The syntax is type Name = ExistingType;.


type does not create a new type:

type Meters = u32;
type Kilograms = u32;

let m: Meters = 3;
let k: Kilograms = 3;

assert_eq!(m, k);

A type can be generic:

type ArcMutex<T> = Arc<Mutex<T>>;

In traits, type is used to declare an associated type:

trait Iterator {
    // associated type declaration
    type Item;
    fn next(&mut self) -> Option<Self::Item>;

struct Once<T>(Option<T>);

impl<T> Iterator for Once<T> {
    // associated type definition
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {