[−][src]Type Definition phantasm::Covariant

type Covariant<T: ?Sized> = Covariant<T>;

Marker zst type that is covariant over T.

"Covariant" means that given F<_>, Super and Sub (where Sub is a subtype of Super), F<Sub> is a subtype of F<Super> (but F<Super> is not a subtype of F<Sub>)

Examples

// This struct is covariant over `T`
struct Test<T>(phantasm::Covariant<T> /* ... */);

let _: Test<i32> = Test(phantasm::Covariant);
let _ = Test(phantasm::Covariant::<i32>);
ⓘThis example deliberately fails to compile
use phantasm::Covariant;
// `F<Super>` is **not** a subtype of `F<Sub>`
fn fail<'l>(arg: Covariant<&'l ()>) -> Covariant<&'static ()> {
    arg
}
use phantasm::Covariant;
// `F<Sub>` **is** a subtype of `F<Super>`
fn pass<'l>(arg: Covariant<&'static ()>) -> Covariant<&'l ()> {
    arg
}

See also