Type Definition phantasm::Contravariant

source · [−]
pub type Contravariant<T: ?Sized> = Contravariant<T>;
Expand description

Marker zero-sized type that is contravariant over T.

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

Examples

use phantasm::Contravariant;

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

let _: Test<i32> = Test(Contravariant /* ... */);
let _ = Test::<i32>(Contravariant /* ... */);
let _ = Test(Contravariant::<i32> /* ... */);
ⓘ
use phantasm::{Contravariant, Lt};

// `F<Sub>` is **not** a subtype of `F<Super>`
fn covariant_fail<'l>(with_sub: Contravariant<Lt<'static>>) {
    let with_super: Contravariant<Lt<'l>> = with_sub; // mismatched types
}
use phantasm::{Contravariant, Lt};

// `F<Super>` **is** a subtype of `F<Sub>`
fn contravariant_pass<'l>(with_super: Contravariant<Lt<'l>>) {
    let with_sub: Contravariant<Lt<'static>> = with_super;
}

See also