Type Definition phantasm::Covariant

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

Marker zero-sized 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

use phantasm::Covariant;

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

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

// `F<Sub>` **is** a subtype of `F<Super>`
fn covariant_pass<'l>(with_sub: Covariant<Lt<'static>>) {
    let with_super: Covariant<Lt<'l>> = with_sub;
}
ⓘ
use phantasm::{Covariant, Lt};

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

See also