Construct a variance marker; equivalent to Default::default.

This type can be any of the following. You generally should not need to explicitly name the type, however.

• PhantomCovariant
• PhantomContravariant
• PhantomInvariant
• PhantomCovariantLifetime
• PhantomContravariantLifetime
• PhantomInvariantLifetime

Example

#![feature(phantom_variance_markers)]

use core::marker::{PhantomCovariant, variance};

struct BoundFn<F, P, R>
where
    F: Fn(P) -> R,
{
    function: F,
    parameter: P,
    return_value: PhantomCovariant<R>,
}

let bound_fn = BoundFn {
    function: core::convert::identity,
    parameter: 5u8,
    return_value: variance(),
};