例如

use type_variance::{Covariant, Contravariant};

// UnaryFunction is a zero-sized type that is covariant to `Arg` and
// contravariant to `Ret`.
struct UnaryFunction<Arg, Ret> {
    arg: Covariant<Arg>,
    ret: Contravariant<Ret>,
}

fn foo<'sup>() {
    // Here, the type &'static() is a subtype of &'sup().
    // Therefore, `Arg` may be replaced with a subtype and `Ret` may be
    // replaced with a supertype.
    let _func: UnaryFunction<&'sup(), &'static()> = UnaryFunction {
        arg: Covariant::<&'static()>::default(),
        ret: Contravariant::<&'sup()>::default(),
    };
}

许可证

该软件包采用 MIT 许可证。