Macro Unused 

macro_rules! Unused {
    ($($type:ty $(: $variance:ident)?),+ $(,)?) => { ... };
}

A macro that allows for the creation of Unused type containers.

A basic example of usage can be found in the crate documentation.

Usage

use unused::Unused;

struct Foo<A, B, C, D, E> {
    unused: Unused!(
        A,
        B,
        C: covariant,
        D: contravariant,
        E: invariant,
    ),
}

Variances

You can have different type variances when working with Unused.

Invariant

Invariance is the default:

let unused: Unused!(u8) = Unused;
// is the same as:
let unused: Unused!(u8: invariant) = Unused;

Covariance and Contravariance

To make Foo covariant over A and contravariant over B:

struct Foo<A, B> {
    unused: Unused!(A: covariant, B: contravariant),
}

Lifetimes

Variance is particularily useful when it comes to lifetimes:

struct Foo<'foo> {
    unused: Unused!(&'foo (): covariant),   
}

fn change_foo_lifetime<'a>(foo: Foo<'static>) -> Foo<'a> {
    foo
}