`&'a mut T` is covariant in `'a` but invariant in `T`. Why must the `T` position be invariant, with a concrete unsoundness it prevents?

fn cheat<'short, 'long: 'short>(r: &mut &'long str) { let local = String::from("oops"); // if &mut T were covariant in T, we could assign // a &'short str into *r here, then read it after `local` drops }