Rust already has the `Unpin` trait, which is identical to `Move` except that it doesn’t pin `&mut T`. I’ve never understood why Rust doesn’t require the `Unpin`

Rust already has the `Unpin` trait, which is identical to `Move` except that it doesn’t pin `&mut T`. I’ve never understood why Rust doesn’t require the `Unpin` trait in `mem::swap` and derivatives instead of having `Pin`. I also don’t really know of any other methods / cases besides those using `mem::swap` where `Pin` and `Unpin` are relevant. It works for `swap` because if you have a self-referential data structure, there’s never a good reason you want to tangle the references. Lastly, normally a mutable reference guarantees there aren’t other references, but a mutable reference to a self-referential data-structure doesn’t have this guarantee because it references itself. Is this why `Pin<&mut T>` is necessary, because a real mutable reference to a set-referential data-structure violates Rust’s not-fully-defined “borrowing rules”? And do we want to keep that specific rule (since we’re mutable borrowing the entire region including the self-referential borrow, and self-referential structures are already unsafe, I doubt it would cause any issues)? Maybe `&pin mut T` should be added, but as its own kind of reference instead of syntax sugar…