Implement CoerceUnsized for Unique<T>

[rphmeier]

It's implemented for raw pointers, references, boxes, Rc, Arc, NonZero, and Shared. Is there a specific reason it can't be implemented for Unique?

As it stands, you can't actually reimplement something like Box the same way the standard library does. Somehow, even though a Box is just a wrapper around Unique, it can be coerced to an unsized type due to some compiler magic.

The following is paraphrased from the Box source, but does not compile:

use std::marker::Unsize;
use std::ops::CoerceUnsized;
use std::ptr::Unique;

pub struct MyBox<T: ?Sized>(Unique<T>);

impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<MyBox<U>> for MyBox<T> {}

fn main() { }

[apasel422]

This should be easy to implement now. I think it was previously blocked on #1234.