[Tamschi]

Something else that I didn't mention in the post is that `Pin<Box<T>>` is `From<Box<T>>` (so it can be cast directly, and here without reallocation or such), and this could also be implemented for (other) exclusively-owning containers with heap storage.

The standard library collections don't do this because they don't have an API that would make it useful. It suspect it's partly because you can't `self: &mut Pin<Self>` in stable, which would be needed to cleanly give them one, but there's also a fair argument for not making them kitchen-sink types while a wrapper could provide this easily.