Add best-effort `owns` method?

[lachlansneff]

AllocRef should have an owns method for determining if a particular allocation came from specific allocation. Obviously, many allocators couldn't know for sure, so it would have to be best effort.

fn owns(&self, ptr: NonNull<u8>, layout: Layout) -> bool;

or

pub enum Owns {
    Yes,
    Maybe,
}

fn owns(&self, ptr: NonNull<u8>, layout: Layout) -> Owns;

This could be very useful for implementing combinatorial allocators, like in https://www.youtube.com/watch?v=LIb3L4vKZ7U.

[Amanieu]

This was previously discussed here: rust-lang/rust#44302

The conclusion was that it isn't implementable (or at least not efficiently) for the majority of allocator designs. This is best left to a separate trait for allocators that specifically support composability.

[lachlansneff]

Ah, that makes sense.