Add `Box::as_non_null`

[theemathas]

Proposal

Problem statement

This is the intersection between rust-lang/rust#129090 (ACP: #355) and rust-lang/rust#130364 (ACP: #418).

Motivating examples or use cases

A user might want to create a pointer to the contents of a Box, like in #355, and then store it as an Option<NonNull<T>>, to get niche optimizations in the case of a None.

Solution sketch

impl<T: ?Sized, A: Allocator> Box<T, A> {
    pub fn as_non_null(b: &mut Self) -> NonNull<T>;
}

Alternatives

The function could take a &Self instead. I am using &mut Self here to be consistent with Vec::as_non_null. (See #440 and #t-opsem > Should `slice/Vec::as_non_null`take `&self` or `&mut self`?)

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

[nia-e]

Per today's libs-api meeting, we are in agreement with the ACP. Feel free to open a tracking issue and implementation PR. Consistency with Vec is probably a good reason to keep the method as &mut Self. Thanks!