Add from_ref and from_mut constructors to core::ptr::NonNull.

[bjoernager]

Relevant tracking issue: #130823

The core::ptr::NonNull type should have the convenience constructors from_ref and from_mut for parity with core::ptr::from_ref and core::ptr::from_mut.

Although the type in question already implements From<&T> and From<&mut T>, these new functions also carry the ability to be used in constant expressions (due to not being behind a trait).

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[tgross35]

This will need an ACP, or at least libs-api signoff. An ACP is just an issue template at https://github.com/rust-lang/libs-team/.

This PR additionally adds the must_use attribute to the new_unchecked and new constructors.

Should probably be moved to its own commit to keep updates from that and updates from the new API separate (e.g. the MIR test needs to be blessed)

[bjoernager]

I've removed the must_use attributes and will PR them for independent review at another time.

[dtolnay]

Thank you!

[dtolnay]

@bors r+

[bors]

📌 Commit 94ab726 has been approved by dtolnay

It is now in the queue for this repository.