Make NonNull::as_ref (and friends) return refs with unbound lifetimes

[thomcc]

Rationale:

1. The documentation for all of these functions claims that this is what the functions already do, as they all come with this comment:

   You must enforce Rust's aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data...

   So I think it's just a bug that they weren't this way already. Note that had it not been for this part, I wouldn't be making this PR, so if we decide we won't take this change, I'll follow it up with a docs PR to fix this.

2. This is how the equivalent raw pointer functions behave.

   They also take self and not &self/&mut self, but that can't be changed compatibly at this point. This is the next best thing.

3. Without this fix, often code that uses these methods will find it has to expand the lifetime of the result.

   (I can't speak for others but even in unsafe-heavy code, needing to do this unexpectedly is a huge red flag -- if Rust thinks something should have a specific lifetime, I assume it's for a reason)

Can this cause existing code to be unsound?

I'm confident this can't cause new unsoundness since the reference exists for at most its lifetime, but you get a borrow checker error if you do something that would require/allow the reference to exist past its lifetime.

Additionally, the aliasing rules of a reference only applies while the reference exists.

This must be the case, as it is required by the rules used by safe code. (That said, the documentation in this file sort of contradicts it, but I think it's just ambiguity between the lifetime 'a in &'a T and lifetime of the &'a T reference itself...)

We are increasing the lifetime of these references, but they should already have hard bounds on that lifetime, or they'd have borrow checker errors.

(CC @RalfJung because I have gone and done the mistake where I say something definitive about aliasing in Rust which is honestly outside the group of things I should make definitive comments about).

Caveats

1. This is insta-stable (except for on the unstable functions ofc). I don't think there's any other alternative.

2. I don't believe this is a breaking change in practice. In theory someone could be assigning NonNull::as_ref to a function pointer of type fn(&NonNull<T>) -> &T. Now they'd need to use a slightly different function pointer type which is (probably) incompatible. This seems pathological, but I guess crater could be used if there are concerns.

3. This has no tests. The old version didn't either that I saw. I could add some stuff that fails to compile without it, if that would be useful.

4. Sometimes the NLL borrow checker gives up and decides lifetimes live till the end of the scope, as opposed to the range where they're used. If this change can cause this to happen more, then my soundness rationale is wrong, and it's likely breaking.

   In practice this seems super unlikely.

Anyway. That was a lot of typing.

Fixes #80183

[rust-highfive]

r? @withoutboats

(rust-highfive has picked a reviewer for you, use r? to override)

[RalfJung]

I agree with the soundness analysis; I cannot imagine any way in which this makes existing code unsound.

Regarding the change itself, I seem to recall one case where I was hit by this as well. My conclusion back then was that this was meant as a kind of safety net (note how getting a mutable reference also requires &mut NonNull). I am not sure if that safety net is really helping anyone, though.

[JohnTitor]

r? @Amanieu

[Amanieu]

as_ref on raw pointers already returns an unbounded lifetime, so I think this change is fine.

@rfcbot fcp merge

[rfcbot]

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @KodrAus
• @dtolnay
• @joshtriplett
• @m-ou-se
• @sfackler

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[dtolnay]

@bors r+

[bors]

📌 Commit cce258b has been approved by dtolnay