Lang: Stabilize usage of rustc_nonnull_optimization_guaranteed on -1

[joshtriplett]

I/O Safety (#87074) uses types which are repr(transparent) wrappers around file descriptors or sockets, and which have a niche at -1 which is an invalid fd/socket. Those types use rustc_nonnull_optimization_guaranteed, which together with the niche, allows using things like Option<OwnedFd> in FFI.

(The attribute remains unstable, and is misnamed at this point since it shouldn't say "nonnull"; it should be something like rustc_niche_optimization_guaranteed, but that shouldn't block this issue.)

Before stabilizing those types, we'd like to confirm T-lang consensus that we have no objections to exposing a type with a guaranteed non-zero niche (in this case, a guaranteed niche at -1). This commits us to continuing to provide some mechanism (even if perma-unstable) for declaring non-zero niches and allowing their use in FFI.

[joshtriplett]

@rfcbot merge

[rfcbot]

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

• @cramertj
• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm

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. 🔔

[joshtriplett]

@sunfishcode Should OwnedSocket have the same optimization applied?

[sunfishcode]

@joshtriplett Yes, it should, and it does.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.