Stabilize unchecked_{add,sub,mul}

[scottmcm]

Tracking issue: #85122

I think we might as well just stabilize these basic three. They're the ones that have nuw/nsw flags in LLVM.

Notably, this doesn't include the potentially-more-complex or -more-situational things like unchecked_neg or unchecked_shr that are under different feature flags.

To quote Ralf #85122 (comment),

Are there any objections to stabilizing at least unchecked_{add,sub,mul}? For those there shouldn't be any surprises about what their safety requirements are.

Semantially these are already available on stable, even in const, via checked_*+unreachable_unchecked. So IMHO we might as well just let people write them directly, rather than try to go through a let Some(x) = x.checked_add(y) else { unsafe { hint::unreachable_unchecked() }}; dance.

I added additional text to each method to attempt to better describe the behaviour and encourage wrapping_* instead.

r? rust-lang/libs-api

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[scottmcm]

Finally managed to get through CI 😅

@rustbot ready

[jhpratt]

It doesn't look like there's been an FCP for this yet. r=me once that passed, though.

@rustbot claim

[Amanieu]

@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
• @dtolnay
• @joshtriplett
• @m-ou-se

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. 🔔

[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.

[jhpratt]

@bors r+

[bors]

📌 Commit 50392cc has been approved by jhpratt

It is now in the queue for this repository.

[bors]

⌛ Testing commit 50392cc with merge faae5f1...

[bors]

☀️ Test successful - checks-actions
Approved by: jhpratt
Pushing faae5f1 to master...

[rust-timer]

Finished benchmarking commit (faae5f1): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	5.3%	[5.3%, 5.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	5.3%	[5.3%, 5.3%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 667.985s -> 668.88s (0.13%)
Artifact size: 315.84 MiB -> 315.90 MiB (0.02%)