Use BinOp::Cmp for iNN::signum

[scottmcm]

This way it can use the nice new LLVM intrinsic in LLVM20.

[rustbot]

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[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @rust-lang/wg-const-eval

[scottmcm]

@RalfJung, can you remind me which teams need to sign off on using this in const?

[RalfJung]

Usually lang.

[RalfJung]

In the interpreter, this operator is implemented for integers and characters. That matches what the validator does. Seems worth mentioning in the doc comment that char is supported.

Other than that, the implementation looks good and doesn't do anything odd, so seems fine to stabilize. Cc @rust-lang/wg-const-eval

[rvolgers]

(drive by suggestion, not a reviewer)

[scottmcm]

Hello lang! I propose that we stabilize the indirect use of the three_way_compare intrinsic in const. This is the one already used in Ord::cmp for iN, uN, and char, but this would be the first stable use of it in const fn, since the trait impls are not yet const themselves.

This doesn't expose anything fundamentally new, since the comparison operators are const-stable on these types and thus open-coding this was already possible. Ralf says that it "seems fine" (#137835 (comment)).

(The PR uses it in the implementation of the stable iNN::signum method, which would be the first accessible use from stable. The intrinsic itself will, of course, remain unstable.)

@rfcbot fcp merge

[rfcbot]

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

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rfcbot reviewed

Looks good to me.

[tmandry]

@rfcbot reviewed

Seems like there's no interesting surface area being stabilized.

[rfcbot]

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