commit 8281302563aa1fc2d6cc0e3bd176817480676ad4
author bors <bors@rust-lang.org> Sat Jul 19 08:03:40 2025 +0000
committer bors <bors@rust-lang.org> Sat Jul 19 08:03:40 2025 +0000
tree a2368b5a26f287d6d18a91eeb90fef20129bfeba
parent 4f674e6df768b18faa27422f97d4eefed806d673
parent f934bcf4f6654887245f334dc2172a2238be3117

Auto merge of #143784 - scottmcm:enums-again-new-ex2, r=dianqk

Simplify discriminant codegen for niche-encoded variants which don't wrap across an integer boundary

Inspired by rust-lang/rust#139729, this attempts to be a much-simpler and more-localized change while still making a difference.  (Specifically, this does not try to solve the problem with select-sinking, leaving that to be fixed by https://github.com/llvm/llvm-project/issues/134024 -- once it gets released -- instead of in rustc's codegen.)

What this *does* improve is checking for the variant in a 3+ variant enum when that variant is the type providing the niche.  Something like `if let Foo::WithBool(_) = ...` previously compiled to `ugt(add(x, -2), 2)`, which is non-trivial to think about because it's depending on the unsigned wrapping to shift the 0/1 up above 2.  With this PR it compiles to just `ult(x, 2)`, which is probably what you'd have written yourself if you were doing it by hand to look for "is this byte a bool?".

That's done by leaving most of the codegen alone, but adding a couple new special cases to the `is_niche` check.  The default looks at the relative discriminant, but in the common cases where there's no wraparound involved, we can just check the original value, rather than the offsetted one.

The first commit just adds some tests, so the best way to see the effect of this change is to look at the second commit and how it updates the test expectations.