perf: UninhabitedEnumBranching avoid n^2

[simonvandel]

Avoid n² complexity. This showed up in a profile for match-stress-enum that has 8192 variants

I have only profiled locally against match-stress-enum, so we should have it perf tested to make sure it does not regress other crates.

[rust-highfive]

r? @estebank

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[estebank]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit de89fb67fb8292b1f27f59ea7d585a3e95c9d520 with merge c7115a3597a7b1fbced7d79b8a23e22359edc73d...

[jonas-schievink]

Suggested change

	FxHashSet::from_iter(variant_discriminants(&layout, discriminant_ty, tcx))
	variant_discriminants(&layout, discriminant_ty, tcx).collect()

..then you can remove the FromIterator import

[Marwes]

variant_discriminants could be changed to return a FxHashSet directly which would avoid some overhead.

[simonvandel]

Addressed in latest commit

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: c7115a3597a7b1fbced7d79b8a23e22359edc73d (c7115a3597a7b1fbced7d79b8a23e22359edc73d)

[rust-timer]

Queued c7115a3597a7b1fbced7d79b8a23e22359edc73d with parent a1dfd24, future comparison URL.

[rust-timer]

Finished benchmarking try commit (c7115a3597a7b1fbced7d79b8a23e22359edc73d): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[simonvandel]

I interpret the perf results as being good.

This PR should not change behavior, so in a perfect deterministic world I would only expect optimized_mir to be affected by this change. I therefore see all regressions reported in LLVM as noise.

For match-stress-enum this is a clear win. All other benchmarks look neutral.

[jonas-schievink]

r=me with #77597 (comment) addressed

[jonas-schievink]

@bors r+

[bors]

📌 Commit e231c47 has been approved by jonas-schievink

[bors]

⌛ Testing commit e231c47 with merge e055f87...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: jonas-schievink
Pushing e055f87 to master...

[Mark-Simulacrum]

As expected, a minor improvement on the match stress test (https://perf.rust-lang.org/compare.html?start=91a79fb29ac78d057d04dbe86be13d5dcc64309a&end=e055f87cdfcac1f4da6c518a547dee459de0aa26&stat=instructions:u). Slight (0.1%) wins on other benchmarks. Great work!