Optimize impl Hash for ObligationCauseData by not hashing ObligationCauseCode variant fields

[the8472]

Split out from #90913 since it's a clear performance win and should be easier to review.

It speeds up hashing for Obligationdeduplication by only hashing the discriminant of the ObligationCauseCode enum instead of its contents. That shouldn't affect hash quality much since there are several other fields in Obligation which should be unique enough, especially the predicate itself which is hashed as an interned pointer.

[matthewjasper]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 78b5f2d with merge 4d6fdfcc90813f646820764dbf3cc985d883d9af...

[bors]

☀️ Try build successful - checks-actions
Build commit: 4d6fdfcc90813f646820764dbf3cc985d883d9af (4d6fdfcc90813f646820764dbf3cc985d883d9af)

[rust-timer]

Queued 4d6fdfcc90813f646820764dbf3cc985d883d9af with parent 6414e0b, future comparison URL.

[the8472]

Huh, this shouldn't need another perf run, I already linked one for this commit in my opening comment.

Split out from #90913 since it's a clear performance win

[rust-timer]

Finished benchmarking commit (4d6fdfcc90813f646820764dbf3cc985d883d9af): comparison url.

Summary: This change led to very large relevant mixed results 🤷 in compiler performance.

• Very large improvement in instruction counts (up to -58.2% on full builds of diesel)
• Moderate regression in instruction counts (up to 1.0% on incr-unchanged builds of wg-grammar)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

[the8472]

The wg-grammar change probably is noise since it didn't show up in the previous perf run on the same commit changes. incr-unchanged runs are quite sensitive to the 1/32-randomized incremental cache verification.

[matthewjasper]

@bors r+

[bors]

📌 Commit 78b5f2d has been approved by matthewjasper

[camelid]

Should this be marked as relnotes-perf? Documenting diesel was sped up by 58%! (cc @rust-lang/rustdoc)

[GuillaumeGomez]

Sounds like it is definitely worth mentioning it indeed!

[bors]

⌛ Testing commit 78b5f2d with merge 18fa434...

[bors]

☀️ Test successful - checks-actions
Approved by: matthewjasper
Pushing 18fa434 to master...

[rust-timer]

Finished benchmarking commit (18fa434): comparison url.

Summary: This change led to very large relevant improvements 🎉 in compiler performance.

• Very large improvement in instruction counts (up to -58.1% on full builds of diesel)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

[nnethercote]

For next time: I suggest leaving a brief comment explaining why this custom hash is used, because it's non-obvious and no fun to have to dig through version control history to understand why.

[GuillaumeGomez]

Please open an issue so it's not forgotten.

[the8472]

I'm not opposed to adding comments but

no fun to have to dig through version control history

seems a bit subjective, for me that's 3 clicks which is hardly onerous. So that didn't quite meet my threshold of adding a comment.