add cache to AmbiguityCausesVisitor

[lcnr]

fixes #135457, alternative to #135524.

cc https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/new-solver.20hang.20.23135457

r? @compiler-errors

[lcnr]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 94bf8f0 with merge 1a372d6...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1a372d6 (1a372d66de385a25aaacfb711a4d1d32af5acd55)

[rust-timer]

Finished benchmarking commit (1a372d6): comparison URL.

Overall result: no relevant changes - no action needed

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 may lead to changes in compiler perf.

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

Instruction count

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

Max RSS (memory usage)

Results (primary 2.5%, secondary 2.5%)

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)	2.5%	[1.6%, 3.4%]	2
Regressions ❌ (secondary)	4.9%	[1.7%, 8.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	2.5%	[1.6%, 3.4%]	2

Cycles

Results (primary -1.6%, secondary 2.8%)

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)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-1.6%	[-2.3%, -1.2%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.6%	[-2.3%, -1.2%]	3

Binary size

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

Bootstrap: 764.62s -> 763.451s (-0.15%)
Artifact size: 326.02 MiB -> 326.07 MiB (0.02%)

[compiler-errors]

@bors r+

[bors]

📌 Commit 0910173 has been approved by compiler-errors

It is now in the queue for this repository.

[Kobzol]

Should we perhaps add the test as a secondary benchmark in rustc-perf instead? If something regresses from seconds to minutes, it will hardly be visible in CI. Last year, the whole CI took like 2 hours longer because of some similar blow-up and we only found out after a couple of days, IIRC.

CC @lqd if you have opinions on the topic :)

[lqd]

"Instead", or in addition to this one?

• "Instead of in this PR", I guess not really: since we also need an in-tree test for the Compilation taking unexpected long after 1.84 #135457 issue itself.
• "In addition to", why not. It should be easier to see it regress on rustc-perf, yeah. As to whether it's needed, I'm not really sure, did we add secondary benchmarks for the other new solver blowups?

[Kobzol]

Yeah sorry, I meant "in addition to". I think that last time we added typenum as a benchmark for the new solver, but that's not exactly a stress test :)

[bors]

⌛ Testing commit 0910173 with merge 8e59cf9...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 8e59cf9 to master...

[rust-timer]

Finished benchmarking commit (8e59cf9): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.5%)

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)	2.5%	[1.5%, 3.4%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[1.5%, 3.4%]	2

Cycles

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

Binary size

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

Bootstrap: 763.854s -> 765.445s (0.21%)
Artifact size: 326.09 MiB -> 326.00 MiB (-0.03%)

[apiraino]

Beta backport accepted as per compiler team on Zulip. A backport PR will be authored by the release team at the end of the current development cycle. Backport labels handled by them.

Stable backport approved as well. Since a dot release seems likely, this is a good candidate for inclusion.

@rustbot label +beta-accepted +stable-accepted