ignore head usages from ignored candidates

[lcnr]

Fixes rust-lang/trait-system-refactor-initiative#210. The test now takes 0.8s to compile, which seems good enough to me. We are actually still walking the entire graph here, we're just avoiding unnecessary reruns.

The basic idea is that if we've only accessed a cycle head inside of a candidate which didn't impact the final result of our goal, we don't need to rerun that cycle head even if is the used provisional result differs from the final result.

We also use this information when rebasing goals over their cycle heads. If a goal doesn't actually depend on the result of that cycle head, rebasing always succeeds. However, we still need to make sure we track the fact that we relied on the cycle head at all to avoid query instability.

It is implemented by tracking the number of HeadUsages for every head while evaluating goals. We then also track the head usages while evaluating a single candidate, which the search graph returns as CandidateHeadUsages. If there is now an always applicable candidate candidate we know that all other candidates with that source did not matter. We then call fn ignore_candidate_head_usages to remove the usages while evaluating this single candidate from the total. If the final HeadUsages end up empty, we know that the result of this cycle head did not matter when evaluating its nested goals.

[rustbot]

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[lcnr]

r? @compiler-errors @BoxyUwU

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[lcnr]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit d49499e with merge 5c09e88…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5c09e88 (5c09e883643532880d70eb19836fb01847e0e5f5, parent: dc0bae1db725fbba8524f195f74f680995fd549e)

[rust-timer]

Finished benchmarking commit (5c09e88): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.7%	[0.2%, 1.1%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.5%, secondary 2.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.0%, 3.1%]	2
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[2.0%, 3.1%]	2

Cycles

Results (secondary 0.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.4%, 2.9%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.7%, -2.3%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 463.057s -> 466.661s (0.78%)
Artifact size: 377.38 MiB -> 377.62 MiB (0.06%)

[lcnr]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit 2d42758 with merge 3a46227…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 546cb5f (546cb5ff81bf3b99c56bc90236840949825cc3c0, parent: a1531335fe2807715fff569904d99602022643a7)

[rust-timer]

Finished benchmarking commit (546cb5f): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	2
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.3%]	3
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	2

Max RSS (memory usage)

Results (primary 1.3%, secondary 1.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	2
Regressions ❌ (secondary)	1.4%	[1.3%, 1.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.3%	[1.3%, 1.3%]	2

Cycles

Results (primary -2.4%, secondary -4.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.4%	[-2.7%, -2.0%]	3
Improvements ✅ (secondary)	-4.9%	[-4.9%, -4.9%]	1
All ❌✅ (primary)	-2.4%	[-2.7%, -2.0%]	3

Binary size

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

Bootstrap: 465.439s -> 465.953s (0.11%)
Artifact size: 377.34 MiB -> 377.51 MiB (0.05%)

[BoxyUwU]

r=me if CI passes

[lcnr]

@bors r=BoxyUwU

uwu

[bors]

📌 Commit 8afe306 has been approved by BoxyUwU

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8afe306 with merge c018ae5...

[bors]

☀️ Test successful - checks-actions
Approved by: BoxyUwU
Pushing c018ae5 to master...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing ba412a6 (parent) -> c018ae5 (this PR)

Test differences

Show 20 test diffs

Stage 1

• [ui] tests/ui/traits/next-solver/cycles/many-where-clauses-with-aliases-hang.rs: [missing] -> pass (J0)

Stage 2

• [ui] tests/ui/traits/next-solver/cycles/many-where-clauses-with-aliases-hang.rs: [missing] -> pass (J1)

Additionally, 18 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: x86_64-gnu-llvm-19-3, x86_64-gnu-llvm-20-3
• J1: aarch64-apple, aarch64-gnu, aarch64-gnu-llvm-19-1, aarch64-msvc-1, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-apple-2, x86_64-gnu, x86_64-gnu-llvm-19, x86_64-gnu-llvm-19-2, x86_64-gnu-llvm-20-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard c018ae5389c49cc4bcb8343d80dd8e7323325410 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. x86_64-apple-1: 6601.9s -> 10539.6s (59.6%)
2. x86_64-apple-2: 5668.3s -> 3988.7s (-29.6%)
3. aarch64-apple: 5284.3s -> 6232.4s (17.9%)
4. dist-powerpc64le-linux-gnu: 5219.4s -> 5755.5s (10.3%)
5. dist-x86_64-apple: 11475.4s -> 10343.9s (-9.9%)
6. dist-loongarch64-linux: 6182.5s -> 6767.6s (9.5%)
7. aarch64-gnu-llvm-19-1: 3643.9s -> 3958.1s (8.6%)
8. pr-check-1: 1673.6s -> 1794.1s (7.2%)
9. dist-loongarch64-musl: 5461.5s -> 5077.1s (-7.0%)
10. dist-aarch64-apple: 7866.2s -> 7324.3s (-6.9%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.