Canonicalizer cleanups #150748

nnethercote · 2026-01-07T06:25:54Z

Some cleanups in and around the canonicalizers, found while I was looking closely at this code.

rustbot · 2026-01-07T06:25:56Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

lcnr · 2026-01-07T11:54:48Z

@bors try @rust-timer queue

r=me after perf, I assume that u measured it to not have an impact, but want to be sure

Canonicalizer cleanups

rust-bors · 2026-01-07T14:19:30Z

☀️ Try build successful (CI)
Build commit: 23ed3e9 (23ed3e95ddb50d9de56e5fba78af875e824ef8d7, parent: d9617c8d9a55773a96b61ba3a4acb107d65615c1)

rust-timer · 2026-01-07T15:00:33Z

Finished benchmarking commit (23ed3e9): 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)	0.2%	[0.2%, 0.3%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.5%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -0.7%)

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)	-	-	0
Improvements ✅ (secondary)	-0.7%	[-0.7%, -0.7%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.7%, secondary -0.2%)

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

	mean	range	count
Regressions ❌ (primary)	2.7%	[2.3%, 3.3%]	3
Regressions ❌ (secondary)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	2.7%	[2.3%, 3.3%]	3

Binary size

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

Bootstrap: 474.542s -> 471.79s (-0.58%)
Artifact size: 390.84 MiB -> 390.85 MiB (0.00%)

It's an empty `Vec` at both call sites, and so is unnecessary.

Variables that are collections of `CanonicalVarKind` are sometimes called `var_kinds` and sometimes called `variables`. The former is much better, because `variables` is (a) non-descript, and (b) often used nearby for collections of `I::GenericArg`. I found the inconsistency made the canonicalization code harder to understand. This commit renames various `variables` things as `var_kinds`.

Currently it's a mutable reference, but it doesn't need to be, because what's passed in is always a mutable reference to an empty `Vec`. This requires returning variables in a few extra places, which is fine. It makes the handling of `variables` the same as the handling of `var_kinds` and `variable_lookup_table`.

It's weird. `clone` is better.

I did some measurements. The current choice of 16 is fine.

This explains why the predicate folding code looks different to the ty/const folding code, something I was wondering.

nnethercote · 2026-01-08T04:04:55Z

The slight regressions on the new-solver benchmarks are due to the "Make Canonicalizer::variables owned" commit. Let's try a possible fix:

@bors try @rust-timer queue

Canonicalizer cleanups

rust-bors · 2026-01-08T06:29:15Z

☀️ Try build successful (CI)
Build commit: 0a07a16 (0a07a161046bb0312b7c8ec7d9794b1fc0628e02, parent: 548e586795f6b6fe089d8329aa5edbf0f5202646)

rust-timer · 2026-01-08T07:10:16Z

Finished benchmarking commit (0a07a16): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.6%	[-0.6%, -0.5%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.1%)

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)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.1%	[-2.1%, -2.1%]	1

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)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.9%	[-4.9%, -4.9%]	1
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

Binary size

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

Bootstrap: 474.664s -> 473.643s (-0.22%)
Artifact size: 390.87 MiB -> 390.87 MiB (-0.00%)

nnethercote · 2026-01-08T07:27:06Z

The final commit fixes the problem. I don't understand why, even though I spent a chunk of today investigating it. I can't work out why cloning a Vec would cause more allocations than extending an empty Vec.

lcnr · 2026-01-08T11:10:48Z

@bors r+ rollup=never

rust-bors · 2026-01-08T11:10:53Z

📌 Commit b222cf3 has been approved by lcnr

It is now in the queue for this repository.

rust-bors · 2026-01-12T02:14:07Z

☀️ Test successful - CI
Approved by: lcnr
Pushing 44a5b55 to main...

github-actions · 2026-01-12T02:17:34Z

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 b68e16c (parent) -> 44a5b55 (this PR)

Test differences

Show 2 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 44a5b55557c26353f388400d7da95527256fe260 --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

dist-aarch64-llvm-mingw: 5365.4s -> 6419.2s (+19.6%)
pr-check-1: 1676.0s -> 1943.5s (+16.0%)
test-various: 6458.2s -> 7459.5s (+15.5%)
aarch64-gnu-debug: 3777.5s -> 4359.1s (+15.4%)
i686-gnu-2: 5068.8s -> 5838.8s (+15.2%)
aarch64-apple: 8437.9s -> 9661.2s (+14.5%)
dist-apple-various: 4167.0s -> 3619.8s (-13.1%)
aarch64-gnu-llvm-20-2: 2709.3s -> 3041.7s (+12.3%)
x86_64-gnu-llvm-20-1: 4128.7s -> 4623.7s (+12.0%)
x86_64-gnu-gcc: 3124.2s -> 3490.7s (+11.7%)

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.

rust-timer · 2026-01-12T03:03:32Z

Finished benchmarking commit (44a5b55): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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%	[0.7%, 0.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (secondary -3.5%)

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)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-4.3%, -2.7%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 473.667s -> 473.154s (-0.11%)
Artifact size: 391.39 MiB -> 391.41 MiB (0.01%)

rustbot assigned lcnr Jan 7, 2026

nnethercote force-pushed the canonicalizer-cleanups branch from 2c8e55e to a24aef0 Compare January 7, 2026 06:27

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nnethercote force-pushed the canonicalizer-cleanups branch from a24aef0 to d0278f9 Compare January 7, 2026 07:19

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rust-bors bot added a commit that referenced this pull request Jan 7, 2026

Auto merge of #150748 - nnethercote:canonicalizer-cleanups, r=<try>

23ed3e9

Canonicalizer cleanups

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 7, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jan 7, 2026

nnethercote added 9 commits January 8, 2026 13:37

Remove some unnecessary braces in patterns.

a4da8e8

Factor out duplicated code in Canonicalizer::finalize.

5ca112f

Remove variables arg from Canonicalizer::canonicalize_response.

0ad7701

It's an empty `Vec` at both call sites, and so is unnecessary.

Avoid using to_vec to clone Vecs.

e4bbfe8

It's weird. `clone` is better.

Update a comment.

eb16456

I did some measurements. The current choice of 16 is fine.

Add comments about predicate folding.

a07f717

This explains why the predicate folding code looks different to the ty/const folding code, something I was wondering.

Tweak variable cloning.

b222cf3

nnethercote force-pushed the canonicalizer-cleanups branch from d0278f9 to b222cf3 Compare January 8, 2026 04:04

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rust-bors bot added a commit that referenced this pull request Jan 8, 2026

Auto merge of #150748 - nnethercote:canonicalizer-cleanups, r=<try>

0a07a16

Canonicalizer cleanups

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 8, 2026

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rustbot removed perf-regression Performance regression. S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jan 8, 2026

rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 8, 2026

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rust-bors bot added the merged-by-bors This PR was explicitly merged by bors. label Jan 12, 2026

rust-bors bot merged commit 44a5b55 into rust-lang:main Jan 12, 2026
13 checks passed

rustbot added this to the 1.94.0 milestone Jan 12, 2026

nnethercote deleted the canonicalizer-cleanups branch January 13, 2026 00:09

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Canonicalizer cleanups #150748

Canonicalizer cleanups #150748

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rust-timer commented Jan 7, 2026

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

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rust-timer commented Jan 8, 2026

Overall result: ✅ improvements - no action needed

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github-actions bot commented Jan 12, 2026

Test differences

Job duration changes

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rust-timer commented Jan 12, 2026

Overall result: ❌ regressions - no action needed

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