Optimize handling of solver errors by ChayimFriedman2 · Pull Request #160160 · rust-lang/rust

ChayimFriedman2 · 2026-07-29T15:23:36Z

Check for the empty case in collect_remaining_errors().
Instead of representing trait errors as Vec<Error>, use a special type

This has multiple advantages:
- Performance. The new type is 1/3 the size of Vec (being equivalent in layout to Option<ThinVec>) and can be kept in a register.
- Type safety. We mark the type #[must_use], and thinks requiring errors take ThinVec, which requires unwrapping the type and verifying there is indeed an error. We still provide conversions to slices, ThinVec, and iteration, because some code needs this and I saw no benefit in changing it, but we deliberately do not provide Deref<Target = [E]> or things like that.

ChayimFriedman2 · 2026-07-29T15:23:41Z

Check for the empty case in `collect_remaining_errors()`

panstromek · 2026-07-29T15:39:02Z


    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
-        #[allow(clippy::iter_skip_zero)]
+        if self.obligations.pending.is_empty() && self.obligations.overflowed.is_empty() {
+            // In 99.983% this is true, so this helps perf.
+            return Vec::new();
+        }
+
        self.obligations


One other thing I'd try later too is that sometimes it helps is to put #[inline] on the function and wrap the rest of the code after the fast path in outline(move || { ... })).

View changes since the review

True, I also wanted to check if making this ThinVec will help since it will return in a register. It may cancel with outlining though.

ChayimFriedman2 · 2026-07-29T15:48:00Z

@bors cancel

rust-bors · 2026-07-29T15:48:03Z

❗ There is currently no auto build in progress on this PR.

Hint: There is a pending try build on this PR. Maybe you meant to cancel it? You can do that using @bors try cancel.

ChayimFriedman2 · 2026-07-29T15:48:14Z

@bors try cancel

rust-bors · 2026-07-29T15:48:18Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/30465642688

ChayimFriedman2 · 2026-07-29T15:53:20Z

@bors try @rust-timer queue

Check for the empty case in `collect_remaining_errors()`

the8472 · 2026-07-29T15:58:01Z

I think it's worth checking this in std's TrustedLen specializations. The code is querying the size-hint anyway, it might as well bail early rather than invoking the next machinery.

ChayimFriedman2 · 2026-07-29T15:59:52Z

It might be interesting, but std's specialization applies to much more diverse range of behaviors. There we can't know if "empty most of the times" is the common case.

rust-bors · 2026-07-29T17:59:18Z

☀️ Try build successful (CI)
Build commit: f5bcd01 (f5bcd011d3725a90befeaac099c3fe9d56706db8)
Base parent: d366396 (d3663963ca08f465d01d283a7199778902623bb9)

rust-timer · 2026-07-29T19:31:57Z

Finished benchmarking commit (f5bcd01): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	-1.6%	[-3.6%, -0.2%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.6%, 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)	0.7%	[0.7%, 0.7%]	1
Improvements ✅ (primary)	-3.6%	[-5.5%, -1.2%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.6%	[-5.5%, -1.2%]	3

Cycles

Results (primary 0.4%, secondary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.5%, 1.4%]	9
Regressions ❌ (secondary)	1.6%	[0.4%, 5.2%]	13
Improvements ✅ (primary)	-1.4%	[-2.3%, -0.4%]	2
Improvements ✅ (secondary)	-1.6%	[-5.5%, -0.5%]	7
All ❌✅ (primary)	0.4%	[-2.3%, 1.4%]	11

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 489.658s -> 489.161s (-0.10%)
Artifact size: 390.17 MiB -> 390.11 MiB (-0.01%)

ChayimFriedman2 · 2026-07-29T19:50:32Z

Okay, so not only this does not regress #160073 this also has a non-negligible improvement. Seems worth to merge, but I'll try to see if I can use ThinVec or something similar before.

panstromek · 2026-07-29T20:08:28Z

Yea, the large-workspace change is likely noise, but the other two are legit. ThinVec might help or not, because it stores len=0 in a static singleton, while Vec keeps it on a stack. Using Option<ThinVec> for the empty case can get around that (and it's the same size).

ChayimFriedman2 · 2026-07-29T20:18:14Z

Option<ThinVec> is what I'm doing (more precisely a custom enum with the same layout). But it's a slightly bigger change.

ChayimFriedman2 · 2026-07-30T00:04:56Z

To the reviewer: The commits are fully separate. I can create separate PRs, but the first commit is very small so I don't think it's needed.

As it's very common.

This has multiple advantages: - Performance. The new type is 1/3 the size of `Vec` (being equivalent in layout to `Option<ThinVec>`) and can be kept in a register. - Type safety. We mark the type `#[must_use]`, and thinks requiring errors take `ThinVec`, which requires unwrapping the type and verifying there is indeed an error. We still provide conversions to slices, `ThinVec`, and iteration, because some code needs this and I saw no benefit in changing it, but we deliberately do not provide `Deref<Target = [E]>` or things like that.

rustbot · 2026-08-06T04:19:15Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

TaKO8Ki · 2026-08-06T16:32:53Z

@bors r+

rust-bors · 2026-08-06T16:32:56Z

📌 Commit 5491ec8 has been approved by TaKO8Ki

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 6. This pull request will be tested once the tree is reopened.

Reason for tree closure: Github problems

rust-bors · 2026-08-07T09:00:38Z

☀️ Test successful - CI
Approved by: TaKO8Ki
Duration: 2h 59m 40s
Pushing ae45457 to main...

github-actions · 2026-08-07T09:03:05Z

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 88f7399 (parent) -> ae45457 (this PR)

Test differences

Show 44 test diffs

44 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 ae45457594a670c59cd4d5591eaa243d9a3d44d5 --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-apple: 1h 28m -> 2h 10m (+47.7%)
x86_64-gnu-aux: 1h 51m -> 2h 34m (+38.5%)
i686-msvc-2: 1h 31m -> 2h 4m (+36.2%)
aarch64-apple-2: 1h 58m -> 2h 40m (+34.9%)
i686-gnu-nopt-1: 2h 18m -> 1h 36m (-30.6%)
dist-riscv64-linux-gnu: 1h 6m -> 1h 26m (+29.0%)
dist-riscv64-linux-musl: 1h 7m -> 1h 27m (+29.0%)
dist-powerpc64-linux-gnu: 1h 7m -> 1h 26m (+28.6%)
test-various: 1h 36m -> 2h 2m (+26.9%)
dist-arm-linux-musl: 1h 20m -> 1h 42m (+26.9%)

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.

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 29, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 29, 2026

Auto merge of #160160 - ChayimFriedman2:if-empty-opt, r=<try>

95b51e3

Check for the empty case in `collect_remaining_errors()`

ChayimFriedman2 mentioned this pull request Jul 29, 2026

Avoid TrustedLen specialization that optimize poorly in collect_remaining_errors #160073

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panstromek reviewed Jul 29, 2026

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ChayimFriedman2 force-pushed the if-empty-opt branch from 1223716 to fbe4c63 Compare July 29, 2026 15:53

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rust-bors Bot pushed a commit that referenced this pull request Jul 29, 2026

Auto merge of #160160 - ChayimFriedman2:if-empty-opt, r=<try>

f5bcd01

Check for the empty case in `collect_remaining_errors()`

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 29, 2026

rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) T-clippy Relevant to the Clippy team. labels Jul 29, 2026

ChayimFriedman2 changed the title ~~Check for the empty case in collect_remaining_errors()~~ Optimize handling of solver errors Jul 30, 2026

ChayimFriedman2 force-pushed the if-empty-opt branch from 6f6965c to 838f636 Compare July 30, 2026 00:10

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ChayimFriedman2 force-pushed the if-empty-opt branch from 838f636 to a41a292 Compare August 2, 2026 00:23

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ChayimFriedman2 force-pushed the if-empty-opt branch from a41a292 to 4fcd53a Compare August 4, 2026 19:10

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ChayimFriedman2 added 2 commits August 6, 2026 03:53

Check for the empty case in collect_remaining_errors()

6733ec7

As it's very common.

ChayimFriedman2 force-pushed the if-empty-opt branch from 4fcd53a to 5491ec8 Compare August 6, 2026 04:19

lcnr mentioned this pull request Aug 6, 2026

Stabilize the next-generation trait solver rust-lang/goals#113

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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 Aug 6, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 7, 2026

rust-bors Bot merged commit ae45457 into rust-lang:main Aug 7, 2026
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This was referenced Aug 7, 2026

leak_check in rustc_const_eval::util::relate_types #155883

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Move fulfillment into rustc_next_trait_solver #160485

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Optimize try_evaluate_obligations #160479

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Overall result: ✅ improvements - no action needed

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