outline unreachable! from Clause::kind by panstromek · Pull Request #163657 · rust-lang/rust

panstromek · 2026-10-02T13:15:48Z

Safe alternative to #163639. unreachable!() expands to panic("internal error: entered unreachable code"), which we've seen before to be worse than plain argument-less call in other hot places.

see https://rust-lang.zulipchat.com/#narrow/channel/247081-t-compiler.2Fperformance/topic/needs.20help.3A.20small.20perf.20improvement.20with.20non-local.20reasoning/with/628660586

panstromek · 2026-10-02T13:16:00Z

@bors try @rust-timer queue

outline unreachable! from Clause::kind

rust-bors · 2026-10-02T14:15:05Z

☀️ Try build successful (CI)
Build commit: 2660fea (2660fea4cf8b7dcc478d2dc474f57411892974f5)
Base parent: c165834 (c1658342b0ea7eab61db1960082668c77f553522)

rust-timer · 2026-10-02T16:44:56Z

Finished benchmarking commit (2660fea): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@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)	-0.4%	[-0.6%, -0.1%]	4
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.2%]	34
All ❌✅ (primary)	-0.4%	[-0.6%, -0.1%]	4

Max RSS (memory usage)

Results (secondary 3.9%)

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)	3.9%	[3.9%, 3.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.4%, secondary 3.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.1%, 2.7%]	2
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.1%, 2.7%]	2

Binary size

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

Bootstrap: 491.121s -> 490.2s (-0.19%)
Artifact size: 408.64 MiB -> 408.63 MiB (-0.00%)

rustbot · 2026-10-02T16:53:25Z

r? @oli-obk

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

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Owners of files modified in this PR: compiler, types
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panstromek · 2026-10-02T16:54:05Z

This is not as good as unreachable_unchecked but it's at least something.

lcnr · 2026-10-02T17:30:14Z

    }

    pub fn kind(self) -> ty::Binder<'tcx, ClauseKind<'tcx>> {
+        #[cold]


can you put a comment here referencing this perf run, r=me after

View changes since the review

Done, thanks ;) but I don't have r+ rights to approve this

unreachable!() expands to panic("internal error: entered unreachable code"), which makes the happy path perform worse than plain argument-less call.

rustbot · 2026-10-03T06:32:22Z

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.

lcnr · 2026-10-03T08:11:13Z

@bors r+ rollup=never

rust-bors · 2026-10-03T08:11:15Z

📌 Commit 22f30e6 has been approved by lcnr

It is now in the queue for this repository.

rust-bors · 2026-10-04T14:45:54Z

☀️ Test successful - CI
Approved by: lcnr
Duration: 3h 8m 16s
Pushing a639ea0 to main...

github-actions · 2026-10-04T14:49:12Z

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 4ddbc06 (parent) -> a639ea0 (this PR)

Test differences

Show 3 test diffs

3 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 a639ea0890c0977b1c45596fd6b5f9a70a1d67da --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

test-x86_64-gnu-llvm-22-1: 43m 46s -> 1h 16m (+75.7%)
test-i686-gnu-1: 1h 23m -> 2h 23m (+72.8%)
dist-x86_64-llvm-mingw: 1h 20m -> 2h 11m (+64.1%)
test-x86_64-gnu-distcheck: 1h 31m -> 2h 25m (+59.6%)
test-aarch64-apple-1: 1h 36m -> 2h 32m (+58.0%)
test-x86_64-gnu-stdlib-semver-check: 11m 21s -> 17m 27s (+53.7%)
dist-x86_64-netbsd: 1h 2m -> 1h 33m (+47.8%)
test-x86_64-gnu-gcc: 58m 22s -> 1h 23m (+43.2%)
dist-i686-msvc: 53m 1s -> 32m 25s (-38.9%)
test-i686-gnu-2: 1h 12m -> 1h 40m (+38.1%)

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 · 2026-10-04T15:29:28Z

Finished benchmarking commit (a639ea0): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

This perf run didn't have 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)	3.5%	[3.3%, 3.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 491.424s -> 489.631s (-0.36%)
Artifact size: 408.69 MiB -> 408.63 MiB (-0.01%)

panstromek · 2026-10-04T15:45:58Z

huh, this is quite funny, there's no speedup anymore. I wonder what happened here.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 2, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 2, 2026

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rust-bors Bot pushed a commit that referenced this pull request Oct 2, 2026

Auto merge of #163657 - panstromek:next-solver-unreachable, r=<try>

2660fea

outline unreachable! from Clause::kind

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 2, 2026

panstromek marked this pull request as ready for review October 2, 2026 16:53

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 2, 2026

rustbot assigned oli-obk Oct 2, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 2, 2026

lcnr reviewed Oct 2, 2026

View reviewed changes

panstromek force-pushed the next-solver-unreachable branch from a5d2421 to 1a98b21 Compare October 3, 2026 04:47

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Outline unreachable from Clause::kind

22f30e6

unreachable!() expands to panic("internal error: entered unreachable code"), which makes the happy path perform worse than plain argument-less call.

panstromek force-pushed the next-solver-unreachable branch from 1a98b21 to 22f30e6 Compare October 3, 2026 06:32

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 Oct 3, 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 Oct 4, 2026

rust-bors Bot merged commit a639ea0 into rust-lang:main Oct 4, 2026
15 checks passed

rustbot added this to the 1.101.0 milestone Oct 4, 2026

hanna-kruppe mentioned this pull request Oct 4, 2026

[EXPERIMENT] Outline some panics with constant messages #163766

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rust-timer commented Oct 2, 2026

Overall result: ✅ improvements - no action needed

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Test differences

Job duration changes

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Overall result: no relevant changes - no action needed

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lcnr Oct 2, 2026 •

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