Fix wrong par_slice implementation

[zetanumbers]

#152375 (comment)

[rustbot]

r? @JonathanBrouwer

rustbot has assigned @JonathanBrouwer.
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

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 68 candidates
• Random selection from 14 candidates

[jieyouxu]

@bors try @rust-timer queue

[JonathanBrouwer]

cc @Zoxc @jieyouxu @lqd
Change looks good to me, r=me
I think we should redo the perf numbers

[JonathanBrouwer]

I think last time rust-timer didn't show any perf impact, how were the statistics in that PR generated?

[lqd]

I think last time rust-timer didn't show any perf impact, how were the statistics in that PR generated?

Zoxc ran rcb and posted that; rustc-perf did show wall time improvements for the multithreaded benchmark we have.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 532510f (532510fc9b4953f08a24a2fd52f36860df85a645, parent: 71e00273c0921e1bc850ae8cc4161fbb44cfa848)

[rust-timer]

Finished benchmarking commit (532510f): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (secondary -5.1%)

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)	-	-	0
Improvements ✅ (secondary)	-5.1%	[-5.1%, -5.1%]	1
All ❌✅ (primary)	-	-	0

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: 485.967s -> 483.907s (-0.42%)
Artifact size: 397.80 MiB -> 397.81 MiB (0.00%)

[jieyouxu]

(Since this unborks parallel-frontend and tree is reasonably light)
@bors r=JonathanBrouwer,petrochenkov,jieyouxu rollup=never p=5

[rust-bors]

📌 Commit 33a8dc1 has been approved by JonathanBrouwer,petrochenkov,jieyouxu

It is now in the queue for this repository.

[rust-bors]

☀️ Test successful - CI
Approved by: JonathanBrouwer,petrochenkov,jieyouxu
Duration: 3h 15m 22s
Pushing 1210e9f to main...

[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 d1a11b6 (parent) -> 1210e9f (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 1210e9fa3ee9c9712fa694f90f7a032f2f0fd786 --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. dist-aarch64-apple: 3h -> 1h 43m (-42.9%)
2. dist-x86_64-apple: 1h 53m -> 2h 20m (+24.3%)
3. aarch64-apple: 4h 8m -> 3h 8m (-24.0%)
4. x86_64-gnu-tools: 52m 4s -> 1h 2m (+20.5%)
5. dist-aarch64-llvm-mingw: 1h 59m -> 1h 35m (-19.6%)
6. x86_64-gnu-llvm-21-2: 1h 26m -> 1h 42m (+18.6%)
7. tidy: 2m 29s -> 2m 53s (+16.1%)
8. dist-apple-various: 1h 41m -> 1h 53m (+11.5%)
9. dist-aarch64-msvc: 1h 38m -> 1h 49m (+10.6%)
10. x86_64-gnu-llvm-21-3: 1h 51m -> 2h 1m (+9.8%)

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]

Finished benchmarking commit (1210e9f): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

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

Cycles

Results (secondary 3.3%)

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)	3.3%	[3.2%, 3.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 483.207s -> 482.345s (-0.18%)
Artifact size: 397.79 MiB -> 397.85 MiB (0.02%)