Allow bounds checks when enumerating IndexSlice to be elided

[Jarcho]

Without this hint, each loop iteration has to separately bounds check the index. See https://godbolt.org/z/zrfPY4Ten for an example.

This is technically a behaviour change, but only in cases where the compiler is going to crash anyways.

[rustbot]

r? @davidtwco

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[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit da0fbc1 with merge 5be67cb...

[bors]

☀️ Try build successful - checks-actions
Build commit: 5be67cb (5be67cb39839ede36ee48cc29046aab68af05c87)

[rust-timer]

Finished benchmarking commit (5be67cb): comparison URL.

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

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.4%	[0.4%, 0.5%]	6
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	8
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	4
All ❌✅ (primary)	-0.3%	[-0.3%, -0.2%]	8

Max RSS (memory usage)

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

Cycles

Results (secondary 2.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

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

Binary size

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

Bootstrap: 771.133s -> 771.692s (0.07%)
Artifact size: 361.91 MiB -> 361.94 MiB (0.01%)

[davidtwco]

This seems like an improvement, there are some small regressions on stress test benchmarks but improvements in real crates.

[davidtwco]

@bors r+

[bors]

📌 Commit da0fbc1 has been approved by davidtwco

It is now in the queue for this repository.

[bors]

⌛ Testing commit da0fbc1 with merge d2b52c5...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing d2b52c5 to master...

[github-actions]

Post-merge analysis result

Test differences

No test diffs found

[rust-timer]

Finished benchmarking commit (d2b52c5): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	6
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	8
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.3%, -0.2%]	8

Max RSS (memory usage)

Results (primary -3.1%, secondary 5.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.4%	[5.4%, 5.4%]	1
Improvements ✅ (primary)	-3.1%	[-3.1%, -3.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.1%	[-3.1%, -3.1%]	1

Cycles

Results (primary -2.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	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

Binary size

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

Bootstrap: 779.867s -> 780.734s (0.11%)
Artifact size: 365.28 MiB -> 365.30 MiB (0.00%)