core: optimize RepeatN

[joboet]

...by adding an optimized implementation of try_fold and fold as well as replacing some unnecessary mem::replace calls with MaybeUninit helper methods.

[rustbot]

r? @thomcc

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[thomcc]

Looks great, thanks.

@bors r+ rollup=never

[bors]

📌 Commit 51d51c8 has been approved by thomcc

It is now in the queue for this repository.

[scottmcm]

curiosity: were you seeing this be a codegen issue? I would guess this was done intentionally to "overwrite" the value with undef so that Miri could catch any further typed use of it. And overwrite-with-undef optimizes out in LLVM, normally.

[joboet]

Ah, that's why it was written this way... I just saw some strange code and wanted to make it more idiomatic. I can go back to the old version and just keep the fold part, if you think there's merit in keeping this.

[bors]

⌛ Testing commit 51d51c8 with merge f08d5c0...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing f08d5c0 to master...

[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 756bff9 (parent) -> f08d5c0 (this PR)

Test differences

Show 8 test diffs

Additionally, 8 doctest diffs were found. These are ignored, as they are noisy.

Job group index

[rust-timer]

Finished benchmarking commit (f08d5c0): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 3.1%)

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

Cycles

Results (primary 2.4%)

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

Binary size

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

Bootstrap: 774.407s -> 774.928s (0.07%)
Artifact size: 365.55 MiB -> 365.54 MiB (-0.00%)