ascii::Char-ify the escaping code in core

[scottmcm]

This means that EscapeIterInner::as_str no longer needs unsafe code, because the type system ensures the internal buffer is only ASCII, and thus valid UTF-8.

Come to think of it, this also gives it a (non-guaranteed) niche.

cc @BurntSushi as potentially interested
ascii::Char tracking issue: #110998

[rustbot]

r? @cuviper

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[cuviper]

LGTM

@bors r+

[bors]

📌 Commit 28449da has been approved by cuviper

It is now in the queue for this repository.

[bors]

⌛ Testing commit 28449da with merge e86fd62...

[bors]

☀️ Test successful - checks-actions
Approved by: cuviper
Pushing e86fd62 to master...

[rust-timer]

Finished benchmarking commit (e86fd62): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.7%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-0.7%, -0.6%]	3
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.4%]	3
All ❌✅ (primary)	0.1%	[-0.7%, 0.7%]	9

Max RSS (memory usage)

Results

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)	2.0%	[2.0%, 2.0%]	1
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-2.7%	[-2.8%, -2.5%]	2
All ❌✅ (primary)	-1.8%	[-1.8%, -1.8%]	1

Cycles

Results

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)	10.4%	[5.4%, 20.2%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	10.4%	[5.4%, 20.2%]	7

Binary size

Results

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)	0.3%	[0.3%, 0.3%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 640.962s -> 641.899s (0.15%)

[lqd]

This PR's wins are noise starting from a couple PRs ago in #109773 (comment) returning to steady state.

I won't mark this as triaged just yet though: we'll see what happens to html5ever in future PRs, as this change doesn't look like its most recent behavior. It's small and only in fast incremental use cases, so it may be fine either way, but the upwards trend on that benchmark is unfortunate: we've already lost half of this month's gains in incr comp.