Constify [u8]::is_ascii (unstably)

[scottmcm]

UTF-8 checking in const fn-stabilized back in 1.63 (#97367), but apparently somehow ASCII checking was never const-ified, despite being simpler.

New constness-tracking issue for is_ascii: #111090

I noticed this working on ascii::Char: #110998

[rustbot]

r? @Mark-Simulacrum

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[BurntSushi]

LGTM

[Noratrieb]

Instead of having to maintain an index, you could also use slice patterns to fake iteration:

while let [next, rest @ ..] = slice {
  slice = rest;
}

[scottmcm]

Well, you got me curious, so I tuned the heck out of it.

Looks like going backwards is the best in both LLVM and cg_gcc: https://rust.godbolt.org/z/K1rKs1T4d

[thomcc]

r? @thomcc

@bors r+

[bors]

📌 Commit 9de3d01 has been approved by thomcc

It is now in the queue for this repository.

[matthiaskrgr]

@bors r-
#111283 (comment)

[scottmcm]

Ok, I think I've fixed up the test. I'll iffy it just in case.
@bors r=thomcc rollup=iffy

[bors]

📌 Commit c8c5a58 has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit c8c5a58 with merge 613a5c9...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 613a5c9 to master...

[rust-timer]

Finished benchmarking commit (613a5c9): 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)

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)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	0.4%	[-0.1%, 0.8%]	2

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.6%	[-2.6%, -2.6%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 653.81s -> 654.562s (0.12%)

[coolreader18]

Hah, I had a precursor in #104126, though this one's much nicer. I'll keep that one open for eq_ignore_ascii_case, I guess.

[scottmcm]

Oh, sorry @coolreader18 -- I didn't even think to look if someone had already done it.

[coolreader18]

No worries! I'm glad it's now possible to do it where const_eval_select isn't necessary