Make skip_whitespace do a single pass

[hkBst]

Offshoot from #136931 that I'd like to see the performance impact of.

r? @nnethercote

[Kobzol]

@bors try @rust-timer queue

[bors]

⌛ Trying commit d15cd2e with merge a13e5e9...

[bors]

☀️ Try build successful - checks-actions
Build commit: a13e5e9 (a13e5e9a6871141d69bd9fd1bcd399e546e5a835)

[rust-timer]

Finished benchmarking commit (a13e5e9): comparison URL.

Overall result: no relevant changes - no action needed

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.

@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 (primary 2.6%)

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

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: 790.438s -> 788.744s (-0.21%)
Artifact size: 347.75 MiB -> 347.73 MiB (-0.01%)

[hkBst]

Closing this in favor of #137275