Bitset cleanups

[nnethercote]

Some minor cleanups I did while working on #147619.

r? @Zalathar

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: c78dd25 (c78dd25d124432abe6198b931b6c3240950c2405, parent: 35456985fab3419baf4bf111b8bbbd5e8565615d)

[rust-timer]

Finished benchmarking commit (c78dd25): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.5%, -0.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.5%, -0.4%]	2

Max RSS (memory usage)

Results (primary -6.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-6.8%	[-6.8%, -6.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-6.8%	[-6.8%, -6.8%]	1

Cycles

Results (primary 4.9%, secondary -2.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	4.9%	[2.9%, 7.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	4.9%	[2.9%, 7.0%]	2

Binary size

Results (secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 474.753s -> 474.463s (-0.06%)
Artifact size: 388.13 MiB -> 388.08 MiB (-0.01%)

[cjgillot]

Should this return a ChunkSize directly? It's mostly used to be cast.

[Zalathar]

If it returns ChunkSize directly, then it's only correct for slices no larger than a chunk, making it incorrect for DenseBitSet::count in the general case.

So if the return value is to be changed, it should be extracted to a separate function with a narrower name.

[cjgillot]

r=me with or without the nit

[Zalathar]

Nit: Instead of a cryptic &**words, I typically find it clearer to write something like words.as_slice() instead.