[perf] Add explicit Iterator::count impl for ChunkBy.

[obi1kenobi]

TODO: fill out description if perf is improved

[obi1kenobi]

cc @Kobzol for perf run

[lqd]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 546c6e5 (546c6e543ac3ec8f80e058d5494d0d388854d0d4)
Base parent: 3c00c96 (3c00c96d3af4d5b5e101e56cc161a608b21366ee)

[rust-timer]

Finished benchmarking commit (546c6e5): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.7%	[0.1%, 1.7%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -0.7%, secondary -5.2%)

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)	-0.7%	[-0.7%, -0.7%]	1
Improvements ✅ (secondary)	-5.2%	[-5.2%, -5.2%]	1
All ❌✅ (primary)	-0.7%	[-0.7%, -0.7%]	1

Cycles

Results (primary 2.9%, secondary -3.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.9%	[2.9%, 2.9%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.4%, -2.6%]	2
All ❌✅ (primary)	2.9%	[2.9%, 2.9%]	1

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: 491.018s -> 490.286s (-0.15%)
Artifact size: 388.45 MiB -> 388.95 MiB (0.13%)

[obi1kenobi]

It's interesting that we see regressions at all. I expected a ~zero impact finding due to chunk_by not being represented in the benchmark suite. I'll try to triage a bit more, but if someone with more experience cares to look more closely I welcome the help!

[Kobzol]

Those two benchmarks are essentially just hello world, it might just be noise, or the stdlib becoming a tiny bit larger to load or something.

A change like this most likely won't be caught by our compile benchmark suite (which doesn't mean it's not worth landing).