Adjust successor iterators.

[nnethercote]

Because #148054 was a slight perf regression.

The problem was seemingly because this iterator structure:

slice_iter.chain(Option_iter.chain(Option_iter))

changed to this:

slice_iter.chain(Option_iter).chain(Option_iter)

The commit also tweaks the slice_iter part, changing into_iter to iter and using [] instead of (&[]), for conciseness and consistency.

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 07d7953 (07d7953ec5ca596313c79b067e4a60e37ef77a99, parent: f977dfc388ea39c9886b7f8c49abce26e6918df6)

[rust-timer]

Finished benchmarking commit (07d7953): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.1%]	26
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.0%]	19
All ❌✅ (primary)	-0.2%	[-0.4%, -0.1%]	26

Max RSS (memory usage)

Results (primary -0.5%, secondary 0.7%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-1.4%	[-2.0%, -0.8%]	2
Improvements ✅ (secondary)	-1.7%	[-1.7%, -1.7%]	1
All ❌✅ (primary)	-0.5%	[-2.0%, 1.3%]	3

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: 473.751s -> 477.517s (0.79%)
Artifact size: 390.43 MiB -> 390.50 MiB (0.02%)