Revert "Streamline iterator chaining when computing successors."

[Zalathar]

• Reverts Streamline iterator chaining when computing successors. #148054

The new chaining structure turned out to have unexpected perf consequences.

---

This reverts commit 3d95159.

r? ghost (until a perf run confirms that the revert is good)

[Zalathar]

@bors try @rust-timer queue

[Zalathar]

Per #148054 (comment) I no longer intend to merge this, but I'll let the perf job continue to confirm that a revert would resolve the perf regression.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 1d89aee (1d89aeeed0bcd8d39f69adb36812312f90750391, parent: f435972085b697a1ece8ee6a1ac76efff8d1df7b)

[rust-timer]

Finished benchmarking commit (1d89aee): 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)	-0.2%	[-0.4%, -0.1%]	31
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.0%]	26
All ❌✅ (primary)	-0.2%	[-0.4%, -0.1%]	31

Max RSS (memory usage)

Results (primary 2.2%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	1.6%	[0.5%, 4.4%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Cycles

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

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: 474.979s -> 474.299s (-0.14%)
Artifact size: 390.46 MiB -> 390.46 MiB (-0.00%)

[Zalathar]

Closing this PR, and leaving any revert/fix up to the original author instead.