Use normal iterators instead of the internal vec::ExtendWith

[scottmcm]

Repeating is TrustedLen, so we don't need another copy of the TrustedLen specialization for extend.

All the codegen tests are passing (and I made one stricter to help be sure), so let's see if perf is also happy with it.

(This is a simpler version of #104596, which tried to do too many things at once.)

r? @ghost

Built atop #104435; will leave draft until I can rebase atop that.

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4820629 with merge a31a4b1059fe70a1571639674d618845dcb8c7dd...

[bors]

☀️ Try build successful - checks-actions
Build commit: a31a4b1059fe70a1571639674d618845dcb8c7dd (a31a4b1059fe70a1571639674d618845dcb8c7dd)

[rust-timer]

Finished benchmarking commit (a31a4b1059fe70a1571639674d618845dcb8c7dd): comparison URL.

Overall result: ❌✅ regressions and improvements - 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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.5%]	14
Regressions ❌ (secondary)	2.9%	[0.2%, 5.1%]	10
Improvements ✅ (primary)	-0.7%	[-1.1%, -0.5%]	9
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-1.1%, 0.5%]	23

Max RSS (memory usage)

Results

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)	2.2%	[0.3%, 5.1%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.8%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[-2.9%, 5.1%]	7

Cycles

Results

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)	0.7%	[0.7%, 0.7%]	1
Regressions ❌ (secondary)	3.9%	[3.7%, 4.4%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.7%	[0.7%, 0.7%]	1

[scottmcm]

A couple of nice wins, but overall too many losses.

Time for yet another different approach :(