Another attempt at Vec::extend_with via iter::repeat_n

[scottmcm]

This time, try specializing RepeatN::fold instead, since for TrivialClone it can just copy it every time.

Since this hasn't gone well in perf the last few times, start in draft and with
r? ghost

(Inspired by #133662 and #104596 but rather different from both of those)

[scottmcm]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: c311241 (c311241ae75ce3e70730757e2a9b994fb92fe911, parent: ba2142a19c1acabe770433418fa395b8a0ef47a4)

[rust-timer]

Finished benchmarking commit (c311241): comparison URL.

Overall result: ❌ regressions - 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)	0.9%	[0.1%, 9.7%]	20
Regressions ❌ (secondary)	0.2%	[0.0%, 0.3%]	11
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.6%]	2
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.8%	[-0.7%, 9.7%]	22

Max RSS (memory usage)

Results (primary 2.9%, secondary -3.2%)

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

	mean	range	count
Regressions ❌ (primary)	2.9%	[1.5%, 4.9%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	2.9%	[1.5%, 4.9%]	3

Cycles

Results (primary 3.8%, secondary 2.1%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[1.4%, 10.1%]	5
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.8%	[1.4%, 10.1%]	5

Binary size

Results (primary 0.0%, secondary -0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 1.8%]	28
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-1.7%, -0.0%]	28
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	0.0%	[-1.7%, 1.8%]	56

Bootstrap: 470.113s -> 473.879s (0.80%)
Artifact size: 388.98 MiB -> 389.00 MiB (0.01%)