Another attempt at `Vec::extend_with` via `iter::repeat_n` by scottmcm · Pull Request #149751 · rust-lang/rust

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

scottmcm · 2025-12-08T00:56:28Z

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)

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

scottmcm · 2025-12-08T01:25:26Z

@bors try @rust-timer queue

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

rust-bors · 2025-12-08T03:43:09Z

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

rust-timer · 2025-12-08T07:07:30Z

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%)

rust-bors · 2026-08-30T00:18:01Z

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 8, 2025

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Dec 8, 2025

Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Dec 8, 2025

Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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Another attempt at Vec::extend_with via iter::repeat_n
3d96b4f
This time, try specializing `RepeatN::fold` instead, since for `TrivialClone` it can just copy it every time.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 8, 2025

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Auto merge of#149751- scottmcm:spec-repeat-n-fold, r=<try>
c311241
Another attempt at `Vec::extend_with` via `iter::repeat_n`

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rust-timer commented Dec 8, 2025

Overall result: ❌ regressions - please read the text below

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Overall result: ❌ regressions - please read the text below

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Overall result: ❌ regressions - please read the text below

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