Reserve items in Extend implementations

[JonathanBrouwer]

This might be a perf win, inspired by @Kobzol's approach in this PR: #162480

[JonathanBrouwer]

@bors try @rust-timer queue

[JonathanBrouwer]

Searched in the compiler for more places to do this
@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5653ff1 (5653ff1472c1fbac81b5ce04e505360b28b0af67)
Base parent: b505807 (b505807a88bdb0dca9c968155f2167a927dddb34)

[Kobzol]

Nice finds! Just noting that in case the data structure was previously cleared or its size reduced, this might overallocate. The exact size to allocate should be something like items.len() - (self.capacity() - self.len()).

[JonathanBrouwer]

Per the documentation of reserve(n), it ensures there is space for at least n more elements than the length (not capacity!) of the current collection.

https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.reserve

So given that, I don't think you're right

[rust-timer]

Finished benchmarking commit (5653ff1): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	6
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.4%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[0.5%, 3.2%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.2%	[-1.7%, -0.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-1.7%, 3.2%]	5

Cycles

Results (primary 0.2%, secondary -3.7%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[0.5%, 2.1%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.7%, -0.4%]	7
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	0.2%	[-0.7%, 2.1%]	14

Binary size

Results (primary -0.0%, 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
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	7
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	7

Bootstrap: 477.178s -> 480.938s (0.79%)
Artifact size: 403.53 MiB -> 403.61 MiB (0.02%)

[rustbot]

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[JonathanBrouwer]

The include-blob results are noise, but if you also show non-relevant results it is quite green, so I think this is worth merging

[Kobzol]

Ah, sorry, I got this wrong again 😆Yeah, looks generally reasonable. Thanks!

@bors r+

[rust-bors]

📌 Commit 5bd43f3 has been approved by Kobzol

It is now in the queue for this repository.

[Kobzol]

r? kobzol