Avoid Iterator::last

[kadiwa4]

Adapters like Filter and Map use the default implementation of Iterator::last which is not short-circuiting (and so does core::str::Split). The predicate function will be run for every single item of the underlying iterator. I hope that removing those calls to last results in slight performance improvements.

[rust-highfive]

r? @oli-obk

(rust-highfive has picked a reviewer for you, use r? to override)

[leonardo-m]

If this is a problem, has Clippy a "performance" suggestion about this? (Dogfooding compiler improvementes into Clippy is generally a good idea).

[kadiwa4]

No, it doesn't look like a lint for that exists.

[Kobzol]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 66211d8 with merge 5bcf9d36a0c3bd58c90f463f91304174af29e909...

[bors]

☀️ Try build successful - checks-actions
Build commit: 5bcf9d36a0c3bd58c90f463f91304174af29e909 (5bcf9d36a0c3bd58c90f463f91304174af29e909)

[rust-timer]

Queued 5bcf9d36a0c3bd58c90f463f91304174af29e909 with parent fa521a4, future comparison URL.

[rust-timer]

Finished benchmarking commit (5bcf9d36a0c3bd58c90f463f91304174af29e909): comparison URL.

Overall result: no relevant changes - no 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.

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

Instruction count

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

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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.0%	[1.0%, 1.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.9%	[2.9%, 4.9%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Footnotes

1. the arithmetic mean of the percent change↩↩²

2. number of relevant changes↩↩²

[oli-obk]

I don't expect this to affect perfbot. We don't have runtime benchmarks for such changes, so you'll have to manually benchmark some examples

[Kobzol]

This PR modifies the compiler code, not stdlib, so it should be visible in perfbot, or not? In any case, the changes look quite reasonable.

[oli-obk]

Oh right. Still, these changes are not in hot paths. It's a good cleanup either way.

@bors r+ rollup

[bors]

📌 Commit 66211d8 has been approved by oli-obk

It is now in the queue for this repository.