Specialize count for range iterators

[clarfonthey]

Since size_hint is already specialized, it feels apt to specialize count as well. Without any specialized version of ExactSizeIterator::len or Step::steps_between, this feels like a more reliable way of accessing this without having to rely on knowing that size_hint is correct.

In my case, this is particularly useful to access the steps_between implementation for char from the standard library without having to compute it manually.

I didn't think it was worth modifying the Step trait to add a version of steps_between that used native overflow checks since this is just doing one subtraction in most cases anyway, and so I decided to make the inclusive version use checked_add so it didn't have this lopsided overflow-checks-but-only-sometimes logic.

[rustbot]

r? @scottmcm

(rustbot has picked a reviewer for you, use r? to override)

[scottmcm]

Since this is an observable change, I'm going to flip it over to
r? rust-lang/libs-api

For example, today

pub fn range_count_demo() -> usize { (0..u128::MAX).count() }

just returns usize::MAX, rather than panics

; playground::range_count_demo
; Function Attrs: mustprogress nofree norecurse nosync nounwind nonlazybind willreturn memory(none) uwtable
define noundef i64 @_ZN10playground16range_count_demo17h49ff333a76b29162E() unnamed_addr #0 personality ptr @rust_eh_personality {
start:
  ret i64 -1
}

https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=7202fd68c70d7dcd0f467880c2cbd48d

[scottmcm]

Hmm, it should be someone on the review rotation, so let's say
r? m-ou-se

[BurntSushi]

It looks like this change is explicitly allowed by the docs in Iterator::count:

The method does no guarding against overflows, so counting elements of an iterator with more than usize::MAX elements either produces the wrong result or panics. If debug assertions are enabled, a panic is guaranteed.

So this seems fine to me.

@rfcbot fcp merge

[clarfonthey]

It looks like this change is explicitly allowed by the docs in Iterator::count:

I should have included this in the description too, but I was also taking this into consideration when deciding to implement this. The main justification for not using the default overflow behaviour (hence making observable changes) is because to do so would require augmenting the Step trait, and I didn't think it'd be worth it.

[BurntSushi]

Trying again...

@rfcbot fcp merge

[ChrisDenton]

You may also need to flip the label to libs-api

[tgross35]

@BurntSushi just to move this thing - want to retry your fcp?

[BurntSushi]

@rfcbot fcp merge

[rfcbot]

Team member @BurntSushi has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[tgross35]

Third time's the charm :)

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[clarfonthey]

Gentle nudge to @m-ou-se to merge when you have the chance, since the FCP has ended.

[dtolnay]

@bors r+

[bors]

📌 Commit 08aa6c9 has been approved by dtolnay

It is now in the queue for this repository.

[bors]

⌛ Testing commit 08aa6c9 with merge 4514fb9...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 4514fb9 to master...

[rust-timer]

Finished benchmarking commit (4514fb9): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 633.405s -> 632.183s (-0.19%)
Artifact size: 318.16 MiB -> 318.14 MiB (-0.01%)