Override StepBy::{try_fold, try_rfold}

[timvermeulen]

Previous PR: #51435

The previous PR was closed in favor of #51601, which was later reverted. I don't think these implementations will make it harder to specialize StepBy<Range<_>> later, so we should be able to land this without any consequences.

This should fix#57517 – in my benchmarks iter and iter.step_by(1) now perform equally well, provided internal iteration is used.

[rust-highfive]

r? @KodrAus

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[Centril]

r? @scottmcm

[scottmcm]

Thanks for the PR! This definitely seems like one that needs try_fold (though I wish LLVM would just peel it). One minor change and it'll be good to go.

[scottmcm]

Can you just use a while let Some(x) = self.iter.nth(self.step) loop here instead? It'll do the same thing, but without introducing a new closure that would regress #62429.

[timvermeulen]

Thanks, I forgot about that. I turned the closures into functions instead, that should have the same benefits, correct? I figured this will make it easier to potentially undo these changes in the future.

[scottmcm]

@bors r+ rollup

[bors]

📌 Commit 78908f2 has been approved by scottmcm