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The extra check caused by the expect() call can, in general, not be optimized away, because the length of the iterator is unknown at compile time, causing a noticable slow-down. Since the check only triggers if the element isn't actually found in the iterator, i.e. it isn't guaranteed to trigger for ill-behaved ExactSizeIterators, it seems reasonable to switch to an implementation that doesn't need the check and just always returns None if the value isn't found. Benchmark: ````rust let v: Vec<u8> = (0..1024*65).map(|_| 0).collect(); b.iter(|| { v.as_slice().iter().rposition(|&c| c == 1) }); ```` Before: ```` test rposition ... bench: 49939 ns/iter (+/- 23) ```` After: ```` test rposition ... bench: 33306 ns/iter (+/- 68) ````
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