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Make Map::last skip closure calls until last element #146674

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Make Map::last skip closure calls until last element #146674

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7 changes: 7 additions & 0 deletions library/core/src/iter/adapters/map.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -112,6 +112,13 @@ where
self.iter.size_hint()
}

fn last(mut self) -> Option<Self::Item>
where
Self: Sized,
{
self.iter.last().map(&mut self.f)
}
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@Mark-Simulacrum Mark-Simulacrum Sep 28, 2025

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I don't think we should do this; it skips calling f on all of the elements up to the last one, which doesn't seem right to me, especially since we do iterate over those elements (i.e., we aren't specialized in some way for DoubleEndedIterator).

If we do proceed with this it'll need libs-api FCP, so nominating this PR.

fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
where
Self: Sized,
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16 changes: 16 additions & 0 deletions library/coretests/tests/iter/sources.rs
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Expand Up @@ -30,6 +30,22 @@ fn test_repeat_take_collect() {
assert_eq!(v, vec![42, 42, 42]);
}

#[test]
#[should_panic = "iterator is infinite"]
fn test_repeat_count() {
repeat(42).count();
}

#[test]
fn test_repeat_last() {
assert_eq!(repeat(42).last(), Some(42));
}

#[test]
fn test_repeat_map_double_last() {
assert_eq!(repeat(42).map(|e| 2 * e).last(), Some(2 * 42));
}

#[test]
fn test_repeat_with() {
#[derive(PartialEq, Debug)]
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