diff --git a/library/alloc/benches/vec.rs b/library/alloc/benches/vec.rs index 91eec10d57593..c93a493cadb0d 100644 --- a/library/alloc/benches/vec.rs +++ b/library/alloc/benches/vec.rs @@ -726,3 +726,9 @@ fn bench_dedup_old_100000(b: &mut Bencher) { fn bench_dedup_new_100000(b: &mut Bencher) { bench_vec_dedup_new(b, 100000); } + +#[bench] +fn bench_flat_map_collect(b: &mut Bencher) { + let v = vec![777u32; 500000]; + b.iter(|| v.iter().flat_map(|color| color.rotate_left(8).to_be_bytes()).collect::>()); +} diff --git a/library/core/src/array/iter.rs b/library/core/src/array/iter.rs index 61ab1b1faff89..f6616399610a6 100644 --- a/library/core/src/array/iter.rs +++ b/library/core/src/array/iter.rs @@ -123,6 +123,27 @@ impl Iterator for IntoIter { (len, Some(len)) } + #[inline] + fn fold(mut self, init: Acc, mut fold: Fold) -> Acc + where + Fold: FnMut(Acc, Self::Item) -> Acc, + { + let data = &mut self.data; + // FIXME: This uses try_fold(&mut iter) instead of fold(iter) because the latter + // would go through the blanket `impl Iterator for &mut I` implementation + // which lacks inline annotations on its methods and adding those would be a larger + // perturbation than using try_fold here. + // Whether it would be beneficial to add those annotations should be investigated separately. + (&mut self.alive) + .try_fold::<_, _, Result<_, !>>(init, |acc, idx| { + // SAFETY: idx is obtained by folding over the `alive` range, which implies the + // value is currently considered alive but as the range is being consumed each value + // we read here will only be read once and then considered dead. + Ok(fold(acc, unsafe { data.get_unchecked(idx).assume_init_read() })) + }) + .unwrap() + } + fn count(self) -> usize { self.len() }