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Rollup merge of #66341 - crgl:vec-deque-extend, r=Amanieu 

Match `VecDeque::extend` to `Vec::extend_desugared`

Currently, `VecDeque::extend` [does not reserve at all](https://github.com/rust-lang/rust/pull/65069#discussion_r333166522). This implementation still runs a check every iteration of the loop, but should reallocate at most once for the common cases where the `size_hint` lower bound is exact. Further optimizations in the future could improve this for some common cases, but given the complexity of the `Vec::extend` implementation it's not immediately clear that this would be worthwhile.
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Mazdak Farrokhzad 2019-12-13 04:21:22 +01:00 committed by GitHub
parent 3eeb8d4f2f 164d1a205d
commit 2ca7b7e539
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1 changed files with 16 additions and 1 deletions
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17
src/liballoc/collections/vec_deque.rs
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@ -2809,7 +2809,22 @@ impl<'a, T> IntoIterator for &'a mut VecDeque<T> {
#[stable(feature = "rust1", since = "1.0.0")] #[stable(feature = "rust1", since = "1.0.0")]
impl<A> Extend<A> for VecDeque<A> { impl<A> Extend<A> for VecDeque<A> {
fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T) { fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T) {
iter.into_iter().for_each(move |elt| self.push_back(elt)); // This function should be the moral equivalent of:
//
// for item in iter.into_iter() {
// self.push_back(item);
// }
let mut iter = iter.into_iter();
while let Some(element) = iter.next() {
if self.len() == self.capacity() {
let (lower, _) = iter.size_hint();
self.reserve(lower.saturating_add(1));
}
let head = self.head;
self.head = self.wrap_add(self.head, 1);
unsafe { self.buffer_write(head, element); }
}
} }
} }