try to reserve space in the Extend impls

Author: cehteh

src/lib.rs

@@ -1082,17 +1082,18 @@ impl<H, T> Extend<T> for HeaderVec<H, T> {
     #[inline]
     #[track_caller]
     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
-        iter.into_iter().for_each(|item| self.push(item));
+        let iter = iter.into_iter();
+        self.reserve(iter.size_hint().0);
+        iter.for_each(|item| self.push(item));
     }
 }
 
 /// Extend implementation that copies elements out of references before pushing them onto the Vec.
-// Note: from std Vec: not implemented here yet
-// This implementation is specialized for slice iterators, where it uses [`copy_from_slice`] to
-// append the entire slice at once.
 impl<'a, H, T: Copy + 'a> Extend<&'a T> for HeaderVec<H, T> {
     #[track_caller]
     fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
-        iter.into_iter().for_each(|item| self.push(*item));
+        let iter = iter.into_iter();
+        self.reserve(iter.size_hint().0);
+        iter.for_each(|item| self.push(*item));
     }
 }