Use v.for_each(|elem| ...) instead of for elem in v in Vec::extend_desugared`.

This makes use of specialized Iterator methods (when available).

Author: pnkfelix

src/liballoc/vec.rs

@@ -1918,26 +1918,27 @@ impl<'a, T: 'a> SpecExtend<&'a T, slice::Iter<'a, T>> for Vec<T>
 }
 
 impl<T> Vec<T> {
-    fn extend_desugared<I: Iterator<Item = T>>(&mut self, mut iterator: I) {
+    fn extend_desugared<I: Iterator<Item = T>>(&mut self, iterator: I) {
         // This is the case for a general iterator.
         //
         // This function should be the moral equivalent of:
         //
         //      for item in iterator {
         //          self.push(item);
         //      }
-        while let Some(element) = iterator.next() {
+        let (lower, _) = iterator.size_hint();
+        self.reserve(lower);
+        // rust-lang/rust#59605: `for_each` call leverages specialized
+        // override, if any, of `for_each` or `fold` for Iterator `I`.
+        iterator.for_each(|element| {
             let len = self.len();
-            if len == self.capacity() {
-                let (lower, _) = iterator.size_hint();
-                self.reserve(lower.saturating_add(1));
-            }
+            if len == self.capacity() { self.reserve(1); }
             unsafe {
                 ptr::write(self.get_unchecked_mut(len), element);
                 // NB can't overflow since we would have had to alloc the address space
                 self.set_len(len + 1);
             }
-        }
+        });
     }
 
     /// Creates a splicing iterator that replaces the specified range in the vector