Rollup merge of rust-lang#41191 - seanmonstar:spec-extend-vec-intoiter, r=alexcrichton

specialize Extend for Vec with IntoIter

Before, `vec.extend(&other_vec)` was quite a bit faster than `vec.extend(other_vec)`. This allows extending by consuming a vec to use the same code as extending from a slice.

Author: frewsxcv

src/libcollections/vec.rs

@@ -1681,7 +1681,7 @@ impl<T, I> SpecExtend<T, I> for Vec<T>
         vector
     }
 
-    fn spec_extend(&mut self, iterator: I) {
+    default fn spec_extend(&mut self, iterator: I) {
         // This is the case for a TrustedLen iterator.
         let (low, high) = iterator.size_hint();
         if let Some(high_value) = high {
@@ -1728,6 +1728,14 @@ impl<T> SpecExtend<T, IntoIter<T>> for Vec<T> {
     }
 }
 
+impl<T> SpecExtend<T, IntoIter<T>> for Vec<T>
+    where T: Copy,
+{
+    fn spec_extend(&mut self, iterator: IntoIter<T>) {
+        self.spec_extend(iterator.as_slice());
+    }
+}
+
 impl<'a, T: 'a, I> SpecExtend<&'a T, I> for Vec<T>
     where I: Iterator<Item=&'a T>,
           T: Clone,