Implement RFC 839 for {HashMap, HashSet}

It appears that these impls were left out of #25989 by mistake.

src/libstd/collections/hash/map.rs

@@ -1583,6 +1583,14 @@ impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>
     }
 }
 
+#[stable(feature = "hash_extend_copy", since = "1.4.0")]
+impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S>
+    where K: Eq + Hash + Copy, V: Copy, S: HashState
+{
+    fn extend<T: IntoIterator<Item=(&'a K, &'a V)>>(&mut self, iter: T) {
+        self.extend(iter.into_iter().map(|(&key, &value)| (key, value)));
+    }
+}
 
 /// `RandomState` is the default state for `HashMap` types.
 ///
@@ -2347,4 +2355,20 @@ mod test_map {
             check(&m);
         }
     }
+
+    #[test]
+    fn test_extend_ref() {
+        let mut a = HashMap::new();
+        a.insert(1, "one");
+        let mut b = HashMap::new();
+        b.insert(2, "two");
+        b.insert(3, "three");
+
+        a.extend(&b);
+
+        assert_eq!(a.len(), 3);
+        assert_eq!(a[&1], "one");
+        assert_eq!(a[&2], "two");
+        assert_eq!(a[&3], "three");
+    }
 }

src/libstd/collections/hash/set.rs

@@ -654,6 +654,16 @@ impl<T, S> Extend<T> for HashSet<T, S>
     }
 }
 
+#[stable(feature = "hash_extend_copy", since = "1.4.0")]
+impl<'a, T, S> Extend<&'a T> for HashSet<T, S>
+    where T: 'a + Eq + Hash + Copy,
+          S: HashState,
+{
+    fn extend<I: IntoIterator<Item=&'a T>>(&mut self, iter: I) {
+        self.extend(iter.into_iter().cloned());
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, S> Default for HashSet<T, S>
     where T: Eq + Hash,
@@ -1325,4 +1335,32 @@ mod test_set {
         assert_eq!(it.next(), Some(&Foo("a", 2)));
         assert_eq!(it.next(), None);
     }
+
+    #[test]
+    fn test_extend_ref() {
+        let mut a = HashSet::new();
+        a.insert(1);
+
+        a.extend(&[2, 3, 4]);
+
+        assert_eq!(a.len(), 4);
+        assert!(a.contains(&1));
+        assert!(a.contains(&2));
+        assert!(a.contains(&3));
+        assert!(a.contains(&4));
+
+        let mut b = HashSet::new();
+        b.insert(5);
+        b.insert(6);
+
+        a.extend(&b);
+
+        assert_eq!(a.len(), 6);
+        assert!(a.contains(&1));
+        assert!(a.contains(&2));
+        assert!(a.contains(&3));
+        assert!(a.contains(&4));
+        assert!(a.contains(&5));
+        assert!(a.contains(&6));
+    }
 }