iter-mut.diff

diff --git a/src/libcollections/btree/map.rs b/src/libcollections/btree/map.rs
index 8c61b06f2b..b194798677 100644
--- a/src/libcollections/btree/map.rs
+++ b/src/libcollections/btree/map.rs
@@ -422,12 +422,16 @@ impl<'a, K: 'a, V: 'a> fmt::Debug for RangeMut<'a, K, V> {
     }
 }
 
-/*#[stable(feature = "collection_debug", since = "1.15.0")]
+#[stable(feature = "collection_debug", since = "1.15.0")]
 impl<'a, K: 'a + fmt::Debug, V: 'a + fmt::Debug> fmt::Debug for RangeMut<'a, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.debug_list().entries(self.clone()).finish()
+        let range = Range {
+            front: self.front.reborrow(),
+            back: self.back.reborrow(),
+        };
+        f.debug_list().entries(range).finish()
     }
-}*/
+}
 
 /// A view into a single entry in a map, which may either be vacant or occupied.
 /// This enum is constructed from the [`entry`] method on [`BTreeMap`].