rollup merge of #19359: japaric/clone-cow

Now we can use `#[deriving(Clone)]` on structs that contain `Cow`.

r? @aturon or anyone else

src/libcore/borrow.rs

@@ -137,6 +137,18 @@ pub enum Cow<'a, T, Sized? B: 'a> where B: ToOwned<T> {
     Owned(T)
 }
 
+impl<'a, T, Sized? B> Clone for Cow<'a, T, B> where B: ToOwned<T> {
+    fn clone(&self) -> Cow<'a, T, B> {
+        match *self {
+            Borrowed(b) => Borrowed(b),
+            Owned(ref o) => {
+                let b: &B = BorrowFrom::borrow_from(o);
+                Owned(b.to_owned())
+            },
+        }
+    }
+}
+
 impl<'a, T, Sized? B> Cow<'a, T, B> where B: ToOwned<T> {
     /// Acquire a mutable reference to the owned form of the data.
     ///