Rollup merge of rust-lang#34305 - Aaronepower:master, r=alexcrichton

Added Default trait for Cow.

Adds a default implementation for Cow. Which is the Owned's default.

src/libcollections/borrow.rs

@@ -15,6 +15,7 @@
 use core::clone::Clone;
 use core::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
 use core::convert::AsRef;
+use core::default::Default;
 use core::hash::{Hash, Hasher};
 use core::marker::Sized;
 use core::ops::Deref;
@@ -248,6 +249,16 @@ impl<'a, B: ?Sized> fmt::Display for Cow<'a, B>
     }
 }
 
+#[stable(feature = "default", since = "1.11.0")]
+impl<'a, B: ?Sized> Default for Cow<'a, B>
+    where B: ToOwned,
+          <B as ToOwned>::Owned: Default
+{
+    fn default() -> Cow<'a, B> {
+        Owned(<B as ToOwned>::Owned::default())
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, B: ?Sized> Hash for Cow<'a, B> where B: Hash + ToOwned {
     #[inline]