std: add default implementations to Option

Author: erickt

src/libstd/option.rs

@@ -43,6 +43,7 @@ let unwrapped_msg = match msg {
 
 use clone::Clone;
 use cmp::{Eq,Ord};
+use default::Default;
 use util;
 use num::Zero;
 use iter;
@@ -349,6 +350,10 @@ impl<T> Option<T> {
     }
 }
 
+impl<T> Default for Option<T> {
+    fn default() -> Option<T> { None }
+}
+
 impl<T:Zero> Option<T> {
     /// Returns the contained value or zero (for this type)
     #[inline]