Add stability attribute

src/libcore/option.rs

@@ -1064,6 +1064,7 @@ impl<T> From<T> for Option<T> {
     }
 }
 
+#[stable(feature = "option_ref_from_ref_option", since = "1.30.0")]
 impl<'a, T> From<&'a Option<T>> for Option<&'a T> {
     fn from(o: &'a Option<T>) -> Option<&'a T> {
         o.as_ref()