Add useful attributes

Author: eopb

src/libcore/num/mod.rs

@@ -4771,7 +4771,10 @@ pub struct ParseIntError {
     pub kind: IntErrorKind,
 }
 
+#[unstable(feature = "int_error_matching",
+           reason = "it can be useful to match errors when making error messages for integer parsing")]
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
 pub enum IntErrorKind {
     Empty,
     InvalidDigit,