add unstable attribute

Author: eopb

src/libcore/num/mod.rs

@@ -4796,6 +4796,10 @@ pub enum IntErrorKind {
 
 impl ParseIntError {
     /// Outputs the detailed cause of parsing an integer failing.
+    #[unstable(feature = "int_error_matching",
+               reason = "it can be useful to match errors when making error messages \
+                         for integer parsing",
+               issue = "22639")]
     pub fn kind(self) -> IntErrorKind {
         self.kind
     }