Set label for unreachable pattern

src/librustc_const_eval/check_match.rs

@@ -335,6 +335,7 @@ fn check_arms(cx: &MatchCheckCtxt,
                         hir::MatchSource::Normal => {
                             let mut err = struct_span_err!(cx.tcx.sess, pat.span, E0001,
                                                            "unreachable pattern");
+                            err.span_label(pat.span, &format!("this is an unreachable pattern"));
                             // if we had a catchall pattern, hint at that
                             for row in &seen.0 {
                                 if pat_is_catchall(&cx.tcx.def_map.borrow(), row[0].0) {

src/test/compile-fail/issue-31221.rs

@@ -22,13 +22,15 @@ fn main() {
         //~^ NOTE this pattern matches any value
         Var2 => (),
         //~^ ERROR unreachable pattern
+        //~^^ NOTE this is an unreachable pattern
     };
     match &s {
         &Var1 => (),
         &Var3 => (),
         //~^ NOTE this pattern matches any value
         &Var2 => (),
         //~^ ERROR unreachable pattern
+        //~^^ NOTE this is an unreachable pattern
     };
     let t = (Var1, Var1);
     match t {
@@ -37,6 +39,7 @@ fn main() {
         //~^ NOTE this pattern matches any value
         anything => ()
         //~^ ERROR unreachable pattern
+        //~^^ NOTE this is an unreachable pattern
     };
     // `_` need not emit a note, it is pretty obvious already.
     let t = (Var1, Var1);
@@ -45,5 +48,6 @@ fn main() {
         _ => (),
         anything => ()
         //~^ ERROR unreachable pattern
+        //~^^ NOTE this is an unreachable pattern
     };
 }