Update E0005 to use a label

Author: pythoneer

src/librustc_const_eval/check_match.rs

@@ -1073,11 +1073,12 @@ fn check_irrefutable(cx: &MatchCheckCtxt, pat: &Pat, is_fn_arg: bool) {
     };
 
     is_refutable(cx, pat, |uncovered_pat| {
-        span_err!(cx.tcx.sess, pat.span, E0005,
+        let pattern_string = pat_to_string(uncovered_pat);
+        struct_span_err!(cx.tcx.sess, pat.span, E0005,
             "refutable pattern in {}: `{}` not covered",
             origin,
-            pat_to_string(uncovered_pat),
-        );
+            pattern_string,
+        ).span_label(pat.span, &format!("pattern `{}` not covered", pattern_string)).emit();
     });
 }