E0165 Update error format #35270

- Fixes #35270
- Part of #35233

r? @jonathandturner

src/librustc_const_eval/check_match.rs

@@ -324,7 +324,10 @@ fn check_arms(cx: &MatchCheckCtxt,
                             let &(ref first_arm_pats, _) = &arms[0];
                             let first_pat = &first_arm_pats[0];
                             let span = first_pat.span;
-                            span_err!(cx.tcx.sess, span, E0165, "irrefutable while-let pattern");
+                            struct_span_err!(cx.tcx.sess, span, E0165,
+                                             "irrefutable while-let pattern")
+                                .span_label(span, &format!("irrefutable pattern"))
+                                .emit();
                         },
 
                         hir::MatchSource::ForLoopDesugar => {

src/test/compile-fail/E0165.rs

@@ -13,6 +13,7 @@ struct Irrefutable(i32);
 fn main() {
     let irr = Irrefutable(0);
     while let Irrefutable(x) = irr { //~ ERROR E0165
+                                     //~| irrefutable pattern
         // ...
     }
 }