New format for E0426

Issue #35799 as a part of issue #35233
?r jonathandturner

src/librustc_resolve/lib.rs

@@ -390,11 +390,13 @@ fn resolve_struct_error<'b, 'a: 'b, 'c>(resolver: &'b Resolver<'a>,
             err
         }
         ResolutionError::UndeclaredLabel(name) => {
-            struct_span_err!(resolver.session,
-                             span,
-                             E0426,
-                             "use of undeclared label `{}`",
-                             name)
+            let mut err = struct_span_err!(resolver.session,
+                                           span,
+                                           E0426,
+                                           "use of undeclared label `{}`",
+                                           name);
+            err.span_label(span, &format!("undeclared label `{}`",&name));
+            err
         }
         ResolutionError::SelfImportsOnlyAllowedWithin => {
             struct_span_err!(resolver.session,

src/test/compile-fail/E0426.rs

@@ -10,6 +10,8 @@
 
 fn main () {
     loop {
-        break 'a; //~ ERROR E0426
+        break 'a;
+        //~^ ERROR E0426
+        //~| NOTE undeclared label `'a`
     }
 }