Improve error message

Author: petrochenkov

src/libsyntax/feature_gate.rs

@@ -867,6 +867,8 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                 self.context.span_handler.span_err(span, "empty tuple structs and enum variants \
                                                           are not allowed, use unit structs and \
                                                           enum variants instead");
+                self.context.span_handler.span_help(span, "remove trailing () to make a unit \
+                                                           struct or unit enum varian");
             }
         }
         visit::walk_struct_def(self, s)

src/test/compile-fail/issue-12560-1.rs

@@ -14,7 +14,9 @@
 
 enum Foo {
     Bar(), //~ ERROR empty tuple structs and enum variants are not allowed
+    //~^ HELP remove trailing () to make a unit struct or unit enum varian
     Baz(), //~ ERROR empty tuple structs and enum variants are not allowed
+    //~^ HELP remove trailing () to make a unit struct or unit enum varian
     Bazar
 }