Fixed a compilefail test for flatten on enums

serde_derive_internals/src/check.rs

@@ -45,9 +45,7 @@ fn check_getter(cx: &Ctxt, cont: &Container) {
 fn check_flatten(cx: &Ctxt, cont: &Container) {
     match cont.data {
         Data::Enum(_) => {
-            if cont.attrs.has_flatten() {
-                cx.error("#[serde(flatten)] is not supported on enums");
-            }
+            debug_assert!(!cont.attrs.has_flatten());
         }
         Data::Struct(_, _) => {
             for field in cont.data.all_fields() {

test_suite/tests/compile-fail/enum-representation/flatten-enum.rs

@@ -10,7 +10,7 @@
 extern crate serde_derive;
 
 #[derive(Serialize)] //~ ERROR: proc-macro derive panicked
-//~^ HELP: #[serde(flatten] is not supported on enums
+//~^ HELP: unknown serde variant attribute `flatten`
 enum Foo {
     #[serde(flatten)]
     Foo {