Produce error message for use of flatten within struct variant

test_suite/tests/compile-fail/conflict/flatten-within-enum.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: #[serde(flatten)] cannot be used within enums
+enum Foo {
+    A {
+        #[serde(flatten)]
+        fields: HashMap<String, String>,
+    }
+}
+
+fn main() {}