Refer to Option through serde::export in generated code

test_suite/tests/test_gen.rs

@@ -522,6 +522,13 @@ fn test_gen() {
     }
     assert::<FlattenWith>();
 
+    #[derive(Serialize, Deserialize)]
+    #[serde(deny_unknown_fields)]
+    struct FlattenDenyUnknown<T> {
+        #[serde(flatten)]
+        t: T,
+    }
+
     #[derive(Serialize, Deserialize)]
     struct StaticStrStruct<'a> {
         a: &'a str,