Fill in missing cfgs
diff --git a/src/lib.rs b/src/lib.rs
index 30d46df..f73939e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -136,6 +136,7 @@
     #[cfg(feature = "fold")]
     pub mod fold;
 
+    #[cfg(any(feature = "full", feature = "derive"))]
     #[path = "../gen_helper.rs"]
     mod helper;
 }