Hyphenate cargo cfgs
diff --git a/src/gen_helper.rs b/src/gen_helper.rs
index 97a6cec..b981f32 100644
--- a/src/gen_helper.rs
+++ b/src/gen_helper.rs
@@ -123,7 +123,7 @@
     }
 }
 
-#[cfg(feature = "visit_mut")]
+#[cfg(feature = "visit-mut")]
 pub mod visit_mut {
     use proc_macro2::Span;
     use visit_mut::VisitMut;