Hyphenate cargo cfgs
diff --git a/src/lib.rs b/src/lib.rs
index 937953c..952af8e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -391,7 +391,7 @@
     #[cfg(feature = "visit")]
     pub mod visit;
 
-    #[cfg(feature = "visit_mut")]
+    #[cfg(feature = "visit-mut")]
     pub mod visit_mut;
 
     #[cfg(feature = "fold")]