diff --git a/syntax/impls.rs b/syntax/impls.rs
index acc99abd..a2ce05b2 100644
--- a/syntax/impls.rs
+++ b/syntax/impls.rs
@@ -1,7 +1,7 @@
 use crate::syntax::{ExternFn, Receiver, Ref, Signature, Slice, Ty1, Type};
 use std::hash::{Hash, Hasher};
 use std::mem;
-use std::ops::Deref;
+use std::ops::{Deref, DerefMut};
 
 impl Deref for ExternFn {
     type Target = Signature;
@@ -11,6 +11,12 @@ impl Deref for ExternFn {
     }
 }
 
+impl DerefMut for ExternFn {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.sig
+    }
+}
+
 impl Hash for Type {
     fn hash<H: Hasher>(&self, state: &mut H) {
         mem::discriminant(self).hash(state);