diff --git a/src/branch.rs b/src/branch.rs
index e4443574a..c7589ede6 100644
--- a/src/branch.rs
+++ b/src/branch.rs
@@ -198,6 +198,7 @@ macro_rules! alt (
 
           // Compile-time hack to ensure that res's E type is not under-specified.
           // This all has no effect at runtime.
+          #[allow(dead_code)]
           fn unify_types<T>(_: &T, _: &T) {}
           if let Err(Err::Error(ref e2)) = out {
             unify_types(&e, e2);