diff --git a/src/file_lines.rs b/src/file_lines.rs
index c6c4cc3e597..1e151a57838 100644
--- a/src/file_lines.rs
+++ b/src/file_lines.rs
@@ -222,7 +222,7 @@ impl ::serde::ser::Serialize for FileLines {
     fn serialize<S>(&self, _: S) -> Result<S::Ok, S::Error>
         where S: ::serde::ser::Serializer
     {
-        panic!("FileLines cannot be serialized. This is a rustfmt bug.");
+        unreachable!("FileLines cannot be serialized. This is a rustfmt bug.");
     }
 }