diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index c7c561156e3..27e9f817894 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -437,10 +437,10 @@ impl IntEncodedWithFixedSize {
 impl Encodable<FileEncoder> for IntEncodedWithFixedSize {
     #[inline]
     fn encode(&self, e: &mut FileEncoder) {
-        let _start_pos = e.position();
+        let start_pos = e.position();
         e.write_array(self.0.to_le_bytes());
-        let _end_pos = e.position();
-        debug_assert_eq!((_end_pos - _start_pos), IntEncodedWithFixedSize::ENCODED_SIZE);
+        let end_pos = e.position();
+        debug_assert_eq!((end_pos - start_pos), IntEncodedWithFixedSize::ENCODED_SIZE);
     }
 }