Fix used_underscore_binding in rustc_serialize

Author: practicalrs

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);
     }
 }