Temporary fix for rustc warning false-positive?

Author: mcarton

tests/compile-fail/derive.rs

@@ -3,7 +3,7 @@
 
 #![deny(warnings)]
 #![allow(dead_code)]
-#![allow(unused_variables)]
+#![allow(unused_variables)] // Temporary fix for rustc false positive. To be removed.
 
 use std::hash::{Hash, Hasher};