Fix the new unused fields in tuple structs detection

Author: saethlin

src/set.rs

@@ -2834,6 +2834,7 @@ mod test_set {
     fn test_replace() {
         use core::hash;
 
+        #[allow(dead_code)] // Having an unused field is the point of this test
         #[derive(Debug)]
         struct Foo(&'static str, i32);