Use an underscore instead of an attribute

Author: topecongiro

src/expr.rs

@@ -2949,8 +2949,7 @@ impl<'a> ToExpr for ast::StructField {
         None
     }
 
-    #[allow(unused_variables)]
-    fn can_be_overflowed(&self, context: &RewriteContext, len: usize) -> bool {
+    fn can_be_overflowed(&self, _: &RewriteContext, _: usize) -> bool {
         false
     }
 }