error: `darth` already exists, having another argument having almost the same name makes code comprehension and documentation more difficult
 --> $DIR/duplicate_underscore_argument.rs:7:23
  |
7 | fn join_the_dark_side(darth: i32, _darth: i32) {}
  |                       ^^^^^
  |
  = note: `-D duplicate-underscore-argument` implied by `-D warnings`