warning: struct is never used: `S`
  --> $DIR/macro-span-replacement.rs:15:9
   |
15 |         $b $a;
   |         ^^^^^^
...
20 |     m!(S struct);
   |     ------------- in this macro invocation
   |
note: lint level defined here
  --> $DIR/macro-span-replacement.rs:11:9
   |
11 | #![warn(unused)]
   |         ^^^^^^
   = note: #[warn(dead_code)] implied by #[warn(unused)]