warning: struct is never constructed: `S`
  --> $DIR/macro-span-replacement.rs:7:14
   |
LL |         $b $a;
   |              ^
...
LL |     m!(S struct);
   |     ------------- in this macro invocation
   |
note: the lint level is defined here
  --> $DIR/macro-span-replacement.rs:3:9
   |
LL | #![warn(unused)]
   |         ^^^^^^
   = note: `#[warn(dead_code)]` implied by `#[warn(unused)]`
   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)