warning: struct `S` is never constructed
  --> $DIR/macro-span-replacement.rs:12:8
   |
LL |     m!(S struct);
   |        ^
   |
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 the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: 1 warning emitted