error: unused return value of `f` that must be used
  --> $DIR/decorate-can-emit-warnings.rs:9:5
   |
LL |     f();
   |     ^^^
   |
   = note: requested on the command line with `-D unused-must-use`
help: use `let _ = ...` to ignore the resulting value
   |
LL |     let _ = f();
   |     +++++++

error: aborting due to 1 previous error