warning: unused `Result` that must be used
  --> $DIR/must-use-in-macro-55516.rs:9:5
   |
LL |     write!(&mut example, "{}", 42);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this `Result` may be an `Err` variant, which should be handled
   = note: `-W unused-must-use` implied by `-W unused`
   = help: to override `-W unused` add `#[allow(unused_must_use)]`
help: use `let _ = ...` to ignore the resulting value
   |
LL |     let _ = write!(&mut example, "{}", 42);
   |     +++++++

warning: 1 warning emitted