warning: unused `std::result::Result` that must be used
  --> $DIR/must-use-in-macro-55516.rs:9:5
   |
LL |     write!(&mut example, "{}", 42);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `-W unused-must-use` implied by `-W unused`
   = note: this `Result` may be an `Err` variant, which should be handled
   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)