emit a better error message for using the macro incorrectly
fixing: https://github.com/EnzymeAD/rust/issues/185
I feel like it's not a perfect message either, so I'm open to suggestions.
But at the end of the day users will need to read the docs anyway, and emitting
multi-line errors each time this gets triggered can probably become annoying?
r? ``@jieyouxu`` since you've reviewed my frontend work back in the days.
Tracking:
- https://github.com/rust-lang/rust/issues/124509
e8994abe52