Pre-RFC: The `#[diagnostic]` attribute namespace and the `#[diagnostic::on_unimplemented]` attribute

[weiznich]

@CAD97

A notable thing this can do which the OP RFC cannot is actually not provide any error message customization in the general case, only providing it for specific cases. It does mean duplicating some text (e.g. the message here) if it should be the same in all cases, but that might be fine tbh.

That's a good suggestion. That will also allow to easily put certain parts of the attribute behind a feature flag. I would expect that to be useful for cases where a certain type is only available behind a feature flag or just for cases where a certain impl is behind a feature flag. I will in cooperate that into the final RFC before opening a PR.

@ekuber

The big benefit of being able to annotate types with on_unimplemented or something similar is being able to customize errors for foreign traits, like extending std::fmt::Display if called on a type from my crate where you should have called .to_string_lossy() instead.

On the other hand you would loose the ability to emit always some error message for a specific trait. I think both use-cases might be important. Maybe add it to future work to allow the attribute in more places? Possibly on types?

@semicoleon

It would be very reasonable to not open that can of worms for an initial feature, but it would probably be good to state that in the RFC. Something like that might also require the ability to introduce type parameters in filters which might need to be considered in the syntax of the initial feature.

That's a good suggestion. I will add that to the "Future possibilities" section.