Callback based API for declaring macro implementations about language

[jakemac53]

I prefer the current approach (unsurprisingly, because I designed it 🤣 ), here are some reasons:

• How does a macro get a "macro context"? Presumably, this essentially adds a 4th phase (for registering which phases you want to run in), with additional round trips and overhead (to register your macro in each phase) that isn't required with the current approach.
• Knowing all the phases a macro wants to operate in ahead of time allows you to more easily only run the necessary phases. As an example, the first phase cannot do introspection and never needs to be re-ran unless the current file changes. If you have to (or have the ability to) register which later phases you are going to run in during that phase, then it is more challenging to avoid redoing this work.
• The callback approach encourages retaining state between phases which we do not want macros to do. The (const) class based approach means you can only share global state, and won't implicitly capture local state like a callback would. In particular, objects retrieved from macro APIs are only valid in the current zone, attempting to cache them and use them in a later phase will cause problems.
• Similarly the callback approach is more prone to memory leaks - these callbacks have to be retained potentially indefinitely (especially if you want to try and avoid re-running earlier phases). And it is very easy for them to retain a bunch of state that isn't actually necessary, or shouldn't have been retained.
• I don't believe it actually makes the code meaningfully better. It won't be significantly shorter (the callbacks will be essentially the same as the implemented methods), the class header is short and replaces the explicit register calls.
• When written as a class the tooling is better because you write the interfaces you want to implement, and the tooling will create the stubs for you to fill in.
• Classes also enable the use of mixins etc which makes them easily composable.

[jakemac53]

I am going to close this one as not planned.