eii: support rustc builtin macro

[boozook]

Please support (implement) builtin attr-macro eii expansion.

[ChayimFriedman2]

I do not know a builtin macro named eii. May you link to it?

[boozook]

@ChayimFriedman2, that's extern_item_impls.

[ChayimFriedman2]

It's new, extremely experimental and seem to not even be implemented properly yet. It's also not trivial to implement (the easiest way I see is to emit a macro 2.0 attribute macro (another experimental feature that we do support), and it's still not trivial. I do not like implementing features at this stage and we generally don't do that, unless not supporting the feature has severe consequences and it's simple to implement, both things not true here.

[boozook]

Understood. 👌🏻
I'll be back for it someday.