One issue with using procedural macros to implement more aggressive changes to the Rust language is that macros are inherently local. There was some discussion in the past of enabling more direct customization of the compiler, e.g. here.
I’m wondering, now that procedural macros have landed, is there any chance of this being on the table again? If it is I’d be happy to write a Rust 2019 post about what I’d use it for if it were available.