Rewriting huge parts of the IDL compiler for Nucom, my implementation of Microsoft’s COM binary interface standard and (in the future) network protocol. The original version was hacked together with a lot of assumptions made in the parser and isn’t very extensible, and I do need to extend it now.

(Nucom is another way to have a stable ABI for Rust, e.g. for a plugin interface, and for Rust/C++ (and more OOP-style C) interop, based on objects with vtable pointers. And hopefully sometime in the future, transparent IPC and networking so you don’t have to load plugins into your process’s memory space if they don’t need to be there, with it working the same as if it were direct calls. All of this I assume you can already do on Windows with the windows-rs crate but it's obviously Windows-only.)

I do have to say though, I need a better way of transforming my syntax tree. Right now I’m just copying the struct definitions over and over for each compile stage and manually writing code to copy everything instead of just the parts I’m actually transforming, and it just seems like there has to be a better way. I might want another proc macro for this.