The codebase is old and almost entirely in Lisp; parts of the runtime are in C, but they're also not the parts that are getting worked on, because they're not where the bugs are (because so much of it is in Lisp).

Unless Rust has gotten runtime reflection without me hearing about it, I don't think writing the compiler in Rust gives you any special powers in interfacing with Rust. You'd get a lot more of a win from parsing the DWARF of a library when you dlopen() it, and using the type information that exposes to be able to use the full Rust ABI rather than the C ABI.

EDIT: Actually, no, even with runtime reflection, that would only help an interpreter, not a compiler.

That still wouldn't give you the ability to perform template instantiations at runtime though -- for that, you'd need to ship the Rust compiler with your Lisp _applications_, since you might be able to invoke a Rust template with arguments at runtime that hadn't been seen before.

If you're looking at work on building a more modern Lisp compiler, check out SICL or Clasp, though as far as I'm aware, those are both early-ish projects.