There's parts of LLVM architecture that are long in the tooth (IMO) (as is the language it's implemented in, IMO).

I had hoped one day to re-implement parts of LLVM itself in Rust; in particular, I've been curious if we can concurrently compile C (and parse C in parallel, or lazily) that haven't been explored in LLVM, and I think might be safer to do in Rust. I don't know enough about grammers to know if it's technically impossible, but a healthy dose of ignorance can sometimes lead to breakthroughs.

LLVM is pretty well designed for test. I was able to implement a lexer for C in Rust that could lex the Linux kernel, and use clang to cross check my implementation (I would compare my interpretation of the token stream against clang's). Just having a standard module system makes having reusable pieces seems like perhaps a better way to compose a toolchain, but maybe folks with more experience with rustc have scars to disagree?