As someone that writes a lot in Rust, I have a longstanding dream to make a compiler for a subset of the language, since I don’t use all features of Rust and my hope is that this way I could have fast debug builds (in terms of compilation time) while I develop, with simplified borrow checker and so on. So then I can iterate faster. That’s one of my dreams. But I have not had time to even look at it yet, as the other code I work on takes precedence at the moment.

I think you would be better served by a simplified, less optimized code generation, as from what I understand LLVM is the thing that takes a lot of time. You can see this pattern in Crystal and Swift for example, that also use LLVM and are also relatively slow to compile, compared to OCaml or Go which have their own backend.