Why would you want to compile rust to lua ? because you may want to run some rust where the only interface is a Lua environnement. I have no clue if it's possible to map enough of the Rust MIR to lua or if a direct Rust to Lua transpiler is the better route, we will see
All the scaffolding for building a codegen backend is taken from rustc_codegen_cranelift.
To build the backend you can do
rustup component add --toolchain nightly rust-src rustc-dev llvm-tools-preview
./y.rs prepare
./y.rs build
And then you can user $cg_lua_dir/build/cargo to build using the codegen backend