Coalton is great. Indeed often it's much easier to just implement on top of either SBCL or ChezScheme; they are very fast and chances are you are not going to make anything as or more performant, better documented etc. However, reimplementation in Rust is not too bad as that's were we need to be for small, fast, low level runtimes. If the goal was 'just' their own language, I feel just a an implementation on top of CL/SBCL + Coalton would've been faster to build, easier to maintain and probably far more performant. Less fun though!