Question 1: is it truely worth trying to use the same syntax tree/parsing code for the compiler as for IDEs, rustfmt, etc.? As matklad says, requirements differ quite a bit.
Question 2: though LLVM is the only option (as far as I am aware) available in the near future to generate well-optimised code, is it the best option for the typical edit-compile-run testing/dev cycle? There’s already been talk of using Cretonne as an alternative backend. (Of course, speed ups to LLVM builds are still welcome.)