Like I said, the success of the project doesn't depend on the feasibility of rewriting HarfBuzz. We're quite pragmatic about using external libraries; in fact, Rust's FFI is the best FFI I've ever used in any non-C/C++ language. Robert's point is simply that the end result will be better if we can make a memory-safe, parallel HarfBuzz along the way, written in a language that improves programmer productivity.