What's the target platform? Looks like it compiles to system-executable, but Mozilla was involved, which makes me wonder if there's browser execution.
It may be that Rust is being designed as if that were a goal, but let's not start any rumors here.
Of course, if it turns out to work amazingly well and they can port all their old code (or interface with it) with relative ease, there's no reason not to jump ship from the hellfires of C++ compilation. They've had major issues with that lately, after all.