Not the OP here, but AFAIK LLVM has been struggling with parallelizing compilation of independent functions due to some shared global context that ideally wouldn't be there. A full rewrite would allow a redesign of this part in a more concurrency-friendly way. So it's conceivable that a concurrency-oriented rewrite would bring nice speedups in wall time, not total CPU time. And Rust might give some more guarantees that there really aren't any hidden shared corners.
In Firefox there've been two failed attempts to make css styling parallel while using C++. Only the parallel rust rewrite, stylo, succeeded.
Rust doesn't replace careful planning of parallel infrastructure or performance optimization, but it makes it possible to maintain the parallel system.
I’m already running make in -j8 and running multiple invocations of clang in parallel, why would I want each one of them using up 800% CPU? Effort spent making a single invocation of clang saturate all my CPU cores is misguided IMO, and the wrong place to be doing parallelization.
