How long are all the rust builds in your distro? If the compiler is 20% faster (my LLVM Clang results), you "just" need 30 hours of building time to get even, and I'm sure there is enough rust around (maybe not all in your distro) to make this worthwhile.
In any case, people could be using the stage2 or stage3 binary and still get the same binaries in the end, they are just alternative versions of the same package that should be compatible.
And using a 192 core CPU is a bit futile, have you looked at how well the work can parallelize? Isn't there some good remote worker solution you could use to have more cores available for the whole fleet without locking a 192 cores machine for a single build? It doesn't look very efficient to me as it is.
Building from zero to rust is about 6 hours on my 192 core local workstation. The extra cores absolutely help. Each linux kernel takes 16 seconds. Rust still takes a few hours. And on a typical laptop, a day or more.
Building rust that far to build mold to then build rust again faster negates any benefits.
In any case, people could be using the stage2 or stage3 binary and still get the same binaries in the end, they are just alternative versions of the same package that should be compatible.
And using a 192 core CPU is a bit futile, have you looked at how well the work can parallelize? Isn't there some good remote worker solution you could use to have more cores available for the whole fleet without locking a 192 cores machine for a single build? It doesn't look very efficient to me as it is.