Can't you just keep using Mold2 for the initial toolchain bootstrap and then build mold3 with Rust as part of the "final" set of toolchains that are to be used to compile everything beyond that? It isn't perfect but it's congruent with how many other components in most open reproducible bootstrapping flows work; ie using GCC 4.4 or whatever just to bootstrap GCC 10. It's annoying but it only needs to be done once and then you just use it to bootstrap newer components forever. Doesn't help with compile times, though.