But we're just talking for bootstrapping Rust here.
The complaint is about bootstrapping, where speed is of little relevance.
It is of extreme relevance for us as we have to re-bootstrap every time we change any dependency of rust, and rust depends on almost everything that is expensive to build in a toolchain tree.
Use the mold binary from the last rebuild and only rebuild again if the new one is different?
If 80% of the tree is in the parent list for mold, the cache hit rate would be 20% - assuming the change distribution is even, which it won’t be.