In short, the entire distro is always built in one-shot at any given commit, and we can only rely on cached binaries from a past release if they or nothing in their supply chains changed. Given rustc depends on almost everything, mold3 would be built far too late to be useful for the most expensive build in the whole tree, which is rust.
Recursive dependencies would break our threat model, so we cannot use any rust tools to bootstrap rust. We bootstrap rust from llvm which we bootstrap from gcc which we bootstrap from tinycc which we bootstrap from M2Planet and so on back to 186 bytes of machine code.
Anyone getting a rust package from stagex must be able to build the entire tree up until that package and get the same hash, with no binary dependencies, thus removing any trust in maintainers.
This is entirely self-inflicted on your part!
> Rust is not actually the right tool for all problems.
Rust is certainly the right tool for this problem, your own decisions notwithstanding.
What _is_ your threat model?
If you can boostrap one version of stagex (so that its mold is trustworthy), I don't see why you can't use that to build another version of stagex.
As you say, you can rely on the cached binary because nothing in the old version's supply chain would change; it is pinned, and as long as you can build that, everything is fine.