The moonshot is fixing the way we deal with shared libraries. We pay an immense disk-space and bandwidth cost every time we rebuild a library which is depended upon by other packages. It is an insane cost. No other distro (except Guix) pays this cost, and it’s getting increasingly impractical.
I’ve said a bit more about how to fix this elsewhere but it’s still in the oven and not done cooking yet.
So long as its symbol table doesn’t change, modifying glibc shouldn’t trigger rebuilds of things downstream of it. It also shouldn’t change the contents of packages downstream of it, except for a tiny intermediate “indirection table”. The only thing that should change in the downstream outpaths is a pointer to that indirection table.
This goes way beyond CA derivations (but works best in combination with them) and better compression. It’s not the same thing as pkgs.replaceDependency but can be used instead of it if the dependency’s symbol table hasn’t changed.