You can use a different path, but then you don't have access to the binary cache, because of the mismatch between your path and the path found in the cached binaries. That's how I understand it.
Perhaps they hardcode full paths into whatever Nix calls its formula files (which are then, if I understand correctly, referred to by their cryptographic hashes, meaning that a file with a different path would have a different hash, and so break the whole hierarchy of formula-to-formula dependency references above it.)
From looking at PR seems that it will utilize some extra space (for derivations that aren't really OS specific) and there might be problems wih deploying from os x to linux, but feels like they are planning to go that route ultimately.
> The main consequence of using a separate prefix for macOS is that you can't have Hydra jobsets anymore containing jobs for macOS and Linux. It would also make it harder to deploy from macOS to Linux.
i.e., if the same package builds for both Linux and macOS, you can't write a single shell script that runs on both Linux Nix and macOS Nix and calls that package? Maybe I'm misunderstanding how people deploy things, but is that a huge problem in practice?
(I don't quite follow the thing about Hydra jobsets. Aren't you building once for Linux and once for macOS anyway? Figuring out how to make the build do that seems like it would subsume any complexity of using /nix vs. /opt/nix.)