Welp, this turns out to be more work than I expected; I'm going to start over bottom-up rather than top-down.

Hey, thanks for working on this! Some things are okay to update, like command line tools, but other things are important to leave older, like libdispatch or Libsystem. We want to support older macOS systems and that means using the older headers.

There are some major changes to this that are included in #56744 and now in staging-next. It might be a good idea to base off of those.

Hey, 10.12's Libc actually has the header that led me down this whole path in the first place, so that's great. I definitely underestimated the complexity here 🤯.

I might continue to plug away at this just to learn the system better—it's looking like we're likely to be deploying nix to 500–1000 macbooks and I'd like to have a handle on how this part of it works.

Yeah we definitely need more visibility and documentation. The difference between "bootstrapping" things like Libsystem and the others are not clearly marked from the less critical apps.

Being stuck on 10.10 for so long with the unavailability of CF was definitively becoming painful. The stdenv is pretty complex because we have to deal with impurities that are out of control of nix as well as the fact that we want binaries to be backwards compatible with (some) previous releases.