[Demo] splice.nix: poor man's implementation of Gentoo-like use-flags

[oxij]

What? Why?

See #56110.

git log

• splice.nix: poor man's implementation of Gentoo-like use-flags

  Now, instantiate mpv, add

  poorMansUseFlags = {
    pulseaudioSupport = false;
  };
  

  (Sure, there is pulseaudio option already, I wanted to propose to set x11Support, but setting it breaks evaluation ATM...)

  Instantiate mpv again. Compare drvs.

nix-instantiate environment

• Host OS: Linux 4.9, SLNOS 19.03
• Nix: nix-env (Nix) 2.1.3
• Multi-user: yes
• Sandbox: yes
• NIXPKGS_CONFIG:

{
  checkMeta = true;
  doCheckByDefault = true;
}

nix-env -qaP diffs

• On x86_64-linux: noop
• On aarch64-linux: noop
• On x86_64-darwin: noop

[nixos-discourse]

This pull request has been mentioned on Nix community. There might be relevant details there:

https://discourse.nixos.org/t/gentoo-like-use-flags/2210/1

[danbst]

On a second thought, this seems to be basic needed change. Just replace poorMansUseFlags with use. Then nixpkgs.config.use will override scope, not pkgs (which can be overriden with packageOverrides or overlays). The only minor issue is the use if (//), which isn't recursive, but well I don't know if this is needed here.

The actual config.use then can be constructed on-the-site - no need to upstream useDefinitions, but still possible to share. As Hydra problem and configuration space explosion problems don't yet have a nice solution, I doubt actual use flags can be upstreamed here.

The consensus on naming things has indeed to go through RFC. But what is nice about this PR, it doesn't need that consensus, so I guess this (this idea, I mean) can be merged without RFC.

We can also cc Graham C. on whether RFC is required for this.

[oxij]

Closed in favor of #56227.

[oxij]

@danbst Yes, this is the crux of #56158 and #56227, but the whole point of use-flags is UI, and this has a very bad one. #56227 is also pretty awesome, IMHO.