bug: nix.package conflict with nix-darwin & NixOS modules

[sellout]

Are you following the right branch?

• My Nixpkgs and Home Manager versions are in sync

Is there an existing issue for this?

• I have searched the existing issues

Issue description

The nix module requires that nix.package is set (it would be helpful if the description explained why it doesn’t have a default), but the Home Manager modules for nix-darwin and NixOS set nix.package unconditionally.

I share most of my Home Manager configuration between multiple accounts and multiple machines. I had been always using home-manager explicitly, but decided to switch to the nix-darwin and NixOS modules for systems that I administer. So my configuration is loaded all three ways – home-manager, darwin-rebuild, and nixos-rebuild.

Since I have nix.packages = pkgs.nix in my shared config, this now errors when I use Home Manager as a module:

error: The option `home-manager.users.greg.nix.package' is defined multiple times while it's expected to be unique.

Definition values:
- In `<unknown-file>': <derivation nix-2.18.5>
- In `/nix/store/hxhym8c5xz6dxkl3d9yppiwlnzk3khn7-source/nixos/common.nix': <derivation nix-2.18.5>
Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

The error makes sense, but I think the fix should be in Home Manager somewhere. There are a few options, in my order of preference:

1. make pkgs.nix the default value in modules/misc/nix.nix, which makes it less likely that a user will override it, and if they do, they can decide how they want to manage this issue;
2. have nixos/common.nix use lib.mkDefault, allowing the user’s setting to take precedence over the system module; or
3. have nixos/common.nix use lib.mkForce, ensuring that the system modue’s version always takes precedence.

Number 1 seems like the best option to me, but considering the default is explicitly set to null, I figure there’s some reason it’s not already done that way. Number 3 seems a bit ill-behaved, in that it will override what a user explicitly put in their config, so I don’t really like that. But again, the documentation doesn’t make it clear why this situation exists.

My workaround is to change nix.package = pkgs.nix to nix.package = lib.mkDefault pkgs.nix, and if that is the correct solution, it would be helpful if the docs were more descriptive.

(BTW, the channels listed by nix-info below make no sense to me. I do everything with flakes, and when I run nix-channel --list, there are no channels listed.)

Maintainer CC

I see no explicit mantainers on either modules/misc/nix.nix or nixos/common.nix, but @rycee (unsurprisingly) touched both recently.

System information

 - system: `"aarch64-darwin"`
 - host os: `Darwin 23.6.0, macOS 14.6.1`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.18.5`
 - channels(root): `"darwin, home-manager-20.09.tar.gz, nixpkgs-21.11-darwin, unstable"`
 - channels(greg): `"darwin, home-manager-22.05.tar.gz, nixpkgs-22.05-darwin, unstable"`
 - nixpkgs: `/nix/store/4a7cgzcnwyr0s82r3mk6gz0fngkdyvrx-source`