nixpkgs: rebase module on latest NixOS

This is based on the current NixOS `nixpkgs` module, adjusted for the
nix-darwin context and without adding the options due for deprecation
in NixOS.

This gives us the ability to set the package set modularly through
`nixpkgs.pkgs` and builds up infrastructure for handling user-specified
Nixpkgs instantiations more robustly.

The cross-compilation options are currently not very useful due to
even Darwin->Darwin cross-compilation not being wholly functional
yet, but it looks feasible to build an `aarch64-darwin` system from
`x86_64-darwin` with some patching and it should be possible to make
cross-compilation more widely supported after the Darwin SDK situation
in Nixpkgs improves.

One casualty is the error for setting `nixpkgs.*` options when
overriding the package set. That could be ported over to this new
scheme, but it'd increase divergence with the NixOS module and reduce
cross-compatibility of configurations, so I lean towards adding it
upstream to NixOS if anything. (But if people want to keep it I can add
it back.)

1 files changed, 3 insertions, 9 deletions

diff --git a/eval-config.nix b/eval-config.nix
index 4b71303..2beb121 100644
--- a/eval-config.nix
+++ b/eval-config.nix
@@ -26,15 +26,9 @@ let
   pkgsModule = { config, inputs, ... }: {
     _file = ./eval-config.nix;
     config = {
-      assertions = [ {
-        # Ensure that nixpkgs.* options are not set when pkgs is set
-        assertion = pkgs == null || (config.nixpkgs.config == { } && config.nixpkgs.overlays == [ ]);
-        message = ''
-          `nixpkgs` options are disabled when `pkgs` is supplied through `darwinSystem`.
-        '';
-      } ];
-
-      _module.args.pkgs = if pkgs != null then pkgs else import inputs.nixpkgs config.nixpkgs;
+      _module.args.pkgs = lib.mkIf (pkgs != null) (lib.mkForce pkgs);
+
+      nixpkgs.source = lib.mkDefault inputs.nixpkgs;
 
       # This permits the configuration to override the passed-in
       # system.