fix nix_path for darwin-option

flake.nix

@@ -32,7 +32,7 @@
         pkgs = throw "nixpkgs eval";
       };
 
-      home-manager-config = {
+      user-config = {
         home-manager.useGlobalPkgs = true;
         home-manager.useUserPackages = true;
         home-manager.extraSpecialArgs = { inherit nurNoPkgs pkgs; inputs = { inherit (inputs); }; };
@@ -49,11 +49,12 @@
     {
       darwinConfigurations = {
         zeus = darwin.lib.darwinSystem {
-          inherit system pkgs;
+          inherit system pkgs inputs;
           modules = [
+            ./modules/nix.nix
             ./system/darwin.nix
             home-manager.darwinModules.home-manager
-            home-manager-config
+            user-config
           ];
         };
       };

modules/nix.nix

@@ -0,0 +1,83 @@
+# https://github.com/LnL7/nix-darwin/issues/277#issuecomment-992866471
+
+{ config, lib, pkgs, inputs, ... }:
+
+with lib;
+
+let
+  cfg = config.nix;
+
+  nixRegistry = builtins.mapAttrs (name: value: { flake = value; }) inputs;
+  etcNixInputs = pkgs.runCommandNoCC "etc-nix-inputs"
+    {
+      inputNames = builtins.attrNames inputs;
+      inputPaths = builtins.map (x: x.outPath) (builtins.attrValues inputs);
+    } ''
+    mkdir -p $out
+    inputNames=($inputNames)
+    inputPaths=($inputPaths)
+    for (( i=0; i<''${#inputNames[@]}; i++)); do
+      source=''${inputPaths[$i]}
+      name=''${inputNames[$i]}
+      if [[ -f $source/default.nix ]]; then
+        ln -s $source $out/$name
+      fi
+    done
+  '';
+in
+
+# Based on flake-utils-plus#nixosModules.autoGenFromInputs
+  # https://github.com/gytis-ivaskevicius/flake-utils-plus/blob/master/lib/options.nix
+  #
+  # We're not using that directly because we don't need the rest of the flake, and to work around
+  # https://github.com/gytis-ivaskevicius/flake-utils-plus/issues/105 and
+  # https://github.com/gytis-ivaskevicius/flake-utils-plus/issues/107
+
+{
+  options = {
+    nix.generateNixPathFromInputs = mkOption {
+      type = types.bool;
+      description = ''
+        If set, NIX_PATH will be generated from available inputs.
+        This requires `nix.linkInputs` to be enabled, and setting this will default
+        `nix.linkInputs` to true.
+      '';
+      default = false;
+      example = true;
+    };
+    nix.generateRegistryFromInputs = mkOption {
+      type = types.bool;
+      description = ''
+        If set, the system Nix registry will be generated from available inputs.
+        Otherwise, the registry will still include the `self` flake.
+      '';
+      default = false;
+      example = true;
+    };
+    nix.linkInputs = mkOption {
+      type = types.bool;
+      description = "If set, inputs will be symlinked into /etc/nix/inputs.";
+      example = true;
+    };
+  };
+
+  config = {
+    assertions = [{
+      assertion = cfg.generateNixPathFromInputs -> cfg.linkInputs;
+      message = "nix.generateNixPathFromInputs requires nix.linkInputs";
+    }];
+
+    nix.linkInputs = mkDefault cfg.generateNixPathFromInputs;
+
+    nix.registry =
+      if cfg.generateRegistryFromInputs
+      then nixRegistry
+      else { self.flake = inputs.self; };
+
+    environment.etc."nix/inputs" = mkIf cfg.linkInputs {
+      source = etcNixInputs;
+    };
+
+    nix.nixPath = mkIf cfg.generateNixPathFromInputs [ "/etc/nix/inputs" ];
+  };
+}

system/darwin.nix

@@ -3,7 +3,16 @@
 {
   environment.systemPackages = with pkgs; [ ];
 
-  nix.package = pkgs.nix;
+  nix = {
+    extraOptions = ''
+      experimental-features = nix-command flakes
+      extra-trusted-users = willem
+    '';
+    generateRegistryFromInputs = true;
+    generateNixPathFromInputs = true;
+    linkInputs = true;
+    package = pkgs.nix;
+  };
 
   programs.bash.enable = true;