nixos integration: add option 'useGlobalPkgs'

PR #1059

doc/installation.xml

@@ -225,6 +225,23 @@ home-manager.useUserPackages = true;
     become the default value in the future.
    </para>
   </note>
+
+  <note>
+   <para>
+    By default, Home Manager uses a private <literal>pkgs</literal> instance
+    that is configured via the <option>home-manager.users.&lt;name&gt;.nixpkgs</option> options.
+    To instead use the global <literal>pkgs</literal> that is configured via
+    the system level <option>nixpkgs</option> options, set
+   </para>
+<programlisting language="nix">
+home-manager.useGlobalPkgs = true;
+</programlisting>
+  <para>
+   This saves an extra Nixpkgs evaluation, adds consistency, and removes the
+   dependency on <envar>NIX_PATH</envar>, which is otherwise used for
+   importing Nixpkgs.
+   </para>
+  </note>
  </section>
  <section xml:id="sec-install-nix-darwin-module">
   <title>nix-darwin module</title>

modules/misc/news.nix

@@ -1351,6 +1351,21 @@ in
           '--description', '--on-event', and more.
         '';
       }
+
+      {
+        time = "2020-03-07T13:11:43+00:00";
+        condition = hostPlatform.isLinux;
+        message = ''
+          The NixOS module has a new option: 'home-manager.useGlobalPkgs'.
+
+          This enables using the system configuration's 'pkgs'
+          argument in Home Manager.
+
+          To learn more, see the installation section of the manual
+
+            https://rycee.gitlab.io/home-manager/#sec-install-nixos-module
+        '';
+      }
     ];
   };
 }

modules/modules.nix

@@ -5,6 +5,9 @@
 
   # Whether to enable module type checking.
 , check ? true
+
+# If disabled, the pkgs attribute passed to this function is used instead.
+, useNixpkgsModule ? true
 }:
 
 with lib;
@@ -27,7 +30,7 @@ let
     (loadModule ./misc/gtk.nix { })
     (loadModule ./misc/lib.nix { })
     (loadModule ./misc/news.nix { })
-    (loadModule ./misc/nixpkgs.nix { })
+    (loadModule ./misc/nixpkgs.nix { condition = useNixpkgsModule; })
     (loadModule ./misc/numlock.nix { condition = hostPlatform.isLinux; })
     (loadModule ./misc/pam.nix { })
     (loadModule ./misc/qt.nix { })
@@ -170,11 +173,14 @@ let
   modules = map (getAttr "file") (filter (getAttr "condition") allModules);
 
   pkgsModule = {
-    config._module.args.baseModules = modules;
-    config._module.args.pkgs = lib.mkDefault pkgs;
-    config._module.check = check;
-    config.lib = lib.hm;
-    config.nixpkgs.system = mkDefault pkgs.system;
+    config = {
+      _module.args.baseModules = modules;
+      _module.args.pkgs = lib.mkDefault pkgs;
+      _module.check = check;
+      lib = lib.hm;
+    } // optionalAttrs useNixpkgsModule {
+      nixpkgs.system = mkDefault pkgs.system;
+    };
   };
 
 in

nixos/default.nix

@@ -15,6 +15,7 @@ let
         imports = import ../modules/modules.nix {
           inherit pkgs;
           lib = extendedLib;
+          useNixpkgsModule = !cfg.useGlobalPkgs;
         };
 
         config = {
@@ -45,6 +46,12 @@ in {
         <option>users.users.‹name?›.packages</option> option.
       '';
 
+      useGlobalPkgs = mkEnableOption ''
+        using the system configuration's <literal>pkgs</literal>
+        argument in Home Manager. This disables the Home Manager
+        options <option>nixpkgs.*</option>
+      '';
+
       backupFileExtension = mkOption {
         type = types.nullOr types.str;
         default = null;