nixos/specialisation: Rephrase in terms of extendModules, noUserModules

By using the new extendModules function to produce the specialisations,
we avoid reimplementing the eval-config.nix logic in reverse and fix
cross compilation support for specialisations in the process.

nixos/lib/eval-config.nix

@@ -51,23 +51,28 @@ let
     };
   };
 
-in rec {
-
-  # Merge the option definitions in all modules, forming the full
-  # system configuration.
-  inherit (lib.evalModules {
+  noUserModules = lib.evalModules {
     inherit prefix check;
-    modules = baseModules ++ extraModules ++ [ pkgsModule ] ++ modules;
+    modules = baseModules ++ extraModules ++ [ pkgsModule ];
     args = extraArgs;
     specialArgs =
       { modulesPath = builtins.toString ../modules; } // specialArgs;
-  }) config options _module type;
+  };
 
   # These are the extra arguments passed to every module.  In
   # particular, Nixpkgs is passed through the "pkgs" argument.
   extraArgs = extraArgs_ // {
-    inherit baseModules extraModules modules;
+    inherit noUserModules baseModules extraModules modules;
   };
 
+in rec {
+
+  # Merge the option definitions in all modules, forming the full
+  # system configuration.
+  inherit (noUserModules.extendModules { inherit modules; })
+    config options _module type;
+
+  inherit extraArgs;
+
   inherit (_module.args) pkgs;
 }

nixos/modules/system/activation/top-level.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, modules, baseModules, specialArgs, ... }:
+{ config, lib, pkgs, extendModules, noUserModules, ... }:
 
 with lib;
 
@@ -11,16 +11,10 @@ let
   # you can provide an easy way to boot the same configuration
   # as you use, but with another kernel
   # !!! fix this
-  children = mapAttrs (childName: childConfig:
-      (import ../../../lib/eval-config.nix {
-        inherit lib baseModules specialArgs;
-        system = config.nixpkgs.initialSystem;
-        modules =
-           (optionals childConfig.inheritParentConfig modules)
-        ++ [ ./no-clone.nix ]
-        ++ [ childConfig.configuration ];
-      }).config.system.build.toplevel
-    ) config.specialisation;
+  children =
+    mapAttrs
+      (childName: childConfig: childConfig.configuration.system.build.toplevel)
+      config.specialisation;
 
   systemBuilder =
     let
@@ -176,7 +170,11 @@ in
         </screen>
       '';
       type = types.attrsOf (types.submodule (
-        { ... }: {
+        local@{ ... }: let
+          extend = if local.config.inheritParentConfig
+            then extendModules
+            else noUserModules.extendModules;
+        in {
           options.inheritParentConfig = mkOption {
             type = types.bool;
             default = true;
@@ -185,7 +183,15 @@ in
 
           options.configuration = mkOption {
             default = {};
-            description = "Arbitrary NixOS configuration options.";
+            description = ''
+              Arbitrary NixOS configuration.
+
+              Anything you can add to a normal NixOS configuration, you can add
+              here, including imports and config values, although nested
+              specialisations will be ignored.
+            '';
+            visible = "shallow";
+            inherit (extend { modules = [ ./no-clone.nix ]; }) type;
           };
         })
       );