Revert "nixos/specialisation: Rephrase in terms of extendModules, noUserModules"

This reverts commit e2bea4427b.

While this commit was probably fine, I want to be conservative
with changes right before the release branch-off.
So far the extendModules commits have been adding and refactoring
expressions that did not affect derivation hashes, whereas this
commit changes the module ordering. I will open a separate PR for
it.

nixos/lib/eval-config.nix

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

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

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, extendModules, noUserModules, ... }:
+{ config, lib, pkgs, modules, baseModules, specialArgs, ... }:
 
 with lib;
 
@@ -11,10 +11,16 @@ 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: childConfig.configuration.system.build.toplevel)
-      config.specialisation;
+  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;
 
   systemBuilder =
     let
@@ -170,11 +176,7 @@ 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;
@@ -183,15 +185,7 @@ in
 
           options.configuration = mkOption {
             default = {};
-            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;
+            description = "Arbitrary NixOS configuration options.";
           };
         })
       );