pkgs/top-level/config.nix: Add warnUndeclaredOptions

pkgs/top-level/config.nix

@@ -1,6 +1,6 @@
 # This file defines the structure of the `config` nixpkgs option.
 
-{ lib, ... }:
+{ config, lib, ... }:
 
 with lib;
 
@@ -28,6 +28,11 @@ let
 
     /* Config options */
 
+    warnUndeclaredOptions = mkOption {
+      description = "Whether to warn when <literal>config</literal> contains an unrecognized attribute.";
+      default = false;
+    };
+
     doCheckByDefault = mkMassRebuild {
       feature = "run <literal>checkPhase</literal> by default";
     };
@@ -113,6 +118,15 @@ let
 
 in {
 
+  freeformType =
+    let t = lib.types.attrsOf lib.types.raw;
+    in t // {
+      merge = loc: defs:
+        lib.mapAttrs
+          (k: v: lib.warnIf config.warnUndeclaredOptions "undeclared Nixpkgs option set: config.${k}" v)
+          (t.merge loc defs);
+    };
+
   inherit options;
 
 }

pkgs/top-level/default.nix

@@ -79,15 +79,13 @@ in let
       ./config.nix
       ({ options, ... }: {
         _file = "nixpkgs.config";
-        # filter-out known options, FIXME: remove this eventually
-        config = builtins.intersectAttrs options config1;
+        config = config1;
       })
     ];
   };
 
   # take all the rest as-is
-  config = lib.showWarnings configEval.config.warnings
-    (config1 // builtins.removeAttrs configEval.config [ "_module" ]);
+  config = lib.showWarnings configEval.config.warnings configEval.config;
 
   # A few packages make a new package set to draw their dependencies from.
   # (Currently to get a cross tool chain, or forced-i686 package.) Rather than