nixos/nixpkgs: fix assertion text & show def files

nixos/modules/misc/nixpkgs.nix

@@ -386,7 +386,10 @@ in
           `nixpkgs.config` options should be passed when creating the instance instead.
 
           Current value:
-          ${lib.generators.toPretty { multiline = true; } opt.config}
+          ${lib.generators.toPretty { multiline = true; } cfg.config}
+
+          Defined in:
+          ${lib.concatMapStringsSep "\n" (file: "  - ${file}") opt.config.files}
         '';
       }
     ];

nixos/modules/misc/nixpkgs/test.nix

@@ -16,6 +16,11 @@ let
     nixpkgs.hostPlatform = "aarch64-linux";
     nixpkgs.buildPlatform = "aarch64-linux";
   };
+  externalPkgsWithConfig = {
+    _file = "ext-pkgs-config.nix";
+    nixpkgs.pkgs = pkgs;
+    nixpkgs.config.allowUnfree = true;
+  };
   ambiguous = {
     _file = "ambiguous.nix";
     nixpkgs.hostPlatform = "aarch64-linux";
@@ -108,6 +113,20 @@ lib.recurseIntoAttrs {
           For a future proof system configuration, we recommend to remove
           the legacy definitions.
         ''];
+    assert builtins.trace (lib.head (getErrors externalPkgsWithConfig))
+      getErrors externalPkgsWithConfig ==
+        [''
+          Your system configures nixpkgs with an externally created instance.
+          `nixpkgs.config` options should be passed when creating the instance instead.
+
+          Current value:
+          {
+            allowUnfree = true;
+          }
+
+          Defined in:
+            - ext-pkgs-config.nix
+        ''];
     assert getErrors {
         nixpkgs.localSystem = pkgs.stdenv.hostPlatform;
         nixpkgs.hostPlatform = pkgs.stdenv.hostPlatform;