tests: add nixpkgs-disabled warning/assertion coverage

Evaluate `nixpkgs-disabled.nix` standalone (the harness always loads the
real `nixpkgs.nix`) to verify the warning and assertion report the offending
`nixpkgs.config`/`nixpkgs.overlays` definition locations.

tests/default.nix

@@ -187,6 +187,7 @@ import nmtSrc {
           ./modules/misc/news
           ./modules/misc/nix
           ./modules/misc/nix-remote-build
+          ./modules/misc/nixpkgs-disabled
           ./modules/misc/specialisation
           ./modules/misc/ssh-auth-sock/default.nix
           ./modules/misc/xdg

tests/modules/misc/nixpkgs-disabled/assertion.nix

@@ -0,0 +1,34 @@
+{ lib, pkgs, ... }:
+let
+  # Both options set trips the assertion; it should list both locations.
+  eval = lib.evalModules {
+    modules = [
+      ../../../../modules/misc/nixpkgs-disabled.nix
+      (pkgs.path + "/nixos/modules/misc/assertions.nix")
+      (pkgs.path + "/nixos/modules/misc/meta.nix")
+      { _module.args.pkgs = pkgs; }
+      ./offending-overlay.nix
+      ./offending-config.nix
+    ];
+  };
+
+  files = lib.showFiles (
+    lib.unique (eval.options.nixpkgs.config.files ++ eval.options.nixpkgs.overlays.files)
+  );
+
+  failed = map (a: a.message) (lib.filter (a: !a.assertion) eval.config.assertions);
+in
+{
+  nmt.script =
+    let
+      expected = pkgs.writeText "nixpkgs-disabled-assertion.expected" ''
+        `nixpkgs` options are disabled when `home-manager.useGlobalPkgs` is enabled.
+        Definitions found in ${files}.
+      '';
+
+      actual = pkgs.writeText "nixpkgs-disabled-assertion.actual" (lib.concatStringsSep "\n--\n" failed);
+    in
+    ''
+      assertFileContent ${actual} ${expected}
+    '';
+}

tests/modules/misc/nixpkgs-disabled/default.nix

@@ -0,0 +1,4 @@
+{
+  nixpkgs-disabled-warning = ./warning.nix;
+  nixpkgs-disabled-assertion = ./assertion.nix;
+}

tests/modules/misc/nixpkgs-disabled/offending-config.nix

@@ -0,0 +1,3 @@
+{
+  nixpkgs.config.allowUnfree = true;
+}

tests/modules/misc/nixpkgs-disabled/offending-overlay.nix

@@ -0,0 +1,3 @@
+{
+  nixpkgs.overlays = [ (_self: _super: { }) ];
+}

tests/modules/misc/nixpkgs-disabled/warning.nix

@@ -0,0 +1,33 @@
+{ lib, pkgs, ... }:
+let
+  # nixpkgs-disabled.nix only loads under `useGlobalPkgs` (useNixpkgsModule =
+  # false), which the standard harness can't toggle; evaluate it standalone.
+  eval = lib.evalModules {
+    modules = [
+      ../../../../modules/misc/nixpkgs-disabled.nix
+      (pkgs.path + "/nixos/modules/misc/assertions.nix")
+      (pkgs.path + "/nixos/modules/misc/meta.nix")
+      { _module.args.pkgs = pkgs; }
+      ./offending-overlay.nix
+    ];
+  };
+
+  files = lib.showFiles eval.options.nixpkgs.overlays.files;
+in
+{
+  nmt.script =
+    let
+      expected = pkgs.writeText "nixpkgs-disabled-warning.expected" ''
+        You have set either `nixpkgs.config` or `nixpkgs.overlays` while using `home-manager.useGlobalPkgs`.
+        This will soon not be possible. Please remove all `nixpkgs` options when using `home-manager.useGlobalPkgs`.
+        Definitions found in ${files}.
+      '';
+
+      actual = pkgs.writeText "nixpkgs-disabled-warning.actual" (
+        lib.concatStringsSep "\n--\n" eval.config.warnings
+      );
+    in
+    ''
+      assertFileContent ${actual} ${expected}
+    '';
+}