examples/testing/nixos-integration: Add no-op validation system

There is also a minimal stage-1 validation since it's important to
compare an "unconfigured" NixOS stage-1 with an "unconfigured"
Mobile NixOS stage-1.

examples/testing/nixos-integration/configuration.nix

@@ -0,0 +1,22 @@
+{ lib, ... }:
+{
+  # This system is not expected to be bootable.
+  fileSystems = {
+    "/" = {
+      device = "tmpfs";
+      fsType = "tmpfs";
+    };
+  };
+  boot.loader.grub.enable = false;
+  boot.loader.generic-extlinux-compatible.enable = false;
+
+  # Documentation build will be different, so comparing `toplevel` would fail.
+  # Different why? New options!
+  documentation.enable = lib.mkOverride 10 false;
+
+  # Unimportant, but keeps the eval log cleaner.
+  system.stateVersion = "22.05";
+
+  # ¯\_(ツ)_/¯
+  services.getty.autologinUser = "root";
+}

examples/testing/nixos-integration/default.nix

@@ -0,0 +1,52 @@
+{ pkgs ? (import ../../../pkgs.nix {})
+}@args':
+let args = args' // { inherit pkgs; }; in
+
+let
+  eval = configuration: import (pkgs.path + "/nixos") {
+    configuration = {
+      imports = [ configuration ];
+    };
+  };
+
+  # A "clean" NixOS eval
+  nixos-eval = eval {
+    imports = [
+      ./configuration.nix
+    ];
+  };
+  # A Mobile NixOS eval that should be a no-op
+  mobile-nixos-eval = eval {
+    imports = [
+      ./configuration.nix
+      (import ../../../lib/configuration.nix { })
+    ];
+    mobile.enable = false;
+  };
+  # A Mobile NixOS eval that should be a no-op
+  mobile-nixos-stage-1-eval = eval {
+    imports = [
+      ./configuration.nix
+      (import ../../../lib/configuration.nix { })
+    ];
+    mobile.enable = false;
+    mobile.boot.stage-1.enable = true;
+  };
+in
+  {
+    inherit
+      nixos-eval
+      mobile-nixos-eval
+      mobile-nixos-stage-1-eval
+    ;
+
+    # Use this output to check that the product works as expected.
+    # (The bogus rootfs will be overriden by the VM config.)
+    default =
+      assert nixos-eval.config.system.build.toplevel == mobile-nixos-eval.config.system.build.toplevel;
+      assert nixos-eval.config.system.build.vm == mobile-nixos-eval.config.system.build.vm;
+      mobile-nixos-eval.config.system.build.vm
+    ;
+
+    mobile-nixos-stage-1 = mobile-nixos-stage-1-eval.config.system.build.vm;
+  }

examples/testing/nixos-integration/diff.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -p nix-diff -i bash
+
+this_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")"; pwd)"
+
+nix-diff \
+	"$(nix-instantiate "$this_dir"/default.nix -A nixos-eval.config.system.build.toplevel)" \
+	"$(nix-instantiate "$this_dir"/default.nix -A mobile-nixos-eval.config.system.build.toplevel)"