nixos: Make config.nix.enable pass test

nixos/modules/installer/tools/tools.nix

@@ -117,7 +117,7 @@ in
     '';
   };
 
-  config = lib.mkIf (!config.system.disableInstallerTools) {
+  config = lib.mkIf (config.nix.enable && !config.system.disableInstallerTools) {
 
     system.nixos-generate-config.configuration = mkDefault ''
       # Edit this configuration file to define what should be installed on

nixos/modules/services/misc/nix-gc.nix

@@ -81,8 +81,14 @@ in
   ###### implementation
 
   config = {
-
-    systemd.services.nix-gc = {
+    assertions = [
+      {
+        assertion = cfg.automatic -> config.nix.enable;
+        message = ''nix.gc.automatic requires nix.enable'';
+      }
+    ];
+
+    systemd.services.nix-gc = lib.mkIf config.nix.enable {
       description = "Nix Garbage Collector";
       script = "exec ${config.nix.package.out}/bin/nix-collect-garbage ${cfg.options}";
       startAt = optional cfg.automatic cfg.dates;

nixos/modules/services/misc/nix-optimise.nix

@@ -37,8 +37,14 @@ in
   ###### implementation
 
   config = {
-
-    systemd.services.nix-optimise =
+    assertions = [
+      {
+        assertion = cfg.automatic -> config.nix.enable;
+        message = ''nix.optimise.automatic requires nix.enable'';
+      }
+    ];
+
+    systemd.services.nix-optimise = lib.mkIf config.nix.enable
       { description = "Nix Store Optimiser";
         # No point this if the nix daemon (and thus the nix store) is outside
         unitConfig.ConditionPathIsReadWrite = "/nix/var/nix/daemon-socket";

nixos/modules/virtualisation/qemu-vm.nix

@@ -796,7 +796,7 @@ in
     # allow `system.build.toplevel' to be included.  (If we had a direct
     # reference to ${regInfo} here, then we would get a cyclic
     # dependency.)
-    boot.postBootCommands =
+    boot.postBootCommands = lib.mkIf config.nix.enable
       ''
         if [[ "$(cat /proc/cmdline)" =~ regInfo=([^ ]*) ]]; then
           ${config.nix.package.out}/bin/nix-store --load-db < ''${BASH_REMATCH[1]}

nixos/tests/without-nix.nix

@@ -6,6 +6,21 @@ import ./make-test-python.nix ({ lib, ... }: {
 
   nodes.machine = { ... }: {
     nix.enable = false;
+    nixpkgs.overlays = [
+      (self: super: {
+        nix = throw "don't want to use pkgs.nix";
+        nixVersions = lib.mapAttrs (k: throw "don't want to use pkgs.nixVersions.${k}") super.nixVersions;
+        # aliases, some deprecated
+        nix_2_3 = throw "don't want to use pkgs.nix_2_3";
+        nix_2_4 = throw "don't want to use pkgs.nix_2_4";
+        nix_2_5 = throw "don't want to use pkgs.nix_2_5";
+        nix_2_6 = throw "don't want to use pkgs.nix_2_6";
+        nixFlakes = throw "don't want to use pkgs.nixFlakes";
+        nixStable = throw "don't want to use pkgs.nixStable";
+        nixUnstable = throw "don't want to use pkgs.nixUnstable";
+        nixStatic = throw "don't want to use pkgs.nixStatic";
+      })
+    ];
   };
 
   testScript = ''