treewide: add and use option nixowos.allowAddingPackages

default.nix

@@ -1,4 +1,5 @@
 {
+  lib,
   ...
 }:
 
@@ -7,4 +8,8 @@
     ./defaults
     ./nvidia.nix
   ];
+
+  options.nixowos = {
+    allowAddingPackages = lib.mkEnableOption "adding to `environment.systemPackages`";
+  };
 }

defaults/nix.nix

@@ -25,9 +25,11 @@ in
     # source: https://nix.dev/manual/nix/2.26/command-ref/new-cli/nix3-store-optimise
     nix.settings.auto-optimise-store = true;
 
-    environment.systemPackages = with pkgs; [
-      nixd
-      nixfmt
-    ];
+    environment.systemPackages =
+      with pkgs;
+      lib.mkIf config.nixowos.allowAddingPackages [
+        nixd
+        nixfmt
+      ];
   };
 }

defaults/plasma.nix

@@ -26,11 +26,13 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    environment.systemPackages = with pkgs; [
-      kdePackages.plasma-keyboard
-      kdePackages.qtvirtualkeyboard
-      wl-clipboard
-    ];
+    environment.systemPackages =
+      with pkgs;
+      lib.mkIf config.nixowos.allowAddingPackages [
+        kdePackages.plasma-keyboard
+        kdePackages.qtvirtualkeyboard
+        wl-clipboard
+      ];
 
     environment.interactiveShellInit =
       lib.mkIf cfg.startOnFirstTTY

defaults/shell.nix

@@ -61,7 +61,7 @@ in
       nixos-rebuild = /* sh */ "nixos-rebuild --ask-sudo-password";
     };
 
-    environment.systemPackages = cfg.extraPackages;
+    environment.systemPackages = lib.mkIf config.nixowos.allowAddingPackages cfg.extraPackages;
 
     # enter dev env on cd
     programs.direnv = lib.mkDefault {

defaults/system.nix

@@ -23,7 +23,7 @@ in
 
   config = lib.mkIf cfg.enable {
 
-    environment.systemPackages = cfg.extraPackages;
+    environment.systemPackages = lib.mkIf config.nixowos.allowAddingPackages cfg.extraPackages;
 
     boot.loader.grub = lib.mkDefault {
       # modern installation