activate-system: remove enable option

Disabling this is not supported as `/run` gets cleared out on every
reboot so it is necessary for ensuring that the `/run/current-system`
symlink exists.

README.md

@@ -172,7 +172,7 @@ Furthermore there's `darwin-option` to introspect the settings of a system and i
 > NOTE: `darwin-option` is only available to non-flake installations.
 
 ```
-$ darwin-option services.activate-system.enable
+$ darwin-option nix.linux-builder.enable
 Value:
 true
 
@@ -180,10 +180,10 @@ Default:
 false
 
 Example:
-no example
+true
 
 Description:
-Whether to activate system at boot time.
+Whether to enable Linux builder.
 ```
 
 There's also a small wiki https://github.com/LnL7/nix-darwin/wiki about

modules/nix/default.nix

@@ -774,7 +774,6 @@ in
 
     # Not in NixOS module
     warnings = [
-      (mkIf (!config.services.activate-system.enable && cfg.distributedBuilds) "services.activate-system is not enabled, a reboot could cause distributed builds to stop working.")
       (mkIf (!cfg.distributedBuilds && cfg.buildMachines != []) "nix.distributedBuilds is not enabled, build machines won't be configured.")
     ];
 

modules/services/activate-system/default.nix

@@ -1,22 +1,11 @@
 { config, lib, pkgs, ... }:
 
-with lib;
-
-let
-  cfg = config.services.activate-system;
-in
-
 {
-  options = {
-    services.activate-system.enable = mkOption {
-      type = types.bool;
-      default = true;
-      description = "Whether to activate system at boot time.";
-    };
-  };
-
-  config = mkIf cfg.enable {
+  imports = [
+    (lib.mkRemovedOptionModule [ "services" "activate-system" "enable" ] "The `activate-system` service is now always enabled as it is necessary for a working `nix-darwin` setup.")
+  ];
 
+  config = {
     launchd.daemons.activate-system = {
       script = ''
         set -e
@@ -41,6 +30,5 @@ in
       serviceConfig.RunAtLoad = true;
       serviceConfig.KeepAlive.SuccessfulExit = false;
     };
-
   };
 }

tests/services-activate-system-changed-label-prefix.nix

@@ -1,7 +1,6 @@
 { config, pkgs, ... }:
 
 {
-  services.activate-system.enable = true;
   launchd.labelPrefix = "org.nix-darwin";
 
   test = ''

tests/services-activate-system.nix

@@ -1,8 +1,6 @@
 { config, pkgs, ... }:
 
 {
-  services.activate-system.enable = true;
-
   test = ''
     echo checking activation service in /Library/LaunchDaemons >&2
     grep "org.nixos.activate-system" ${config.out}/Library/LaunchDaemons/org.nixos.activate-system.plist