nixos/xserver: improve DM error message when X off

Previously, if you, for example, set
services.xserver.displayManager.sddm.enable, but forgot to set
services.xserver.enable, you would get an error message that looked like
this:

    error: attribute 'display-manager' missing

Which was not particularly helpful.

Using assertions, we can make this message much better.

1 files changed, 5 insertions, 0 deletions

diff --git a/nixos/modules/services/x11/display-managers/lightdm.nix b/nixos/modules/services/x11/display-managers/lightdm.nix
index 3ab4f26399f..afa0cebbc52 100644
--- a/nixos/modules/services/x11/display-managers/lightdm.nix
+++ b/nixos/modules/services/x11/display-managers/lightdm.nix
@@ -189,6 +189,11 @@ in
   config = mkIf cfg.enable {
 
     assertions = [
+      { assertion = xcfg.enable;
+        message = ''
+          LightDM requires services.xserver.enable to be true
+        '';
+      }
       { assertion = cfg.autoLogin.enable -> cfg.autoLogin.user != null;
         message = ''
           LightDM auto-login requires services.xserver.displayManager.lightdm.autoLogin.user to be set