From b716673f971311c40cd037c95860ea36c6e9ff2c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B8rn=20Forsman?= <bjorn.forsman@gmail.com>
Date: Thu, 3 Sep 2015 22:22:00 +0200
Subject: [PATCH] nixos/gdm: add assertion for the autoLogin.user option

Or else users may see this unfriendly message:

  error: cannot coerce null to a string, at .../nixos/modules/services/x11/display-managers/gdm.nix:107:49
---
 nixos/modules/services/x11/display-managers/gdm.nix | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/nixos/modules/services/x11/display-managers/gdm.nix b/nixos/modules/services/x11/display-managers/gdm.nix
index b51c3dd28bf..55af2ecbb76 100644
--- a/nixos/modules/services/x11/display-managers/gdm.nix
+++ b/nixos/modules/services/x11/display-managers/gdm.nix
@@ -68,6 +68,13 @@ in
 
   config = mkIf cfg.gdm.enable {
 
+    assertions = [
+      { assertion = let autoLogin = cfg.gdm.autoLogin; in
+          if autoLogin.enable then autoLogin.user != null else true;
+        message = "GDM auto-login requires services.xserver.displayManager.gdm.autoLogin.user to be set";
+      }
+    ];
+
     services.xserver.displayManager.slim.enable = false;
 
     users.extraUsers.gdm =