xserver.displayManager: change default

Switch from slim to lightdm as the display-manager.
If plasma5 is used as desktop-manager use sdddm.
If gnome3 is used as desktop-manager use gdm.

Based on #12516

Author: Lassulus

nixos/doc/manual/release-notes/rl-1803.xml

@@ -74,7 +74,13 @@ following incompatible changes:</para>
 
 <title>Other Notable Changes</title>
 
-<itemizedlist>
+  <itemizedlist>
+    <listitem>
+    <para>The default display manager is now LightDM.
+    To use SLiM set <literal>services.xserver.displayManager.slim.enable</literal>
+    to <literal>true</literal>.
+  </listitem>
+
   <listitem>
     <para>
       ZNC option <option>services.znc.mutable</option> now defaults to <literal>true</literal>.

nixos/modules/services/x11/desktop-managers/gnome3.nix

@@ -166,6 +166,8 @@ in {
         '';
       };
 
+    services.xserver.displayManager.gdm.enable = mkDefault true;
+
     services.xserver.updateDbusEnvironment = true;
 
     environment.variables.GIO_EXTRA_MODULES = [ "${lib.getLib pkgs.gnome3.dconf}/lib/gio/modules"

nixos/modules/services/x11/desktop-managers/plasma5.nix

@@ -190,6 +190,7 @@ in
       ];
 
       services.xserver.displayManager.sddm = {
+        enable = mkDefault true;
         theme = mkDefault "breeze";
       };
 

nixos/modules/services/x11/display-managers/auto.nix

@@ -41,10 +41,12 @@ in
 
   config = mkIf cfg.enable {
 
-    services.xserver.displayManager.slim = {
+    services.xserver.displayManager.lightdm = {
       enable = true;
-      autoLogin = true;
-      defaultUser = cfg.user;
+      autoLogin = {
+        enable = true;
+        user = cfg.user;
+      };
     };
 
   };

nixos/modules/services/x11/display-managers/gdm.nix

@@ -87,8 +87,6 @@ in
       }
     ];
 
-    services.xserver.displayManager.slim.enable = false;
-
     users.extraUsers.gdm =
       { name = "gdm";
         uid = config.ids.uids.gdm;

nixos/modules/services/x11/display-managers/lightdm.nix

@@ -187,8 +187,6 @@ in
       }
     ];
 
-    services.xserver.displayManager.slim.enable = false;
-
     services.xserver.displayManager.job = {
       logsXsession = true;
 

nixos/modules/services/x11/display-managers/sddm.nix

@@ -202,8 +202,6 @@ in
       }
     ];
 
-    services.xserver.displayManager.slim.enable = false;
-
     services.xserver.displayManager.job = {
       logsXsession = true;
 

nixos/modules/services/x11/display-managers/slim.nix

@@ -49,7 +49,7 @@ in
 
       enable = mkOption {
         type = types.bool;
-        default = config.services.xserver.enable;
+        default = false;
         description = ''
           Whether to enable SLiM as the display manager.
         '';

nixos/modules/services/x11/xserver.nix

@@ -525,6 +525,15 @@ in
 
   config = mkIf cfg.enable {
 
+    services.xserver.displayManager.lightdm.enable =
+      let dmconf = cfg.displayManager;
+          default = !( dmconf.auto.enable
+                    || dmconf.gdm.enable
+                    || dmconf.sddm.enable
+                    || dmconf.slim.enable
+                    || dmconf.xpra.enable );
+      in mkIf (default) true;
+
     hardware.opengl.enable = mkDefault true;
 
     services.xserver.videoDrivers = mkIf (cfg.videoDriver != null) [ cfg.videoDriver ];

nixos/tests/gnome3-gdm.nix

@@ -11,7 +11,6 @@ import ./make-test.nix ({ pkgs, ...} : {
 
       services.xserver.enable = true;
 
-      services.xserver.displayManager.slim.enable = false;
       services.xserver.displayManager.gdm = {
         enable = true;
         autoLogin = {