nixos/xwayland: add new module and allow configuring a default font path

- Add option `programs.xwayland.defaultFontPath`
- Modify sway to enable Xwayland

nixos/modules/programs/sway.nix

@@ -86,8 +86,7 @@ in {
     extraPackages = mkOption {
       type = with types; listOf package;
       default = with pkgs; [
-        swaylock swayidle
-        xwayland alacritty dmenu
+        swaylock swayidle alacritty dmenu
         rxvt-unicode # For backward compatibility (old default terminal)
       ];
       defaultText = literalExample ''
@@ -104,6 +103,7 @@ in {
         Extra packages to be installed system wide.
       '';
     };
+
   };
 
   config = mkIf cfg.enable {
@@ -130,6 +130,7 @@ in {
     programs.dconf.enable = mkDefault true;
     # To make a Sway session available if a display manager like SDDM is enabled:
     services.xserver.displayManager.sessionPackages = [ swayPackage ];
+    programs.xwayland.enable = mkDefault true;
   };
 
   meta.maintainers = with lib.maintainers; [ gnidorah primeos colemickens ];

nixos/modules/programs/xwayland.nix

@@ -0,0 +1,45 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.xwayland;
+
+in
+
+{
+  options.programs.xwayland = {
+
+    enable = mkEnableOption ''
+      Xwayland X server allows running X programs on a Wayland compositor.
+    '';
+
+    defaultFontPath = mkOption {
+      type = types.str;
+      default = optionalString config.fonts.fontDir.enable
+        "/run/current-system/sw/share/X11/fonts";
+      description = ''
+        Default font path. Setting this option causes Xwayland to be rebuilt.
+      '';
+    };
+
+    package = mkOption {
+      type = types.path;
+      description = "The Xwayland package";
+    };
+
+  };
+
+  config = mkIf cfg.enable {
+
+    # Needed by some applications for fonts and default settings
+    environment.pathsToLink = [ "/share/X11" ];
+
+    environment.systemPackages = [ cfg.package ];
+
+    programs.xwayland.package = pkgs.xwayland.override (oldArgs: {
+      inherit (cfg) defaultFontPath;
+    });
+
+  };
+}