sway: do not use pkgs.sway when cfg.package = null

- Implicitly disable checkConfig when `cfg.package = null` as we don’t
  have any exe to use for the check
- Implicitly disable `swaymsg reload` on activation, since we have no
  exe to use for running it

See https://github.com/nix-community/home-manager/issues/5307

modules/services/window-managers/i3-sway/sway.nix

@@ -258,7 +258,6 @@ let
 
   variables = concatStringsSep " " cfg.systemd.variables;
   extraCommands = concatStringsSep " && " cfg.systemd.extraCommands;
-  swayPackage = if cfg.package == null then pkgs.sway else cfg.package;
   systemdActivation = ''
     exec "${pkgs.dbus}/bin/dbus-update-activation-environment --systemd ${variables}; ${extraCommands}"'';
 
@@ -269,7 +268,7 @@ let
     checkPhase = lib.optionalString cfg.checkConfig ''
       export DBUS_SESSION_BUS_ADDRESS=/dev/null
       export XDG_RUNTIME_DIR=$(mktemp -d)
-      ${pkgs.xvfb-run}/bin/xvfb-run ${swayPackage}/bin/sway --config "$target" --validate --unsupported-gpu
+      ${pkgs.xvfb-run}/bin/xvfb-run ${cfg.package}/bin/sway --config "$target" --validate --unsupported-gpu
     '';
 
     text = concatStringsSep "\n"
@@ -362,7 +361,8 @@ in {
         'wrapperFeatures', 'extraSessionCommands', and 'extraOptions'.
         Set to `null` to not add any Sway package to your
         path. This should be done if you want to use the NixOS Sway
-        module to install Sway.
+        module to install Sway. Beware setting to `null` will also disable
+        reloading Sway when new config is activated.
       '';
     };
 
@@ -478,9 +478,9 @@ in {
 
     checkConfig = mkOption {
       type = types.bool;
-      default = true;
-      description =
-        "If enabled (the default), validates the generated config file.";
+      default = cfg.package != null;
+      defaultText = literalExpression "cfg.package != null";
+      description = "If enabled, validates the generated config file.";
     };
 
     extraConfig = mkOption {
@@ -515,6 +515,11 @@ in {
       assertions = [
         (hm.assertions.assertPlatform "wayland.windowManager.sway" pkgs
           platforms.linux)
+        {
+          assertion = cfg.checkConfig -> cfg.package != null;
+          message =
+            "programs.sway.checkConfig requires non-null programs.sway.package";
+        }
       ];
 
       home.packages = optional (cfg.package != null) cfg.package
@@ -522,10 +527,10 @@ in {
 
       xdg.configFile."sway/config" = {
         source = configFile;
-        onChange = ''
+        onChange = lib.optionalString (cfg.package != null) ''
           swaySocket="''${XDG_RUNTIME_DIR:-/run/user/$UID}/sway-ipc.$UID.$(${pkgs.procps}/bin/pgrep --uid $UID -x sway || true).sock"
           if [ -S "$swaySocket" ]; then
-            ${swayPackage}/bin/swaymsg -s $swaySocket reload
+            ${cfg.package}/bin/swaymsg -s $swaySocket reload
           fi
         '';
       };