mpd: use XDG music dir if XDG user dirs are enabled

If the user has enabled the XDG user directories module then we can
use the XDG music directory in the MPD module. Otherwise we'll leave
the option undefined so that the user is forced to define the
directory to use.

This applies to state version 22.11 and above.

Fixes #3225

modules/services/mpd.nix

@@ -52,11 +52,18 @@ in {
 
       musicDirectory = mkOption {
         type = with types; either path str;
-        default = "${config.home.homeDirectory}/music";
-        defaultText = "$HOME/music";
+        defaultText = literalExpression ''
+          ''${home.homeDirectory}/music    if state version < 22.11
+          ''${xdg.userDirs.music}          if xdg.userDirs.enable == true
+          undefined                      otherwise
+        '';
         apply = toString; # Prevent copies to Nix store.
         description = ''
           The directory where mpd reads music from.
+          </para><para>
+          If <xref linkend="opt-xdg.userDirs.enable"/> is
+          <literal>true</literal> then the defined XDG music directory is used.
+          Otherwise, you must explicitly specify a value.
         '';
       };
 
@@ -146,6 +153,17 @@ in {
       (lib.hm.assertions.assertPlatform "services.mpd" pkgs lib.platforms.linux)
     ];
 
+    services.mpd = mkMerge [
+      (mkIf (versionAtLeast config.home.stateVersion "22.11"
+        && config.xdg.userDirs.enable) {
+          musicDirectory = mkOptionDefault config.xdg.userDirs.music;
+        })
+
+      (mkIf (versionOlder config.home.stateVersion "22.11") {
+        musicDirectory = mkOptionDefault "${config.home.homeDirectory}/music";
+      })
+    ];
+
     systemd.user.services.mpd = {
       Unit = {
         After = [ "network.target" "sound.target" ];
@@ -164,6 +182,7 @@ in {
           ${pkgs.bash}/bin/bash -c "${pkgs.coreutils}/bin/mkdir -p '${cfg.dataDir}' '${cfg.playlistDirectory}'"'';
       };
     };
+
     systemd.user.sockets.mpd = mkIf cfg.network.startWhenNeeded {
       Socket = {
         ListenStream = let