pkgs-lib/formats/systemd: make format a function (#448460)

nixos/doc/manual/release-notes/rl-2511.section.md

@@ -275,6 +275,8 @@
 
 - The `boot.readOnlyNixStore` has been removed. Control over bind mount options on `/nix/store` is now offered by the `boot.nixStoreMountOpts` option.
 
+- Direct use of `pkgs.formats.systemd` has been deprecated, and should now be instantiated with `pkgs.formats.systemd { }` similarly to other items in `pkgs.formats`.
+
 - The Postfix module has been updated and likely requires configuration changes:
   - The `services.postfix.sslCert` and `sslKey` options were removed and you now need to configure
     - [services.postfix.settings.main.smtpd_tls_chain_files](#opt-services.postfix.settings.main.smtpd_tls_chain_files) for server certificates,

nixos/modules/system/boot/systemd/journald-remote.nix

@@ -7,7 +7,7 @@
 
 let
   cfg = config.services.journald.remote;
-  format = pkgs.formats.systemd;
+  format = pkgs.formats.systemd { };
 
   cliArgs = lib.cli.toGNUCommandLineShell { } {
     inherit (cfg) output;

nixos/modules/system/boot/systemd/journald-upload.nix

@@ -7,7 +7,7 @@
 
 let
   cfg = config.services.journald.upload;
-  format = pkgs.formats.systemd;
+  format = pkgs.formats.systemd { };
 in
 {
   meta.maintainers = [ lib.maintainers.raitobezarius ];

pkgs/pkgs-lib/formats.nix

@@ -425,10 +425,22 @@ optionalAttrs allowAliases aliases
     let
       mkValueString = mkValueStringDefault { };
       mkKeyValue = k: v: if v == null then "# ${k} is unset" else "${k} = ${mkValueString v}";
+
+      rawFormat = ini {
+        listsAsDuplicateKeys = true;
+        inherit mkKeyValue;
+      };
     in
-    ini {
-      listsAsDuplicateKeys = true;
-      inherit mkKeyValue;
+    rawFormat
+    // {
+      generate =
+        name: value:
+        lib.warn
+          "Direct use of `pkgs.formats.systemd` has been deprecated, please use `pkgs.formats.systemd { }` instead."
+          rawFormat.generate
+          name
+          value;
+      __functor = self: { }: rawFormat;
     };
 
   keyValue =