modules: fix modular services with strict nixpkgs modules

[DaVinci42]

Description

Fixes evaluation of modular services when the imported nixpkgs systemd service module does not accept every argument passed by the Home Manager module system.

Minimal reproducer:

{
  inputs = {
    # Nixpkgs master as of 2026-07-25 06:57:37 UTC.
    nixpkgs.url = "github:NixOS/nixpkgs/9e3461d3ed9792395b1ebb8e99a9bb7f866cda7b";
    # Home Manager master as of 2026-07-24 17:12:39 UTC.
    home-manager.url = "github:nix-community/home-manager/079a3b5d1aa6a719920a51316253b7d6dd22738d";
    home-manager.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs =
    {
      nixpkgs,
      home-manager,
      ...
    }:
    let
      pkgs = nixpkgs.legacyPackages.x86_64-linux;
    in
    {
      homeConfigurations.repro = home-manager.lib.homeManagerConfiguration {
        inherit pkgs;
        modules = [
          {
            home.username = "me";
            home.homeDirectory = "/home/me";
            home.stateVersion = "25.11";
            programs.home-manager.enable = true;
          }
        ];
      };
    };
}

Build command:

nix build .#homeConfigurations.repro.activationPackage --show-trace

This currently fails while generating the Home Manager manpage/options:

error: function 'anonymous lambda' called with unexpected argument 'lib'
at .../nixos/modules/system/service/systemd/service.nix:4:1:
     3| # This makes service modules self-contained, allowing mixing of Nixpkgs versions.
     4| { pkgs }:
       | ^

This happens because modules/services-modular/service.nix imports the nixpkgs systemd service module as a module path. The module system then calls it with the full Home Manager argument set. Some nixpkgs revisions expose that file as a stricter function and reject extra arguments such as lib or specialArgs.

The fix imports the nixpkgs module function explicitly and calls it with only the arguments declared by that function. This preserves compatibility with different nixpkgs signatures while avoiding accidental argument injection by the module system.

Checklist

• Change is backwards compatible.
• Code formatted with nix fmt or
  nix-shell -A dev --run treefmt.
• Code tested through nix build .#test-all
  or a targeted nix run .#tests -- <pattern>.
• Test cases updated/added.

[KiaraGrouwstra]

hey there, sorry again for NixOS/nixpkgs#518860 having broken downstream consumption like here.

for what it's worth, the underlying breaking change was reverted at NixOS/nixpkgs#545519, and the refile at NixOS/nixpkgs#545521 would lack the breaking change.

i think the underlying issue here is in file-based imports being fragile, hence i'd filed a few PRs to shift toward going thru root-level entrypoints instead:

• lib side: lib/services: expose portable service lib as lib.services NixOS/nixpkgs#507052
• nixos side: nixos/modular-services: split system-specific config out of pkgs.<pkg>.services.<service> NixOS/nixpkgs#540863

on the approach proposed in the current PR, i think the tough part is that the breaking change in this particular case was maybe less about adding or removing an argument, more about the module there getting wrapped in a closure, which then maybe raises the question for me on whether the approach here wouldn't like break again with the (now-landed) revert at NixOS/nixpkgs#545519 - or, for that matter, with the refile at NixOS/nixpkgs#545521.

in that sense, i wonder if the fix here would help more than just that revert upstream.

[DaVinci42]

hey there, sorry again for NixOS/nixpkgs#518860 having broken downstream consumption like here.

for what it's worth, the underlying breaking change was reverted at NixOS/nixpkgs#545519, and the refile at NixOS/nixpkgs#545521 would lack the breaking change.

i think the underlying issue here is in file-based imports being fragile, hence i'd filed a few PRs to shift toward going thru root-level entrypoints instead:

* `lib` side: [lib/services: expose portable service lib as `lib.services` NixOS/nixpkgs#507052](https://github.com/NixOS/nixpkgs/pull/507052)

* nixos side: [nixos/modular-services: split system-specific config out of `pkgs.<pkg>.services.<service>` NixOS/nixpkgs#540863](https://github.com/NixOS/nixpkgs/pull/540863)

on the approach proposed in the current PR, i think the tough part is that the breaking change in this particular case was maybe less about adding or removing an argument, more about the module there getting wrapped in a closure, which then maybe raises the question for me on whether the approach here wouldn't like break again with the (now-landed) revert at NixOS/nixpkgs#545519 - or, for that matter, with the refile at NixOS/nixpkgs#545521.

in that sense, i wonder if the fix here would help more than just that revert upstream.

Thanks for the context! Since the follow up PRs are still pending, I’ll hold off for now.
Once those merged, I’ll test again and see whether this workaround is still needed.

[DaVinci42]

Tested again with the latest nixpkgs, and the build works fine now.
Close this one.

[pyrox0]

hey there, sorry again for NixOS/nixpkgs#518860 having broken downstream consumption like here.

for what it's worth, the underlying breaking change was reverted at NixOS/nixpkgs#545519, and the refile at NixOS/nixpkgs#545521 would lack the breaking change.

i think the underlying issue here is in file-based imports being fragile, hence i'd filed a few PRs to shift toward going thru root-level entrypoints instead:

* `lib` side: [lib/services: expose portable service lib as `lib.services` NixOS/nixpkgs#507052](https://github.com/NixOS/nixpkgs/pull/507052)

* nixos side: [nixos/modular-services: split system-specific config out of `pkgs.<pkg>.services.<service>` NixOS/nixpkgs#540863](https://github.com/NixOS/nixpkgs/pull/540863)

on the approach proposed in the current PR, i think the tough part is that the breaking change in this particular case was maybe less about adding or removing an argument, more about the module there getting wrapped in a closure, which then maybe raises the question for me on whether the approach here wouldn't like break again with the (now-landed) revert at NixOS/nixpkgs#545519 - or, for that matter, with the refile at NixOS/nixpkgs#545521.

in that sense, i wonder if the fix here would help more than just that revert upstream.

Just FYI, neither the revert or refile landed in the latest nixpkgs-unstable, breaking evaluation in a very significant way. Is there a way to avoid this with home-manager right now?

[KiaraGrouwstra]

@pyrox0 thanks for notifying me! i filed a fix now at #9714.