How to home-manager build from a nixosConfiguration and not form a home.nix

[proxemy]

For testing, introspection and migration purposes, I would like to be able to build a home directory from an existing nixosConfiguration.HOST.home-manager.users.USER in a flake.

Most guides assume a cleanly separated project structure, where a single home.nix is the entry module bundling everything around home-manager.users.USER. With that, a simple homeConfiguration.USER with a modules import like modules = [ ./home.nix ]; is sufficient to build a result home directory via home-manager build –flake .#USER but this approach has two drawbacks.

1. Logically, your home configuration is likely tied to the underlying system configuration, forcefully splitting these can create unnecessary headache.

In my most recent case, I was building a nas_client.nix that was defining system mounts as well as thunar bookmarks to them via home-manager. I broke out of a clean home.nix.

2. It kind of contradicts the spirit of the nixos module system. Order (location) of modules should not matter when evaluated by tools.

For tinkering, or maybe better understanding, there is a full demo flake below.
Attempt 2 seems to be promising to become a hacky way to achieve the goal of a homeConfiguraion.USER definition in a flake that can be build with home-manager build –flake .#USER but I am lacking the knowledge to get past the weird error.

I am sorry if i missed a spelled out solution to this. Otherwise, if a hacky solution is possible, maybe update the docs with it.

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";

    home-manager = {
      url = "github:nix-community/home-manager";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs =
    {
      self,
      nixpkgs,
      home-manager,
    }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
      username = "USER";
      hostname = "HOST";

    in
    rec {
      # Attempt 1
      # Define regular nixosConfiguration and reach into it to build home selectively
      nixosConfigurations.${hostname} = nixpkgs.lib.nixosSystem {

        modules = [
          # home-manager module
          home-manager.nixosModules.home-manager
          {
            home-manager.users.${username}.home = {
              inherit username;
              stateVersion = "24.11";
              homeDirectory = "/home/${username}";
            };
          }

          (
            # regular nixos module
            { ... }:
            {
              nixpkgs.hostPlatform = system;
              boot.isContainer = true;
              users.users.${username}.isNormalUser = true;
            }
          )
        ];
      };

      # Attempt 2
      # Wrap the HM part of the existing nixosConfiguraion into a homeConfiguration
      homeConfigurations.${username} = home-manager.lib.homeManagerConfiguration {
        inherit pkgs;
        modules = [
          nixosConfigurations.${hostname}.config.home-manager.users.${username}
        ];
      };

      devShells.${system}.default = pkgs.mkShell {
        buildInputs = [
          pkgs.nixos-generators
          pkgs.home-manager
        ];
        shellHook = ''
          set -x
          alias ATTEMP_1="nix build .#nixosConfigurations.${hostname}.config.system.build.toplevel" # or something
          alias ATTEMP_2="home-manager build --flake .#${username}"
          set +x
        '';
      };
    };
}