default.nix

let
  pkgs = import <nixpkgs> { };
  lib = pkgs.lib;

  targetFS =
    { targetFSes }:
    pkgs.symlinkJoin {
      name = "fs";
      paths = targetFSes;
    };

  passwd =
    { users }:
    let
      concatMapStrings = lib.strings.concatMapStrings;
      combiner = (x: x + ":x:0:0::/home/${x}:/run/current-system-/sw/bin/bash\n");
      content = concatMapStrings combiner users;
    in
    pkgs.stdenv.mkDerivation {
      pname = "passwd";
      name = "passwd";

      phases = [ "installPhase" ];

      installPhase = ''
        mkdir -p $out/etc/
        cat <<EOF > $out/etc/passwd
        ${content}'';
    };

  infuse = (import ./lib/infuse.nix { inherit lib; }).v1.infuse;
in
lib.fix (
  final:
  infuse { } [
    {
      fs.__assign = pkgs.callPackage targetFS {
        targetFSes = [ ];
      };
    }

    {
      passwd.__assign = pkgs.callPackage passwd {
        users = [
          "root"
          "toor"
        ];
      };

      fs.__input.targetFSes.__append = [ final.passwd ];
    }

    {
      passwd.__input.users.__append = [ "haylin" ];
    }
  ]
)

result-etc-passwd

root:x:0:0::/home/root:/run/current-system-/sw/bin/bash
toor:x:0:0::/home/toor:/run/current-system-/sw/bin/bash
haylin:x:0:0::/home/haylin:/run/current-system-/sw/bin/bash