Like

❯ nix-build nixos/tests/systemd-initrd-simple.nix -A nodes.machine.config.system.build.initialRamdisk
error: the expression selected by the selection path 'nodes.machine.config.system.build.initialRamdisk' should be a set but is a function

?

I dunno if there's a smarter way to do it, but I've just been testing with this file:

import ./nixos { configuration = { lib, config, pkgs, ... }: {
  imports = [./nixos/modules/virtualisation/qemu-vm.nix];
  boot.initrd.systemd = {
    enable = true;
    emergencyHashedPassword = "";
    # additionalUpstreamUnits = ["cryptsetup.target"];
    # package = pkgs.systemdMinimal.override { withCryptsetup = true; };
    # services.foo = {
    #   wantedBy = ["initrd.target"];
    #   after = ["local-fs.target"];
    #   before = ["initrd.target" "initrd-cleanup.service"];
    #   serviceConfig.ExecStart = "/bin/systemctl emergency";
    #   serviceConfig.Type = "oneshot";
    # };
  };
  virtualisation = {
    graphics = false;
    memorySize = 8192;
    cores = 4;
  };
  documentation.enable = false;
  services.getty.autologinUser = "root";
}; }

I dunno if there's a smarter way to do it, but I've just been testing with this file:

import ./nixos { configuration = { lib, config, pkgs, ... }: {
  imports = [./nixos/modules/virtualisation/qemu-vm.nix];
  boot.initrd.systemd = {
    enable = true;
    emergencyHashedPassword = "";
    # services.foo = {
    #   wantedBy = ["initrd.target"];
    #   after = ["local-fs.target"];
    #   before = ["initrd.target" "initrd-cleanup.service"];
    #   serviceConfig.ExecStart = "/bin/systemctl emergency";
    #   serviceConfig.Type = "oneshot";
    # };
  };
  virtualisation = {
    graphics = false;
    memorySize = 8192;
    cores = 4;
  };
  documentation.enable = false;
  services.getty.autologinUser = "root";
}; }

$ systemd-analyze
Startup finished in 392ms (kernel) + 1.902s (initrd) + 6.241s (userspace) = 8.536s
multi-user.target reached after 6.166s in userspace

    boot.initrd.systemd.objects = [
      { object = pkgs.busybox; }
      { object = pkgs.writeShellScript "nixos-activation" ''
        set -x
        init=
        for o in $(</proc/cmdline); do
            case $o in
                boot.debugtrace)
                    # Show each command.
                    set -x
                    ;;
                init=*)
                    set -- $(IFS==; echo $o)
                    init=$2
                    ;;
            esac
        done
        closure="$(${pkgs.busybox}/bin/dirname "$init")"


        ${pkgs.busybox}/bin/mkdir -p /sysroot/run
        ${pkgs.busybox}/bin/mount --bind /run /sysroot/run
        ${pkgs.busybox}/bin/chroot /sysroot $closure/systemd-activation
        ${pkgs.busybox}/bin/umount /sysroot/run
      ''; symlink = "/nixos-activation"; }