baseline.nix

{ system ? builtins.currentSystem
, baseModulesFile ? ../nixos/modules/module-list.nix
}:
let
  evalNixOS = import ../nixos/lib/eval-config.nix {
    system = system;

    baseModules = import baseModulesFile;

    modules = [
      ({ modulesPath, ... }: {
        # these are required for nixos to evaluate
        boot.loader.systemd-boot.enable = true;
        fileSystems."/".fsType = "tmpfs";
      })
    ];
  };
in
evalNixOS.config.system.build

reduce.rb

#!/usr/bin/env ruby

require "json"
require "pathname"
require "pp"
require "tempfile"

def test(modules)
  # Write the modules to a file as nix code
  tmpfile = Tempfile.new
  tmpfile.puts "["
  modules.each do |path|
    tmpfile.puts "  #{path}"
  end
  tmpfile.puts "]"
  tmpfile.flush
  system("nix-instantiate baseline.nix --arg baseModulesFile #{tmpfile.path} -A toplevel")
end

PWD = Pathname.pwd

needed = []

modules = JSON.load `nix-instantiate --eval --expr '(map toString (import ../nixos/modules/module-list.nix))' --strict --json`

while modules.size > 0 do
  try = modules.sample
  modules.delete(try)

  puts "# testing #{try}"

  if !test(needed + modules)
    # keep it, it's needed
    needed.push try
  end

  p modules.size, needed
end