module ActiveSupport::Dependencies

def load_missing_constant(from_mod, const_name)
  unless qualified_const_defined?(from_mod.name) && Inflector.constantize(from_mod.name).equal?(from_mod)
    raise ArgumentError, "A copy of #{from_mod} has been removed from the module tree but is still active!"
  end
  qualified_name = qualified_name_for(from_mod, const_name)
  path_suffix = qualified_name.underscore
  file_path = search_for_file(path_suffix)
  if file_path
    expanded = File.expand_path(file_path)
    expanded.sub!(/\.rb\z/, "")
    if loading.include?(expanded)
      raise "Circular dependency detected while autoloading constant #{qualified_name}"
    else
      require_or_load(expanded, qualified_name)
      if from_mod.const_defined?(const_name, false)
        log("constant #{qualified_name} autoloaded from #{expanded}.rb")
        return from_mod.const_get(const_name)
      else
        raise LoadError, "Unable to autoload constant #{qualified_name}, expected #{file_path} to define it"
      end
    end
  elsif mod = autoload_module!(from_mod, const_name, qualified_name, path_suffix)
    return mod
  elsif (parent = from_mod.module_parent) && parent != from_mod &&
        ! from_mod.module_parents.any? { |p| p.const_defined?(const_name, false) }
    # If our parents do not have a constant named +const_name+ then we are free
    # to attempt to load upwards. If they do have such a constant, then this
    # const_missing must be due to from_mod::const_name, which should not
    # return constants from from_mod's parents.
    begin
      # Since Ruby does not pass the nesting at the point the unknown
      # constant triggered the callback we cannot fully emulate constant
      # name lookup and need to make a trade-off: we are going to assume
      # that the nesting in the body of Foo::Bar is [Foo::Bar, Foo] even
      # though it might not be. Counterexamples are
      #
      #   class Foo::Bar
      #     Module.nesting # => [Foo::Bar]
      #   end
      #
      # or
      #
      #   module M::N
      #     module S::T
      #       Module.nesting # => [S::T, M::N]
      #     end
      #   end
      #
      # for example.
      return parent.const_missing(const_name)
    rescue NameError => e
      raise unless e.missing_name? qualified_name_for(parent, const_name)
    end
  end
  name_error = NameError.new("uninitialized constant #{qualified_name}", const_name)
  name_error.set_backtrace(caller.reject { |l| l.starts_with? __FILE__ })
  raise name_error
end