test.ex

defmodule Test do
  Module.register_attribute __MODULE__, :config, accumulate: true, persist: false

  @before_compile __MODULE__
  
  defmacro __before_compile__(_) do
    quote do
      Module.put_attribute(__MODULE__, :config, { :foo, "foo" })
    end
  end
  
  def env, do: @config
end

# == Compilation error on file lib/test.ex ==
# ** (UndefinedFunctionError) undefined function: Test.__before_compile__/1