I think you can generate the @moduledoc as part of @before_compile. The Macro.Env.t term passed into the before_compile callback should have information about what functions exist in the module.

Well, if you meant GenStage.init/1 callback then it can return different values depending on passed arguments. So unless you return always the same value, then it is not possible to achieve what you want. If you always return the same value, independently of the argument, then you can do:

defmodule A do
  @init_return …

  @moduledoc """
  #{make_mermaid_chart(@init_return)}
  """

  def init(_), do: @init_return
end