Is there a way to match controller action in conn struct?

In the docs I’ve already seen that Phoenix.Controller.action_name/1 returns the action name as an atom and raises an error if unavailable.

I also know that inside a controller I can do something like:

plug :my_plug when action in [:index]

But I would rather like to match it as below because I will be calling the plug inside my router:

  defp my_plug(conn = %{private: %{:phoenix_action => :index}}, _),
    do: put_flash(conn, :info, ":index action matched !!!!")

  defp my_plug(conn, _), do: put_flash(conn, :info, "Matching failed !!!!")

I’m doing that because after inspecting conn in the console I saw something like
%{..., private: %{..., :phoenix_action => :index, ....}, ...}.

Unfortunately the matching always fails.

Are there other ways to do this?