Ok, now I added elixir embedding in heex, as well as
elixir->heex->elixir that e.g. highlights '@entries' in:

  def unordered_list(assigns) do
    ~H"""
    <ul>
      <li :for={entry <- @entries}>{render_slot(@inner_block, entry)}</li>
    </ul>
    """
  end

that works well.