Just to add: this also works for rendering components to plain HTML, without having to define a separate template / view, for example:

DevApp.ProductComponent.render(%{product: %{id: 1}}) 
|> Phoenix.HTML.Safe.to_iodata() 
|> IO.iodata_to_binary()

You can try

Phoenix.Template.render_to_string(<module_name>, function name in string, "html", assigns)

for example

Phoenix.Template.render_to_string(MyAppWeb.TaskHTML, "show_task", "html", task: task)

Nice finding, thanks for sharing!

I found one more usable API that achieves the same, with the advantage of a compilation error in case the template name/function is invalid:

MyModule.my_function_component(%{some: assign_value})
|> Phoenix.LiveViewTest.rendered_to_string()

Phoenix.LiveViewTest.rendered_to_string/1’s implementation is, no surprise, similar to the first solution in this thread:

  def rendered_to_string(rendered) do
    rendered
    |> Phoenix.HTML.html_escape()
    |> Phoenix.HTML.safe_to_string()
  end

Example usage

Passing some HTML code to my template, for example an embed code that the user can copy:

  def mount(%{"id" => id}, _session, socket) do
    socket =
      socket
      |> assign(
        id: id,
        embed_code: embed_code(%{id: id}) |> Phoenix.LiveViewTest.rendered_to_string()
        # embed_code: Phoenix.Template.render_to_string(__MODULE__, "embed_code", "html", %{id: id})
      )

    {:ok, socket}
  end

  attr :id, :string, required: true

  def embed_code(assigns) do
    ~H"""
    ...<%= @id %>...
    """
  end