Raise when :router given to use Phoenix.VerifiedRoutes is not a literal

[novaugust]

What is this

Warn Raises when non-literal modules are passed for the :router option with VerifiedRoutes

Background

In an effort to avoid compilation-cycles, we had code where the VerifiedRoutes router was being created dynamically.
While it technically worked, that could create scenarios where it looked like routes were being enforced/verified at compile time when they really weren't, as the compiler was not aware of the dependencies between the modules.

I've since updated our code to not have that problem, but thought it might be worth having a warning come out of Phoenix itself.

There's already existing code that does similar work within the private build_route function:

phoenix/lib/phoenix/verified_routes.ex

Lines 932 to 943 in cb69bb8

	router =
	case Macro.expand(router, env) do
	mod when is_atom(mod) ->
	mod
	other ->
	raise ArgumentError, """
	expected router to be to module, got: #{inspect(other)}
	If your router is not defined at compile-time, use unverified_path/3 instead.
	"""
	end

I'm wondering if the intention there was the same as what i have here, save that that missed cases like this where the value could be expanded into a module.

Example

Given a dynamically resolved router:

defmodule MyAppWeb.SubMod.NeatCode do
  use Phoenix.VerifiedRoutes,
    router: __MODULE__ |> Module.split() |> List.replace_at(-1, "Router") |> Module.concat()

  def my_path, do: ~p"/my_path"
end

in our setup, I could get this buggy behaviour due to the missing explicit compile-time dependency:

1. have the example code above
2. delete the relevant action from the router
3. compile & get a warning about the invalid ~p"/my_path" route
4. restore the route to the router
5. compile, and still get the warning

Assisted by: claude-sonnet-5