pipe_through [SomePlugModule] should not introduce a compile-time dependency

[marcandre]

Environment

$ elixir -v 
Erlang/OTP 24 [erts-12.0.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]

Elixir 1.12.1 (compiled with Erlang/OTP 22)

• Phoenix version (mix deps): 1.5.10

Expected behavior

  scope "/", HelloWeb do
    pipe_through [:browser, HelloWeb.Plugs.SomePlug]
    # ...

This should not creates a compile-time dependency to the file where SomePlug is defined but it does:

    $ mix xref graph --source lib/hello_web/router.ex --label=compile
    lib/hello_web/router.ex
    ├── lib/hello_web.ex (compile)
    ├── lib/hello_web/plugs/some_plug.ex (compile)
    └── lib/hello_web/telemetry.ex (compile)

Note that the equivalent code below does not create such a compile-time dependency:

  pipeline :some_pipe do
    plug HelloWeb.Plugs.SomePlug
  end

  scope "/", HelloWeb do
    pipe_through [:browser, :some_pipe]
    # ...

$ mix xref graph --source lib/hello_web/router.ex --label=compile
lib/hello_web/router.ex
├── lib/hello_web.ex (compile)
└── lib/hello_web/telemetry.ex (compile)

Basic project showing this: with compile-time dependency and without (marcandre/basic_phoenix@916e769)

[marcandre]

Let me know if I should make a PR for this

[marcandre]

Confirmed still present in master as of today (0da9b5c)