`:for` creates compile-time dependencies when defining protocol implementations

[ribanez7]

Environment

• Elixir & Erlang/OTP versions (elixir --version): Elixir 1.13.0
• Operating system: any

Current behavior

:for in protocol implementations creates transitive compile-time dependencies. Here you can see an example, within a basic newly created phoenix app.

Ecto.Schema wrapper:

defmodule ProtoComp.Schema do
  defmacro __using__(_) do
    quote do
      use Ecto.Schema

      @timestamps_opts type: :utc_datetime_usec
    end
  end
end

Schema:

defmodule ProtoComp.Things.Chair do
  use ProtoComp.Schema
  import Ecto.Changeset

  schema "chairs" do
    field :attributes, :string
    field :color, :string

    timestamps()
  end

  @doc false
  def changeset(chair, attrs) do
    chair
    |> cast(attrs, [:color, :attributes])
    |> validate_required([:color, :attributes])
  end
end

Protocol:

defprotocol ProtoCompWeb.UseCases.GrabWith do
  def accessories(thing)
end

Implementation:

defimpl ProtoCompWeb.UseCases.GrabWith, for: ProtoComp.Things.Chair do
  def accessories(_), do: "use your hands"
end

With this setup, when running mix xref graph --label compile-connected the result is that the Chair schema is a transitive dependency for the protocol implementation.

However, if I remove the Ecto.Schema wrapper and use it directly, that compile-time dependency disappears:

defmodule ProtoComp.Things.Chair do
  use Ecto.Schema
  import Ecto.Changeset

  @timestamps_opts type: :utc_datetime_usec

  schema "chairs" do
    field :attributes, :string
  ...
end

There is a thread explaining that issue also in the elixirforum: https://elixirforum.com/t/wrapping-ecto-schema-in-macro-results-in-transitive-compile-time-dependency/46611

And you can check the repo created to reproduce the issue here: https://github.com/ribanez7/protocols_compilation

Expected behavior

:for should not add a compile-time dependency. I expect to run mix xref graph --label compile-connected within a similar setup, using protocols implemented by ecto schemas, and not having them as transitive dependencies.