Because the warning comes from elixir 1.7, supposedly (I haven’t seen this warning in elixir 1.6), and phoenix v1.3 was released before elixir 1.7.

The warning is about a keyword list in aliases in your mix.exs. If you check it, you’d see something like

defp aliases do
  [
    # ...
    "test":  #...
  ]
end

there.

The warning says that "test": can safely be replaced with test:.

If it is only possible to create with quotes, it shouldn’t create any warning. If it indeed creates a warning and is not possible without quotes, please file a bug at GitHub - elixir-lang/elixir: Elixir is a dynamic, functional language for building scalable and maintainable applications · GitHub.

edit

For me :"$" doesn’t create a warning. I’m on Elixir 1.7.3 compiled and running on OTP 21.

I’ve removed the quotes from the atom but it still moans about the line… my guess is that it doesn’t like the other quoted “test” either, but I assume that is a command parameter that needs to be quoted?

  defp aliases do
    [
      "ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
      "ecto.reset": ["ecto.drop", "ecto.setup"],
      test: ["ecto.create --quiet", "ecto.migrate", "test"]
    ]
  end