If I have a mix.exs file something like:
mix.exs
defmodule Mix.Tasks.My_task do use Mix.Task @shortdoc \"Perform my task\" def run(_) do IO.
It looks like you can define inside the project block (mix.exs) using default_task:
def project do [default_task: "run"] end
More info: https://github.com/elixir-lang/elixir/blob/f3f64cdba205253ca0bbc0ce6a5ffffdd500ffb48f/lib/mix/lib/mix/project.ex#L266-L280