How to default ExUnit.CaseTemplate to `async: true`

I’ve been using Elixir for a while and only today realized that the default async value for use ExUnit.Case is false.

It seems I’m not the only one who thought this was the case: Default value for ExUnit.Case async

Anyways, I spent some time looking at the code of ExUnit.Case as well as ExUnit.CaseTemplate, and was able to figure out a way to make async: true the default. So I thought I’d share it here:

defmodule MyApp.Case do
  @moduledoc false

  use ExUnit.CaseTemplate

  # Override `ExUnit.CaseTemplate.__using__` so we can manipulate opts and
  # default to `async: true`.
  #
  # For reference, see the original `defmacro __using__` that is injected
  # by ExUnit.CaseTemplate:
  # https://github.com/elixir-lang/elixir/blob/main/lib/ex_unit/lib/ex_unit/case_template.ex#L142
  defmacro __using__(opts) do
    parent = ExUnit.CaseTemplate.__proxy__(__MODULE__, default_async_true(opts))
    injected_frontmatter = frontmatter()
    {:__block__, [], [parent, injected_frontmatter]}
  end

  defp default_async_true(opts) do
    case Keyword.fetch(opts, :async) do
      {:ok, false} ->
        # If `async: false` is specified, leave opts alone.
        opts

      _ ->
        # If `async: false` is not specified, set `async: true`
        Keyword.put(opts, :async, true)
    end
  end

  defp frontmatter do
    quote do
      # Here's where you'd put useful aliases that normally go in `using do`
    end
  end
end

Now if you do use MyApp.Case, it’ll get async: true by default!