`assert_push` weirdness

README.md

If I run the tests with the code in bad.ex, I get the following:

warning: variable expected_song is unused
  test/channels/collusion_channel_test.exs:16

If I use good.ex, it works. (naming!)

This is a macro that's doing a match behind the scenes. If I am not careful to write a test that fails first, I'll think this works when it doesn't because it binds whatever I pass. For instance, if I change the test such that expected_song = %{}, which is not what the channel sends me, it will still pass without the hat obviously (though it does throw a warning)

This is super weird to me.

bad.ex

    test "sends the collusion's state to the client on join", %{id: id} do
      expected_song = Song.init(id)
      assert_push "collusion:state", expected_song
    end

good.ex

    test "sends the collusion's state to the client on join", %{id: id} do
      expected_song = Song.init(id)
      assert_push "collusion:state", ^expected_song
    end