Keep erts when pruning load paths (#12642)

Author: josevalim

lib/mix/lib/mix/app_loader.ex

@@ -97,8 +97,8 @@ defmodule Mix.AppLoader do
 
   defp extra_apps(config) do
     case Keyword.get(config, :language, :elixir) do
-      :elixir -> [:ex_unit, :iex, :mix, :elixir]
-      :erlang -> [:compiler]
+      :elixir -> [:ex_unit, :iex, :mix, :elixir, :erts]
+      :erlang -> [:compiler, :erts]
     end
   end
 

lib/mix/test/mix/tasks/compile_test.exs

@@ -335,6 +335,9 @@ defmodule Mix.Tasks.CompileTest do
     in_fixture("no_mixfile", fn ->
       assert Mix.Task.run("compile", []) == {:ok, []}
       assert :code.where_is_file(~c"parsetools.app") == :non_existing
+
+      # Make sure erts is also kept
+      assert {:docs_v1, _, _, _, _, _, _} = Code.fetch_docs(:zlib)
     end)
   end