Fix erts handling when loading apps

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, :erts]
-      :erlang -> [:compiler, :erts]
+      :elixir -> [:ex_unit, :iex, :mix, :elixir]
+      :erlang -> [:compiler]
     end
   end
 
@@ -136,8 +136,12 @@ defmodule Mix.AppLoader do
   end
 
   # We have processed all apps.
-  defp traverse_apps([], _seen, _deps_children, _builtin_paths, _lib_path) do
-    []
+  defp traverse_apps([], seen, _deps_children, builtin_paths, _lib_path) do
+    # We want to keep erts in the load path but it doesn't require to be loaded.
+    case builtin_paths do
+      %{erts: path} when not is_map_key(seen, :erts) -> [{:erts, path}]
+      %{} -> []
+    end
   end
 
   defp app_children(app) do

lib/mix/lib/mix/tasks/compile.all.ex

@@ -31,7 +31,7 @@ defmodule Mix.Tasks.Compile.All do
     {loaded_paths, loaded_modules} =
       Mix.AppLoader.load_apps(apps, deps, config, {[], []}, fn {app, path}, {paths, mods} ->
         paths = if path, do: [path | paths], else: paths
-        mods = if app_cache, do: [{app, Application.spec(app, :modules)} | mods], else: mods
+        mods = if app_cache, do: [{app, Application.spec(app, :modules) || []} | mods], else: mods
         {paths, mods}
       end)
 

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

@@ -336,8 +336,9 @@ defmodule Mix.Tasks.CompileTest do
       assert Mix.Task.run("compile", []) == {:ok, []}
       assert :code.where_is_file(~c"parsetools.app") == :non_existing
 
-      # Make sure erts is also kept
+      # Make sure erts is also kept but not loaded
       assert {:docs_v1, _, _, _, _, _, _} = Code.fetch_docs(:zlib)
+      assert Application.spec(:erts, :vsn) == nil
     end)
   end