Add a default hook name for add_compile_callback

lib/module.ex

@@ -109,7 +109,7 @@ defmodule Module do
   # can be updated throughout the module definition and therefore, the final
   # value of the data can only be compiled using a compiation callback,
   # which will read the final value of :some_data and compile to a function.
-  def add_compile_callback(module, target, fun) do
+  def add_compile_callback(module, target, fun // :__compiling__) do
     assert_already_compiled!(:add_compile_callback, module)
     new   = { target, fun }
     table = table_for(module)

test/elixir/module_test.exs

@@ -5,9 +5,14 @@ defmodule ModuleTest::ToBeUsed do
 
   def __using__(target) do
     Module.merge_data target, callback: false
+    Module.add_compile_callback(target, __MODULE__)
     Module.add_compile_callback(target, __MODULE__, :callback)
   end
 
+  def __compiling__(target) do
+    Module.merge_data target, compiling: true
+  end
+
   defmacro callback(target) do
     value = Orddict.fetch(Module.read_data(target), :callback, nil)
     Module.merge_data target, callback: true
@@ -47,6 +52,10 @@ defmodule ModuleTest do
 
   def test_compile_callback_hook do
     false = ModuleTest::ToUse.original_value
-    [callback: true] = ModuleTest::ToUse.__data__
+    true  = Orddict.fetch ModuleTest::ToUse.__data__, :callback, false
+  end
+
+  def test_default_compile_callback_hook do
+    true  = Orddict.fetch ModuleTest::ToUse.__data__, :compiling, false
   end
 end