Fix issue with nested binaries in Macro.to_string/1, closes #1522

Author: José Valim

lib/elixir/lib/macro.ex

@@ -197,7 +197,10 @@ defmodule Macro do
 
   # Bits containers
   def to_string({ :<<>>, _, args }) do
-    "<<" <> Enum.map_join(args, ", ", to_string(&1)) <> ">>"
+    case Enum.map_join(args, ", ", to_string(&1)) do
+      "<" <> rest -> "<< <" <> rest  <> " >>"
+      rest -> "<<" <> rest <> ">>"
+    end
   end
 
   # Tuple containers

lib/elixir/test/elixir/macro_test.exs

@@ -325,6 +325,7 @@ defmodule MacroTest do
     assert Macro.to_string(quote do: { 1, 2, 3 })   == "{1, 2, 3}"
     assert Macro.to_string(quote do: [ 1, 2, 3 ])   == "[1, 2, 3]"
     assert Macro.to_string(quote do: << 1, 2, 3 >>) == "<<1, 2, 3>>"
+    assert Macro.to_string(quote do: << <<1>> >>) == "<< <<1>> >>"
   end
 
   test :binary_ops_to_string do