Add index reflection to private records

Example:
defrecordp :user, [:name, age]
user(:age) #=> 2

Author: ericmj

lib/elixir/lib/record.ex

@@ -453,14 +453,24 @@ defmodule Record do
     Module.put_attribute(module, :record_optimizable, functions)
   end
 
+  @doc false
+  # Implements the access macro used by records.
+  # It returns the index of the given field.
+  def access(atom, fields, arg, _caller) when is_atom(arg) do
+    if index = find_index(fields, arg, 0) do
+      index + 1
+    else
+      raise ArgumentError, message: "record #{inspect atom} does not have the key: #{inspect arg}"
+    end
+  end
+
   # Implements the access macro used by records.
   # It returns a quoted expression that defines
   # a record or a match in case the record is
   # inside a match.
-  @doc false
   def access(atom, fields, keyword, caller) do
     unless is_keyword(keyword) do
-      raise ArgumentError, message: "expected contents inside brackets to be a keyword list, got: #{inspect keyword}"
+      raise ArgumentError, message: "expected contents inside brackets to be a keyword list or an atom, got: #{inspect keyword}"
     end
 
     in_match = caller.in_match?

lib/elixir/test/elixir/kernel/errors_test.exs

@@ -491,7 +491,7 @@ defmodule Kernel.ErrorsTest do
   end
 
   test :invalid_access_protocol_not_keywords do
-    assert_raise ArgumentError, "expected contents inside brackets to be a keyword list, got: [0]", fn ->
+    assert_raise ArgumentError, "expected contents inside brackets to be a keyword list or an atom, got: [0]", fn ->
       defmodule ErrorsTest do
         def sample(Kernel.ErrorsTest.Config[0]), do: true
       end

lib/elixir/test/elixir/record/private_test.exs

@@ -45,6 +45,14 @@ defmodule Record.PrivateTest do
       _user(user, [:age, :name])
     end
 
+    def name_ix() do
+      _user(:name)
+    end
+
+    def age_ix() do
+      _user(:age)
+    end
+
     def my_new() do
       _my_user()
     end
@@ -77,6 +85,14 @@ defmodule Record.PrivateTest do
       _my_user(user, [:age, :name])
     end
 
+    def my_name_ix() do
+      _my_user(:name)
+    end
+
+    def my_age_ix() do
+      _my_user(:age)
+    end
+
     def file_info() do
       _file_info()
     end
@@ -128,6 +144,13 @@ defmodule Record.PrivateTest do
     assert elem(record, 0) == Macros
   end
 
+  test "defrecordp access index" do
+    assert Macros.name_ix == 1
+    assert Macros.age_ix == 2
+    assert Macros.my_name_ix == 1
+    assert Macros.my_age_ix == 2
+  end
+
   test "defrecordp with dynamic arguments" do
     assert [:_file_info|_] = Macros.file_info() |> tuple_to_list
     assert { :_dynamic, nil } = Macros.dynamic()