Add class to deprecate instance variables

Add ActiveSupport::Deprecation::DeprecatedInstanceVariable class to
deprecate instance variables of primitive types such as stings.

activesupport/lib/active_support/deprecation.rb

@@ -175,6 +175,20 @@ module ActiveSupport
           ActiveSupport::Deprecation.warn("#{@var} is deprecated! Call #{@method}.#{called} instead of #{@var}.#{called}. Args: #{args.inspect}", callstack)
         end
     end
+
+    class DeprecatedInstanceVariable < Delegator #:nodoc:
+      def initialize(value, method)
+        super(value)
+        @method = method
+        @value = value
+      end
+
+      def __getobj__
+        ActiveSupport::Deprecation.warn("Instance variable @#{@method} is deprecated! Call instance method #{@method} instead.")
+        @value
+      end
+    end
+
   end
 end
 

activesupport/test/deprecation_test.rb

@@ -149,3 +149,13 @@ class DeprecationTest < Test::Unit::TestCase
     assert_nil @last_message
   end
 end
+
+class DeprecatedIvarTest < Test::Unit::TestCase
+
+  def test_deprecated_ivar
+    @action = ActiveSupport::Deprecation::DeprecatedInstanceVariable.new("fubar", :foobar)
+
+    assert_deprecated(/Instance variable @foobar is deprecated! Call instance method foobar instead/) { assert_equal "fubar", @action }
+  end
+
+end