Some performance tweaks to ActiveSupport::Memoizable

Signed-off-by: NJoshua Peek <josh@joshpeek.com>

activesupport/lib/active_support/memoizable.rb

@@ -7,10 +7,12 @@ def self.included(base) #:nodoc:
     module ClassMethods
       def memoize(symbol)
         original_method = "_unmemoized_#{symbol}"
+        raise "Already memoized #{symbol}" if instance_methods.map(&:to_s).include?(original_method)
+
         alias_method original_method, symbol
         class_eval <<-EOS, __FILE__, __LINE__
           def #{symbol}
-            if instance_variable_defined?(:@#{symbol})
+            if defined? @#{symbol}
               @#{symbol}
             else
               @#{symbol} = #{original_method}
@@ -22,7 +24,7 @@ def #{symbol}
 
     def freeze
       methods.each do |method|
-        if m = method.to_s.match(/^_unmemoized_(.*)/)
+        if m = method.to_s.match(/\A_unmemoized_(.*)/)
           send(m[1]).freeze
         end
       end

activesupport/test/memoizable_test.rb

@@ -41,5 +41,9 @@ def test_memoization_frozen_with_nil_value
       person.freeze
       assert_equal nil, person.age
     end
+
+    def test_double_memoization
+      assert_raise(RuntimeError) { Person.memoize :name }
+    end
   end
 end