Add a sketch implementation of args diffing

Related #434. This uses the differ that is now in RSpec support to
perform argument diffing when arguments don't match.

Things worth noting/still to do:

* Special case behaviour needs to be added for the no arguments matcher
  and the any arguments matcher, at the moment something like this:
       @@ -1,2 +1,2 @@
       -[#<RSpec::Mocks::ArgumentMatchers::NoArgsMatcher:0x000001024ec728>]
       +[{:bees=>:foo}]
  gets printed, and that's obviously not great.
* Need to work out what to do in the "similar args" case where there are
  potientially multiple calls. I think the best thing to do would be to
  would be to only diff if there's one call.

There's some refactoring that could be done here but I want to finish
implementing the "similar args" implementation before I do that so that
I'm sure that I'm refactoring the right thing.

lib/rspec/mocks/error_generator.rb

@@ -40,28 +40,29 @@ def raise_unexpected_message_error(message, *args)
       def raise_unexpected_message_args_error(expectation, *args)
         expected_args = format_args(*expectation.expected_args)
         actual_args = format_received_args(*args)
-        __raise "#{intro} received #{expectation.message.inspect} with " \
-                "unexpected arguments\n  expected: #{expected_args}\n" \
-                "       got: #{actual_args}"
+        diff = diff_message(expectation.expected_args, args)
+        message = "#{intro} received #{expectation.message.inspect} #{unexpected_arguments_message(expected_args, actual_args)}"
+        message << " #{diff}" unless diff.empty?
+        __raise message
       end
 
       # @private
       def raise_missing_default_stub_error(expectation, *args)
         expected_args = format_args(*expectation.expected_args)
         actual_args = format_received_args(*args)
-        __raise "#{intro} received #{expectation.message.inspect} with " \
-                "unexpected arguments\n  expected: #{expected_args}\n" \
-                "       got: #{actual_args}\n Please stub a default value " \
-                "first if message might be received with other args as well. \n"
+        diff = diff_message(expectation.expected_args, args)
+        message = "#{intro} received #{expectation.message.inspect} #{unexpected_arguments_message(expected_args, actual_args)}"
+        message << " #{diff}" unless diff.empty?
+        message << "\n Please stub a default value first if message might be received with other args as well. \n"
+        __raise message
       end
 
       # @private
       def raise_similar_message_args_error(expectation, *args_for_multiple_calls)
         expected_args = format_args(*expectation.expected_args)
         actual_args = args_for_multiple_calls.map { |a| format_received_args(*a) }.join(", ")
-        __raise "#{intro} received #{expectation.message.inspect} with " \
-                "unexpected arguments\n  expected: #{expected_args}\n" \
-                "       got: #{actual_args}"
+        actual_args = args_for_multiple_calls.collect {|a| format_received_args(*a)}.join(", ")
+        __raise "#{intro} received #{expectation.message.inspect} #{unexpected_arguments_message(expected_args, actual_args)}"
       end
 
       # rubocop:disable Style/ParameterLists
@@ -194,6 +195,14 @@ def self.raise_double_negation_error(wrapped_expression)
 
     private
 
+      def unexpected_arguments_message(expected_args_string, actual_args_string)
+        "with unexpected arguments\n  expected: #{expected_args_string}\n       got: #{actual_args_string}"
+      end
+
+      def diff_message(expected_args, actual_args)
+        RSpec::Support::Differ.new(:color => true).diff(actual_args, expected_args)
+      end
+
       def intro
         if @name
           "Double #{@name.inspect}"

spec/rspec/mocks/double_spec.rb

@@ -257,7 +257,7 @@ def @double.method_with_default_argument(arg={}); end
           expect {
             @double.method_with_default_argument(nil)
             verify @double
-          }.to raise_error(RSpec::Mocks::MockExpectationError, "Double \"test double\" received :method_with_default_argument with unexpected arguments\n  expected: ({})\n       got: (nil)")
+          }.to raise_error(RSpec::Mocks::MockExpectationError, "Double \"test double\" received :method_with_default_argument with unexpected arguments\n  expected: ({})\n       got: (nil) \e[0m\n\e[0m\e[34m@@ -1,2 +1,2 @@\n\e[0m\e[31m-[{}]\n\e[0m\e[32m+[nil]\n\e[0m")
         end
       end