add instance_exec for Ruby 1.8.6

lib/rspec/core.rb

@@ -1,4 +1,5 @@
 require 'rspec/core/kernel_extensions'
+require 'rspec/core/instance_exec'
 require 'rspec/core/object_extensions'
 require 'rspec/core/load_path'
 require 'rspec/core/deprecation'

lib/rspec/core/example_group.rb

@@ -2,6 +2,7 @@ module RSpec
   module Core
     class ExampleGroup
       extend  Hooks
+      include InstanceExec
       include Subject
       include Let
       include Pending

lib/rspec/core/instance_exec.rb

@@ -0,0 +1,30 @@
+module RSpec
+  module Core
+    module InstanceExec
+      unless respond_to?(:instance_exec)
+        # based on Bounded Spec InstanceExec (Mauricio Fernandez)
+        # http://eigenclass.org/hiki/bounded+space+instance_exec
+        # - uses singleton_class instead of global InstanceExecHelper module
+        # - this keeps it scoped to classes/modules that include this module
+        # - only necessary for ruby 1.8.6
+        def instance_exec(*args, &block)
+          singleton_class = (class << self; self; end)
+          begin
+            orig_critical, Thread.critical = Thread.critical, true
+            n = 0
+            n += 1 while respond_to?(method_name="__instance_exec#{n}")
+            singleton_class.module_eval{ define_method(method_name, &block) }
+          ensure
+            Thread.critical = orig_critical
+          end
+          begin
+            return send(method_name, *args)
+          ensure
+            singleton_class.module_eval{ remove_method(method_name) } rescue nil
+          end
+        end
+      end
+    end
+  end
+end
+