Renamed instance_exec to instance_eval_with_args.

We don't want to expose instance_exec as a public API; it's just for RSpec's internal use.  I also added some guard conditionals to the implementation so that we only use the 1.8.6 instance_exec hack if it's really necessary.

lib/rspec/core.rb

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

lib/rspec/core/example_group.rb

@@ -2,7 +2,7 @@ module RSpec
   module Core
     class ExampleGroup
       extend  Hooks
-      include InstanceExec
+      include InstanceEvalWithArgs
       include Subject
       include Let
       include Pending
@@ -192,7 +192,7 @@ def self.eval_before_alls(example_group_instance)
       def self.eval_around_eachs(example_group_instance, wrapped_example)
         around_hooks.reverse.inject(wrapped_example) do |wrapper, hook|
           def wrapper.run; call; end
-          lambda { example_group_instance.instance_exec(wrapper, &hook) }
+          lambda { example_group_instance.instance_eval_with_args(wrapper, &hook) }
         end
       end
 

lib/rspec/core/instance_eval_with_args.rb

@@ -0,0 +1,38 @@
+module RSpec
+  module Core
+    module InstanceEvalWithArgs
+      # 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_eval_with_args(*args, &block)
+        return instance_exec(*args, &block) if respond_to?(:instance_exec)
+
+        # If there are no args and the block doesn't expect any, there's no
+        # need to fake instance_exec with our hack below.
+        # Notes:
+        #   * lambda { }.arity # => -1
+        #   * lambda { || }.arity # => 0
+        #   * lambda { |*a| }.arity # -1
+        return instance_eval(&block) if block.arity < 1 && args.size.zero?
+
+        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
+