module RSpec::Mocks::InstanceExec

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