>>>>   (mapcar 'car (region-bounds))
>>>
>>> Sounds reasonable.
>>
>> Then here is the implementation that is more less in line with
>> how other rectangular functions are implemented:
>>
>> diff --git a/lisp/rect.el b/lisp/rect.el
>> +(advice-add 'region-beginning :around #'rectangle--region-beginning)
>
> This causes a strange compilation error:
>   Symbol's function definition is void: easy-menu-define

This patch avoids such problem: