bug#25777: 25.1; [PATCH] `rectangle--pos-cols' should not move point

[Noam Postavsky <npostavs@users.sourceforge.net>]

On Fri, Mar 3, 2017 at 11:44 AM, Drew Adams <drew.adams@oracle.com> wrote:
>> >
>> > No, I don't think so.  What did you have in mind?  It can
>> > reset window parameter `rectangle--point-crutches' or variable
>> > `rectangle--mark-crutches', but I don't think those actions are
>> > worth mentioning.  Do you?
>>
>> I thought they might be important.  I'm not really sure what the
>> user-visible effect of those are though.
>
> AFAICT, those make use of recorded "crutches" (which are
> nowhere described explicitly, but which apparently associate
> a buffer position with a display column).  And that, for a
> given window.
>
> They seem important for proper handling of a rectangle
> regarded with respect to display (columns are a display
> thing in this context).
>
> Put differently, and comparing the code and doc for Emacs 25
> with previous releases, it seems that Emacs 25 started
> treating rectangles, in at least some cases, wrt _display_
> and not just buffer position: respect of wide chars, window,
> places past eol,

Ah, with this hint I figured out what the user-visible side-effect is,
from 'emacs -Q' do:

C-x SPC
C-3 C-p M-f ;; the bottom 2 lines of the rectangle now extend past eol
C-x C-x ;; point is now past eol
M-: (save-excursion (rectangle--pos-cols 1 3)) RET ;; => (0 . 2)
C-f ;; Now point jumps to column 1 instead of 8.

So calling rectangle--pos-cols with START and END not corresponding to
the current rectangle can mess with the selection.

>
> The use case I have, in `modeline-posn.el', is about the
> rectangular region.  It tries to report on displayed
> columns, not buffer positions.  So I think that for my
> use case, at least, I need the full-blown
> `rectangle--pos-cols' code (under whatever name).

Perhaps you want this:

    (defun rectangle-columns ()
      "Return the current rectangle's columns.
    The return value is a cons of the form (START-COLUMN . END-COLUMN)."
      (save-excursion (rectangle--pos-cols (region-beginning) (region-end))))

Which should call rectangle--pos-cols only with positions
corresponding to the current rectangle, and so won't be able to affect
the selection (I think?).