bug#32523: 27.0.50; Emacs hangs when killing rectangle

[Eli Zaretskii <eliz@gnu.org>]

> From: Joseph Mingrone <jrm@ftfl.ca>
> Date: Fri, 24 Aug 2018 21:59:48 -0300
> 
> Here is a recipe to make Emacs (nearly) hang indefinitely.
> 
> 1. emacs -Q
> 
> 2. Visit the file found https://ftfl.ca/misc/big_file_hangs_emacs.txt
> 
> 3. M-x toggle-truncate-lines
> 
> 4. Use rectangle-mark-mode (C-x SPC) to mark the rectangle that starts
>    at the top left of the file (point 1), and includes the leading white
>    space, the line numbers, and the space after the line numbers (point
>    468848).
> 
> 5. Kill the rectangle with C-x r k.
> 
> For me, the Emacs process will continue to use 100% CPU and Emacs is
> almost completely unresponsive and has to be killed.  Some actions such
> as saving the file may complete, but only after a few minutes.

It doesn't hang, it just takes very long to finish that operation (3
min on my machine with an unoptimized build; should be something like
1 to 1.5 min in an optimized build).

This belongs to the "Emacs is very slow with long lines" class of
problems: the file has 2900-character lines.  If this file will never
include any text, I suggest to visit it with
"M-x find-file-literally", then the problem of slowness will go away.