bug#48734: 28.0.50; Performance regression in `string-width`?

[Eli Zaretskii]

> Date: Mon, 31 May 2021 17:28:44 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: larsi@gnus.org, 48734@debbugs.gnu.org
> 
> > (benchmark-run 1
> >   (let ((str))
> >     (with-temp-buffer
> >       (insert-file-contents "~/2591-0.txt")
> >       (setq str (string-replace "\n" " " (buffer-string))))
> >     (print (string-width str)))) ;;;; beware this now hangs
> > 
> > I waited a minute for it to finish before killing Emacs.
> 
> Why would someone want to measure the visible width of a 550KB string?
> Is that a real-life use case?
> 
> But I think I see the reason, and will try to improve this.

Turns out I completely misunderstood how find_automatic_composition
works (because its API is deceptively similar to that of
find_composition, and the crucial differences aren't documented).  So
I will need to restructure the code in lisp_string_width to deal
correctly with automatic compositions; stay tuned.