bug#55205: 28.1.50; completion--replace illegally mutates completion candidates

[Eli Zaretskii <eliz@gnu.org>]

> Resent-From: Lars Ingebrigtsen <larsi@gnus.org>
> Original-Sender: "Debbugs-submit" <debbugs-submit-bounces@debbugs.gnu.org>
> Resent-CC: bug-gnu-emacs@gnu.org
> Resent-Sender: help-debbugs@gnu.org
> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Sun, 01 May 2022 13:53:59 +0200
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, 55205@debbugs.gnu.org
> 
> Daniel Mendler <mail@daniel-mendler.de> writes:
> 
> > The function completion--replace mutates the replacement string, it
> > strips the text properties.
> 
> I don't think that's, strictly speaking, illegal.  :-)
> 
> Anyway, I agree that it's unfortunate that completion destructively
> modifies the strings it's handed, and this has been discussed
> extensively over the years (and there's probably several bug reports
> open about that, although I can't find them now).
> 
> I don't remember why we're doing that, but I seem to vaguely recall that
> there's a reason...  Anybody?

I don't understand how can completion work in general without
destructively modifying strings.  Isn't that obvious?