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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> I have no idea.  The way you present this is waaay above my level of
> understanding.
>
> Completion takes text typed by the user and produces strings that the
> user could possibly mean by typing what he/she typed.  Some part(s) of
> the candidates can legitimately come from what the user typed, some
> other part(s) could be invented by the completion machinery more or
> less out of thin air.  Why should anyone expect this meat-grinder to
> refrain from destructively modifying any of the involved strings?

Because the overwhelming majority of strings are never modified.
It's very unusual to modify a string by side effect (as opposed to
creating a new string object via `concat`, `substring`, ...).
This is true in most languages, AFAICT, but it's definitely true in
ELisp.


        Stefan