bug#28625: [PROPOSED] Speed up (format "%s" STRING) and the like

[Eli Zaretskii]

> From: Philipp Stephani <p.stephani2@gmail.com>
> Date: Sat, 07 Oct 2017 10:56:52 +0000
> Cc: jwiegley@gmail.com, eggert@cs.ucla.edu, 28625@debbugs.gnu.org
> 
>  > Shouldn't this at least be documented in the "Incompatible Lisp changes" 
> section of NEWS?
> 
>  If it's incompatible, sure. Is it? Can you show the effect of the
>  incompatibility?
> 
> $ emacs -Q -batch -eval '(let* ((a "a") (b (format "%s" a))) (aset a 0 ?b) 
> (print b))'
> 
> With Emacs 25.3 this prints "a", with current master it prints "b". 

You could see the same behavior with earlier versions when the string
was empty, so we just expanded this behavior, not really changed it.

However, I see your point, so I moved the entry into the Incompatible
Lisp Changes section.