bug#64391: buffer narrowing slowdown regression in emacs 29

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 09 Jul 2023 16:04:09 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: acohen@ust.hk, 64391@debbugs.gnu.org, mattias.engdegard@gmail.com, 
>     monnier@iro.umontreal.ca, bugs@gnus.org
> 
> On a second thought, I believe its better to not replace LABEL with TAG, 
> because that would mean changing that word in many places, including 
> places in which such a change would make the text less understandable, 
> e.g. the docstring of narrow-to-region:

LABEL implies a string or a symbol, whereas :label can accept "any
object that is not nil".  The description of 'catch' uses TAG.

> --- a/doc/lispref/positions.texi
> +++ b/doc/lispref/positions.texi
> @@ -1169,9 +1169,10 @@ Narrowing
> 
>   @cindex labeled narrowing
>   @cindex labeled restriction
> -When the optional argument @var{label}, a symbol, is present, the
> -narrowing is @dfn{labeled}.  A labeled narrowing differs from a
> -non-labeled one in several ways:
> +When the optional argument @var{label}, which is evaluated to get the
> +label to use and must not be @code{nil},

What "must not be nil": the label or the result of its evaluation?