bug#64391: buffer narrowing slowdown regression in emacs 29

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

> I'm sorry, I don't understand your question.  The only way (except by using
> internal functions) to enter a labeled restriction is to use the
> with-restriction special form with its optional label argument.

Nitpick: it's not a special form, it's a macro.  There's a *big*
difference because adding a special form requires changing
`macroexpand-all`, the compiler, yadayada, and it introduces backward
incompatibilities with packages doing their own code-walks.

> Indeed, but I'd say it's clear enough from the context that "symbol" means
> a quoted symbol here.

Other nitpick: nil can also be quoted :-)
BTW, "LABEL is a symbol" sends the wrong message (a quoted
symbol evaluates to a symbol but it's not itself a symbol).
IOW the docstring should clarify that LABEL is an expression that's
evaluated at runtime (and should return a symbol).
While I'm here, is it important that LABEL evaluates to a symbol?
Or is it like `catch/throw` where we expect most uses to use a symbol
but where any other (non-nil) value works as well?


        Stefan


diff --git a/lisp/subr.el b/lisp/subr.el
index 85adef5b689..f8a34796584 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3964,11 +3964,11 @@ with-restriction
 
 The current restrictions, if any, are restored upon return.
 
-When the optional :label LABEL argument is present, in which
-LABEL is a symbol, inside BODY, `narrow-to-region' and `widen'
-can be used only within the START and END limits.  To gain access
-to other portions of the buffer, use `without-restriction' with the
-same LABEL argument.
+When the optional :label LABEL argument is present (in which
+LABEL evaluates to a non-nil symbol) inside BODY, `narrow-to-region'
+and `widen' can be used only within the START and END limits.
+To gain access to other portions of the buffer, use `without-restriction'
+with the same LABEL argument.
 
 \(fn START END [:label LABEL] BODY)"
   (declare (indent 2) (debug t))