Add internal function to enter a labeled restriction

* src/editfns.c (Finternal__labeled_narrow_to_region): New
function.  A specific function is necessary to avoid unnecessary
slowdowns when 'narrow-to-region'/'widen' are called in a loop.
(Fnarrow_to_region): Remove the call to Fset, which has been moved
into Finternal__labeled_narrow_to_region.
(labeled_narrow_to_region): Use the new function.
(syms_of_editfns): Add the symbol of the new function.

* lisp/subr.el (internal--with-restriction): Use the new function.

1 files changed, 3 insertions, 2 deletions

diff --git a/lisp/subr.el b/lisp/subr.el
index 85adef5b689..0b397b7bebf 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -3980,8 +3980,9 @@ same LABEL argument.
 (defun internal--with-restriction (start end body &optional label)
   "Helper function for `with-restriction', which see."
   (save-restriction
-    (narrow-to-region start end)
-    (if label (internal--label-restriction label))
+    (if label
+        (internal--labeled-narrow-to-region start end label)
+      (narrow-to-region start end))
     (funcall body)))
 
 (defmacro without-restriction (&rest rest)