* lisp/emacs-lisp/syntax.el (syntax-propertize): ignore is like nil

(syntax-ppss-after-change-function): Mark it as obsolete.
* lisp/font-lock.el (font-lock-apply-syntactic-highlight):
Use syntax-ppss-flush-cache instead.

Author: monnier

lisp/emacs-lisp/syntax.el

@@ -286,7 +286,7 @@ END) suitable for `syntax-propertize-function'."
 (defun syntax-propertize (pos)
   "Ensure that syntax-table properties are set until POS (a buffer point)."
   (when (< syntax-propertize--done pos)
-    (if (null syntax-propertize-function)
+    (if (memq syntax-propertize-function '(nil ignore))
         (setq syntax-propertize--done (max (point-max) pos))
       ;; (message "Needs to syntax-propertize from %s to %s"
       ;;          syntax-propertize--done pos)
@@ -404,7 +404,8 @@ These are valid when the buffer has no restriction.")
 (defvar-local syntax-ppss-narrow-start nil
   "Start position of the narrowing for `syntax-ppss-narrow'.")
 
-(defalias 'syntax-ppss-after-change-function 'syntax-ppss-flush-cache)
+(define-obsolete-function-alias 'syntax-ppss-after-change-function
+  #'syntax-ppss-flush-cache "27.1")
 (defun syntax-ppss-flush-cache (beg &rest ignored)
   "Flush the cache of `syntax-ppss' starting at position BEG."
   ;; Set syntax-propertize to refontify anything past beg.

lisp/font-lock.el

@@ -1501,7 +1501,7 @@ see `font-lock-syntactic-keywords'."
       ;; Flush the syntax-cache.  I believe this is not necessary for
       ;; font-lock's use of syntax-ppss, but I'm not 100% sure and it can
       ;; still be necessary for other users of syntax-ppss anyway.
-      (syntax-ppss-after-change-function start)
+      (syntax-ppss-flush-cache start)
       (cond
        ((not override)
 	;; Cannot override existing fontification.