Re: master 2399541: Remove font-lock toggle from font-lock-update

[Gregory Heytings <gregory@heytings.org>]

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Thanks for your detailed reply!

>
> Yes, there are three cases: - font-lock-mode is nil - font-lock-mode is 
> t but the font-lock machinery is not activated (e.g. in text-mode with 
> the default config). - font-lock fully activated.
>

With this indication and another mail you sent to Eli, I think I finally 
understand what you mean.  One thing I'm not entirely sure is whether the 
second case is (and font-lock-mode (not font-lock-keywords)) or (and 
font-lock-mode (not font-lock-fontified)), but my guess is that 
font-lock-fontified is an internal variable and that it is safer to use 
font-lock-keywords here.

>> +Otherwise, with prefix ARG, toggle Font Lock mode."
>
> Is this behavior useful?
>

I think it is, yes, and I think it makes sense to use the prefix argument 
for that.  M-x font-lock-mode does not always produce the expected effect, 
which can be puzzling, so having a way to "do what I mean" in a command is 
useful.

I attach the patch, updated with your suggestions.

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From 49d758e5a91f19615fe1e7858b013c88d95da0a4 Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Mon, 29 Mar 2021 09:31:15 +0000
Subject: [PATCH] Improve font-lock-update and rename it to font-lock-dwim

* lisp/font-lock.el (font-lock-dwim): Renamed from 'font-lock-update'.
Only refontify the region when it is active, and act more cautiously.

* list/font-lock.el (font-lock-update-region): New function that calls
the appropriate function depending on the current Font Lock state.

* lisp/bindings.el (ctl-x-x-map): Adjust the command name.

* etc/NEWS: Adjust the command name.
---
 etc/NEWS          |  4 ++--
 lisp/bindings.el  |  2 +-
 lisp/font-lock.el | 33 +++++++++++++++++++++++++--------
 3 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 2d66a93474..4747d49ac5 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -94,7 +94,7 @@ useful on systems such as FreeBSD which ships only with "etc/termcap".
 * Changes in Emacs 28.1
 
 +++
-** New command 'font-lock-update', bound to 'C-x x f'.
+** New command 'font-lock-dwim', bound to 'C-x x f'.
 This command updates the syntax highlighting in this buffer.
 
 ** The new NonGNU ELPA archive is enabled by default alongside GNU ELPA.
@@ -255,7 +255,7 @@ The 'C-x x' keymap now holds keystrokes for various buffer-oriented
 commands.  The new keystrokes are 'C-x x g' ('revert-buffer'),
 'C-x x r' ('rename-buffer'), 'C-x x u' ('rename-uniquely'), 'C-x x n'
 ('clone-buffer'), 'C-x x i' ('insert-buffer'), 'C-x x t'
-('toggle-truncate-lines') and 'C-x x f' ('font-lock-update').
+('toggle-truncate-lines') and 'C-x x f' ('font-lock-dwim').
 
 ---
 ** Commands 'set-frame-width' and 'set-frame-height' can now get their
diff --git a/lisp/bindings.el b/lisp/bindings.el
index 6eac528eb6..6f25e9738a 100644
--- a/lisp/bindings.el
+++ b/lisp/bindings.el
@@ -1432,7 +1432,7 @@ ctl-x-map
 
 (defvar ctl-x-x-map
   (let ((map (make-sparse-keymap)))
-    (define-key map "f" #'font-lock-update)
+    (define-key map "f" #'font-lock-dwim)
     (define-key map "g" #'revert-buffer)
     (define-key map "r" #'rename-buffer)
     (define-key map "u" #'rename-uniquely)
diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 82915d8c8b..4fb66fc45a 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -1120,16 +1120,33 @@ font-lock-ensure
     (funcall font-lock-ensure-function
              (or beg (point-min)) (or end (point-max)))))
 
-(defun font-lock-update (&optional arg)
-  "Updates the syntax highlighting in this buffer.
-Refontify the accessible portion of this buffer, or enable Font Lock mode
-in this buffer if it is currently disabled.  With prefix ARG, toggle Font
-Lock mode."
+(defun font-lock-update-region (&optional beg end)
+  "Update the syntax highlighting between BEG and END.
+Refontify depending on the current Font Lock mode state.
+Font Lock mode can be disabled, in which case the text is unfontified,
+or enabled but inactive, in which case the text is refontified,
+or enabled and active, in which case the text fontification is flushed.
+If the region is not specified, it defaults to the entire accessible
+portion of the current buffer."
+  (let ((b (or beg (point-min)))
+        (e (or end (point-max))))
+    (if (not font-lock-mode)
+        (font-lock-unfontify-region b e)
+      (if (not font-lock-keywords)
+          (font-lock-fontify-region b e)
+        (font-lock-flush b e)))))
+
+(defun font-lock-dwim (&optional arg)
+  "Update the syntax highlighting in this buffer.
+Refontify the region if it is active, or the accessible portion of the
+buffer.  Otherwise, with prefix ARG, toggle Font Lock mode."
   (interactive "P")
   (save-excursion
-    (if (and (not arg) font-lock-mode)
-        (font-lock-fontify-region (point-min) (point-max))
-      (font-lock-unfontify-region (point-min) (point-max))
+    (if (not arg)
+        (if (use-region-p)
+            (font-lock-update-region (region-beginning) (region-end))
+          (font-lock-update-region))
+      (font-lock-update-region)
       (font-lock-mode 'toggle))))
 
 (defun font-lock-default-fontify-buffer ()
-- 
2.30.2