bug#56637: 28.1.90; [FR] Allow a way around font-lock-mode being unconditionally disabled in " *hidden*" buffers

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

> More seriously, I didn't mean to remove it willy-nilly, but that it's
> a desirable goal and the question is how to get there.

Of course, for that we need to know what effect this code has in
practice, so I tried the patch below a the first few messages I got
were:

    Enabling font-lock for mode minibuffer-inactive-mode in hidden buffer #<buffer  *Minibuf-0*>
    Enabling font-lock for mode fundamental-mode in hidden buffer #<buffer  *load*>
    Enabling font-lock for mode fundamental-mode in hidden buffer #<buffer  *Echo Area 0*>
    Enabling font-lock for mode fundamental-mode in hidden buffer #<buffer  *Echo Area 1*>
    Enabling font-lock for mode fundamental-mode in hidden buffer #<buffer  *load*-628722>
    Enabling font-lock for mode checkdoc-output-mode in hidden buffer #<buffer  *checkdoc-temp*>
    Enabling font-lock for mode minibuffer-mode in hidden buffer #<buffer  *Minibuf-1*>
    Enabling font-lock for mode minibuffer-inactive-mode in hidden buffer #<buffer  *Minibuf-0*>
    Enabling font-lock for mode minibuffer-inactive-mode in hidden buffer #<buffer  *Minibuf-1*>

Maybe it would be mostly "harmless" to enable font-lock in those buffers/modes,
but even if it is, it would clearly be wasteful.

Side note: the *checkdoc-temp* seems like an odd case.  Its major mode is
clearly designed for a "normal/visible" buffer (and IIUC it is indeed
used in buffers that are displayed), so I'm not sure how *checkdoc-temp*
ends up using that mode.


        Stefan


diff --git a/lisp/font-core.el b/lisp/font-core.el
index f92d1e38306..5678ab22da5 100644
--- a/lisp/font-core.el
+++ b/lisp/font-core.el
@@ -132,12 +132,15 @@ font-lock-mode
   ;; Don't turn on Font Lock mode if we don't have a display (we're running a
   ;; batch job) or if the buffer is invisible (the name starts with a space).
   (when (or noninteractive (eq (aref (buffer-name) 0) ?\s))
-    (setq font-lock-mode nil))
+    ;; (setq font-lock-mode nil)
+    (when font-lock-mode
+      (message "Enabling font-lock for mode %S in hidden buffer %S"
+               major-mode (current-buffer))))
   (funcall font-lock-function font-lock-mode)
   ;; Arrange to unfontify this buffer if we change major mode later.
   (if font-lock-mode
-      (add-hook 'change-major-mode-hook 'font-lock-change-mode nil t)
-    (remove-hook 'change-major-mode-hook 'font-lock-change-mode t)))
+      (add-hook 'change-major-mode-hook #'font-lock-change-mode nil t)
+    (remove-hook 'change-major-mode-hook #'font-lock-change-mode t)))
 
 ;; Get rid of fontification for the old major mode.
 ;; We do this when changing major modes.
@@ -270,7 +273,7 @@ global-font-lock-mode
   font-lock-mode turn-on-font-lock-if-desired
   ;; What was this :extra-args thingy for?  --Stef
   ;; :extra-args (dummy)
-  :initialize 'custom-initialize-delay
+  :initialize #'custom-initialize-delay
   :init-value (not (or noninteractive emacs-basic-display))
   :group 'font-lock
   :version "22.1")