bug#48752: 28.0.50; Feature request: Hook hide-ifdef into font-lock

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

The use of hide-ifdef is a bit outdated and it requires to manually
call: `M-x hide-ifdefs` to update the buffer

#if 0
<CODE>
#endif

M-x hide-ifdef-mode
M-x hide-ifdef-toggle-shadowing

does not shadows/hides the region <CODE>.

It requires to explicitly call: M-x hide-ifdefs.

Then if we change #if 0 -> #if 1;

the shadow does not disappears/appears either until we call: M-x
hide-ifdefs again.

This part may be automatized. Calling hide-ifdefs every time is annoying
and unneeded. Vim and most of other editors already do the
re-fontification automatically. So we can take that behavior as a good
starting point.

Emacs is smart enough to recognize when the user modifies a
cpp-macro. And multi-line code highlight is simple like /* */ does... 

I think font lock already has all the functionalities we need to add
this feature in a more dynamic way. But some time ago (2008) there was a
message in the mailing list advising not to implement hide-ifdef with
font-lock due to performance concerns... but I am not sure if those are
applicable still today.




In GNU Emacs 28.0.50 (build 28, x86_64-pc-linux-gnu, GTK+ Version 3.24.29, cairo version 1.17.4)
  of 2021-05-29 built on Ergus
Repository revision: e3fc16fd71a46990503a71b11a2ad592e99d9edb
Repository branch: master
System Description: Arch Linux

Configured using:
  'configure --prefix=/home/ergo/.local/ --with-mailutils --with-json
  --with-x-toolkit=gtk3 --with-xft --with-wide-int --with-modules
  --with-cairo --with-harfbuzz --with-native-compilation'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER
PNG RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM
GTK3 ZLIB

Important settings:
   value of $LANG: en_US.UTF-8
   locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
   global-auto-revert-mode: t
   xclip-mode: t
   yas-global-mode: t
   yas-minor-mode: t
   show-paren-mode: t
   repeat-mode: t
   flyspell-mode: t
   company-mode: t
   flycheck-mode: t
   counsel-mode: t
   ivy-mode: t
   electric-pair-mode: t
   composable-mark-mode: t
   composable-mode: t
   xterm-mouse-mode: t
   winner-mode: t
   save-place-mode: t
   which-key-mode: t
   override-global-mode: t
   delete-selection-mode: t
   savehist-mode: t
   global-display-fill-column-indicator-mode: t
   display-fill-column-indicator-mode: t
   global-display-line-numbers-mode: t
   display-line-numbers-mode: t
   eldoc-mode: t
   mouse-wheel-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   size-indication-mode: t
   column-number-mode: t
   line-number-mode: t
   transient-mark-mode: t

Load-path shadows:
/usr/share/emacs/site-lisp/cmake-mode hides /home/ergo/.emacs.d/elpa/cmake-mode-20210104.1831/cmake-mode
/usr/share/emacs/site-lisp/notmuch-crypto hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-crypto
/usr/share/emacs/site-lisp/notmuch-compat hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-compat
/usr/share/emacs/site-lisp/notmuch-hello hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-hello
/usr/share/emacs/site-lisp/notmuch hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch
/usr/share/emacs/site-lisp/notmuch-show hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-show
/usr/share/emacs/site-lisp/notmuch-maildir-fcc hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-maildir-fcc
/usr/share/emacs/site-lisp/coolj hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/coolj
/usr/share/emacs/site-lisp/notmuch-draft hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-draft
/usr/share/emacs/site-lisp/notmuch-tree hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-tree
/usr/share/emacs/site-lisp/notmuch-parser hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-parser
/usr/share/emacs/site-lisp/notmuch-lib hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-lib
/usr/share/emacs/site-lisp/notmuch-mua hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-mua
/usr/share/emacs/site-lisp/notmuch-message hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-message
/usr/share/emacs/site-lisp/notmuch-address hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-address
/usr/share/emacs/site-lisp/notmuch-wash hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-wash
/usr/share/emacs/site-lisp/notmuch-tag hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-tag
/usr/share/emacs/site-lisp/notmuch-print hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-print
/usr/share/emacs/site-lisp/notmuch-query hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-query
/usr/share/emacs/site-lisp/notmuch-jump hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-jump
/usr/share/emacs/site-lisp/notmuch-company hides /home/ergo/.emacs.d/elpa/notmuch-20210515.1210/notmuch-company
/home/ergo/.emacs.d/elpa/transient-20210525.1141/transient hides /home/ergo/.local/share/emacs/28.0.50/lisp/transient

Features:
(shadow sort notmuch-company notmuch-lib notmuch-version notmuch-compat
mm-view mml-smime smime dig mailcap mail-extr emacsbug message rmc puny
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs auth-source password-cache time-date mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils autorevert
filenotify xclip yasnippet-snippets yasnippet thingatpt amx comp
comp-cstr warnings s paren repeat flyspell-correct-ivy flyspell-correct
flyspell ispell company-keywords company-gtags company-dabbrev-code
company-dabbrev company-files company-semantic company-template
company-capf company flycheck json map find-func subr-x dash counsel xdg
xref project dired-x dired dired-loaddefs compile text-property-search
comint ansi-color swiper ivy-avy avy ivy flx ivy-faces ivy-overlay colir
color pcase term/tmux term/xterm xterm jka-compr elec-pair init
composable composable-mark xt-mouse simple-16-theme winner ring
saveplace diminish edmacro kmacro which-key advice configmail cl-extra
help-mode use-package use-package-ensure use-package-delight
use-package-diminish use-package-bind-key bind-key use-package-core
disp-table delsel savehist easy-mmode display-fill-column-indicator
display-line-numbers info ede/auto eieio-base cl-seq seq eieio byte-opt
bytecomp byte-compile cconv eieio-core cl-macs gv eieio-loaddefs
cl-loaddefs cl-lib tex-site rx slime-autoloads early-init iso-transl
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
easymenu timer select scroll-bar mouse jit-lock font-lock syntax
font-core term/tty-colors frame minibuffer cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
cl-preloaded nadvice button loaddefs faces cus-face macroexp files
window text-properties overlay sha1 md5 base64 format env code-pages
mule custom widget hashtable-print-readable backquote threads dbusbind
inotify lcms2 dynamic-setting system-font-setting font-render-setting
cairo move-toolbar gtk x-toolkit x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 255105 10285)
  (symbols 48 18525 1)
  (strings 32 58700 7082)
  (string-bytes 1 2146750)
  (vectors 16 29784)
  (vector-slots 8 590763 10019)
  (floats 8 189 1037)
  (intervals 56 1022 0)
  (buffers 992 11))