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

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]

The use of hide-ifdef is a bit outdated and it requires to manually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
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   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
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   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
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   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
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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
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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
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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
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inotify lcms2 dynamic-setting system-font-setting font-render-setting
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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))

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

[Eli Zaretskii]

> Date: Sun, 30 May 2021 22:26:06 +0200
> From:  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.

Did you try customizing hide-ifdef-initially to a non-nil value?

> 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.

As mentioned on emacs-devel, how exactly to automate this should be
carefully thought-out, because blindly using the JIT font-lock
mechanism will cause code blocks to appear and disappear several time
as long as the user types the preprocessor directive.  It sounds like
a no-brainer for a simple "#if 0" (which btw is rarely if ever used in
real-life production code), but think about typing something like
this:

  #if (defined __has_attribute \
       && (!defined __clang_minor__ \
	   || 3 < __clang_major__ + (5 <= __clang_minor__)))

(This is a real-life example from our conf_post.h, and there are a lot
like it all over our code.)  Depending on how each of the symbols are
defined, the block under this could appear and disappear several times
until you end typing.  I'm quite sure we want to avoid this annoying
effect as much as possible.

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]

On Mon, May 31, 2021 at 03:01:42PM +0300, Eli Zaretskii wrote:
>> Date: Sun, 30 May 2021 22:26:06 +0200
>> From:  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.
>
>Did you try customizing hide-ifdef-initially to a non-nil value?
>
>> 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.
>
>As mentioned on emacs-devel, how exactly to automate this should be
>carefully thought-out, because blindly using the JIT font-lock
>mechanism will cause code blocks to appear and disappear several time
>as long as the user types the preprocessor directive.  It sounds like
>a no-brainer for a simple "#if 0" (which btw is rarely if ever used in
>real-life production code), but think about typing something like
>this:
>
>  #if (defined __has_attribute \
>       && (!defined __clang_minor__ \
>	   || 3 < __clang_major__ + (5 <= __clang_minor__)))
>
>(This is a real-life example from our conf_post.h, and there are a lot
>like it all over our code.)  Depending on how each of the symbols are
>defined, the block under this could appear and disappear several times
>until you end typing.  I'm quite sure we want to avoid this annoying
>effect as much as possible.

Hi Eli:

I understand that emacs tries to do something smarter than what vim and
the other editors do, but this is limiting the basic trivial use case. I
don;t actually think many people are using the hide-ifdef
package... specially not new users.

In the general case I understand that the block under this could appear
and disappear, but that will be also a feedback for the user to know if
the region will be enabled or not.

For example in the case we have 

#if

#else

#endif

That may be also very useful to know which branch will be compiled. I
don't intend to get something very complex here, but at least the most
basic highlight with trivial symbols may work.

We use emacs to develop code, so, our code is not really production most
of the time. And the #if 0 is very common in many code I have seen
around.

While the

/*
<CODE>
// */

Approach I use is not common at all.

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

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> As mentioned on emacs-devel, how exactly to automate this should be
> carefully thought-out, because blindly using the JIT font-lock
> mechanism will cause code blocks to appear and disappear several time
> as long as the user types the preprocessor directive.

When the code is actually made invisible the appear/disappear could be
quite annoying, indeed, but I think Ergus is mostly concerned about the
case where `hide-ifdef-shadow` is activated, where it would make sense
to make it behave like comments and strings, I think.


        Stefan