bug#42805: 27.1; cc-mode/syntax/font-lock bug

[Gustavo Tavares Cabral <gustavotcabral@gmail.com>]

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Hi,

I have found two related issues. Both are evident in csharp-mode, but
can easily be reproduced in c-mode.

Issue 1 (cc-mode.el):

  When c-multiline-string-start-char is set, font locking breaks after
  changes in string literals. The issue started after commit 2ca12bb3de:
  I believe that '(c-remove-string-fences (car end-limits))' should be
  '(c-remove-string-fences (1- (cdr end-limits)))'

  Steps to reproduce:
    1. Open a .c file;
    2. Eval '(setq c-multiline-string-start-char ?@)';
    3. Delete/insert double quotes.

  I think this issue is related:
  https://github.com/Fuco1/smartparens/issues/992


Issue 2 (maybe syntax.el):

  When syntax-propertize-function is set (to '(lambda (beg end) nil)',
  for example), a similar problem occurs if electric-pair-mode (or even
  smartparens) is enabled. The symptoms started after commit 585fb95739,
  but I don't know if it is related.

  Steps to reproduce:
    1. Open a .c file;
    2. Eval '(setq syntax-propertize-function (lambda (beg end) nil)';
    3. Enable electric-pair-mode;
    4. Insert/delete empty strings.




In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30,
cairo version 1.16.0)
of 2020-08-06 built on gustavo-Lenovo-ideapad-330-15IKB
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Ubuntu 18.04.5 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Configured using:
'configure --with-x-toolkit=gtk3 --with-modules --with-sound=yes
--with-file-notification=yes --with-mailutils --with-cairo'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS PDUMPER LCMS2
GMP

Important settings:
value of $LANG: en
value of $XMODIFIERS:
locale-coding-system: iso-latin-1-unix

Major mode: Lisp Interaction

Minor modes in effect:
tooltip-mode: t
global-eldoc-mode: t
eldoc-mode: t
electric-indent-mode: t
mouse-wheel-mode: t
tool-bar-mode: t
menu-bar-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
line-number-mode: t
transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search seq
byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date subr-x
cl-loaddefs cl-lib 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 timer select scroll-bar mouse jit-lock font-lock
syntax facemenu 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 loaddefs button faces cus-face macroexp files
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 emacs)

Memory information:
((conses 16 44286 8232)
(symbols 48 5980 1)
(strings 32 15464 904)
(string-bytes 1 505487)
(vectors 16 9275)
(vector-slots 8 124610 9084)
(floats 8 20 48)
(intervals 56 256 67)
(buffers 1000 12))

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