master 697942f: CC Mode: Fix wrong value of comment-start-skip, fixing half of bug #41952

[Alan Mackenzie]

branch: master
commit 697942f9a0d875cb8b00e6de155a34354db1dcba
Author: Alan Mackenzie <acm@muc.de>
Commit: Alan Mackenzie <acm@muc.de>

    CC Mode: Fix wrong value of comment-start-skip, fixing half of bug #41952
    
    Also add functions to enable correct use of CC Mode's filling functionality
    from major modes which don't initialize CC Mode fully.  These modes are
    currently js-mode and mhtml-mode.
    
    * lisp/progmodes/cc-langs.el (comment-start-skip): Replace "\\(" by "\\(?:" 
so
    that (match-end 1) isn't falsely taken to be the start of the comment.
    
    * lisp/progmodes/cc-engine.el (c-foreign-truncate-lit-pos-cache)
    (c-foreign-init-lit-pos-cache): New functions.
---
 lisp/progmodes/cc-engine.el | 18 ++++++++++++++++++
 lisp/progmodes/cc-langs.el  |  2 +-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/cc-engine.el b/lisp/progmodes/cc-engine.el
index 1977ead..fec1065 100644
--- a/lisp/progmodes/cc-engine.el
+++ b/lisp/progmodes/cc-engine.el
@@ -3186,6 +3186,24 @@ comment at the start of cc-engine.el for more info."
        c-semi-near-cache-limit (min c-semi-near-cache-limit pos)
        c-full-near-cache-limit (min c-full-near-cache-limit pos)))
 
+(defun c-foreign-truncate-lit-pos-cache (beg _end)
+  "Truncate CC Mode's literal cache.
+
+This function should be added to the `before-change-functions'
+hook by major modes that use CC Mode's filling functionality
+without initializing CC Mode.  Currently (2020-06) these are
+js-mode and mhtml-mode."
+  (c-truncate-lit-pos-cache beg))
+
+(defun c-foreign-init-lit-pos-cache ()
+  "Initialize CC Mode's literal cache.
+
+This function should be called from the mode functions of major
+modes which use CC Mode's filling functionality without
+initializing CC Mode.  Currently (2020-06) these are js-mode and
+mhtml-mode."
+  (c-truncate-lit-pos-cache 1))
+
 
 ;; A system for finding noteworthy parens before the point.
 
diff --git a/lisp/progmodes/cc-langs.el b/lisp/progmodes/cc-langs.el
index 814a85c..b77bf33 100644
--- a/lisp/progmodes/cc-langs.el
+++ b/lisp/progmodes/cc-langs.el
@@ -1769,7 +1769,7 @@ ender."
 `comment-start-skip' is initialized from this."
   ;; Default: Allow the last char of the comment starter(s) to be
   ;; repeated, then allow any amount of horizontal whitespace.
-  t (concat "\\("
+  t (concat "\\(?:"
            (c-concat-separated
             (mapcar (lambda (cs)
                       (when cs