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bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escape

From: Stefan Monnier
Subject: bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped.
Date: Thu, 19 Nov 2020 17:47:40 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

>> So, yeah, you can add yet-another-hack on top of the other syntax.c
>> hacks if you want, but there's a good chance it will only ever be used
>> by CC-mode.  It will take a lot more code changes in syntax.c than
>> a quick tweak to your Elisp code to search for "\*/".
> OK, here's the patch.

I think the patch agrees with my assessment above (even though it's
still missing a etc/NEWS entry, adjustment to the docstring of
modify-syntax-entry and to the .texi manual).

I really can't understand why you resist so much the use of
a `syntax-table` property on those rare \\\n sequences.


PS: Also, I just noticed that `gcc -Wall` warns about the use of such
multiline comments, so it doesn't seem to be a very popular feature.

PPS: For reference, I just tried to add support for it in sm-c-mode
and this is the resulting code:

@@ -312,7 +315,15 @@ E.g. a #define nested within 2 #ifs will be turned into 
\"#  define\"."
                                'syntax-table (string-to-syntax "|"))
             (put-text-property (match-beginning 2) (match-end 2)
                                'syntax-table (string-to-syntax "|")))
-          (sm-c--cpp-syntax-propertize end)))))
+          (sm-c--cpp-syntax-propertize end))))
+    ("\\\\\\(\n\\)"
+     (1 (let ((ppss (save-excursion (syntax-ppss (match-beginning 0)))))
+          (when (and (nth 4 ppss)        ;Within a comment
+                     (null (nth 7 ppss)) ;Within a // comment
+                     (save-excursion     ;The \ is not itself escaped
+                       (goto-char (match-beginning 0))
+                       (zerop (mod (skip-chars-backward "\\\\") 2))))
+            (string-to-syntax "."))))))
    (point) end))
 (defun sm-c-syntactic-face-function (ppss)

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