Subject: bug#22246: 24.4; C-Mode: Syntax-Highlighting for „//“-Comment-Line continued with „\“

From: Alan Mackenzie <[email-masked]>
Date: 28 Dec 2015 17:55:51 -0000

Hello, Andreas.

There's now at least a provisional fix to this bug in the Emacs
repository release branch.

In article <[email-masked]> you wrote:
> [-- text/plain, encoding quoted-printable, charset: utf-8, 114 lines --]

> In Standard-C, it is allowed to continue Lines with a Backslash („\“).
> This even works for Single-Line-Comments („//“). But the C-Mode of Emacs
> does not colour the Continuation-Line right in this Case.

Thanks for taking the trouble to report this bug.

> In GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.5)
>  of 2015-03-07 on trouble, modified by Debian

I've converted the fix to work on Emacs 24.4, but after applying the
patch, you'll need (i) to rebuild the Emacs binary, and also (ii) to
rebuild CC Mode.  If you want any help on these, feel free to send me a
private email.

Here's the patch for Emacs 24.4:

--- src/syntax.c~	2015-12-28 17:31:54.658565440 +0000
+++ src/syntax.c	2015-12-28 16:26:02.057742664 +0000
@@ -139,6 +139,7 @@
 
 static Lisp_Object Qsyntax_table_p;
 static Lisp_Object Qsyntax_table, Qscan_error;
+static Lisp_Object Qcomment_end_can_be_escaped;
 
 /* This is the internal form of the parse state used in parse-partial-sexp.  */
 
@@ -734,8 +735,10 @@
 		   || SYNTAX_FLAGS_COMMENT_NESTED (syntax) != comnested))
 	continue;
 
-      /* Ignore escaped characters, except comment-enders.  */
-      if (code != Sendcomment && char_quoted (from, from_byte))
+      /* Ignore escaped characters, except comment-enders which cannot
+         be escaped.  */
+      if ((Vcomment_end_can_be_escaped || code != Sendcomment)
+          && char_quoted (from, from_byte))
 	continue;
 
       switch (code)
@@ -2286,7 +2289,8 @@
       if (code == Sendcomment
 	  && SYNTAX_FLAGS_COMMENT_STYLE (syntax, 0) == style
 	  && (SYNTAX_FLAGS_COMMENT_NESTED (syntax) ?
-	      (nesting > 0 && --nesting == 0) : nesting < 0))
+	      (nesting > 0 && --nesting == 0) : nesting < 0)
+          && !(Vcomment_end_can_be_escaped && char_quoted (from, from_byte)))
 	/* we have encountered a comment end of the same style
 	   as the comment sequence which began this comment
 	   section */
@@ -3641,6 +3645,12 @@
 In both cases, LIMIT bounds the search. */);
   Vfind_word_boundary_function_table = Fmake_char_table (Qnil, Qnil);
 
+  DEFVAR_BOOL ("comment-end-can-be-escaped", Vcomment_end_can_be_escaped,
+               doc: /* Non-nil means an escaped ender inside a comment doesn't end the comment.  */);
+  Vcomment_end_can_be_escaped = 0;
+  DEFSYM (Qcomment_end_can_be_escaped, "comment-end-can-be-escaped");
+  Fmake_variable_buffer_local (Qcomment_end_can_be_escaped);
+
   defsubr (&Ssyntax_table_p);
   defsubr (&Ssyntax_table);
   defsubr (&Sstandard_syntax_table);
--- lisp/progmodes/cc-langs.el~	2014-06-14 23:51:42.000000000 +0000
+++ lisp/progmodes/cc-langs.el	2015-12-28 16:18:41.226762430 +0000
@@ -1380,6 +1380,14 @@
 	    "\\)\\s *"))
 (c-lang-setvar comment-start-skip (c-lang-const comment-start-skip))
 
+(c-lang-defconst comment-end-can-be-escaped
+  "When non-nil, escaped EOLs inside comments are valid.
+This works in Emacs >= 25.1."
+  t nil
+  (c c++ objc) t)
+(c-lang-setvar comment-end-can-be-escaped
+	       (c-lang-const comment-end-can-be-escaped))
+
 (c-lang-defconst c-syntactic-ws-start
   ;; Regexp matching any sequence that can start syntactic whitespace.
   ;; The only uncertain case is '#' when there are cpp directives.

-- 
Alan Mackenzie (Nuremberg, Germany).