CC Mode (some languages): handle string lines ending in \\

In C, C++, Objective C, and Pike modes, regard \\ in a string
at EOL as a backslash followed by an escaped newline.  In the
other languages, this remains regarded as an escaped backslash
followed by an invalid string terminator.

* lisp/progmodes/cc-defs.el (c-is-escaped, c-will-be-escaped):
Amend to observe the changed notion of escaped newlines.

* lisp/progmodes/cc-langs.el (c-string-escaped-newlines):
Unused, removed.
(c-escaped-newline-takes-precedence): New lang const and var.
(c-string-innards-re-alist): Amend, using the above new lang
var.

Author: Alan Mackenzie

lisp/progmodes/cc-defs.el

@@ -425,25 +425,38 @@ to it is returned.  This function does not modify the point or the mark."
 (defvar lookup-syntax-properties)       ;XEmacs.
 
 (defmacro c-is-escaped (pos)
-  ;; Are there an odd number of backslashes before POS?
+  ;; Is the character following POS escaped?
   (declare (debug t))
   `(save-excursion
      (goto-char ,pos)
-     (not (zerop (logand (skip-chars-backward "\\\\") 1)))))
+     (if (and c-escaped-newline-takes-precedence
+	      (memq (char-after) '(?\n ?\r)))
+	 (eq (char-before) ?\\)
+       (not (zerop (logand (skip-chars-backward "\\\\") 1))))))
 
 (defmacro c-will-be-escaped (pos beg end)
   ;; Will the character after POS be escaped after the removal of (BEG END)?
   ;; It is assumed that (>= POS END).
   (declare (debug t))
   `(save-excursion
      (let ((-end- ,end)
+	   (-pos- ,pos)
 	   count)
-       (goto-char ,pos)
-       (setq count (skip-chars-backward "\\\\" -end-))
-       (when (eq (point) -end-)
-	 (goto-char ,beg)
-	 (setq count (+ count (skip-chars-backward "\\\\"))))
-       (not (zerop (logand count 1))))))
+       (if (and c-escaped-newline-takes-precedence
+		(memq (char-after -pos-) '(?\n ?\r)))
+	   (eq (char-before (if (eq -pos- -end-)
+				,beg
+			      -pos-))
+	       ?\\)
+	 (goto-char -pos-)
+	 (setq count
+	       (if (> -pos- -end-)
+		   (skip-chars-backward "\\\\" -end-)
+		 0))
+	 (when (eq (point) -end-)
+	   (goto-char ,beg)
+	   (setq count (+ count (skip-chars-backward "\\\\"))))
+	 (not (zerop (logand count 1)))))))
 
 (defmacro c-will-be-unescaped (beg)
   ;; Would the character after BEG be unescaped?

lisp/progmodes/cc-langs.el

@@ -1071,14 +1071,6 @@ Currently (2022-09) just C++ Mode uses this."
   ;; matched.
   t nil)
 
-(c-lang-defconst c-string-escaped-newlines
-  "Set if the language support backslash escaped newlines inside string
-literals."
-  t nil
-  (c c++ objc pike) t)
-(c-lang-defvar c-string-escaped-newlines
-  (c-lang-const c-string-escaped-newlines))
-
 (c-lang-defconst c-multiline-string-start-char
   "Set if the language supports multiline string literals without escaped
 newlines.  If t, all string literals are multiline.  If a character,
@@ -1095,6 +1087,18 @@ further directions."
 (c-lang-defvar c-multiline-string-start-char
   (c-lang-const c-multiline-string-start-char))
 
+(c-lang-defconst c-escaped-newline-takes-precedence
+  "Set if the language resolves escaped newlines first.
+This makes a difference in a string like \"...\\\\\n\".  When
+this variable is nil, the first backslash escapes the second,
+leaving an unterminated string.  When it's non-nil, the string is
+continued onto the next line, and the first backslash escapes
+whatever begins that next line."
+  t nil
+  (c c++ objc pike) t)
+(c-lang-defvar c-escaped-newline-takes-precedence
+  (c-lang-const c-escaped-newline-takes-precedence))
+
 (c-lang-defconst c-string-innards-re-alist
   ;; An alist of regexps matching the innards of a string, the key being the
   ;; string's delimiter.
@@ -1105,9 +1109,12 @@ further directions."
   t (mapcar (lambda (delim)
 	      (cons
 	       delim
-	       (concat "\\(\\\\\\(.\\|\n\\)\\|[^\\\n\r"
-		       (string delim)
-		       "]\\)*")))
+	       (concat
+		(if (c-lang-const c-escaped-newline-takes-precedence)
+		    "\\(\\\\\\(\\\\?\n\\|.\\)\\|[^\\\n\r"
+		  "\\(\\\\\\(\n\\|.\\)\\|[^\\\n\r")
+		(string delim)
+		"]\\)*")))
 	    (and
 	     (or (null (c-lang-const c-multiline-string-start-char))
 		 (c-characterp (c-lang-const c-multiline-string-start-char)))