Teach AUTOs to process escaped backslashes at the end of quoted strings (veripool#1831)

  "...\\..." is a string containing a backslash: \
  "...\"..." is a string containing a double-quote: "

When parsing double-quoted strings, we must detect embedded escaped backslashes
and double-quotes. Detecting when a string terminates with one or more of these
characters is tricky. Strings terminating in "...\"" are not uncommon, and have
reasonably robust support, but strings terminating in "...\\" are quite rare,
and were quite broken.

This patch aims to improve the detection of "...\\" (or "...\\\\", or any even
number of backslashes in a row immediately preceding a double-quote string
terminator.

Author: acr4

verilog-mode.el

@@ -3608,38 +3608,46 @@ This creates v-cmts properties where comments are in force."
     (save-match-data
       (verilog-save-buffer-state
        (let (pt)
-	 (goto-char beg)
-	 (while (< (point) end)
-	   (cond ((looking-at "//")
-		  (setq pt (point))
-		  (or (search-forward "\n" end t)
-		      (goto-char end))
-		  ;; "1+": The leading // or /* itself isn't considered as
-		  ;; being "inside" the comment, so that a (search-backward)
-		  ;; that lands at the start of the // won't mis-indicate
-		  ;; it's inside a comment.  Also otherwise it would be
-		  ;; hard to find a commented out /*AS*/ vs one that isn't
-		  (put-text-property (1+ pt) (point) 'v-cmts t))
-		 ((looking-at "/\\*")
-		  (setq pt (point))
-		  (or (search-forward "*/" end t)
-		      ;; No error - let later code indicate it so we can
-		      ;; use inside functions on-the-fly
-		      ;;(error "%s: Unmatched /* */, at char %d"
-		      ;;       (verilog-point-text) (point))
-		      (goto-char end))
-		  (put-text-property (1+ pt) (point) 'v-cmts t))
-		 ((looking-at "\"")
-		  (setq pt (point))
-                  (or (re-search-forward "[^\\]\"" end t)  ; don't forward-char first, since we look for a non backslash first
-		      ;; No error - let later code indicate it so we can
-		      (goto-char end))
-		  (put-text-property (1+ pt) (point) 'v-cmts t))
-		 (t
-		  (forward-char 1)
-		  (if (re-search-forward "[/\"]" end t)
-		      (backward-char 1)
-		    (goto-char end))))))))))
+         (goto-char beg)
+         (while (< (point) end)
+           (cond ((looking-at "//")
+                  (setq pt (point))
+                  (or (search-forward "\n" end t)
+                     (goto-char end))
+                  ;; "1+": The leading // or /* itself isn't considered as
+                  ;; being "inside" the comment, so that a (search-backward)
+                  ;; that lands at the start of the // won't mis-indicate
+                  ;; it's inside a comment.  Also otherwise it would be
+                  ;; hard to find a commented out /*AS*/ vs one that isn't
+                  (put-text-property (1+ pt) (point) 'v-cmts t))
+                 ((looking-at "/\\*")
+                  (setq pt (point))
+                  (or (search-forward "*/" end t)
+                     ;; No error - let later code indicate it so we can
+                     ;; use inside functions on-the-fly
+                     ;;(error "%s: Unmatched /* */, at char %d"
+                     ;;       (verilog-point-text) (point))
+                     (goto-char end))
+                  (put-text-property (1+ pt) (point) 'v-cmts t))
+                 ((looking-at "\"")
+                  (setq pt (point))
+                  (setq pt2 nil)
+                  ;; Move forward to the first non-escaped (backslashed) double-quote ("). That closes this string.
+                  ;;   skip: \" (an escaped double-quote)
+                  ;;   match: \\" (a backslash as the last character of a string)
+                  (while (not pt2)
+                    (or (re-search-forward "\\\\" end t)
+                       (re-search-forward "[^\\]\"" end t)
+                       (goto-char end))
+                    (if (looking-at "\\\\")
+                        (forward-char 2)
+                      (setq pt2 (point))))
+                  (put-text-property (1+ pt) (point) 'v-cmts t))
+                 (t
+                  (forward-char 1)
+                  (if (re-search-forward "[/\"]" end t)
+                      (backward-char 1)
+                    (goto-char end))))))))))
 
 (defun verilog-scan ()
   "Parse the buffer, marking all comments with properties.