Don't insert string delimiter inside strings.

Author: kurnevsky

rust-mode.el

@@ -1137,7 +1137,7 @@ raw string, or to END, whichever comes first."
     (rust--char-literal-rx (1 "\"") (2 "\""))
     ;; Raw strings.
     ("\\(r\\)#*\""
-     (1 (prog1 "|"
+     (1 (prog1 (if (nth 8 (syntax-ppss (match-beginning 0))) nil (string-to-syntax "|"))
 	  (goto-char (match-end 0))
 	  (rust--syntax-propertize-raw-string end))))
     ("[<>]"