Initial commit. Allow wanted fontification of open string in any mode.scratch/fontify-open-string

The wanted fontification is for the string face to end at the first unescaped
newline.  This is achieved by a new syntax flag `s' on NL, which means
"terminate any open string".

src/syntax.c (SYNTAX_FLAGS_CLOSE_STRING, back_maybe_string): New functions.
(Fstring_to_syntax, Finternal_describe_syntax_value, scan_lists)
(scan_sexps_forward): Adapt to handle the `s' flag.

lisp/font-lock.el (font-lock-warn-open-string): New defcustom.
(font-lock-fontify-syntactically-region): Enhance to fontify " with
warning-face.

lisp/progmodes/sh-script.el (sh-mode-syntax-table): Add flag `s' to syntax
entry for \n.

2 files changed, 24 insertions, 2 deletions

diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index be9fb4dc93f..f2b7fef5c23 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -287,6 +287,16 @@ If a number, only buffers greater than this size have fontification messages."
                  (integer :tag "size"))
   :group 'font-lock
   :version "24.1")
+
+(defcustom font-lock-warn-open-string t
+  "Fontify the opening quote of an unterminated string with warning face?
+This is done when this variable is non-nil.
+
+This works only when the syntax-table entry for newline contains the flag `s'
+\(see page \"xxx\" in the Elisp manual)."
+  :type 'boolean
+  :group 'font-lock
+  :version "27.1")
 
 
 ;; Originally these variable values were face names such as `bold' etc.
@@ -1597,18 +1607,30 @@ START should be at the beginning of a line."
               (replace-regexp-in-string "^ *" "" comment-end))))
         ;; Find the `start' state.
         (state (syntax-ppss start))
-        face beg)
+        face beg in-string s-c-start)
     (if loudly (message "Fontifying %s... (syntactically...)" (buffer-name)))
     ;;
     ;; Find each interesting place between here and `end'.
     (while
         (progn
           (when (or (nth 3 state) (nth 4 state))
+            (setq s-c-start (nth 8 state))
+            (setq in-string (nth 3 state))
             (setq face (funcall font-lock-syntactic-face-function state))
             (setq beg (max (nth 8 state) start))
             (setq state (parse-partial-sexp (point) end nil nil state
                                             'syntax-table))
             (when face (put-text-property beg (point) 'face face))
+;;;; NEW STOUGH, 2018-06-29
+            (put-text-property s-c-start (1+ s-c-start)
+                               'face
+                               (if (and font-lock-warn-open-string
+                                        in-string
+                                        (not (nth 3 state))
+                                        (not (eq in-string (char-before))))
+                                   'font-lock-warning-face
+                                 face))
+;;;; END OF NEW STOUGH
             (when (and (eq face 'font-lock-comment-face)
                        (or font-lock-comment-start-skip
                            comment-start-skip))
diff --git a/lisp/progmodes/sh-script.el b/lisp/progmodes/sh-script.el
index aaa86b5816f..bf760e0a6cc 100644
--- a/lisp/progmodes/sh-script.el
+++ b/lisp/progmodes/sh-script.el
@@ -429,7 +429,7 @@ name symbol."
 (defvar sh-mode-syntax-table
   (sh-mode-syntax-table ()
         ?\# "<"
-        ?\n ">#"
+        ?\n ">#s"
         ?\" "\"\""
         ?\' "\"'"
         ?\` "\"`"