* lisp/apropos.el (apropos-read-pattern): Fix error message for empty string.

[bpt/emacs.git] / lisp / jit-lock.el

;;; jit-lock.el --- just-in-time fontification  -*- lexical-binding: t -*-

;; Copyright (C) 1998, 2000-2014 Free Software Foundation, Inc.

;; Author: Gerd Moellmann <gerd@gnu.org>
;; Keywords: faces files
;; Package: emacs

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; Just-in-time fontification, triggered by C redisplay code.

;;; Code:


(eval-when-compile
  (defmacro with-buffer-prepared-for-jit-lock (&rest body)
    "Execute BODY in current buffer, overriding several variables.
Preserves the `buffer-modified-p' state of the current buffer."
    (declare (debug t))
    `(let ((inhibit-point-motion-hooks t))
       (with-silent-modifications
         ,@body))))
\f
;;; Customization.

(defgroup jit-lock nil
  "Font Lock support mode to fontify just-in-time."
  :version "21.1"
  :group 'font-lock)

(defcustom jit-lock-chunk-size 500
  "Jit-lock fontifies chunks of at most this many characters at a time.

This variable controls both display-time and stealth fontification."
  :type 'integer
  :group 'jit-lock)


(defcustom jit-lock-stealth-time nil
  "Time in seconds to wait before beginning stealth fontification.
Stealth fontification occurs if there is no input within this time.
If nil, stealth fontification is never performed.

The value of this variable is used when JIT Lock mode is turned on."
  :type '(choice (const :tag "never" nil)
		 (number :tag "seconds" :value 16))
  :group 'jit-lock)


(defcustom jit-lock-stealth-nice 0.5
  "Time in seconds to pause between chunks of stealth fontification.
Each iteration of stealth fontification is separated by this amount of time,
thus reducing the demand that stealth fontification makes on the system.
If nil, means stealth fontification is never paused.
To reduce machine load during stealth fontification, at the cost of stealth
taking longer to fontify, you could increase the value of this variable.
See also `jit-lock-stealth-load'."
  :type '(choice (const :tag "never" nil)
		 (number :tag "seconds"))
  :group 'jit-lock)


(defcustom jit-lock-stealth-load
  (if (condition-case nil (load-average) (error)) 200)
  "Load in percentage above which stealth fontification is suspended.
Stealth fontification pauses when the system short-term load average (as
returned by the function `load-average' if supported) goes above this level,
thus reducing the demand that stealth fontification makes on the system.
If nil, means stealth fontification is never suspended.
To reduce machine load during stealth fontification, at the cost of stealth
taking longer to fontify, you could reduce the value of this variable.
See also `jit-lock-stealth-nice'."
  :type (if (condition-case nil (load-average) (error))
	    '(choice (const :tag "never" nil)
		     (integer :tag "load"))
	  '(const :format "%t: unsupported\n" nil))
  :group 'jit-lock)


(defcustom jit-lock-stealth-verbose nil
  "If non-nil, means stealth fontification should show status messages."
  :type 'boolean
  :group 'jit-lock)


(defvaralias 'jit-lock-defer-contextually 'jit-lock-contextually)
(defcustom jit-lock-contextually 'syntax-driven
  "If non-nil, means fontification should be syntactically true.
If nil, means fontification occurs only on those lines modified.  This
means where modification on a line causes syntactic change on subsequent lines,
those subsequent lines are not refontified to reflect their new context.
If t, means fontification occurs on those lines modified and all
subsequent lines.  This means those subsequent lines are refontified to reflect
their new syntactic context, after `jit-lock-context-time' seconds.
If any other value, e.g., `syntax-driven', means syntactically true
fontification occurs only if syntactic fontification is performed using the
buffer mode's syntax table, i.e., only if `font-lock-keywords-only' is nil.

The value of this variable is used when JIT Lock mode is turned on."
  :type '(choice (const :tag "never" nil)
		 (const :tag "always" t)
		 (other :tag "syntax-driven" syntax-driven))
  :group 'jit-lock)

(defcustom jit-lock-context-time 0.5
  "Idle time after which text is contextually refontified, if applicable."
  :type '(number :tag "seconds")
  :group 'jit-lock)

(defcustom jit-lock-defer-time nil ;; 0.25
  "Idle time after which deferred fontification should take place.
If nil, fontification is not deferred."
  :group 'jit-lock
  :type '(choice (const :tag "never" nil)
	         (number :tag "seconds")))
\f
;;; Variables that are not customizable.

(defvar-local jit-lock-mode nil
  "Non-nil means Just-in-time Lock mode is active.")

(defvar-local jit-lock-functions nil
  "Functions to do the actual fontification.
They are called with two arguments: the START and END of the region to fontify.")

(defvar-local jit-lock-context-unfontify-pos nil
  "Consider text after this position as contextually unfontified.
If nil, contextual fontification is disabled.")

(defvar jit-lock-stealth-timer nil
  "Timer for stealth fontification in Just-in-time Lock mode.")
(defvar jit-lock-stealth-repeat-timer nil
  "Timer for repeated stealth fontification in Just-in-time Lock mode.")
(defvar jit-lock-context-timer nil
  "Timer for context fontification in Just-in-time Lock mode.")
(defvar jit-lock-defer-timer nil
  "Timer for deferred fontification in Just-in-time Lock mode.")

(defvar jit-lock-defer-buffers nil
  "List of buffers with pending deferred fontification.")
(defvar jit-lock-stealth-buffers nil
  "List of buffers that are being fontified stealthily.")
\f
;;; JIT lock mode

(defun jit-lock-mode (arg)
  "Toggle Just-in-time Lock mode.
Turn Just-in-time Lock mode on if and only if ARG is non-nil.
Enable it automatically by customizing group `font-lock'.

When Just-in-time Lock mode is enabled, fontification is different in the
following ways:

- Demand-driven buffer fontification triggered by Emacs C code.
  This means initial fontification of the whole buffer does not occur.
  Instead, fontification occurs when necessary, such as when scrolling
  through the buffer would otherwise reveal unfontified areas.  This is
  useful if buffer fontification is too slow for large buffers.

- Stealthy buffer fontification if `jit-lock-stealth-time' is non-nil.
  This means remaining unfontified areas of buffers are fontified if Emacs has
  been idle for `jit-lock-stealth-time' seconds, while Emacs remains idle.
  This is useful if any buffer has any deferred fontification.

- Deferred context fontification if `jit-lock-contextually' is
  non-nil.  This means fontification updates the buffer corresponding to
  true syntactic context, after `jit-lock-context-time' seconds of Emacs
  idle time, while Emacs remains idle.  Otherwise, fontification occurs
  on modified lines only, and subsequent lines can remain fontified
  corresponding to previous syntactic contexts.  This is useful where
  strings or comments span lines.

Stealth fontification only occurs while the system remains unloaded.
If the system load rises above `jit-lock-stealth-load' percent, stealth
fontification is suspended.  Stealth fontification intensity is controlled via
the variable `jit-lock-stealth-nice'."
  (setq jit-lock-mode arg)
  (cond
   ((buffer-base-buffer)
    ;; We're in an indirect buffer.  This doesn't work because jit-lock relies
    ;; on the `fontified' text-property which is shared with the base buffer.
    (setq jit-lock-mode nil)
    (message "Not enabling jit-lock: it does not work in indirect buffer"))

   (jit-lock-mode ;; Turn Just-in-time Lock mode on.

    ;; Mark the buffer for refontification.
    (jit-lock-refontify)

    ;; Install an idle timer for stealth fontification.
    (when (and jit-lock-stealth-time (null jit-lock-stealth-timer))
      (setq jit-lock-stealth-timer
            (run-with-idle-timer jit-lock-stealth-time t
                                 'jit-lock-stealth-fontify)))

    ;; Create, but do not activate, the idle timer for repeated
    ;; stealth fontification.
    (when (and jit-lock-stealth-time (null jit-lock-stealth-repeat-timer))
      (setq jit-lock-stealth-repeat-timer (timer-create))
      (timer-set-function jit-lock-stealth-repeat-timer
                          'jit-lock-stealth-fontify '(t)))

    ;; Init deferred fontification timer.
    (when (and jit-lock-defer-time (null jit-lock-defer-timer))
      (setq jit-lock-defer-timer
            (run-with-idle-timer jit-lock-defer-time t
                                 'jit-lock-deferred-fontify)))

    ;; Initialize contextual fontification if requested.
    (when (eq jit-lock-contextually t)
      (unless jit-lock-context-timer
        (setq jit-lock-context-timer
              (run-with-idle-timer jit-lock-context-time t
                                   'jit-lock-context-fontify)))
      (setq jit-lock-context-unfontify-pos
            (or jit-lock-context-unfontify-pos (point-max))))

    ;; Setup our hooks.
    (add-hook 'after-change-functions 'jit-lock-after-change nil t)
    (add-hook 'fontification-functions 'jit-lock-function))

   ;; Turn Just-in-time Lock mode off.
   (t
    ;; Cancel our idle timers.
    (when (and (or jit-lock-stealth-timer jit-lock-defer-timer
                   jit-lock-context-timer)
               ;; Only if there's no other buffer using them.
               (not (catch 'found
                      (dolist (buf (buffer-list))
                        (with-current-buffer buf
                          (when jit-lock-mode (throw 'found t)))))))
      (when jit-lock-stealth-timer
        (cancel-timer jit-lock-stealth-timer)
        (setq jit-lock-stealth-timer nil))
      (when jit-lock-context-timer
        (cancel-timer jit-lock-context-timer)
        (setq jit-lock-context-timer nil))
      (when jit-lock-defer-timer
        (cancel-timer jit-lock-defer-timer)
        (setq jit-lock-defer-timer nil)))

    ;; Remove hooks.
    (remove-hook 'after-change-functions 'jit-lock-after-change t)
    (remove-hook 'fontification-functions 'jit-lock-function))))

(define-minor-mode jit-lock-debug-mode
  "Minor mode to help debug code run from jit-lock.
When this minor mode is enabled, jit-lock runs as little code as possible
during redisplay and moves the rest to a timer, where things
like `debug-on-error' and Edebug can be used."
  :global t :group 'jit-lock
  (when jit-lock-defer-timer
    (cancel-timer jit-lock-defer-timer)
    (setq jit-lock-defer-timer nil))
  (when jit-lock-debug-mode
    (setq jit-lock-defer-timer
          (run-with-idle-timer 0 t #'jit-lock--debug-fontify))))

(defvar jit-lock--debug-fontifying nil)

(defun jit-lock--debug-fontify ()
  "Fontify what was deferred for debugging."
  (when (and (not jit-lock--debug-fontifying)
             jit-lock-defer-buffers (not memory-full))
    (let ((jit-lock--debug-fontifying t)
          (inhibit-debugger nil))       ;FIXME: Not sufficient!
      ;; Mark the deferred regions back to `fontified = nil'
      (dolist (buffer jit-lock-defer-buffers)
        (when (buffer-live-p buffer)
          (with-current-buffer buffer
            ;; (message "Jit-Debug %s" (buffer-name))
            (with-buffer-prepared-for-jit-lock
                (let ((pos (point-min)))
                  (while
                      (progn
                        (when (eq (get-text-property pos 'fontified) 'defer)
                          (let ((beg pos)
                                (end (setq pos (next-single-property-change
                                                pos 'fontified
                                                nil (point-max)))))
                            (put-text-property beg end 'fontified nil)
                            (jit-lock-fontify-now beg end)))
                        (setq pos (next-single-property-change
                                   pos 'fontified)))))))))
      (setq jit-lock-defer-buffers nil))))

(defun jit-lock-register (fun &optional contextual)
  "Register FUN as a fontification function to be called in this buffer.
FUN will be called with two arguments START and END indicating the region
that needs to be (re)fontified.
If non-nil, CONTEXTUAL means that a contextual fontification would be useful."
  (add-hook 'jit-lock-functions fun nil t)
  (when (and contextual jit-lock-contextually)
    (setq-local jit-lock-contextually t))
  (jit-lock-mode t))

(defun jit-lock-unregister (fun)
  "Unregister FUN as a fontification function.
Only applies to the current buffer."
  (remove-hook 'jit-lock-functions fun t)
  (unless jit-lock-functions (jit-lock-mode nil)))

;; This function is used to prevent font-lock-fontify-buffer from
;; fontifying eagerly the whole buffer.  This is important for
;; things like CWarn mode which adds/removes a few keywords and
;; does a refontify (which takes ages on large files).
(defun jit-lock-refontify (&optional beg end)
  "Force refontification of the region BEG..END (default whole buffer)."
  (with-buffer-prepared-for-jit-lock
   (save-restriction
     (widen)
     (put-text-property (or beg (point-min)) (or end (point-max))
			'fontified nil))))
\f
;;; On demand fontification.

(defun jit-lock-function (start)
  "Fontify current buffer starting at position START.
This function is added to `fontification-functions' when `jit-lock-mode'
is active."
  (when (and jit-lock-mode (not memory-full))
    (if (null jit-lock-defer-timer)
	;; No deferral.
	(jit-lock-fontify-now start (+ start jit-lock-chunk-size))
      ;; Record the buffer for later fontification.
      (unless (memq (current-buffer) jit-lock-defer-buffers)
	(push (current-buffer) jit-lock-defer-buffers))
      ;; Mark the area as defer-fontified so that the redisplay engine
      ;; is happy and so that the idle timer can find the places to fontify.
      (with-buffer-prepared-for-jit-lock
       (put-text-property start
			  (next-single-property-change
			   start 'fontified nil
			   (min (point-max) (+ start jit-lock-chunk-size)))
			  'fontified 'defer)))))

(defun jit-lock-fontify-now (&optional start end)
  "Fontify current buffer from START to END.
Defaults to the whole buffer.  END can be out of bounds."
  (with-buffer-prepared-for-jit-lock
   (save-excursion
     (unless start (setq start (point-min)))
     (setq end (if end (min end (point-max)) (point-max)))
     ;; This did bind `font-lock-beginning-of-syntax-function' to
     ;; nil at some point, for an unknown reason.  Don't do this; it
     ;; can make highlighting slow due to expensive calls to
     ;; `parse-partial-sexp' in function
     ;; `font-lock-fontify-syntactically-region'.  Example: paging
     ;; from the end of a buffer to its start, can do repeated
     ;; `parse-partial-sexp' starting from `point-min', which can
     ;; take a long time in a large buffer.
     (let ((orig-start start) next)
       (save-match-data
	 ;; Fontify chunks beginning at START.  The end of a
	 ;; chunk is either `end', or the start of a region
	 ;; before `end' that has already been fontified.
	 (while (and start (< start end))
	   ;; Determine the end of this chunk.
	   (setq next (or (text-property-any start end 'fontified t)
			  end))

	   ;; Decide which range of text should be fontified.
	   ;; The problem is that START and NEXT may be in the
	   ;; middle of something matched by a font-lock regexp.
	   ;; Until someone has a better idea, let's start
	   ;; at the start of the line containing START and
	   ;; stop at the start of the line following NEXT.
	   (goto-char next)  (setq next (line-beginning-position 2))
	   (goto-char start) (setq start (line-beginning-position))

           ;; Make sure the contextual refontification doesn't re-refontify
           ;; what's already been refontified.
           (when (and jit-lock-context-unfontify-pos
                      (< jit-lock-context-unfontify-pos next)
                      (>= jit-lock-context-unfontify-pos start)
                      ;; Don't move boundary forward if we have to
                      ;; refontify previous text.  Otherwise, we risk moving
                      ;; it past the end of the multiline property and thus
                      ;; forget about this multiline region altogether.
                      (not (get-text-property start 'jit-lock-defer-multiline)))
             (setq jit-lock-context-unfontify-pos next))

	   ;; Fontify the chunk, and mark it as fontified.
	   ;; We mark it first, to make sure that we don't indefinitely
	   ;; re-execute this fontification if an error occurs.
	   (put-text-property start next 'fontified t)
	   (condition-case err
	       (run-hook-with-args 'jit-lock-functions start next)
	     ;; If the user quits (which shouldn't happen in normal on-the-fly
	     ;; jit-locking), make sure the fontification will be performed
	     ;; before displaying the block again.
	     (quit (put-text-property start next 'fontified nil)
		   (funcall 'signal (car err) (cdr err))))

           ;; The redisplay engine has already rendered the buffer up-to
           ;; `orig-start' and won't notice if the above jit-lock-functions
           ;; changed the appearance of any part of the buffer prior
           ;; to that.  So if `start' is before `orig-start', we need to
           ;; cause a new redisplay cycle after this one so that any changes
           ;; are properly reflected on screen.
           ;; To make such repeated redisplay happen less often, we can
           ;; eagerly extend the refontified region with
           ;; jit-lock-after-change-extend-region-functions.
           (when (< start orig-start)
	     (run-with-timer 0 nil #'jit-lock-force-redisplay
                             (copy-marker start) (copy-marker orig-start)))

	   ;; Find the start of the next chunk, if any.
	   (setq start (text-property-any next end 'fontified nil))))))))

(defun jit-lock-force-redisplay (start end)
  "Force the display engine to re-render START's buffer from START to END.
This applies to the buffer associated with marker START."
  (when (marker-buffer start)
    (with-current-buffer (marker-buffer start)
      (with-buffer-prepared-for-jit-lock
       (when (> end (point-max))
         (setq end (point-max) start (min start end)))
       (when (< start (point-min))
         (setq start (point-min) end (max start end)))
       ;; Don't cause refontification (it's already been done), but just do
       ;; some random buffer change, so as to force redisplay.
       (put-text-property start end 'fontified t)))))
\f
;;; Stealth fontification.

(defsubst jit-lock-stealth-chunk-start (around)
  "Return the start of the next chunk to fontify around position AROUND.
Value is nil if there is nothing more to fontify."
  (if (zerop (buffer-size))
      nil
    (let* ((next (text-property-not-all around (point-max) 'fontified t))
           (prev (previous-single-property-change around 'fontified))
           (prop (get-text-property (max (point-min) (1- around))
                                    'fontified))
           (start (cond
                   ((null prev)
                    ;; There is no property change between AROUND
                    ;; and the start of the buffer.  If PROP is
                    ;; non-nil, everything in front of AROUND is
                    ;; fontified, otherwise nothing is fontified.
                    (if (eq prop t)
                        nil
                      (max (point-min)
                           (- around (/ jit-lock-chunk-size 2)))))
                   ((eq prop t)
                    ;; PREV is the start of a region of fontified
                    ;; text containing AROUND.  Start fontifying a
                    ;; chunk size before the end of the unfontified
                    ;; region in front of that.
                    (max (or (previous-single-property-change prev 'fontified)
                             (point-min))
                         (- prev jit-lock-chunk-size)))
                   (t
                    ;; PREV is the start of a region of unfontified
                    ;; text containing AROUND.  Start at PREV or
                    ;; chunk size in front of AROUND, whichever is
                    ;; nearer.
                    (max prev (- around jit-lock-chunk-size)))))
           (result (cond ((null start) next)
                         ((null next) start)
                         ((< (- around start) (- next around)) start)
                         (t next))))
      result)))

(defun jit-lock-stealth-fontify (&optional repeat)
  "Fontify buffers stealthily.
This function is called repeatedly after Emacs has become idle for
`jit-lock-stealth-time' seconds.  Optional argument REPEAT is expected
non-nil in a repeated invocation of this function."
  ;; Cancel timer for repeated invocations.
  (unless repeat
    (cancel-timer jit-lock-stealth-repeat-timer))
  (unless (or executing-kbd-macro
	      memory-full
	      (window-minibuffer-p)
	      ;; For first invocation set up `jit-lock-stealth-buffers'.
	      ;; In repeated invocations it's already been set up.
	      (null (if repeat
			jit-lock-stealth-buffers
		      (setq jit-lock-stealth-buffers (buffer-list)))))
    (let ((buffer (car jit-lock-stealth-buffers))
	  (delay 0)
	  minibuffer-auto-raise
	  message-log-max
	  start)
      (if (and jit-lock-stealth-load
	       (> (car (load-average)) jit-lock-stealth-load))
	  ;; Wait a little if load is too high.
	  (setq delay jit-lock-stealth-time)
	(if (buffer-live-p buffer)
	    (with-current-buffer buffer
	      (if (and jit-lock-mode
		       (setq start (jit-lock-stealth-chunk-start (point))))
		  ;; Fontify one block of at most `jit-lock-chunk-size'
		  ;; characters.
		  (with-temp-message (if jit-lock-stealth-verbose
					 (concat "JIT stealth lock "
						 (buffer-name)))
		    (jit-lock-fontify-now start
					  (+ start jit-lock-chunk-size))
		    ;; Run again after `jit-lock-stealth-nice' seconds.
		    (setq delay (or jit-lock-stealth-nice 0)))
		;; Nothing to fontify here.  Remove this buffer from
		;; `jit-lock-stealth-buffers' and run again immediately.
		(setq jit-lock-stealth-buffers (cdr jit-lock-stealth-buffers))))
	  ;; Buffer is no longer live.  Remove it from
	  ;; `jit-lock-stealth-buffers' and run again immediately.
	  (setq jit-lock-stealth-buffers (cdr jit-lock-stealth-buffers))))
      ;; Call us again.
      (when jit-lock-stealth-buffers
	(timer-set-idle-time jit-lock-stealth-repeat-timer (current-idle-time))
	(timer-inc-time jit-lock-stealth-repeat-timer delay)
	(timer-activate-when-idle jit-lock-stealth-repeat-timer t)))))

\f
;;; Deferred fontification.

(defun jit-lock-deferred-fontify ()
  "Fontify what was deferred."
  (when (and jit-lock-defer-buffers (not memory-full))
    ;; Mark the deferred regions back to `fontified = nil'
    (dolist (buffer jit-lock-defer-buffers)
      (when (buffer-live-p buffer)
	(with-current-buffer buffer
	  ;; (message "Jit-Defer %s" (buffer-name))
	  (with-buffer-prepared-for-jit-lock
	   (let ((pos (point-min)))
	     (while
		 (progn
		   (when (eq (get-text-property pos 'fontified) 'defer)
		     (put-text-property
		      pos (setq pos (next-single-property-change
				     pos 'fontified nil (point-max)))
		      'fontified nil))
		   (setq pos (next-single-property-change
                              pos 'fontified)))))))))
    (setq jit-lock-defer-buffers nil)
    ;; Force fontification of the visible parts.
    (let ((jit-lock-defer-timer nil))
      ;; (message "Jit-Defer Now")
      (sit-for 0)
      ;; (message "Jit-Defer Done")
      )))


(defun jit-lock-context-fontify ()
  "Refresh fontification to take new context into account."
  (unless memory-full
    (dolist (buffer (buffer-list))
      (with-current-buffer buffer
	(when jit-lock-context-unfontify-pos
	  ;; (message "Jit-Context %s" (buffer-name))
	  (save-restriction
            ;; Don't be blindsided by narrowing that starts in the middle
            ;; of a jit-lock-defer-multiline.
	    (widen) 
	    (when (and (>= jit-lock-context-unfontify-pos (point-min))
		       (< jit-lock-context-unfontify-pos (point-max)))
	      ;; If we're in text that matches a complex multi-line
	      ;; font-lock pattern, make sure the whole text will be
	      ;; redisplayed eventually.
	      ;; Despite its name, we treat jit-lock-defer-multiline here
	      ;; rather than in jit-lock-defer since it has to do with multiple
	      ;; lines, i.e. with context.
	      (when (get-text-property jit-lock-context-unfontify-pos
				       'jit-lock-defer-multiline)
		(setq jit-lock-context-unfontify-pos
		      (or (previous-single-property-change
			   jit-lock-context-unfontify-pos
			   'jit-lock-defer-multiline)
			  (point-min))))
	      (with-buffer-prepared-for-jit-lock
	       ;; Force contextual refontification.
	       (remove-text-properties
		jit-lock-context-unfontify-pos (point-max)
		'(fontified nil jit-lock-defer-multiline nil)))
	      (setq jit-lock-context-unfontify-pos (point-max)))))))))

(defvar jit-lock-start) (defvar jit-lock-end) ; Dynamically scoped variables.
(defvar jit-lock-after-change-extend-region-functions nil
  "Hook that can extend the text to refontify after a change.
This is run after every buffer change.  The functions are called with
the three arguments of `after-change-functions': START END OLD-LEN.
The extended region to refontify is returned indirectly by modifying
the variables `jit-lock-start' and `jit-lock-end'.

Note that extending the region this way is not strictly necessary, except
that the nature of the redisplay code tends to otherwise leave some of
the rehighlighted text displayed with the old highlight until the next
redisplay (see comment about repeated redisplay in `jit-lock-fontify-now').")

(defun jit-lock-after-change (start end old-len)
  "Mark the rest of the buffer as not fontified after a change.
Installed on `after-change-functions'.
START and END are the start and end of the changed text.  OLD-LEN
is the pre-change length.
This function ensures that lines following the change will be refontified
in case the syntax of those lines has changed.  Refontification
will take place when text is fontified stealthily."
  (when (and jit-lock-mode (not memory-full))
    (let ((jit-lock-start start)
          (jit-lock-end end))
      (with-buffer-prepared-for-jit-lock
          (run-hook-with-args 'jit-lock-after-change-extend-region-functions
                              start end old-len)
          ;; Make sure we change at least one char (in case of deletions).
          (setq jit-lock-end (min (max jit-lock-end (1+ start)) (point-max)))
          ;; Request refontification.
          (put-text-property jit-lock-start jit-lock-end 'fontified nil))
      ;; Mark the change for deferred contextual refontification.
      (when jit-lock-context-unfontify-pos
        (setq jit-lock-context-unfontify-pos
              ;; Here we use `start' because nothing guarantees that the
              ;; text between start and end will be otherwise refontified:
              ;; usually it will be refontified by virtue of being
              ;; displayed, but if it's outside of any displayed area in the
              ;; buffer, only jit-lock-context-* will re-fontify it.
              (min jit-lock-context-unfontify-pos jit-lock-start))))))

(provide 'jit-lock)

;;; jit-lock.el ends here