bug#13837: 24.2; Make it possible to turn whitespace-mode only when there are no existing problems

[Reuben Thomas]

I like global-whitespace-mode, but it has a downside: it keeps
activating in files that have whitespace problems, but which don't
belong to me, so I don't want to fix them. Typically I want to make a
small edit to someone else's code, and they, entirely reasonably, don't
want a big diff.

The following code implements a solution to that, which is to check if
the buffer is whitespace-clean before activating whitespace-mode.

Unfortunately, my current solution is non-optional: it requires
modifying whitespace-turn-on-if-enabled. This could be fixed, for
example by adding a preference.

The other part of the solution is easier to integrate: I've factored out
part of whitespace-report-region into whitespace-test-region, in a way
which is easy to use to implement whitespace-report-region.

If maintainers are interested in adding this feature, I'd be happy to
take guidance on how to modify my code into a patch (currently it just
sits in my .emacs).

;; Make whitespace-turn-on-if-enabled to turn on only if no
;; whitespace problems in current buffer.
(require 'whitespace)
(defun whitespace-test-region (start end)
  "Find whether there are whitespace problems in a region.

Return nil if there is no whitespace problem; otherwise, return
non-nil.

A whitespace problem is one of the following:

* If `indent-tabs-mode' is non-nil:
   empty                1. empty lines at beginning of buffer.
   empty                2. empty lines at end of buffer.
   trailing             3. SPACEs or TABs at end of line.
   indentation          4. 8 or more SPACEs at beginning of line.
   space-before-tab     5. SPACEs before TAB.
   space-after-tab      6. 8 or more SPACEs after TAB.

* If `indent-tabs-mode' is nil:
   empty                1. empty lines at beginning of buffer.
   empty                2. empty lines at end of buffer.
   trailing             3. SPACEs or TABs at end of line.
   indentation          4. TABS at beginning of line.
   space-before-tab     5. SPACEs before TAB.
   space-after-tab      6. 8 or more SPACEs after TAB.

See `whitespace-style' for documentation.
See also `whitespace-cleanup' and `whitespace-cleanup-region' for
cleaning up these problems."
  (save-excursion
    (save-match-data                ;FIXME: Why?
      (let* ((has-bogus nil)
             (rstart    (min start end))
             (rend      (max start end))
             (bogus-list
              (mapcar
               #'(lambda (option)
                   (when force
                     (add-to-list 'whitespace-style (car option)))
                   (goto-char rstart)
                   (let ((regexp
                          (cond
                           ((eq (car option) 'indentation)
                            (whitespace-indentation-regexp))
                           ((eq (car option) 'indentation::tab)
                            (whitespace-indentation-regexp 'tab))
                           ((eq (car option) 'indentation::space)
                            (whitespace-indentation-regexp 'space))
                           ((eq (car option) 'space-after-tab)
                            (whitespace-space-after-tab-regexp))
                           ((eq (car option) 'space-after-tab::tab)
                            (whitespace-space-after-tab-regexp 'tab))
                           ((eq (car option) 'space-after-tab::space)
                            (whitespace-space-after-tab-regexp 'space))
                           (t
                            (cdr option)))))
                     (and (re-search-forward regexp rend t)
                          (setq has-bogus t))))
               whitespace-report-list)))
        has-bogus))))

(defun whitespace-turn-on-if-enabled ()
  (when (cond
         ((eq whitespace-global-modes t))
         ((listp whitespace-global-modes)
          (if (eq (car-safe whitespace-global-modes) 'not)
              (not (memq major-mode (cdr whitespace-global-modes)))
            (memq major-mode whitespace-global-modes)))
         (t nil))
    (let (inhibit-quit)
      ;; Don't turn on whitespace mode if...
      (or
       ;; ...we don't have a display (we're running a batch job)
       noninteractive
       ;; ...or if the buffer is invisible (name starts with a space)
       (eq (aref (buffer-name) 0) ?\ )
       ;; ...or if the buffer is temporary (name starts with *)
       (and (eq (aref (buffer-name) 0) ?*)
            ;; except the scratch buffer.
            (not (string= (buffer-name) "*scratch*")))
       ;; Otherwise, turn on whitespace mode.
       (whitespace-find-problems-region (point-min) (point-max))
       (whitespace-turn-on)))))


In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.13)
 of 2012-12-13 on komainu, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11300000
Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 
'--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.2/site-lisp:/usr/share/emacs/site-lisp'
 '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
 '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
 'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
 --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: en_GB.UTF-8
  value of $LC_NUMERIC: en_GB.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: en_GB.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

Minor modes in effect:
  recentf-mode: t
  show-paren-mode: t
  server-mode: t
  savehist-mode: t
  minibuffer-electric-default-mode: t
  iswitchb-mode: t
  icomplete-mode: t
  global-whitespace-mode: t
  global-auto-revert-mode: t
  dtrt-indent-mode: t
  desktop-save-mode: t
  TeX-PDF-mode: t
  TeX-source-correlate-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
SPC SPC C-x C-s C-x k <return> C-x k <return> <down-mouse-1> 
<mouse-1> C-x b <return> C-x b C-s C-a C-] C-] C-] 
C-x b f u n s <return> C-a C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-SPC 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-p C-g C-p C-p C-p 
C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p C-p 
C-p C-p C-p C-p C-n <up> <up> <down> <right> <right> 
<right> <right> <right> <right> <right> <right> <right> 
<right> <right> <right> <right> <right> <right> <right> 
<left> <left> <left> <left> <left> <left> <left> <M-backspace> 
M a k e C-x C-s C-a C-SPC C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-p M-w M-x r e p o r t - b <backspace> e m a c s - 
b u g <return>

Recent messages:
Saving file /home/rrt/Software/hacked/cw/bar.c...
Wrote /home/rrt/Software/hacked/cw/bar.c
Quit
call-interactively: No recursive edit is in progress [2 times]
Mark set
Quit
Saving file /home/rrt/.emacs.d/funs.el...
Wrote /home/rrt/.emacs.d/funs.el
Mark activated
Saved text from ";; Make whitespace-turn-on-if-enabled to"

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/usr/share/emacs/24.2/site-lisp/cdargs hides /usr/share/emacs/site-lisp/cdargs
/usr/share/emacs/site-lisp/rst hides /usr/share/emacs/24.2/lisp/textmodes/rst
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/home/rrt/local/share/emacs/site-lisp/flymake hides 
/usr/share/emacs/24.2/lisp/progmodes/flymake

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mailheader sendmail rfc2047 rfc2045 ietf-drums mail-utils find-func
help-mode view cperl-mode sh-script executable nroff-mode conf-mode
newcomment misearch multi-isearch jka-compr rect todoo noutline outline
lua-mode autoconf autoconf-mode make-mode vc-git cc-mode cc-fonts
cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars cc-defs
flymake compile comint ansi-color ring face-remap flyspell ispell
smart-quotes auto-dictionary-autoloads c-eldoc-autoloads
dictionary-autoloads diff-git-autoloads dired-isearch-autoloads
full-ack-autoloads guess-style-autoloads kill-ring-search-autoloads
magit-autoloads mv-shell-autoloads tumble-autoloads
http-post-simple-autoloads package tabulated-list completing-help
recentf tree-widget wid-edit uniquify paren server savehist
minibuf-eldef iswitchb icomplete whitespace autorevert dtrt-indent
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auth-source eieio byte-opt bytecomp byte-compile cconv macroexp
gnus-util password-cache url-vars mm-util mail-prsvr tex dbus xml
regexp-opt remember user-site-loaddefs advice advice-preload yasnippet
help-fns derived edmacro kmacro easymenu assoc cl muse-autoloads
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emacs-goodies-loaddefs easy-mmode preview-latex tex-site auto-loads
time-date tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
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facemenu font-core frame cham georgian utf-8-lang misc-lang vietnamese
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jka-cmpr-hook help simple abbrev minibuffer loaddefs button faces
cus-face files text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget hashtable-print-readable backquote
make-network-process dbusbind dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)

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