Re: hi-lock mode and special buffers

[Chong Yidong]

After studying hi-lock.el more closely, I think there is more than one
problem at work here.

First of all, `M-x highlight-regexp' turns on hi-lock-mode in all
buffers.  This is unnecessary.  I suggest the following fix:

  1. Using define-minor-mode, define a buffer-local minor-mode
     `hi-lock-buffer-mode' that does the same stuff as the current
     `hi-lock-mode'.

  2. Using define-global-minor-mode, define `hi-lock-mode' as a global
     mode that turns on `hi-lock-buffer-mode'.

This will avoid having to set font-lock-defaults for all buffers, only
the local buffer.  The global-minor-mode will turn on the local mode,
which can set font-lock-defaults just for that buffer.  So the Gnus
group buffer won't lose when you use highlight-regexp elsewhere in
Emacs.  (This was why my change made the bug surface -- it made
hi-lock-mode really take effect everywhere in Emacs.)

This does not address the incompatibility between the Gnus group
buffer and hi-lock-mode.  The reason for *that* is that Gnus tweaks
the 'face property directly, instead of using font lock.  On the other
hand, hi-lock works by adding the regexp that you want to highlight to
font-lock-keywords, and calling font-lock-fontify-buffer.  This wipes
the existing face properties.  (Incidentally, you don't need hi-lock
for this; go to the Gnus group and run M-x font-lock-fontify-buffer).

So we need another fix for this problem.  I suggest making hi-lock
highlight faces on the first pass by altering the face properties in
the buffer directly.  At the same time, it adds the desired regexp to
font-lock-keywords, but does *not* call font-lock-fontify-buffer.
When the buffer is subsequently changed, the changes will be fontified
by the font-lock engine.

Sound like a plan?