Re: [PATCH] Remove obsolete fast-lock and lazy-lock libraries

[Jeff Norden]

Alan Mackenzie writes:
> This doesn't make sense, I think.  font-lock-support-mode is a function,
> and if a user has this set to a non-existent function, the correct
> response is surely to tell her with an error message, rather than
> executing a different function.
>
> The only reason anybody nowadays would set f-l-s-m to 'fast-lock-mode
> would be for debugging.  Let's not make that debugging any more
> difficult than it already is.

Stefan Monnier writes:
> IMNSHO, that variable should not be a defcustom any more and we
> shouldn't encourage users to touch it.  IOW we should mark it obsolete.
>
> We could OTOH add a new command to enter a form of
> `font-lock-debug-mode` which could disable the use of jit-lock (but not
> necessarily in exactly the same way as setting `font-lock-support-mode`
> would, tho it would probably be the most obvious immediate choice in the
> short term).

I guess my suggestion was to change f-l-s-m to a boolean flag in order
to make debugging *easier*.  My own experience, as someone previously
unfamiliar with the font-lock code, was that it took me a while to
discover that setting f-l-s-m to nil is the way to use font-lock
directly, without jit-lock "support".  Neither setting the jit-lock-mode
variable nor calling (jit-lock-mode nil) does this.  The latter will
turn off jit-lock, but doesn't tell font-lock to install its default
change functions.

I was also thinking that treating any non-nil value by using jit-Lock
would provide better backward compatibility.  I later realized that this
is actually a moot point.  The only way that setting f-l-s-m to
'fast-lock-mode will currently work is to also (require 'fast-lock), and
that is going to crash-and-burn once fast-lock-mode.el is gone.

Personally, I like Stefan M's suggestion of creating a new function or
variable and marking f-l-s-m as obsolete, but Eli's point that this
could be disruptive to people who are currently using f-l-s-m for
debugging is also well taken.  Anything that makes it clear how to
disable Jit Lock while still using Font Lock seem like an improvement
to me.

Thanks,
-Jeff