Re: master 2399541: Remove font-lock toggle from font-lock-update

[Gregory Heytings <gregory@heytings.org>]

>> One thing I'm not entirely sure is whether the second case is (and 
>> font-lock-mode (not font-lock-keywords)) or (and font-lock-mode (not 
>> font-lock-fontified)), but my guess is that font-lock-fontified is an 
>> internal variable and that it is safer to use font-lock-keywords here.
>
> I'm not entirely sure what is the best way to detect this middle-point 
> either.  The code that decides whether to activate the font-lock 
> machinery calls `font-lock-specified-p` for that, but maybe there are 
> corner cases where the machinery can be activated even when 
> `font-lock-specified-p` returns nil?  Similarly, I'm not sure if 
> `font-lock-fontified` is always non-nil when the font-lock machinery is 
> activated and always nil when it isn't.
>
> IOW, someone needs to look carefully at the code to find out (and 
> presumably then document the result e.g. by adding a function that 
> returns this info, or with comments, or by adding a variable which keeps 
> track of this info or ...).
>

I guess that Someone^TM is me? ;-)  I'll have a look.

>>>> +Otherwise, with prefix ARG, toggle Font Lock mode."
>>>
>>> Is this behavior useful?
>>
>> I think it is, yes, and I think it makes sense to use the prefix 
>> argument for that.  M-x font-lock-mode does not always produce the 
>> expected effect, which can be puzzling, so having a way to "do what I 
>> mean" in a command is useful.
>
> Could you describe what you mean by "does not always produce the 
> expected effect" here?  [ And maybe how the prefix ARG to 
> `font-lock-dwim` avoids those problems?  ]
>

By "does not always produce the expected effect", I mean for example that 
M-x font-lock-mode in a text-mode or fundamental-mode buffer does not 
remove then fontification from a piece of code that was killed-yanked from 
a prog-mode buffer, and M-x font-lock-mode again (which re-enables font 
lock mode) still does not remove the fontification.  From a certain point 
of view, this is perhaps expected, but from a user point of view it is 
not.

With the prefix ARG, font-lock-update-region is called before toggling 
font-lock-mode, which ensures that the fontification is coherent with the 
current major-mode before disabling/enabling font-lock-mode.