Re: Selective font-locking?

[JD Smith <jdtsmith@gmail.com>]

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> On Apr 12, 2021, at 10:07 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
> Calling `font-lock-flush` or `font-lock-ensure` from font-lock-keywords
> is quite odd.  I'd call something like `font-lock-fontify-region` instead.

Using `font-lock-fontify-region` instead causes Emacs to become mostly unresponsive.  Sending a USR2 reveals:

Debugger entered--entering a function:
* #f(compiled-function () #<bytecode 0x1fed77d1f329>)()
  font-lock-default-fontify-region(188 189 nil)
  font-lock-fontify-region(188 189)
  #f(compiled-function (fun) #<bytecode 0x1fed77d1f2f9>)(font-lock-fontify-region)
  run-hook-wrapped(#f(compiled-function (fun) #<bytecode 0x1fed77d1f2f9>) font-lock-fontify-region)
  jit-lock--run-functions(188 189)
  jit-lock-fontify-now(188 688)
  jit-lock-function(188)
  redisplay_internal\ \(C\ function\)()  

> There's one thing with which you might want to be careful, tho, which is
> the `syntax-ppss` state.  You might want to `narrow-to-region` around
> the call to `font-lock-fontify-region` (maybe narrow to pmark...(point-max)?).
> 
> This is because in a shell buffer, some of the past interactions may
> have been truncated (e.g. by `comint-truncate-buffer`), so you may end
> up with (point-min) being in the middle of a string or something.
> [ Similar problems can occur if the prompt itself contains funny characters
>  like unmatched quotes.  or if past interactions include output which
>  is not lexically valid Python code.  ]

Well hmm, this is a bummer.  l tested for this issue by inserting an entirely unmatched quote:

In [12]: print(chr(39))
'

and this does affect the syntax (everything is a string).  But unfortunately narrowing as follows doesn’t seem to fix this:

	    (save-restriction
	      (narrow-to-region pmark (point-max))
	      (with-syntax-table python-mode-syntax-table
		(font-lock-flush start limit)
		(font-lock-ensure start limit)))))))

I’m not sure it’s the same thing, but I found a related issue with `indent-for-tab-command'.  In attempting to ignore the prompt for computing indentation, I narrowed to a region which excluded it, but indent.el calls `indent--funcall-widened’, which undoes my narrowing!  

Is there any way to specify "narrow to this region and don’t let anybody widen it(!)"?

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