bug#32322: 27.0.50; Connect flymake to next-error-function when enabled

[Juri Linkov]

>> I know that some other packages utilize the 'next-error-function' to get
>> support for 'next-error' and 'previous-error'. I wonder if it would make
>> sense to set this up by default when the flymake mode is enabled. I have
>> currently enabled it by:
>>
>>   (setq next-error-function 'flymake-goto-next-error)
>>
>> And it seems to work flawlessly.
>
> I don't think it would be appropriate for flymake-mode to set that
> itself, since that variable is mostly just meant for other
> compilation-line major modes to set buffer-locally.  But I can see how
> it might be convenient for users that mainly just use flymake and not
> the regular `M-x compile', so I've now mentioned this in the
> flymake-mode doc string in Emacs 29.

The problem with enabling next-error-function in buffers
where flymake-mode is activated is that there is no evident
visual indication that next-error is used to navigate errors
in the current buffer.  In other modes like compilation
the enabled next-error-function is usually indicated
by the presence of the compilation window with a list of errors
nearby.