Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> @@ -389,7 +396,9 @@ reftex-TeX-master-file
>>       (t
>>        ;; Use buffer file name.
>>        (setq master (buffer-file-name))))
>> -    (expand-file-name master)))
>> +    (if reftex--suppress-nonfile-error
>> +        "<none>.tex"
>> +      (expand-file-name master))))
>
> I think TRT would be to return the buffer when in a non-file buffer and
> then to adjust callers accordingly.

I agree that the right approach is to make RefTeX work properly in
non-file buffers.  To explore this, I implemented the approach you
suggested (see the attached patch).  It's a big change that I'm unlikely
to stress-test in my "day job", where for many years I've used a cheap
substitute [1] for RefTeX that I find more efficient for my purposes.
(Any feedback, either on the patch or on how I might better integrate
[1] into the ecosystem, would of course be welcome.)

This bug report arose from a simpler issue: C-x v v C-c C-w in a tex
document (with AUCTeX installed) makes vc activate LaTeX-mode in a
temporary non-file buffer, where AUCTeX style hooks attempt to load
RefTeX (catastrophically).  For this narrow issue, the workaround
provided by Ikumi suffices.

[1] https://github.com/ultronozm/czm-tex-ref.el