bug#73484: 31.0.50; Abolishing etags-regen-file-extensions

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 26 Sep 2024 01:30:55 +0300
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> > We want to replace etags-regen-file-extensions with enabling etags's
> > hashbang detection support.  That requires disabling its Fortran
> > fallback.
> 
> Thanks, a fuller plan would look something like this:
> 
> - Implement the --no-fortran-fallback flag in etags. Or an environment 
> variable, or etc. Use it conditionally in etags-regen-mode.
> - Revisit the default lists of extensions that etags recognizes, keeping 
> in mind the recent thread we talking this about in - e.g. *.a seems out 
> of place for ASM (someone more familiar with assembly dialects please 
> feel free to correctme).
> - Add new possible value t to etags-regen-file-extensions, and switch 
> the default to it.

I understand that we need to disable the Fortran and C fallbacks to
avoid false positives, but what do we want to do if the fallbacks are
disabled and no suitable language parser is found using the file name?
Just skip the file and do nothing? emit a warning? something else?

I also don't understand why enabling the etags' shebang detection
requires to disable the Fortran and C fallbacks: etags looks for
shebang _before_ it falls back to Fortran and C, so what am I missing?