bug#46055: [PATCH] Add rust lang to etags

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: pot@gnu.org,  stefan@marxist.se,  contact@parouby.fr,
>   46055@debbugs.gnu.org
> Date: Tue, 18 May 2021 15:44:50 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > The test suite uses the same source files for 7 different runs, each
> > time invoking etags with different command-line switches.  So whenever
> > you add a new source file, you need to run make, and each time it
> > fails, copy the left-over ETAGS file into ETAGS.good_1, ETAGS.good_2,
> > etc., one by one.
> 
> Ah, I see.  Putting this info in a README in that directory would be
> helpful for the next person who wonders how it works, though.

Done.