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

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: pot@gnu.org,  stefan@marxist.se,  contact@parouby.fr,
>   46055@debbugs.gnu.org
> Date: Mon, 17 May 2021 18:08:09 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Just say "make" in that directory, it should finish with no errors.
> 
> If I do that, I get an enormous diff in the terminal, and it then
> finishes with:
> 
>  y-src/parse.c,520
>  #define YYBISON 4,64
> make[1]: *** [Makefile:63: ediff_1] Error 1
> make[1]: Leaving directory '/home/larsi/src/emacs/trunk/test/manual/etags'
> make: *** [Makefile:54: check] Error 2

Try again, I just fixed this.

The problem is that people make mechanic changes to all our sources,
and forget that files in this particular directory shall NOT be
changed, or else if they are, the '*good*' files with expected results
should be updated at that very moment.  Examples of such changes
include: yearly update of copyright years, removal of obsolete
variables across the board, spelling fixes, etc.  Just do

  git log test/manual/etags/c-src/abbrev.c

and you will see what I mean.  Each such change breaks this test
suite, and requires manual inspection of the diffs to convince
ourselves that the new results are correct and not bugs.