[Bug rust/113499] crab1 fails to link when configuring with --disable-plugin

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113499

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Arthur Cohen from comment #6)
> (In reply to Richard Biener from comment #5)
> > (In reply to Thomas Schwinge from comment #4)
> > > If I understood Arthur correctly, GCC/Rust is going to effectively require
> > > 'dlopen' (and therefore '--enable-plugin'?), so that means, if the latter's
> > > not available we have to auto-disable Rust language front end if enabled
> > > '--enable-languages=all' vs. raise a 'configure'-time error if enabled via
> > > explicit '--enable-languages=rust'?
> > 
> > Not sure - --enable-plugin is not about dlopen, it's about exporting all
> > GCCs internal symbols for use by a dlopened shared module.  Is the
> > macro processing requiring this or is it rather self-contained?
> > 
> > Being able to dlopen() is something different.
> 
> No, it does not require this and is rather self contained. Macro expansion
> needs to be able to dlopen() compiled Rust libraries, which contain a
> specific type of function our frontend calls as a macro. So we always need
> to dlopen().
> 
> Is there anything similar in other frontends? If so, how does it work on
> such platforms which do not support dlopen()?

I'm not aware of other frontends absolutely requiring dlopen(), but the
Modula-2 frontend uses a plugin for some extended diagnostics (but that
requires a "real" plugin, with exporting symbols from GCC).

I'm not aware of any abstraction for host shared module opening (to cover
windows for example), I suppose in the end we need to have such a thing
(like libiberty pex_* for execve/wait).  Maybe there's functionality in
gnulib for this.

For now I suggest to look for dlopen(), there's

# Some systems need dlopen
save_LIBS="$LIBS"
LIBS=
AC_SEARCH_LIBS(dlopen, dl) 
DL_LIB="$LIBS"
LIBS="$save_LIBS"
AC_SUBST(DL_LIB)

in gcc/configure.ac, so adding DL_LIB to the crab1 link should possibly
suffice.

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