[Bug c/28901] -Wunused-variable ignores unused const initialised variables

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28901

--- Comment #31 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Mark Wielaard from comment #29)
> > > +                   || filename_cmp (main_input_filename,
> > > +                                    DECL_SOURCE_FILE (decl)) == 0)))
> > 
> > Better use MAIN_FILE_P(DECL_SOURCE_LOCATION (decl))
> 
> MAIN_FILE_P doesn't take a source location, but a line_map_ordinary, which
> seems more cumbersome to get at from a decl then the filename to compare
> against.

Ah, yes, sorry. One needs to lookup the map for the location before being able
to use MAIN_FILE_P. I was hoping that there was already a convenient overload
or similar function in input.h or line-map.h, but alas, there isn't.

I think it is still worth it to add something like in_main_file_at(location_t),
similar to the existing in_system_header_at(), even if you wish to use
filename_cmp within it. Somebody else in the future may decide to replace that
implementation with something using MAIN_FILE_P, since it is much faster.