bug#17813: 24.3.91; bad linum display with `text-scale-adjust'

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: shigeru.fukaya@gmail.com,  17813@debbugs.gnu.org,  miles@gnu.org
> Date: Fri, 20 Jun 2014 09:35:32 -0400
> 
> >> I think the right solution is to scale the margin width by the ratio
> >> "linum font size / base font size".  Then users can set their `linum'
> >> face the way want and choose between (1) and (2).
> >> But it turns out that face-attribute can't be used for that because it
> >> does not pay attention to face-remapping-alist.
> > Not sure what you mean by that: attributes don't care about remapping,
> > because remapping doesn't change the attributes, it only changes their
> > values.
> 
> I mean the following:
> 
>  a way to know "if I apply face `foo' (and only face `foo') to a chunk
>  of text in the current buffer, what will it look like: will it be bold?
>  what will be its size? ..."
> 
> `face-attribute' doesn't work here because it fails to take into account
> the effect of face-remapping-alist.

APIs that need to take face remapping into account need to be
specifically programmed to do that.  Some were, some weren't;
face-font belongs to the former.

> >   (aref (font-info (face-font 'default)) 2)
> 
> Indeed, that seems to work, thanks.
> 
> > But note that font's size and its "width" (which is actually the
> > average width recorded in the font file) are not identical.  We don't
> > have a Lisp API to get the average width of a non-default font;
> 
> Indeed, that's an additional problem.
> 
> > perhaps we should.
> 
> I'm all for it.

Would it be OK to add that as the last element to the array returned
by font-info?