bug#49797: 28.0.50; Setting face to custom fontset doesn't work

[Eli Zaretskii]

> From: Yuan Fu <casouri@gmail.com>
> Date: Wed, 6 Oct 2021 11:56:18 -0700
> Cc: Kenichi Handa <handa@gnu.org>,
>  49797@debbugs.gnu.org
> 
> >>        fontset = FONTSET_FROM_ID (face->fontset);
> >> 
> >> Then find an appropriate font from it
> >> 
> >>        rfont_def = fontset_font (fontset, c, face, id);
> > 
> > Yes, AFAIU that's the part where we check if the current face's
> > fontset's font, made for the ASCII characters, happens to have a glyph
> > for the non-ASCII character we need to display.
> 
> That is not, the part you mentioned is before:
> 
>       if (face->ascii_face->font)
>       {
>         XSETFONT (font_object, face->ascii_face->font);
>         if (font_has_char (f, font_object, c))
>           return face->ascii_face->id;
>       }

No, this is where we try to use the ASCII font for symbols and
punctuation characters.

> And what about the comment I mentioned? Did you read it?

Yes, but it has no relevance to the issue at hand.  That whole part is
about displaying symbol and punctuation characters.

> In any case, 
> 
>     rfont_def = fontset_font (fontset, c, face, id); 
> 
> finds a font in the fontset that can display character c. That’s hard to 
> ignore.

I didn't.

> > As mentioned earlier, user code is not supposed to set the :fontset
> > attribute directly, which is why it isn't documented.
> 
> Finternal_set_lisp_face_attribute has code that let me set :fontset. Why is 
> it there if Lisp is not supposed to set :fontset?

How is that relevant?  That something is possible doesn't mean it's
recommended.

> Plus, Emacs doesn’t set :fontset by itself

It does, via the Lisp attributes and LFACE_FONTSET_INDEX.  That's what
matters on the C level.