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

[Yuan Fu]

> On Oct 6, 2021, at 12:02 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> 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.

You are right.

> 
>> 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.

It IMO reveals the writer’s intent that fontset can be used to force a face to 
display certain characters with certain font. I won’t insist on proving this, 
tho.

> 
>> 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.

Ok.

> 
>>> 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.

Ok.

> 
>> 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.

Ok.

Still, I believe fontset is supposed to be used as I described. Let’s wait for 
Handa-san.

Thanks,
Yuan