How can I apply a user-defined fontset to a face?

Question

Say I create a fontset and populate it with fonts:

(create-fontset-from-fontset-spec
 (font-xlfd-name (font-spec :registry "fontset-mine")))
(set-fontset-font "fontset-mine" 'latin "Noto Serif")
(set-fontset-font "fontset-mine" 'kana "Noto Serif CJK JP")

I can't find a way to get a face to use this fontset. For example, if I set it like (set-face-attribute 'my-test-face nil :font "fontset-mine"), then my-test-face will show kana scripts in Noto Sans CJK KR, not JP (presumably a fallback specified by fontset-standard or something).

According to investigations by Yuan Fu, when set-face-attribute or similar is given a fontset as :font, it merely extracts the ASCII font of that set and copies its family, height, etc. onto the face (corroborated by a comparison of describe-face before and after set-face-attribute). This means I can't use it to set more than one font to a face simultaneously; anything not supported by the one font that is set, will use some Emacs-wide fallback. The bug report talks about setting the :fontset attribute, but that doesn't seem to do anything either. (EDIT: this last statement was wrong; see the answer.)

So my question is: is there a way to apply user-defined fontsets to any faces? By user-defined, I mean those created in init.el with create-fontset-from-fontset-spec, as opposed to built-in ones like fontset-standard. If not, are user-defined fontsets useful for anything?

My particular use case is as follows. I want to switch between these sets of fonts, depending on the type of document I'm editing.

• ETbb for latin, Noto Serif CJK JP for kana.
• Noto Sans for latin, Noto Sans CJK JP for kana.

(There are lots more scripts I'm interested in, but I'll focus on two.) Right now I can produce a face that has any of the following, none of which is what I want.

• ETbb for latin and some fallback font for kana
• Noto Sans for latin and some fallback font for kana
• Some fallback font for latin and Noto Serif CJK JP for kana
• Some fallback font for latin and Noto Sans CJK JP for kana

The point is, without a way to apply a fontset as one indivisible unit, I can only control latin or kana, not both; one of them will get specified by default, not by me.

Answer 1

Yikes. I had a fresh look at this after a good night's sleep, and it turns out I was wrong to say that setting the :fontset attribute doesn't work. My test code had a critical typo, and I also tried custom-theme-set-faces etc. in addition to set-face-attribute, and then mixed things up. Sorry for the confusion.

To set the record straight:

• You can apply a fontset to a face by setting its :fontset attribute with set-face-attribute. The face will then use all fonts in the set as expected. EDIT: the values of :font, :family etc. override :fontset, so if you're modifying a pre-existing face like variable-pitch, be sure to adjust those other attributes as well.
• However, you cannot set the :fontset attribute using custom-theme-set-faces or defface, which ignore that attribute.
• The manual currently says in some places that you can pass in a fontset for the :font attribute or the font argument of set-face-font, but this is misleading at best: the fontset will be stripped of the "set" part and be treated as just an ASCII font.

I think the second bullet means that there's no way to enable/disable fontsets with themes, but someone more knowledgeable on themes will have to set me straight on this.