Re: Using face names as FACESPEC in font-lock-keywords does not work

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 22 Jun 2022 08:33:49 +0000
> From: jan.synacek@posteo.org
> 
> I have the following piece of code that is supposed to add highlighting 
> for CPP-like conditionals in my mode:
> 
> (setq-local font-lock-defaults
>              (let* ((cpp-rx (rx line-start
>                                 "#" (or "if" "ifdef" "else" "elif" 
> "endif")
>                                 word-boundary))
>                     (keywords
>                      `((,cpp-rx . xref-file-header))))
>                (list keywords)))
> 
> This piece of code does not work and does not use the 'xref-file-header' 
> face.
> 
> 
> In the Elisp manual (24.6.2 Search-based Fontification) it says:
> "
>    -- Variable: font-lock-keywords
>    ...
> 
> ...
> Each element of ‘font-lock-keywords’ should have one of these forms:
> ...
> ‘(MATCHER . FACESPEC)’
>       In this kind of element, FACESPEC is an expression whose value
>       specifies the face to use for highlighting.  In the simplest case,
>       FACESPEC is a Lisp variable (a symbol) whose value is a face name.
> ...
> "
> 
> I find the language here quite confusing. 'xref-file-header' is a symbol 
> that evaluates to itself, which also happens to be the face name, and is 
> technically an expression. But apparently, that's not enough, as it 
> seems that the symbol itself has to have a value cell in this particular 
> case.

The documentation you cite is of font-lock-keywords, but your code
uses font-lock-defaults, a variable whose value is supposed to be
something very different.  If that is a typo, then which of these two
is correct?

> Also, in 40.12.2 Defining Faces, it says:
> "
>     People are sometimes tempted to create a variable whose value is a
> face name.  In the vast majority of cases, this is not necessary; the
> usual procedure is to define a face with ‘defface’, and then use its
> name directly.
> "
> 
> This piece of documentation doesn't really help, too.

Please note that it describes a different use case: how to _define_ a
face, not how to use faces in font-lock.  Faces can be used in Emacs
in contexts completely unrelated to font-lock.

> So my question is: Am I doing something wrong? Or does this case 
> (setting font-lock-keywords) really
> require a variable which holds the face name instead of using the face 
> name directly?

My suggestion is to use one of the font-lock faces, not something
unrelated.  I don't really understand how xref-file-header face could
be related to highlighting CPP-like conditionals in the first place.
If you like its colors, why not customize one of the rarely-used
font-lock faces to have those colors, and then use that?

But if you insist on using the above, and if you must understand how
to force Emacs to do what you want in that case, that's possibly a
separate discussion, so please do indicate your intentions.  (And yes,
faces are a tricky subject and can easily confuse in corner cases, and
your code uses some "advanced" techniques, so a little wonder you bump
into that.)