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

From: jan . synacek
Subject: Using face names as FACESPEC in font-lock-keywords does not work
Date: Wed, 22 Jun 2022 08:33:49 +0000


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")
                    `((,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:
     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.

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. It seems that my case is exactly the minority case where it's actually needed. Using faces like 'font-lock-string-face' works, because they are defined exactly as the documentation says is not necessary. And evaluating "(defvar xref-file-header 'xref-file-header)" fixes the highlighting, too.

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?

In any case, I think that the documentation deserves a bit of sharpening.

Jan Synáček

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