bug#50041: Add font-lock-doc-markup-face

["Mattias Engdegård" <mattiase@acm.org>]

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Add a standard face for mark-up syntax and keywords inside code documentation (doc comments/strings) whose body text is typically set in font-lock-doc-face. This could include various tags, brackets, quotes, keywords or other syntactically special characters.

Justification:

- A standard face means that users and theme authors can set it once for all modes needing it.
- Having a face that harmonises with font-lock-doc-face reduces the risk of bad ergonomics and aesthetics.
- Setting doc mark-up in a distinct face makes in-source documentation more readable.
- It also aids writing documentation by providing feed-back to the programmer in the same way as fontifying regular language constructs.

The CC modes have the foresight of already using `font-lock-doc-markup-face` if defined, anticipating its eventual addition, defaulting to `font-lock-constant-face`. I therefore propose that the new face will inherit from `font-lock-constant-face` by default.

Proposed patch attached.


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From 6265617a428a60db9ca932a2bff33c70a94a9bc0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mattias=20Engdeg=C3=A5rd?= <mattiase@acm.org>
Date: Fri, 13 Aug 2021 12:47:39 +0200
Subject: [PATCH] Add font-lock-doc-markup-face

This face is intended for mark-up syntax and constructs inside text
using font-lock-doc-face; ie, documentation comments and strings.

* lisp/font-lock.el (font-lock-doc-markup-face): New face and variable.
* lisp/cus-theme.el (custom-theme--listed-faces): Add it to the list.
* doc/lispref/modes.texi (Faces for Font Lock): Document it.
* etc/NEWS: Mention it.
---
 doc/lispref/modes.texi | 5 +++++
 etc/NEWS               | 7 +++++++
 lisp/cus-theme.el      | 2 +-
 lisp/font-lock.el      | 8 ++++++++
 4 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index d48c9cc1af..7a651b6390 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -3447,6 +3447,11 @@ Faces for Font Lock
 for documentation strings in the code.  This inherits, by default, from
 @code{font-lock-string-face}.
 
+@item font-lock-doc-markup-face
+@vindex font-lock-doc-markup-face
+for mark-up constructs in text using @code{font-lock-doc-face}.
+This inherits, by default, from @code{font-lock-constant-face}.
+
 @item font-lock-negation-char-face
 @vindex font-lock-negation-char-face
 for easily-overlooked negation characters.
diff --git a/etc/NEWS b/etc/NEWS
index 26ede71523..6fce7c6a43 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -149,6 +149,13 @@ invoked with the '--declarations' command-line option.
 ** New command 'font-lock-update', bound to 'C-x x f'.
 This command updates the syntax highlighting in this buffer.
 
++++
+** A new standard face 'font-lock-doc-markup-face'.
+Intended for documentation mark-up syntax and tags inside text that
+uses 'font-lock-doc-face'.  This would typically be used in
+documentation comments for systems such as Doxygen, Haddock, Javadoc etc.
+By default this face inherits from 'font-lock-constant-face'.
+
 ** The new NonGNU ELPA archive is enabled by default alongside GNU ELPA.
 
 +++
diff --git a/lisp/cus-theme.el b/lisp/cus-theme.el
index f4885d0f52..7457d9e323 100644
--- a/lisp/cus-theme.el
+++ b/lisp/cus-theme.el
@@ -66,7 +66,7 @@ custom-theme--listed-faces
   shadow secondary-selection trailing-whitespace
   font-lock-builtin-face font-lock-comment-delimiter-face
   font-lock-comment-face font-lock-constant-face
-  font-lock-doc-face font-lock-function-name-face
+  font-lock-doc-face font-lock-doc-markup-face font-lock-function-name-face
   font-lock-keyword-face font-lock-negation-char-face
   font-lock-preprocessor-face font-lock-regexp-grouping-backslash
   font-lock-regexp-grouping-construct font-lock-string-face
diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 4dc42d9cf6..32b7819533 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -312,6 +312,9 @@ font-lock-string-face
 (defvar font-lock-doc-face		'font-lock-doc-face
   "Face name to use for documentation.")
 
+(defvar font-lock-doc-markup-face       'font-lock-doc-markup-face
+  "Face name to use for documentation mark-up.")
+
 (defvar font-lock-keyword-face		'font-lock-keyword-face
   "Face name to use for keywords.")
 
@@ -2006,6 +2009,11 @@ font-lock-doc-face
   "Font Lock mode face used to highlight documentation."
   :group 'font-lock-faces)
 
+(defface font-lock-doc-markup-face
+  '((t :inherit font-lock-constant-face))
+  "Font Lock mode face used to highlight documentation mark-up."
+  :group 'font-lock-faces)
+
 (defface font-lock-keyword-face
   '((((class grayscale) (background light)) :foreground "LightGray" :weight bold)
     (((class grayscale) (background dark))  :foreground "DimGray" :weight bold)
-- 
2.21.1 (Apple Git-122.3)