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Restrict symbol prettification to displayable glyphs

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* lisp/international/mule.el (char-displayable-on-frame-p): New
function used to determine whether a character can be
meaningfully displayed on a given frame.
* doc/lispref/display.texi (Fontsets): Document it.
* lisp/progmodes/prog-mode.el
(prettify-symbols--composition-displayable-p): New function used
to restrict to displayable prettification symbols.  This
prevents issues with missing characters appearing as boxes.
(prettify-symbols--make-keywords): Use it.  (Bug#77381)
This commit is contained in:
Paul Nelson 2025-03-31 15:37:14 +02:00 • committed by Eli Zaretskii
parent ef6203b64a
commit 2d0b5f34a0
4 changed files with 73 additions and 2 deletions
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31
lisp/progmodes/prog-mode.el
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@ -230,10 +230,37 @@ Regexp match data 0 specifies the characters to be composed."
;; Return nil because we're not adding any face property.
nil)
(defun prettify-symbols--composition-displayable-p (composition)
"Return non-nil if COMPOSITION can be displayed with the current fonts.
COMPOSITION can be a single character, a string, or a sequence (vector or
list) of characters and composition rules as described in the documentation
of `prettify-symbols-alist' and `compose-region'."
(cond
((characterp composition)
(char-displayable-on-frame-p composition))
((stringp composition)
(seq-every-p #'char-displayable-on-frame-p composition))
((seqp composition)
;; check that every even-indexed element is displayable
(seq-every-p
(lambda (idx-elt)
(if (evenp (car idx-elt))
(char-displayable-on-frame-p (cdr idx-elt))
t))
(seq-map-indexed #'cons composition)))
(t
;; silently ignore invalid compositions
t)))
(defun prettify-symbols--make-keywords ()
(if prettify-symbols-alist
`((,(regexp-opt (mapcar 'car prettify-symbols-alist) t)
(0 (prettify-symbols--compose-symbol ',prettify-symbols-alist))))
(let ((filtered-alist
(seq-filter
(lambda (elt)
(prettify-symbols--composition-displayable-p (cdr elt)))
prettify-symbols-alist)))
`((,(regexp-opt (mapcar 'car filtered-alist) t)
(0 (prettify-symbols--compose-symbol ',filtered-alist)))))
nil))
(defvar-local prettify-symbols--keywords nil)