Improve shorthands-font-lock-shorthands (bug#67390)

Add font locking to the shorthand prefix of a given printed symbol
name by checking if any of the shorthand prefixes in
read-symbol-shorthands are a prefix for that print name.  Although
this does more string comparisons, it didn't prove to be any slower
than the existing approach, and is more correct.

This version is more accurate when highlighting files with many
overlapping shorthands.   Given:

;; Local Variables:
;; read-symbol-shorthands: (("bc-" . "breadcrumb-")
;;                          ("aw-" . "ace-window-")
;;                          ("zorglub/" . "ace-window-")
;;                          ("he//" . "hyperdrive-entry--")
;;                          ("h//"  . "hyperdrive--")
;;                          ("he/"  . "hyperdrive-entry-")
;;                          ("h/"   . "hyperdrive-"))
;; End:

The following are correct highlights on print names

'(zorglub/blerh ; hilits "zorglub/" reads to 'ace-window-blerh'
  he/foo        ; hilits "he/"      reads to 'hyperdrive-entry-foo'
  he//bar       ; hilits "he//"     reads to 'hyperdrive-entry--bar'
  h/coiso       ; hilits "h/"       reads to 'hyperdrive-coiso'
  h//thingy     ; hilits "h//"      reads to 'hyperdrive--thingy'
  bc-yo         ; hilits "bc-"      reads to 'breadcrumb-yo'
  aw-thingy     ; hilits "aw-"      reads to 'ace-window-thingy'
  )

Co-authored-by: Jonas Bernoulli <jonas@bernoul.li>
Co-authored-by: Joseph Turner <joseph@ushin.org>

* lisp/emacs-lisp/shorthands.el (shorthands-font-lock-shorthands):

1 files changed, 11 insertions, 23 deletions

diff --git a/lisp/emacs-lisp/shorthands.el b/lisp/emacs-lisp/shorthands.el
index 6348aaccf93..379fb0baec9 100644
--- a/lisp/emacs-lisp/shorthands.el
+++ b/lisp/emacs-lisp/shorthands.el
@@ -52,38 +52,26 @@
   :version "28.1"
   :group 'font-lock-faces)
 
-(defun shorthands--mismatch-from-end (str1 str2)
-  "Tell index of first mismatch in STR1 and STR2, from end.
-The index is a valid 0-based index on STR1.  Returns nil if STR1
-equals STR2.  Return 0 if STR1 is a suffix of STR2."
-  (cl-loop with l1 = (length str1) with l2 = (length str2)
-           for i from 1
-           for i1 = (- l1 i) for i2 = (- l2 i)
-           while (eq (aref str1 i1) (aref str2 i2))
-           if (zerop i2) return (if (zerop i1) nil i1)
-           if (zerop i1) return 0
-           finally (return i1)))
-
 (defun shorthands-font-lock-shorthands (limit)
+  "Font lock until LIMIT considering `read-symbol-shorthands'."
   (when read-symbol-shorthands
     (while (re-search-forward
             (concat "\\_<\\(" (rx lisp-mode-symbol) "\\)\\_>")
             limit t)
       (let* ((existing (get-text-property (match-beginning 1) 'face))
+             (print-name (match-string 1))
              (probe (and (not (memq existing '(font-lock-comment-face
                                                font-lock-string-face)))
-                         (intern-soft (match-string 1))))
-             (sname (and probe (symbol-name probe)))
-             (mismatch (and sname (shorthands--mismatch-from-end
-                                   (match-string 1) sname)))
-             (guess (and mismatch (1+ mismatch))))
-        (when guess
-          (when (and (< guess (1- (length (match-string 1))))
-                     ;; In bug#67390 we allow other separators
-                     (eq (char-syntax (aref (match-string 1) guess)) ?_))
-            (setq guess (1+ guess)))
+                         (intern-soft print-name)))
+             (symbol-name (and probe (symbol-name probe)))
+             (prefix (and symbol-name
+                          (not (string-equal print-name symbol-name))
+                          (car (assoc print-name
+                                      read-symbol-shorthands
+                                      #'string-prefix-p)))))
+        (when prefix
           (add-face-text-property (match-beginning 1)
-                                  (+ (match-beginning 1) guess)
+                                  (+ (match-beginning 1) (length prefix))
                                   'elisp-shorthand-font-lock-face))))))
 
 (font-lock-add-keywords 'emacs-lisp-mode '((shorthands-font-lock-shorthands)) t)