From 046db64044268ec62dd63681c821edb11297f905 Mon Sep 17 00:00:00 2001
From: Jonas Bernoulli <jonas@bernoul.li>
Date: Mon, 27 Apr 2026 21:11:52 +0200
Subject: [PATCH] Call format-spec substitution functions in current buffer

* lisp/format-spec.el (format-spec): Call format-spec substitution
functions in current buffer.  I.e., in the same buffer in which
substitution expressions are evaluated.  Previously functions were
instead called in the temporary buffer used to deal with FORMAT.
---
 lisp/format-spec.el | 116 +++++++++++++++++++++++---------------------
 1 file changed, 60 insertions(+), 56 deletions(-)

diff --git a/lisp/format-spec.el b/lisp/format-spec.el
index b08c7a1cdee..d2c47bf26ab 100644
--- a/lisp/format-spec.el
+++ b/lisp/format-spec.el
@@ -87,62 +87,66 @@ any occurrences of \"%%\" in FORMAT verbatim in the result.
 
 If SPLIT, instead of returning a single string, a list of strings
 is returned, where each format spec is its own element."
-  (with-temp-buffer
-    (let ((split-start (point-min))
-          (split-result nil))
-      (insert format)
-      (goto-char (point-min))
-      (while (search-forward "%" nil t)
-        (cond
-         ;; Quoted percent sign.
-         ((= (following-char) ?%)
-          (when (memq ignore-missing '(nil ignore delete))
-            (delete-char 1)))
-         ;; Valid format spec.
-         ((looking-at (rx (? (group (+ (in " 0<>^_-"))))
-                          (? (group (+ digit)))
-                          (? (group ?. (+ digit)))
-                          (group alpha)))
-          (let* ((beg (point))
-                 (end (match-end 0))
-                 (flags (match-string 1))
-                 (width (match-string 2))
-                 (trunc (match-string 3))
-                 (char (string-to-char (match-string 4)))
-                 (text (let ((res (cdr (assq char specification))))
-                         (if (functionp res) (funcall res) res))))
-            (when (and split
-                       (not (= (1- beg) split-start)))
-              (push (buffer-substring split-start (1- beg)) split-result))
-            (cond (text
-                   ;; Handle flags.
-                   (setq text (format-spec--do-flags
-                               (format "%s" text)
-                               (format-spec--parse-flags flags)
-                               (and width (string-to-number width))
-                               (and trunc (car (read-from-string trunc 1)))))
-                   ;; Insert first, to preserve text properties.
-                   (insert-and-inherit text)
-                   ;; Delete the specifier body.
-                   (delete-region (point) (+ end (length text)))
-                   ;; Delete the percent sign.
-                   (delete-region (1- beg) beg))
-                  ((eq ignore-missing 'delete)
-                   ;; Delete the whole format spec.
-                   (delete-region (1- beg) end))
-                  ((not ignore-missing)
-                   (error "Invalid format character: `%%%c'" char)))
-            (when split
-              (push (buffer-substring (1- beg) (point)) split-result)
-              (setq split-start (point)))))
-         ;; Signal an error on bogus format strings.
-         ((not ignore-missing)
-          (error "Invalid format string"))))
-      (if (not split)
-          (buffer-string)
-        (unless (= split-start (point-max))
-          (push (buffer-substring split-start (point-max)) split-result))
-        (nreverse split-result)))))
+  (let ((orig-buffer (current-buffer)))
+    (with-temp-buffer
+      (let ((split-start (point-min))
+            (split-result nil))
+        (insert format)
+        (goto-char (point-min))
+        (while (search-forward "%" nil t)
+          (cond
+           ;; Quoted percent sign.
+           ((= (following-char) ?%)
+            (when (memq ignore-missing '(nil ignore delete))
+              (delete-char 1)))
+           ;; Valid format spec.
+           ((looking-at (rx (? (group (+ (in " 0<>^_-"))))
+                            (? (group (+ digit)))
+                            (? (group ?. (+ digit)))
+                            (group alpha)))
+            (let* ((beg (point))
+                   (end (match-end 0))
+                   (flags (match-string 1))
+                   (width (match-string 2))
+                   (trunc (match-string 3))
+                   (char (string-to-char (match-string 4)))
+                   (text (let ((res (cdr (assq char specification))))
+                           (if (functionp res)
+                               (with-current-buffer orig-buffer
+                                 (funcall res))
+                             res))))
+              (when (and split
+                         (not (= (1- beg) split-start)))
+                (push (buffer-substring split-start (1- beg)) split-result))
+              (cond (text
+                     ;; Handle flags.
+                     (setq text (format-spec--do-flags
+                                 (format "%s" text)
+                                 (format-spec--parse-flags flags)
+                                 (and width (string-to-number width))
+                                 (and trunc (car (read-from-string trunc 1)))))
+                     ;; Insert first, to preserve text properties.
+                     (insert-and-inherit text)
+                     ;; Delete the specifier body.
+                     (delete-region (point) (+ end (length text)))
+                     ;; Delete the percent sign.
+                     (delete-region (1- beg) beg))
+                    ((eq ignore-missing 'delete)
+                     ;; Delete the whole format spec.
+                     (delete-region (1- beg) end))
+                    ((not ignore-missing)
+                     (error "Invalid format character: `%%%c'" char)))
+              (when split
+                (push (buffer-substring (1- beg) (point)) split-result)
+                (setq split-start (point)))))
+           ;; Signal an error on bogus format strings.
+           ((not ignore-missing)
+            (error "Invalid format string"))))
+        (if (not split)
+            (buffer-string)
+          (unless (= split-start (point-max))
+            (push (buffer-substring split-start (point-max)) split-result))
+          (nreverse split-result))))))
 
 (defun format-spec--do-flags (str flags width trunc)
   "Return STR formatted according to FLAGS, WIDTH, and TRUNC.