Implement field numbers in format strings

A field number explicitly specifies the argument to be formatted.
This is especially important for potential localization work, since
grammars of various languages dictate different word orders.

* src/editfns.c (Fformat): Update documentation.
(styled_format): Implement field numbers.

* doc/lispref/strings.texi (Formatting Strings): Document field numbers.

* lisp/emacs-lisp/bytecomp.el (byte-compile-format-warn): Adapt.

* test/src/editfns-tests.el (format-with-field): New unit test.

1 files changed, 8 insertions, 3 deletions

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 12a7d4afc2a..e5b9b47b1d0 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -1375,10 +1375,15 @@ extra args."
     (let ((nfields (with-temp-buffer
                      (insert (nth 1 form))
                      (goto-char (point-min))
-                     (let ((n 0))
+                     (let ((i 0) (n 0))
                        (while (re-search-forward "%." nil t)
-                         (unless (eq ?% (char-after (1+ (match-beginning 0))))
-                           (setq n (1+ n))))
+                         (backward-char)
+                         (unless (eq ?% (char-after))
+                           (setq i (if (looking-at "\\([0-9]+\\)\\$")
+                                       (string-to-number (match-string 1) 10)
+                                     (1+ i))
+                                 n (max n i)))
+                         (forward-char))
                        n)))
           (nargs (- (length form) 2)))
       (unless (= nargs nfields)