Fix and extend format-spec (bug#41758)

* lisp/format-spec.el: Use lexical-binding.  Remove dependence on
subr-x.el.
(format-spec-make): Clarify docstring.
(format-spec--parse-modifiers): Rename to...
(format-spec--parse-flags): ...this and simplify.  In particular,
don't bother parsing :space-pad which is redundant and unused.
(format-spec--pad): Remove, replacing with...
(format-spec--do-flags): ...this new helper function which performs
more of format-spec's supported text manipulation.
(format-spec): Autoload.  Allow optional argument to take on special
values 'ignore' and 'delete' for more control over what happens when
a replacement for a format specification isn't provided.  Bring back
proper support for a precision modifier similar to that of 'format'.

* lisp/battery.el (battery-format): Rewrite in terms of format-spec.
(battery-echo-area-format, battery-mode-line-format): Mention
support of format-spec syntax in docstrings.

* doc/lispref/strings.texi (Custom Format Strings):
* etc/NEWS: Document and announce these changes.

* lisp/dired-aux.el (dired-do-compress-to):
* lisp/erc/erc-match.el (erc-log-matches):
* lisp/erc/erc.el (erc-update-mode-line-buffer):
* lisp/gnus/gnus-sieve.el (gnus-sieve-update):
* lisp/gnus/gssapi.el (open-gssapi-stream):
* lisp/gnus/mail-source.el (mail-source-fetch-file)
(mail-source-fetch-directory, mail-source-fetch-pop)
(mail-source-fetch-imap):
* lisp/gnus/message.el (message-insert-formatted-citation-line):
* lisp/image-dired.el:
* lisp/net/eww.el:
* lisp/net/imap.el (imap-kerberos4-open, imap-gssapi-open)
(imap-shell-open):
* lisp/net/network-stream.el (network-stream-open-shell):
* lisp/obsolete/tls.el (open-tls-stream):
* lisp/textmodes/tex-mode.el:
Remove extraneous loads and autoloads of format-spec now that it is
autoloaded and simplify its uses where possible.

* test/lisp/battery-tests.el (battery-format): Test new format-spec
support.
* test/lisp/format-spec-tests.el (test-format-spec): Rename to...
(format-spec) ...this, extending test cases.
(test-format-unknown): Rename to...
(format-spec-unknown): ...this, extending test cases.
(test-format-modifiers): Rename to...
(format-spec-flags): ...this.
(format-spec-make, format-spec-parse-flags, format-spec-do-flags)
(format-spec-do-flags-truncate, format-spec-do-flags-pad)
(format-spec-do-flags-chop, format-spec-do-flags-case): New tests.

1 files changed, 17 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index d702f758f23..4d730228139 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -461,6 +461,16 @@ In Emacs 24.3, the variable 'dbus-event-error-hooks' was renamed to
 The old names, which were kept as obsolete aliases of the new names,
 have now been removed.
 
+** Battery
+
+---
+*** A richer syntax can be used to format battery status information.
+The user options 'battery-mode-line-format' and
+'battery-echo-area-format' now support the full formatting syntax of
+the function 'format-spec' documented under '(elisp) Custom Format
+Strings'.  The new syntax includes specifiers for padding and
+truncation, amongst other things.
+
 
 * New Modes and Packages in Emacs 28.1
 
@@ -578,6 +588,13 @@ for encoding and decoding without having to bind
 It controls, whether 'process-file' returns a string when a remote
 process is interrupted by a signal.
 
++++
+** The behavior of 'format-spec' is now closer to that of 'format'.
+In order for the two functions to behave more consistently,
+'format-spec' now pads and truncates based on string width rather than
+length, and also supports format specifications that include a
+truncating precision field, such as '%.2a'.
+
 
 * Changes in Emacs 28.1 on Non-Free Operating Systems