bug#66742: 30.0.50; transient-mark-mode is not enabled after re-dumping Emacs

[Eli Zaretskii <eliz@gnu.org>]

> Cc: gerd.moellmann@gmail.com, 66742@debbugs.gnu.org
> Date: Thu, 26 Oct 2023 16:10:53 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > I thought that we should expect the value to
> > be evaluated unless explicitly wrapped into (eval-when-compile ...).
> 
> At what time do you think defcustom's value is evaluated?
> 
> > Also, I just looked up defvar instances across Emacs and I am seeing
> > multiple cases when the value is calculated dynamically, assuming
> > loading time:
> 
> Of course, it's loading time.  But AFAIU you were talking about
> evaluation during dumping, i.e. when Emacs is built.  This is only
> relevant for defcustom's in packages that are preloaded, because those
> packages are loaded at build time, not at run time.  So for those
> packages, and only for those packages, any defcustom whose default
> value is not a constant, should be re-evaluated at startup, to make
> sure the value is suitable for the system on which Emacs runs.
> 
> > cvs-diff-program, diff-use-changed-face, url-mime-encoding-string,
> > url-dav-lock-identifier, type-break-time-last-command,
> > reftex-section-numbers, ispell-library-directory, artist-pointer-shape,
> > so-long-version, shadow-system-name, bdf-cache-file,
> > vhdl-mode-menu-list, sh-shell, js-js-tmpdir, gud-sdb-needs-tags,
> > tetris-next-x, tetris-score-y, <a number of Org mode variables>,
> > vip-startup-file, vi-scroll-amount, tramp-cache-read-persistent-data,
> > tramp-archive-enabled, socks-username, newsticker--latest-update-time,
> > dictionary-color-support, mh-uncompface-executable, mh-swish++-binary,
> > mh-swish-binary, mh-mairix-binary, mh-namazu-binary, mh-grep-binary,
> > mh-spamassassin-executable, mh-sa-learn-executable,
> > mh-bogofilter-executable, mh-spamprobe-executable, mh-pgp-support-flag,
> > kkc-init-file-name, image-dired-queue-active-limit,
> > mm-temp-files-cache-file, erc-autoaway-last-sent-time,
> > viper-custom-file-name, viper-current-frame-saved,
> > copyright-current-year, cl--random-state (!!! that can easily cause
> > subtle issues), dframe-have-timer-flag, todo-files, todo-archives,
> > todo-done-separator, diary-font-lock-keywords, math-expr-opers,
> > calc-gnuplot-display, archive-7z-program.
> 
> This is not relevant, since packages that aren't preloaded will be
> loaded at run time, and therefore the values will be evaluated in the
> correct context.  If you are talking about these, then I don't
> understand what prompted you to raise this issue to begin with, and in
> a thread that discusses dumping.

To clarify: a defcustom whose default value is not a constant needs to
be re-evaluated during startup, if we want the dumped Emacs to be able
to run on another system or with different environment than at dump
time.