bug#57639: [PATCH] Add new command 'toggle-theme'

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> > I think you'd basically put something like the following into
>> > `loaddefs-generate--emacs-batch' --
>> >
>> > (let ((lisp-mode-autoload-regexp
>> >         "^;;;###\\(\\(noexist\\)-\\)?\\(theme-autoload\\)"))

Hmm... `lisp-mode-autoload-regexp` is defined as a `defconst`.  If we're
going to set it to some other value, we should change its definition to
a `defvar`, no?

Also, this code should come with a comment explaining why we're doing
this silly dance (it took me a while to go from that code to here).
[ I don't understand the "noexist" thingy, BTW.  Is that intended to be
  a regexp that will never match?  Should it use `regexp-unmatchable`
  then?  Or why not just "^;;;###\\(?3:theme-autoload\\)"?  ]

>> >   (loaddefs-generate
>> >     (expand-file-name "../etc/themes/" lisp-directory)
>> >     (expand-file-name "theme-loaddefs.el" lisp-directory)))

Note that if one of those ;;;###theme-autoload cookies is placed in
front of a function definition, it will still result in a broken:

    (autoload 'FUNCTIONNAME "etc/themes/FILENAME" ...)

which fails because "etc/themes/FILENAME" isn't found in `load-path`.

>> > Then you have to teach `loaddefs-generate--make-autoload' to generate
>> > the correct forms as a result of the `deftheme' -- which should be some
>> > `put's.
>> 
>> OK, it looks like this worked.  Just to recap, these are the proposed
>> changes:
>
> What, if anything, does this mean for themes that aren't bundled with
> Emacs?
>
> Also, this needs a NEWS entry.

I see this got the following entry:

    ** Themes have special autoload cookies.
    All build-in themes are scraped for ;;;###theme-autoload cookies that
    are loaded along with the regular auto-loaded code.

but I can't see any good reason why Emacs users should care about that.
It seems like a purely internal hack (and given the restrictions on
what can be autoloaded this way, we probably shouldn't advertise it too
loudly, unless accompanied with appropriate warnings).


        Stefan