Limit what M-x TAB lists?

[Lars Ingebrigtsen <larsi@gnus.org>]

Here's today's unrealistic idea: Wouldn't it be nice if `M-x fooTAB' was
more useful?

For instance, if I type `M-x vc-dTAB' in this buffer, I get the
following: 

Possible completions are:
vc-default-check-headers 	vc-default-mark-resolved
vc-default-revision-completion-table 	vc-delete-file
vc-diff 	vc-dir
vc-dir-delete-file 	vc-dir-find-file
vc-dir-find-file-other-window 	vc-dir-hide-state
vc-dir-hide-up-to-date 	vc-dir-ignore
vc-dir-isearch 	vc-dir-isearch-regexp
vc-dir-kill-dir-status-process 	vc-dir-kill-line
vc-dir-mark 	vc-dir-mark-all-files
vc-dir-menu 	vc-dir-mode
vc-dir-next-directory 	vc-dir-next-line
vc-dir-previous-directory 	vc-dir-previous-line
vc-dir-query-replace-regexp 	vc-dir-refresh
vc-dir-search 	vc-dir-show-fileentry
vc-dir-toggle-mark 	vc-dir-unmark
vc-dir-unmark-all-files 	vc-dir-unmark-file-up

Out of all this, I think there's two actual commands I can use here --
the rest are just commands bound to keystrokes in VC mode.

It would be really nice for discoverability if only those two applicable
commands were listed.

And it doesn't really seem unachievable.  There's oodles of ways of
achieving this that would require more or less work.

1) Just make M-x list ;;;###autoloaded functions.

That would require us adding those cookies to more functions, though.

2) New form like (command-interactive 'vc-mode "p")

This would say "this command is only valid in VC mode buffers", so it
would not be listed by M-x in other modes.  This would be pretty
flexible, but would be a lot of code churn.  But it wouldn't require a
global Armageddon date, but one could add this at leisure, and M-x would
gradually become nicer.

3) New form like (global-interactive "p")

Kinda like the opposite of 2), but here we mark commands as being
global.  It would be less churn, but be more Armageddon-ey.  Unless we
add cookies per file that says that "this file is now new-style".  Kinda
like

(pragma (local-interactive 'vc-mode))

at the start or something.

The added bonus of both 2) and 3) might be that commands would refuse to
run unless called in the right mode buffers.  Which might mean fewer bug
reports down the line.

There's a bunch of commands (like `quit-window') that apply to many
(special) modes, but we could figure something out for those, too...

(Oops.  I almost ran `M-x quit-window' in this buffer.  I wonder what
would have happened...)

-- 
(domestic pets only, the antidote for overdose, milk.)
  bloggy blog http://lars.ingebrigtsen.no/