bug#47150: 28.0.50; Incorrect major-mode in minibuffer

[Stefan Monnier <monnier@iro.umontreal.ca>]

[ Hi, perpetrator of `minibuffer-inactive-mode` speaking.  ]

> minibuffer-inactive-mode: the critical thing here is "inactive", which
> means "doing nothing", or "not in use", or even "sleeping".  The
> opposite word is "active".  From its name, this major mode was never
> intended for use in active minibuffers,

That's right.

> but somehow nobody noticed that the buffer never got shifted out of
> minibuffer-inactive-mode when it came to be used again.

I did notice, but it didn't seem to cause any harm and I didn't want to
get into the discussion in which we are now, so I left things as
they stood.

> I've been fixing things in minibuf.c recently, and when I discovered
> this anomaly, I fixed it, so that an active minibuffer now runs in
> fundamental-mode, as originally intended.  I did wonder why there was no
> "minibuffer-mode".  But it was clear from the code that whoever wrote it
> intended minibuffers to use fundamental-mode whilst active.

I'm in favor of introducing a `minibuffer-mode`.

Part of the question is also when and how that mode is activated (since
activating such a mode has the effect of deleting the local variables).
I think we should call `minibuffer-mode` every time we (re)activate
a minibuffer.

>> For my case, I want automatic paren pairing and editing in
>> eval-expression.
> That does indeed suggest we really want a minibuffer-mode, rather than
> just fundamental-mode.  But surely, the parenthesis pairing will be
> dependant on the sort of text you're typing into the minibuffer, so it
> can't really be connected with, say, minibuffer-mode.

The way I see it, `eval-expression` would want to use a new major mode
that derives from `minibuffer-mode`.  And more generally
`read-from-minibuffer` should accept an argument that says which major
mode to use (I think it'd make sense to re-use the `keymap` argument
for that: if that argument is `functionp`, then treat it as a major
mode, if it's `keymapp` then use it as the keymap).
It would also provide a cleaner way to do what we currently do via the
`minibuffer-with-setup-hook` hack.

>> Plus we also need a keymap for it, which is
>> minibuffer-inactive-mode-map.
> No.  That keymap is very low functionality, and almost useless, as it's
> intended to be.

Indeed, the purpose of that keymap is that you can press `f` (for
example) into a minibuffer-only frame to open a file, but only when
there's no active minibuffer in that minibuffer-only frame.

>> It seems to me the minibuffer is always inactive? I tried M-x,
>> M-!, M-:, all reports minibuffer-inactive-mode in Emacs 27.1.  Is this
>> a mistake and the offending commit was trying to fix this
>> inconsistency?
> Very much so!

BTW: thank you for that.

> So, a quick summary: (i) the change in the minibuffer's major mode to
> fundamental-mode was intended; (ii) there may be some problems in some
> packages because of this;

The minibuffer used to be "always" in fundamental mode in Emacs<24
(since there was no `minibuffer-inactive-mode` back then), so I'm not
too worried.


        Stefan