bug#23010: 25.1.50; Isearch help key to indicate current search state/mode

[Drew Adams <drew.adams@oracle.com>]

This report was requested by Kaushal Modi in bug #22991.  See that
bug for more context.

This request is for a help key, to show the current state/modes of
Isearch.  The info shown should include all of the possible toggle
states, such as case folding, whitespace folding, regexp/literal, and
char folding, as well as indicate the various toggle keys for these.

This is a _temporary_ stopgap.  What is needed for the next release is a
well designed implementation that provides for easy configuration by
users and switching among _sets_ of character equivalence classes,
toggling of such sets, and toggling of individual equivalence classes.
A useful and supple design will no doubt change the needs for both state
toggling keys (probably at least one "toggle" prefix key) and any help
display about the current state/mode.

For now, the state/mode help could be bound to a new, dedicated key.  Or
it could added to the current mode help, `C-h m'.

Unless it were added to the top of the `C-h m' output, a separate help
key (and separate *Help* content) would probably be better, so users can
easily see all of the state info without scrolling the help window etc.

It is important to provide a succinct summary in a few lines, and then
follow that by any additional detailed info that might be helpful.

Such additional info could give details about the behavior of the
different modes, as well as info about how to change among them.  Yes,
some of that info might repeat some of what is in the `C-h m' output,
but that's OK.

Another possibility might be to add this info to the regular `C-h m'
output, but have a new help key that takes you directly to the part of
that output that shows the current modes and explains the possibile
modes, i.e., scrolls to that part.  That would have the advantage of
providing this info in the context of isearch help in general.

Again, it is likely that the help output for this and the key(s) for
displaying it will change soon.  The char-folding feature is very
immature, and will hopefully be developed quite a bit further.  What
directions that might take are unknown now.


In GNU Emacs 25.1.50.1 (i686-pc-mingw32)
 of 2015-12-10
Repository revision: 6148555ee5a3d0139ae517803718b3e0357933c7
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --prefix=/c/Devel/emacs/snapshot/trunk --enable-checking=yes
 --enable-check-lisp-object-type --without-compress-install 'CFLAGS=-Og
 -ggdb3' LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''