There are really two different features there. Either could be provided
on its own by Emacs. But `rec-edit.el' rolls them together in a minor
1. You can use the same key, `C-M-c', to enter a recursive edit as to
exit. When already in a recursive edit, it exits or, if you use a
prefix arg, it enters a new recursive edit. In other words, it does
(if (or (< (recursion-depth) 1)
2. It optionally highlights the mode-line indication, `[...]', to make
clear that you're in a recursive edit, and to indicate the depth.
Whether this highlighting is done is controlled by a Boolean user
Emacs generally discourages, or doesn't encourage, initiating a
recursive edit. That's understandable. But sometimes it's useful, and
a major reason it's discouraged is because it's easy for users to not
realize they're in a recursive edit, or not know how to exit.
This minor mode aims to mitigate those downsides. And it's off by
default, so presumably a user of it wants to make use of recursive
editing but wants to be aware when doing that.
I think something like this would be helpful for Emacs to provide.
In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.17763
`configure --without-dbus --host=x86_64-w64-mingw32
--without-compress-install 'CFLAGS=-O2 -static -g3''
bug#38535: 26.3; Optionally make recursive editing more noticeable
> > Recursive editing is sometimes useful.
> > Its main disadvantage is that you might not
> > be aware that you've entered or exited a
> > recursive edit.
> But there is minibuffer-depth-indicate-mode that shows the recursion
> depth in the minibuffer prompt when recursive minibuffers are enabled.
Minibuffer recursion is indicated as you say.
It is not indicated by [...] in the mode line.
"Minibuffer input is a special kind of
recursive editing." (elisp) `Recursive Editing'
This is about a non-minibuffer recursive edit,
which is indicated only by a not-very-noticeable
[...] in the mode line.
Those two are handled very differently in terms
of UI and interaction.
For the same reason that we highlight the
minibuffer indication of minibuffer depth, minor
mode `rec-edit-mode' can highlight the [...]
brackets. ("Can", because an option controls
whether the highlighting occurs.)