Access to readline completions without line editing

[sbaugh]

Hi,

GNU Emacs wraps many readline-using programs, but disables readline in
favor of its own line-editing commands.  However, it would be great for
Emacs to still be able to access readline completions on demand; then
Emacs would display those completions to the user in Emacs-y ways.
Right now there's no support in Emacs for accessing these completions,
so completion in such programs is usually reimplemented inside Emacs in
ad-hoc ways, or isn't supported at all.

How should we access these completions if we want to disable
line-editing?  What do you recommend?  Is there an existing mechanism
which would be suitable for this?  Or could a new feature for this be
added to readline?

Thanks!

P.S. a few links to some limited prior discussion:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=26661
https://github.com/szermatt/emacs-bash-completion/issues/45#issuecomment-1336463080