Hello,

when switching to a new in-buffer completion user interface package
which is solely based on completion-at-point, I observed that email
address completion in message-mode happens in the minibuffer, regardless
of what UI on top of completion-at-point I use. Also, EUDC's ability to
aggregate search results from several servers, which was recently added
with commit 0470a4a939772c4bd25123b15f5eadab41f8bee5, was not kicking
in, i.e. I was presented results from a single EUDC server only.

Browsing through message.el quickly brought me to the sixth of the 21
FIXMEs in that file, which reads on line 3183:

  ;; FIXME: merge the completion tables from ecomplete/bbdb/...?
  ;;(add-hook 'completion-at-point-functions #'message-ecomplete-capf nil t)
  (add-hook 'completion-at-point-functions #'message-completion-function nil t)

Thus, the most straightforward solution to my problem seemed to make a
new EUDC function that can be added to completion-at-point-functions,
and to update message-mode to actually add that function to
completion-at-point-functions. This is what the below attached patch
does.

As a result, both of my issues are fixed: the new completion-at-point
user interface presents the email addresses, and I get results from all
configured EUDC servers.

For EUDC, the added value is that it gains a function for
completion-at-point-functions, which it didn't have before. The new
function also makes use of EUDC's new "try all servers" feature,
merging the search results from several sources.

For message-mode, the added value is that the FIXME regarding the
merging of email address completion tables is addressed, as this is now
done by EUDC. Further sources for email addresses can be added to EUDC
by writing EUDC back-ends for them, which is not difficult in my
experience. Any new email address sources will thus become available in
message-mode without any further changes in message-mode itself (neither
code nor config).


Hoping to have helped, and looking forward to your thoughts,

  --alexander