>> BTW, there is another TODO item in minibuffer.el:
>>
>> ;;   - indicate how to display the completions in *Completions* (turn
>> ;;     \n into something else, add special boundaries between
>> ;;     completions).  E.g. when completing from the kill-ring.
>>
>> Such pre-processing can be performed by the caller.
>> For a long time I used the command 'yank-from-kill-ring'
>> bound to 'C-M-y' that relies on query-replace-descr
>> to display newlines as ^J and displays long completions
>> with an ellipsis at the end and an ellipsis on leading whitespace.
>
> Looks great, thanks.  Any reason you bind it to C-M-y instead
> of to "M-y that doesn't immediately follow C-y"?

Oops, I don't remember why I forgot about no-op M-y.

Here is a better patch.  There is still a small problem:
sometimes it's handy to slightly edit a previous string
from kill-ring before inserting it.  But with completing-read
it's not easy to insert the space character, e.g. after 'M-p'
while editing, typing 'SPC' errors with "[No matches]"
and doesn't insert a space character.

Could you recommend a more lightweight version of completing-read
that doesn't override the 'SPC' key?