>> Before knowing what's the best approach, I think we should clearly
>> decide what would be the "ideal" new API.  E.g. should it return "any
>> string" and then it'd be up to the infrastructure code to store
>> side-info about what is the corresponding candidate's actual text?
>
> After trying different variants, it turned out that the most convenient
> is to use annotation-function that can contain a placeholder for the
> completion candidate.  Currently "%s" is used as a placeholder,
> for example, "prefix %s suffix" but it can be any string.

Actually, I discovered that unfortunately annotation-function
is unsuitable for formatting data in a columnar format.
It needs to receive a complete list of completions to be able
to adjust their widths for more optimal overall columnar layout.

So a new function is needed that is like annotation-function
but accepts a list of all completions.  There is an existing
similar function 'display-sort-function' that could be abused
to add prefix/suffix annotations, but maybe a separate function
would be preferable for cleaner API?