>> Do you want to use the completion-annotations face conditionally only
>> for annotations, i.e. when only the suffix is provided by the client?
>> Because when a prefix is provided as well, then it's not an annotation
>> anymore, so the completion-annotations face is not applicable to prefixes.
>
> I see, personally I think of all strings besides the completions themselves
> as annotations ;) Makes sense to do it only for the suffix then.
>
>> Doing this is not something new, we already have the same logic
>> in minibuffer-message:
>>      (unless (or (null minibuffer-message-properties)
>>                  ;; Don't overwrite the face properties the caller has set
>>                  (text-properties-at 0 message))
>>        (setq message (apply #'propertize message minibuffer-message-properties)))
>> Is this logic suitable for completion-annotations?
>
> I guess this could also be used, the version I posted earlier only checks
> for the face property and then also check the whole string:
>
> (if (text-property-not-all 0 (length str) 'face nil str)
>         str
>       (propertize str 'face face))
>
> When only the face matters my proposed version might be better?

I agree its purpose is quite different from the example above.

Then maybe something like this should do what you want: