>> It shouldn't be limited to hi-lock.  It should be
>> for `read-face-name'.
>
> So I put the affixion thing into read-file-name and made hi-lock call
> read-file-name instead of doing the completion itself.

This has such regression that before it supported the following workflow:
in the minibuffer 'M-n M-n ...' used to pull hi-lock faces from the ordered
"future history" list.  But read-face-name doesn't support this feature.

If highlight-regexp should use read-face-name, then this feature
could be added to read-face-name.  Here is the simplest patch,
but it would be a tough task to explain the need of an additional
DEFAULTS arg in the documentation.  An alternative would be to
rewrite read-face-name and keep cdr of the existing arg DEFAULT
for the future history of the minibuffer.