bug#68214: Completion sorting customization by category

[Daniel Mendler via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>> Why do we need this memoization?  If it's performance sensitive wouldn't
>>> it more effective to cache at a higher level (e.g. to memoize
>>> `completion-metadata-get`)?
>>
>> The memoization ensures that `completion-metadata-get' doesn't allocate
>> and performs a symbol lookup only, as has been the case before the
>> addition. The function is called often from completion UIs which
>> auto-update on every keypress.
>
> I understand there is a benefit.  I just wonder how important it is in
> practice (if it's done 2-3 times per keypress it's likely lost in the
> noise) and whether it's the best place to put it.

Well, for comparison, in Corfu people asked me not to repeatedly
recreate timer objects on every key press and instead reuse them, so the
accumulated garbage matters somewhat. I think it is better not to
degrade functions which have been cheap before, such that they become
significantly more costly, even if the function may not matter that much
in the greater scheme. I'd appreciate if we keep the function as is,
unless you are bothered too much by it. I've ported this back to
compat-30 already and it is in the emacs-30 branch. But maybe add
with-memoization for conciseness?

Daniel