bug#68214: Completion sorting customization by category

[Daniel Mendler <mail@daniel-mendler.de>]

On 1/3/24 17:07, Juri Linkov wrote:
>> This function could be used to look up the other meta data
>> functions too, `display-sort-function`, `annotation-function`,
>> `affixation-function`, `group-function`, etc.
> 
> All these meta data functions could be added later to
> completion-category-overrides after pushing the current patch.

Makes sense, the `display-sort-function' is a good start. I suggest to
at least add the `cycle-sort-function' too. The cycle threshold can be
customized too.

For later - in some cases one may also want to customize the
`annotation-' or `affixation-function'. We have a whole package which
does only that - see Marginalia on GNU ELPA. In contrast to sorting, for
annotations one has to work a bit harder, since one may not have access
to locally let-bound candidate-related data, which is only accessible by
the completion table closure and not globally.

> Maybe customization of completion-category-overrides could support
> a catch-all category `nil` for completions without metadata, like
> e.g. `nil` can be used in .dir-locals.el as a catch-all for all modes.
> But I'm not sure how useful this would be.

Okay. I suggest t instead of nil for the catch-all category. This seems
more aligned with other customization options or Lisp language
constructs like `cond'.

Daniel