bug#73766: 30.0.91; (documentation 'pcase) can take several seconds

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

>> >> There's still a remaining performance issue for things like
>> >> `eldoc-mode`, admittedly where we'd really want it to be instantaneous,
>> >> but I think this should be fixed by making `eldoc` call `documentation`
>> >> in a special way to skips those useless computations (eldoc just wants
>> >> a "short doc" anyway).  This affects other functions using the
>> >> `function-documentation` property in a similar way.
>> > Does it mean that this slowdown of eldoc was also caused by commit
>> > 052c2ce0284c5193c9d6768a45a9b3508af51230, and if so, does the patch
>> > you've shown previously (and which you consider not safe enough for
>> > emacs-30) solve the eldoc slowdown as well?
>> 
>> What I meant is that the 4s slowdown affects both `C-h f` and eldoc,
>> and that the patch I sent which reduces it to 0.12s is IMO sufficient
>> for the `C-h f` case but may still make Emacs feel occasionally a bit
>> sluggish when eldoc hits this 0.12s.
>
> If the slowdown of eldoc is significant, maybe we will be better with
> reverting the change which introduced the regression on emacs-30,
> and then solving on master with your more thorough fix.

The "quickfix" I installed on `emacs-30` should not suffer from
a significant slowdown, no, so I think we're good for `emacs-30`.

The "better" fix I installed on `master`, may still cause a bit of
sluggishness with eldoc, OTOH.

I think the "right" fix for that is to extend `documentation` or `C-h f`
such that the `pcase` doc can distinguish between the (quick to
generate) doc about `pcase` itself and the table of Pcase patterns
defined with `pcase-defmacro` which is appended to the end of the
`pcase` doc (and which can take a bit of time to generate but is not
needed for eldoc).

It would also be good to be able to show the table in a kind of
abbreviated form (one line per Pcase pattern) with a way to
unfold/refold specific patterns' doc.

The same applies to the list of methods displayed in the doc of generic
functions and in the doc of types.


        Stefan