bug#61066: [PATCH] Add inlay hint support to eglot

[Eli Zaretskii <eliz@gnu.org>]

> From: Dimitri Belopopsky <dimitri@belopopsky.com>
> Date: Wed, 25 Jan 2023 23:34:02 +0100
> 
> I've been working on adding support for inlay hints inside eglot using overlays.
> Here is a working patch, but I'm still missing a few things:
> 
> - I can't figure out a way to show the hints on a document without causing lags or timeouts from the lsp
> server
> - I'm currently updating the hints by sending the whole file each time (to make sure all hints get updated
> correctly). I'm not sure on how to make this more efficient (or if it even necessary).
> 
> On the implementation side:
> - implemented with overlays as a minor model, enabled by default
> - shows all hints supported by the protocol
> - there is a customisation to disable the minor mode if the user doesn't want the feature
> 
> I'd love to get a few points to finish this patch, and of course any ideas and feedbacks are welcome!

Thank you for working on this important feature.

AFAIU, inlay hints provide information of the same kind as ElDoc and
in similar manner from the display and UX POV.  So I think this
feature should work via ElDoc, not as a separate from-the-scratch
implementation.  ElDoc is already capable of using Eglot-supplied
information, so perhaps the only feature we need to add is the
capability of ElDoc to (optionally) display the information in
overlays near point.  (I thought we already had such a capability in
eldoc.el, but it looks like I was dreaming, because I cannot find it
there.)

The advantage of basing this on ElDoc is that then we will be able to
provide similar features from information sources other than Eglot.

João, WDYT?