bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Dmitry Gutov]

On 25.05.2020 20:04, João Távora wrote:
> Hi Stefan, Dmitry, Andrii and maintainers,
>
> Moving the discussion that started in
> https://github.com/joaotavora/eglot/pull/459  to the bug tracker, and
> attaching the two patches that contain what I think is a decent
> short-term solution to the eldoc/async problems.

Here's a modified approach that doesn't use a global var and should make 
it easier to transition to using futures.

Patch attached. Example of usage:

(add-hook 'eldoc-documentation-functions
          #'test-eldoc-async 0 t)

(defun test-eldoc-async ()
  (cons :async
        (lambda (cb)
          (funcall cb "doc here!"))))

If you like, we could simplify the returned objects to be just FETCHER 
(as documented in the patch) rather than (:async . FETCHER). But the 
latter seems more explicit.

There also exist a possible modification of this patch with a bit of 
functional programming where both calls to eldoc--handle-multiline 
happen from inside of eldoc-documentation-default's definition.

eldoc-async-with-lexical-callback.diff
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