Eglot cannot work with default completion-in-region?

[Spencer Baugh <sbaugh@janestreet.com>]

Hi,

The recent commit d376462c7183752bf44b9bd20bf5020fe7eaf75a prompted by
issue https://github.com/joaotavora/eglot/issues/1339 says:

>I declare it impossible to make C-M-i use of 'try-completion' behave
>sanely with LSP in its current state.  YMMV.  Use a completion tooltip,
>like Company.

So completion from Eglot, which is built into Emacs, is broken out of
the box?  It has a hard dependency on using company-mode or similar
third-party packages?

If this is the case, perhaps Eglot should make it more clear that it
cannot be used without also installing and using company-mode or some
other package.  Perhaps it should abort rather than running if the user
is using the default completion-in-region.

Alternatively, as someone who uses default completion-in-region and
would like to use Eglot, is there some way to make the common case
behave correctly?

I guess the issue is that in the LSP protocol, there's a difference
between the "sortText" and "filterText" which are used for displaying
completions and letting the user choose them, and the
"insertText"/"textEdit" which are used for inserting them.

So Eglot has this somewhat hacky code which runs in :exit-function to
delete the completion after completion-in-region inserts it, and insert
a different string instead:

(cond (textEdit
       ;; Revert buffer back to state when the edit
       ;; was obtained from server. If a `proxy'
       ;; "bar" was obtained from a buffer with
       ;; "foo.b", the LSP edit applies to that
       ;; state, _not_ the current "foo.bar".
       (delete-region orig-pos (point))
       (insert (substring bounds-string (- orig-pos (car bounds))))
       (eglot--dbind ((TextEdit) range newText) textEdit
         (pcase-let ((`(,beg . ,end)
                      (eglot-range-region range)))
           (delete-region beg end)
           (goto-char beg)
           (funcall (or snippet-fn #'insert) newText))))
      (snippet-fn
       ;; A snippet should be inserted, but using plain
       ;; `insertText'.  This requires us to delete the
       ;; whole completion, since `insertText' is the full
       ;; completion's text.
       (delete-region (- (point) (length proxy)) (point))
       (funcall snippet-fn (or insertText label))))

This is code which is often broken, especially with default
completion-in-region.

However, the common case is that sortText==insertText/textEdit.  In that
case, this code is not necessary, and Eglot doesn't need an
:exit-function at all.

Can Eglot detect this and avoid this somewhat hacky code in that case?
It should be a performance improvement and simplification anyway for all
completion frameworks.

(BTW, besides the issue with default completion-in-region, I also ran
into this while trying to write an OCaml-specific wrapper for
eglot-completion-at-point function which adds some additional
completions to those returned from the LSP.  But if those completions
are chosen, then the Eglot exit-function breaks when it tries to look up
the insertText/textEdit.  My LSP doesn't use textEdit at all, though, so
this is another unnecessary breakage)