bug#68699: Eglot completion has stopped working in some cases

[Brennan Vincent <brennan@umanwizard.com>]

The issue is fixed for me on latest master, which includes the patch you
mentioned.

João Távora <joaotavora@gmail.com> writes:

> On Thu, Jan 25, 2024 at 12:43 AM João Távora <joaotavora@gmail.com> wrote:
>>
>> On Wed, Jan 24, 2024 at 8:37 PM Brennan Vincent <brennan@umanwizard.com> wrote:
>>
>> > For example, in a Rust crate with the following main file:
>> >
>> > fn foo_len() {
>> > }
>> >
>> > fn main() {
>> >     len
>> > }
>>
>> I've reproduced this with latest master:
>>
>> ~/Source/Emacs/emacs/src/emacs -Q ~/MoreSource/Rust/hello/src/main.rs
>> -f rust-ts-mode -f eglot
>>
>> where main.rs is the provided snippet.
>>
>> Pressing C-M-i with point after 'len' yields the error.
>>
>> The error only happens in partial-completion is in the completion-styles
>> variable, so a workaround is to remove partial-completion from
>> completion-styles, at least in Eglot enabled buffers.
>>
>> I will look now for the underlying cause.
>
> Stefan, this is down to partial-completion somehow thinking
> that "\\'len" should match at least some LSP-provided
> completion candidate, otherwise it's an "internal error"
>
> I wish I could turn off all the styles from the table but I can
> only add to them.  I simply cannot write a reasonable
> try-completion thing for LSP completions.  LSP completions
> are really designed to be navigated using a company-like
> popup.  Without it they should almost always use the *Completions*
> buffer, and rarely ever Emacs partial completion shenanigans.
>
> Nevertheless, I need to plug this bug somehow in Eglot.
>
> Brennan, can you try this patch I've just pushed?
>
> diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
> index 511000927cf..c5fbf5eb9d5 100644
> --- a/lisp/progmodes/eglot.el
> +++ b/lisp/progmodes/eglot.el
> @@ -3060,9 +3060,13 @@ eglot--dumb-flex
>             finally (cl-return comp)))
>
>  (defun eglot--dumb-allc (pat table pred _point) (funcall table pat pred t))
> +(defun eglot--dumb-tryc (pat table pred point)
> +  (if-let ((probe (funcall table pat pred nil)))
> +      (cons probe (length probe))
> +    (cons pat point)))
>
>  (add-to-list 'completion-category-defaults '(eglot-capf (styles
> eglot--dumb-flex)))
> -(add-to-list 'completion-styles-alist '(eglot--dumb-flex ignore
> eglot--dumb-allc))
> +(add-to-list 'completion-styles-alist '(eglot--dumb-flex
> eglot--dumb-tryc eglot--dumb-allc))
>
>  (defun eglot-completion-at-point ()
>    "Eglot's `completion-at-point' function."
>
> João