Support clangd's editsNearCursor

[rohhamh]

Normally one would put -> in front of a pointer variable to get the completion candidates.
However it's also possible to get them by putting a ., by setting editsNearCursor client capability as it seems.

An example of such edit is the following screenshot from neovim, the listed toJsonArray needs a -> but I'm getting it with a . instead.

To get the same behaviour, I set the editsNearCursor cap with the following code (this is provided by llm, is there a better way to do this? I couldn't find any documentation)

(after! lsp-mode
  (defun my/lsp-modify-clangd (caps)
    (setf (alist-get 'editsNearCursor
                     (alist-get 'completion
                                (alist-get 'textDocument caps)))
          t)
    caps)

  (advice-add 'lsp--client-capabilities
              :filter-return #'my/lsp-modify-clangd))

and also made this change (also provided by llm)

(after! lsp-clangd
  (advice-add 'lsp-completion--guess-prefix :filter-return
              (lambda (start)
                (when (and start
                           (char-after start)
                           (= (char-after start) ?.))
                  (setq start (1+ start)))
                start))
  )

The conclusion the llm made when it finally worked is as follows:

clangd's editsNearCursor returns completion items with a textEdit whose range starts before the . so it can rewrite it to ->. lsp-completion--guess-prefix faithfully returns that pre-. position, making the prefix ".", which doesn't match any member name, so company shows nothing.
The one-liner advice skips past the . so the prefix becomes "", and since :company-prefix-length t was already set, company doesn't filter on it and shows all the candidates.

The way I understand it is that some prefix (. here) is expected among all the candidates of clangd's response for completions to work currently.

I would be very happy to know if

• There's a better way to set capabilities, and if you'd include editsNearCursor as default for clangd (or make it more easily configurable)
• There's' a more straightforward way that the same result as (advice-add 'lsp-completion--guess-prefix :filter-return ...) can be achieved which I couldn't find (seems nobody cares about editsNearCursor on the web).
• You think this would make sense for a feature

Thank you for what you're doing.