lsp-completion--exit-fn: works with non props candidate

[kiennq]

In case of just using default completion-at-point (in that case completion-in-region-function will be completion--in-region), the :exit-function will be called with selected item as string without any properties that we built up during filtering phase.
We need a way to re-associated the striped string with the full one.

The propose here is:

• Remember the last completion results in lsp-completion--last-result, use that to trace back to full string in :exit-function.
• Make try-completion will always return nil (no exact completion item) so that the selection will always shown and prevent early clean up of completion cache.

This will also server as first step for #1894

[yyoncho]

will be called with selected item as string without any properties that we built up during filtering phase.

Isn't that emacs bug?

[kiennq]

will be called with selected item as string without any properties that we built up during filtering phase.

Isn't that emacs bug?

Is it? I think there's no guarantee that the string's properties will be preserved when passed to :exit-function.

For that's by design since the completion items will be displayed in other buffer in case of default completion-in-region-function.

Thus it will mostly cannot retain the property when selecting one of them.

Even if this's a bug (not likely), we still need to work-around in lsp-mode since it will not be fixed in pre Emacs 28.

[yyoncho]

Hm, some of the previous capf implementations used to preserve the properties.

[yyoncho]

The text properties used to work. I even filed a helm issue - emacs-helm/helm#2265 (comment) . Do you know what broke them?

[kiennq]

AFAIK, ivy, company, emacs-helm are preserving text properties for :exit-function.

However, the default function for completion-in-region-function (which is completion--in-region) doesn't do that.
I've checked Emacs25 and it seems that behavior is there too, so it's pretty much default behavior.
https://github.com/emacs-mirror/emacs/blob/c3ff6712ad24fcf45874dc0665a8606e9b2208a4/lisp/minibuffer.el#L1005

I think you can test this by disable helm-mode and manually trigger completion-at-point (bind to C-M-i), then select item with mouse