Re: dabbrev-select-buffers-function project aware.

[Ergus]

On Mon, Jan 17, 2022 at 06:59:46PM +0100, Ergus wrote:
> Hi:
>
> I am trying to limit dabbrev to know about current project context qhen
> there is project-root.
>
> I am trying to implement a function to substitute
> `dabbrev--same-major-mode-p`, to add an extra condition to check if both
> buffers are in the same project. But some time in the past I reported
> some performance issue that was fixed, but I am wondering if this
> addition may worst instead of improve performance for dabbrev
> completion.
>
> Do we recommend anything special?
>
> Best,
> Ergus

Somehow in reply to myself, this is what I did, maybe someone could
suggest an improvement.

```
(defun my/dabbrev--select-project-buffers ()
  "Dabbrev list of buffers in the same project and apply the filters."
  (dabbrev-filter-elements
   buffer (project-buffers (project-current t))
   (and (not (eq (current-buffer) buffer))
        (not (dabbrev--ignore-buffer-p buffer))
        (boundp 'dabbrev-friend-buffer-function)
        (funcall dabbrev-friend-buffer-function buffer))))

(setq-default dabbrev-check-all-buffers nil
              dabbrev-ignored-buffer-regexps "^[ *]"
              dabbrev-select-buffers-function 
#'my/dabbrev--select-project-buffers)
```

If we want to add such functionality to dabbrev itself maybe we could
consider adding a variable like dabbrev-list-buffers that we can set to
#'list-buffers, (project-buffers (project-current t)) or anything like
that. I am wondering because setting dabbrev-friend-buffer-function
impacted performance negatively when using tramp like used to happen
project.el some months ago.

BTW: I am wondering that `project-buffers` does not filter the buffers
list, so the user gets all the ^[ *].+* buffers as "project's"
buffers. I understand this is less restrictive, but I don't know if it
is conceptually correct to get *Packages* or *Help* as project's
buffers for example.