Fix register preview for position register with nil buffer

[nick4f42]

The consult register preview would error when a register entry was #<marker in no buffer>, which could happen if you killed the buffer a marker pointed to.

[minad]

Hi, thanks for this!

...which could happen if you killed the buffer a marker pointed to.

When does this happen? Marker registers are usually swapped out with file-query registers when the corresponding buffer is killed. See:

https://github.com/emacs-mirror/emacs/blob/30bc867aecc59265b6e315acf459f8d79c423bca/lisp/register.el#L623-L624

[nick4f42]

If I create a non-file-visiting buffer "foo", create a mark register in it, then kill "foo", I see the issue. I didn't realize there was a hook to update the register when the buffer is killed, but it looks like it only handles file-visiting buffers:

https://github.com/emacs-mirror/emacs/blob/30bc867aecc59265b6e315acf459f8d79c423bca/lisp/register.el#L730

Maybe Emacs should delete the register if there's no file, but I still think consult should handle the nil buffer case to be safe.

[minad]

Okay, thanks. Fixed in b48ff6bf0527baeb6bfd07c6da9d303ff0b79c3d. I decided to filter out such invalid registers early on, since they can be considered empty.

[nick4f42]

Thanks!