bug#75072: [PATCH] Set marker insertion type for Imenu markers

[Eli Zaretskii <eliz@gnu.org>]

> From: Morgan Willcock <morgan@ice9.digital>
> Date: Tue, 24 Dec 2024 22:58:08 +0000
> 
> The internal Imenu function for creating an Imenu index uses markers by
> default, but the marker insertion type is left at the default type which
> does not advance the marker position when additional characters are
> inserted at the marker position.  Attached is a patch which sets the
> marker insertion type to advance the marker position in the same
> scenario.
> 
> This change means that the Imenu index will continue to be valid instead
> of the marker becoming desynced from the buffer contents, i.e. jumping
> to an index position and inserting characters will keep the marker in
> the correct place.
> 
> I can't think of a situation where the previous marker behaviour was
> intentional, but it is likely that the issue wouldn't be seen by anyone
> who sets imenu-auto-rescan to t or in modes which do not use both
> imenu-prev-index-position-function and imenu-extract-index-name-function
> to create the Imenu index.
> 
> I've assumed this would go on the master branch.

Yes.  But I have one comment below.

> -                              (if imenu-use-markers (point-marker) (point)))
> +                              (if imenu-use-markers
> +                                  (let ((marker (point-marker)))
> +                                    (set-marker-insertion-type marker t)
> +                                    marker)
> +                                (point)))

I think it is cleaner to use copy-marker here.