Separate bugs reports for sure, Eli, but I first thought it useful to
discuss holistically to illustrate. Let's consider this bug report to be
solely about project.

Dmitry, I think the following patch to project--write-project-list might
suffice to avoid this in the future:

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index f2a27ff91dd..f9b3b8891bc 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1824,7 +1824,7 @@ project--read-project-list
                (lambda (elem)
                  (let ((name (car elem)))
                    (list (if (file-remote-p name) name
-                           (abbreviate-file-name name)))))
+                           (file-name-as-directory (abbreviate-file-name
name))))))
                (condition-case nil
                    (read (current-buffer))
                  (end-of-file

This causes /Users/xxx to appear as /Users/xxx/ in the project file and
then project--read-project-list produces ~/ rather than ~ for which
file-name-directory produces nil.

(defun project--read-project-list ()
...
                           (file-name-as-directory (abbreviate-file-name
name))))))

Note: I did not consider the tramp/remote file cases in this analysis.

On Sat, Feb 1, 2025 at 3:50 AM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Ship Mints <shipmints@gmail.com>
> > Date: Fri, 31 Jan 2025 19:10:47 -0500
> >
> > The behaviors below can be seen on 29, 30, 31 (I don't have an earlier
> Emacs to test but these issues
> > probably are long-standing). Let's discuss these as I suspect there may
> be historical reasons for these
> > behaviors that I'm unaware of, or maybe these are legitimate, even if
> old. It's also possible that I'm
> > misunderstanding something.
>
> Thanks, but please don't mix separate issues in the same bug report.
> If you think file-name-directory has a bug (I disagree), please report
> a separate bug against file-name-directory.  If you think file-equal-p
> should accept nil as its argument (I'm not sure I will agree, but
> maybe you will convince), please report a separate feature-request bug
> about that function.
>
> > Issue 3:
> >
> > project-forget-projects-under crashes when `project-list-file' contains
> "/home/username", which appears in
> > my remembered project list.
>
> It doesn't crash, it signals an error.  Crashing means the entire
> Emacs session goes down in flames.
>
> > (defun project--read-project-list ()
> > ...
> >    (abbreviate-file-name name) ; converts "/home/username" to "~"
> >
> > (defun project-forget-projects-under ()
> > ...
> >       (dolist (proj (project-known-project-roots))
> >         (when (file-equal-p (file-name-directory proj) dir) ; <--- boom
> on nil
> >
> > I'm happy to submit patches for any or all of these including guarding
> project-forget-projects-under to check
> > for nil and/or amending project--read-project-list to convert "~" to
> "~/" as workarounds, with each submitted
> > against a discrete bug number.
>
> I think this should be solved in project.el, but let's hear what
> Dmitry (CC'ed) thinks.
>