I don't think this is the same problem, in
`project-forget-projects-under' there is no manual recursive descent,
just some duplicated code. We could also re-write it to look like this:
diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 95db9d0ef4c..5f1cce160b2 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1905,15 +1905,12 @@ project-forget-projects-under
forgotten projects."
(interactive "DDirectory: \nP")
(let ((count 0))
- (if recursive
- (dolist (proj (project-known-project-roots))
- (when (file-in-directory-p proj dir)
- (project-forget-project proj)
- (setq count (1+ count))))
- (dolist (proj (project-known-project-roots))
- (when (file-equal-p (file-name-directory proj) dir)
- (project-forget-project proj)
- (setq count (1+ count)))))
+ (dolist (proj (project-known-project-roots))
+ (when (if recursive
+ (file-in-directory-p proj dir)
+ (file-equal-p (file-name-directory proj) dir))
+ (project-forget-project proj)
+ (setq count (1+ count))))
(if (zerop count)
(message "No projects were forgotten")
(project--write-project-list)
But that would incur a branch in every iteration of `dolist'.