Add a `transient' project type

* lisp/progmodes/project.el (project-roots): Implement for the
`transient' project type (bug#23224).
(project-current): Instead of signaling an error, return a
transient project instance rooted in the chosen directory.

1 files changed, 8 insertions, 2 deletions

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 1251bca2491..9c8a88c80fc 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -101,7 +101,9 @@ that it is not applicable, or a project instance.")
 (defun project-current (&optional maybe-prompt dir)
   "Return the project instance in DIR or `default-directory'.
 When no project found in DIR, and MAYBE-PROMPT is non-nil, ask
-the user for a different directory to look in."
+the user for a different directory to look in.  If that directory
+is not a part of a detectable project either, return a
+`transient' project instance rooted in it."
   (unless dir (setq dir default-directory))
   (let ((pr (project--find-in-directory dir)))
     (cond
@@ -110,7 +112,8 @@ the user for a different directory to look in."
       (setq dir (read-directory-name "Choose the project directory: " dir nil t)
             pr (project--find-in-directory dir))
       (unless pr
-        (user-error "No project found in `%s'" dir))))
+        (message "Using '%s' as a transient project root" dir)
+        (setq pr (cons 'transient dir)))))
     pr))
 
 (defun project--find-in-directory (dir)
@@ -182,6 +185,9 @@ to find the list of ignores for each directory."
        (t
 	(complete-with-action action all-files string pred))))))
 
+(cl-defmethod project-roots ((project (head transient)))
+  (list (cdr project)))
+
 (defgroup project-vc nil
   "Project implementation using the VC package."
   :version "25.1"