bug#23224: 25.0.92; Can xref-find-references work without a "project"?

[Dmitry Gutov <dgutov@yandex.ru>]

On 04/05/2016 06:23 PM, Eli Zaretskii wrote:

> I think we shouldn't fail the operation in this case, but instead use
> the directory specified by the user as the single project root.

Sure, fine by me.

> +   (let* ((pr (project-current t nil 'noerror))
> +          (roots (ignore-errors (project-roots pr)))
> +          (eroots (ignore-errors (project-external-roots pr))))
> +     ;; If we didn't find any project files in PR, default to PR
> +     ;; itself as the project's single root.
> +     (if (and (null roots) (null eroots))
> +         (setq roots (list (expand-file-name pr))))
> +     (append roots eroots))))

I'm going to assume you're teasing here. :)

See the counter-proposal below, it should provide a more universal 
benefit along the same line of reasoning. Unless we come upon situations 
where we *do* want to abort with an error after asking the user for a 
project and not finding a "real" one.

If 'transient' doesn't sound great, alternative suggestions welcome.

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 1251bca..9c8a88c 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -101,7 +101,9 @@ project-find-functions
  (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 @@ project-current
        (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 @@ project--find-in-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"