[Fix #1591] Add project.el integration

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### New features
+
+* [#1591](https://github.com/bbatsov/projectile/issues/1591): Add `project.el` integration that will make Projectile the default provider for project lookup.
+
 ## 2.6.0 (2022-10-25)
 
 ### New features

doc/modules/ROOT/pages/usage.adoc

@@ -379,3 +379,17 @@ You can add additional commands to the commander like this:
 ----
 
 Place such snippets after ``projectile-mode``'s init code.
+
+== Using Projectile with project.el
+
+Starting with version 2.7 Projectile bundles some integration with
+`project.el` that makes `project.el` use by default Projectile's
+project lookup function (`projectile-project-root`) and project file
+lookup function (`projectile-project-files`).
+
+That's useful as some packages (e.g. `eglot`) support natively only
+`project.el`'s API for project discovery. Fortunately, `project.el`
+makes it easy to install additional project lookup functions and that's
+exactly what Projectile does.
+
+TIP: You can read more about the integration https://github.com/bbatsov/projectile/issues/1591[here].

projectile.el

@@ -5987,6 +5987,38 @@ Otherwise behave as if called interactively.
 ;;;###autoload
 (define-obsolete-function-alias 'projectile-global-mode 'projectile-mode "1.0")
 
+;;;; project.el integration
+;;
+;; Projectile will become the default provider for
+;; project.el project and project files lookup.
+;; See https://github.com/bbatsov/projectile/issues/1591 for
+;; more details.
+
+;; it's safe to require this directly, as it was added in Emacs 25.1
+(require 'project)
+
+(cl-defmethod project-root ((project (head projectile)))
+  (cdr project))
+
+(cl-defmethod project-files ((project (head projectile)) &optional dirs)
+  (let ((root (project-root project)))
+    ;; Make paths absolute and ignore the optional dirs argument,
+    ;; see https://github.com/bbatsov/projectile/issues/1591#issuecomment-896423965
+    ;; That's needed because Projectile uses relative paths for project files
+    ;; and project.el expects them to be absolute.
+    ;; FIXME: That's probably going to be very slow in large projects.
+    (mapcar (lambda (f)
+              (concat f root))
+            (projectile-project-files root))))
+
+(defun project-projectile (dir)
+  "Return Projectile project of form ('projectile . root-dir) for DIR."
+  (let ((root (projectile-project-root dir)))
+    (when root
+      (cons 'projectile root))))
+
+(add-hook 'project-find-functions #'project-projectile)
+
 (provide 'projectile)
 
 ;;; projectile.el ends here