This patch add support for "plain project" in project.el. Plain project is a
kind of project without any VC backend but should be.

To mark a directoy as project, put an empty magic file .emacs-project under
the
directory, and project.el should be responsible for it.

~~~~

From cb0a67cfacf141a8b1955c08c3f459bcac801a39 Mon Sep 17 00:00:00 2001
From: Zhu Zihao <all_but_last@163.com>
Date: Thu, 28 May 2020 11:04:44 +0800
Subject: [PATCH] Support plain project marked with file .emacs-project

* lisp/progmodes/project.el (project-try-plain): New function
* lisp/progmodes/project.el (project-root): New dispatch variants
* lisp/progmodes/project.el (project-find-functions): Add project-try-plain
---
 lisp/progmodes/project.el | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 88f73e4fb31..4c1810aeb56 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -94,7 +94,7 @@

 (require 'cl-generic)

-(defvar project-find-functions (list #'project-try-vc)
+(defvar project-find-functions (list #'project-try-plain #'project-try-vc)
   "Special hook to find the project containing a given directory.
 Each functions on this hook is called in turn with one
 argument (the directory) and should return either nil to mean
@@ -194,6 +194,18 @@ project-files
    (or dirs
        (list (project-root project)))))

+(defun project-try-plain (dir)
+  "Return the plain project instance of current DIR.
+
+A directory with magic file \".emacs-project\" will be recognized as
+plain project."
+  (pcase (locate-dominating-file dir ".emacs-project")
+    (`nil nil)
+    (root (cons 'plain root))))
+
+(cl-defmethod project-root ((project (head plain)))
+  (cdr project))
+
 (defun project--files-in-directory (dir ignores &optional files)
   (require 'find-dired)
   (require 'xref)
-- 
2.26.2