I recently learnt about the Emacs package project.el, which is used to figure out which files and directories belong to the same project. This is used under the covers by Eglot, for example.

In practice, a project is recognized by looking for Git repositories, which is a decent first approximation that often just works.

But what if the detection fails? For example, maybe you want to anchor your project-based commands in a parent directory that contains multiple Git repositories.

Luckily, we can provide our own entry to the project-find-functions hook, and look for a .project.el file in the parent directories:

;; Returns the parent directory containing a .project.el file, if any,
;; to override the standard project.el detection logic when needed.
(defun zkj-project-override (dir)
  (let ((override (locate-dominating-file dir ".project.el")))
    (if override
      (cons 'vc override)
      nil)))

(use-package project
  ;; Cannot use :hook because 'project-find-functions does not end in -hook
  ;; Cannot use :init (must use :config) because otherwise
  ;; project-find-functions is not yet initialized.
  :config
  (add-hook 'project-find-functions #'zkj-project-override))

Now, we can use touch .project.el in any directory to make project.el recognize the directory as project root!

By the way, in case you are unfamiliar, the configuration above uses use-package, which is a great way to (lazily, i.e. quickly!) load and configure Emacs packages.