gnu: emacs: Find packages in "~/.guix-profile".

* gnu/packages/emacs.scm (emacs)[inputs]: Add 'guix-src' input.
  [arguments]: Add 'install-site-start' phase.

Co-authored-by: Alex Kost <alezost@gmail.com>

gnu/packages/emacs.scm

@@ -52,6 +52,7 @@
   #:use-module (gnu packages xml)
   #:use-module (gnu packages glib)
   #:use-module (gnu packages acl)
+  #:use-module (gnu packages package-management)
   #:use-module (gnu packages perl)
   #:use-module (gnu packages pdf)
   #:use-module (gnu packages xiph)
@@ -74,7 +75,8 @@
                             (search-patch "emacs-source-date-epoch.patch")))))
     (build-system glib-or-gtk-build-system)
     (arguments
-     '(#:phases (modify-phases %standard-phases
+     `(#:phases
+       (modify-phases %standard-phases
          (add-before 'configure 'fix-/bin/pwd
            (lambda _
              ;; Use `pwd', not `/bin/pwd'.
@@ -87,6 +89,26 @@
              (let ((out (assoc-ref outputs "out")))
                (delete-file
                 (string-append out "/share/info/info.info.gz"))
+               #t)))
+         (add-after 'install 'install-site-start
+           ;; Copy guix-emacs.el from Guix and add it to site-start.el.  This
+           ;; way, Emacs packages provided by Guix and installed in
+           ;; ~/.guix-profile/share/emacs/site-lisp/guix.d/PACKAGE-VERSION are
+           ;; automatically found.
+           (lambda* (#:key inputs outputs #:allow-other-keys)
+             (let* ((guix-src (assoc-ref inputs "guix-src"))
+                    (out      (assoc-ref outputs "out"))
+                    (lisp-dir (string-append out "/share/emacs/"
+                                             ,(version-major+minor version)
+                                             "/site-lisp"))
+                    (unpack   (assoc-ref %standard-phases 'unpack)))
+               (mkdir "guix")
+               (with-directory-excursion "guix"
+                 (apply unpack (list #:source guix-src))
+                 (install-file "emacs/guix-emacs.el" lisp-dir))
+               (with-output-to-file (string-append lisp-dir "/site-start.el")
+                 (lambda ()
+                   (display "(require 'guix-emacs nil t)")))
                #t))))))
     (inputs
      `(("gnutls" ,gnutls)
@@ -112,7 +134,8 @@
        ("libice" ,libice)
        ("libsm" ,libsm)
        ("alsa-lib" ,alsa-lib)
-       ("dbus" ,dbus)))
+       ("dbus" ,dbus)
+       ("guix-src" ,(package-source guix))))
     (native-inputs
      `(("pkg-config" ,pkg-config)
        ("texinfo" ,texinfo)))