build: Add 'as-derivation' target.

* build-aux/compile-as-derivation.scm: New file.
* Makefile.am (as-derivation): New target.
(EXTRA_DIST): Add compile-as-derivation.scm.

Makefile.am

@@ -470,6 +470,7 @@ EXTRA_DIST =						\
   build-aux/hydra/guix-modular.scm			\
   build-aux/check-available-binaries.scm		\
   build-aux/check-final-inputs-self-contained.scm	\
+  build-aux/compile-as-derivation.scm			\
   build-aux/generate-authors.scm			\
   build-aux/test-driver.scm				\
   build-aux/update-guix-package.scm			\
@@ -535,6 +536,13 @@ $(guix_install_go_files): install-nobase_dist_guilemoduleDATA
 install-data-hook: set-bootstrap-executable-permissions
 	touch "$(DESTDIR)$(guileobjectdir)/guix/config.go"
 
+# Assuming Guix is already installed and the daemon is up and running, this
+# rule builds from $(srcdir), creating and building derivations.
+as-derivation:
+	$(AM_V_at)echo "Building Guix in Guix..." ;		\
+	$(GUILE) --no-auto-compile				\
+	  "$(top_srcdir)/build-aux/compile-as-derivation.scm"	\
+	  "$(abs_top_srcdir)"
 
 SUBDIRS = po/guix po/packages
 BUILT_SOURCES =
@@ -763,7 +771,7 @@ cuirass-jobs.scm: $(GOBJECTS)
 .PHONY: gen-ChangeLog gen-AUTHORS gen-tarball-version
 .PHONY: assert-no-store-file-names assert-binaries-available
 .PHONY: assert-final-inputs-self-contained
-.PHONY: clean-go make-go
+.PHONY: clean-go make-go as-derivation
 .PHONY: update-guix-package update-NEWS release
 
 ## -------------- ##

build-aux/compile-as-derivation.scm

@@ -0,0 +1,53 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2018 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+;; Build Guix using Guix.
+
+(use-modules (srfi srfi-26))
+
+;; Add ~/.config/guix/latest to the search path.
+(add-to-load-path
+ (and=> (or (getenv "XDG_CONFIG_HOME")
+            (and=> (getenv "HOME")
+                   (cut string-append <> "/.config")))
+        (cut string-append <> "/guix/latest")))
+
+(use-modules (guix) (guix ui)
+             (guix git-download)
+             (ice-9 match))
+
+(match (command-line)
+  ((program source)
+   (with-error-handling
+     (with-store store
+       (let* ((script (string-append source "/build-aux/build-self.scm"))
+              (build  (primitive-load script))
+              (git?   (git-predicate source)))
+         (run-with-store store
+           ;; TODO: Extract #:version and #:commit using Guile-Git.
+           (mlet* %store-monad ((source (interned-file source "guix-source"
+                                                       #:select? git?
+                                                       #:recursive? #t))
+                                (drv    (build source)))
+             (mbegin %store-monad
+               (show-what-to-build* (list drv))
+               (built-derivations (list drv))
+               (with-monad %store-monad
+                 (display (derivation->output-path drv))
+                 (newline)
+                 (return drv))))))))))