emacs: Disable grafts when dry-run is enabled.

* emacs/guix-main.scm (process-package-actions): Set grafting according
to 'dry-run?'.
* guix/scripts.scm (build-package): Disable grafts when 'dry-run?' is #t.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

emacs/guix-main.scm

@@ -46,6 +46,7 @@
  (guix)
  (guix combinators)
  (guix git-download)
+ (guix grafts)
  (guix packages)
  (guix profiles)
  (guix licenses)
@@ -930,15 +931,16 @@ OUTPUTS is a list of package outputs (may be an empty list)."
          (new-manifest (manifest-perform-transaction
                         manifest transaction)))
     (unless (and (null? install) (null? remove))
-      (with-store store
-        (set-build-options store
-                           #:print-build-trace #f
-                           #:use-substitutes? use-substitutes?)
-        (show-manifest-transaction store manifest transaction
-                                   #:dry-run? dry-run?)
-        (build-and-use-profile store profile new-manifest
-                               #:use-substitutes? use-substitutes?
-                               #:dry-run? dry-run?)))))
+      (parameterize ((%graft? (not dry-run?)))
+        (with-store store
+          (set-build-options store
+                             #:print-build-trace #f
+                             #:use-substitutes? use-substitutes?)
+          (show-manifest-transaction store manifest transaction
+                                     #:dry-run? dry-run?)
+          (build-and-use-profile store profile new-manifest
+                                 #:use-substitutes? use-substitutes?
+                                 #:dry-run? dry-run?))))))
 
 (define (delete-generations* profile generations)
   "Delete GENERATIONS from PROFILE.

guix/scripts.scm

@@ -1,7 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014 Deck Pickard <deck.r.pickard@gmail.com>
-;;; Copyright © 2015 Alex Kost <alezost@gmail.com>
+;;; Copyright © 2015, 2016 Alex Kost <alezost@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,6 +19,7 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix scripts)
+  #:use-module (guix grafts)
   #:use-module (guix utils)
   #:use-module (guix ui)
   #:use-module (guix store)
@@ -105,11 +106,13 @@ true."
                         #:rest build-options)
   "Build PACKAGE using BUILD-OPTIONS acceptable by 'set-build-options'.
 Show what and how will/would be built."
-  (mbegin %store-monad
+  (mlet %store-monad ((grafting? ((lift0 %graft? %store-monad))))
     (apply set-build-options*
            #:use-substitutes? use-substitutes?
            (strip-keyword-arguments '(#:dry-run?) build-options))
-    (mlet %store-monad ((derivation (package->derivation package)))
+    (mlet %store-monad ((derivation (package->derivation
+                                     package #:graft? (and (not dry-run?)
+                                                           grafting?))))
       (mbegin %store-monad
         (maybe-build (list derivation)
                      #:use-substitutes? use-substitutes?