packages: Raise an error condition a cross builder is needed but unavailable.

* guix/packages.scm (&package-cross-build-system-error): New condition type.
  (package-cross-derivation): Raise &package-cross-build-system-error
  when the build system doesn't support cross builds.
* guix/ui.scm (call-with-error-handling): Add
  package-cross-build-system-error? case.
* tests/packages.scm ("package-cross-derivation, no cross builder"): New test.

1 files changed, 9 insertions, 0 deletions

diff --git a/guix/ui.scm b/guix/ui.scm
index cf94aa0818..cd47989ca6 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -23,6 +23,7 @@
   #:use-module (guix store)
   #:use-module (guix config)
   #:use-module (guix packages)
+  #:use-module (guix build-system)
   #:use-module (guix derivations)
   #:use-module ((guix licenses) #:select (license? license-name))
   #:use-module (srfi srfi-1)
@@ -152,6 +153,14 @@ General help using GNU software: <http://www.gnu.org/gethelp/>"))
                (leave (_ "~a:~a:~a: package `~a' has an invalid input: ~s~%")
                       file line column
                       (package-full-name package) input)))
+            ((package-cross-build-system-error? c)
+             (let* ((package (package-error-package c))
+                    (loc     (package-location package))
+                    (system  (package-build-system package)))
+               (leave (_ "~a: ~a: build system `~a' does not support cross builds~%")
+                      (location->string loc)
+                      (package-full-name package)
+                      (build-system-name system))))
             ((nix-connection-error? c)
              (leave (_ "failed to connect to `~a': ~a~%")
                     (nix-connection-error-file c)