gexp: Catch and report non-self-quoting gexp inputs.

Previously we would, for example, generate build scripts in the store;
when trying to run them, we'd get a 'read' error due to the presence
of #<foo> syntax in there.

* guix/gexp.scm (gexp->sexp)[self-quoting?]: New procedure.
[reference->sexp]: Check whether the argument in a <gexp-input> box is
self-quoting.  Raise a '&gexp-input-error' condition if it's not.
* tests/gexp.scm ("lower-gexp, non-self-quoting input"): New test.

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/gexp.scm b/tests/gexp.scm
index 5c013d838d9..50d09486599 100644
--- a/tests/gexp.scm
+++ b/tests/gexp.scm
@@ -871,6 +871,13 @@
                    (eq? (derivation-input-derivation (lowered-gexp-guile lexp))
                         (%guile-for-build)))))))
 
+(test-eq "lower-gexp, non-self-quoting input"
+  +
+  (guard (c ((gexp-input-error? c)
+             (gexp-error-invalid-input c)))
+    (run-with-store %store
+      (lower-gexp #~(foo #$+)))))
+
 (test-assertm "gexp->derivation #:references-graphs"
   (mlet* %store-monad
       ((one (text-file "one" (random-text)))