diff --git a/src/libutil/rust-ffi.hh b/src/libutil/rust-ffi.hh
index a488b96d6..663758bfc 100644
--- a/src/libutil/rust-ffi.hh
+++ b/src/libutil/rust-ffi.hh
@@ -81,17 +81,4 @@ struct CBox
     }
 };
 
-// Grrr, this is only needed because 'extern "C"' functions don't
-// support non-POD return types (and CBox has a destructor so it's not
-// POD).
-template<typename T>
-struct CBox2
-{
-    T * ptr;
-    CBox<T> use()
-    {
-        return CBox(ptr);
-    }
-};
-
 }
diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index f7d3ad417..2cc7793fd 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -2,14 +2,16 @@
 #include "compression.hh"
 
 extern "C" {
-    rust::CBox2<rust::Result<std::tuple<>>> unpack_tarfile(rust::Source source, rust::StringSlice dest_dir);
+    rust::Result<std::tuple<>> *
+    unpack_tarfile(rust::Source source, rust::StringSlice dest_dir);
 }
 
 namespace nix {
 
 void unpackTarfile(Source & source, const Path & destDir)
 {
-    unpack_tarfile(source, destDir).use()->unwrap();
+    rust::Source source2(source);
+    rust::CBox(unpack_tarfile(source2, destDir))->unwrap();
 }
 
 void unpackTarfile(const Path & tarFile, const Path & destDir,