diff --git a/src/pkg/io/ioutil/ioutil.go b/src/pkg/io/ioutil/ioutil.go
index fffa132..dd50d96 100644
--- a/src/pkg/io/ioutil/ioutil.go
+++ b/src/pkg/io/ioutil/ioutil.go
@@ -104,6 +104,10 @@
 
 type devNull int
 
+// devNull implements ReaderFrom as an optimization so io.Copy to
+// ioutil.Discard can avoid doing unnecessary work.
+var _ io.ReaderFrom = devNull(0)
+
 func (devNull) Write(p []byte) (int, os.Error) {
 	return len(p), nil
 }