diff --git a/examples/cargo/lib.rs b/examples/cargo/lib.rs
new file mode 100644
index 0000000..3bb21d3
--- /dev/null
+++ b/examples/cargo/lib.rs
@@ -0,0 +1,39 @@
+// Copyright 2020 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// theoretically this should be safe, as unit tests are built without
+// optimizations
+#![allow(clippy::assertions_on_constants)]
+mod test {
+    #[test]
+    fn test_env_contents() {
+        assert_eq!(env!("FOO"), "BAR");
+        assert_eq!(env!("BAR"), "FOO");
+    }
+
+    #[test]
+    fn test_cfg_contents() {
+        assert!(cfg!(foobar));
+    }
+
+    #[test]
+    fn test_rustc_contents() {
+        assert!(cfg!(blah = "bleh"));
+    }
+
+    #[test]
+    fn test_access_data() {
+        assert!(cfg!(data = "Yeah!"));
+    }
+}