Add a trivial test

src/lib.rs

@@ -15,3 +15,14 @@ pub fn cast<T: 'static, R: 'static>(t: T) -> Result<R, T> {
         Err(t)
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_cast() {
+        assert_eq!(cast::<_, &'static str>("foo"), Ok("foo"));
+        assert_eq!(cast::<_, &'static str>(4), Err(4));
+    }
+}