Implement passing ownership of string to Rust
diff --git a/tests/ffi/tests.cc b/tests/ffi/tests.cc
index 65640df..c94e34d 100644
--- a/tests/ffi/tests.cc
+++ b/tests/ffi/tests.cc
@@ -100,7 +100,7 @@
   r_take_unique_ptr(std::unique_ptr<C>(new C{2020}));
   r_take_ref_c(C{2020});
   r_take_str(rust::Str("2020"));
-  // TODO r_take_rust_string(rust::String("2020"));
+  r_take_rust_string(rust::String("2020"));
   r_take_unique_ptr_string(
       std::unique_ptr<std::string>(new std::string("2020")));