Implement fallible C++ functions
diff --git a/tests/ffi/tests.h b/tests/ffi/tests.h
index 7cff6fa..6a5f802 100644
--- a/tests/ffi/tests.h
+++ b/tests/ffi/tests.h
@@ -36,4 +36,8 @@
 void c_take_rust_string(rust::String s);
 void c_take_unique_ptr_string(std::unique_ptr<std::string> s);
 
+void c_try_return_void();
+size_t c_try_return_primitive();
+size_t c_fail_return_primitive();
+
 } // namespace tests