I'm not pcwalton, but he is correct. The Rust compiler only allows objects that implement the Sync trait to cross thread boundaries. Rc<T> is not Sync, while Arc<T> is. Sync is a marker trait, with impls automatically generated by the compiler for types that are comprised entirely of Sync types, or manually (and unsafely) specified by the author. As a result, Rc<T> cannot be misused from multiple threads without unsafe.

To learn more about this, look here: https://doc.rust-lang.org/beta/nomicon/send-and-sync.html