There is one case where Rust breaks this rule for the sake of simpler syntax. Send of Futures: async fn send_peekaboo() { let _r = std::rc::Rc::ne

There is one case where Rust breaks this rule for the sake of simpler syntax. Send of Futures: async fn send_peekaboo() { let _r = std::rc::Rc::new(()); // async {}.await; } fn is_it_send(_: impl Send) {} is_it_send(send_peekaboo()); If you uncomment the .await line, this crates a possibility of moving thread-unsafe Rc across threads, and the Future stops being thread-safe. Rust catches this problem correctly, but it is a case where function body leaks into the signature.