error: future cannot be sent between threads safely
 --> tests/fail/not_send.rs:4:1
  |
4 | async fn handler() {
  | ^^^^^ future returned by `handler` is not `Send`
  |
  = help: within `impl Future`, the trait `Send` is not implemented for `Rc<()>`
note: future is not `Send` as this value is used across an await
 --> tests/fail/not_send.rs:6:5
  |
5 |     let rc = std::rc::Rc::new(());
  |         -- has type `Rc<()>` which is not `Send`
6 |     async {}.await;
  |     ^^^^^^^^^^^^^^ await occurs here, with `rc` maybe used later
7 | }
  | - `rc` is later dropped here
note: required by a bound in `check`
 --> tests/fail/not_send.rs:4:1
  |
4 | async fn handler() {
  | ^^^^^ required by this bound in `check`