error: coroutine cannot be sent between threads safely
  --> $DIR/drop-yield-twice.rs:7:5
   |
LL |     assert_send(|| {
   |     ^^^^^^^^^^^ coroutine is not `Send`
   |
   = help: within `{coroutine@$DIR/drop-yield-twice.rs:7:17: 7:19}`, the trait `Send` is not implemented for `Foo`
note: coroutine is not `Send` as this value is used across a yield
  --> $DIR/drop-yield-twice.rs:9:9
   |
LL |         let guard = Foo(42);
   |             ----- has type `Foo` which is not `Send`
LL |         yield;
   |         ^^^^^ yield occurs here, with `guard` maybe used later
note: required by a bound in `assert_send`
  --> $DIR/drop-yield-twice.rs:15:19
   |
LL | fn assert_send<T: Send>(_: T) {}
   |                   ^^^^ required by this bound in `assert_send`

error: aborting due to 1 previous error