error: generator cannot be sent between threads safely
  --> $DIR/drop-yield-twice.rs:7:17
   |
LL |       assert_send(|| {
   |  _________________^
LL | |         let guard = Foo(42);
LL | |         yield;
LL | |         drop(guard);
LL | |         yield;
LL | |     })
   | |_____^ generator is not `Send`
   |
   = help: within `[generator@$DIR/drop-yield-twice.rs:7:17: 7:19]`, the trait `Send` is not implemented for `Foo`
note: generator 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
...
LL |     })
   |     - `guard` is later dropped here
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 previous error