error[E0038]: the trait `AsyncFnMut` cannot be made into an object
  --> $DIR/dyn-pos.rs:3:16
   |
LL | fn foo(x: &dyn AsyncFn()) {}
   |                ^^^^^^^^^ `AsyncFnMut` cannot be made into an object
   |
note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> $SRC_DIR/core/src/ops/async_function.rs:LL:COL
   |
   = note: the trait cannot be made into an object because it contains the generic associated type `CallRefFuture`
   = help: the following types implement the trait, consider defining an enum where each variant holds one of these types, implementing `AsyncFnMut` for this new enum and using it instead:
             &F
             &mut F
             std::boxed::Box<F, A>

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0038`.