error[E0307]: invalid `self` parameter type: `&dyn Foo`
  --> $DIR/inference_var_self_argument.rs:5:24
   |
LL |     async fn foo(self: &dyn Foo) {
   |                        ^^^^^^^^
   |
   = note: type of `self` must be `Self` or a type that dereferences to it
   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)

error[E0038]: the trait `Foo` cannot be made into an object
  --> $DIR/inference_var_self_argument.rs:5:5
   |
LL |     async fn foo(self: &dyn Foo) {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Foo` 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>
  --> $DIR/inference_var_self_argument.rs:5:14
   |
LL | trait Foo {
   |       --- this trait cannot be made into an object...
LL |     async fn foo(self: &dyn Foo) {
   |              ^^^ ...because method `foo` is `async`
   = help: consider moving `foo` to another trait

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0038, E0307.
For more information about an error, try `rustc --explain E0038`.