error[E0277]: the size for values of type `impl ?Sized` cannot be known at compilation time
  --> $DIR/rpit-not-sized.rs:1:13
   |
LL | fn foo() -> impl ?Sized {
   |             ^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `impl ?Sized`
   = note: the return type of a function must have a statically known size

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.