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`.