error: the `foo` method cannot be invoked on a trait object
  --> $DIR/by-value-trait-object-safety.rs:18:7
   |
LL |     fn foo(self) -> String where Self: Sized;
   |                                        ----- this has a `Sized` requirement
...
LL |     x.foo();
   |       ^^^

error: aborting due to previous error