error[E0038]: the trait `Expr` cannot be made into an object
  --> $DIR/object-safety-issue-22040.rs:12:23
   |
LL |     elements: Vec<Box<dyn Expr + 'x>>,
   |                       ^^^^^^^^^^^^^ the trait `Expr` cannot be made into an object
   |
   = note: the trait cannot be made into an object because it cannot use `Self` as a type parameter in the supertraits or `where`-clauses

error: aborting due to previous error

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