error[E0038]: the trait `Expr` cannot be made into an object
  --> $DIR/object-safety-issue-22040.rs:22:23
   |
LL |     elements: Vec<Box<Expr+ 'x>>,
   |                       ^^^^^^^^ the trait `Expr` cannot be made into an object
   |
   = note: the trait 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`.