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>>,
   |                       ^^^^^^^^^^^^^ `Expr` cannot be made into an object
   |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> $DIR/object-safety-issue-22040.rs:5:21
   |
LL | trait Expr: Debug + PartialEq {
   |       ----          ^^^^^^^^^ ...because it uses `Self` as a type parameter
   |       |
   |       this trait cannot be made into an object...
   = help: only type `SExpr<'x>` implements the trait, consider using it directly instead

error: aborting due to 1 previous error

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