error[E0072]: recursive type `Expr` has infinite size
  --> $DIR/recursive-enum-memory-32326.rs:5:1
   |
LL | enum Expr {
   | ^^^^^^^^^
LL |     Plus(Expr, Expr),
   |          ---- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL |     Plus(Box<Expr>, Expr),
   |          ++++    +

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0072`.