error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> $DIR/regions-creating-enums4.rs:7:5
   |
LL |     Ast::Add(x, y)
   |     ^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime `'a` as defined on the function body at 6:16...
  --> $DIR/regions-creating-enums4.rs:6:16
   |
LL | fn mk_add_bad2<'a,'b>(x: &'a Ast<'a>, y: &'a Ast<'a>, z: &Ast) -> Ast<'b> {
   |                ^^
   = note: ...so that the expression is assignable:
           expected &Ast<'_>
              found &Ast<'a>
note: but, the lifetime must be valid for the lifetime `'b` as defined on the function body at 6:19...
  --> $DIR/regions-creating-enums4.rs:6:19
   |
LL | fn mk_add_bad2<'a,'b>(x: &'a Ast<'a>, y: &'a Ast<'a>, z: &Ast) -> Ast<'b> {
   |                   ^^
   = note: ...so that the expression is assignable:
           expected Ast<'b>
              found Ast<'_>

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.