error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> $DIR/regions-creating-enums4.rs:17:5
   |
LL |     ast::add(x, y) //~ ERROR cannot infer
   |     ^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the function body at 16:16...
  --> $DIR/regions-creating-enums4.rs:16:16
   |
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 16:19...
  --> $DIR/regions-creating-enums4.rs:16:19
   |
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`.