error[E0623]: lifetime mismatch
  --> $DIR/regions-creating-enums3.rs:17:5
   |
LL | fn mk_add_bad1<'a,'b>(x: &'a ast<'a>, y: &'b ast<'b>) -> ast<'a> {
   |                                          -----------     -------
   |                                          |
   |                                          this parameter and the return type are declared with different lifetimes...
LL |     ast::add(x, y) //~ ERROR 17:5: 17:19: lifetime mismatch [E0623]
   |     ^^^^^^^^^^^^^^ ...but data from `y` is returned here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0623`.