error[E0597]: `x` does not live long enough
  --> tests/ui/example_opaque_vs_drop.rs:18:1
   |
18 | / opaque! {
19 | |     impl Opaque for FooKey {
20 | |         type Kind<'a> = Foo<'a>;
21 | |     }
22 | | }
   | | ^
   | | |
   | | borrowed value does not live long enough
   | |_`x` dropped here while still borrowed
   |   borrow might be used here, when `x` is dropped and runs the destructor for type `UBCheck<Foo<'_>>`
   |
   = note: this error originates in the macro `opaque` (in Nightly builds, run with -Z macro-backtrace for more info)