error[E0277]: the size for values of type `V` cannot be known at compilation time
  --> $DIR/issue-unsized-tail-restatic-ice-122488.rs:5:61
   |
LL | struct ArenaSet<U: Deref, V: ?Sized = <U as Deref>::Target>(V, U);
   |                           --------------------------------  ^ doesn't have a size known at compile-time
   |                           |
   |                           this type parameter needs to be `Sized`
   |
   = note: only the last field of a struct may have a dynamically sized type
   = help: change the field's type to have a statically known size
help: consider removing the `?Sized` bound to make the type parameter `Sized`
   |
LL - struct ArenaSet<U: Deref, V: ?Sized = <U as Deref>::Target>(V, U);
LL + struct ArenaSet<U: Deref, V = <U as Deref>::Target>(V, U);
   |
help: borrowed types always have a statically known size
   |
LL | struct ArenaSet<U: Deref, V: ?Sized = <U as Deref>::Target>(&V, U);
   |                                                             +
help: the `Box` type always has a statically known size and allocates its contents in the heap
   |
LL | struct ArenaSet<U: Deref, V: ?Sized = <U as Deref>::Target>(Box<V>, U);
   |                                                             ++++ +

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.