error: unreachable pattern
  --> $DIR/issue-12116.rs:15:9
   |
LL |         &IntList::Cons(val, box ref next_list) => tail(next_list),
   |         -------------------------------------- matches all the relevant values
LL |         &IntList::Cons(val, box IntList::Nil)  => IntList::Cons(val, Box::new(IntList::Nil)),
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no value can reach this
   |
note: the lint level is defined here
  --> $DIR/issue-12116.rs:4:9
   |
LL | #![deny(unreachable_patterns)]
   |         ^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error