warning: lifetime parameter `'a` never used
  --> $DIR/unused-lifetimes.rs:15:28
   |
LL | pub fn by_value(_x: unsafe<'a> S) -> usize {
   |                     -------^^-- help: elide the unused lifetime
   |
note: the lint level is defined here
  --> $DIR/unused-lifetimes.rs:5:9
   |
LL | #![warn(unused_lifetimes)]
   |         ^^^^^^^^^^^^^^^^

warning: 1 warning emitted