warning: lifetime parameter `'b` never used
  --> $DIR/unused-lifetimes-2.rs:15:32
   |
LL | pub fn by_value(_x: unsafe<'a, 'b> &'a S) -> usize {
   |                              --^^
   |                              |
   |                              help: elide the unused lifetime
   |
note: the lint level is defined here
  --> $DIR/unused-lifetimes-2.rs:5:9
   |
LL | #![warn(unused_lifetimes)]
   |         ^^^^^^^^^^^^^^^^

warning: 1 warning emitted