error: lifetime may not live long enough
  --> $DIR/underscore-lifetime-elison-mismatch.rs:1:42
   |
LL | fn foo(x: &mut Vec<&'_ u8>, y: &'_ u8) { x.push(y); }
   |                    -           -         ^^^^^^^^^ argument requires that `'1` must outlive `'2`
   |                    |           |
   |                    |           let's call the lifetime of this reference `'1`
   |                    let's call the lifetime of this reference `'2`
   |
help: consider introducing a named lifetime parameter
   |
LL | fn foo<'a>(x: &mut Vec<&'a u8>, y: &'a u8) { x.push(y); }
   |       ++++              ~~          ~~

error: aborting due to 1 previous error