error[E0106]: missing lifetime specifier
  --> $DIR/elision.rs:4:70
   |
LL | fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
   |         ------------------------------------------------             ^^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the signature does not say which one of `x`'s 2 lifetimes it is borrowed from
help: consider introducing a named lifetime parameter
   |
LL - fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
LL + fn f<'a>(x: &'a mut dyn Iterator<Item: Iterator<Item = &'a ()>>) -> Option<&'a ()> { x.next() }
   |

error: associated type bounds are not allowed in `dyn` types
  --> $DIR/elision.rs:4:27
   |
LL | fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: use `impl Trait` to introduce a type instead
   |
LL - fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
LL + fn f(x: &mut dyn Iterator<Item = impl Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
   |

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0106`.