error[E0106]: missing lifetime specifier
  --> $DIR/elision.rs:5:70
   |
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<'a>(x: &'a mut dyn Iterator<Item: Iterator<Item = &'a ()>>) -> Option<&'a ()> { x.next() }
   |     ++++     ++                                         ~~                  ~~

error[E0308]: mismatched types
  --> $DIR/elision.rs:5:79
   |
LL | fn f(x: &mut dyn Iterator<Item: Iterator<Item = &'_ ()>>) -> Option<&'_ ()> { x.next() }
   |                           -----------------------------      --------------   ^^^^^^^^ expected `&()`, found type parameter `impl Iterator<Item = &'_ ()>`
   |                           |                                  |
   |                           |                                  expected `Option<&'static ()>` because of return type
   |                           this type parameter
   |
   = note: expected enum `Option<&'static ()>`
              found enum `Option<impl Iterator<Item = &'_ ()>>`

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0106, E0308.
For more information about an error, try `rustc --explain E0106`.