error[E0308]: mismatched types
  --> $DIR/retslot-cast.rs:24:5
   |
LL |     inner(x) //~ ERROR mismatched types
   |     ^^^^^^^^ expected trait `std::iter::Iterator<Item=()>`, found trait `std::iter::Iterator<Item=()> + std::marker::Send`
   |
   = note: expected type `std::option::Option<&dyn std::iter::Iterator<Item=()>>`
              found type `std::option::Option<&dyn std::iter::Iterator<Item=()> + std::marker::Send>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.