1 error[E0308]: mismatched types
2 --> $DIR/retslot-cast.rs:13:5
5 | ^^^^^^^^ expected trait `Iterator<Item = ()>`, found trait `Iterator<Item = ()> + Send`
7 = note: expected enum `Option<&dyn Iterator<Item = ()>>`
8 found enum `Option<&dyn Iterator<Item = ()> + Send>`
10 error: aborting due to previous error
12 For more information about this error, try `rustc --explain E0308`.