error[E0308]: mismatched types --> $DIR/retslot-cast.rs:13:5 | LL | -> Option<&Iterator> { | -------------------------- expected `Option<&dyn Iterator>` because of return type ... LL | inner(x) | ^^^^^^^^ expected trait `Iterator`, found trait `Iterator + Send` | = note: expected enum `Option<&dyn Iterator>` found enum `Option<&dyn Iterator + Send>` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0308`.