error[E0321]: cross-crate traits with a default impl, like `Send`, can only be implemented for a struct/enum type, not `&'static Foo` --> $DIR/empty_span.rs:7:5 | LL | unsafe impl Send for &'static Foo { } | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type error: aborting due to previous error For more information about this error, try `rustc --explain E0321`.