1 error[E0277]: `std::rc::Rc<std::cell::RefCell<A>>` cannot be sent between threads safely
2 --> $DIR/issue-7013.rs:36:19
4 LL | let a = A {v: box B{v: None} as Box<Foo+Send>};
5 | ^^^^^^^^^^^^^^ `std::rc::Rc<std::cell::RefCell<A>>` cannot be sent between threads safely
7 = help: within `B`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::cell::RefCell<A>>`
8 = note: required because it appears within the type `std::option::Option<std::rc::Rc<std::cell::RefCell<A>>>`
9 = note: required because it appears within the type `B`
10 = note: required for the cast to the object type `dyn Foo + std::marker::Send`
12 error: aborting due to previous error
14 For more information about this error, try `rustc --explain E0277`.