error[E0277]: `Rc<RefCell<A>>` cannot be sent between threads safely
  --> $DIR/issue-7013.rs:24:19
   |
LL |     let a = A {v: Box::new(B{v: None}) as Box<dyn Foo + Send>};
   |                   ^^^^^^^^^^^^^^^^^^^^ `Rc<RefCell<A>>` cannot be sent between threads safely
   |
   = help: within `B`, the trait `Send` is not implemented for `Rc<RefCell<A>>`
note: required because it appears within the type `Option<Rc<RefCell<A>>>`
  --> $SRC_DIR/core/src/option.rs:LL:COL
note: required because it appears within the type `B`
  --> $DIR/issue-7013.rs:8:8
   |
LL | struct B {
   |        ^
   = note: required for the cast from `Box<B>` to `Box<dyn Foo + Send>`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.