error[E0277]: `std::rc::Rc<i8>` cannot be sent between threads safely
  --> $DIR/builtin-superkinds-simple.rs:6:6
   |
LL | trait Foo : Send { }
   |             ---- required by this bound in `Foo`
LL | 
LL | impl Foo for std::rc::Rc<i8> { }
   |      ^^^ `std::rc::Rc<i8>` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `std::rc::Rc<i8>`

error: aborting due to previous error

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