error[E0277]: `Rc` cannot be sent between threads safely --> $DIR/builtin-superkinds-simple.rs:6:6 | LL | impl Foo for std::rc::Rc { } | ^^^ `Rc` cannot be sent between threads safely | = help: the trait `Send` is not implemented for `Rc` note: required by a bound in `Foo` --> $DIR/builtin-superkinds-simple.rs:4:13 | LL | trait Foo : Send { } | ^^^^ required by this bound in `Foo` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.