/// [arc]: ../../std/sync/struct.Arc.html
/// [ub]: ../../reference/behavior-considered-undefined.html
#[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(stage0, lang = "send")]
#[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"]
pub unsafe trait Send {
// empty.