pub fn enter() -> Result<Enter, EnterError>
Marks the current thread as being within the dynamic extent of an executor.
Executor implementations should call this function before beginning to
execute a tasks, and drop the returned
Enter value after
completing task execution:
use futures::executor::enter; let enter = enter().expect("..."); /* run task */ drop(enter);
Doing so ensures that executors aren't accidentally invoked in a nested fashion.
Returns an error if the current thread is already marked, in which case the caller should panic with a tailored error message.