Function park 

pub unsafe fn park(
    key: usize,
    validate: impl FnOnce() -> bool,
    before_sleep: impl FnOnce(),
    timed_out: impl FnOnce(usize, bool),
    park_token: ParkToken,
    timeout: Option<Instant>,
) -> ParkResult

Parks the current thread in the queue associated with the given key.

The validate function is called while the queue is locked and can abort the operation by returning false. If validate returns true then the current thread is appended to the queue and the queue is unlocked.

The before_sleep function is called after the queue is unlocked but before the thread is put to sleep. The thread will then sleep until it is unparked or the given timeout is reached.

The timed_out function is also called while the queue is locked, but only if the timeout was reached. It is passed the key of the queue it was in when it timed out, which may be different from the original key if unpark_requeue was called. It is also passed a bool which indicates whether it was the last thread in the queue.

Safety

You should only call this function with an address that you control, since you could otherwise interfere with the operation of other synchronization primitives.

The validate and timed_out functions are called while the queue is locked and must not panic or call into any function in parking_lot.

The before_sleep function is called outside the queue lock and is allowed to call unpark_one, unpark_all, unpark_requeue or unpark_filter, but it is not allowed to call park or panic.