Type Alias NextDeadlineFn 

pub type NextDeadlineFn = unsafe extern "C" fn(ctx: *mut c_void) -> u64;

When this source will next need the executor, in the executor’s clock, as microseconds FROM NOW.

u64::MAX means “nothing pending” — the source does not shorten the park. That is how an ASYNC source spells itself: a socket or a DDS listener contributes no deadline and instead breaks the park by signalling the wake primitive, so async and deadline sources compose without a second mechanism.