Function spawn 

pub unsafe fn spawn<T: 'static + Send, A: 'static>(
    stack: &'static mut [u8],
    priority: u8,
    entry: fn(&'static A) -> T,
    arg: &'static A,
) -> Result<TaskHandle, SpawnError>

Spawn a preemptive task with its own stack.

entry receives arg (a 'static reference — no heap allocation needed since the argument just needs to outlive the task, and tasks never exit). Returns the assigned task id, or None if the task registry (tcb::MAX_PTASKS) is full.

Safety

stack must not be shared with any other task and must remain valid (i.e. 'static) for as long as the task runs.