Associative constants are lazy

[Ddystopia]

Hello, looking at rtic_sync::channel I saw this:

impl<T, const N: usize> Channel<T, N> {
    const _CHECK: () = assert!(N < 256, "This queue support a maximum of 255 entries");

    const INIT_SLOTS: UnsafeCell<MaybeUninit<T>> = UnsafeCell::new(MaybeUninit::uninit());

    /// Create a new channel.
    pub const fn new() -> Self {
        Self {
            freeq: UnsafeCell::new(Deque::new()),
            readyq: UnsafeCell::new(Deque::new()),
            receiver_waker: WakerRegistration::new(),
            slots: [Self::INIT_SLOTS; N],
            wait_queue: WaitQueue::new(),
            receiver_dropped: UnsafeCell::new(false),
            num_senders: UnsafeCell::new(0),
        }
    }

    /// Split the queue into a `Sender`/`Receiver` pair.
    pub fn split(&mut self) -> (Sender<'_, T, N>, Receiver<'_, T, N>) {
        // Fill free queue
        for idx in 0..N as u8 {
            debug_assert!(!self.freeq.get_mut().is_full());

            // SAFETY: This safe as the loop goes from 0 to the capacity of the underlying queue.
            unsafe {
                self.freeq.get_mut().push_back_unchecked(idx);
            }
        }

        debug_assert!(self.freeq.get_mut().is_full());

        // There is now 1 sender
        *self.num_senders.get_mut() = 1;

        (Sender(self), Receiver(self))
    }

    fn access<'a>(&'a self, _cs: critical_section::CriticalSection) -> UnsafeAccess<'a, N> {
        // SAFETY: This is safe as are in a critical section.
        unsafe {
            UnsafeAccess {
                freeq: &mut *self.freeq.get(),
                readyq: &mut *self.readyq.get(),
                receiver_dropped: &mut *self.receiver_dropped.get(),
                num_senders: &mut *self.num_senders.get(),
            }
        }
    }
}
   

Here _CHECK is actually ignored - it is never executed. And I indeed can create channel with capacity 300.

How to fix it

In new this line should be added:

        Self::_CHECK;

This will force _CHECK to be computed, and assert fired. Note, that this should be done in every constructor of the type, even private.

Other code

I don't know the codebase, but you probably do. Every such place should be fixed, so I will not pr this particular change.

[perlindgren]

Good catch!