Function try_enqueue 

pub fn try_enqueue<T>(
    tx: &Sender<T>,
    item: T,
    closed: &AtomicBool,
    last_error: &Mutex<Option<Error>>,
    message: &str,
) -> bool

Enqueue a publication without blocking. On a full queue, mark the subscription closed and return Error::QueueOverflow — never silently drop.