Function fanout_mpsc_producer 

pub fn fanout_mpsc_producer<const N: usize>(
    shared: &SharedFanoutLayoutCell<N>,
) -> Result<FanoutMpscProducer<N>, Error>

Create an MPSC producer handle over a fanout layout.

Returns Err if the region is not initialized or is not v1-compatible.

The returned handle borrows the region by raw pointer; the caller must keep the underlying SharedFanoutLayout<N> alive (and mapped) for as long as the handle is used.