fn fanout<Si>(self, other: Si) -> Fanout<Self, Si>
    where
        Self: Sized,
        Item: Clone,
        Si: Sink<Item, Error = Self::Error>,
    {
        assert_sink::<Item, Self::Error, _>(Fanout::new(self, other))
    }
Fanout items to multiple sinks.

This adapter clones each incoming item and forwards it to both this as well as
the other sink at the same time.
futures-util/src/sink/mod.rs