fn sink_map_err<E, F>(self, f: F) -> SinkMapErr<Self, F>
    where
        F: FnOnce(Self::Error) -> E,
        Self: Sized,
    {
        assert_sink::<Item, E, _>(SinkMapErr::new(self, f))
    }
Transforms the error returned by the sink.
futures-util/src/sink/mod.rs