fn right_stream<B>(self) -> Either<B, Self>
    where
        B: Stream<Item = Self::Item>,
        Self: Sized,
    {
        assert_stream::<Self::Item, _>(Either::Right(self))
    }
Wrap this stream in an `Either` stream, making it the right-hand variant
of that `Either`.

This can be used in combination with the `left_stream` method to write `if`
statements that evaluate to different streams in different branches.
futures-util/src/stream/stream/mod.rs