Function copy_bidirectional_with_sizes 

pub async fn copy_bidirectional_with_sizes<A, B>(
    reader: A,
    writer: B,
    a_to_b_size: usize,
    b_to_a_size: usize,
) -> (Result<u64, Error>, Result<u64, Error>)
where
    A: Splittable,
    <A as Splittable>::ReadHalf: AsyncRead,
    <A as Splittable>::WriteHalf: AsyncWrite,
    B: Splittable,
    <B as Splittable>::ReadHalf: AsyncRead,
    <B as Splittable>::WriteHalf: AsyncWrite,

Available on crate feature io only.

Asynchronously copies data bidirectionally between two pairs of reader and writer with specified buffer sizes.

This function is like copy_bidirectional, but allows you to specify the buffer sizes for each direction of copying.