For now the Bytes::from_fixedarray allocates closures because it uses Bytes::makei:

pub fn Bytes::from_fixedarray(arr : FixedArray[Byte], len? : Int) -> Bytes {
  let len = match len {
    None => arr.length()
    Some(x) => x
  }
  Bytes::makei(len, i => arr[i])
}

Using simple memcpy or, better, no-op will be much better as there will be no allocate when writing each individual element.