Function reserve_and_pad 

fn reserve_and_pad<A: Allocator>(
    pos_mut: &mut u64,
    vec: &mut Vec<u8, A>,
    buf_len: usize,
) -> Result<usize>

🔬This is a nightly-only experimental API. (alloc_io #154046)

Reserves the required space, and pads the vec with 0s if necessary.