Rust VecDeque "wraparound" bug

main.rs

extern crate rand;

use std::collections::VecDeque;

fn handle_ack(q: &mut VecDeque<u8>, bytes_ackd: u32)
{
    for _i in 0..bytes_ackd
    {
        q.pop_front();
    }
}

fn extend_and_check(q: &mut VecDeque<u8>, amount: usize)
{
    let (size1a, size1b) = { let (a,b) = q.as_slices(); (a.len(), b.len()) };
    if size1b != 0
    {
        panic!("bizarreness BEFORE: {}, {}", size1a, size1b);
    }
    let mut raw = [0u8 ; 80000];
    for i in 0..amount
    {
        raw[i] = (i % 256) as u8;
    }
    q.extend(&raw[0..amount]);
    let (size2a, size2b) = { let (a,b) = q.as_slices(); (a.len(), b.len()) };
    if size2b != 0
    {
        panic!("bizarreness AFTER: {}, {}", size2a, size2b);
    }
}

fn print_lens(q: &VecDeque<u8>)
{
    let (a,b) = q.as_slices();
    println!("{}\t{}", a.len(), b.len());
}

fn random_up_to_len(q: &VecDeque<u8>) -> u32
{
    let raw_amt = (rand::random::<u16>() % 20000) as u32;
    let (a,_b) = q.as_slices();
    let q_len = a.len() as u32;
    if raw_amt > q_len { q_len } else { raw_amt }
}

fn main()
{
    let mut q = VecDeque::with_capacity(1460*50);

    loop
    {
        if rand::random::<bool>()
        {
            extend_and_check(&mut q, (rand::random::<u16>() % 20000) as usize);
        }
        else
        {
            let ack_len = random_up_to_len(&q) as u32;
            handle_ack(&mut q, ack_len);
        }
        print_lens(&q);
    }
}