#[derive(BinRead)]
#[br(little)]
// Is 7+ bytes but needs to get padded to `arbitrary`
pub struct Bar {
  f1: u8,
  f2: u16,
  len: u32,
  #[br(count = len)]
  v: Vec<u8>
}
#[derive(BinRead)]
pub struct Foo {
  arbitrary: u8,
  len: u8,
  #[br(pad_size_to = arbitrary, count = len)]
  pub list: Vec<Bar>
}

// passing [
//   12, 2,
//   1, 2, 0, // f1, f2
//   1, 0, 0, 0, // len
//   42, // list
//   0, 0, 0, 0, // padding
//   4, 5, 0, // f1, f2
//   2, 0, 0, 0, // len
//   41, 43 // list
//   0, 0, 0 // padding
// ]
// should return Foo { arbitrary: 8, len: 2, list: vec![Bar {f1: 1, f2: 2, len: 1, list: vec![42]}] }