// in core lib
trait Seq<T> {
    fn len(self) -> usize;
    fn get(self, i: usize) -> T; // or just use the core::ops::Index trait for this
}

struct CalldataView<T>
where
  T: Decode,
{
  offset: usize,
  len: usize,
}
impl<T> Seq<T> for CalldataView<T>
where T: Decode
{
  fn len(self) -> usize { self.len }
  fn get(self, i: usize) -> T {
    T::decode_from(ptr: CalldataPtr<T>::new(self.offset + i))
  }
}

fn example() -> u64 {
  let nums = CalldataView<u64>::at_offset(4)
  let mut sum = 0
  for n in nums {
    sum += n
  }
  sum
}
// desugars to:
let len = nums.len() // or Seq::len(nums)
let mut i = 0
while i < len {
  sum += nums.get(i) // or Seq::get(nums, i)
  i += 1
}