Why does btreemap's iter not implement count?

Question

I wonder about the Iterator for Iter implementation of BTreeMaps Iter:

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
    type Item = (&'a K, &'a V);

    fn next(&mut self) -> Option<(&'a K, &'a V)> {
        if self.length == 0 {
            None
        } else {
            self.length -= 1;
            Some(unsafe { self.range.next_unchecked() })
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.length, Some(self.length))
    }

    fn last(mut self) -> Option<(&'a K, &'a V)> {
        self.next_back()
    }

    fn min(mut self) -> Option<(&'a K, &'a V)>
    where
        (&'a K, &'a V): Ord,
    {
        self.next()
    }

    fn max(mut self) -> Option<(&'a K, &'a V)>
    where
        (&'a K, &'a V): Ord,
    {
        self.next_back()
    }
}

Only some functions are implemented, and count should be very easy to implement. Just return len. Why use next?

I'd love to know some details about iterator design, maybe I've been missing something key this whole time.