fix: Fix Vec Iter::count() after next_back() (#250)

There is a bug in the current implementation that returns the count
until the end of the vector even if some elements have been taken from
the iterator using `next_back()` (see the testcase in the PR).

src/base_vec.rs

@@ -379,11 +379,10 @@ where
     }
 
     fn count(self) -> usize {
-        let n = self.vec.len().saturating_sub(self.range.start);
-        if n > usize::MAX as u64 {
-            panic!("The number of items in the vec {n} does not fit into usize");
-        }
-        n as usize
+        min(self.vec.len(), self.range.end)
+            .saturating_sub(self.range.start)
+            .try_into()
+            .expect("Cannot express count as usize")
     }
 
     fn nth(&mut self, n: usize) -> Option<T> {

src/vec/tests.rs

@@ -229,6 +229,27 @@ fn test_iter() {
     assert_eq!(sv.iter().skip(usize::MAX).count(), 0);
 }
 
+#[test]
+fn test_iter_count() {
+    let sv = StableVec::<u64, M>::new(M::default()).unwrap();
+    sv.push(&1).unwrap();
+    sv.push(&2).unwrap();
+    sv.push(&3).unwrap();
+    sv.push(&4).unwrap();
+    {
+        let mut iter = sv.iter();
+        iter.next_back();
+        assert_eq!(iter.count(), 3);
+    }
+    {
+        let mut iter = sv.iter();
+        iter.next_back();
+        sv.pop(); // this pops the element that we iterated through on the previous line
+        sv.pop();
+        assert_eq!(iter.count(), 2);
+    }
+}
+
 // A struct with a bugg implementation of storable where the max_size can
 // smaller than the serialized size.
 #[derive(Clone, Ord, PartialOrd, Eq, PartialEq)]