Fix Vec Iter::count() after next_back

src/base_vec.rs

@@ -379,11 +379,7 @@ 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
+        self.range.count()
     }
 
     fn nth(&mut self, n: usize) -> Option<T> {

src/vec/tests.rs

@@ -227,6 +227,13 @@ fn test_iter() {
     assert_eq!(sv.iter().skip(3).count(), 0);
     assert_eq!(sv.iter().skip(4).count(), 0);
     assert_eq!(sv.iter().skip(usize::MAX).count(), 0);
+
+    {
+        assert_eq!(sv.len(), 3);
+        let mut iter = sv.iter();
+        iter.next_back();
+        assert_eq!(iter.by_ref().count(), 2);
+    }
 }
 
 // A struct with a bugg implementation of storable where the max_size can