Test and fix to vec-final

Fixed Iterator implementation for Drain
Added some tests to code

src/vec-final.md

@@ -289,7 +289,7 @@ pub struct Drain<'a, T: 'a> {
 
 impl<'a, T> Iterator for Drain<'a, T> {
     type Item = T;
-    fn next(&mut self) -> Option<T> { self.iter.next_back() }
+    fn next(&mut self) -> Option<T> { self.iter.next() }
     fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
 }
 
@@ -305,4 +305,75 @@ impl<'a, T> Drop for Drain<'a, T> {
 }
 
 # fn main() {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn create_push_pop() {
+        let mut v = Vec::new();
+        v.push(1);
+        assert_eq!(1, v.len());
+        assert_eq!(1, v[0]);
+        for i in v.iter_mut() {
+            *i += 1;
+        }
+        v.insert(0, 1);
+        let x = v.pop();
+        assert_eq!(Some(2), x);
+        assert_eq!(1, v.len());
+        v.push(10);
+        let x = v.remove(0);
+        assert_eq!(1, x);
+        assert_eq!(1, v.len());
+    }
+
+    #[test]
+    fn iter_test() {
+        let mut v = Vec::new();
+        for i in 0..10 {
+            v.push(Box::new(i))
+        }
+        let mut iter = v.into_iter();
+        let first = iter.next().unwrap();
+        let last = iter.next_back().unwrap();
+        drop(iter);
+        assert_eq!(0, *first);
+        assert_eq!(9, *last);
+    }
+
+    #[test]
+    fn test_drain() {
+        let mut v = Vec::new();
+        for i in 0..10 {
+            v.push(Box::new(i))
+        }
+        {
+            let mut drain = v.drain();
+            let first = drain.next().unwrap();
+            let last = drain.next_back().unwrap();
+            assert_eq!(0, *first);
+            assert_eq!(9, *last);
+        }
+        assert_eq!(0, v.len());
+        v.push(Box::new(1));
+        assert_eq!(1, *v.pop().unwrap());
+    }
+
+    #[test]
+    fn test_zst() {
+        let mut v = Vec::new();
+        for _i in 0..10 {
+            v.push(())
+        }
+
+        let mut count = 0;
+
+        for _ in v.into_iter() {
+            count += 1
+        }
+
+        assert_eq!(10, count);
+    }
+}
 ```