refactor: VecDeques Iter fields to private

Made the fields of VecDeque's Iter private by creating a Iter::new(...) function to create a new instance of Iter and migrating usage to use Iter::new(...).

library/alloc/src/collections/vec_deque/iter.rs

@@ -13,9 +13,15 @@
 /// [`iter`]: super::VecDeque::iter
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Iter<'a, T: 'a> {
-    pub(crate) ring: &'a [MaybeUninit<T>],
-    pub(crate) tail: usize,
-    pub(crate) head: usize,
+    ring: &'a [MaybeUninit<T>],
+    tail: usize,
+    head: usize,
+}
+
+impl<'a, T> Iter<'a, T> {
+    pub(super) fn new(ring: &'a [MaybeUninit<T>], tail: usize, head: usize) -> Self {
+        Iter { ring, tail, head }
+    }
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]

library/alloc/src/collections/vec_deque/mod.rs

@@ -1009,7 +1009,7 @@ pub fn allocator(&self) -> &A {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn iter(&self) -> Iter<'_, T> {
-        Iter { tail: self.tail, head: self.head, ring: unsafe { self.buffer_as_slice() } }
+        Iter::new(unsafe { self.buffer_as_slice() }, self.tail, self.head)
     }
 
     /// Returns a front-to-back iterator that returns mutable references.
@@ -1188,12 +1188,8 @@ pub fn range<R>(&self, range: R) -> Iter<'_, T>
         R: RangeBounds<usize>,
     {
         let (tail, head) = self.range_tail_head(range);
-        Iter {
-            tail,
-            head,
-            // The shared reference we have in &self is maintained in the '_ of Iter.
-            ring: unsafe { self.buffer_as_slice() },
-        }
+        // The shared reference we have in &self is maintained in the '_ of Iter.
+        Iter::new(unsafe { self.buffer_as_slice() }, tail, head)
     }
 
     /// Creates an iterator that covers the specified mutable range in the deque.
@@ -1309,16 +1305,15 @@ pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>
         self.head = drain_tail;
 
         let deque = NonNull::from(&mut *self);
-        let iter = Iter {
-            tail: drain_tail,
-            head: drain_head,
+        unsafe {
             // Crucially, we only create shared references from `self` here and read from
             // it.  We do not write to `self` nor reborrow to a mutable reference.
             // Hence the raw pointer we created above, for `deque`, remains valid.
-            ring: unsafe { self.buffer_as_slice() },
-        };
+            let ring = self.buffer_as_slice();
+            let iter = Iter::new(ring, drain_tail, drain_head);
 
-        unsafe { Drain::new(drain_head, head, iter, deque) }
+            Drain::new(drain_head, head, iter, deque)
+        }
     }
 
     /// Clears the deque, removing all values.