Rollup merge of #89245 - DeveloperC286:iter_mut_fields_to_private, r=joshtriplett

refactor: make VecDeque's IterMut fields module-private, not just crate-private

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

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

@@ -13,10 +13,21 @@ use super::{count, wrap_index, RingSlices};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct IterMut<'a, T: 'a> {
     // Internal safety invariant: the entire slice is dereferencable.
-    pub(crate) ring: *mut [T],
-    pub(crate) tail: usize,
-    pub(crate) head: usize,
-    pub(crate) phantom: PhantomData<&'a mut [T]>,
+    ring: *mut [T],
+    tail: usize,
+    head: usize,
+    phantom: PhantomData<&'a mut [T]>,
+}
+
+impl<'a, T> IterMut<'a, T> {
+    pub(super) unsafe fn new(
+        ring: *mut [T],
+        tail: usize,
+        head: usize,
+        phantom: PhantomData<&'a mut [T]>,
+    ) -> Self {
+        IterMut { ring, tail, head, phantom }
+    }
 }
 
 // SAFETY: we do nothing thread-local and there is no interior mutability,

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

@@ -1000,12 +1000,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
         // SAFETY: The internal `IterMut` safety invariant is established because the
         // `ring` we create is a dereferencable slice for lifetime '_.
-        IterMut {
-            tail: self.tail,
-            head: self.head,
-            ring: ptr::slice_from_raw_parts_mut(self.ptr(), self.cap()),
-            phantom: PhantomData,
-        }
+        let ring = ptr::slice_from_raw_parts_mut(self.ptr(), self.cap());
+
+        unsafe { IterMut::new(ring, self.tail, self.head, PhantomData) }
     }
 
     /// Returns a pair of slices which contain, in order, the contents of the
@@ -1192,12 +1189,9 @@ impl<T, A: Allocator> VecDeque<T, A> {
 
         // SAFETY: The internal `IterMut` safety invariant is established because the
         // `ring` we create is a dereferencable slice for lifetime '_.
-        IterMut {
-            tail,
-            head,
-            ring: ptr::slice_from_raw_parts_mut(self.ptr(), self.cap()),
-            phantom: PhantomData,
-        }
+        let ring = ptr::slice_from_raw_parts_mut(self.ptr(), self.cap());
+
+        unsafe { IterMut::new(ring, tail, head, PhantomData) }
     }
 
     /// Creates a draining iterator that removes the specified range in the