make as_leaf return a raw pointer, to reduce aliasing assumptions

Author: RalfJung

library/alloc/src/collections/btree/node.rs

@@ -316,7 +316,9 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     /// Note that, despite being safe, calling this function can have the side effect
     /// of invalidating mutable references that unsafe code has created.
     pub fn len(&self) -> usize {
-        self.as_leaf().len as usize
+        // Crucially, we only access the `len` field here. There might be outstanding mutable references
+        // to keys/values that we must not invalidate.
+        unsafe { (*self.as_leaf()).len as usize }
     }
 
     /// Returns the height of this node in the whole tree. Zero height denotes the
@@ -334,11 +336,14 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     /// If the node is a leaf, this function simply opens up its data.
     /// If the node is an internal node, so not a leaf, it does have all the data a leaf has
     /// (header, keys and values), and this function exposes that.
-    fn as_leaf(&self) -> &LeafNode<K, V> {
+    ///
+    /// Returns a raw ptr to avoid invalidating other references to this node
+    /// (such as during iteration).
+    fn as_leaf(&self) -> *const LeafNode<K, V> {
         // The node must be valid for at least the LeafNode portion.
         // This is not a reference in the NodeRef type because we don't know if
         // it should be unique or shared.
-        unsafe { self.node.as_ref() }
+        self.node.as_ptr()
     }
 
     /// Borrows a view into the keys stored in the node.
@@ -361,7 +366,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     pub fn ascend(
         self,
     ) -> Result<Handle<NodeRef<BorrowType, K, V, marker::Internal>, marker::Edge>, Self> {
-        let parent_as_leaf = self.as_leaf().parent as *const LeafNode<K, V>;
+        let parent_as_leaf = unsafe { (*self.as_leaf()).parent as *const LeafNode<K, V> };
         if let Some(non_zero) = NonNull::new(parent_as_leaf as *mut _) {
             Ok(Handle {
                 node: NodeRef {
@@ -370,7 +375,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
                     root: self.root,
                     _marker: PhantomData,
                 },
-                idx: unsafe { usize::from(*self.as_leaf().parent_idx.as_ptr()) },
+                idx: unsafe { usize::from(*(*self.as_leaf()).parent_idx.as_ptr()) },
                 _marker: PhantomData,
             })
         } else {
@@ -475,13 +480,13 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
 impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
     fn into_key_slice(self) -> &'a [K] {
         unsafe {
-            slice::from_raw_parts(MaybeUninit::slice_as_ptr(&self.as_leaf().keys), self.len())
+            slice::from_raw_parts(MaybeUninit::slice_as_ptr(&(*self.as_leaf()).keys), self.len())
         }
     }
 
     fn into_val_slice(self) -> &'a [V] {
         unsafe {
-            slice::from_raw_parts(MaybeUninit::slice_as_ptr(&self.as_leaf().vals), self.len())
+            slice::from_raw_parts(MaybeUninit::slice_as_ptr(&(*self.as_leaf()).vals), self.len())
         }
     }
 }