CoAlloc: Workaround invariant detection defect rust-lang/rust #108751.

Author: peter-lyons-kehl

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

@@ -46,6 +46,20 @@ const KV_IDX_CENTER: usize = B - 1;
 const EDGE_IDX_LEFT_OF_CENTER: usize = B - 1;
 const EDGE_IDX_RIGHT_OF_CENTER: usize = B;
 
+/// Workaround https://github.com/rust-lang/rust/issues/108751
+macro_rules! leaf_node_capacity {
+    () => {
+        11
+    }; // instead of: CAPACITY
+}
+
+/// Workaround https://github.com/rust-lang/rust/issues/108751
+macro_rules! internal_node_capacity {
+    () => {
+        12
+    }; // instead of: 2 * B
+}
+
 /// The underlying representation of leaf nodes and part of the representation of internal nodes.
 struct LeafNode<K, V> {
     /// We want to be covariant in `K` and `V`.
@@ -61,8 +75,20 @@ struct LeafNode<K, V> {
 
     /// The arrays storing the actual data of the node. Only the first `len` elements of each
     /// array are initialized and valid.
-    keys: [MaybeUninit<K>; CAPACITY],
-    vals: [MaybeUninit<V>; CAPACITY],
+    keys: [MaybeUninit<K>; leaf_node_capacity!()], // @FIXME leaf_node_capacity!() workaround for https://github.com/rust-lang/rust/issues/108751
+    vals: [MaybeUninit<V>; leaf_node_capacity!()],
+}
+
+/// @FIXME Remove once we remove leaf_node_capacity!() workaround for https://github.com/rust-lang/rust/issues/108751
+#[test]
+#[allow(dead_code)]
+fn assert_leaf_node_capacity() {
+    fn leaf_node() -> LeafNode<char, bool> {
+        loop {}
+    }
+    let node = leaf_node();
+    let _keys: [MaybeUninit<char>; CAPACITY] = node.keys;
+    let _vals: [MaybeUninit<bool>; CAPACITY] = node.vals;
 }
 
 impl<K, V> LeafNode<K, V> {
@@ -100,7 +126,18 @@ struct InternalNode<K, V> {
     /// The pointers to the children of this node. `len + 1` of these are considered
     /// initialized and valid, except that near the end, while the tree is held
     /// through borrow type `Dying`, some of these pointers are dangling.
-    edges: [MaybeUninit<BoxedNode<K, V>>; 2 * B],
+    edges: [MaybeUninit<BoxedNode<K, V>>; internal_node_capacity!()], // @FIXME internal_node_capacity!() workaround for https://github.com/rust-lang/rust/issues/108751
+}
+
+/// @FIXME Remove once we remove internal_node_capacity!() workaround for https://github.com/rust-lang/rust/issues/108751
+#[test]
+#[allow(dead_code)]
+fn assert_internal_node_capacity() {
+    fn internal_node() -> InternalNode<char, bool> {
+        loop {}
+    }
+    let node = internal_node();
+    let _edges: [MaybeUninit<BoxedNode<char, bool>>; 2 * B] = node.edges;
 }
 
 impl<K, V> InternalNode<K, V> {