Auto merge of rust-lang#82440 - ssomers:btree_fix_casts, r=Mark-Simulacrum

BTree: no longer define impossible casts

Casts to leaf to internal only make sense when the original has a chance of being the thing it's cast to.

r? `@Mark-Simulacrum`

Author: bors

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

@@ -460,7 +460,7 @@ impl<K, V> NodeRef<marker::Dying, K, V, marker::LeafOrInternal> {
     }
 }
 
-impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
+impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
     /// Unsafely asserts to the compiler the static information that this node is a `Leaf`.
     unsafe fn cast_to_leaf_unchecked(self) -> NodeRef<marker::Mut<'a>, K, V, marker::Leaf> {
         debug_assert!(self.height == 0);
@@ -472,7 +472,9 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         debug_assert!(self.height > 0);
         NodeRef { height: self.height, node: self.node, _marker: PhantomData }
     }
+}
 
+impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
     /// Temporarily takes out another, mutable reference to the same node. Beware, as
     /// this method is very dangerous, doubly so since it may not immediately appear
     /// dangerous.
@@ -751,15 +753,17 @@ impl<BorrowType, K, V, NodeType, HandleType>
     }
 }
 
-impl<'a, K, V, NodeType, HandleType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, HandleType> {
+impl<'a, K, V, Type> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, Type> {
     /// Unsafely asserts to the compiler the static information that the handle's node is a `Leaf`.
     pub unsafe fn cast_to_leaf_unchecked(
         self,
-    ) -> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, HandleType> {
+    ) -> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, Type> {
         let node = unsafe { self.node.cast_to_leaf_unchecked() };
         Handle { node, idx: self.idx, _marker: PhantomData }
     }
+}
 
+impl<'a, K, V, NodeType, HandleType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, HandleType> {
     /// Temporarily takes out another, mutable handle on the same location. Beware, as
     /// this method is very dangerous, doubly so since it may not immediately appear
     /// dangerous.
@@ -1509,15 +1513,13 @@ impl<BorrowType, K, V> Handle<NodeRef<BorrowType, K, V, marker::Internal>, marke
     }
 }
 
-impl<BorrowType, K, V, HandleType>
-    Handle<NodeRef<BorrowType, K, V, marker::LeafOrInternal>, HandleType>
-{
+impl<BorrowType, K, V, Type> Handle<NodeRef<BorrowType, K, V, marker::LeafOrInternal>, Type> {
     /// Checks whether the underlying node is an `Internal` node or a `Leaf` node.
     pub fn force(
         self,
     ) -> ForceResult<
-        Handle<NodeRef<BorrowType, K, V, marker::Leaf>, HandleType>,
-        Handle<NodeRef<BorrowType, K, V, marker::Internal>, HandleType>,
+        Handle<NodeRef<BorrowType, K, V, marker::Leaf>, Type>,
+        Handle<NodeRef<BorrowType, K, V, marker::Internal>, Type>,
     > {
         match self.node.force() {
             ForceResult::Leaf(node) => {