Rollup merge of #74874 - ssomers:btree_cleanup_8, r=Mark-Simulacrum

BTreeMap: define forget_type only when relevant

Similar to `forget_node_type` for handles.
No effect on generated code, apart maybe from the superfluous calls that might not have been optimized away.

r? @Mark-Simulacrum

Author: JohnTitor

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

@@ -19,7 +19,7 @@ impl<BorrowType, K, V> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::E
                 Ok(internal_kv) => return Ok(internal_kv),
                 Err(last_edge) => match last_edge.into_node().ascend() {
                     Ok(parent_edge) => parent_edge.forget_node_type(),
-                    Err(root) => return Err(root.forget_type()),
+                    Err(root) => return Err(root),
                 },
             }
         }
@@ -40,7 +40,7 @@ impl<BorrowType, K, V> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::E
                 Ok(internal_kv) => return Ok(internal_kv),
                 Err(last_edge) => match last_edge.into_node().ascend() {
                     Ok(parent_edge) => parent_edge.forget_node_type(),
-                    Err(root) => return Err(root.forget_type()),
+                    Err(root) => return Err(root),
                 },
             }
         }

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

@@ -307,12 +307,6 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
         self.height
     }
 
-    /// Removes any static information about whether this node is a `Leaf` or an
-    /// `Internal` node.
-    pub fn forget_type(self) -> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
-        NodeRef { height: self.height, node: self.node, root: self.root, _marker: PhantomData }
-    }
-
     /// Temporarily takes out another, immutable reference to the same node.
     fn reborrow(&self) -> NodeRef<marker::Immut<'_>, K, V, Type> {
         NodeRef { height: self.height, node: self.node, root: self.root, _marker: PhantomData }
@@ -1357,6 +1351,20 @@ unsafe fn move_edges<K, V>(
     }
 }
 
+impl<BorrowType, K, V> NodeRef<BorrowType, K, V, marker::Leaf> {
+    /// Removes any static information asserting that this node is a `Leaf` node.
+    pub fn forget_type(self) -> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
+        NodeRef { height: self.height, node: self.node, root: self.root, _marker: PhantomData }
+    }
+}
+
+impl<BorrowType, K, V> NodeRef<BorrowType, K, V, marker::Internal> {
+    /// Removes any static information asserting that this node is an `Internal` node.
+    pub fn forget_type(self) -> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
+        NodeRef { height: self.height, node: self.node, root: self.root, _marker: PhantomData }
+    }
+}
+
 impl<BorrowType, K, V> Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge> {
     pub fn forget_node_type(
         self,