Define nicer Rust tree type and coerce

rust/lib.rs

@@ -42,7 +42,20 @@ extern "C" fn add(a: i64, b: i64) -> i64 {
 }
 
 #[repr(C)]
-#[derive(Clone, Copy)]
+enum BTreeBox {
+    Leaf { value: i64 },
+    Fork { left: Box<BTreeBox>, right: Box<BTreeBox> },
+}
+impl BTreeBox {
+    fn sum(&self) -> i64 {
+        match self {
+            BTreeBox::Leaf { value } => *value,
+            BTreeBox::Fork { left, right } => left.sum() + right.sum(),
+        }
+    }
+}
+
+#[repr(C)]
 enum BTree {
     Leaf {
         value: i64,
@@ -55,8 +68,5 @@ enum BTree {
 
 #[unsafe(no_mangle)]
 extern "C" fn sum_tree(t: BTree) -> i64 {
-    match t {
+    (unsafe { std::mem::transmute::<_, &BTreeBox>(&t) }).sum()
-        BTree::Leaf { value } => value,
-        BTree::Fork { left, right } => unsafe { sum_tree(*left) + sum_tree(*right) },
-    }
 }