Make Rust tree type polymorphic

rust/lib.rs

@@ -1,6 +1,9 @@
 #![allow(dead_code)]
 
-use std::ffi::{CStr, c_char};
+use std::{
+    ffi::{CStr, c_char},
+    ops::Add,
+};
 
 fn say_hello(name: &str) {
     println!("Hello from Rust, {name}!");
@@ -42,12 +45,20 @@ extern "C" fn add(a: i64, b: i64) -> i64 {
 }
 
 #[repr(C)]
-enum BTree {
-    Leaf { value: i64 },
-    Fork { left: Box<BTree>, right: Box<BTree> },
+enum BTree<A> {
+    Leaf {
+        value: A,
+    },
+    Fork {
+        left: Box<BTree<A>>,
+        right: Box<BTree<A>>,
+    },
 }
-impl BTree {
-    fn sum(&self) -> i64 {
+impl<A> BTree<A>
+where
+    A: Add<Output = A> + Copy,
+{
+    fn sum(&self) -> A {
         match self {
             BTree::Leaf { value } => *value,
             BTree::Fork { left, right } => left.sum() + right.sum(),
@@ -68,5 +79,5 @@ enum BTreeC {
 
 #[unsafe(no_mangle)]
 extern "C" fn sum_tree(t: BTreeC) -> i64 {
-    (unsafe { std::mem::transmute::<_, &BTree>(&t) }).sum()
+    (unsafe { std::mem::transmute::<_, &BTree<i64>>(&t) }).sum()
 }