Rust compiled by wasm-pack

example.rs

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
extern {
  pub fn callback(res: i32);
}

#[wasm_bindgen]
pub fn add(a: i32, b: i32) -> i32 {
  return a + b;
}

#[wasm_bindgen]
pub fn add_callback(a: i32, b: i32) {
  callback(a + b)
}

example.wat

(module
  (type (;0;) (func (param i32)))
  (type (;1;) (func (param i32 i32)))
  (type (;2;) (func (param i32 i32) (result i32)))
  (import "wbg" "__wbg_callback_330497ee23f1aa0e" (func (;0;) (type 0)))
  (func (;1;) (type 1) (param i32 i32)
    local.get 0
    local.get 1
    i32.add
    call 0)
  (func (;2;) (type 2) (param i32 i32) (result i32)
    local.get 0
    local.get 1
    i32.add)
  (memory (;0;) 17)
  (export "memory" (memory 0))
  (export "add" (func 2))
  (export "add_callback" (func 1)))