HelloWorld in WebAssembly

a.wat

(module
    (import "host" "print_char" (func $print_char (param i32)))
    (func (export "main")
        i32.const 72
        call $print_char
        i32.const 101
        call $print_char
        i32.const 108
        call $print_char
        i32.const 108
        call $print_char
        i32.const 111
        call $print_char
        i32.const 32
        call $print_char
        i32.const 87
        call $print_char
        i32.const 111
        call $print_char
        i32.const 114
        call $print_char
        i32.const 108
        call $print_char
        i32.const 100
        call $print_char
        i32.const 33
        call $print_char
        i32.const 10
        call $print_char
    )
)

main.rs

use std::fs;

use wasmtime::{Engine, Func, Instance, Module, Store};

fn main() {
    let wat = fs::read_to_string("a.wat").unwrap();

    let engine = Engine::default();
    let module = Module::new(&engine, &wat).unwrap();

    let mut store = Store::new(&engine, 4);

    let print_char = Func::wrap(&mut store, |x: i32| print!("{}", x as u8 as char));

    let instance = Instance::new(&mut store, &module, &[print_char.into()]).unwrap();

    let wasm_main = instance
        .get_typed_func::<(), (), _>(&mut store, "main")
        .unwrap();
    wasm_main.call(&mut store, ()).unwrap();
}