Adding wasm logic

Adding logic on the wasm / Rust side is very much just like writing a (C compatible) shared library.

Let's begin simple. This function creates a Lua instance and returns the raw pointer to it.

#[no_mangle]
pub extern "C" fn lua_new() -> *mut mlua::Lua {
    let lua = mlua::Lua::new();
    Box::into_raw(Box::new(lua))
}

Alright, good. Now we have a Lua instance, but no way to use it, so let us create one.
The function takes the pointer to the Lua struct we create in the new_lua function as well as an arbitrary string, which should be lua code, as parameters. It then executes this string via the Lua instance and may write to stderr if an error occurs.

#[no_mangle]
pub unsafe extern "C" fn lua_execute(lua: *mut mlua::Lua, to_execute: *const std::ffi::c_char) {
    // casting the raw pointer of the created lua instance back to a usable Rust struct
    let lua: &mut mlua::Lua = &mut *lua;
    // converting the c string into a `CStr` (which then can be converted to a `String`)
    let to_execute = std::ffi::CStr::from_ptr(to_execute);
    
    // execute the input code via the lua interpreter
    if let Err(err) = lua.load(&to_execute.to_string_lossy().to_string()).exec() {
        // because emscripten wraps stderr, we are able to catch the error on the js
        // side just fine
        eprintln!("{}", err)
    }
}

Okay, this looks great! In theory. So let's head over to the next page to see how to compile the code to make it actually usable via Javascript.