type Async_cb = fn (x []u8, mut y []u8) int

fn async_cb(b []u8, mut res []u8) int {
    if b.len > 0 {
        res << b
    }
    return 0
}

struct Ep_arg {
mut:
    sfd int
    cb  Async_cb
}

fn test_struct_fn_field_can_be_used_directly() {
    buf := [u8(1), 2, 3]
    mut res := []u8{}
    res << 0x88
    async_cb(buf[0..2], mut res)
    data := Ep_arg{
        sfd: 1234
        cb:  async_cb
    }
    data.cb(buf[1..2], mut res)
    res << 0x99
    eprintln(res)
    assert res == [u8(0x88), 0x01, 0x02, 0x02, 0x99]
}