Type annotation needed when use byteorder with vector

Question

I need to build a packet to send to the server. I use byteorder crate for this. One of my field contains usize data, so I use typecasting to convert it to the i16:

use byteorder::WriteBytesExt;

fn main () {
    let login = "test";
    let packet_length = 30 + (login.len() as i16);
    let mut packet = Vec::new();
    packet.write_u8(0x00);
    packet.write_i16(packet_length);
    packet.append(&mut Vec::from(String::from("game name ").as_bytes_mut()));
    // ... rest code
}

But I got an error:

packet.write_i16(packet_length);
 |     ^^^^^^^^^ cannot infer type for type parameter `T` declared on the associated function `write_i16`

This is playground to reproduce. https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1e5dd29816a128f7561e06f7c825864d

I am confused where I should to add type annotation here, could you please help?