How do I insert a dynamic byte string into a vector? - rust

Question

I need to create packet to send to the server. For this purpose I use vector with byteorder crate. When I try to append string, Rust compiler tells I use unsafe function and give me an error.
use byteorder::{LittleEndian, 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::<LittleEndian>(packet_length);
packet.append(&mut Vec::from(String::from("game name ").as_bytes_mut()));
// ... rest code
}
The error is:
packet.append(&mut Vec::from(String::from("game name ").as_bytes_mut()));
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
This is playground to reproduce: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=381c6d14660d47beaece15d068b3dc6a
What is the correct way to insert some string as bytes into vector ?

Answer 1

The unsafe function called was as_bytes_mut(). This creates a mutable reference with exclusive access to the bytes representing the string, allowing you to modify them. You do not really need a mutable reference in this case, as_bytes() would have sufficed.
However, there is a more idiomatic way. Vec<u8> also functions as a writer (it implements std::io::Write), so you can use one of its methods, or even the write! macro, to write encoded text on it.
use std::io::Write;
use byteorder::{LittleEndian, WriteBytesExt};
fn main () -> Result<(), std::io::Error> {
let login = "test";
let packet_length = 30 + (login.len() as i16);
let mut packet = Vec::new();
packet.write_u8(0x00)?;
packet.write_i16::<LittleEndian>(packet_length)?;
let game_name = String::from("game name");
write!(packet, "{} ", game_name)?;
Ok(())
}
Playground
See also:
Use write! macro with a string instead of a string literal
What's the de-facto way of reading and writing files in Rust 1.x?

Answer 2

You can use .extend() on the Vec and pass in the bytes representation of the String:
use byteorder::{LittleEndian, 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::<LittleEndian>(packet_length);
let string = String::from("game name ");
packet.extend(string.as_bytes());
}
Playground