You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: threads.rs
    path of file: ./repos/os_kernel_lab/user/src/bin
    
    the code of the file until where you have to start completion: #![no_std]
#![no_main]

#[macro_use]
extern crate user_lib;
extern crate alloc;

use alloc::vec;
use user_lib::{exit, thread_create, waittid};

pub fn thread_a() -> ! {
    for _ in 0
    
pub fn main() -> i32 {
    let v = vec![
        thread_create(thread_a as usize, 0),
        thread_create(thread_b as usize, 0),
        thread_create(thread_c as usize, 0),
    ];
    for tid in v.iter() {
        let exit_code = waittid(*tid as usize);
        println!("thread#{} exited with code {}", tid, exit_code);
    }
    println!("main thread exited.");
    0
}