Rust CondVar 交替打印|ifbiu blog

```rust
use std::sync::{Arc, Condvar, Mutex};
use std::thread;
use std::time::Duration;

#[derive(Clone, Debug)]
enum Next {
    Child,
    Main,
}
fn main() {
    let next = Arc::new(Mutex::new(Next::Main));
    let cond = Arc::new(Condvar::new());

let next2 = next.clone();
    let cond2 = cond.clone();

for i in 1..=3 {
        eprintln!("main:\t{}", i);
        let mut next_flag = next.lock().unwrap();
        *next_flag = Next::Child; // 下一个进行打印的是child线程
        drop(next_flag);
        cond.notify_one();
        thread::sleep(Duration::from_secs(1));
        //睡一秒, 给child线程提供上锁的机会.
    }
    handle.join().unwrap();
}
```