Rust — 为什么这里有警告 "value captured by ` online` is never read"？

Question

为什么 Rust 编译器会发出此警告？

value captured by online is never read

use std::{thread, time::Duration};
use rand::Rng;

fn main() {
    let mut online = false;

    thread::spawn(move || {
        loop {
            if let Some(_) = get_rand_option() {
                online = true;    // why is there a warning here?
            } else {
                online = false;
            }

            if online {           // is the value not being read here?
                println!("Yee");
            } else {
                println!("...");
            }
        }
    });

    thread::sleep(Duration::from_millis(100000));
}

fn get_rand_option() -> Option<i32> {
    rand::thread_rng().gen::<Option<i32>>()
}

Answer 1

Rust 足够聪明，知道您将在使用该变量之前对其进行初始化，因此它会告诉您在此之前为其赋值是多余的。我还将变量声明移动到闭包内，否则它会永久移动到闭包内。

请注意，对于这个最小的示例，您最好在循环内声明 online，因为您永远不会跳出它或使用上一次迭代的值:let online =如果让……

use std::{thread, time::Duration};
use rand::Rng;

fn main() {

    thread::spawn(|| {
        let mut online: bool;
        loop {
            // using the fact that ‘if let’ is an expression
            online = if let Some(_) = get_rand_option() {
                true
            } else {
                false
            };

            if online {         
                println!("Yee");
            } else {
                println!("...");
            }
        }
    });

    thread::sleep(Duration::from_millis(100000));
}

fn get_rand_option() -> Option<i32> {
    rand::thread_rng().gen::<Option<i32>>()
}