如何在模式匹配中使用 box 关键字? [英] How do I use the box keyword in pattern matching?

问题描述

此代码显示在 The Rust编程语言:

This code is shown in The Rust Programming Language:

#![feature(box_syntax, box_patterns)]

fn main() {
    let b = Some(box 5);
    match b {
        Some(box n) if n < 0 => {
            println!("Box contains negative number {}", n);
        }
        Some(box n) if n >= 0 => {
            println!("Box contains non-negative number {}", n);
        }
        None => {
            println!("No box");
        }
        _ => unreachable!(),
    }
}

但是当我运行它时，出现以下错误:

But when I run it, the following error occurs:

error[E0554]: #[feature] may not be used on the stable release channel

我也试过

fn main() {
    let b = Some(box 5);
}

error: box expression syntax is experimental; 

是不是因为我的 Rust 版本不是最新的?如何获取 Box::new() 中的内容?我试过了

Is it because my version of Rust is not the latest? How I can get the content in Box::new()? I tried

fn main() {
    let b = Some(Box::new(5));
    match b {
        Some(Box::new(y)) => print!("{:?}", y), 

        _ => print!("{:?}", 1),
    }
}

error[E0164]: `Box::new` does not name a tuple variant or a tuple struct
--> main.rs:6:14
  |
6 |         Some(Box::new(y)) => print!("{:?}", y), 
  |              ^^^^^^^^^^^ not a tuple variant or struct

推荐答案

您正在使用 #[feature] 而那些只能与 夜间 Rust 编译器.我认为目前不可能在稳定的 Rust 中匹配 Box，但 nightly 允许采用以下方式进行匹配(就像您在开始时尝试的那样):

You are using a #[feature] and those can only be used with a nightly Rust compiler. I don't think it is currently possible to match against a Box in stable Rust, but nightly allows the following way of doing it (like you attempted in the beginning):

#![feature(box_patterns)]

fn main() {
    let b = Some(Box::new(5));
    match b {
        Some(box y) => print!("{:?}", y),
        _ => print!("{:?}", 1),
    }
}

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