我应该用 ; 结束一个表达式吗?在一个循环内? [英] Should I end an expression with ; inside a loop?

问题描述

我刚开始学习 Rust，经常遇到一些我找不到任何答案的问题.我真的不知道如何以及在哪里发布我的问题，所以我会尝试.

I just started learning Rust and I often have some questions that I can't find any answer. I don't really know how and where to post my question, so I'll try SO.

我开始阅读网站上的 Rust 文档，并做了猜谜游戏示例.

I started to read the Rust documentation on the website and I did the Guessing game example.

我意识到循环内的 match cmp 表达式可以变成一个语句，一切仍然有效.所以我想知道为什么和哪个版本应该是首选?

I realized that the match cmp expression inside the loop could be turned into a statement and everything still works. So I wonder why and what version should really be preferred?

use rand::Rng;
use std::cmp::Ordering;
use std::io;

fn main() {
    println!("Guess the number!");

    let secret_number = rand::thread_rng().gen_range(1, 101);

    loop {
        println!("Please input your guess.");

        let mut guess = String::new();

        io::stdin()
            .read_line(&mut guess)
            .expect("Failed to read line");

        let guess: u32 = match guess.trim().parse() {
            Ok(num) => num,
            Err(_) => continue,
        };

        println!("You guessed: {}", guess);

        match guess.cmp(&secret_number) {
            Ordering::Less => println!("Too small!"),
            Ordering::Greater => println!("Too big!"),
            Ordering::Equal => {
                println!("You win!");
                break;
            }
        } // <=== here should it be ended as statement (;) or not?
    }
}

推荐答案

match、if、loop 和其他具有 {} 块由 Rust 编译器特别处理.当这些表达式作为表达式语句出现时，即不是作为更大表达式的一部分，并且有块的表达式的类型是()，你不必把;放在后面将其与以下语句分开.

match, if, loop, and other expressions that have {} blocks are treated specially by the Rust compiler. When these expressions occur as expression statements, that is, not as part of a larger expression, and the type of the expression that has the block is (), you do not have to put ; after it to separate it from a following statement.

这不是没有块的表达式语句的情况，它总是必须通过;与下面的语句(如果存在)分开，即使它们的类型是().

This is not the case for expression statements without blocks, which always must be separated from the following statement (if one exists) by ;, even if their type is ().

因为这个规则存在，在Rust中通常不把;放在match之后，if、unsafe 等仅用于副作用，或在 for 和 while 循环(它们是总是只用于副作用，因为它们总是返回 ()).

Because this rule exists, it is usual in Rust not to put ; after match, if, unsafe, etc. when they are used only for side effects, or after for and while loops (which are always used only for side effects, since they always return ()).

也就是说，rustfmt 和 Clippy 似乎都可以使用额外的 ;，所以如果您出于审美原因更喜欢它，那么使用它就不太可能冒犯任何人.

That said, both rustfmt and Clippy seem to be fine with the extra ;, so if you prefer it there for aesthetic reasons, you're unlikely to offend anyone by using it.

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