添加引用和数字值时了解(自动?)取消引用/强制 [英] Understanding (automatic?) Deref/coercion when adding references and values of numbers

问题描述

如何理解下面这段代码?我是 Rust 的新手，但有 C/Haskell 和一点 C++ 的背景知识.我能找到的唯一参考是 deref coercions.

How to make sense of the following piece of code? I'm new to Rust but have background on C/Haskell and a little bit C++. The only reference I can find is to deref coercions.

fn main() {
    let xs: [u32; 4] = [0, 1, 2, 3];
    let mut i: u32 = 0;
    for x in xs.iter() {
        if i > *x {     // It looks x is an iterator. Understood.            
            i = i + x;  // no error. (coerced)
                        //Quote: "Rust will do this as many times
                        //       as possible until the types match."
            i = i + *x; // no error (explicit deref)
            i += x;     // error about u32/&u32 mismatch. Why the magic failed?
            i += *x;    // no error (explicit deref)
        }
    }
    println!("{}", i);
}

推荐答案

这里没有 auto-deref 或 coercion，i + x 工作只是因为 u32 实现了两者Add 和 Add<&u32>.如果您查看 u32 的文档，你会发现以下四个特征实现:

There is no auto-deref or coercion here, i + x works simply because u32 implements both Add<u32> and Add<&u32>. If you check the docs for u32, you'll find the following four trait impls:

impl Add<u32> for u32
impl<'a> Add<u32> for &'a u32
impl<'a> Add<&'a u32> for u32
impl<'a, 'b> Add<&'a u32> for &'b u32

u32 只实现了 AddAssign 而不是 AddAssign<&u32> (这是一个错误 和 将在 1.18 或 1.19 中修复 修复它导致回归，这意味着这个 impl 可能需要等待 Rust 2.0)，所以 i += x 是一个错误.

u32 only implements AddAssign<u32> but not AddAssign<&u32> (this is a bug and will be fixed in 1.18 or 1.19 fixing it causes regression which means this impl probably needs to wait for Rust 2.0), so i += x is an error.

impl AddAssign<u32> for u32
//impl<'a> AddAssign<&'a u32> for u32 <-- is missing.

<小时>

为什么自动解引用不会发生?— Auto-deref 仅在它是接收者时发生，即方法调用 foo.bar() 中的self".x 不是self"参数，+ 不是方法调用.所以这里没有自动取消引用.有关详细信息，请参阅 Rust 的确切自动取消引用规则是什么?.

---

Why does auto-dereferencing not happen? — Auto-deref only happens when it is a receiver i.e. the "self" in a method call foo.bar(). x is not a "self" argument and + is not a method call. So there's no auto-deref here. See What are Rust's exact auto-dereferencing rules? for detail.

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