Rust 模式匹配如何确定绑定变量是引用还是值? [英] How does Rust pattern matching determine if the bound variable will be a reference or a value?

问题描述

use crate::List::{Cons, Nil};

#[derive(Debug)]
struct Foo {}

#[derive(Debug)]
enum List {
    Cons(i32, Foo),
    Nil,
}

impl List {
    fn tail(&self) -> Option<&Foo> {
        match self {
            Cons(_, item) => Some(item), // why `item` is of type `&Foo`?
            Nil => None,
        }
    }
}

如评论中所述，为什么 item 是 &Foo 类型?item 的类型是 &Foo 而不是 Foo 的规则是什么?

As stated in the comment, why is item of type &Foo? What is the rule that says item will be type &Foo rather than Foo?

我知道将 item 设为 Foo 是没有意义的；&self 说 self 是一个引用，所以从引用中移出一个值是没有意义的，但是有没有明确定义规则的规范?

I understand that it does not make sense for item to be Foo; &self says self is a reference, so it does not make sense to move a value out of a reference, but are there any specifications that define the rules clearly?

推荐答案

RFC 2005(又名匹配人体工程学)引入了该规则.

RFC 2005 (a.k.a. match ergonomics) introduced the rule.

在实施更改之前，有两种方法可以编写此match.

Before the change was implemented, there were 2 ways to write this match.

1. 匹配 self 并在每个模式前加上 & 以解构"引用.

1. Match on self and prefix each pattern with & to "destructure" the reference.

fn tail(&self) -> Option<&Foo> {
    match self {
        &Cons(_, ref item) => Some(item),
        &Nil => None,
    }
}

匹配 *self 并且不要在每个模式前加上 &(因为 *self 不是引用).

Match on *self and don't prefix each pattern with & (because *self is not a reference).

fn tail(&self) -> Option<&Foo> {
    match *self {
        Cons(_, ref item) => Some(item),
        Nil => None,
    }
}

然而，在这两种情况下，我们都需要编写ref item，否则我们会得到error[E0507]: cannot move out of Bored content.

Yet, in both cases, we need to write ref item, otherwise we'll get error[E0507]: cannot move out of borrowed content.

然而，在您编写的 match 中，被匹配的表达式是一个引用(类型 &List)，但模式不是引用模式(如1. 以上).这就是匹配人体工程学发挥作用的地方:规则说，当引用与非引用模式匹配时，该模式中的绑定通过引用而不是通过值绑定(即，好像它们以 refref).

However, in the match you've written, the expression being matched is a reference (type &List) but the patterns are not reference patterns (as in 1. above). This is where match ergonomics kick in: the rule says that when a reference is matched with a non-reference pattern, the bindings within that pattern bind by reference rather than by value (i.e. as if they were prefixed with ref).

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