为什么宏卫生不能阻止多个const定义之间的冲突? [英] Why does macro hygiene not prevent collisions between multiple const definitions?

问题描述

我认为卫生"可以防止在宏m!中定义的X之间发生冲突，但事实并非如此.我有什么误会?

I thought "hygiene" would prevent collisions between Xs defined within my macro m! but that turned out not to be the case. What am I misunderstanding?

macro_rules! m {
    ($e:expr) => {
        const X: i32 = $e;
    };
}

m!(0);
m!(1);

fn main() {
    m!(2);
    m!(3);
}

游乐场

错误消息:

error[E0428]: the name `X` is defined multiple times
 --> src/main.rs:3:9
  |
3 |         const X: i32 = $e;
  |         ^^^^^^^^^^^^^^^^^^
  |         |
  |         `X` redefined here
  |         previous definition of the value `X` here
...
7 | m!(0);
  | ------ in this macro invocation
  |
  = note: `X` must be defined only once in the value namespace of this module

推荐答案

来自 Rust编程语言(第一版)的

此[即重命名]用于let绑定和循环标签，但不适用于项目

This [i.e. renaming] holds for let bindings and loop labels, but not for items

Rust参考定义了项目:

The Rust reference defines items:

一个物品是一个箱子的组成部分.物品放在箱子里组织 由一组嵌套的模块组成.每个板条箱都只有一个最外侧" 匿名模块；箱子中所有其他物品的路径都在 板条箱的模块树.

An item is a component of a crate. Items are organized within a crate by a nested set of modules. Every crate has a single "outermost" anonymous module; all further items within the crate have paths within the module tree of the crate.

项目完全在编译时确定，通常保持固定 在执行过程中，可能驻留在只读存储器中.

Items are entirely determined at compile-time, generally remain fixed during execution, and may reside in read-only memory.

有几种项目:

• 模块
• extern crate声明
• use声明
• 功能定义
• 类型定义
• 结构定义
• 枚举定义
• 联盟定义
• 常数项
• 静态物品
• 特征定义
• 实现
• extern块

• modules
• extern crate declarations
• use declarations
• function definitions
• type definitions
• struct definitions
• enumeration definitions
• union definitions
• constant items
• static items
• trait definitions
• implementations
• extern blocks

这很有道理:如果您在宏中引入一个项目，您可能想从其他项目/模块/框架(因此在宏之外)中实际使用它，但是如果您不知道它，则无法使用名称，因此编译器无法重命名.

This makes sense: if you you introduce an item in a macro you presumably want to actually use it from other items/modules/crates (and thus outside the macro), but you can't if you don't know its name, so the compiler can't rename it.

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