为什么“不能泄漏私有类型”？仅适用于结构而不适用于枚举？

问题描述

在，它们表明 pub枚举无法容纳私有的 struct ： ，

In Learning Rust With Entirely Too Many Linked Lists, they show that a pub enum can't hold a private struct:,

struct Node {
    elem: i32,
    next: List,
}

pub enum List {
    Empty,
    More(Box<Node>),
}

这将导致编译器抱怨：

error[E0446]: private type `Node` in public interface
 --> src/main.rs:8:10
  |
8 |     More(Box<Node>),
  |          ^^^^^^^^^^ can't leak private type

但是此代码不会即使 Link 是私有的，也会导致错误：

But this code will not cause an error even though Link is private:

pub struct List {
    head: Link,
}

enum Link {
    Empty,
    More(Box<Node>),
}

struct Node {
    elem: i32,
    next: Link,
}

此差异的原因是什么？为什么私有枚举不会在私有结构引起错误的呢？

What is the reason for this discrepancy? Why does a private enum not cause an error while a private struct does?

推荐答案

在第一个示例中，枚举列表是公开的。这意味着枚举变体 More 也是公开的。但是，更多不能被外部代码使用，因为 Node 不是公开的。因此，您所拥有的东西在外部是可见的，但实际上并不能被使用，这可能不是您想要的。

In the first example, the enum List is public. That means that the enum variant More is also public. However, More cannot be used by external code because Node isn't public. Thus, you have a thing that is externally visible, but can't actually be used, which is probably not what you wanted.

在第二个示例，结构 List 是公共的。但是， head 字段是不公开的。因此， Link 是否公开并不重要，因为外部代码看不到 head 字段首先。

In the second example, the struct List is public. However, the head field is not public. Thus, it doesn't matter whether Link is public or not, because external code can't see the head field in the first place.

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