为什么不能在方法体中使用“ Self”来引用枚举的变量？ [英] Why can't `Self` be used to refer to an enum's variant in a method body?

问题描述

该问题现在已过时，因为已实现了此功能。 相关答案。

This question is now obsolete because this feature has been implemented. Related answer.

以下Rust代码无法编译：

The following Rust code fails to compile:

enum Foo {
    Bar,
}

impl Foo {
    fn f() -> Self {
        Self::Bar
    }
}

错误消息使我感到困惑：

The error message confuses me:

error[E0599]: no associated item named `Bar` found for type `Foo` in the current scope
 --> src/main.rs:7:9
  |
7 |         Self::Bar
  |         ^^^^^^^^^

可以使用 Foo 而不是 Self ，但这使我感到奇怪，因为 Self 应该是指正在执行的类型（忽略特征），在这种情况下为 Foo 。

The problem can be fixed by using Foo instead of Self, but this strikes me as strange since Self is supposed to refer to the type that is being implemented (ignoring traits), which in this case is Foo.

enum Foo {
    Bar,
}

impl Foo {
    fn f() -> Self {
        Foo::Bar
    }
}

为什么 Self 在这种情况下不能使用？ Self 到底可以在哪里使用 ^* ？还有什么我可以用来避免在方法主体中重复类型名称的东西？

Why can't Self be used in this situation? Where exactly can Self be used^*? Is there anything else I can use to avoid repeating the type name in the method body?

^{* 特质，其中 Self 指实现特质的任何类型。}

^{* I'm ignoring usage in traits, where Self refers to whatever type implements the trait.}

推荐答案

要注意的重要一点是错误表示关联的项目。 enum Foo {Baz} 没有关联的项目。一个特征可以具有一个关联的项目：

An important thing to note is that the error said associated item. enum Foo { Baz } doesn't have associated items. A trait can have an associated item:

trait FooBaz { type Baz }
//             ^~~~~~~~ - associated item

总结一下：

在这种情况下为什么不能使用 Self ？

Why can't Self be used in this situation?

由于此问题。 RFC 2338 已尚未实施。

Self 似乎充当类型别名，尽管进行了一些修改。

Self seems to act as a type alias, albeit with some modifications.

自我到底在哪里使用？

自我只能用于特征和 impl 中。这段代码：

Self can only be used in traits and impls. This code:

struct X {
    f: i32,
    x: &Self,
}

输出以下内容：

error[E0411]: cannot find type `Self` in this scope
 --> src/main.rs:3:9
  |
3 |     x: &Self,
  |         ^^^^ `Self` is only available in traits and impls

这可能是暂时情况，将来可能会改变！

This is possibly a temporary situation and might change in the future!

更准确地说， Self 应该仅用作方法签名的一部分（例如 fn self_in_self_out（& self）-> Self ）或访问关联的类型：

More precisely, Self should be used only as part of method signature (e.g. fn self_in_self_out(&self) -> Self) or to access an associated type:

enum Foo {
    Baz,
}

trait FooBaz {
    type Baz;

    fn b(&self) -> Self::Baz; // Valid use of `Self` as method argument and method output
}


impl FooBaz for Foo {
    type Baz = Foo;

    fn b(&self) -> Self::Baz {
        let x = Foo::Baz as Self::Baz; // You can use associated type, but it's just a type
        x
    }
}

我认为 user4815162342覆盖了其余的最佳答案。

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