Rust 编译器看不到 Sized 的结构 [英] Rust compiler does not see structure as Sized

问题描述

我试图定义一个特征如下:

I am trying to define a trait as follows:

pub struct Parameter<A: Parameterisable>(&'static str, Box<A>);

pub trait Parameterisable {
    // Some functions
}

impl Parameterisable for i32 {}
impl Parameterisable for f64 {}

pub struct ParameterCollection(Vec<Parameter<Parameterisable>>);

即参数集合是不同类型参数的混合.但是编译会出现以下错误:

That is, the parameter collection is a mixture of parameters of different types. However compiling gives the following error:

error[E0277]: the trait bound `Parameterisable + 'static: std::marker::Sized` is not satisfied
  --> src/main.rs:10:32
   |
10 | pub struct ParameterCollection(Vec<Parameter<Parameterisable>>);
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ trait `Parameterisable + 'static: std::marker::Sized` not satisfied
   |
   = note: `Parameterisable + 'static` does not have a constant size known at compile-time
   = note: required by `Parameter`

我从这篇文章得知Vec 必须是 Sized，但似乎 Parameter 应该被调整大小(因为 Box)所以我如何说服 RustParameter 是 Sized 类型的编译器?

I am aware from this post that Vec must be Sized, but it seems that Parameter should be sized (because of the Box) so how do I convince the Rust compiler that Parameter is a Sized type?

推荐答案

Rust 中的类型参数有一个 隐式Sized绑定，除非另有说明(通过添加?Sized绑定).

Type parameters in Rust have an implicit Sized bound unless otherwise specified (by adding a ?Sized bound).

所以 Parameter 结构声明是有效的:

So the Parameter struct declaration is effectively:

pub struct Parameter<A: Parameterisable+Sized>(&'static str, Box<A>);

请注意，Parameter 本身总是有大小的，因为 &'static str 和 Box 总是有大小的；边界只是说 T 也必须调整大小.

Note that Parameter<T> is always itself sized, since &'static str and Box<A> are always sized; the bound just says that T must also be sized.

错误消息支持了这一点；它是说 Parameterisable 不是 Sized，不是 Parameter 不是 Sized.

The error message backs this up; it's saying that Parameterisable is not Sized, not that Parameter<Parametrerisable> is not Sized.

所以正确的更改是添加 ?Sized 绑定:

So the correct change is to add the ?Sized bound:

pub struct Parameter<A: Parameterisable+?Sized>(&'static str, Box<A>);

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