如何使用关联常量来定义数组的长度？

Question

我有一个特征，它表示可以通过 UDP 套接字发送的实体：

pub trait ToNetEnt {
    const NET_SIZE: usize;

    fn from_net(data: &[u8]) -> Self;
    fn to_net(&self) -> &[u8];
}

虽然有一个关联常量 NET_SIZE，但我无法在方法中使用它：

pub fn req<T: ToNetEnt>(&self) -> T {
    let buf: [u8; T::NET_SIZE];
}

因为它导致了这个错误：

error[E0599]: no associated item named `NET_SIZE` found for type `T` in the current scope
  --> src/main.rs:10:23
   |
10 |         let buf: [u8; T::NET_SIZE];
   |                       ^^^^^^^^^^^ associated item not found in `T`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `NET_SIZE`, perhaps you need to implement it:
           candidate #1: `ToNetEnt`

我可以在此上下文中使用关联常量吗？

Answer 1

要访问关联常量，您需要明确说明它来自哪个特征，使用这种稍微麻烦的语法：<T as ToNetEnt>::NET_SIZE.

Answer 2

在撰写本文时，这是不可能的。但让我们先修复您的代码：

pub fn req<T: ToNetEnt>(&self) -> T {
    let buf: [u8; <T as ToNetEnt>::NET_SIZE];
    ...
}

playground

这应该是正确的语法，但目前无法编译：

error[E0277]: the trait bound `T: ToNetEnt` is not satisfied
 --> src/main.rs:6:19
  |
6 |     let buf: [u8; <T as ToNetEnt>::NET_SIZE];
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `ToNetEnt` is not implemented for `T`
  |
  = help: consider adding a `where T: ToNetEnt` bound

这只是针对问题的错误错误消息。见 this comment on GitHub:

[U]sing an associated const as an array size does not appear to work:
pub trait Can {
    const SIZE: usize;
}

fn f<T: Can>(t: T) {
    // error[E0277]: the trait bound `T: Can` is not satisfied
    let x = [0u8; <T as Can>::SIZE];
}
There error message is clearly wrong, so this is either a bug in rustc or unimplemented functionality resulting in a bogus error message.

直接在结构中定义 const NET_SIZE 时可以解决此问题。

您可以在针对此特定错误的 GitHub 问题中阅读更多相关信息：Array lengths don't support generic type parameters (#43408)。

如何使用关联常量来定义数组的长度？

How to use associated constants to define the length of an array?

function

traits

rust