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

问题

I've got a trait which represents an entity that can be sent through UDP sockets:

pub trait ToNetEnt {
    const NET_SIZE: usize;

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

Though there is an associated constant NET_SIZE, I couldn't use it in methods:

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

because it causes this error:

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`

Can I use an associated constant in this context?

回答1:

At time of writing this is not possible. But let's first fix your code:

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

playground

This should be the right syntax, but does not compile at the moment:

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

which is simply a wrong error message to the problem. See 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.

You can workaround this when defining the const NET_SIZE in your struct directly.

You can read more about this at the GitHub issue for this specific bug: Array lengths don't support generic type parameters (#43408).

回答2:

To access an associated constant, you need to explicitly say which trait it is coming from, using this slightly cumbersome syntax: <T as ToNetEnt>::NET_SIZE.

来源：https://stackoverflow.com/questions/51131229/how-to-use-associated-constants-to-define-the-length-of-an-array

标签

function

rust

traits