类型绑定中的特征“未实现”？

Question

Consider the following code, which uses a trait to make the length of an array fixed in the type.

use std::marker::PhantomData;

pub trait MyLen {
    const LEN: usize;
}

pub struct Data<L: MyLen> {
    bytes: [u8; <L as MyLen>::LEN],
    _phantom: PhantomData<L>,
}

I could swear I saw similar code work a couple days ago, but now I'm seeing

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `L: MyLen` is not satisfied
 --> src/lib.rs:8:17
  |
4 |     const LEN: usize;
  |     ----------------- required by `MyLen::LEN`
...
8 |     bytes: [u8; <L as MyLen>::LEN],
  |                 ^^^^^^^^^^^^^^^^^ the trait `MyLen` is not implemented for `L`
  |
help: consider further restricting this bound
  |
7 | pub struct Data<L: MyLen + MyLen> {
  |                          ^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`

This error message doesn't make sense to me, especially not the suggestion. Any ideas what I'm doing wrong?

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=49c9a11106a46125eaa9612560966bac

Answer 1

First of all, you'll need to use theconst_generics feature to be able to do that. However, that's not entirely enough, as you'll also need to use the const_evaluatable_checked feature .

When you add the const_generics feature, then the issue becomes apparent with a more descriptive compiler error.

error: constant expression depends on a generic parameter
  --> src\main.rs:37:12
   |
37 |     bytes: [u8; <L as MyLen>::LEN],
   |            ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this may fail depending on what value the parameter takes

error: constant expression depends on a generic parameter
  --> src\main.rs:34:30
   |
34 |     [u8; <L as MyLen>::LEN]: Sized,
   |                              ^^^^^
   |
   = note: this may fail depending on what value the parameter takes

The missing part is where [u8; <L as MyLen>::LEN]: where [u8; <L as MyLen>::LEN]: (or where [u8; <L as MyLen>::LEN]: Sized ) which requires the const_evaluatable_checked feature. The where clause is needed, to ensure that LEN doesn't evaluate into something that overflows or otherwise invalidates that array.

You can read more about it on this HackMD post by the Rust Compiler Team.

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]
#![allow(incomplete_features)]

use std::marker::PhantomData;

pub trait MyLen {
    const LEN: usize;
}

pub struct Data<L: MyLen>
where
    [u8; <L as MyLen>::LEN]: ,
    // or
    // [u8; <L as MyLen>::LEN]: Sized,
{
    bytes: [u8; <L as MyLen>::LEN],
    _phantom: PhantomData<L>,
}

Which of course requires a nightly compiler to build.