rust - 无法实现自定义特征的 io::Read

Question

我想为我的自定义特征实现 std::io::Read 特征。这是我的尝试:-

use std::io::Read;

trait Bar {
    fn foo(&self);
}

impl<T: Bar> Read for T {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        Ok(0)
    }
}

Playground

错误:-

   Compiling playground v0.0.1 (/playground)
error[E0119]: conflicting implementations of trait `std::io::Read` for type `&mut _`:
 --> src/lib.rs:7:1
  |
7 | impl<T: Bar> Read for T {
  | ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `std`:
          - impl<R> std::io::Read for &mut R
            where R: std::io::Read, R: ?Sized;
  = note: downstream crates may implement trait `Bar` for type `&mut _`

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
 --> src/lib.rs:7:6
  |
7 | impl<T: Bar> Read for T {
  |      ^ type parameter `T` must be used as the type parameter for some local type
  |
  = note: implementing a foreign trait is only possible if at least one of the types for which is it implemented is local
  = note: only traits defined in the current crate can be implemented for a type parameter

error: aborting due to 2 previous errors

Answer 1

目前不允许这样做，如注释中所述:

= note: downstream crates may implement trait `Bar` for type `&mut _`

您正在尝试为所有实现 Bar 的类型实现 Read，但您不能保证 Bar 的所有实现(目前或将来)现在还没有或不会实现Read。例如，您的 crate 的用户可能会为他们自己的类型之一实现 Bar。

第二个错误中解释了另一个问题:

= note: implementing a foreign trait is only possible if at least one of the types for which is it implemented is local
= note: only traits defined in the current crate can be implemented for a type parameter

仅当您的包中定义了特征(Read 未定义)或类型时，您才可以实现该特征您要实现它的目的是在您的箱子中定义的。这意味着您无法为 T 实现 Read，因为 T 实际上是任何类型，包括来自其他 crate 。

解决此问题的方法是添加一个函数，该函数从任意 T: Bar 公开 Read 实现:

use std::io::Read;

trait Bar {
    fn foo(&self);
    fn as_reader(&self) -> BarRead<&Self> {
        BarRead(self)
    }
}

pub struct BarRead<T>(T);

impl<T: Bar> Read for BarRead<T> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        Ok(0)
    }
}