rust - 无法实现io::Read以获得自定义特征

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

如注释中所述，当前不允许这样做:

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

您正在尝试为实现 Read的所有类型实现 Bar，但是不能保证 Bar的所有实现(现在或将来)都不会(或不会)已经实现 Read。例如，您的 crate 用户可能为自己的一种类型实现 Bar。
第二个错误说明了另一个问题:

= 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

仅当在 crate 中定义了特征(未指定 Read)或在 crate 中定义了要实现其特征的类型时，才可以实现该特征。这意味着您不能为 Read实现 T，因为 T实际上是任何类型，包括来自其他 crate 的任意类型。
一种解决方法是添加一个函数，该函数从任意 Read中公开 T: Bar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<'a, T: Bar> Read for BarRead<&'a T> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        Ok(0)
    }
}