如何在特征的默认方法中使用“调试”格式化程序打印“ self”？

Question

我定义了一个特征：

pub trait SaySomething {
    fn say(&self) {
        println!("{:?} can't speak", self);
    }
}

我得到了错误：

error[E0277]: `Self` doesn't implement `std::fmt::Debug`
 --> src/lib.rs:3:38
  |
3 |         println!("{:?} can't speak", self);
  |                                      ^^^^ `Self` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
  |
  = help: the trait `std::fmt::Debug` is not implemented for `Self`
  = help: consider adding a `where Self: std::fmt::Debug` bound
  = note: required because of the requirements on the impl of `std::fmt::Debug` for `&Self`
  = note: required by `std::fmt::Debug::fmt`

所以我尝试了：

use std::fmt::Debug;

pub trait SaySomething {
    fn say<Self: Debug>(self: &Self) {
        println!("{:?} can't speak", self);
    }
}

我得到了错误：

error: expected identifier, found keyword `Self`
 --> src/lib.rs:4:12
  |
4 |     fn say<Self: Debug>(self: &Self) {
  |            ^^^^ expected identifier, found keyword

error[E0194]: type parameter `Self` shadows another type parameter of the same name
 --> src/lib.rs:4:12
  |
3 | / pub trait SaySomething {
4 | |     fn say<Self: Debug>(self: &Self) {
  | |            ^^^^ shadows another type parameter
5 | |         println!("{:?} can't speak", self);
6 | |     }
7 | | }
  | |_- first `Self` declared here

error[E0307]: invalid method receiver type: &Self
 --> src/lib.rs:4:31
  |
4 |     fn say<Self: Debug>(self: &Self) {
  |                               ^^^^^
  |
  = note: type must be `Self` or a type that dereferences to it
  = help: consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`

Answer 1

您需要将特征限制为实现Debug的类型。

pub trait SaySomething: std::fmt::Debug {
    fn say(&self) {
        println!("{:?} can't speak", self);
    }
}