如何将 Debug 与特征对象一起使用？

Question

我想在

Debug

结构上实现

Post

特征，该结构有一个特征对象作为值，但我从下面得到堆栈溢出错误：

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

use std::fmt::{Debug, Display, Formatter, Result as fmtResult};

#[derive(Debug)]
pub struct Post {
    state: Option<Box<dyn State>>,
    content: String,
}

trait State {
    fn request_review(self: Box<Self>) -> Box<dyn State>;
    fn approve(self: Box<Self>) -> Box<dyn State>;
    fn content<'a>(&self, post: &'a Post) -> &'a str {
        ""
    }
}

impl Debug for dyn State {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmtResult {
        write!(f, "{}", self)
    }
}

impl Display for dyn State {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmtResult {
        write!(f, "{}", self)
    }
}

Answer 1

如果你想要任何东西实现

State

也实现

Debug

，那么你可以这样做：

trait State: Debug {
        // ^^^^^^^
    fn request_review(self: Box<Self>) -> Box<dyn State>;
    fn approve(self: Box<Self>) -> Box<dyn State>;
    ...
}

然后

#[derive(Debug)]

将在

Post

上运行，无需任何手动实现。 游乐场上的完整代码。