相关疑难解决方法(0)

我看到它std::convert::Into有一个实现任何实现的东西std::convert::From:

impl<T, U> Into<U> for T where U: From<T>

有更具体的实现From,虽然Into目前只有3个特定的实现,这使得它似乎是From默认实现的主流决定.我确信有些时候我只想实现Into而不是From,但我没有看到它们.

这个测试代码(围栏):

use std::fmt::{Display, Formatter, Error};

struct MyLocalType;

type MyResult = Result<MyLocalType, String>;

impl Display for MyResult {
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        f.write_str("some test string")
    }
}

fn main() { 
    let r: MyResult = Ok(MyLocalType); 
    println!("{}" , r); 
}

生成此错误消息:

<anon>:7:1: 11:2 error: the impl does not reference any types defined in this crate; only traits defined in the current crate can be implemented for arbitrary types [E0117]
<anon>:7 impl Display for MyResult { …