这是我的测试代码:
use std::error::Error;
use std::fmt;
struct Handler {
error: String
}
#[derive(Debug)]
struct SpecificError;
impl fmt::Display for SpecificError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "SpecificError")
}
}
impl Error for SpecificError {}
impl<E: Error> From<E> for Handler {
fn from(e: E) -> Self {
Handler { error: format!("{}", e) }
}
}
fn fail1() -> Result<(), SpecificError> {
Err(SpecificError)
}
fn fail2() -> Result<(), Box<Error>> {
Err(Box::new(SpecificError))
}
fn handler() -> Result<(), Handler> {
fail1()?;
fail2()?;
Ok(())
}
对fail1()
的调用很好,但是对fail2()
的调用不能编译:
error[E0277]: the size for values of type `dyn std::error::Error` cannot be known at compilation time
--> src/main.rs:35:5
|
35 | fail2()?;
| ^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `dyn std::error::Error`
= note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<dyn std::error::Error>`
= note: required because of the requirements on the impl of `std::convert::From<std::boxed::Box<dyn std::error::Error>>` for `Handler`
= note: required by `std::convert::From::from`
我同意编译器dyn Error
在编译时没有已知的大小,但我不明白为什么这是相关的,因为我试图转换的类型是Box<dyn Error>
,它的编码大小已知时间。
TL; DR:我很确定你不能以通用的方式。
我不明白为什么这是相关的,因为我试图转换的类型是
Box<dyn Error>
,它在编译时确实有一个大小。
这不是它抱怨的地方。再次查看错误消息(稍微清理一下):
the trait `Sized` is not implemented for `dyn Error`
required because of the requirements on the impl of `Error` for `Box<dyn Error>`
required because of the requirements on the impl of `From<Box<dyn Error>>` for `Handler`
required by `From::from`
第二行是重要的一行。这是您问题的简单再现:
use std::error::Error;
fn example<E: Error>() {}
fn main() {
example::<Box<dyn Error>>();
}
error[E0277]: the size for values of type `dyn std::error::Error` cannot be known at compilation time
--> src/main.rs:6:5
|
6 | example::<Box<dyn Error>>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `dyn std::error::Error`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<dyn std::error::Error>`
note: required by `example`
--> src/main.rs:3:1
|
3 | fn example<E: Error>() {}
| ^^^^^^^^^^^^^^^^^^^^^^
Error
只是implemented for Box<T>
when T
is Sized
并实施Error
本身:
impl<T: Error> Error for Box<T> {
// ...
}
换句话说,Box<dyn Error>
没有实施Error
。
有人可能认为你可以为From
添加Box<Error>
的第二个实现,但这是不允许的:
upstream crates may add new impl of trait `std::error::Error` for type
`std::boxed::Box<(dyn std::error::Error + 'static)>` in future versions
我必须提供的最佳替代方案是为您需要支持的每种具体类型实施From
:
impl From<SpecificError> for Handler {
fn from(e: SpecificError) -> Self {
Handler { error: format!("{}", e) }
}
}
impl From<Box<dyn Error>> for Handler {
fn from(e: Box<dyn Error>) -> Self {
Handler { error: format!("{}", e) }
}
}
宏可以减少这里的样板。