rust - 用特征别名替换特征绑定(bind)说 "the size for values cannot be known at compilation time"

Question

这个问题在这里已经有了答案:





Sized is not implemented for the type Fn

(1 个回答)


2年前关闭。




在阅读了 trait aliases 之后，我试图切换这段代码:

pub fn authorize<LoadClient>(get_client: LoadClient) -> Result<String, ()> 
where
   LoadClient: FnOnce(String) -> Result<Client, LoadingError>,
{
    unimplemented!()
}

到

#![feature(trait_alias)]

trait LoadClient = FnOnce(String) -> Result<Client, ClientLoadingError>;

pub fn authorize(get_client: LoadClient) -> Result<String, ()> {
    unimplemented!()
}

这给了我一个错误:

warning: trait objects without an explicit `dyn` are deprecated
   --> src/oauth2/mod.rs:396:21
    |
396 |         get_client: LoadClient,
    |                     ^^^^^^^^^^ help: use `dyn`: `dyn LoadClient`
    |
    = note: `#[warn(bare_trait_objects)]` on by default

error[E0277]: the size for values of type `(dyn std::ops::FnOnce(std::string::String) -> std::result::Result<oauth2::Client, oauth2::ClientLoadingError> + 'static)` cannot be known at compilation time
   --> src/oauth2/mod.rs:396:9
    |
396 |         get_client: LoadClient,
    |         ^^^^^^^^^^ doesn't have a size known at compile-time
    |

是否可以以这种方式使用特征别名？此函数在其他地方使用，因此最好有一个共享定义，而不是在每个使用它的地方重新定义它。

这与 Can match on Result here be replaced with map_err and "?" 有关

Answer 1

您仍然需要将其用作通用参数:

#![feature(trait_alias)]

pub trait LoadClient = FnOnce(String) -> Result<Client, ClientLoadingError>;

pub fn authorize<T>(get_client: T) -> Result<String, ()>
where
    T: LoadClient,
{
    unimplemented!()
}

您还需要公开 trait 别名，因为它是公共(public)接口(interface)的一部分。