Rust E0277错误：FnMut闭包签名不匹配如何正确传handler

我为多个结构体实现了Processor trait以复用相似逻辑，该trait提供collect_responses函数，接收handler向量并返回各handler生成的响应向量。当前代码无法编译，抛出如下错误：

error[E0277]: expected a `FnMut<(HandlerInput<'_>,)>` closure, found `dyn for<'r, 's> FnMut(&'r 
mut HandlerInput<'s>) -> Result<HandlerOutput, std::io::Error>`
   --> src/lib.rs:146:18
    |
146 |             self.process(handler, vec![]);
    |                  ^^^^^^^ expected an `FnMut<(HandlerInput<'_>,)>` closure, found `dyn for<'r, 's> FnMut(&'r mut HandlerInput<'s>) -> Result<HandlerOutput, std::io::Error>`
    |
    = help: the trait `FnMut<(HandlerInput<'_>,)>` is not implemented for `dyn for<'r, 's> 
    FnMut(&'r mut HandlerInput<'s>) -> Result<HandlerOutput, std::io::Error>`
    = note: expected a closure with arguments `(&mut HandlerInput<'_>,)`
           found a closure with arguments `(HandlerInput<'_>,)`
    = note: required because of the requirement

沙箱示例可查看完整上下文。

涉及的trait代码如下：

pub trait Processor {
    fn process_input(input: HandlerInput) -> Result<Vec<HandlerOutput>, Error>;

    fn collect_responses(
        handlers: Vec<Box<dyn FnMut(&mut HandlerInput) -> Result<HandlerOutput, Error>>>,
        input: HandlerInput
    ) -> Result<Vec<HandlerOutput>, Error> {
        let responses = handlers
            .iter()
            .map(|func| func(&mut input))
            .filter(|r| match r {
                Ok(_) => true,
                _ => false,
            })
            .map(|r| r.unwrap())
            .collect::<Vec<HandlerOutput>>();

        return if responses.is_empty() {
            Err(
                Error::new(
                    ErrorKind::Other,
                    "No successful response"
                )
            )
        } else {
            Ok(responses)
        }
    }
}

报错位置的方法代码如下：

fn process<F>(&mut self, handler: F, data: Vec<u8>)
    where
        F: FnMut(HandlerInput) -> Result<HandlerOutput, Error>
{
    let response: Result<HandlerOutput, Error> = handler(HandlerInput {
        session: &mut self.session,
        data: Some(data),
    });

    match response.unwrap() {
        HandlerOutput::Data(result) => {
            self.handle_write(result).unwrap();
        },
        HandlerOutput::ConnectionRequest(host, port) => {
            self.connect(&host[..], port);
        },
        HandlerOutput::Void => {},
    }
}

更新： 调用process方法的代码如下：

let handlers = AuthProcessor::init();
for handler in handlers {
    self.process(handler, vec![]);
}

请问如何才能正确向self.process传入handler参数？

核心问题是闭包参数签名不匹配：

• 你存入Vec的handler类型是Box<dyn FnMut(&mut HandlerInput) -> Result<HandlerOutput, Error>>，要求接收&mut HandlerInput可变引用作为参数
• 但process方法的泛型约束要求F: FnMut(HandlerInput) -> Result<HandlerOutput, Error>，要求接收HandlerInput所有权作为参数
  两者是完全独立的FnMut实现，编译器无法自动转换，因此直接传参会报类型不匹配错误。

统一两处的参数签名即可，二选一：

方案1（最小改动）：修改process方法约束，匹配现有handler签名 #

和collect_responses的调用逻辑保持一致，将process的泛型约束改为接收可变引用，调整内部调用逻辑：

fn process<F>(&mut self, mut handler: F, data: Vec<u8>)
    where
        F: FnMut(&mut HandlerInput) -> Result<HandlerOutput, Error>
{
    let mut input = HandlerInput {
        session: &mut self.session,
        data: Some(data),
    };
    let response: Result<HandlerOutput, Error> = handler(&mut input);

    match response.unwrap() {
        HandlerOutput::Data(result) => {
            self.handle_write(result).unwrap();
        },
        HandlerOutput::ConnectionRequest(host, port) => {
            self.connect(&host[..], port);
        },
        HandlerOutput::Void => {},
    }
}

方案2：修改handler集合类型，匹配现有process签名 #

如果你确实需要handler获取HandlerInput的所有权，就把collect_responses的参数类型、AuthProcessor::init返回的Vec元素类型统一改成Box<dyn FnMut(HandlerInput) -> Result<HandlerOutput, Error>>。注意这种写法下调用多个handler时，需要为每个handler单独构造新的HandlerInput实例，否则第一个handler拿走所有权后后续变量无法使用。

内容的提问来源于stack exchange，提问作者Sergio Ivanuzzo