如何修复Actix Web中Result枚举类型参数E无法推导的编译错误？

我正在开发一款在线LaTeX编辑器，用户可在浏览器中编辑LaTeX文档。当用户点击「编译」按钮时，希望将服务器端的LaTeX编译日志推送给客户端，因此选择使用SSE实现该功能。编写了如下Actix Web服务端SSE API代码：

use actix_web::{middleware::Logger, web, App, HttpRequest, HttpResponse, HttpServer};
use futures::StreamExt;

async fn sse_handler(req: HttpRequest, stream: web::Payload) -> HttpResponse {
    let mut res = HttpResponse::Ok()
        .content_type("text/event-stream")
        .streaming(Box::pin(stream.map(|chunk| {
            Ok(actix_web::web::Bytes::from(format!(
                "data: {}\n\n",
                String::from_utf8(chunk.unwrap().to_vec()).unwrap()
            )))
        })));

    if req.headers().get("cache-control").is_none() {
        res.headers_mut().insert(
            actix_web::http::header::CACHE_CONTROL,
            actix_web::http::header::HeaderValue::from_static("no-cache"),
        );
    }

    res
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| {
        App::new()
            .wrap(Logger::default())
            .route("/events", web::get().to(sse_handler))
    })
    .bind("127.0.0.1:8080")?
    .run()
    .await
}

编译器报错：

error[E0283]: type annotations needed
   --> src/main.rs:8:13
    |
7   |         .streaming(Box::pin(stream.map(|chunk| {
    |          --------- type must be known at this point
8   |             Ok(actix_web::web::Bytes::from(format!(
    |             ^^ cannot infer type of the type parameter `E` declared on the enum `Result`
    |
    = note: cannot satisfy `_: Into<Box<(dyn std::error::Error + 'static)>>`
note: required by a bound in `HttpResponseBuilder::streaming`
   --> /Users/xiaoqiangjiang/.cargo/registry/src/mirrors.tuna.tsinghua.edu.cn-df7c3c540f42cdbd/actix-web-4.3.1/src/response/builder.rs:324:12
    |
324 |         E: Into<BoxError> + 'static,
    |            ^^^^^^^^^^^^^^ required by this bound in `HttpResponseBuilder::streaming`
help: consider specifying the generic arguments
    |
8   |             Ok::<actix_web::web::Bytes, E>(actix_web::web::Bytes::from(format!(
    |               ++++++++++++++++++++++++++++

Cargo.toml配置：

[package]
name = "rust-learn"
version = "0.1.0"
edition = "2018"

[dependencies]
tokio = { version = "1.17.0", features = ["full"] }
serde = { version = "1.0.64", features = ["derive"] }
serde_json = "1.0.64"
actix-web = "4"
futures = "0.3"

修复方案 #

错误原因

编译器无法推断Result枚举中的错误类型E，因为streaming方法要求错误类型必须满足Into<BoxError>约束。同时原代码逻辑错误：直接将客户端请求的Payload作为SSE流返回，这和推送服务器端编译日志的需求不符，且使用unwrap()处理错误既不安全，也导致类型推断失败。

修复后的代码

use actix_web::{middleware::Logger, web, App, HttpRequest, HttpResponse, HttpServer, error::BoxError};
use futures::StreamExt;
use std::convert::Infallible;

async fn sse_handler(req: HttpRequest) -> HttpResponse {
    // 生成服务器端编译日志的异步流
    let log_stream = async_stream::stream! {
        // 模拟实际编译过程中的日志输出
        let compile_logs = [
            "正在初始化编译环境...",
            "加载LaTeX模板与依赖包...",
            "编译主文档，生成中间文件...",
            "生成PDF文档，优化文件大小...",
            "编译完成，可下载PDF文件"
        ];

        for log in compile_logs {
            // 模拟编译步骤间隔，实际场景替换为真实编译日志的异步读取
            tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
            yield Ok(actix_web::web::Bytes::from(format!("data: {}\n\n", log)));
        }
    };

    let mut res = HttpResponse::Ok()
        .content_type("text/event-stream")
        .streaming(Box::pin(log_stream));

    if req.headers().get("cache-control").is_none() {
        res.headers_mut().insert(
            actix_web::http::header::CACHE_CONTROL,
            actix_web::http::header::HeaderValue::from_static("no-cache"),
        );
    }

    res
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| {
        App::new()
            .wrap(Logger::default())
            .route("/events", web::get().to(sse_handler))
    })
    .bind("127.0.0.1:8080")?
    .run()
    .await
}

关键修复点

1. 修正流逻辑：移除客户端Payload参数，改为生成服务器端的编译日志流，符合SSE推送服务器数据的需求。
2. 明确错误类型：使用Infallible作为错误类型（模拟场景无错误），若实际编译中有错误，可替换为BoxError并在流中处理错误分支。
3. 安全错误处理：移除unwrap()调用，避免运行时panic，同时让编译器能正确推断类型。
4. 添加异步流依赖：使用async-stream crate创建异步流，需在Cargo.toml中添加该依赖。

更新后的Cargo.toml

[package]
name = "rust-learn"
version = "0.1.0"
edition = "2018"

[dependencies]
tokio = { version = "1.17.0", features = ["full"] }
serde = { version = "1.0.64", features = ["derive"] }
serde_json = "1.0.64"
actix-web = "4"
futures = "0.3"
async-stream = "0.3" # 添加异步流依赖

内容的提问来源于stack exchange，提问作者Dolphin