Issue migrating ServeDir handler from Axum 0.6 to 0.7

[JasonGH17]

• I have looked for existing issues (including closed) about this

Bug Report

Recently started to migrate to Axum 0.7
I had a static file hosting handler setup, but when updating to 0.7 it started throwing an error at compile time.

Compile error:

type annotations needed
cannot satisfy `<_ as axum::body::HttpBody>::Data == axum::body::Bytes`
required for `HandlerService<fn() -> impl Future<Output = (StatusCode, &str)> {not_found}, ((),), ()>` to implement `tower::Service<axum::http::Request<_>>`
1 redundant requirement hidden
required for `ServeDir<SetStatus<HandlerService<fn() -> impl Future<Output = (StatusCode, &str)> {not_found}, ((),), ()>>>` to implement `tower::Service<axum::http::Request<_>>`

Version

axum: 0.7.8

Platform

Windows (64-bit)

Crates

tower: 0.4.13
tower-http: 0.5.2

Description

The snippet of code:

use axum::{
    handler::HandlerWithoutStateExt,
    http::{HeaderValue, StatusCode},
    middleware::from_fn_with_state,
    Router,
};
use tower::ServiceExt;
use tower_http::services::ServeDir;

pub fn setup_static_fs_handler() -> Router {
    async fn not_found() -> (StatusCode, &'static str) {
        (StatusCode::NOT_FOUND, "Resource not found")
    }
    let not_found_service = not_found.into_service();

    let serve = ServeDir::new("data")
        .precompressed_gzip()
        .not_found_service(not_found_service)
        .map_response(|mut res| {
            if res.status() != StatusCode::NOT_FOUND {
                res.headers_mut()
                    .insert("Cache-Control", HeaderValue::from_static("max-age=3600"));
            }
            res
        });

    Router::new().nest_service("/", serve)
}

[jplatte]

Looks like the Rust compiler can't infer the body type for the request. Where exactly is the error message pointing?

[JasonGH17]

It's pointing to
.map_response(|mut res| {

[JasonGH17]

If it matters too, I'm using Rust v1.78.0-nightly

[jplatte]

This is a really tricky one! I played around with it locally to find a solution. You can add a map_request before map_response to explicitly specify the request body type that the compiler fails to infer:

.map_request(|req: http::Request<axum::body::Body>| req)

There are other solutions, but I found them to be way too complicated. I would love if we could do something about this being so weird, but I'm not really certain how this could be fixed in tower / axum.