Trait issues with Tower layers

[blinsay]

Summary

I've been having trouble trying to use tower-http's path normalization middleware and I keep running into gnarly trait compatibility issues. Here's an repro slightly modified from the README example and the example in the docs.

I'd love a pointer and suggestions on how to debug this. I didn't see one, but it'd be awesome to have a #[debug_layer] macro equivalent of #[debug_handler].

use std::sync::Arc;

use axum::ServiceExt;
use axum::{Router, extract::State, response::IntoResponse, routing::get};
use tower::ServiceBuilder;
use tower_http::ServiceBuilderExt;
use tower_http::request_id::MakeRequestUuid;

#[tokio::main]
async fn main() {
    let app = Router::new()
        .route("/", get(root))
        .route("/hello", get(hello))
        .with_state(Arc::new(AppState {
            thing: "buddy".to_string(),
        }));

    let middleware = ServiceBuilder::new()
        .set_x_request_id(MakeRequestUuid)
        .layer(tower_http::normalize_path::NormalizePathLayer::trim_trailing_slash())
        .propagate_x_request_id();
    let app = middleware.layer(app);

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app.into_make_service())
        .await
        .unwrap();
}

struct AppState {
    thing: String,
}

async fn root() -> &'static str {
    "hey pal"
}

async fn hello(state: State<Arc<AppState>>) -> impl IntoResponse {
    format!("hello {thing}", thing = state.thing)
}

here's my Cargo.toml

[package]
name = "axum-tower-slashes"
version = "0.1.0"
edition = "2024"

[dependencies]
axum = { version = "0.8.4", features = ["json"] }
tokio = { version = "1.45.1", features = ["full"] }
tower = "0.5.2"
tower-http = { version = "0.6.6", features = ["catch-panic", "normalize-path", "request-id", "util"] }

The compiler output is fairly inscrutable:

$ cargo run
   Compiling axum-tower-slashes v0.1.0 (/Users/benl/scratch/axum-tower-slashes)
error[E0599]: the method `into_make_service` exists for struct `ServiceBuilder<Stack<Router<_>, Stack<..., ...>>>`, but its trait bounds were not satisfied
   --> src/main.rs:22:37
    |
22  |     let app = middleware.layer(app).into_make_service();
    |                                     ^^^^^^^^^^^^^^^^^ method cannot be called on `ServiceBuilder<Stack<Router<_>, Stack<..., ...>>>` due to unsatisfied trait bounds
    |
   ::: /Users/benl/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tower-0.5.2/src/builder/mod.rs:106:1
    |
106 | pub struct ServiceBuilder<L> {
    | ---------------------------- doesn't satisfy `_: Service<_>` or `_: ServiceExt<_>`
    |
    = note: the following trait bounds were not satisfied:
            `ServiceBuilder<Stack<Router<_>, Stack<PropagateRequestIdLayer, Stack<NormalizePathLayer, Stack<SetRequestIdLayer<MakeRequestUuid>, Identity>>>>>: Service<_>`
            which is required by `ServiceBuilder<Stack<Router<_>, Stack<PropagateRequestIdLayer, Stack<NormalizePathLayer, Stack<SetRequestIdLayer<MakeRequestUuid>, Identity>>>>>: axum::ServiceExt<_>`
            `&ServiceBuilder<Stack<Router<_>, Stack<PropagateRequestIdLayer, Stack<NormalizePathLayer, Stack<SetRequestIdLayer<MakeRequestUuid>, Identity>>>>>: Service<_>`
            which is required by `&ServiceBuilder<Stack<Router<_>, Stack<PropagateRequestIdLayer, Stack<NormalizePathLayer, Stack<SetRequestIdLayer<MakeRequestUuid>, Identity>>>>>: axum::ServiceExt<_>`
            `&mut ServiceBuilder<Stack<Router<_>, Stack<PropagateRequestIdLayer, Stack<NormalizePathLayer, Stack<SetRequestIdLayer<MakeRequestUuid>, Identity>>>>>: Service<_>`
            which is required by `&mut ServiceBuilder<Stack<Router<_>, Stack<PropagateRequestIdLayer, Stack<NormalizePathLayer, Stack<SetRequestIdLayer<MakeRequestUuid>, Identity>>>>>: axum::ServiceExt<_>`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `axum-tower-slashes` (bin "axum-tower-slashes") due to 1 previous error

axum version

0.8.4

[mladedav]

You need to change let app = middleware.layer(app); to let app = middleware.service(app);.

[blinsay]

It did work on me when I tried with latest main where we started allowing more body types inside serve so I'm confident that you just need to map one of the http bodies to axum::body::Body.

Is there a recommended way to apply a group of middleware without becoming a tower internals expert?

[blinsay]

To map the body, should I be using tower's map_request, axum's map_request, or something else entirely?

[blinsay]

@mladedav ooops, my bad. I was confused because I tried your suggestion to use ServiceExt::<axum::extract::Request> and but was importing tower's ServiceExt. With axum::ServiceExt it works perfectly. Thank you!

[mladedav]

Is there a recommended way to apply a group of middleware without becoming a tower internals expert?

I have to say I never had to use more than one middleware outside the router (path normalization), I think you could just put the request-x-id ones inside the router and there should not be any problems. That's beside the point of the original question but kind of due to that I don't really have any recommendation on how to do this any other way.

So my only recommendation would be to put as much stuff inside the router so it runs after routing instead of wrapping the Router.

The error you faced later will be fixed in next version of axum, so I guess that lowers the bar of being able to do this at least a little. Sadly, I don't think there is a good way for us to improve the error message. Although trying to make something like debug_router could be an interesting approach. Though I don't think we have had enough issues like this one to really prioritize it.

To map the body, should I be using ...

There's map_{request,response}_body on ServiceBuilder behind specific features of tower-http, so that would be the easiest if you had to do that. See the discussion I linked before, the original post uses it.

Glad it works for you now.

[blinsay]

I have to say I never had to use more than one middleware outside the router (path normalization), I think you could just put the request-x-id ones inside the router and there should not be any problems. That's beside the point of the original question but kind of due to that I don't really have any recommendation on how to do this any other way.

I've been trying hard not to care. I wanted one way to re-use a stack of middleware across a handful of small HTTP services, and having to do path normalization outside the Router made trying to do everything outside the Router feel like the path of least resistance.

It's the only real sore spot we've hit dealing with HTTP. Thanks for all your hard work on axum. 😸