using layer RequestDecompressionLayer with Router.merge

[yanns]

In the following repository, the layer RequestDecompressionLayer is being used to decompress requests' bodies.

It's working great. (see other question)

This code is compiling:

    let app: Router = Router::new()
        .route("/", post(root))
        .layer(
            ServiceBuilder::new()
                .layer(HandleErrorLayer::new(|_: BoxError| async move {
                    (StatusCode::INTERNAL_SERVER_ERROR, "Unhandled server error")
                }))
                .layer(RequestDecompressionLayer::new())
        );

but if I want to use the merge function, the code does not compile anymore:

    let otherRouter: Router = Router::new().route("/test", post(root));
    let app: Router = Router::new()
        .route("/", post(root))
        .merge(otherRouter)
        .layer(
            ServiceBuilder::new()
                .layer(HandleErrorLayer::new(|_: BoxError| async move {
                    (StatusCode::INTERNAL_SERVER_ERROR, "Unhandled server error")
                }))
                .layer(RequestDecompressionLayer::new()),
        );

The compilation error is the following:

error[E0277]: the trait bound `HandleError<RequestDecompression<Route>, [closure@src/main.rs:19:46: 19:59], _>: Service<Request<_>>` is not satisfied
   --> src/main.rs:18:13
    |
17  |           .layer(
    |            ----- required by a bound introduced by this call
18  | /             ServiceBuilder::new()
19  | |                 .layer(HandleErrorLayer::new(|_: BoxError| async move {
20  | |                     (StatusCode::INTERNAL_SERVER_ERROR, "Unhandled server error")
21  | |                 }))
22  | |                 .layer(RequestDecompressionLayer::new()),
    | |________________________________________________________^ the trait `Service<Request<_>>` is not implemented for `HandleError<RequestDecompression<Route>, [closure@src/main.rs:19:46: 19:59], _>`
    |
    = help: the following other types implement trait `Service<Request>`:
              HandleError<S, F, ()>
              HandleError<S, F, (T1,)>
              HandleError<S, F, (T1, T2)>
              HandleError<S, F, (T1, T2, T3)>
              HandleError<S, F, (T1, T2, T3, T4)>
              HandleError<S, F, (T1, T2, T3, T4, T5)>
              HandleError<S, F, (T1, T2, T3, T4, T5, T6)>
              HandleError<S, F, (T1, T2, T3, T4, T5, T6, T7)>
            and 9 others
note: the method call chain might not have had the expected associated types
   --> src/main.rs:19:18
    |
18  |               ServiceBuilder::new()
    |               --------------------- this expression has type `ServiceBuilder<Identity>`
19  |                   .layer(HandleErrorLayer::new(|_: BoxError| async move {
    |  __________________^
20  | |                     (StatusCode::INTERNAL_SERVER_ERROR, "Unhandled server error")
21  | |                 }))
    | |___________________^ `Layer::Service` is `HandleError<_, [[email protected]:19:46], _>` here
22  |                   .layer(RequestDecompressionLayer::new()),
    |                    --------------------------------------- `Layer::Service` remains `HandleError<RequestDecompression<_>, [[email protected]:19:46], _>` here
note: required by a bound in `Router::<S, B>::layer`
   --> /Users/yannsimon/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.6.20/src/routing/mod.rs:236:21
    |
233 |     pub fn layer<L, NewReqBody>(self, layer: L) -> Router<S, NewReqBody>
    |            ----- required by a bound in this associated function
...
236 |         L::Service: Service<Request<NewReqBody>> + Clone + Send + 'static,
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Router::<S, B>::layer`

For more information about this error, try `rustc --explain E0277`.

In general, I find it quite difficult to fix issues with types on Layer. Any hints appreciated.

[davidpdrsn]

The issue is not merge, its the type annotation in

let other_router: Router = Router::new().route("/test", post(|| async {}));
//              ^^^^^^^^

The issue is that RequestDecompressionLayer requires a service that accepts a DecompressionBody<B> (where B is some generic body type).

But when you write other_router: Router you're setting the default body type param for Router which is B. Thus you're saying other_router: Router<(), axum::body::Body> (() is the default state). Router<(), axum::body::Body> doesn't accept DecompressionBody<B>, it only accepts axum::body::Body.

The solution is to leave off the type annotation and do

#[tokio::main]
async fn main() {
    let other_router = Router::new().route("/test", post(|| async {}));

    let app = Router::new()
        .route("/", post(|| async {}))
        .merge(other_router)
        .layer(
            ServiceBuilder::new()
                .layer(HandleErrorLayer::new(|_: BoxError| async move {
                    (StatusCode::INTERNAL_SERVER_ERROR, "Unhandled server error")
                }))
                .layer(RequestDecompressionLayer::new()),
        );

    axum::Server::bind(&"0.0.0.0:3000".parse().unwrap())
        .serve(app.into_make_service())
        .await
        .expect("server failed");
}

Now the compiler is able to infer that the request body type must be DecompressionBody<axum::body::Body> so everything works.

If you need to return other_router from a function you can do this

fn make_other_router<B>() -> Router<(), B>
where
    B: axum::body::HttpBody + Send + 'static,
{
    Router::new().route("/test", post(|| async {}))
}

[yanns]

Thanks a lot!!

[yanns]

This new signature for make_other_router is giving me some headaches. It does not compile whenever I want to use the request body.

Ex:

fn make_other_router<B>() -> Router<(), B>
where
    B: axum::body::HttpBody + Send + 'static,
{
    Router::new().route("/test", post(root))
}

async fn root(Json(value): Json<Value>) -> Json<Value> {
    Json(value)
}

Compilation error:

error[E0277]: `<B as axum::body::HttpBody>::Data` cannot be sent between threads safely
   --> src/main.rs:36:39
    |
36  |     Router::new().route("/test", post(root))
    |                                  ---- ^^^^ `<B as axum::body::HttpBody>::Data` cannot be sent between threads safely
    |                                  |
    |                                  required by a bound introduced by this call
    |
    = help: the trait `std::marker::Send` is not implemented for `<B as axum::body::HttpBody>::Data`
    = note: required for `axum::Json<serde_json::Value>` to implement `axum::extract::FromRequest<(), B>`
    = note: required for `fn(axum::Json<serde_json::Value>) -> impl std::future::Future<Output = axum::Json<serde_json::Value>> {root}` to implement `axum::handler::Handler<(axum_core::extract::private::ViaRequest, axum::Json<serde_json::Value>), (), B>`
note: required by a bound in `axum::routing::post`
   --> /Users/yannsimon/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.6.20/src/routing/method_routing.rs:407:1
    |
407 | top_level_handler_fn!(post, POST);
    | ^^^^^^^^^^^^^^^^^^^^^^----^^^^^^^
    | |                     |
    | |                     required by a bound in this function
    | required by this bound in `post`
    = note: this error originates in the macro `top_level_handler_fn` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider further restricting the associated type
    |
34  |     B: axum::body::HttpBody + Send + 'static, <B as axum::body::HttpBody>::Data: std::marker::Send
    |                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[davidpdrsn]

Add the bounds the compile suggests in

34 | B: axum::body::HttpBody + Send + 'static, ::Data: std::marker::Send

[yanns]

You're right, this is better:

fn make_other_router<B>() -> Router<(), B>
where
    B: axum::body::HttpBody + Send + 'static,
    B::Data: Send,
{
    Router::new().route("/test", post(root))
}

Now I guess I need to add constraint on B::Error:

error[E0277]: the trait bound `<B as axum::body::HttpBody>::Error: std::error::Error` is not satisfied
   --> src/main.rs:44:39
    |
44  |     Router::new().route("/test", post(root))
    |                                  ---- ^^^^ the trait `std::error::Error` is not implemented for `<B as axum::body::HttpBody>::Error`
    |                                  |
    |                                  required by a bound introduced by this call
    |
    = note: required for `std::boxed::Box<(dyn std::error::Error + std::marker::Send + std::marker::Sync + 'static)>` to implement `std::convert::From<<B as axum::body::HttpBody>::Error>`
    = note: required for `<B as axum::body::HttpBody>::Error` to implement `std::convert::Into<std::boxed::Box<(dyn std::error::Error + std::marker::Send + std::marker::Sync + 'static)>>`
    = note: required for `axum::Json<serde_json::Value>` to implement `axum::extract::FromRequest<(), B>`
    = note: required for `fn(axum::Json<serde_json::Value>) -> impl std::future::Future<Output = axum::Json<serde_json::Value>> {root}` to implement `axum::handler::Handler<(axum_core::extract::private::ViaRequest, axum::Json<serde_json::Value>), (), B>`
note: required by a bound in `axum::routing::post`
   --> /Users/yannsimon/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.6.20/src/routing/method_routing.rs:407:1
    |
407 | top_level_handler_fn!(post, POST);
    | ^^^^^^^^^^^^^^^^^^^^^^----^^^^^^^
    | |                     |
    | |                     required by a bound in this function
    | required by this bound in `post`
    = note: this error originates in the macro `top_level_handler_fn` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider further restricting the associated type
    |
42  |     B::Data: Send, <B as axum::body::HttpBody>::Error: std::error::Error
    |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is working:

fn make_other_router<B>() -> Router<(), B>
where
    B: axum::body::HttpBody + Send + 'static,
    B::Data: Send,
    B::Error: Into<BoxError>,
{
    Router::new().route("/test", post(root))
}

yanns/axum-gzip@c3c8433