Problems with type inference

[GigaFirmDev]

Lately, I was dealing with this problem where rust cannot infer the type generics of the route. I even had to resort to axum examples to check if it was a mistake but the result stayed the same:

let app = Router::new()
        // `GET /` goes to `root`
        .route("/", get(root))
        // `POST /users` goes to `create_user`
        .route("/users", post(create_user));

This is the portion of the code that is causing me trouble. This is what rust said about it:

error[E0283]: type annotations needed
   --> src/main.rs:14:15
    |
14  |     let app = Router::new()
    |               ^^^^^^^^^^^ cannot infer type of the type parameter `B` declared on the struct `Router`
    |
    = note: cannot satisfy `_: HttpBody`
    = help: the following types implement trait `HttpBody`:
              &mut T
              Body
              Box<T>
              Pin<P>
              Request<B>
              Response<B>
              StreamBody<S>
              axum::body::Empty<D>
            and 7 others
note: required by a bound in `Router::<S, B>::new`
   --> PATH
    |
105 |     B: HttpBody + Send + 'static,
    |        ^^^^^^^^ required by this bound in `Router::<S, B>::new`
...
112 |     pub fn new() -> Self {
    |            --- required by a bound in this associated function
help: consider specifying the generic arguments
    |
14  |     let app = Router::<S, B>::new()
    |                     ++++++++

For more information about this error, try `rustc --explain E0283`.
error: could not compile `backend` (bin "backend") due to previous error

[davidpdrsn]

If you actually run the router with

axum::Server::bind(&addr)
    .serve(app.into_make_service())
    .await
    .unwrap();

the compile is able to infer the types.

[davidpdrsn]

@jplatte The trait bounds on serve requires the request body type to be Body:

Though its not exactly obvious 😅

[jplatte]

I just saw that there's still a B parameter and assumed the method was generic. Both the use of Body and B being inside generic parameter lists of private types doesn't make this one easier to figure out 😬

[davidpdrsn]

I believe B is the response body.

[davidpdrsn]

I just saw that there's still a B parameter and assumed the method was generic. Both the use of Body and B being inside generic parameter lists of private types doesn't make this one easier to figure out 😬

hehe yes

[jplatte]

So hyper determines the fixed request body, and axum determines the fixed response body. Really looking forward to hyper 1.0 / axum 0.7 to get most people off of this overly-generic function 😄