Help: consider constraining the associated type `<T as ValidateArgs<'a>>::Args` to `&'life0 S`

[sibu-github]

I am trying to write a custom extractor which will also validate the JSON body with validator crate. To validate some of the fields I need custom function with args. Here is my code -

#[tokio::main]
async fn main() {
    println!("Starting...");
    dotenv().ok();
    let uri = std::env::var("MONGODB_URI").expect("MONGODB_URI not found in .env file");
    let min_pool = std::env::var("MONGODB_MIN_POOL_SIZE").unwrap_or_default();
    let max_pool = std::env::var("MONGODB_MAX_POOL_SIZE").unwrap_or_default();
    let min_pool = min_pool.parse::<u32>().unwrap_or(5);
    let max_pool = max_pool.parse::<u32>().unwrap_or(10);
    let timeout = Duration::from_secs(5);
    // create the mongodb client options
    let mut client_options = ClientOptions::parse(uri).await.unwrap();
    client_options.max_pool_size = Some(max_pool);
    client_options.min_pool_size = Some(min_pool);
    client_options.connect_timeout = Some(timeout);
    // create the client
    let db_client = Client::with_options(client_options).unwrap();
    let app = Router::new()
        .route("/", get(handler).post(post_handler))
        .with_state(db_client);
    let addr = SocketAddr::from(([0, 0, 0, 0], 3000));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

pub struct ValidatedBody<T>(pub T);

#[async_trait]
impl<'a, S, B, T> FromRequest<S, B> for ValidatedBody<T>
where
    B: Send + 'static,
    S: Send + Sync,
    T: ValidateArgs<'a> + 'static,
    Json<T>: FromRequest<(), B>,
{
    type Rejection = (StatusCode, &'static str);

    async fn from_request(req: Request<B>, state: &S) -> Result<Self, Self::Rejection> {
        let Json(data) = req
            .extract::<Json<T>, _>()
            .await
            .map_err(|_| (StatusCode::BAD_REQUEST, "Invalid JSON body"))?;
        data.validate_args(state)
            .map_err(|_| (StatusCode::BAD_REQUEST, "Invalid JSON body"))?;
        Ok(Self(data))
    }
}

async fn handler() -> &'static str {
    "Hello from server"
}

#[derive(Debug, Serialize, Deserialize, Validate)]
struct MyReqBody {
    #[validate(custom(function = "validate_name", arg = "&'v_a Client"))]
    name: String,
}

fn validate_name(name: &str, db_client: &Client) -> Result<(), ValidationError> {
    Ok(())
}

async fn post_handler(
    State(client): State<Client>,
    ValidatedBody(req_body): ValidatedBody<MyReqBody>,
) -> impl IntoResponse {
    println!("{:?}", req_body);
    (StatusCode::OK, Json(serde_json::json!({"success": true})))
}

The problem is that inside from_request function I am not able to call data.validate_args with one argument.
This is the error I am getting -

error[E0308]: mismatched types
   --> src/main.rs:60:28
    |
60  |         data.validate_args(state)
    |              ------------- ^^^^^ expected associated type, found `&S`
    |              |
    |              arguments to this function are incorrect
    |
    = note: expected associated type `<T as ValidateArgs<'a>>::Args`
                     found reference `&'life0 S`
    = help: consider constraining the associated type `<T as ValidateArgs<'a>>::Args` to `&'life0 S`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
note: associated function defined here
   --> /Users/nebuchadnezzar/.cargo/registry/src/github.com-1ecc6299db9ec823/validator-0.16.0/src/traits.rs:211:8
    |
211 |     fn validate_args(&self, args: Self::Args) -> Result<(), ValidationErrors>;
    |        ^^^^^^^^^^^^^

Is there any way I can achieve this?

[quinedot]

I'm not familiar with the crate, so this is somewhat of a long shot, but does the following change help?

-impl<'a, S, B, T> FromRequest<S, B> for ValidatedBody<T>
+impl<S, B, T> FromRequest<S, B> for ValidatedBody<T>
 where
     B: Send + 'static,
     S: Send + Sync,
-    T: ValidateArgs<'a> + 'static,
+    T: 'static + for<'any> ValidateArgs<'any, Args = &'any S>,
     Json<T>: FromRequest<(), B>,

Edit: Guess I'll throw in an explanation of how I got there as it may be more generally useful, whether this works or not. I looked at the trait definition, then looking at you're code you're doing

    async fn from_request(req: Request<B>, state: &S) -> Result<Self, Self::Rejection> {
        // ...
        data.validate_args(state)
            .map_err(|_| (StatusCode::BAD_REQUEST, "Invalid JSON body"))?;

So you need ValidateArgs::Args to be &S with the same lifetime as the state parameter:

impl<'a, S, B, T> ...
where
    T: 'static + ValidateArgs<'a, Args = &'a S>,
//                                ^^^^^^^^^^^^
{
    async fn from_request(req: Request<B>, state: &'a S) -> Result<Self, Self::Rejection> {
//                                                 ^^^

But that won't work because this is a trait implementation where you need from_request to be able to accept a state with any arbitrary lifetime (you can't change it to state: &'a S). So instead you need the higher-ranked bound I've suggested above, so ValidateArgs::Args can match state's type for any arbitrary lifetime.

Whether it works or not probably depends on how the trait is derived.