How to fix "the trait Factory<_, _, _> is not implemented for {function}" error in actix_web?

I have this simple code:

use actix_web::{get, post, web, App, HttpResponse, HttpServer, Responder};
use serde::{Deserialize, Serialize};
use serde_json::{json};
use validator::{Validate, ValidationError};

#[derive(Deserialize, Serialize, Validate)]
struct Auth {
    #[validate(length(min = 4))]
    username: String,
    password: String,
}

#[post("/auth")]
async fn auth(form: web::Json<Auth>) -> Result<serde_json::Value, ValidationError> {
    Ok(json!{ "success" })
}

This keeps giving me this error:

error[E0277]: the trait bound `fn(Json<Auth>) -> impl std::future::Future {auth}: Factory<_, _, _>` is not satisfied
  --> src/main.rs:77:10
   |
77 | async fn auth(form: web::Json<Auth>) -> Result<serde_json::Value, ValidationError> {
   |          ^^^^ the trait `Factory<_, _, _>` is not implemented for `fn(Json<Auth>) -> impl std::future::Future {auth}`

What am I doing wrong?

I figured it out. Changing Result<serde_json::Value, ValidationError> to HttpResponse and returning the json in HttpResponse worked.

#[post("/auth")]
async fn auth(form: web::Json<Auth>) -> HttpResponse {
    match form.validate() {
        Ok(_) => HttpResponse::Ok().json(json!{ {"success":true}}),
        Err(e) => HttpResponse::NotAcceptable().body(format!("{}", e))
    }
}