actix-website/examples/errors/src/recommend_one.rs

// <recommend-one>
use actix_web::{
    error, get,
    http::{header::ContentType, StatusCode},
    App, HttpResponse, HttpServer,
};
use derive_more::derive::{Display, Error};

#[derive(Debug, Display, Error)]
enum UserError {
    #[display("Validation error on field: {field}")]
    ValidationError { field: String },
}

impl error::ResponseError for UserError {
    fn error_response(&self) -> HttpResponse {
        HttpResponse::build(self.status_code())
            .insert_header(ContentType::html())
            .body(self.to_string())
    }
    fn status_code(&self) -> StatusCode {
        match *self {
            UserError::ValidationError { .. } => StatusCode::BAD_REQUEST,
        }
    }
}
// </recommend-one>

#[get("/")]
async fn index() -> Result<&'static str, UserError> {
    Err(UserError::ValidationError {
        field: "bad stuff".to_string(),
    })
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| App::new().service(index))
        .bind(("127.0.0.1", 8080))?
        .run()
        .await
}