actix-web/guide/src/qs_4_5.md

Errors

Actix uses Error type and ResponseError trait for handling handler's errors. Any error that implements ResponseError trait can be returned as error value. Handler can return Result object, actix by default provides Responder implementation for compatible result types. Here is implementation definition:

impl<T: Responder, E: Into<Error>> Responder for Result<T, E>

And any error that implements ResponseError can be converted into Error object. For example if handler function returns io::Error, it would be converted into HttpInternalServerError response. Implementation for io::Error is provided by default.

# extern crate actix_web;
# use actix_web::*;
use std::io;

fn index(req: HttpRequest) -> io::Result<fs::NamedFile> {
    Ok(fs::NamedFile::open("static/index.html")?)
}
#
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }

Custom error response

To add support for custom errors, all we need to do is just implement ResponseError trait for custom error. ResponseError trait has default implementation for error_response() method, it generates 500 response.

# extern crate actix_web;
#[macro_use] extern crate failure;
use actix_web::*;

#[derive(Fail, Debug)]
#[fail(display="my error")]
struct MyError {
   name: &'static str
}

/// Use default implementation for `error_response()` method
impl error::ResponseError for MyError {}

fn index(req: HttpRequest) -> Result<&'static str, MyError> {
    Err(MyError{name: "test"})
}
#
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }

In this example index handler always returns 500 response. But it is easy to return different responses for different type of errors.

# extern crate actix_web;
#[macro_use] extern crate failure;
use actix_web::*;

#[derive(Fail, Debug)]
enum MyError {
   #[fail(display="internal error")]
   InternalError,
   #[fail(display="bad request")]
   BadClientData,
   #[fail(display="timeout")]
   Timeout,
}

impl error::ResponseError for MyError {
    fn error_response(&self) -> HttpResponse {
       match *self {
          MyError::InternalError => HttpResponse::new(
              StatusCode::INTERNAL_SERVER_ERROR, Body::Empty),
          MyError::BadClientData => HttpResponse::new(
              StatusCode::BAD_REQUEST, Body::Empty),
          MyError::Timeout => HttpResponse::new(
              StatusCode::GATEWAY_TIMEOUT, Body::Empty),
       }
    }
}

fn index(req: HttpRequest) -> Result<&'static str, MyError> {
    Err(MyError::BadClientData)
}
#
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }

Error helpers

Actix provides set of error helper types. It is possible to use them to generate specific error responses. We can use helper types for the first example with custom error.

# extern crate actix_web;
#[macro_use] extern crate failure;
use actix_web::*;

#[derive(Debug)]
struct MyError {
   name: &'static str
}

fn index(req: HttpRequest) -> Result<&'static str> {
    let result: Result<&'static str, MyError> = Err(MyError{name: "test"});
    
    Ok(result.map_err(|e| error::ErrorBadRequest(e))?)
}
# fn main() {
#     Application::new()
#         .resource(r"/a/index.html", |r| r.f(index))
#         .finish();
# }

In this example BAD REQUEST response get generated for MyError error.

Error logging

Actix logs all errors with WARN log level. If log level set to DEBUG and RUST_BACKTRACE is enabled, backtrace get logged. The Error type uses cause's error backtrace if available, if the underlying failure does not provide a backtrace, a new backtrace is constructed pointing to that conversion point (rather than the origin of the error). This construction only happens if there is no underlying backtrace; if it does have a backtrace no new backtrace is constructed.

You can enable backtrace and debug logging with following command:

>> RUST_BACKTRACE=1 RUST_LOG=actix_web=debug cargo run