actix-web/guide/src/qs_4_5.md

# Errors

Actix uses [`Error` type](../actix_web/error/struct.Error.html) 
and [`ResponseError` trait](../actix_web/error/trait.ResponseError.html) 
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:

```rust,ignore
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.

```rust
# 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.

```rust
# 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.

```rust
# 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.

```rust
# 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
```