Struct std::error::Report

pub struct Report<E = Box<dyn Error>> { /* private fields */ }

🔬This is a nightly-only experimental API. (error_reporter #90172)

An error reporter that prints an error and its sources.

Report also exposes configuration options for formatting the error sources, either entirely on a single line, or in multi-line format with each source on a new line.

Report only requires that the wrapped error implement Error. It doesn’t require that the wrapped error be Send, Sync, or 'static.

Examples

#![feature(error_reporter)]
use std::error::{Error, Report};
use std::fmt;

#[derive(Debug)]
struct SuperError {
    source: SuperErrorSideKick,
}

impl fmt::Display for SuperError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "SuperError is here!")
    }
}

impl Error for SuperError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Some(&self.source)
    }
}

#[derive(Debug)]
struct SuperErrorSideKick;

impl fmt::Display for SuperErrorSideKick {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "SuperErrorSideKick is here!")
    }
}

impl Error for SuperErrorSideKick {}

fn get_super_error() -> Result<(), SuperError> {
    Err(SuperError { source: SuperErrorSideKick })
}

fn main() {
    match get_super_error() {
        Err(e) => println!("Error: {}", Report::new(e)),
        _ => println!("No error"),
    }
}

This example produces the following output:

Error: SuperError is here!: SuperErrorSideKick is here!

Output consistency

Report prints the same output via Display and Debug, so it works well with Result::unwrap/Result::expect which print their Err variant via Debug:

#![feature(error_reporter)]
use std::error::Report;

get_super_error().map_err(Report::new).unwrap();

This example produces the following output:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: SuperError is here!: SuperErrorSideKick is here!', src/error.rs:34:40
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Return from main

Report also implements From for all types that implement Error; this when combined with the Debug output means Report is an ideal starting place for formatting errors returned from main.

#![feature(error_reporter)]
use std::error::Report;

fn main() -> Result<(), Report<SuperError>> {
    get_super_error()?;
    Ok(())
}

This example produces the following output:

Error: SuperError is here!: SuperErrorSideKick is here!

Note: Reports constructed via ? and From will be configured to use the single line output format. If you want to make sure your Reports are pretty printed and include backtrace you will need to manually convert and enable those flags.

#![feature(error_reporter)]
use std::error::Report;

fn main() -> Result<(), Report<SuperError>> {
    get_super_error()
        .map_err(Report::from)
        .map_err(|r| r.pretty(true).show_backtrace(true))?;
    Ok(())
}

This example produces the following output:

Error: SuperError is here!

Caused by:
      SuperErrorSideKick is here!

Implementations

impl<E> Report<E>where
    Report<E>: From<E>,

pub fn new(error: E) -> Report<E>

🔬This is a nightly-only experimental API. (error_reporter #90172)

Create a new Report from an input error.

impl<E> Report<E>

pub fn pretty(self, pretty: bool) -> Self

🔬This is a nightly-only experimental API. (error_reporter #90172)

Enable pretty-printing the report across multiple lines.

Examples

#![feature(error_reporter)]
use std::error::Report;

let error = SuperError { source: SuperErrorSideKick };
let report = Report::new(error).pretty(true);
eprintln!("Error: {report:?}");

This example produces the following output:

Error: SuperError is here!

Caused by:
      SuperErrorSideKick is here!

When there are multiple source errors the causes will be numbered in order of iteration starting from the outermost error.

#![feature(error_reporter)]
use std::error::Report;

let source = SuperErrorSideKickSideKick;
let source = SuperErrorSideKick { source };
let error = SuperError { source };
let report = Report::new(error).pretty(true);
eprintln!("Error: {report:?}");

This example produces the following output:

Error: SuperError is here!

Caused by:
   0: SuperErrorSideKick is here!
   1: SuperErrorSideKickSideKick is here!

pub fn show_backtrace(self, show_backtrace: bool) -> Self

🔬This is a nightly-only experimental API. (error_reporter #90172)

Display backtrace if available when using pretty output format.

Examples

Note: Report will search for the first Backtrace it can find starting from the outermost error. In this example it will display the backtrace from the second error in the sources, SuperErrorSideKick.

#![feature(error_reporter)]
#![feature(provide_any)]
#![feature(error_generic_member_access)]
use std::any::Demand;
use std::error::Report;
use std::backtrace::Backtrace;

#[derive(Debug)]
struct SuperErrorSideKick {
    backtrace: Backtrace,
}

impl SuperErrorSideKick {
    fn new() -> SuperErrorSideKick {
        SuperErrorSideKick { backtrace: Backtrace::force_capture() }
    }
}

impl Error for SuperErrorSideKick {
    fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
        demand.provide_ref::<Backtrace>(&self.backtrace);
    }
}

// The rest of the example is unchanged ...

let source = SuperErrorSideKick::new();
let error = SuperError { source };
let report = Report::new(error).pretty(true).show_backtrace(true);
eprintln!("Error: {report:?}");

This example produces something similar to the following output:

Error: SuperError is here!

Caused by:
      SuperErrorSideKick is here!

Stack backtrace:
   0: rust_out::main::_doctest_main_src_error_rs_1158_0::SuperErrorSideKick::new
   1: rust_out::main::_doctest_main_src_error_rs_1158_0
   2: rust_out::main
   3: core::ops::function::FnOnce::call_once
   4: std::sys_common::backtrace::__rust_begin_short_backtrace
   5: std::rt::lang_start::{{closure}}
   6: std::panicking::try
   7: std::rt::lang_start_internal
   8: std::rt::lang_start
   9: main
  10: __libc_start_main
  11: _start

Trait Implementations

impl<E> Debug for Report<E>where
    Report<E>: Display,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<E> Display for Report<E>where
    E: Error,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<E> From<E> for Report<E>where
    E: Error,

fn from(error: E) -> Self

Converts to this type from the input type.