failure 0.1.8

Experimental error handling abstraction.

failure - a new error management story

Notice: failure is deprecated. If you liked failure's API, consider using:

  • Anyhow is a good replacement for failure::Error.
  • thiserror is a good, near drop-in replacement for #[derive(Fail)].

Build Status Latest Version docs

failure is designed to make it easier to manage errors in Rust. It is intended to replace error management based on std::error::Error with a new system based on lessons learned over the past several years, including those learned from experience with quick-error and error-chain.

failure provides two core components:

  • Fail: A new trait for custom error types.
  • Error: A struct which any type that implements Fail can be cast into.


Failure is currently evolving as a library. First of all there is work going on in Rust itself to fix the error trait secondarily the original plan for Failure towards 1.0 is unlikely to happen in the current form.

As such the original master branch towards 1.0 of failure was removed and master now represents the future iteration steps of 0.1 until it's clear what happens in the stdlib.

The original 1.0 branch can be found in evolution/1.0.


extern crate serde;
extern crate toml;

#[macro_use] extern crate failure;
#[macro_use] extern crate serde_derive;

use std::collections::HashMap;
use std::path::PathBuf;
use std::str::FromStr;

use failure::Error;

// This is a new error type that you've created. It represents the ways a
// toolchain could be invalid.
// The custom derive for Fail derives an impl of both Fail and Display.
// We don't do any other magic like creating new types.
#[derive(Debug, Fail)]
enum ToolchainError {
    #[fail(display = "invalid toolchain name: {}", name)]
    InvalidToolchainName {
        name: String,
    #[fail(display = "unknown toolchain version: {}", version)]
    UnknownToolchainVersion {
        version: String,

pub struct ToolchainId {
    // ... etc

impl FromStr for ToolchainId {
    type Err = ToolchainError;

    fn from_str(s: &str) -> Result<ToolchainId, ToolchainError> {
        // ... etc

pub type Toolchains = HashMap<ToolchainId, PathBuf>;

// This opens a toml file containing associations between ToolchainIds and
// Paths (the roots of those toolchains).
// This could encounter an io Error, a toml parsing error, or a ToolchainError,
// all of them will be thrown into the special Error type
pub fn read_toolchains(path: PathBuf) -> Result<Toolchains, Error>
    use std::fs::File;
    use std::io::Read;

    let mut string = String::new();
    File::open(path)?.read_to_string(&mut string)?;

    let toml: HashMap<String, PathBuf> = toml::from_str(&string)?;

    let toolchains = toml.iter().map(|(key, path)| {
        let toolchain_id = key.parse()?;
        Ok((toolchain_id, path))
    }).collect::<Result<Toolchains, ToolchainError>>()?;



Both failure and failure_derive are intended to compile on all stable versions of Rust newer than 1.31.0, as well as the latest beta and the latest nightly. If either crate fails to compile on any version newer than 1.31.0, please open an issue.

failure is no_std compatible, though some aspects of it (primarily the Error type) will not be available in no_std mode.


failure is licensed under the terms of the MIT License or the Apache License 2.0, at your choosing.

Code of Conduct

Contribution to the failure crate is organized under the terms of the Contributor Covenant, the maintainer of failure, @withoutboats, promises to intervene to uphold that code of conduct.