failure 0.1.0

Experimental error handling abstraction.

failure - a new error management story

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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.


extern crate failure;
extern crate serde;
extern crate toml;

#[macro_use] extern crate failure_derive;
#[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 Derive.
// 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 1.18.0, as well as the latest beta and the latest nightly. If either crate fails to compile on any version newer than 1.18.0, 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.