Crate eyre

Expand description

This library provides eyre::Report, a trait object based error handling type for easy idiomatic error handling and reporting in Rust applications.

This crate is a fork of anyhow with a support for customized error reports. For more details on customization checkout the docs on eyre::EyreHandler.

Custom Report Handlers

The heart of this crate is it’s ability to swap out the Handler type to change what information is carried alongside errors and how the end report is formatted. This crate is meant to be used alongside companion crates that customize it’s behavior. Below is a list of known crates that export report handlers for eyre and short summaries of what features they provide.

stable-eyre: Switches the backtrace type from std’s to backtrace-rs’s so that it can be captured on stable. The report format is identical to DefaultHandler’s report format.
color-eyre: Captures a backtrace::Backtrace and a tracing_error::SpanTrace. Provides a Section trait for attaching warnings and suggestions to error reports. The end report is then pretty printed with the help of color-backtrace, color-spantrace, and ansi_term. Check out the README on color-eyre for details on the report format.
simple-eyre: A minimal EyreHandler that captures no additional information, for when you do not wish to capture Backtraces with errors.
jane-eyre: A report handler crate that exists purely for the pun. Currently just re-exports color-eyre.

Usage Recommendations and Stability Considerations

We recommend users do not re-export types from this library as part their own public API for libraries with external users. The main reason for this is that it will make your library API break if we ever bump the major version number on eyre and your users upgrade the eyre version they use in their application code before you upgrade your own eyre dep version¹.

However, even beyond this API stability hazard, there are other good reasons to avoid using eyre::Report as your public error type.

You export an undocumented error interface that is otherwise still accessible via downcast, making it hard for users to react to specific errors while not preventing them from depending on details you didn’t mean to make part of your public API.
- This in turn makes the error types of all libraries you use a part of your public API as well, and makes changing any of those libraries into an undetectable runtime breakage.
If many of your errors are constructed from strings you encourage your users to use string comparision for reacting to specific errors which is brittle and turns updating error messages into a potentially undetectable runtime breakage.

Details

Use Result<T, eyre::Report>, or equivalently eyre::Result<T>, as the return type of any fallible function.

Within the function, use ? to easily propagate any error that implements the std::error::Error trait.

use eyre::Result;

fn get_cluster_info() -> Result<ClusterMap> {
    let config = std::fs::read_to_string("cluster.json")?;
    let map: ClusterMap = serde_json::from_str(&config)?;
    Ok(map)
}

Wrap a lower level error with a new error created from a message to help the person troubleshooting understand what the chain of failures that occured. A low-level error like “No such file or directory” can be annoying to debug without more information about what higher level step the application was in the middle of.

use eyre::{WrapErr, Result};

fn main() -> Result<()> {
    ...
    it.detach().wrap_err("Failed to detach the important thing")?;

    let content = std::fs::read(path)
        .wrap_err_with(|| format!("Failed to read instrs from {}", path))?;
    ...
}

Error: Failed to read instrs from ./path/to/instrs.json

Caused by:
    No such file or directory (os error 2)

Downcasting is supported and can be by value, by shared reference, or by mutable reference as needed.

// If the error was caused by redaction, then return a
// tombstone instead of the content.
match root_cause.downcast_ref::<DataStoreError>() {
    Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
    None => Err(error),
}

If using the nightly channel, a backtrace is captured and printed with the error if the underlying error type does not already provide its own. In order to see backtraces, they must be enabled through the environment variables described in std::backtrace:
- If you want panics and errors to both have backtraces, set RUST_BACKTRACE=1;
- If you want only errors to have backtraces, set RUST_LIB_BACKTRACE=1;
- If you want only panics to have backtraces, set RUST_BACKTRACE=1 and RUST_LIB_BACKTRACE=0.
The tracking issue for this feature is rust-lang/rust#53487.

Eyre works with any error type that has an impl of std::error::Error, including ones defined in your crate. We do not bundle a derive(Error) macro but you can write the impls yourself or use a standalone macro like thiserror.

use thiserror::Error;

#[derive(Error, Debug)]
pub enum FormatError {
    #[error("Invalid header (expected {expected:?}, got {found:?})")]
    InvalidHeader {
        expected: String,
        found: String,
    },
    #[error("Missing attribute: {0}")]
    MissingAttribute(String),
}

One-off error messages can be constructed using the eyre! macro, which supports string interpolation and produces an eyre::Report.
```
return Err(eyre!("Missing attribute: {}", missing));
```
On newer versions of the compiler (e.g. 1.58 and later) this macro also supports format args captures.
```
return Err(eyre!("Missing attribute: {missing}"));
```

No-std support

NOTE: tests are currently broken for no_std so I cannot guarantee that everything works still. I’m waiting for upstream fixes to be merged rather than fixing them myself, so bear with me.

In no_std mode, the same API is almost all available and works the same way. To depend on Eyre in no_std mode, disable our default enabled “std” feature in Cargo.toml. A global allocator is required.

[dependencies]
eyre = { version = "0.6", default-features = false }

Since the ?-based error conversions would normally rely on the std::error::Error trait which is only available through std, no_std mode will require an explicit .map_err(Report::msg) when working with a non-Eyre error type inside a function that returns Eyre’s error type.

Comparison to failure

The eyre::Report type works something like failure::Error, but unlike failure ours is built around the standard library’s std::error::Error trait rather than a separate trait failure::Fail. The standard library has adopted the necessary improvements for this to be possible as part of RFC 2504.

Comparison to thiserror

Use eyre if you don’t think you’ll do anything with an error other than report it. This is common in application code. Use thiserror if you think you need an error type that can be handled via match or reported. This is common in library crates where you don’t know how your users will handle your errors.

Compatibility with `anyhow`

This crate does its best to be usable as a drop in replacement of anyhow and vice-versa by re-exporting all of the renamed APIs with the names used in anyhow, though there are some differences still.

`Context` and `Option`

As part of renaming Context to WrapErr we also intentionally do not implement WrapErr for Option. This decision was made because wrap_err implies that you’re creating a new error that saves the old error as its source. With Option there is no source error to wrap, so wrap_err ends up being somewhat meaningless.

Instead eyre intends for users to use the combinator functions provided by std for converting Options to Results. So where you would write this with anyhow:

use anyhow::Context;

let opt: Option<()> = None;
let result = opt.context("new error message");

With eyre we want users to write:

use eyre::{eyre, Result};

let opt: Option<()> = None;
let result: Result<()> = opt.ok_or_else(|| eyre!("new error message"));

NOTE: However, to help with porting we do provide a ContextCompat trait which implements context for options which you can import to make existing .context calls compile.