eyre

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 by @dtolnay with a support for customized Reports. For more details on customization checkout the docs on eyre::EyreContext. For an example on how to implement a custom context check out stable-eyre which implements a minimal custom context for capturing backtraces on stable.

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));
  

No-std support

NOTE: tests are currently broken for no_std so I cannot guaruntee 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 = "0.4", 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 care what error type your functions return, you just want it to be easy. This is common in application code. Use thiserror if you are a library that wants to design your own dedicated error type(s) so that on failures the caller gets exactly the information that you choose.

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.

There are two main incompatibilities that you might encounter when porting a codebase from anyhow to eyre:

• type inference errors when using eyre!
• .context not being implemented for Option

Type Inference Errors

The type inference issue is caused by the generic parameter, which isn't present in anyhow::Error. Specifically, the following works in anyhow:

use anyhow::anyhow;

// Works
let val = get_optional_val().ok_or_else(|| anyhow!("failed to get value")).unwrap_err();

Where as with eyre! this will fail due to being unable to infer the type for the Context parameter. The solution to this problem, should you encounter it, is to give the compiler a hint for what type it should be resolving to, either via your return type or a type annotation.

use eyre::eyre;

// Broken
let val = get_optional_val().ok_or_else(|| eyre!("failed to get value")).unwrap();

// Works
let val: Report = get_optional_val().ok_or_else(|| eyre!("failed to get value")).unwrap();

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"));

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.

License