Crate eyre

This library provides eyre::Report, a trait object based error type for easy idiomatic error handling 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.

[dependencies]
eyre = "0.3"

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::Report 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)
  }

• Create new errors from messages to help the person troubleshooting the error understand where things went wrong. A low-level error like "No such file or directory" can be annoying to directly and often benefit from being wrapped with higher level error messages.

  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

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 = "0.3", default-features = false }

Since the ?-based error conversions would normally rely on the std::error::Report 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.

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.

It is not 100% compatible because there are some cases where eyre encounters type inference errors but it should mostly work as a drop in replacement. Specifically, the following works in anyhow:

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

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.

// 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();

Re-exports

pub use eyre as format_err;

pub use eyre as anyhow;

pub use Report as Error;

pub use WrapErr as Context;

Macros

bail	Return early with an error.
ensure	Return early with an error if a condition is not satisfied.
eyre	Construct an ad-hoc error from a string.

Structs

Chain	Iterator of a chain of source errors.
DefaultContext	The default provided context for eyre::Report.
Report	The core error reporting type of the library, a wrapper around a dynamic error reporting type.

Traits

EyreContext	Context trait for customizing eyre::Report
WrapErr	Provides the wrap_err method for Result.

Type Definitions

Result

Result<T, Error>