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 with 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 its 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 its 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 Help 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 of it.
  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 the chain of failures that occurred. 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 rust >1.65, a backtrace is captured and printed with the
  error.

  On nightly eyre will use the underlying error's backtrace if it has one.

  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

No-std support was removed in 2020 in [commit 608a16a] due to unaddressed upstream breakages.
[commit 608a16a]:
eyre-rs@608a16a

Backtrace support

The built in default handler has support for capturing backtrace using rustc-1.65 or later.

Backtraces are captured when an error is converted to an eyre::Report (such as using ? or eyre!).

If using the nightly toolchain, backtraces will also be captured and accessed from other errors using error_generic_member_access if available.

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.

Disabling the compatibility layer

The anyhow compatibility layer is enabled by default.
If you do not need anyhow compatibility, it is advisable
to disable the "anyhow" feature:

eyre = { version = "0.6", default-features = false, features = ["auto-install", "track-caller"] }

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 offers OptionExt::ok_or_eyre to yield static errors from None,
and intends for users to use the combinator functions provided by
std, converting Options to Results, for dynamic errors.
So where you would write this with
anyhow:

use anyhow::Context;

let opt: Option<()> = None;
let result_static = opt.context("static error message");
let result_dynamic = opt.with_context(|| format!("{} error message", "dynamic"));

With eyre we want users to write:

use eyre::{eyre, OptionExt, Result};

let opt: Option<()> = None;
let result_static: Result<()> = opt.ok_or_eyre("static error message");
let result_dynamic: Result<()> = opt.ok_or_else(|| eyre!("{} error message", "dynamic"));

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.

License