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Announcing error-stack v0.2

Introducing customizable outputs, related errors, multiple sources, and much more to error handling in Rust

October 3rd, 2022

Tim DiekmannSenior Platform Engineer, HASH

This is an old blog post and may contain code which does not compile with the current error-stack version. Please check out the latest documentation and examples for more information.

Back in June we announced the initial public release of error-stack, our context-aware Rust error library. We developed this to help address a lot of challenges we encountered when implementing error handling in our simulation engine. In the time since then we’ve started using it in our entity-graph query layer that we’re building as a datastore for HASH. Across that development we’ve learned a lot of lessons, and in joint efforts with open-source contributors we’re ready to release a new version of the library.

Since our last release we’ve been fixing user-reported bugs, reviewing open-source contributor’s pull requests, and adding new features. Version 0.2 is available now and includes important changes from std, a completely redesigned output with new customization options, support for related errors and multiple error sources, compatibility with anyhow and eyre, and a number of other improvements.

What's new?

Changes from std in v0.2

  • The Provider API has landed and replaces our vendored-in implementation.
  • Backtrace was stabilized with Rust 1.65(🎉): we now support Backtraces on a non-nightly channel starting with 1.65-beta.
  • If the root context already contains a backtrace, no new Backtrace is captured. Otherwise, backtraces are captured automatically when RUST_BACKTRACE or RUST_LIB_BACKTRACE env-vars are set.
  • All std-Error types now implement Provider. The blanket implementation on Context will re-use the Error::provide values.
  • Error is now available in core::error, for nightly channels.

A redesigned output with a hook interface for customization

The output in v0.2 has been completely redesigned by Bilal Mahmoud (@indietyp) — huge kudos! It’s now much easier to read compared to v0.1, with visual cues to represent the information hierarchy at a glance, and more information is provided.

error-stack v0.2:

Error: experiment error: could not run experiment
 ├╴unable to set up experiments
 ├─▶ invalid experiment description
 │   ├╴examples/
 │   ╰╴experiment 2 could not be parsed
 ╰─▶ invalid digit found in string
     ╰╴"3o" could not be parsed as experiment

error-stack v0.1:

Error: experiment error: could not run experiment
              at examples/
       - unable to setup experiments

 Caused by:
    0: invalid experiment description
              at examples/
       - experiment 2 could not be parsed
    1: invalid digit found in string
              at examples/
       - "3o" could not be parsed as experiment

To enable granular control over the output format, hooks are now set for specific types instead of for the whole Report. Values provided through Context::provide can be printed, along with attachments, by defining a hook for their types. And it’s now possible to selectively define outputs per attachment. For example:

// Note: neither `Debug`, nor `Display` is implemented
 struct Suggestion(&'static str);

 Report::install_debug_hook::<Suggestion>(|value, context| {
     context.push_body(format!("suggestion: {}", value.0));

 let report = read("config.txt").attach(Suggestion("check if `config.txt` exists"));
no such file or directory
 ╰╴suggestion: check if `config.txt` exists

Report outputs now consist of two sections: a Body and an Appendix. The Body provides top-level information while the Appendix includes additional, more verbose details, like a backtrace. Hooks can be used to write to both of these sections.

experiment error: could not run experiment
 ├╴backtrace with 19 frames (1)
 ├╴experiment 3 has no valid description
 ╰╴unable to set up experiments


 Backtrace No. 1
    0: std::backtrace_rs::backtrace::libunwind::trace
              at /rustc/8b705839cd656d202e920efa8769cbe43a5ee269/library/std/src/../../backtrace/src/backtrace/
    1: std::backtrace_rs::backtrace::trace_unsynchronized
              at /rustc/8b705839cd656d202e920efa8769cbe43a5ee269/library/std/src/../../backtrace/src/backtrace/
    2: std::backtrace::Backtrace::create
              at /rustc/8b705839cd656d202e920efa8769cbe43a5ee269/library/std/src/
    3: core::ops::function::FnOnce::call_once
              at /rustc/8b705839cd656d202e920efa8769cbe43a5ee269/library/core/src/ops/
    18: _main

For a full guide to using hooks, please refer to the fmt module documentation.

Related errors and multiple error sources

error-stack v0.2 introduces support for related errors and multiple error sources, which can be invaluable when multiple errors occur at the same time, e.g. in loops or in multi-threaded applications. Reports are no longer a linear stream of events. You can now create a tree of different Reports, enabling the capture of multiple simultaneous errors without losing information.

This is also shown in the output:

Error: experiment error: could not run experiment
 ├╴Unable to set up experiments
 ├─▶ invalid experiment description
 │   ├╴examples/
 │   ╰╴experiment 2 could not be parsed
 ╰┬▶ invalid digit found in string
  │  ├╴examples/
  │  ╰╴"3o" could not be parsed as experiment
  ╰▶ invalid digit found in string
     ╰╴"4a" could not be parsed as experiment

 experiment error: could not run experiment
 ├╴experiment 3 has no valid description
 ╰╴unable to set up experiments

Again, a huge thanks to Bilal Mahmoud (@indietyp) for his work.

Improved documentation, Termination, anyhow and eyre

We’ve made a few other improvements too:

  • The error-stack documentation has been greatly improved and we’re always open to more feedback (create an Issue or join us in Discord).

  • Termination has been implemented for Reports: it will return the ExitCode provided by any Context if present, or ExitCode::FAILURE will be returned.

  • Compatibility for anyhow and eyre has been added to convert their types into Report.

    • We’ve provided a trait to convert Result<_, anyhow::Error> or Result<_, eyre::Report> to Result<_, Report<anyhow::Error>> or Result<_, Report<eyre::Report>>.
    • The additional sources of anyhow::Error and eyre::Report are also attached as frames to maintain their history.

    This should make it much easier to try out error-stack in existing codebases which utilize these libraries, as well as improving the migration process.

Breaking changes

We care a lot about breaking changes and will always try to conform to SemVer. A few principles guide our approach:

  • we’re not afraid to make breaking changes if it will improve the usability and functionality of the library.
  • we try to maintain consistency. E.g. we’ve deprecated Report::span_trace to avoid special casing SpanTrace and keep the API the same as for backtraces.
  • we try to avoid duplication. If one feature is superseded by another, we’ll deprecate the old one and remove it later, as with the new hook system.

Version 0.2 of error-stack introduces these breaking changes and deprecations:

  • The minimal supported Rust version was bumped to 1.63 to greatly simplify maintenance.
  • We switched to the upstream implementation of Provider, so if you were on nightly and used our provider API, you need to change the imports to core::any::.
  • Remove the unused features hooksfutures, and futures-core
  • Report::backtrace (and Report::span_trace for consistency) were deprecated because you cannot get Backtrace from Error anymore.
    • non-nightly channels should use Report::downcast_ref::<Backtrace> / Report::downcast_ref::<SpanTrace>.
    • nightly channels should useReport::request_ref::<Backtrace> / Report::request_ref::<SpanTrace>.
  • The format of the Debug output has changed significantly. If you had a dependency on the format of the Debug output (which we don’t recommend), it might break.
  • We deprecated IntoReport::report. Please use IntoReport::into_report instead.
  • We deprecated the following as part of the changes we made to how hooks work:
    • instead of Report::set_debug_hook please use Report::install_debug_hook.
    • Please don’t use Report::set_display_hook.
  • Because multiple error sources are now possible, we deprecated:
    • Frame::source; please use Frame::sources instead.
    • Frame::source_mut; please use Frame::sources_mut instead.

What’s next?

We’ve got lots of new features planned, including:

  • adding support for serde to provide a hook-based interface to serialize Reports.
  • adding support for defmt.
  • adding support for WASM environments.
  • continuing to iterate on the ergonomics of how we support related errors and multiple sources.

Please let us know what you think!

We’ll also be using many of the new features of error-stack v0.2 in our own products, namely HASH (learn more or view on GitHub). The new hook system will greatly simplify the creation of useful reports, and the support for related errors will allow us not to return the error the first time it occurs, but to give the user a comprehensive list of errors.

Get involved

HASH, the company behind error-stack, is 100% committed to open source and we’d love to work with you. Ask a question, file an issue, or open a PR on the error-stack repo, or any of our other projects. Also check out the Block Protocol, a new standard for building composable interfaces and interoperable types for a more linked and open semantic web.


This crate is published under the MIT License.

View and star error-stack on GitHub >

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