Eyre
====
[![Build Status][actions-badge]][actions-url]
[](https://crates.io/crates/eyre)
[](https://docs.rs/eyre)
This library provides [`eyre::Report`][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.
```rust
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.
```rust
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))?;
...
}
```
```console
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.
```rust
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].
[`std::backtrace`]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
[rust-lang/rust#53487]: https://github.com/rust-lang/rust/issues/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].
```rust
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`.
```rust
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.
```toml
[dependencies]
eyre = { version = "0.3", 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].
[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
## 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.
[thiserror]: https://github.com/dtolnay/thiserror
## 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:
```rust
// Works
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.
```rust
// Broken
// Works
let val: Report = get_optional_val.ok_or_else(|| eyre!("failed to get value")).unwrap();
```
[Report]: https://docs.rs/eyre/*/eyre/struct.Report.html
[`eyre::EyreContext`]: https://docs.rs/eyre/*/eyre/trait.EyreContext.html
[`eyre::WrapErr`]: https://docs.rs/eyre/*/eyre/trait.WrapErr.html
[`anyhow::Context`]: https://docs.rs/anyhow/*/anyhow/trait.Context.html
[`anyhow`]: https://github.com/dtolnay/anyhow
[`tracing_error::SpanTrace`]: https://docs.rs/tracing-error/*/tracing_error/struct.SpanTrace.html
[`stable-eyre`]: https://docs.rs/stable-eyre
[actions-badge]: https://github.com/yaahc/eyre/workflows/Continuous%20integration/badge.svg
[actions-url]: https://github.com/yaahc/eyre/actions?query=workflow%3A%22Continuous+integration%22
#### License
<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>
<br>
<sub>
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.
</sub>