Crate anyhow[−][src]

This library provides anyhow::Error, a trait object based error type for easy idiomatic error handling in Rust applications.

Details

Use Result<T, anyhow::Error>, or equivalently anyhow::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 anyhow::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)
}

Attach context 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 debug without more context about what higher level step the application was in the middle of.

use anyhow::{Context, Result};

fn main() -> Result<()> {
    ...
    it.detach().context("Failed to detach the important thing")?;

    let content = std::fs::read(path)
        .with_context(|| 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),
}

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, the RUST_LIB_BACKTRACE=1 environment variable must be defined.

Anyhow 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 anyhow! macro, which supports string interpolation and produces an anyhow::Error.
```
return Err(anyhow!("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 Anyhow in no_std mode, disable our default enabled “std” feature in Cargo.toml. A global allocator is required.

[dependencies]
anyhow = { version = "1.0", 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(Error::msg) when working with a non-Anyhow error type inside a function that returns Anyhow’s error type.