| commit | 1431a75978fc1218bf132b4588585c45dcb1ab4f | [log] [tgz] |
|---|---|---|
| author | Android Build Coastguard Worker <[email protected]> | Fri Jul 07 04:42:56 2023 +0000 |
| committer | Android Build Coastguard Worker <[email protected]> | Fri Jul 07 04:42:56 2023 +0000 |
| tree | 3f8485458be804c8c9ca12fff724617701513d03 | |
| parent | b723550e854fc714a3ddc35b35f3f4b4e93448c3 [diff] | |
| parent | 52eb4adffa2688928e7bf037045c570899b864ba [diff] |
Snap for 10453563 from 52eb4adffa2688928e7bf037045c570899b864ba to mainline-appsearch-release Change-Id: I17bb647eee0a52c7c9eb6431552f3134894267ba
This library provides anyhow::Error, a trait object based error type for easy idiomatic error handling in Rust applications.
[dependencies] anyhow = "1.0"
Compiler support: requires rustc 1.39+
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), }
If using the nightly channel, or stable with features = ["backtrace"], 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:
RUST_BACKTRACE=1;RUST_LIB_BACKTRACE=1;RUST_BACKTRACE=1 and RUST_LIB_BACKTRACE=0.The tracking issue for this feature is rust-lang/rust#53487.
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));
A bail! macro is provided as a shorthand for the same early return.
bail!("Missing attribute: {}", missing);
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.
The anyhow::Error 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.
Use Anyhow 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.