Crate anyerr 

A trait object based error type for easy idiomatic error handling in Rust applications.

It provides Error, a trait object based error type, and the Result alias.

Details

• Use Result<T, anyerr::Error>, or equivalently anyerr::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 anyerr::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 anyerr::{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, 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.

• This crate works with any error type that has an impl of std::error::Error, including ones defined in your crate. It does not bundle a derive(Error) macro, but you can write the impls yourself or use the standalone thiserror crate in this workspace.

  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 anyerr::Error.

  return Err(anyhow!("Missing attribute: {}", missing));

  A bail! macro is provided as a shorthand for the same early return.

  bail!("Missing attribute: {}", missing);

No-std support

In no_std mode, almost all of the same API is available and works the same way. Disable the default enabled std feature to use it in no_std mode. A global allocator is required.

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-anyerr error type inside a function that returns anyerr’s error type.

Re-exports

pub use anyhow as format_err;

Macros

anyhow

Construct an ad-hoc error from a string or existing non-anyhow error value.

bail

Return early with an error.

ensure

Return early with an error if a condition is not satisfied.

Structs

Chainstd

Iterator of a chain of source errors.

Error

The Error type, a wrapper around a dynamic error type.

Traits

Context

Provides the context method for Result.

Functions

Ok

Equivalent to Ok::<_, anyerr::Error>(value).

Type Aliases

Result

Result<T, Error>