This library provides [anyhow::Error][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.

  # pub trait Deserialize {}
  #
  # mod serde_json {
  #     use super::Deserialize;
  #     use std::io;
  #
  #     pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> {
  #         unimplemented!()
  #     }
  # }
  #
  # struct ClusterMap;
  #
  # impl Deserialize for ClusterMap {}
  #
  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)
  }
  #
  # fn main() {}
  

• 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.

  # struct It;
  #
  # impl It {
  #     fn detach(&self) -> Result<()> {
  #         unimplemented!()
  #     }
  # }
  #
  use anyhow::{Context, Result};
  
  fn main() -> Result<()> {
      # return Ok(());
      #
      # const _: &str = stringify! {
      ...
      # };
      #
      # let it = It;
      # let path = "./path/to/instrs.json";
      #
      it.detach().context("Failed to detach the important thing")?;
  
      let content = std::fs::read(path)
          .with_context(|| format!("Failed to read instrs from {}", path))?;
      #
      # const _: &str = stringify! {
      ...
      # };
      #
      # Ok(())
  }
  

  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.

  # use anyhow::anyhow;
  # use std::fmt::{self, Display};
  # use std::task::Poll;
  #
  # #[derive(Debug)]
  # enum DataStoreError {
  #     Censored(()),
  # }
  #
  # impl Display for DataStoreError {
  #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
  #         unimplemented!()
  #     }
  # }
  #
  # impl std::error::Error for DataStoreError {}
  #
  # const REDACTED_CONTENT: () = ();
  #
  # let error = anyhow!("...");
  # let root_cause = &error;
  #
  # let ret =
  // 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 Rust ≥ 1.65, 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.

• 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.

  # use anyhow::{anyhow, Result};
  #
  # fn demo() -> Result<()> {
  #     let missing = "...";
  return Err(anyhow!("Missing attribute: {}", missing));
  #     Ok(())
  # }
  

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

  # use anyhow::{bail, Result};
  #
  # fn demo() -> Result<()> {
  #     let missing = "...";
  bail!("Missing attribute: {}", missing);
  #     Ok(())
  # }
  

No-std support

In no_std mode, almost all of the same API is 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 }

With versions of Rust older than 1.81, no_std mode may require an additional .map_err(Error::msg) when working with a non-Anyhow error type inside a function that returns Anyhow's error type, as the trait that ?-based error conversions are defined by is only available in std in those old versions.