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//! This library provides [`anyhow::Error`][Error], a trait object based error //! type for easy idiomatic error handling in Rust applications. //! //! <br> //! //! # 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.jsox"; //! # //! 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(()) //! } //! ``` //! //! ```console //! Error: failed to read instrs from ./path/to/instrs.jsox //! //! 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), //! } //! # ; //! ``` //! //! - 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]. //! //! [thiserror]: https://github.com/dtolnay/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(()) //! # } //! ``` //! //! <br> //! //! # Acknowledgements //! //! The implementation of the `anyhow::Error` type is originally forked from //! `fehler::Exception` ([https://github.com/withoutboats/fehler][fehler]). This //! library exposes it under the more standard `Error` / `Result` terminology //! rather than the `throw!` / `#[throws]` / `Exception` language of exceptions. //! //! [fehler]: https://github.com/withoutboats/fehler #![doc(html_root_url = "https://docs.rs/anyhow/1.0.17")] #![cfg_attr(backtrace, feature(backtrace))] #![allow(clippy::new_ret_no_self, clippy::wrong_self_convention)] #[macro_use] mod backtrace; mod context; mod error; mod kind; mod macros; #[cfg(not(feature = "std"))] compile_error!("no_std support is not implemented yet"); use crate::error::ErrorImpl; use std::error::Error as StdError; use std::fmt::Display; use std::mem::ManuallyDrop; /// The `Error` type, a wrapper around a dynamic error type. /// /// `Error` works a lot like `Box<dyn std::error::Error>`, but with these /// differences: /// /// - `Error` requires that the error is `Send`, `Sync`, and `'static`. /// - `Error` guarantees that a backtrace is available, even if the underlying /// error type does not provide one. /// - `Error` is represented as a narrow pointer — exactly one word in /// size instead of two. pub struct Error { inner: ManuallyDrop<Box<ErrorImpl<()>>>, } /// Iterator of a chain of source errors. /// /// This type is the iterator returned by [`Error::chain`]. /// /// # Example /// /// ``` /// use anyhow::Error; /// use std::io; /// /// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> { /// for cause in error.chain() { /// if let Some(io_error) = cause.downcast_ref::<io::Error>() { /// return Some(io_error.kind()); /// } /// } /// None /// } /// ``` pub struct Chain<'a> { next: Option<&'a (dyn StdError + 'static)>, } /// `Result<T, Error>` /// /// This is a reasonable return type to use throughout your application but also /// for `fn main`; if you do, failures will be printed along with any /// [context][Context] and a backtrace if one was captured. /// /// `anyhow::Result` may be used with one *or* two type parameters. /// /// ```rust /// use anyhow::Result; /// /// # const IGNORE: &str = stringify! { /// fn demo1() -> Result<T> {...} /// // ^ equivalent to std::result::Result<T, anyhow::Error> /// /// fn demo2() -> Result<T, OtherError> {...} /// // ^ equivalent to std::result::Result<T, OtherError> /// # }; /// ``` /// /// # Example /// /// ``` /// # pub trait Deserialize {} /// # /// # mod serde_json { /// # use super::Deserialize; /// # use std::io; /// # /// # pub fn from_str<T: Deserialize>(json: &str) -> io::Result<T> { /// # unimplemented!() /// # } /// # } /// # /// # #[derive(Debug)] /// # struct ClusterMap; /// # /// # impl Deserialize for ClusterMap {} /// # /// use anyhow::Result; /// /// fn main() -> Result<()> { /// # return Ok(()); /// let config = std::fs::read_to_string("cluster.json")?; /// let map: ClusterMap = serde_json::from_str(&config)?; /// println!("cluster info: {:#?}", map); /// Ok(()) /// } /// ``` pub type Result<T, E = Error> = std::result::Result<T, E>; /// Provides the `context` method for `Result`. /// /// This trait is sealed and cannot be implemented for types outside of /// `anyhow`. /// /// # Example /// /// ``` /// use anyhow::{Context, Result}; /// use std::fs; /// use std::path::PathBuf; /// /// pub struct ImportantThing { /// path: PathBuf, /// } /// /// impl ImportantThing { /// # const IGNORE: &'static str = stringify! { /// pub fn detach(&mut self) -> Result<()> {...} /// # }; /// # fn detach(&mut self) -> Result<()> { /// # unimplemented!() /// # } /// } /// /// pub fn do_it(mut it: ImportantThing) -> Result<Vec<u8>> { /// it.detach().context("failed to detach the important thing")?; /// /// let path = &it.path; /// let content = fs::read(path) /// .with_context(|| format!("failed to read instrs from {}", path.display()))?; /// /// Ok(content) /// } /// ``` /// /// When printed, the outermost context would be printed first and the lower /// level underlying causes would be enumerated below. /// /// ```console /// Error: failed to read instrs from ./path/to/instrs.jsox /// /// caused by: /// No such file or directory (os error 2) /// ``` pub trait Context<T, E>: context::private::Sealed { /// Wrap the error value with additional context. fn context<C>(self, context: C) -> Result<T, Error> where C: Display + Send + Sync + 'static; /// Wrap the error value with additional context that is evaluated lazily /// only once an error does occur. fn with_context<C, F>(self, f: F) -> Result<T, Error> where C: Display + Send + Sync + 'static, F: FnOnce() -> C; } // Not public API. Referenced by macro-generated code. #[doc(hidden)] pub mod private { use crate::Error; use std::fmt::{Debug, Display}; #[cfg(backtrace)] use std::backtrace::Backtrace; pub use crate::kind::{AdhocKind, TraitKind}; pub fn new_adhoc<M>(message: M) -> Error where M: Display + Debug + Send + Sync + 'static, { Error::from_adhoc(message, backtrace!()) } }