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//! This is a companion crate to the [`failure`](https://boats.gitlab.io/failure/intro.html) //! crate, which aims to recover the ergonomics of //! [`error_chain`](https://github.com/rust-lang-nursery/error-chain). It does this //! by bringing back: //! * `chain_err` //! * non-verbose `Error`, `ErrorKind` pairs //! * support for `bail!` and `ensure!` with custom `ErrorKind`-s //! //! The `failure` library recommends three different patterns for errors. This //! library implementes th complex one (and the most useful one) but without all the [boilerplate](https://boats.gitlab.io/failure/error-errorkind.html). //! //! ### What it looks like //! //! ```rust //! mod errors { //! // errors.rs //! use failchain::{BoxedError, ChainErrorKind}; //! use failure::Fail; //! use std::path::PathBuf; //! use std::result::Result as StdResult; //! //! pub type Error = BoxedError<ErrorKind>; // Use `UnboxedError` instead for //! // non-allocating, but bigger `Error`. //! pub type Result<T> = StdResult<T, Error>; //! //! #[derive(Clone, Eq, PartialEq, Debug, Fail)] //! pub enum ErrorKind { //! #[fail(display = "Metadata I/O error {:?}.", 0)] //! MetadataIo(PathBuf), //! //! #[fail(display = "Corrupt metadata file: {}", 0)] //! CorruptMetadata(String), //! //! #[fail(display = "WAD I/O error {:?}.", 0)] //! WadIo(PathBuf), //! //! #[fail(display = "Corrupt WAD file: {}", 0)] //! CorruptWad(String), //! } //! //! impl ChainErrorKind for ErrorKind { //! type Error = Error; //! } //! } //! //! mod main { //! // main.rs //! use super::errors::{ErrorKind, Result}; //! use failchain::{ //! bail, //! ensure, //! ResultExt, // for `chain_err`, //! }; //! use std::fs::File; //! use std::io::Read; //! use std::path::Path; //! //! fn validate_metadata(path: &Path, metadata: &str) -> Result<()> { //! // `ensure` is like `assert` (or failure's ensure), except it allows you to //! // specify the `ErrorKind`. //! ensure!( //! !metadata.is_empty(), //! ErrorKind::CorruptMetadata(format!("metadata file {:?} is empty", path)) //! ); //! //! // a special mode of `ensure` works for functions that return `ErrorKind`-s //! // and take a single string as argument: //! ensure!( //! metadata.len() > 100, //! ErrorKind::CorruptMetadata, // Any FnOnce(String) -> ErrorKind //! "metadata file {:?} is too long", // Rest of arguments are format args. //! path, //! ); //! //! // `bail` is like `ensure`, but without the condition and always returns //! // early. //! bail!( //! ErrorKind::CorruptMetadata, //! "validation isn't actually implemented" //! ); //! } //! //! fn load(wad: &Path, metadata: &Path) -> Result<()> { //! // `chain_err` stashes the original error as the `cause` of a new error. //! let wad_file = File::open(wad).chain_err(|| ErrorKind::WadIo(wad.to_owned()))?; //! //! let mut metadata_content = String::new(); //! //! // `chain_inspect_err` stashes the original error as the `cause` of the new //! // error, but it first allows the callback to inspect it. //! let metadata_file = File::open(metadata) //! .and_then(|mut file| file.read_to_string(&mut metadata_content)) //! .chain_inspect_err(|_io_error| ErrorKind::MetadataIo(metadata.to_owned()))?; //! //! validate_metadata(metadata, &metadata_content)?; //! //! Ok(()) //! } //! } //! ``` use failure::{Backtrace, Context, Fail}; use std::fmt; /// Trait which must be implemented by `ErrorKind`-s. /// /// The `Error` associated type should select between `UnboxedError<Self>` or `BoxedError<Self>`. pub trait ChainErrorKind: Fail + Sized { type Error: Fail + From<Context<Self>>; } /// An error type which stores the backtrace, cause pointer and error kind inline. /// /// This is potentially a very large object, but it doesn't allocate on creation unlike /// `BoxedError`. #[derive(Debug)] pub struct UnboxedError<ErrorKindT: Fail> { inner: Context<ErrorKindT>, } /// An error type which stores the backtrace, cause pointer and error kind behind a `Box`. /// /// The size of this object is always one pointer. It's therefore smaller than `UnboxedError`, but /// requires an allocation when created. #[derive(Debug)] pub struct BoxedError<ErrorKindT: Fail> { inner: Box<Context<ErrorKindT>>, } impl<ErrorKindT: Fail> UnboxedError<ErrorKindT> { pub fn kind(&self) -> &ErrorKindT { self.inner.get_context() } } /// Extension trait which adds the family of `.chain_err` methods to `Result` objects. pub trait ResultExt: Sized { type Success; type Error: Fail; /// Replace the error in a Result with a new error built from `map`'s `ErrorKind` output. /// /// The original error is stored as the `cause`/`source` of the new one. fn chain_err<ErrorKindT: ChainErrorKind>( self, map: impl FnOnce() -> ErrorKindT, ) -> Result<Self::Success, ErrorKindT::Error> { self.chain_inspect_err(|_| map()) } /// Like `chain_err`, but the callback is given an opportunity to inspect the original error. fn chain_inspect_err<ErrorKindT: ChainErrorKind>( self, map: impl FnOnce(&mut Self::Error) -> ErrorKindT, ) -> Result<Self::Success, ErrorKindT::Error>; } impl<SuccessT, ErrorT: Fail> ResultExt for Result<SuccessT, ErrorT> { type Success = SuccessT; type Error = ErrorT; fn chain_inspect_err<ErrorKindT: ChainErrorKind>( self, chain: impl FnOnce(&mut ErrorT) -> ErrorKindT, ) -> Result<Self::Success, ErrorKindT::Error> { self.map_err(|mut initial_error| { let kind = chain(&mut initial_error); initial_error.context(kind).into() }) } } /// Returns early with an error built from an error kind. /// /// Examples /// --- /// /// ```rust /// // With an ErrorKind. /// bail!(ErrorKind::CorruptFile(format!("the file at {:?} is corrupt", file_path))) /// /// // With an ErrorKind and format string (equivalent ot the above.) /// bail!(ErrorKind::CorruptFile, "the file at {:?} is corrupt", file_path) /// ``` #[macro_export] macro_rules! bail { ($e:expr) => { return Err($e.into()); }; ($e:expr,) => { return Err($e.into()); }; ($kind:expr, $fmt:expr) => { return Err($kind(($fmt).to_owned()).into()); }; ($kind:expr, $fmt:expr, $($arg:tt)+) => { return Err($kind(format!($fmt, $($arg)+)).into()); }; } /// Returns early with an error built from an error kind if a given condition is false. /// /// Examples /// --- /// /// ```rust /// // With an ErrorKind. /// ensure!( /// file_path != corrupt_file_path, /// ErrorKind::CorruptFile(format!("the file at {:?} is corrupt", file_path)) /// ); /// /// // With an ErrorKind and format string (equivalent ot the above.) /// ensure!( /// file_path != corrupt_file_path, /// ErrorKind::CorruptFile, /// "the file at {:?} is corrupt", /// file_path, /// ); /// ``` #[macro_export] macro_rules! ensure { ($cond:expr, $e:expr, $fmt:expr, $($arg:tt)+) => { if !($cond) { $crate::bail!($e, $fmt, $($arg)+); } }; ($cond:expr, $e:expr, $fmt:expr) => { if !($cond) { $crate::bail!($e, $fmt); } }; ($cond:expr, $e:expr,) => { if !($cond) { $crate::bail!($e); } }; ($cond:expr, $e:expr) => { if !($cond) { $crate::bail!($e); } }; } impl<ErrorKindT: Fail> Fail for UnboxedError<ErrorKindT> { fn cause(&self) -> Option<&Fail> { self.inner.cause() } fn backtrace(&self) -> Option<&Backtrace> { self.inner.backtrace() } } impl<ErrorKindT: Fail> fmt::Display for UnboxedError<ErrorKindT> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { self.inner.fmt(f) } } impl<ErrorKindT: Fail> From<ErrorKindT> for UnboxedError<ErrorKindT> { fn from(kind: ErrorKindT) -> Self { Self::from(Context::new(kind)) } } impl<ErrorKindT: Fail> From<Context<ErrorKindT>> for UnboxedError<ErrorKindT> { fn from(inner: Context<ErrorKindT>) -> Self { Self { inner } } } impl<ErrorKindT: Fail> BoxedError<ErrorKindT> { pub fn kind(&self) -> &ErrorKindT { self.inner.get_context() } } impl<ErrorKindT: Fail> Fail for BoxedError<ErrorKindT> { fn cause(&self) -> Option<&Fail> { self.inner.cause() } fn backtrace(&self) -> Option<&Backtrace> { self.inner.backtrace() } } impl<ErrorKindT: Fail> fmt::Display for BoxedError<ErrorKindT> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { self.inner.fmt(f) } } impl<ErrorKindT: Fail> From<ErrorKindT> for BoxedError<ErrorKindT> { fn from(kind: ErrorKindT) -> Self { Self::from(Context::new(kind)) } } impl<ErrorKindT: Fail> From<Context<ErrorKindT>> for BoxedError<ErrorKindT> { fn from(inner: Context<ErrorKindT>) -> Self { Self { inner: Box::new(inner), } } }