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#![deny(missing_docs)] //! A library for consistent and reliable error handling //! //! error-chain makes it easy to take full advantage of Rust's //! powerful error handling features without the overhead of //! maintaining boilerplate error types and conversions. It implements //! an opinionated strategy for defining your own error types, as well //! as conversions from others' error types. //! //! ## Quick start //! //! If you just want to set up your new project with error-chain, //! follow the [quickstart.rs] template, and read this [intro] //! to error-chain. //! //! [quickstart.rs]: https://github.com/brson/error-chain/blob/master/examples/quickstart.rs //! [intro]: http://brson.github.io/2016/11/30/starting-with-error-chain //! //! ## Why error chain? //! //! * error-chain is easy to configure. Handle errors robustly with minimal //! effort. //! * Basic error handling requires no maintenance of custom error types //! nor the `From` conversions that make `?` work. //! * error-chain scales from simple error handling strategies to more //! rigorous. Return formatted strings for simple errors, only //! introducing error variants and their strong typing as needed for //! advanced error recovery. //! * error-chain makes it trivial to correctly manage the [cause] of //! the errors generated by your own code. This is the "chaining" //! in "error-chain". //! //! [cause]: https://doc.rust-lang.org/std/error/trait.Error.html#method.cause //! //! ## Principles of error-chain //! //! error-chain is based on the following principles: //! //! * No error should ever be discarded. This library primarily //! makes it easy to "chain" errors with the `chain_err` method. //! * Introducing new errors is trivial. Simple errors can be introduced //! at the error site with just a string. //! * Handling errors is possible with pattern matching. //! * Conversions between error types are done in an automatic and //! consistent way - `From` conversion behavior is never specified //! explicitly. //! * Errors implement Send. //! * Errors can carry backtraces. //! //! Similar to other libraries like [error-type] and [quick-error], //! this library introduces the error chaining mechanism originally //! employed by Cargo. The `error_chain!` macro declares the types //! and implementation boilerplate necessary for fulfilling a //! particular error-handling strategy. Most importantly it defines a //! custom error type (called `Error` by convention) and the `From` //! conversions that let the `try!` macro and `?` operator work. //! //! This library differs in a few ways from previous error libs: //! //! * Instead of defining the custom `Error` type as an enum, it is a //! struct containing an `ErrorKind` (which defines the //! `description` and `display` methods for the error), an opaque, //! optional, boxed `std::error::Error + Send + 'static` object //! (which defines the `cause`, and establishes the links in the //! error chain), and a `Backtrace`. //! * The macro also defines a `ResultExt` trait that defines a //! `chain_err` method. This method on all `std::error::Error + Send + 'static` //! types extends the error chain by boxing the current //! error into an opaque object and putting it inside a new concrete //! error. //! * It provides automatic `From` conversions between other error types //! defined by the `error_chain!` that preserve type information, //! and facilitate seamless error composition and matching of composed //! errors. //! * It provides automatic `From` conversions between any other error //! type that hides the type of the other error in the `cause` box. //! * If `RUST_BACKTRACE` is enabled, it collects a single backtrace at //! the earliest opportunity and propagates it down the stack through //! `From` and `ResultExt` conversions. //! //! To accomplish its goals it makes some tradeoffs: //! //! * The split between the `Error` and `ErrorKind` types can make it //! slightly more cumbersome to instantiate new (unchained) errors, //! requiring an `Into` or `From` conversion; as well as slightly //! more cumbersome to match on errors with another layer of types //! to match. //! * Because the error type contains `std::error::Error + Send + 'static` objects, //! it can't implement `PartialEq` for easy comparisons. //! //! ## Declaring error types //! //! Generally, you define one family of error types per crate, though //! it's also perfectly fine to define error types on a finer-grained //! basis, such as per module. //! //! Assuming you are using crate-level error types, typically you will //! define an `errors` module and inside it call `error_chain!`: //! //! ``` //! # #[macro_use] extern crate error_chain; //! mod other_error { //! error_chain! {} //! } //! //! error_chain! { //! // The type defined for this error. These are the conventional //! // and recommended names, but they can be arbitrarily chosen. //! // //! // It is also possible to leave this section out entirely, or //! // leave it empty, and these names will be used automatically. //! types { //! Error, ErrorKind, ResultExt, Result; //! } //! //! // Without the `Result` wrapper: //! // //! // types { //! // Error, ErrorKind, ResultExt; //! // } //! //! // Automatic conversions between this error chain and other //! // error chains. In this case, it will e.g. generate an //! // `ErrorKind` variant called `Another` which in turn contains //! // the `other_error::ErrorKind`, with conversions from //! // `other_error::Error`. //! // //! // Optionally, some attributes can be added to a variant. //! // //! // This section can be empty. //! links { //! Another(other_error::Error, other_error::ErrorKind) #[cfg(unix)]; //! } //! //! // Automatic conversions between this error chain and other //! // error types not defined by the `error_chain!`. These will be //! // wrapped in a new error with, in the first case, the //! // `ErrorKind::Fmt` variant. The description and cause will //! // forward to the description and cause of the original error. //! // //! // Optionally, some attributes can be added to a variant. //! // //! // This section can be empty. //! foreign_links { //! Fmt(::std::fmt::Error); //! Io(::std::io::Error) #[cfg(unix)]; //! } //! //! // Define additional `ErrorKind` variants. The syntax here is //! // the same as `quick_error!`, but the `from()` and `cause()` //! // syntax is not supported. //! errors { //! InvalidToolchainName(t: String) { //! description("invalid toolchain name") //! display("invalid toolchain name: '{}'", t) //! } //! } //! } //! //! # fn main() {} //! ``` //! //! Each section, `types`, `links`, `foreign_links`, and `errors` may //! be omitted if it is empty. //! //! This populates the module with a number of definitions, //! the most important of which are the `Error` type //! and the `ErrorKind` type. An example of generated code can be found in the //! [example_generated](example_generated) module. //! //! ## Returning new errors //! //! Introducing new error chains, with a string message: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! # error_chain! {} //! fn foo() -> Result<()> { //! Err("foo error!".into()) //! } //! ``` //! //! Introducing new error chains, with an `ErrorKind`: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! error_chain! { //! errors { FooError } //! } //! //! fn foo() -> Result<()> { //! Err(ErrorKind::FooError.into()) //! } //! ``` //! //! Note that the return type is the typedef `Result`, which is //! defined by the macro as `pub type Result<T> = //! ::std::result::Result<T, Error>`. Note that in both cases //! `.into()` is called to convert a type into the `Error` type; both //! strings and `ErrorKind` have `From` conversions to turn them into //! `Error`. //! //! When the error is emitted inside a `try!` macro or behind the //! `?` operator, the explicit conversion isn't needed; `try!` will //! automatically convert `Err(ErrorKind)` to `Err(Error)`. So the //! below is equivalent to the previous: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! # error_chain! { errors { FooError } } //! fn foo() -> Result<()> { //! Ok(try!(Err(ErrorKind::FooError))) //! } //! //! fn bar() -> Result<()> { //! Ok(try!(Err("bogus!"))) //! } //! ``` //! //! ## The `bail!` macro //! //! The above method of introducing new errors works but is a little //! verbose. Instead we can use the `bail!` macro, which, much like `try!` //! and `?`, performs an early return with conversions. With `bail!` the //! previous examples look like: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! # error_chain! { errors { FooError } } //! fn foo() -> Result<()> { //! if true { //! bail!(ErrorKind::FooError); //! } else { //! Ok(()) //! } //! } //! //! fn bar() -> Result<()> { //! if true { //! bail!("bogus!"); //! } else { //! Ok(()) //! } //! } //! ``` //! //! ## Chaining errors //! //! To extend the error chain: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! # error_chain! {} //! # fn do_something() -> Result<()> { unimplemented!() } //! # fn test() -> Result<()> { //! let res: Result<()> = do_something().chain_err(|| "something went wrong"); //! # Ok(()) //! # } //! ``` //! //! `chain_err` can be called on any `Result` type where the contained //! error type implements `std::error::Error + Send + 'static`. If //! the `Result` is an `Err` then `chain_err` evaluates the closure, //! which returns *some type that can be converted to `ErrorKind`*, //! boxes the original error to store as the cause, then returns a new //! error containing the original error. //! //! ## Matching errors //! //! error-chain error variants are matched with simple patterns. //! `Error` is a tuple struct and its first field is the `ErrorKind`, //! making dispatching on error kinds relatively compact: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() { //! error_chain! { //! errors { //! InvalidToolchainName(t: String) { //! description("invalid toolchain name") //! display("invalid toolchain name: '{}'", t) //! } //! } //! } //! //! match Error::from("error!") { //! Error(ErrorKind::InvalidToolchainName(_), _) => { } //! Error(ErrorKind::Msg(_), _) => { } //! } //! # } //! ``` //! //! Chained errors are also matched with (relatively) compact syntax //! //! ``` //! # #[macro_use] extern crate error_chain; //! mod utils { //! error_chain! { //! errors { //! BadStuff { //! description("bad stuff") //! } //! } //! } //! } //! //! mod app { //! error_chain! { //! links { //! Utils(::utils::Error, ::utils::ErrorKind); //! } //! } //! } //! //! //! # fn main() { //! match app::Error::from("error!") { //! app::Error(app::ErrorKind::Utils(utils::ErrorKind::BadStuff), _) => { } //! _ => { } //! } //! # } //! ``` //! //! ## Foreign links //! //! Errors that do not conform to the same conventions as this library //! can still be included in the error chain. They are considered "foreign //! errors", and are declared using the `foreign_links` block of the //! `error_chain!` macro. `Error`s are automatically created from //! foreign errors by the `try!` macro. //! //! Foreign links and regular links have one crucial difference: //! `From` conversions for regular links *do not introduce a new error //! into the error chain*, while conversions for foreign links *always //! introduce a new error into the error chain*. So for the example //! above all errors deriving from the `temp::Error` type will be //! presented to the user as a new `ErrorKind::Temp` variant, and the //! cause will be the original `temp::Error` error. In contrast, when //! `rustup_utils::Error` is converted to `Error` the two `ErrorKind`s //! are converted between each other to create a new `Error` but the //! old error is discarded; there is no "cause" created from the //! original error. //! //! ## Backtraces //! //! If the `RUST_BACKTRACE` environment variable is set to anything //! but ``0``, the earliest non-foreign error to be generated creates //! a single backtrace, which is passed through all `From` conversions //! and `chain_err` invocations of compatible types. To read the //! backtrace just call the `backtrace()` method. //! //! Backtrace generation can be disabled by turning off the `backtrace` feature. //! //! ## Iteration //! //! The `iter` method returns an iterator over the chain of error boxes. //! //! [error-type]: https://github.com/DanielKeep/rust-error-type //! [quick-error]: https://github.com/tailhook/quick-error #[cfg(feature = "backtrace")] extern crate backtrace; use std::error; use std::iter::Iterator; #[cfg(feature = "backtrace")] use std::sync::Arc; use std::fmt; #[cfg(feature = "backtrace")] pub use backtrace::Backtrace; #[cfg(not(feature = "backtrace"))] /// Dummy type used when the `backtrace` feature is disabled. pub type Backtrace = (); #[macro_use] mod quick_error; #[macro_use] mod error_chain; #[macro_use] mod quick_main; pub use quick_main::ExitCode; #[cfg(feature = "example_generated")] pub mod example_generated; /// Iterator over the error chain using the `Error::cause()` method. pub struct ErrorChainIter<'a>(pub Option<&'a error::Error>); impl<'a> Iterator for ErrorChainIter<'a> { type Item = &'a error::Error; fn next<'b>(&'b mut self) -> Option<&'a error::Error> { match self.0.take() { Some(e) => { self.0 = e.cause(); Some(e) } None => None, } } } /// Returns a backtrace of the current call stack if `RUST_BACKTRACE` /// is set to anything but ``0``, and `None` otherwise. This is used /// in the generated error implementations. #[cfg(feature = "backtrace")] #[doc(hidden)] pub fn make_backtrace() -> Option<Arc<Backtrace>> { match std::env::var_os("RUST_BACKTRACE") { Some(ref val) if val != "0" => Some(Arc::new(Backtrace::new())), _ => None, } } /// This trait is implemented on all the errors generated by the `error_chain` /// macro. pub trait ChainedError: error::Error + Send + 'static { /// Associated kind type. type ErrorKind; /// Constructs an error from a kind, and generates a backtrace. fn from_kind(kind: Self::ErrorKind) -> Self where Self: Sized; /// Constructs a chained error from another error and a kind, and generates a backtrace. fn with_chain<E, K>(error: E, kind: K) -> Self where Self: Sized, E: ::std::error::Error + Send + 'static, K: Into<Self::ErrorKind>; /// Returns the kind of the error. fn kind(&self) -> &Self::ErrorKind; /// Iterates over the error chain. fn iter(&self) -> ErrorChainIter; /// Returns the backtrace associated with this error. fn backtrace(&self) -> Option<&Backtrace>; /// Returns an object which implements `Display` for printing the full /// context of this error. /// /// The full cause chain and backtrace, if present, will be printed. fn display<'a>(&'a self) -> Display<'a, Self> { Display(self) } /// Creates an error from its parts. #[doc(hidden)] fn new(kind: Self::ErrorKind, state: State) -> Self where Self: Sized; /// Returns the first known backtrace, either from its State or from one /// of the errors from `foreign_links`. #[cfg(feature = "backtrace")] #[doc(hidden)] fn extract_backtrace(e: &(error::Error + Send + 'static)) -> Option<Arc<Backtrace>> where Self: Sized; } /// A struct which formats an error for output. #[derive(Debug)] pub struct Display<'a, T: 'a + ?Sized>(&'a T); impl<'a, T> fmt::Display for Display<'a, T> where T: ChainedError { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { try!(writeln!(fmt, "Error: {}", self.0)); for e in self.0.iter().skip(1) { try!(writeln!(fmt, "Caused by: {}", e)); } if let Some(backtrace) = self.0.backtrace() { try!(writeln!(fmt, "{:?}", backtrace)); } Ok(()) } } /// Common state between errors. #[derive(Debug)] #[doc(hidden)] pub struct State { /// Next error in the error chain. pub next_error: Option<Box<error::Error + Send>>, /// Backtrace for the current error. #[cfg(feature = "backtrace")] pub backtrace: Option<Arc<Backtrace>>, } impl Default for State { #[cfg(feature = "backtrace")] fn default() -> State { State { next_error: None, backtrace: make_backtrace(), } } #[cfg(not(feature = "backtrace"))] fn default() -> State { State { next_error: None } } } impl State { /// Creates a new State type #[cfg(feature = "backtrace")] pub fn new<CE: ChainedError>(e: Box<error::Error + Send>) -> State { let backtrace = CE::extract_backtrace(&*e).or_else(make_backtrace); State { next_error: Some(e), backtrace: backtrace, } } /// Creates a new State type #[cfg(not(feature = "backtrace"))] pub fn new<CE: ChainedError>(e: Box<error::Error + Send>) -> State { State { next_error: Some(e) } } /// Returns the inner backtrace if present. #[cfg(feature = "backtrace")] pub fn backtrace(&self) -> Option<&Backtrace> { self.backtrace.as_ref().map(|v| &**v) } /// Returns the inner backtrace if present. #[cfg(not(feature = "backtrace"))] pub fn backtrace(&self) -> Option<&Backtrace> { None } } /// Exits a function early with an error /// /// The `bail!` macro provides an easy way to exit a function. /// `bail!(expr)` is equivalent to writing. /// /// ``` /// # #[macro_use] extern crate error_chain; /// # error_chain! { } /// # fn main() { } /// # fn foo() -> Result<()> { /// # let expr = ""; /// return Err(expr.into()); /// # } /// ``` /// /// And as shorthand it takes a formatting string a la `println!`: /// /// ``` /// # #[macro_use] extern crate error_chain; /// # error_chain! { } /// # fn main() { } /// # fn foo() -> Result<()> { /// # let n = 0; /// bail!("bad number: {}", n); /// # } /// ``` /// /// # Examples /// /// Bailing on a custom error: /// /// ``` /// # #[macro_use] extern crate error_chain; /// # fn main() {} /// error_chain! { /// errors { FooError } /// } /// /// fn foo() -> Result<()> { /// if bad_condition() { /// bail!(ErrorKind::FooError); /// } /// /// Ok(()) /// } /// /// # fn bad_condition() -> bool { true } /// ``` /// /// Bailing on a formatted string: /// /// ``` /// # #[macro_use] extern crate error_chain; /// # fn main() {} /// error_chain! { } /// /// fn foo() -> Result<()> { /// if let Some(bad_num) = bad_condition() { /// bail!("so bad: {}", bad_num); /// } /// /// Ok(()) /// } /// /// # fn bad_condition() -> Option<i8> { None } /// ``` #[macro_export] macro_rules! bail { ($e:expr) => { return Err($e.into()); }; ($fmt:expr, $($arg:tt)+) => { return Err(format!($fmt, $($arg)+).into()); }; } /// Exits a function early with an error if the condition is not satisfied /// /// The `ensure!` macro is a convenience helper that provides a way to exit /// a function with an error if the given condition fails. /// /// As an example, `ensure!(condition, "error code: {}", errcode)` is equivalent to /// /// ``` /// # #[macro_use] extern crate error_chain; /// # error_chain! { } /// # fn main() { } /// # fn foo() -> Result<()> { /// # let errcode = 0u8; /// # let condition = true; /// if !condition { /// bail!("error code: {}", errcode); /// } /// # Ok(()) /// # } /// ``` /// /// See documentation for `bail!` macro for further details. #[macro_export] macro_rules! ensure { ($cond:expr, $e:expr) => { if !($cond) { bail!($e); } }; ($cond:expr, $fmt:expr, $($arg:tt)+) => { if !($cond) { bail!($fmt, $($arg)+); } }; } #[doc(hidden)] pub mod mock { error_chain!{} }