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#![deny(missing_docs)] #![doc(html_root_url = "https://docs.rs/error-chain/0.12.1")] //! 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/rust-lang-nursery/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 `?` 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_chain`] 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. Define custom responses with the //! // `description` and `display` calls. //! errors { //! InvalidToolchainName(t: String) { //! description("invalid toolchain name") //! display("invalid toolchain name: '{}'", t) //! } //! //! // You can also add commas after description/display. //! // This may work better with some editor auto-indentation modes: //! UnknownToolchainVersion(v: String) { //! description("unknown toolchain version"), // note the , //! display("unknown toolchain version: '{}'", v), // trailing comma is allowed //! } //! } //! } //! //! # 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/index.html) 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 behind the `?` operator, the explicit conversion //! isn't needed; `Err(ErrorKind)` will automatically be converted 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(Err(ErrorKind::FooError)?) //! } //! //! fn bar() -> Result<()> { //! Ok(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 performs an early return //! with conversions done automatically. //! //! 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 //! error-chain supports extending an error chain by appending new errors. //! This can be done on a Result or on an existing Error. //! //! 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`, as long as //! the [`Result`] type's corresponding [`ResultExt`] trait is in scope. 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. //! //! Calling [`chain_err`][Error_chain_err] on an existing [`Error`] instance has //! the same signature and produces the same outcome as being called on a //! [`Result`] matching the properties described above. This is most useful when //! partially handling errors using the [`map_err`] function. //! //! To chain an error directly, use [`with_chain`]: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! # error_chain! {} //! # fn do_something() -> Result<()> { unimplemented!() } //! # fn test() -> Result<()> { //! let res: Result<()> = //! do_something().map_err(|e| Error::with_chain(e, "something went wrong")); //! # Ok(()) //! # } //! ``` //! //! ## Linking errors //! //! To convert an error from another error chain to this error chain: //! //! ``` //! # #[macro_use] extern crate error_chain; //! # fn main() {} //! # mod other { error_chain! {} } //! error_chain! { //! links { //! OtherError(other::Error, other::ErrorKind); //! } //! } //! //! fn do_other_thing() -> other::Result<()> { unimplemented!() } //! //! # fn test() -> Result<()> { //! let res: Result<()> = do_other_thing().map_err(|e| e.into()); //! # Ok(()) //! # } //! ``` //! //! The [`Error`] and [`ErrorKind`] types implements [`From`] for the corresponding //! types of all linked error chains. Linked errors do not introduce a new //! cause to the error chain. //! //! ## 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), _) => { } //! _ => { } //! } //! # } //! ``` //! //! ## Inspecting errors //! //! An error-chain error contains information about the error itself, a backtrace, and the chain //! of causing errors. For reporting purposes, this information can be accessed as follows. //! //! ``` //! # #[macro_use] extern crate error_chain; //! use error_chain::ChainedError; // for e.display_chain() //! //! error_chain! { //! errors { //! InvalidToolchainName(t: String) { //! description("invalid toolchain name") //! display("invalid toolchain name: '{}'", t) //! } //! } //! } //! //! # fn main() { //! // Generate an example error to inspect: //! let e = "xyzzy".parse::<i32>() //! .chain_err(|| ErrorKind::InvalidToolchainName("xyzzy".to_string())) //! .unwrap_err(); //! //! // Get the brief description of the error: //! assert_eq!(e.description(), "invalid toolchain name"); //! //! // Get the display version of the error: //! assert_eq!(e.to_string(), "invalid toolchain name: 'xyzzy'"); //! //! // Get the full cause and backtrace: //! println!("{}", e.display_chain().to_string()); //! // Error: invalid toolchain name: 'xyzzy' //! // Caused by: invalid digit found in string //! // stack backtrace: //! // 0: 0x7fa9f684fc94 - backtrace::backtrace::libunwind::trace //! // at src/backtrace/libunwind.rs:53 //! // - backtrace::backtrace::trace<closure> //! // at src/backtrace/mod.rs:42 //! // 1: 0x7fa9f6850b0e - backtrace::capture::{{impl}}::new //! // at out/capture.rs:79 //! // [..] //! # } //! ``` //! //! The [`Error`] and [`ErrorKind`] types also allow programmatic access to these elements. //! //! ## 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 `?` operator. //! //! 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 [`std::fmt::Error`] type will be //! presented to the user as a new [`ErrorKind`] variant, and the //! cause will be the original [`std::fmt::Error`] error. In contrast, when //! `other_error::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. //! //! The Backtrace contains a Vec of [`BacktraceFrame`]s that can be operated //! on directly. For example, to only see the files and line numbers of code //! within your own project. //! //! ``` //! # #[macro_use] //! # extern crate error_chain; //! # mod errors { //! # error_chain! { //! # foreign_links { //! # Io(::std::io::Error); //! # } //! # } //! # } //! # use errors::*; //! # #[cfg(feature="backtrace")] //! # fn main() { //! if let Err(ref e) = open_file() { //! if let Some(backtrace) = e.backtrace() { //! let frames = backtrace.frames(); //! for frame in frames.iter() { //! for symbol in frame.symbols().iter() { //! if let (Some(file), Some(lineno)) = (symbol.filename(), symbol.lineno()) { //! if file.display().to_string()[0..3] == "src".to_string(){ //! println!("{}:{}", file.display().to_string(), lineno); //! } //! } //! } //! } //! } //! }; //! # } //! # #[cfg(not(feature="backtrace"))] //! # fn main() { } //! //! fn open_file() -> Result<()> { //! std::fs::File::open("does_not_exist")?; //! Ok(()) //! } //! ``` //! //! ## 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 //! [`display_chain`]: trait.ChainedError.html#method.display_chain //! [`error_chain!`]: macro.error_chain.html //! [`bail!`]: macro.bail.html //! [`Backtrace`]: struct.Backtrace.html //! [`Error`]: example_generated/struct.Error.html //! [`with_chain`]: example_generated/struct.Error.html#method.with_chain //! [Error_chain_err]: example_generated/struct.Error.html#method.chain_err //! [`cause`]: example_generated/struct.Error.html#method.cause //! [`backtrace`]: example_generated/struct.Error.html#method.backtrace //! [`iter`]: example_generated/struct.Error.html#method.iter //! [`ErrorKind`]: example_generated/enum.ErrorKind.html //! [`description`]: example_generated/enum.ErrorKind.html#method.description //! [`Result`]: example_generated/type.Result.html //! [`ResultExt`]: example_generated/trait.ResultExt.html //! [`chain_err`]: example_generated/trait.ResultExt.html#tymethod.chain_err //! [`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html //! [`Send`]: https://doc.rust-lang.org/std/marker/trait.Send.html //! [`Into`]: https://doc.rust-lang.org/std/convert/trait.Into.html //! [`From`]: https://doc.rust-lang.org/std/convert/trait.From.html //! [`PartialEq`]: https://doc.rust-lang.org/std/cmp/trait.PartialEq.html //! [`std::fmt::Error`]: https://doc.rust-lang.org/std/fmt/struct.Error.html //! [`.into()`]: https://doc.rust-lang.org/std/convert/trait.Into.html#tymethod.into //! [`map_err`]: https://doc.rust-lang.org/std/result/enum.Result.html#method.map_err //! [`BacktraceFrame`]: https://docs.rs/backtrace/0.3.2/backtrace/struct.BacktraceFrame.html use std::error; use std::iter::Iterator; use std::fmt; #[macro_use] mod impl_error_chain_kind; #[macro_use] mod error_chain; #[macro_use] mod quick_main; pub use quick_main::ExitCode; #[cfg(feature = "example_generated")] pub mod example_generated; mod backtrace; pub use backtrace::Backtrace; #[doc(hidden)] pub use backtrace::InternalBacktrace; #[derive(Debug)] /// Iterator over the error chain using the `Error::cause()` method. pub struct Iter<'a>(Option<&'a error::Error>); impl<'a> Iter<'a> { /// Returns a new iterator over the error chain using `Error::cause()`. pub fn new(err: Option<&'a error::Error>) -> Iter<'a> { Iter(err) } } impl<'a> Iterator for Iter<'a> { type Item = &'a error::Error; fn next<'b>(&'b mut self) -> Option<&'a error::Error> { match self.0.take() { Some(e) => { self.0 = match () { #[cfg(not(has_error_source))] () => e.cause(), #[cfg(has_error_source)] () => e.source(), }; Some(e) } None => 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) -> Iter; /// 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_chain<'a>(&'a self) -> DisplayChain<'a, Self> { DisplayChain(self) } /// Extends the error chain with a new entry. fn chain_err<F, EK>(self, error: F) -> Self where F: FnOnce() -> EK, EK: Into<Self::ErrorKind>; /// 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`. #[doc(hidden)] fn extract_backtrace(e: &(error::Error + Send + 'static)) -> Option<InternalBacktrace> where Self: Sized; } /// A struct which formats an error for output. #[derive(Debug)] pub struct DisplayChain<'a, T: 'a + ?Sized>(&'a T); impl<'a, T> fmt::Display for DisplayChain<'a, T> where T: ChainedError { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { // Keep `try!` for 1.10 support 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. pub backtrace: InternalBacktrace, } impl Default for State { fn default() -> State { State { next_error: None, backtrace: InternalBacktrace::new(), } } } impl State { /// Creates a new State type pub fn new<CE: ChainedError>(e: Box<error::Error + Send>) -> State { let backtrace = CE::extract_backtrace(&*e) .unwrap_or_else(InternalBacktrace::new); State { next_error: Some(e), backtrace: backtrace, } } /// Returns the inner backtrace if present. pub fn backtrace(&self) -> Option<&Backtrace> { self.backtrace.as_backtrace() } } /// 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(local_inner_macros)] 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!{} }