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//! An experimental new error-handling library. Guide-style introduction //! is available [here](https://boats.gitlab.io/failure/). //! //! The primary items exported by this library are: //! //! - `Fail`: a new trait for custom error types in Rust. //! - `Error`: a wrapper around `Fail` types to make it easy to coalesce them //! at higher levels. //! //! As a general rule, library authors should create their own error types and //! implement `Fail` for them, whereas application authors should primarily //! deal with the `Error` type. There are exceptions to this rule, though, in //! both directions, and users should do whatever seems most appropriate to //! their situation. //! //! ## Backtraces //! //! Backtraces are disabled by default. To turn backtraces on, enable //! the `backtrace` Cargo feature and set the `RUST_BACKTRACE` environment //! variable to a non-zero value (this also enables backtraces for panics). //! Use the `RUST_FAILURE_BACKTRACE` variable to enable or disable backtraces //! for `failure` specifically. #![cfg_attr(not(feature = "std"), no_std)] #![deny(missing_docs)] #![deny(warnings)] #![cfg_attr(feature = "small-error", feature(extern_types, allocator_api))] macro_rules! with_std { ($($i:item)*) => ($(#[cfg(feature = "std")]$i)*) } macro_rules! without_std { ($($i:item)*) => ($(#[cfg(not(feature = "std"))]$i)*) } // Re-export libcore using an alias so that the macros can work without // requiring `extern crate core` downstream. #[doc(hidden)] pub extern crate core as _core; mod as_fail; mod backtrace; #[cfg(feature = "std")] mod box_std; mod compat; mod context; mod result_ext; use core::any::TypeId; use core::fmt::{Debug, Display}; pub use as_fail::AsFail; pub use backtrace::Backtrace; pub use compat::Compat; pub use context::Context; pub use result_ext::ResultExt; #[cfg(feature = "failure_derive")] #[allow(unused_imports)] #[macro_use] extern crate failure_derive; #[cfg(feature = "failure_derive")] #[doc(hidden)] pub use failure_derive::*; with_std! { extern crate core; mod sync_failure; pub use sync_failure::SyncFailure; mod error; use std::error::Error as StdError; pub use error::Error; /// A common result with an `Error`. pub type Fallible<T> = Result<T, Error>; mod macros; mod error_message; pub use error_message::err_msg; } /// The `Fail` trait. /// /// Implementors of this trait are called 'failures'. /// /// All error types should implement `Fail`, which provides a baseline of /// functionality that they all share. /// /// `Fail` has no required methods, but it does require that your type /// implement several other traits: /// /// - `Display`: to print a user-friendly representation of the error. /// - `Debug`: to print a verbose, developer-focused representation of the /// error. /// - `Send + Sync`: Your error type is required to be safe to transfer to and /// reference from another thread /// /// Additionally, all failures must be `'static`. This enables downcasting. /// /// `Fail` provides several methods with default implementations. Two of these /// may be appropriate to override depending on the definition of your /// particular failure: the `cause` and `backtrace` methods. /// /// The `failure_derive` crate provides a way to derive the `Fail` trait for /// your type. Additionally, all types that already implement /// `std::error::Error`, and are also `Send`, `Sync`, and `'static`, implement /// `Fail` by a blanket impl. pub trait Fail: Display + Debug + Send + Sync + 'static { /// Returns the "name" of the error. /// /// This is typically the type name. Not all errors will implement /// this. This method is expected to be most useful in situations /// where errors need to be reported to external instrumentation systems /// such as crash reporters. fn name(&self) -> Option<&str> { None } /// Returns a reference to the underlying cause of this failure, if it /// is an error that wraps other errors. /// /// Returns `None` if this failure does not have another error as its /// underlying cause. By default, this returns `None`. /// /// This should **never** return a reference to `self`, but only return /// `Some` when it can return a **different** failure. Users may loop /// over the cause chain, and returning `self` would result in an infinite /// loop. fn cause(&self) -> Option<&Fail> { None } /// Returns a reference to the `Backtrace` carried by this failure, if it /// carries one. /// /// Returns `None` if this failure does not carry a backtrace. By /// default, this returns `None`. fn backtrace(&self) -> Option<&Backtrace> { None } /// Provides context for this failure. /// /// This can provide additional information about this error, appropriate /// to the semantics of the current layer. That is, if you have a /// lower-level error, such as an IO error, you can provide additional context /// about what that error means in the context of your function. This /// gives users of this function more information about what has gone /// wrong. /// /// This takes any type that implements `Display`, as well as /// `Send`/`Sync`/`'static`. In practice, this means it can take a `String` /// or a string literal, or another failure, or some other custom context-carrying /// type. fn context<D>(self, context: D) -> Context<D> where D: Display + Send + Sync + 'static, Self: Sized, { Context::with_err(context, self) } /// Wraps this failure in a compatibility wrapper that implements /// `std::error::Error`. /// /// This allows failures to be compatible with older crates that /// expect types that implement the `Error` trait from `std::error`. fn compat(self) -> Compat<Self> where Self: Sized, { Compat { error: self } } #[doc(hidden)] #[deprecated(since = "0.1.2", note = "please use the 'iter_chain()' method instead")] fn causes(&self) -> Causes where Self: Sized, { Causes { fail: Some(self) } } #[doc(hidden)] #[deprecated( since = "0.1.2", note = "please use the 'find_root_cause()' method instead" )] fn root_cause(&self) -> &Fail where Self: Sized, { find_root_cause(self) } #[doc(hidden)] fn __private_get_type_id__(&self) -> TypeId { TypeId::of::<Self>() } } impl Fail { /// Attempts to downcast this failure to a concrete type by reference. /// /// If the underlying error is not of type `T`, this will return `None`. pub fn downcast_ref<T: Fail>(&self) -> Option<&T> { if self.__private_get_type_id__() == TypeId::of::<T>() { unsafe { Some(&*(self as *const Fail as *const T)) } } else { None } } /// Attempts to downcast this failure to a concrete type by mutable /// reference. /// /// If the underlying error is not of type `T`, this will return `None`. pub fn downcast_mut<T: Fail>(&mut self) -> Option<&mut T> { if self.__private_get_type_id__() == TypeId::of::<T>() { unsafe { Some(&mut *(self as *mut Fail as *mut T)) } } else { None } } /// Returns the "root cause" of this `Fail` - the last value in the /// cause chain which does not return an underlying `cause`. /// /// If this type does not have a cause, `self` is returned, because /// it is its own root cause. /// /// This is equivalent to iterating over `iter_causes()` and taking /// the last item. pub fn find_root_cause(&self) -> &Fail { find_root_cause(self) } /// Returns a iterator over the causes of this `Fail` with the cause /// of this fail as the first item and the `root_cause` as the final item. /// /// Use `iter_chain` to also include the fail itself. pub fn iter_causes(&self) -> Causes { Causes { fail: self.cause() } } /// Returns a iterator over all fails up the chain from the current /// as the first item up to the `root_cause` as the final item. /// /// This means that the chain also includes the fail itself which /// means that it does *not* start with `cause`. To skip the outermost /// fail use `iter_causes` instead. pub fn iter_chain(&self) -> Causes { Causes { fail: Some(self) } } /// Deprecated alias to `find_root_cause`. #[deprecated( since = "0.1.2", note = "please use the 'find_root_cause()' method instead" )] pub fn root_cause(&self) -> &Fail { find_root_cause(self) } /// Deprecated alias to `iter_chain`. #[deprecated(since = "0.1.2", note = "please use the 'iter_chain()' method instead")] pub fn causes(&self) -> Causes { Causes { fail: Some(self) } } } #[cfg(feature = "std")] impl<E: StdError + Send + Sync + 'static> Fail for E {} #[cfg(feature = "std")] impl Fail for Box<Fail> { fn cause(&self) -> Option<&Fail> { (**self).cause() } fn backtrace(&self) -> Option<&Backtrace> { (**self).backtrace() } } /// A iterator over the causes of a `Fail` pub struct Causes<'f> { fail: Option<&'f Fail>, } impl<'f> Iterator for Causes<'f> { type Item = &'f Fail; fn next(&mut self) -> Option<&'f Fail> { self.fail.map(|fail| { self.fail = fail.cause(); fail }) } } fn find_root_cause(mut fail: &Fail) -> &Fail { while let Some(cause) = fail.cause() { fail = cause; } fail }