failure/lib.rs
1//! An experimental new error-handling library. Guide-style introduction
2//! is available [here](https://boats.gitlab.io/failure/).
3//!
4//! The primary items exported by this library are:
5//!
6//! - `Fail`: a new trait for custom error types in Rust.
7//! - `Error`: a wrapper around `Fail` types to make it easy to coalesce them
8//! at higher levels.
9//!
10//! As a general rule, library authors should create their own error types and
11//! implement `Fail` for them, whereas application authors should primarily
12//! deal with the `Error` type. There are exceptions to this rule, though, in
13//! both directions, and users should do whatever seems most appropriate to
14//! their situation.
15//!
16//! ## Backtraces
17//!
18//! Backtraces are disabled by default. To turn backtraces on, enable
19//! the `backtrace` Cargo feature and set the `RUST_BACKTRACE` environment
20//! variable to a non-zero value (this also enables backtraces for panics).
21//! Use the `RUST_FAILURE_BACKTRACE` variable to enable or disable backtraces
22//! for `failure` specifically.
23#![cfg_attr(not(feature = "std"), no_std)]
24#![deny(missing_docs)]
25#![deny(warnings)]
26#![cfg_attr(feature = "small-error", feature(extern_types, allocator_api))]
27
28macro_rules! with_std { ($($i:item)*) => ($(#[cfg(feature = "std")]$i)*) }
29macro_rules! without_std { ($($i:item)*) => ($(#[cfg(not(feature = "std"))]$i)*) }
30
31// Re-export libcore using an alias so that the macros can work without
32// requiring `extern crate core` downstream.
33#[doc(hidden)]
34pub extern crate core as _core;
35
36mod as_fail;
37mod backtrace;
38#[cfg(feature = "std")]
39mod box_std;
40mod compat;
41mod context;
42mod result_ext;
43
44use core::any::TypeId;
45use core::fmt::{Debug, Display};
46
47pub use as_fail::AsFail;
48pub use backtrace::Backtrace;
49pub use compat::Compat;
50pub use context::Context;
51pub use result_ext::ResultExt;
52
53#[cfg(feature = "failure_derive")]
54#[allow(unused_imports)]
55#[macro_use]
56extern crate failure_derive;
57
58#[cfg(feature = "failure_derive")]
59#[doc(hidden)]
60pub use failure_derive::*;
61
62with_std! {
63 extern crate core;
64
65 mod sync_failure;
66 pub use sync_failure::SyncFailure;
67
68 mod error;
69
70 use std::error::Error as StdError;
71
72 pub use error::Error;
73
74 /// A common result with an `Error`.
75 pub type Fallible<T> = Result<T, Error>;
76
77 mod macros;
78 mod error_message;
79 pub use error_message::err_msg;
80}
81
82/// The `Fail` trait.
83///
84/// Implementors of this trait are called 'failures'.
85///
86/// All error types should implement `Fail`, which provides a baseline of
87/// functionality that they all share.
88///
89/// `Fail` has no required methods, but it does require that your type
90/// implement several other traits:
91///
92/// - `Display`: to print a user-friendly representation of the error.
93/// - `Debug`: to print a verbose, developer-focused representation of the
94/// error.
95/// - `Send + Sync`: Your error type is required to be safe to transfer to and
96/// reference from another thread
97///
98/// Additionally, all failures must be `'static`. This enables downcasting.
99///
100/// `Fail` provides several methods with default implementations. Two of these
101/// may be appropriate to override depending on the definition of your
102/// particular failure: the `cause` and `backtrace` methods.
103///
104/// The `failure_derive` crate provides a way to derive the `Fail` trait for
105/// your type. Additionally, all types that already implement
106/// `std::error::Error`, and are also `Send`, `Sync`, and `'static`, implement
107/// `Fail` by a blanket impl.
108pub trait Fail: Display + Debug + Send + Sync + 'static {
109 /// Returns the "name" of the error.
110 ///
111 /// This is typically the type name. Not all errors will implement
112 /// this. This method is expected to be most useful in situations
113 /// where errors need to be reported to external instrumentation systems
114 /// such as crash reporters.
115 fn name(&self) -> Option<&str> {
116 None
117 }
118
119 /// Returns a reference to the underlying cause of this failure, if it
120 /// is an error that wraps other errors.
121 ///
122 /// Returns `None` if this failure does not have another error as its
123 /// underlying cause. By default, this returns `None`.
124 ///
125 /// This should **never** return a reference to `self`, but only return
126 /// `Some` when it can return a **different** failure. Users may loop
127 /// over the cause chain, and returning `self` would result in an infinite
128 /// loop.
129 fn cause(&self) -> Option<&dyn Fail> {
130 None
131 }
132
133 /// Returns a reference to the `Backtrace` carried by this failure, if it
134 /// carries one.
135 ///
136 /// Returns `None` if this failure does not carry a backtrace. By
137 /// default, this returns `None`.
138 fn backtrace(&self) -> Option<&Backtrace> {
139 None
140 }
141
142 /// Provides context for this failure.
143 ///
144 /// This can provide additional information about this error, appropriate
145 /// to the semantics of the current layer. That is, if you have a
146 /// lower-level error, such as an IO error, you can provide additional context
147 /// about what that error means in the context of your function. This
148 /// gives users of this function more information about what has gone
149 /// wrong.
150 ///
151 /// This takes any type that implements `Display`, as well as
152 /// `Send`/`Sync`/`'static`. In practice, this means it can take a `String`
153 /// or a string literal, or another failure, or some other custom context-carrying
154 /// type.
155 fn context<D>(self, context: D) -> Context<D>
156 where
157 D: Display + Send + Sync + 'static,
158 Self: Sized,
159 {
160 Context::with_err(context, self)
161 }
162
163 /// Wraps this failure in a compatibility wrapper that implements
164 /// `std::error::Error`.
165 ///
166 /// This allows failures to be compatible with older crates that
167 /// expect types that implement the `Error` trait from `std::error`.
168 fn compat(self) -> Compat<Self>
169 where
170 Self: Sized,
171 {
172 Compat { error: self }
173 }
174
175 #[doc(hidden)]
176 #[deprecated(since = "0.1.2", note = "please use the 'iter_chain()' method instead")]
177 fn causes(&self) -> Causes
178 where
179 Self: Sized,
180 {
181 Causes { fail: Some(self) }
182 }
183
184 #[doc(hidden)]
185 #[deprecated(
186 since = "0.1.2",
187 note = "please use the 'find_root_cause()' method instead"
188 )]
189 fn root_cause(&self) -> &dyn Fail
190 where
191 Self: Sized,
192 {
193 find_root_cause(self)
194 }
195
196 #[doc(hidden)]
197 fn __private_get_type_id__(&self) -> TypeId {
198 TypeId::of::<Self>()
199 }
200}
201
202impl dyn Fail {
203 /// Attempts to downcast this failure to a concrete type by reference.
204 ///
205 /// If the underlying error is not of type `T`, this will return `None`.
206 pub fn downcast_ref<T: Fail>(&self) -> Option<&T> {
207 if self.__private_get_type_id__() == TypeId::of::<T>() {
208 unsafe { Some(&*(self as *const dyn Fail as *const T)) }
209 } else {
210 None
211 }
212 }
213
214 /// Attempts to downcast this failure to a concrete type by mutable
215 /// reference.
216 ///
217 /// If the underlying error is not of type `T`, this will return `None`.
218 pub fn downcast_mut<T: Fail>(&mut self) -> Option<&mut T> {
219 if self.__private_get_type_id__() == TypeId::of::<T>() {
220 unsafe { Some(&mut *(self as *mut dyn Fail as *mut T)) }
221 } else {
222 None
223 }
224 }
225
226 /// Returns the "root cause" of this `Fail` - the last value in the
227 /// cause chain which does not return an underlying `cause`.
228 ///
229 /// If this type does not have a cause, `self` is returned, because
230 /// it is its own root cause.
231 ///
232 /// This is equivalent to iterating over `iter_causes()` and taking
233 /// the last item.
234 pub fn find_root_cause(&self) -> &dyn Fail {
235 find_root_cause(self)
236 }
237
238 /// Returns a iterator over the causes of this `Fail` with the cause
239 /// of this fail as the first item and the `root_cause` as the final item.
240 ///
241 /// Use `iter_chain` to also include the fail itself.
242 pub fn iter_causes(&self) -> Causes {
243 Causes { fail: self.cause() }
244 }
245
246 /// Returns a iterator over all fails up the chain from the current
247 /// as the first item up to the `root_cause` as the final item.
248 ///
249 /// This means that the chain also includes the fail itself which
250 /// means that it does *not* start with `cause`. To skip the outermost
251 /// fail use `iter_causes` instead.
252 pub fn iter_chain(&self) -> Causes {
253 Causes { fail: Some(self) }
254 }
255
256 /// Deprecated alias to `find_root_cause`.
257 #[deprecated(
258 since = "0.1.2",
259 note = "please use the 'find_root_cause()' method instead"
260 )]
261 pub fn root_cause(&self) -> &dyn Fail {
262 find_root_cause(self)
263 }
264
265 /// Deprecated alias to `iter_chain`.
266 #[deprecated(since = "0.1.2", note = "please use the 'iter_chain()' method instead")]
267 pub fn causes(&self) -> Causes {
268 Causes { fail: Some(self) }
269 }
270}
271
272#[cfg(feature = "std")]
273impl<E: StdError + Send + Sync + 'static> Fail for E {}
274
275#[cfg(feature = "std")]
276impl Fail for Box<dyn Fail> {
277 fn cause(&self) -> Option<&dyn Fail> {
278 (**self).cause()
279 }
280
281 fn backtrace(&self) -> Option<&Backtrace> {
282 (**self).backtrace()
283 }
284}
285
286/// A iterator over the causes of a `Fail`
287pub struct Causes<'f> {
288 fail: Option<&'f dyn Fail>,
289}
290
291impl<'f> Iterator for Causes<'f> {
292 type Item = &'f dyn Fail;
293 fn next(&mut self) -> Option<&'f dyn Fail> {
294 self.fail.map(|fail| {
295 self.fail = fail.cause();
296 fail
297 })
298 }
299}
300
301fn find_root_cause(mut fail: &dyn Fail) -> &dyn Fail {
302 while let Some(cause) = fail.cause() {
303 fail = cause;
304 }
305
306 fail
307}