1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Traits for working with Errors.
13 //! # The `Error` trait
15 //! `Error` is a trait representing the basic expectations for error values,
16 //! i.e. values of type `E` in `Result<T, E>`. At a minimum, errors must provide
17 //! a description, but they may optionally provide additional detail (via
18 //! `Display`) and cause chain information:
21 //! use std::fmt::Display;
23 //! trait Error: Display {
24 //! fn description(&self) -> &str;
26 //! fn cause(&self) -> Option<&Error> { None }
30 //! The `cause` method is generally used when errors cross "abstraction
31 //! boundaries", i.e. when a one module must report an error that is "caused"
32 //! by an error from a lower-level module. This setup makes it possible for the
33 //! high-level module to provide its own errors that do not commit to any
34 //! particular implementation, but also reveal some of its implementation for
35 //! debugging via `cause` chains.
37 #![stable(feature = "rust1", since = "1.0.0")]
39 // A note about crates and the facade:
41 // Originally, the `Error` trait was defined in libcore, and the impls
42 // were scattered about. However, coherence objected to this
43 // arrangement, because to create the blanket impls for `Box` required
44 // knowing that `&str: !Error`, and we have no means to deal with that
45 // sort of conflict just now. Therefore, for the time being, we have
46 // moved the `Error` trait into libstd. As we evolve a sol'n to the
47 // coherence challenge (e.g., specialization, neg impls, etc) we can
48 // reconsider what crate these items belong in.
53 use fmt::{self, Debug, Display};
54 use marker::{Send, Sync, Reflect};
57 use option::Option::{self, Some, None};
58 use result::Result::{self, Ok, Err};
61 use string::{self, String};
63 /// Base functionality for all errors in Rust.
64 #[stable(feature = "rust1", since = "1.0.0")]
65 pub trait Error: Debug + Display + Reflect {
66 /// A short description of the error.
68 /// The description should not contain newlines or sentence-ending
69 /// punctuation, to facilitate embedding in larger user-facing
71 #[stable(feature = "rust1", since = "1.0.0")]
72 fn description(&self) -> &str;
74 /// The lower-level cause of this error, if any.
75 #[stable(feature = "rust1", since = "1.0.0")]
76 fn cause(&self) -> Option<&Error> { None }
78 /// Get the `TypeId` of `self`
80 #[unstable(feature = "error_type_id",
81 reason = "unclear whether to commit to this public implementation detail",
83 fn type_id(&self) -> TypeId where Self: 'static {
88 #[stable(feature = "rust1", since = "1.0.0")]
89 impl<'a, E: Error + 'a> From<E> for Box<Error + 'a> {
90 fn from(err: E) -> Box<Error + 'a> {
95 #[stable(feature = "rust1", since = "1.0.0")]
96 impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<Error + Send + Sync + 'a> {
97 fn from(err: E) -> Box<Error + Send + Sync + 'a> {
102 #[stable(feature = "rust1", since = "1.0.0")]
103 impl From<String> for Box<Error + Send + Sync> {
104 fn from(err: String) -> Box<Error + Send + Sync> {
106 struct StringError(String);
108 impl Error for StringError {
109 fn description(&self) -> &str { &self.0 }
112 impl Display for StringError {
113 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
114 Display::fmt(&self.0, f)
118 Box::new(StringError(err))
122 #[stable(feature = "rust1", since = "1.0.0")]
123 impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a> {
124 fn from(err: &'b str) -> Box<Error + Send + Sync + 'a> {
125 From::from(String::from(err))
129 #[stable(feature = "rust1", since = "1.0.0")]
130 impl Error for str::ParseBoolError {
131 fn description(&self) -> &str { "failed to parse bool" }
134 #[stable(feature = "rust1", since = "1.0.0")]
135 impl Error for str::Utf8Error {
136 fn description(&self) -> &str {
137 "invalid utf-8: corrupt contents"
141 #[stable(feature = "rust1", since = "1.0.0")]
142 impl Error for num::ParseIntError {
143 fn description(&self) -> &str {
148 #[stable(feature = "rust1", since = "1.0.0")]
149 impl Error for num::ParseFloatError {
150 fn description(&self) -> &str {
155 #[stable(feature = "rust1", since = "1.0.0")]
156 impl Error for string::FromUtf8Error {
157 fn description(&self) -> &str {
162 #[stable(feature = "rust1", since = "1.0.0")]
163 impl Error for string::FromUtf16Error {
164 fn description(&self) -> &str {
169 // copied from any.rs
170 impl Error + 'static {
171 /// Returns true if the boxed type is the same as `T`
172 #[stable(feature = "error_downcast", since = "1.3.0")]
174 pub fn is<T: Error + 'static>(&self) -> bool {
175 // Get TypeId of the type this function is instantiated with
176 let t = TypeId::of::<T>();
178 // Get TypeId of the type in the trait object
179 let boxed = self.type_id();
181 // Compare both TypeIds on equality
185 /// Returns some reference to the boxed value if it is of type `T`, or
186 /// `None` if it isn't.
187 #[stable(feature = "error_downcast", since = "1.3.0")]
189 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
192 // Get the raw representation of the trait object
193 let to: TraitObject = transmute(self);
195 // Extract the data pointer
196 Some(&*(to.data as *const T))
203 /// Returns some mutable reference to the boxed value if it is of type `T`, or
204 /// `None` if it isn't.
205 #[stable(feature = "error_downcast", since = "1.3.0")]
207 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
210 // Get the raw representation of the trait object
211 let to: TraitObject = transmute(self);
213 // Extract the data pointer
214 Some(&mut *(to.data as *const T as *mut T))
222 impl Error + 'static + Send {
223 /// Forwards to the method defined on the type `Any`.
224 #[stable(feature = "error_downcast", since = "1.3.0")]
226 pub fn is<T: Error + 'static>(&self) -> bool {
227 <Error + 'static>::is::<T>(self)
230 /// Forwards to the method defined on the type `Any`.
231 #[stable(feature = "error_downcast", since = "1.3.0")]
233 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
234 <Error + 'static>::downcast_ref::<T>(self)
237 /// Forwards to the method defined on the type `Any`.
238 #[stable(feature = "error_downcast", since = "1.3.0")]
240 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
241 <Error + 'static>::downcast_mut::<T>(self)
245 impl Error + 'static + Send + Sync {
246 /// Forwards to the method defined on the type `Any`.
247 #[stable(feature = "error_downcast", since = "1.3.0")]
249 pub fn is<T: Error + 'static>(&self) -> bool {
250 <Error + 'static>::is::<T>(self)
253 /// Forwards to the method defined on the type `Any`.
254 #[stable(feature = "error_downcast", since = "1.3.0")]
256 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
257 <Error + 'static>::downcast_ref::<T>(self)
260 /// Forwards to the method defined on the type `Any`.
261 #[stable(feature = "error_downcast", since = "1.3.0")]
263 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
264 <Error + 'static>::downcast_mut::<T>(self)
270 #[stable(feature = "error_downcast", since = "1.3.0")]
271 /// Attempt to downcast the box to a concrete type.
272 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error>> {
275 // Get the raw representation of the trait object
276 let raw = Box::into_raw(self);
277 let to: TraitObject =
278 transmute::<*mut Error, TraitObject>(raw);
280 // Extract the data pointer
281 Ok(Box::from_raw(to.data as *mut T))
291 #[stable(feature = "error_downcast", since = "1.3.0")]
292 /// Attempt to downcast the box to a concrete type.
293 pub fn downcast<T: Error + 'static>(self: Box<Self>)
294 -> Result<Box<T>, Box<Error + Send>> {
295 let err: Box<Error> = self;
296 <Error>::downcast(err).map_err(|s| unsafe {
297 // reapply the Send marker
298 transmute::<Box<Error>, Box<Error + Send>>(s)
303 impl Error + Send + Sync {
305 #[stable(feature = "error_downcast", since = "1.3.0")]
306 /// Attempt to downcast the box to a concrete type.
307 pub fn downcast<T: Error + 'static>(self: Box<Self>)
308 -> Result<Box<T>, Box<Self>> {
309 let err: Box<Error> = self;
310 <Error>::downcast(err).map_err(|s| unsafe {
311 // reapply the Send+Sync marker
312 transmute::<Box<Error>, Box<Error + Send + Sync>>(s)
323 #[derive(Debug, PartialEq)]
325 #[derive(Debug, PartialEq)]
328 impl fmt::Display for A {
329 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
333 impl fmt::Display for B {
334 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
340 fn description(&self) -> &str { "A-desc" }
343 fn description(&self) -> &str { "A-desc" }
349 let mut a = &mut a as &mut (Error + 'static);
350 assert_eq!(a.downcast_ref::<A>(), Some(&A));
351 assert_eq!(a.downcast_ref::<B>(), None);
352 assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
353 assert_eq!(a.downcast_mut::<B>(), None);
355 let a: Box<Error> = Box::new(A);
356 match a.downcast::<B>() {
357 Ok(..) => panic!("expected error"),
358 Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),