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 = "string_box_error", since = "1.7.0")]
123 impl From<String> for Box<Error> {
124 fn from(str_err: String) -> Box<Error> {
125 let err1: Box<Error + Send + Sync> = From::from(str_err);
126 let err2: Box<Error> = err1;
131 #[stable(feature = "rust1", since = "1.0.0")]
132 impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a> {
133 fn from(err: &'b str) -> Box<Error + Send + Sync + 'a> {
134 From::from(String::from(err))
138 #[stable(feature = "string_box_error", since = "1.7.0")]
139 impl<'a> From<&'a str> for Box<Error> {
140 fn from(err: &'a str) -> Box<Error> {
141 From::from(String::from(err))
145 #[stable(feature = "rust1", since = "1.0.0")]
146 impl Error for str::ParseBoolError {
147 fn description(&self) -> &str { "failed to parse bool" }
150 #[stable(feature = "rust1", since = "1.0.0")]
151 impl Error for str::Utf8Error {
152 fn description(&self) -> &str {
153 "invalid utf-8: corrupt contents"
157 #[stable(feature = "rust1", since = "1.0.0")]
158 impl Error for num::ParseIntError {
159 fn description(&self) -> &str {
164 #[stable(feature = "rust1", since = "1.0.0")]
165 impl Error for num::ParseFloatError {
166 fn description(&self) -> &str {
171 #[stable(feature = "rust1", since = "1.0.0")]
172 impl Error for string::FromUtf8Error {
173 fn description(&self) -> &str {
178 #[stable(feature = "rust1", since = "1.0.0")]
179 impl Error for string::FromUtf16Error {
180 fn description(&self) -> &str {
185 // copied from any.rs
186 impl Error + 'static {
187 /// Returns true if the boxed type is the same as `T`
188 #[stable(feature = "error_downcast", since = "1.3.0")]
190 pub fn is<T: Error + 'static>(&self) -> bool {
191 // Get TypeId of the type this function is instantiated with
192 let t = TypeId::of::<T>();
194 // Get TypeId of the type in the trait object
195 let boxed = self.type_id();
197 // Compare both TypeIds on equality
201 /// Returns some reference to the boxed value if it is of type `T`, or
202 /// `None` if it isn't.
203 #[stable(feature = "error_downcast", since = "1.3.0")]
205 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
208 // Get the raw representation of the trait object
209 let to: TraitObject = transmute(self);
211 // Extract the data pointer
212 Some(&*(to.data as *const T))
219 /// Returns some mutable reference to the boxed value if it is of type `T`, or
220 /// `None` if it isn't.
221 #[stable(feature = "error_downcast", since = "1.3.0")]
223 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
226 // Get the raw representation of the trait object
227 let to: TraitObject = transmute(self);
229 // Extract the data pointer
230 Some(&mut *(to.data as *const T as *mut T))
238 impl Error + 'static + Send {
239 /// Forwards to the method defined on the type `Any`.
240 #[stable(feature = "error_downcast", since = "1.3.0")]
242 pub fn is<T: Error + 'static>(&self) -> bool {
243 <Error + 'static>::is::<T>(self)
246 /// Forwards to the method defined on the type `Any`.
247 #[stable(feature = "error_downcast", since = "1.3.0")]
249 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
250 <Error + 'static>::downcast_ref::<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_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
257 <Error + 'static>::downcast_mut::<T>(self)
261 impl Error + 'static + Send + Sync {
262 /// Forwards to the method defined on the type `Any`.
263 #[stable(feature = "error_downcast", since = "1.3.0")]
265 pub fn is<T: Error + 'static>(&self) -> bool {
266 <Error + 'static>::is::<T>(self)
269 /// Forwards to the method defined on the type `Any`.
270 #[stable(feature = "error_downcast", since = "1.3.0")]
272 pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
273 <Error + 'static>::downcast_ref::<T>(self)
276 /// Forwards to the method defined on the type `Any`.
277 #[stable(feature = "error_downcast", since = "1.3.0")]
279 pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
280 <Error + 'static>::downcast_mut::<T>(self)
286 #[stable(feature = "error_downcast", since = "1.3.0")]
287 /// Attempt to downcast the box to a concrete type.
288 pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error>> {
291 // Get the raw representation of the trait object
292 let raw = Box::into_raw(self);
293 let to: TraitObject =
294 transmute::<*mut Error, TraitObject>(raw);
296 // Extract the data pointer
297 Ok(Box::from_raw(to.data as *mut T))
307 #[stable(feature = "error_downcast", since = "1.3.0")]
308 /// Attempt to downcast the box to a concrete type.
309 pub fn downcast<T: Error + 'static>(self: Box<Self>)
310 -> Result<Box<T>, Box<Error + Send>> {
311 let err: Box<Error> = self;
312 <Error>::downcast(err).map_err(|s| unsafe {
313 // reapply the Send marker
314 transmute::<Box<Error>, Box<Error + Send>>(s)
319 impl Error + Send + Sync {
321 #[stable(feature = "error_downcast", since = "1.3.0")]
322 /// Attempt to downcast the box to a concrete type.
323 pub fn downcast<T: Error + 'static>(self: Box<Self>)
324 -> Result<Box<T>, Box<Self>> {
325 let err: Box<Error> = self;
326 <Error>::downcast(err).map_err(|s| unsafe {
327 // reapply the Send+Sync marker
328 transmute::<Box<Error>, Box<Error + Send + Sync>>(s)
339 #[derive(Debug, PartialEq)]
341 #[derive(Debug, PartialEq)]
344 impl fmt::Display for A {
345 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
349 impl fmt::Display for B {
350 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
356 fn description(&self) -> &str { "A-desc" }
359 fn description(&self) -> &str { "A-desc" }
365 let mut a = &mut a as &mut (Error + 'static);
366 assert_eq!(a.downcast_ref::<A>(), Some(&A));
367 assert_eq!(a.downcast_ref::<B>(), None);
368 assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
369 assert_eq!(a.downcast_mut::<B>(), None);
371 let a: Box<Error> = Box::new(A);
372 match a.downcast::<B>() {
373 Ok(..) => panic!("expected error"),
374 Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),