1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Traits for working with Errors.
//!
//! # The `Error` trait
//!
//! `Error` is a trait representing the basic expectations for error values,
//! i.e. values of type `E` in [`Result<T, E>`]. At a minimum, errors must provide
//! a description, but they may optionally provide additional detail (via
//! [`Display`]) and cause chain information:
//!
//! ```
//! use std::fmt::Display;
//!
//! trait Error: Display {
//!     fn description(&self) -> &str;
//!
//!     fn cause(&self) -> Option<&Error> { None }
//! }
//! ```
//!
//! The [`cause`] method is generally used when errors cross "abstraction
//! boundaries", i.e.  when a one module must report an error that is "caused"
//! by an error from a lower-level module. This setup makes it possible for the
//! high-level module to provide its own errors that do not commit to any
//! particular implementation, but also reveal some of its implementation for
//! debugging via [`cause`] chains.
//!
//! [`Result<T, E>`]: ../result/enum.Result.html
//! [`Display`]: ../fmt/trait.Display.html
//! [`cause`]: trait.Error.html#method.cause

#![stable(feature = "rust1", since = "1.0.0")]

// A note about crates and the facade:
//
// Originally, the `Error` trait was defined in libcore, and the impls
// were scattered about. However, coherence objected to this
// arrangement, because to create the blanket impls for `Box` required
// knowing that `&str: !Error`, and we have no means to deal with that
// sort of conflict just now. Therefore, for the time being, we have
// moved the `Error` trait into libstd. As we evolve a sol'n to the
// coherence challenge (e.g., specialization, neg impls, etc) we can
// reconsider what crate these items belong in.

use any::TypeId;
use cell;
use char;
use fmt::{self, Debug, Display};
use mem::transmute;
use num;
use str;
use string;

/// Base functionality for all errors in Rust.
#[stable(feature = "rust1", since = "1.0.0")]
pub trait Error: Debug + Display {
    /// A short description of the error.
    ///
    /// The description should only be used for a simple message.
    /// It should not contain newlines or sentence-ending punctuation,
    /// to facilitate embedding in larger user-facing strings.
    /// For showing formatted error messages with more information see
    /// [`Display`].
    ///
    /// [`Display`]: ../fmt/trait.Display.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    ///
    /// match "xc".parse::<u32>() {
    ///     Err(e) => {
    ///         println!("Error: {}", e.description());
    ///     }
    ///     _ => println!("No error"),
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn description(&self) -> &str;

    /// The lower-level cause of this error, if any.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::fmt;
    ///
    /// #[derive(Debug)]
    /// struct SuperError {
    ///     side: SuperErrorSideKick,
    /// }
    ///
    /// impl fmt::Display for SuperError {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(f, "SuperError is here!")
    ///     }
    /// }
    ///
    /// impl Error for SuperError {
    ///     fn description(&self) -> &str {
    ///         "I'm the superhero of errors!"
    ///     }
    ///
    ///     fn cause(&self) -> Option<&Error> {
    ///         Some(&self.side)
    ///     }
    /// }
    ///
    /// #[derive(Debug)]
    /// struct SuperErrorSideKick;
    ///
    /// impl fmt::Display for SuperErrorSideKick {
    ///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    ///         write!(f, "SuperErrorSideKick is here!")
    ///     }
    /// }
    ///
    /// impl Error for SuperErrorSideKick {
    ///     fn description(&self) -> &str {
    ///         "I'm SuperError side kick!"
    ///     }
    /// }
    ///
    /// fn get_super_error() -> Result<(), SuperError> {
    ///     Err(SuperError { side: SuperErrorSideKick })
    /// }
    ///
    /// fn main() {
    ///     match get_super_error() {
    ///         Err(e) => {
    ///             println!("Error: {}", e.description());
    ///             println!("Caused by: {}", e.cause().unwrap());
    ///         }
    ///         _ => println!("No error"),
    ///     }
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    fn cause(&self) -> Option<&Error> { None }

    /// Get the `TypeId` of `self`
    #[doc(hidden)]
    #[unstable(feature = "error_type_id",
               reason = "unclear whether to commit to this public implementation detail",
               issue = "27745")]
    fn type_id(&self) -> TypeId where Self: 'static {
        TypeId::of::<Self>()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, E: Error + 'a> From<E> for Box<Error + 'a> {
    fn from(err: E) -> Box<Error + 'a> {
        Box::new(err)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, E: Error + Send + Sync + 'a> From<E> for Box<Error + Send + Sync + 'a> {
    fn from(err: E) -> Box<Error + Send + Sync + 'a> {
        Box::new(err)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl From<String> for Box<Error + Send + Sync> {
    fn from(err: String) -> Box<Error + Send + Sync> {
        #[derive(Debug)]
        struct StringError(String);

        impl Error for StringError {
            fn description(&self) -> &str { &self.0 }
        }

        impl Display for StringError {
            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
                Display::fmt(&self.0, f)
            }
        }

        Box::new(StringError(err))
    }
}

#[stable(feature = "string_box_error", since = "1.7.0")]
impl From<String> for Box<Error> {
    fn from(str_err: String) -> Box<Error> {
        let err1: Box<Error + Send + Sync> = From::from(str_err);
        let err2: Box<Error> = err1;
        err2
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b> From<&'b str> for Box<Error + Send + Sync + 'a> {
    fn from(err: &'b str) -> Box<Error + Send + Sync + 'a> {
        From::from(String::from(err))
    }
}

#[stable(feature = "string_box_error", since = "1.7.0")]
impl<'a> From<&'a str> for Box<Error> {
    fn from(err: &'a str) -> Box<Error> {
        From::from(String::from(err))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for str::ParseBoolError {
    fn description(&self) -> &str { "failed to parse bool" }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for str::Utf8Error {
    fn description(&self) -> &str {
        "invalid utf-8: corrupt contents"
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for num::ParseIntError {
    fn description(&self) -> &str {
        self.__description()
    }
}

#[unstable(feature = "try_from", issue = "33417")]
impl Error for num::TryFromIntError {
    fn description(&self) -> &str {
        self.__description()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for num::ParseFloatError {
    fn description(&self) -> &str {
        self.__description()
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for string::FromUtf8Error {
    fn description(&self) -> &str {
        "invalid utf-8"
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl Error for string::FromUtf16Error {
    fn description(&self) -> &str {
        "invalid utf-16"
    }
}

#[stable(feature = "str_parse_error2", since = "1.8.0")]
impl Error for string::ParseError {
    fn description(&self) -> &str {
        match *self {}
    }
}

#[stable(feature = "decode_utf16", since = "1.9.0")]
impl Error for char::DecodeUtf16Error {
    fn description(&self) -> &str {
        "unpaired surrogate found"
    }
}

#[stable(feature = "box_error", since = "1.7.0")]
impl<T: Error> Error for Box<T> {
    fn description(&self) -> &str {
        Error::description(&**self)
    }

    fn cause(&self) -> Option<&Error> {
        Error::cause(&**self)
    }
}

#[stable(feature = "fmt_error", since = "1.11.0")]
impl Error for fmt::Error {
    fn description(&self) -> &str {
        "an error occurred when formatting an argument"
    }
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Error for cell::BorrowError {
    fn description(&self) -> &str {
        "already mutably borrowed"
    }
}

#[stable(feature = "try_borrow", since = "1.13.0")]
impl Error for cell::BorrowMutError {
    fn description(&self) -> &str {
        "already borrowed"
    }
}

#[unstable(feature = "try_from", issue = "33417")]
impl Error for char::CharTryFromError {
    fn description(&self) -> &str {
        "converted integer out of range for `char`"
    }
}

// copied from any.rs
impl Error + 'static {
    /// Returns true if the boxed type is the same as `T`
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn is<T: Error + 'static>(&self) -> bool {
        // Get TypeId of the type this function is instantiated with
        let t = TypeId::of::<T>();

        // Get TypeId of the type in the trait object
        let boxed = self.type_id();

        // Compare both TypeIds on equality
        t == boxed
    }

    /// Returns some reference to the boxed value if it is of type `T`, or
    /// `None` if it isn't.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
        if self.is::<T>() {
            unsafe {
                Some(&*(self as *const Error as *const T))
            }
        } else {
            None
        }
    }

    /// Returns some mutable reference to the boxed value if it is of type `T`, or
    /// `None` if it isn't.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
        if self.is::<T>() {
            unsafe {
                Some(&mut *(self as *mut Error as *mut T))
            }
        } else {
            None
        }
    }
}

impl Error + 'static + Send {
    /// Forwards to the method defined on the type `Any`.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn is<T: Error + 'static>(&self) -> bool {
        <Error + 'static>::is::<T>(self)
    }

    /// Forwards to the method defined on the type `Any`.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
        <Error + 'static>::downcast_ref::<T>(self)
    }

    /// Forwards to the method defined on the type `Any`.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
        <Error + 'static>::downcast_mut::<T>(self)
    }
}

impl Error + 'static + Send + Sync {
    /// Forwards to the method defined on the type `Any`.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn is<T: Error + 'static>(&self) -> bool {
        <Error + 'static>::is::<T>(self)
    }

    /// Forwards to the method defined on the type `Any`.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
        <Error + 'static>::downcast_ref::<T>(self)
    }

    /// Forwards to the method defined on the type `Any`.
    #[stable(feature = "error_downcast", since = "1.3.0")]
    #[inline]
    pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
        <Error + 'static>::downcast_mut::<T>(self)
    }
}

impl Error {
    #[inline]
    #[stable(feature = "error_downcast", since = "1.3.0")]
    /// Attempt to downcast the box to a concrete type.
    pub fn downcast<T: Error + 'static>(self: Box<Self>) -> Result<Box<T>, Box<Error>> {
        if self.is::<T>() {
            unsafe {
                let raw: *mut Error = Box::into_raw(self);
                Ok(Box::from_raw(raw as *mut T))
            }
        } else {
            Err(self)
        }
    }
}

impl Error + Send {
    #[inline]
    #[stable(feature = "error_downcast", since = "1.3.0")]
    /// Attempt to downcast the box to a concrete type.
    pub fn downcast<T: Error + 'static>(self: Box<Self>)
                                        -> Result<Box<T>, Box<Error + Send>> {
        let err: Box<Error> = self;
        <Error>::downcast(err).map_err(|s| unsafe {
            // reapply the Send marker
            transmute::<Box<Error>, Box<Error + Send>>(s)
        })
    }
}

impl Error + Send + Sync {
    #[inline]
    #[stable(feature = "error_downcast", since = "1.3.0")]
    /// Attempt to downcast the box to a concrete type.
    pub fn downcast<T: Error + 'static>(self: Box<Self>)
                                        -> Result<Box<T>, Box<Self>> {
        let err: Box<Error> = self;
        <Error>::downcast(err).map_err(|s| unsafe {
            // reapply the Send+Sync marker
            transmute::<Box<Error>, Box<Error + Send + Sync>>(s)
        })
    }
}

#[cfg(test)]
mod tests {
    use super::Error;
    use fmt;

    #[derive(Debug, PartialEq)]
    struct A;
    #[derive(Debug, PartialEq)]
    struct B;

    impl fmt::Display for A {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "A")
        }
    }
    impl fmt::Display for B {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "B")
        }
    }

    impl Error for A {
        fn description(&self) -> &str { "A-desc" }
    }
    impl Error for B {
        fn description(&self) -> &str { "A-desc" }
    }

    #[test]
    fn downcasting() {
        let mut a = A;
        let mut a = &mut a as &mut (Error + 'static);
        assert_eq!(a.downcast_ref::<A>(), Some(&A));
        assert_eq!(a.downcast_ref::<B>(), None);
        assert_eq!(a.downcast_mut::<A>(), Some(&mut A));
        assert_eq!(a.downcast_mut::<B>(), None);

        let a: Box<Error> = Box::new(A);
        match a.downcast::<B>() {
            Ok(..) => panic!("expected error"),
            Err(e) => assert_eq!(*e.downcast::<A>().unwrap(), A),
        }
    }
}