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
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
use crate::backtrace::Backtrace;
use crate::chain::Chain;
#[cfg(any(feature = "std", anyhow_no_ptr_addr_of))]
use crate::ptr::Mut;
use crate::ptr::{Own, Ref};
use crate::{Error, StdError};
use alloc::boxed::Box;
use core::any::TypeId;
use core::fmt::{self, Debug, Display};
use core::mem::ManuallyDrop;
#[cfg(not(anyhow_no_ptr_addr_of))]
use core::ptr;
use core::ptr::NonNull;
#[cfg(backtrace)]
use std::error::{self, Request};

#[cfg(feature = "std")]
use core::ops::{Deref, DerefMut};

impl Error {
    /// Create a new error object from any error type.
    ///
    /// The error type must be threadsafe and `'static`, so that the `Error`
    /// will be as well.
    ///
    /// If the error type does not provide a backtrace, a backtrace will be
    /// created here to ensure that a backtrace exists.
    #[cfg(feature = "std")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
    #[cold]
    #[must_use]
    pub fn new<E>(error: E) -> Self
    where
        E: StdError + Send + Sync + 'static,
    {
        let backtrace = backtrace_if_absent!(&error);
        Error::from_std(error, backtrace)
    }

    /// Create a new error object from a printable error message.
    ///
    /// If the argument implements std::error::Error, prefer `Error::new`
    /// instead which preserves the underlying error's cause chain and
    /// backtrace. If the argument may or may not implement std::error::Error
    /// now or in the future, use `anyhow!(err)` which handles either way
    /// correctly.
    ///
    /// `Error::msg("...")` is equivalent to `anyhow!("...")` but occasionally
    /// convenient in places where a function is preferable over a macro, such
    /// as iterator or stream combinators:
    ///
    /// ```
    /// # mod ffi {
    /// #     pub struct Input;
    /// #     pub struct Output;
    /// #     pub async fn do_some_work(_: Input) -> Result<Output, &'static str> {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// #
    /// # use ffi::{Input, Output};
    /// #
    /// use anyhow::{Error, Result};
    /// use futures::stream::{Stream, StreamExt, TryStreamExt};
    ///
    /// async fn demo<S>(stream: S) -> Result<Vec<Output>>
    /// where
    ///     S: Stream<Item = Input>,
    /// {
    ///     stream
    ///         .then(ffi::do_some_work) // returns Result<Output, &str>
    ///         .map_err(Error::msg)
    ///         .try_collect()
    ///         .await
    /// }
    /// ```
    #[cold]
    #[must_use]
    pub fn msg<M>(message: M) -> Self
    where
        M: Display + Debug + Send + Sync + 'static,
    {
        Error::from_adhoc(message, backtrace!())
    }

    #[cfg(feature = "std")]
    #[cold]
    pub(crate) fn from_std<E>(error: E, backtrace: Option<Backtrace>) -> Self
    where
        E: StdError + Send + Sync + 'static,
    {
        let vtable = &ErrorVTable {
            object_drop: object_drop::<E>,
            object_ref: object_ref::<E>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_mut: object_mut::<E>,
            object_boxed: object_boxed::<E>,
            object_downcast: object_downcast::<E>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_downcast_mut: object_downcast_mut::<E>,
            object_drop_rest: object_drop_front::<E>,
            #[cfg(all(not(backtrace), feature = "backtrace"))]
            object_backtrace: no_backtrace,
        };

        // Safety: passing vtable that operates on the right type E.
        unsafe { Error::construct(error, vtable, backtrace) }
    }

    #[cold]
    pub(crate) fn from_adhoc<M>(message: M, backtrace: Option<Backtrace>) -> Self
    where
        M: Display + Debug + Send + Sync + 'static,
    {
        use crate::wrapper::MessageError;
        let error: MessageError<M> = MessageError(message);
        let vtable = &ErrorVTable {
            object_drop: object_drop::<MessageError<M>>,
            object_ref: object_ref::<MessageError<M>>,
            #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))]
            object_mut: object_mut::<MessageError<M>>,
            object_boxed: object_boxed::<MessageError<M>>,
            object_downcast: object_downcast::<M>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_downcast_mut: object_downcast_mut::<M>,
            object_drop_rest: object_drop_front::<M>,
            #[cfg(all(not(backtrace), feature = "backtrace"))]
            object_backtrace: no_backtrace,
        };

        // Safety: MessageError is repr(transparent) so it is okay for the
        // vtable to allow casting the MessageError<M> to M.
        unsafe { Error::construct(error, vtable, backtrace) }
    }

    #[cold]
    pub(crate) fn from_display<M>(message: M, backtrace: Option<Backtrace>) -> Self
    where
        M: Display + Send + Sync + 'static,
    {
        use crate::wrapper::DisplayError;
        let error: DisplayError<M> = DisplayError(message);
        let vtable = &ErrorVTable {
            object_drop: object_drop::<DisplayError<M>>,
            object_ref: object_ref::<DisplayError<M>>,
            #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))]
            object_mut: object_mut::<DisplayError<M>>,
            object_boxed: object_boxed::<DisplayError<M>>,
            object_downcast: object_downcast::<M>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_downcast_mut: object_downcast_mut::<M>,
            object_drop_rest: object_drop_front::<M>,
            #[cfg(all(not(backtrace), feature = "backtrace"))]
            object_backtrace: no_backtrace,
        };

        // Safety: DisplayError is repr(transparent) so it is okay for the
        // vtable to allow casting the DisplayError<M> to M.
        unsafe { Error::construct(error, vtable, backtrace) }
    }

    #[cfg(feature = "std")]
    #[cold]
    pub(crate) fn from_context<C, E>(context: C, error: E, backtrace: Option<Backtrace>) -> Self
    where
        C: Display + Send + Sync + 'static,
        E: StdError + Send + Sync + 'static,
    {
        let error: ContextError<C, E> = ContextError { context, error };

        let vtable = &ErrorVTable {
            object_drop: object_drop::<ContextError<C, E>>,
            object_ref: object_ref::<ContextError<C, E>>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_mut: object_mut::<ContextError<C, E>>,
            object_boxed: object_boxed::<ContextError<C, E>>,
            object_downcast: context_downcast::<C, E>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_downcast_mut: context_downcast_mut::<C, E>,
            object_drop_rest: context_drop_rest::<C, E>,
            #[cfg(all(not(backtrace), feature = "backtrace"))]
            object_backtrace: no_backtrace,
        };

        // Safety: passing vtable that operates on the right type.
        unsafe { Error::construct(error, vtable, backtrace) }
    }

    #[cfg(feature = "std")]
    #[cold]
    pub(crate) fn from_boxed(
        error: Box<dyn StdError + Send + Sync>,
        backtrace: Option<Backtrace>,
    ) -> Self {
        use crate::wrapper::BoxedError;
        let error = BoxedError(error);
        let vtable = &ErrorVTable {
            object_drop: object_drop::<BoxedError>,
            object_ref: object_ref::<BoxedError>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_mut: object_mut::<BoxedError>,
            object_boxed: object_boxed::<BoxedError>,
            object_downcast: object_downcast::<Box<dyn StdError + Send + Sync>>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_downcast_mut: object_downcast_mut::<Box<dyn StdError + Send + Sync>>,
            object_drop_rest: object_drop_front::<Box<dyn StdError + Send + Sync>>,
            #[cfg(all(not(backtrace), feature = "backtrace"))]
            object_backtrace: no_backtrace,
        };

        // Safety: BoxedError is repr(transparent) so it is okay for the vtable
        // to allow casting to Box<dyn StdError + Send + Sync>.
        unsafe { Error::construct(error, vtable, backtrace) }
    }

    // Takes backtrace as argument rather than capturing it here so that the
    // user sees one fewer layer of wrapping noise in the backtrace.
    //
    // Unsafe because the given vtable must have sensible behavior on the error
    // value of type E.
    #[cold]
    unsafe fn construct<E>(
        error: E,
        vtable: &'static ErrorVTable,
        backtrace: Option<Backtrace>,
    ) -> Self
    where
        E: StdError + Send + Sync + 'static,
    {
        let inner: Box<ErrorImpl<E>> = Box::new(ErrorImpl {
            vtable,
            backtrace,
            _object: error,
        });
        // Erase the concrete type of E from the compile-time type system. This
        // is equivalent to the safe unsize coercion from Box<ErrorImpl<E>> to
        // Box<ErrorImpl<dyn StdError + Send + Sync + 'static>> except that the
        // result is a thin pointer. The necessary behavior for manipulating the
        // underlying ErrorImpl<E> is preserved in the vtable provided by the
        // caller rather than a builtin fat pointer vtable.
        let inner = Own::new(inner).cast::<ErrorImpl>();
        Error { inner }
    }

    /// Wrap the error value with additional context.
    ///
    /// For attaching context to a `Result` as it is propagated, the
    /// [`Context`][crate::Context] extension trait may be more convenient than
    /// this function.
    ///
    /// The primary reason to use `error.context(...)` instead of
    /// `result.context(...)` via the `Context` trait would be if the context
    /// needs to depend on some data held by the underlying error:
    ///
    /// ```
    /// # use std::fmt::{self, Debug, Display};
    /// #
    /// # type T = ();
    /// #
    /// # impl std::error::Error for ParseError {}
    /// # impl Debug for ParseError {
    /// #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// # impl Display for ParseError {
    /// #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// #
    /// use anyhow::Result;
    /// use std::fs::File;
    /// use std::path::Path;
    ///
    /// struct ParseError {
    ///     line: usize,
    ///     column: usize,
    /// }
    ///
    /// fn parse_impl(file: File) -> Result<T, ParseError> {
    ///     # const IGNORE: &str = stringify! {
    ///     ...
    ///     # };
    ///     # unimplemented!()
    /// }
    ///
    /// pub fn parse(path: impl AsRef<Path>) -> Result<T> {
    ///     let file = File::open(&path)?;
    ///     parse_impl(file).map_err(|error| {
    ///         let context = format!(
    ///             "only the first {} lines of {} are valid",
    ///             error.line, path.as_ref().display(),
    ///         );
    ///         anyhow::Error::new(error).context(context)
    ///     })
    /// }
    /// ```
    #[cold]
    #[must_use]
    pub fn context<C>(self, context: C) -> Self
    where
        C: Display + Send + Sync + 'static,
    {
        let error: ContextError<C, Error> = ContextError {
            context,
            error: self,
        };

        let vtable = &ErrorVTable {
            object_drop: object_drop::<ContextError<C, Error>>,
            object_ref: object_ref::<ContextError<C, Error>>,
            #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))]
            object_mut: object_mut::<ContextError<C, Error>>,
            object_boxed: object_boxed::<ContextError<C, Error>>,
            object_downcast: context_chain_downcast::<C>,
            #[cfg(anyhow_no_ptr_addr_of)]
            object_downcast_mut: context_chain_downcast_mut::<C>,
            object_drop_rest: context_chain_drop_rest::<C>,
            #[cfg(all(not(backtrace), feature = "backtrace"))]
            object_backtrace: context_backtrace::<C>,
        };

        // As the cause is anyhow::Error, we already have a backtrace for it.
        let backtrace = None;

        // Safety: passing vtable that operates on the right type.
        unsafe { Error::construct(error, vtable, backtrace) }
    }

    /// Get the backtrace for this Error.
    ///
    /// In order for the backtrace to be meaningful, one of the two environment
    /// variables `RUST_LIB_BACKTRACE=1` or `RUST_BACKTRACE=1` must be defined
    /// and `RUST_LIB_BACKTRACE` must not be `0`. Backtraces are somewhat
    /// expensive to capture in Rust, so we don't necessarily want to be
    /// capturing them all over the place all the time.
    ///
    /// - If you want panics and errors to both have backtraces, set
    ///   `RUST_BACKTRACE=1`;
    /// - If you want only errors to have backtraces, set
    ///   `RUST_LIB_BACKTRACE=1`;
    /// - If you want only panics to have backtraces, set `RUST_BACKTRACE=1` and
    ///   `RUST_LIB_BACKTRACE=0`.
    ///
    /// # Stability
    ///
    /// Standard library backtraces are only available on the nightly channel.
    /// Tracking issue: [rust-lang/rust#53487][tracking].
    ///
    /// On stable compilers, this function is only available if the crate's
    /// "backtrace" feature is enabled, and will use the `backtrace` crate as
    /// the underlying backtrace implementation.
    ///
    /// ```toml
    /// [dependencies]
    /// anyhow = { version = "1.0", features = ["backtrace"] }
    /// ```
    ///
    /// [tracking]: https://github.com/rust-lang/rust/issues/53487
    #[cfg(any(backtrace, feature = "backtrace"))]
    #[cfg_attr(doc_cfg, doc(cfg(any(nightly, feature = "backtrace"))))]
    pub fn backtrace(&self) -> &impl_backtrace!() {
        unsafe { ErrorImpl::backtrace(self.inner.by_ref()) }
    }

    /// An iterator of the chain of source errors contained by this Error.
    ///
    /// This iterator will visit every error in the cause chain of this error
    /// object, beginning with the error that this error object was created
    /// from.
    ///
    /// # Example
    ///
    /// ```
    /// use anyhow::Error;
    /// use std::io;
    ///
    /// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
    ///     for cause in error.chain() {
    ///         if let Some(io_error) = cause.downcast_ref::<io::Error>() {
    ///             return Some(io_error.kind());
    ///         }
    ///     }
    ///     None
    /// }
    /// ```
    #[cfg(feature = "std")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
    #[cold]
    pub fn chain(&self) -> Chain {
        unsafe { ErrorImpl::chain(self.inner.by_ref()) }
    }

    /// The lowest level cause of this error &mdash; this error's cause's
    /// cause's cause etc.
    ///
    /// The root cause is the last error in the iterator produced by
    /// [`chain()`][Error::chain].
    #[cfg(feature = "std")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
    pub fn root_cause(&self) -> &(dyn StdError + 'static) {
        self.chain().last().unwrap()
    }

    /// Returns true if `E` is the type held by this error object.
    ///
    /// For errors with context, this method returns true if `E` matches the
    /// type of the context `C` **or** the type of the error on which the
    /// context has been attached. For details about the interaction between
    /// context and downcasting, [see here].
    ///
    /// [see here]: trait.Context.html#effect-on-downcasting
    pub fn is<E>(&self) -> bool
    where
        E: Display + Debug + Send + Sync + 'static,
    {
        self.downcast_ref::<E>().is_some()
    }

    /// Attempt to downcast the error object to a concrete type.
    pub fn downcast<E>(mut self) -> Result<E, Self>
    where
        E: Display + Debug + Send + Sync + 'static,
    {
        let target = TypeId::of::<E>();
        let inner = self.inner.by_mut();
        unsafe {
            // Use vtable to find NonNull<()> which points to a value of type E
            // somewhere inside the data structure.
            #[cfg(not(anyhow_no_ptr_addr_of))]
            let addr = match (vtable(inner.ptr).object_downcast)(inner.by_ref(), target) {
                Some(addr) => addr.by_mut().extend(),
                None => return Err(self),
            };
            #[cfg(anyhow_no_ptr_addr_of)]
            let addr = match (vtable(inner.ptr).object_downcast_mut)(inner, target) {
                Some(addr) => addr.extend(),
                None => return Err(self),
            };

            // Prepare to read E out of the data structure. We'll drop the rest
            // of the data structure separately so that E is not dropped.
            let outer = ManuallyDrop::new(self);

            // Read E from where the vtable found it.
            let error = addr.cast::<E>().read();

            // Drop rest of the data structure outside of E.
            (vtable(outer.inner.ptr).object_drop_rest)(outer.inner, target);

            Ok(error)
        }
    }

    /// Downcast this error object by reference.
    ///
    /// # Example
    ///
    /// ```
    /// # use anyhow::anyhow;
    /// # use std::fmt::{self, Display};
    /// # use std::task::Poll;
    /// #
    /// # #[derive(Debug)]
    /// # enum DataStoreError {
    /// #     Censored(()),
    /// # }
    /// #
    /// # impl Display for DataStoreError {
    /// #     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
    /// #         unimplemented!()
    /// #     }
    /// # }
    /// #
    /// # impl std::error::Error for DataStoreError {}
    /// #
    /// # const REDACTED_CONTENT: () = ();
    /// #
    /// # let error = anyhow!("...");
    /// # let root_cause = &error;
    /// #
    /// # let ret =
    /// // If the error was caused by redaction, then return a tombstone instead
    /// // of the content.
    /// match root_cause.downcast_ref::<DataStoreError>() {
    ///     Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
    ///     None => Err(error),
    /// }
    /// # ;
    /// ```
    pub fn downcast_ref<E>(&self) -> Option<&E>
    where
        E: Display + Debug + Send + Sync + 'static,
    {
        let target = TypeId::of::<E>();
        unsafe {
            // Use vtable to find NonNull<()> which points to a value of type E
            // somewhere inside the data structure.
            let addr = (vtable(self.inner.ptr).object_downcast)(self.inner.by_ref(), target)?;
            Some(addr.cast::<E>().deref())
        }
    }

    /// Downcast this error object by mutable reference.
    pub fn downcast_mut<E>(&mut self) -> Option<&mut E>
    where
        E: Display + Debug + Send + Sync + 'static,
    {
        let target = TypeId::of::<E>();
        unsafe {
            // Use vtable to find NonNull<()> which points to a value of type E
            // somewhere inside the data structure.

            #[cfg(not(anyhow_no_ptr_addr_of))]
            let addr =
                (vtable(self.inner.ptr).object_downcast)(self.inner.by_ref(), target)?.by_mut();

            #[cfg(anyhow_no_ptr_addr_of)]
            let addr = (vtable(self.inner.ptr).object_downcast_mut)(self.inner.by_mut(), target)?;

            Some(addr.cast::<E>().deref_mut())
        }
    }

    #[cfg(backtrace)]
    pub(crate) fn provide<'a>(&'a self, request: &mut Request<'a>) {
        unsafe { ErrorImpl::provide(self.inner.by_ref(), request) }
    }

    // Called by thiserror when you have `#[source] anyhow::Error`. This provide
    // implementation includes the anyhow::Error's Backtrace if any, unlike
    // deref'ing to dyn Error where the provide implementation would include
    // only the original error's Backtrace from before it got wrapped into an
    // anyhow::Error.
    #[cfg(backtrace)]
    #[doc(hidden)]
    pub fn thiserror_provide<'a>(&'a self, request: &mut Request<'a>) {
        Self::provide(self, request);
    }
}

#[cfg(feature = "std")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
impl<E> From<E> for Error
where
    E: StdError + Send + Sync + 'static,
{
    #[cold]
    fn from(error: E) -> Self {
        let backtrace = backtrace_if_absent!(&error);
        Error::from_std(error, backtrace)
    }
}

#[cfg(feature = "std")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
impl Deref for Error {
    type Target = dyn StdError + Send + Sync + 'static;

    fn deref(&self) -> &Self::Target {
        unsafe { ErrorImpl::error(self.inner.by_ref()) }
    }
}

#[cfg(feature = "std")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
impl DerefMut for Error {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unsafe { ErrorImpl::error_mut(self.inner.by_mut()) }
    }
}

impl Display for Error {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        unsafe { ErrorImpl::display(self.inner.by_ref(), formatter) }
    }
}

impl Debug for Error {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        unsafe { ErrorImpl::debug(self.inner.by_ref(), formatter) }
    }
}

impl Drop for Error {
    fn drop(&mut self) {
        unsafe {
            // Invoke the vtable's drop behavior.
            (vtable(self.inner.ptr).object_drop)(self.inner);
        }
    }
}

struct ErrorVTable {
    object_drop: unsafe fn(Own<ErrorImpl>),
    object_ref: unsafe fn(Ref<ErrorImpl>) -> Ref<dyn StdError + Send + Sync + 'static>,
    #[cfg(all(feature = "std", anyhow_no_ptr_addr_of))]
    object_mut: unsafe fn(Mut<ErrorImpl>) -> &mut (dyn StdError + Send + Sync + 'static),
    object_boxed: unsafe fn(Own<ErrorImpl>) -> Box<dyn StdError + Send + Sync + 'static>,
    object_downcast: unsafe fn(Ref<ErrorImpl>, TypeId) -> Option<Ref<()>>,
    #[cfg(anyhow_no_ptr_addr_of)]
    object_downcast_mut: unsafe fn(Mut<ErrorImpl>, TypeId) -> Option<Mut<()>>,
    object_drop_rest: unsafe fn(Own<ErrorImpl>, TypeId),
    #[cfg(all(not(backtrace), feature = "backtrace"))]
    object_backtrace: unsafe fn(Ref<ErrorImpl>) -> Option<&Backtrace>,
}

// Safety: requires layout of *e to match ErrorImpl<E>.
unsafe fn object_drop<E>(e: Own<ErrorImpl>) {
    // Cast back to ErrorImpl<E> so that the allocator receives the correct
    // Layout to deallocate the Box's memory.
    let unerased = e.cast::<ErrorImpl<E>>().boxed();
    drop(unerased);
}

// Safety: requires layout of *e to match ErrorImpl<E>.
unsafe fn object_drop_front<E>(e: Own<ErrorImpl>, target: TypeId) {
    // Drop the fields of ErrorImpl other than E as well as the Box allocation,
    // without dropping E itself. This is used by downcast after doing a
    // ptr::read to take ownership of the E.
    let _ = target;
    let unerased = e.cast::<ErrorImpl<ManuallyDrop<E>>>().boxed();
    drop(unerased);
}

// Safety: requires layout of *e to match ErrorImpl<E>.
unsafe fn object_ref<E>(e: Ref<ErrorImpl>) -> Ref<dyn StdError + Send + Sync + 'static>
where
    E: StdError + Send + Sync + 'static,
{
    // Attach E's native StdError vtable onto a pointer to self._object.

    let unerased = e.cast::<ErrorImpl<E>>();

    #[cfg(not(anyhow_no_ptr_addr_of))]
    return Ref::from_raw(NonNull::new_unchecked(
        ptr::addr_of!((*unerased.as_ptr())._object) as *mut E,
    ));

    #[cfg(anyhow_no_ptr_addr_of)]
    return Ref::new(&unerased.deref()._object);
}

// Safety: requires layout of *e to match ErrorImpl<E>, and for `e` to be derived
// from a `&mut`
#[cfg(all(feature = "std", anyhow_no_ptr_addr_of))]
unsafe fn object_mut<E>(e: Mut<ErrorImpl>) -> &mut (dyn StdError + Send + Sync + 'static)
where
    E: StdError + Send + Sync + 'static,
{
    // Attach E's native StdError vtable onto a pointer to self._object.
    &mut e.cast::<ErrorImpl<E>>().deref_mut()._object
}

// Safety: requires layout of *e to match ErrorImpl<E>.
unsafe fn object_boxed<E>(e: Own<ErrorImpl>) -> Box<dyn StdError + Send + Sync + 'static>
where
    E: StdError + Send + Sync + 'static,
{
    // Attach ErrorImpl<E>'s native StdError vtable. The StdError impl is below.
    e.cast::<ErrorImpl<E>>().boxed()
}

// Safety: requires layout of *e to match ErrorImpl<E>.
unsafe fn object_downcast<E>(e: Ref<ErrorImpl>, target: TypeId) -> Option<Ref<()>>
where
    E: 'static,
{
    if TypeId::of::<E>() == target {
        // Caller is looking for an E pointer and e is ErrorImpl<E>, take a
        // pointer to its E field.

        let unerased = e.cast::<ErrorImpl<E>>();

        #[cfg(not(anyhow_no_ptr_addr_of))]
        return Some(
            Ref::from_raw(NonNull::new_unchecked(
                ptr::addr_of!((*unerased.as_ptr())._object) as *mut E,
            ))
            .cast::<()>(),
        );

        #[cfg(anyhow_no_ptr_addr_of)]
        return Some(Ref::new(&unerased.deref()._object).cast::<()>());
    } else {
        None
    }
}

// Safety: requires layout of *e to match ErrorImpl<E>.
#[cfg(anyhow_no_ptr_addr_of)]
unsafe fn object_downcast_mut<E>(e: Mut<ErrorImpl>, target: TypeId) -> Option<Mut<()>>
where
    E: 'static,
{
    if TypeId::of::<E>() == target {
        // Caller is looking for an E pointer and e is ErrorImpl<E>, take a
        // pointer to its E field.
        let unerased = e.cast::<ErrorImpl<E>>().deref_mut();
        Some(Mut::new(&mut unerased._object).cast::<()>())
    } else {
        None
    }
}

#[cfg(all(not(backtrace), feature = "backtrace"))]
fn no_backtrace(e: Ref<ErrorImpl>) -> Option<&Backtrace> {
    let _ = e;
    None
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, E>>.
#[cfg(feature = "std")]
unsafe fn context_downcast<C, E>(e: Ref<ErrorImpl>, target: TypeId) -> Option<Ref<()>>
where
    C: 'static,
    E: 'static,
{
    if TypeId::of::<C>() == target {
        let unerased = e.cast::<ErrorImpl<ContextError<C, E>>>().deref();
        Some(Ref::new(&unerased._object.context).cast::<()>())
    } else if TypeId::of::<E>() == target {
        let unerased = e.cast::<ErrorImpl<ContextError<C, E>>>().deref();
        Some(Ref::new(&unerased._object.error).cast::<()>())
    } else {
        None
    }
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, E>>.
#[cfg(all(feature = "std", anyhow_no_ptr_addr_of))]
unsafe fn context_downcast_mut<C, E>(e: Mut<ErrorImpl>, target: TypeId) -> Option<Mut<()>>
where
    C: 'static,
    E: 'static,
{
    if TypeId::of::<C>() == target {
        let unerased = e.cast::<ErrorImpl<ContextError<C, E>>>().deref_mut();
        Some(Mut::new(&mut unerased._object.context).cast::<()>())
    } else if TypeId::of::<E>() == target {
        let unerased = e.cast::<ErrorImpl<ContextError<C, E>>>().deref_mut();
        Some(Mut::new(&mut unerased._object.error).cast::<()>())
    } else {
        None
    }
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, E>>.
#[cfg(feature = "std")]
unsafe fn context_drop_rest<C, E>(e: Own<ErrorImpl>, target: TypeId)
where
    C: 'static,
    E: 'static,
{
    // Called after downcasting by value to either the C or the E and doing a
    // ptr::read to take ownership of that value.
    if TypeId::of::<C>() == target {
        let unerased = e
            .cast::<ErrorImpl<ContextError<ManuallyDrop<C>, E>>>()
            .boxed();
        drop(unerased);
    } else {
        let unerased = e
            .cast::<ErrorImpl<ContextError<C, ManuallyDrop<E>>>>()
            .boxed();
        drop(unerased);
    }
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, Error>>.
unsafe fn context_chain_downcast<C>(e: Ref<ErrorImpl>, target: TypeId) -> Option<Ref<()>>
where
    C: 'static,
{
    let unerased = e.cast::<ErrorImpl<ContextError<C, Error>>>().deref();
    if TypeId::of::<C>() == target {
        Some(Ref::new(&unerased._object.context).cast::<()>())
    } else {
        // Recurse down the context chain per the inner error's vtable.
        let source = &unerased._object.error;
        (vtable(source.inner.ptr).object_downcast)(source.inner.by_ref(), target)
    }
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, Error>>.
#[cfg(anyhow_no_ptr_addr_of)]
unsafe fn context_chain_downcast_mut<C>(e: Mut<ErrorImpl>, target: TypeId) -> Option<Mut<()>>
where
    C: 'static,
{
    let unerased = e.cast::<ErrorImpl<ContextError<C, Error>>>().deref_mut();
    if TypeId::of::<C>() == target {
        Some(Mut::new(&mut unerased._object.context).cast::<()>())
    } else {
        // Recurse down the context chain per the inner error's vtable.
        let source = &mut unerased._object.error;
        (vtable(source.inner.ptr).object_downcast_mut)(source.inner.by_mut(), target)
    }
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, Error>>.
unsafe fn context_chain_drop_rest<C>(e: Own<ErrorImpl>, target: TypeId)
where
    C: 'static,
{
    // Called after downcasting by value to either the C or one of the causes
    // and doing a ptr::read to take ownership of that value.
    if TypeId::of::<C>() == target {
        let unerased = e
            .cast::<ErrorImpl<ContextError<ManuallyDrop<C>, Error>>>()
            .boxed();
        // Drop the entire rest of the data structure rooted in the next Error.
        drop(unerased);
    } else {
        let unerased = e
            .cast::<ErrorImpl<ContextError<C, ManuallyDrop<Error>>>>()
            .boxed();
        // Read the Own<ErrorImpl> from the next error.
        let inner = unerased._object.error.inner;
        drop(unerased);
        let vtable = vtable(inner.ptr);
        // Recursively drop the next error using the same target typeid.
        (vtable.object_drop_rest)(inner, target);
    }
}

// Safety: requires layout of *e to match ErrorImpl<ContextError<C, Error>>.
#[cfg(all(not(backtrace), feature = "backtrace"))]
#[allow(clippy::unnecessary_wraps)]
unsafe fn context_backtrace<C>(e: Ref<ErrorImpl>) -> Option<&Backtrace>
where
    C: 'static,
{
    let unerased = e.cast::<ErrorImpl<ContextError<C, Error>>>().deref();
    let backtrace = ErrorImpl::backtrace(unerased._object.error.inner.by_ref());
    Some(backtrace)
}

// NOTE: If working with `ErrorImpl<()>`, references should be avoided in favor
// of raw pointers and `NonNull`.
// repr C to ensure that E remains in the final position.
#[repr(C)]
pub(crate) struct ErrorImpl<E = ()> {
    vtable: &'static ErrorVTable,
    backtrace: Option<Backtrace>,
    // NOTE: Don't use directly. Use only through vtable. Erased type may have
    // different alignment.
    _object: E,
}

// Reads the vtable out of `p`. This is the same as `p.as_ref().vtable`, but
// avoids converting `p` into a reference.
unsafe fn vtable(p: NonNull<ErrorImpl>) -> &'static ErrorVTable {
    // NOTE: This assumes that `ErrorVTable` is the first field of ErrorImpl.
    *(p.as_ptr() as *const &'static ErrorVTable)
}

// repr C to ensure that ContextError<C, E> has the same layout as
// ContextError<ManuallyDrop<C>, E> and ContextError<C, ManuallyDrop<E>>.
#[repr(C)]
pub(crate) struct ContextError<C, E> {
    pub context: C,
    pub error: E,
}

impl<E> ErrorImpl<E> {
    fn erase(&self) -> Ref<ErrorImpl> {
        // Erase the concrete type of E but preserve the vtable in self.vtable
        // for manipulating the resulting thin pointer. This is analogous to an
        // unsize coercion.
        Ref::new(self).cast::<ErrorImpl>()
    }
}

impl ErrorImpl {
    pub(crate) unsafe fn error(this: Ref<Self>) -> &(dyn StdError + Send + Sync + 'static) {
        // Use vtable to attach E's native StdError vtable for the right
        // original type E.
        (vtable(this.ptr).object_ref)(this).deref()
    }

    #[cfg(feature = "std")]
    pub(crate) unsafe fn error_mut(this: Mut<Self>) -> &mut (dyn StdError + Send + Sync + 'static) {
        // Use vtable to attach E's native StdError vtable for the right
        // original type E.

        #[cfg(not(anyhow_no_ptr_addr_of))]
        return (vtable(this.ptr).object_ref)(this.by_ref())
            .by_mut()
            .deref_mut();

        #[cfg(anyhow_no_ptr_addr_of)]
        return (vtable(this.ptr).object_mut)(this);
    }

    #[cfg(any(backtrace, feature = "backtrace"))]
    pub(crate) unsafe fn backtrace(this: Ref<Self>) -> &Backtrace {
        // This unwrap can only panic if the underlying error's backtrace method
        // is nondeterministic, which would only happen in maliciously
        // constructed code.
        this.deref()
            .backtrace
            .as_ref()
            .or_else(|| {
                #[cfg(backtrace)]
                return error::request_ref::<Backtrace>(Self::error(this));
                #[cfg(not(backtrace))]
                return (vtable(this.ptr).object_backtrace)(this);
            })
            .expect("backtrace capture failed")
    }

    #[cfg(backtrace)]
    unsafe fn provide<'a>(this: Ref<'a, Self>, request: &mut Request<'a>) {
        if let Some(backtrace) = &this.deref().backtrace {
            request.provide_ref(backtrace);
        }
        Self::error(this).provide(request);
    }

    #[cold]
    pub(crate) unsafe fn chain(this: Ref<Self>) -> Chain {
        Chain::new(Self::error(this))
    }
}

impl<E> StdError for ErrorImpl<E>
where
    E: StdError,
{
    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        unsafe { ErrorImpl::error(self.erase()).source() }
    }

    #[cfg(backtrace)]
    fn provide<'a>(&'a self, request: &mut Request<'a>) {
        unsafe { ErrorImpl::provide(self.erase(), request) }
    }
}

impl<E> Debug for ErrorImpl<E>
where
    E: Debug,
{
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        unsafe { ErrorImpl::debug(self.erase(), formatter) }
    }
}

impl<E> Display for ErrorImpl<E>
where
    E: Display,
{
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        unsafe { Display::fmt(ErrorImpl::error(self.erase()), formatter) }
    }
}

impl From<Error> for Box<dyn StdError + Send + Sync + 'static> {
    #[cold]
    fn from(error: Error) -> Self {
        let outer = ManuallyDrop::new(error);
        unsafe {
            // Use vtable to attach ErrorImpl<E>'s native StdError vtable for
            // the right original type E.
            (vtable(outer.inner.ptr).object_boxed)(outer.inner)
        }
    }
}

impl From<Error> for Box<dyn StdError + Send + 'static> {
    fn from(error: Error) -> Self {
        Box::<dyn StdError + Send + Sync>::from(error)
    }
}

impl From<Error> for Box<dyn StdError + 'static> {
    fn from(error: Error) -> Self {
        Box::<dyn StdError + Send + Sync>::from(error)
    }
}

#[cfg(feature = "std")]
impl AsRef<dyn StdError + Send + Sync> for Error {
    fn as_ref(&self) -> &(dyn StdError + Send + Sync + 'static) {
        &**self
    }
}

#[cfg(feature = "std")]
impl AsRef<dyn StdError> for Error {
    fn as_ref(&self) -> &(dyn StdError + 'static) {
        &**self
    }
}