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
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
#![doc = include_str!("error.md")]
#![stable(feature = "error_in_core", since = "1.81.0")]

use crate::any::TypeId;
use crate::fmt::{Debug, Display, Formatter, Result};

/// `Error` is a trait representing the basic expectations for error values,
/// i.e., values of type `E` in [`Result<T, E>`].
///
/// Errors must describe themselves through the [`Display`] and [`Debug`]
/// traits. Error messages are typically concise lowercase sentences without
/// trailing punctuation:
///
/// ```
/// let err = "NaN".parse::<u32>().unwrap_err();
/// assert_eq!(err.to_string(), "invalid digit found in string");
/// ```
///
/// Errors may provide cause information. [`Error::source()`] is generally
/// used when errors cross "abstraction boundaries". If one module must report
/// an error that is caused by an error from a lower-level module, it can allow
/// accessing that error via [`Error::source()`]. This makes it possible for the
/// high-level module to provide its own errors while also revealing some of the
/// implementation for debugging.
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
#[rustc_has_incoherent_inherent_impls]
#[allow(multiple_supertrait_upcastable)]
pub trait Error: Debug + Display {
    /// Returns the lower-level source of this error, if any.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::error::Error;
    /// use std::fmt;
    ///
    /// #[derive(Debug)]
    /// struct SuperError {
    ///     source: 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 source(&self) -> Option<&(dyn Error + 'static)> {
    ///         Some(&self.source)
    ///     }
    /// }
    ///
    /// #[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 get_super_error() -> Result<(), SuperError> {
    ///     Err(SuperError { source: SuperErrorSideKick })
    /// }
    ///
    /// fn main() {
    ///     match get_super_error() {
    ///         Err(e) => {
    ///             println!("Error: {e}");
    ///             println!("Caused by: {}", e.source().unwrap());
    ///         }
    ///         _ => println!("No error"),
    ///     }
    /// }
    /// ```
    #[stable(feature = "error_source", since = "1.30.0")]
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        None
    }

    /// Gets the `TypeId` of `self`.
    #[doc(hidden)]
    #[unstable(
        feature = "error_type_id",
        reason = "this is memory-unsafe to override in user code",
        issue = "60784"
    )]
    fn type_id(&self, _: private::Internal) -> TypeId
    where
        Self: 'static,
    {
        TypeId::of::<Self>()
    }

    /// ```
    /// if let Err(e) = "xc".parse::<u32>() {
    ///     // Print `e` itself, no need for description().
    ///     eprintln!("Error: {e}");
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[deprecated(since = "1.42.0", note = "use the Display impl or to_string()")]
    fn description(&self) -> &str {
        "description() is deprecated; use Display"
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    #[deprecated(
        since = "1.33.0",
        note = "replaced by Error::source, which can support downcasting"
    )]
    #[allow(missing_docs)]
    fn cause(&self) -> Option<&dyn Error> {
        self.source()
    }

    /// Provides type-based access to context intended for error reports.
    ///
    /// Used in conjunction with [`Request::provide_value`] and [`Request::provide_ref`] to extract
    /// references to member variables from `dyn Error` trait objects.
    ///
    /// # Example
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    /// use core::fmt;
    /// use core::error::{request_ref, Request};
    ///
    /// #[derive(Debug)]
    /// enum MyLittleTeaPot {
    ///     Empty,
    /// }
    ///
    /// #[derive(Debug)]
    /// struct MyBacktrace {
    ///     // ...
    /// }
    ///
    /// impl MyBacktrace {
    ///     fn new() -> MyBacktrace {
    ///         // ...
    ///         # MyBacktrace {}
    ///     }
    /// }
    ///
    /// #[derive(Debug)]
    /// struct Error {
    ///     backtrace: MyBacktrace,
    /// }
    ///
    /// impl fmt::Display for Error {
    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         write!(f, "Example Error")
    ///     }
    /// }
    ///
    /// impl std::error::Error for Error {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         request
    ///             .provide_ref::<MyBacktrace>(&self.backtrace);
    ///     }
    /// }
    ///
    /// fn main() {
    ///     let backtrace = MyBacktrace::new();
    ///     let error = Error { backtrace };
    ///     let dyn_error = &error as &dyn std::error::Error;
    ///     let backtrace_ref = request_ref::<MyBacktrace>(dyn_error).unwrap();
    ///
    ///     assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
    ///     assert!(request_ref::<MyLittleTeaPot>(dyn_error).is_none());
    /// }
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    #[allow(unused_variables)]
    fn provide<'a>(&'a self, request: &mut Request<'a>) {}
}

mod private {
    // This is a hack to prevent `type_id` from being overridden by `Error`
    // implementations, since that can enable unsound downcasting.
    #[unstable(feature = "error_type_id", issue = "60784")]
    #[derive(Debug)]
    pub struct Internal;
}

#[unstable(feature = "never_type", issue = "35121")]
impl Error for ! {}

// Copied from `any.rs`.
impl dyn Error + 'static {
    /// Returns `true` if the inner 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 (`self`).
        let concrete = self.type_id(private::Internal);

        // Compare both `TypeId`s on equality.
        t == concrete
    }

    /// Returns some reference to the inner 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>() {
            // SAFETY: `is` ensures this type cast is correct
            unsafe { Some(&*(self as *const dyn Error as *const T)) }
        } else {
            None
        }
    }

    /// Returns some mutable reference to the inner 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>() {
            // SAFETY: `is` ensures this type cast is correct
            unsafe { Some(&mut *(self as *mut dyn Error as *mut T)) }
        } else {
            None
        }
    }
}

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

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

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

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

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

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

impl dyn Error {
    /// Returns an iterator starting with the current error and continuing with
    /// recursively calling [`Error::source`].
    ///
    /// If you want to omit the current error and only use its sources,
    /// use `skip(1)`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(error_iter)]
    /// use std::error::Error;
    /// use std::fmt;
    ///
    /// #[derive(Debug)]
    /// struct A;
    ///
    /// #[derive(Debug)]
    /// struct B(Option<Box<dyn Error + 'static>>);
    ///
    /// 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 {}
    ///
    /// impl Error for B {
    ///     fn source(&self) -> Option<&(dyn Error + 'static)> {
    ///         self.0.as_ref().map(|e| e.as_ref())
    ///     }
    /// }
    ///
    /// let b = B(Some(Box::new(A)));
    ///
    /// // let err : Box<Error> = b.into(); // or
    /// let err = &b as &(dyn Error);
    ///
    /// let mut iter = err.sources();
    ///
    /// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
    /// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
    /// assert!(iter.next().is_none());
    /// assert!(iter.next().is_none());
    /// ```
    #[unstable(feature = "error_iter", issue = "58520")]
    #[inline]
    pub fn sources(&self) -> Source<'_> {
        // You may think this method would be better in the Error trait, and you'd be right.
        // Unfortunately that doesn't work, not because of the object safety rules but because we
        // save a reference to self in Sources below as a trait object. If this method was
        // declared in Error, then self would have the type &T where T is some concrete type which
        // implements Error. We would need to coerce self to have type &dyn Error, but that requires
        // that Self has a known size (i.e., Self: Sized). We can't put that bound on Error
        // since that would forbid Error trait objects, and we can't put that bound on the method
        // because that means the method can't be called on trait objects (we'd also need the
        // 'static bound, but that isn't allowed because methods with bounds on Self other than
        // Sized are not object-safe). Requiring an Unsize bound is not backwards compatible.

        Source { current: Some(self) }
    }
}

/// Requests a value of type `T` from the given `impl Error`.
///
/// # Examples
///
/// Get a string value from an error.
///
/// ```rust
/// #![feature(error_generic_member_access)]
/// use std::error::Error;
/// use core::error::request_value;
///
/// fn get_string(err: &impl Error) -> String {
///     request_value::<String>(err).unwrap()
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<'a, T>(err: &'a (impl Error + ?Sized)) -> Option<T>
where
    T: 'static,
{
    request_by_type_tag::<'a, tags::Value<T>>(err)
}

/// Requests a reference of type `T` from the given `impl Error`.
///
/// # Examples
///
/// Get a string reference from an error.
///
/// ```rust
/// #![feature(error_generic_member_access)]
/// use core::error::Error;
/// use core::error::request_ref;
///
/// fn get_str(err: &impl Error) -> &str {
///     request_ref::<str>(err).unwrap()
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<'a, T>(err: &'a (impl Error + ?Sized)) -> Option<&'a T>
where
    T: 'static + ?Sized,
{
    request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>>(err)
}

/// Request a specific value by tag from the `Error`.
fn request_by_type_tag<'a, I>(err: &'a (impl Error + ?Sized)) -> Option<I::Reified>
where
    I: tags::Type<'a>,
{
    let mut tagged = Tagged { tag_id: TypeId::of::<I>(), value: TaggedOption::<'a, I>(None) };
    err.provide(tagged.as_request());
    tagged.value.0
}

///////////////////////////////////////////////////////////////////////////////
// Request and its methods
///////////////////////////////////////////////////////////////////////////////

/// `Request` supports generic, type-driven access to data. Its use is currently restricted to the
/// standard library in cases where trait authors wish to allow trait implementors to share generic
/// information across trait boundaries. The motivating and prototypical use case is
/// `core::error::Error` which would otherwise require a method per concrete type (eg.
/// `std::backtrace::Backtrace` instance that implementors want to expose to users).
///
/// # Data flow
///
/// To describe the intended data flow for Request objects, let's consider two conceptual users
/// separated by API boundaries:
///
/// * Consumer - the consumer requests objects using a Request instance; eg a crate that offers
/// fancy `Error`/`Result` reporting to users wants to request a Backtrace from a given `dyn Error`.
///
/// * Producer - the producer provides objects when requested via Request; eg. a library with an
/// an `Error` implementation that automatically captures backtraces at the time instances are
/// created.
///
/// The consumer only needs to know where to submit their request and are expected to handle the
/// request not being fulfilled by the use of `Option<T>` in the responses offered by the producer.
///
/// * A Producer initializes the value of one of its fields of a specific type. (or is otherwise
/// prepared to generate a value requested). eg, `backtrace::Backtrace` or
/// `std::backtrace::Backtrace`
/// * A Consumer requests an object of a specific type (say `std::backtrace::Backtrace`). In the
/// case of a `dyn Error` trait object (the Producer), there are functions called `request_ref` and
/// `request_value` to simplify obtaining an `Option<T>` for a given type.
/// * The Producer, when requested, populates the given Request object which is given as a mutable
/// reference.
/// * The Consumer extracts a value or reference to the requested type from the `Request` object
/// wrapped in an `Option<T>`; in the case of `dyn Error` the aforementioned `request_ref` and `
/// request_value` methods mean that `dyn Error` users don't have to deal with the `Request` type at
/// all (but `Error` implementors do). The `None` case of the `Option` suggests only that the
/// Producer cannot currently offer an instance of the requested type, not it can't or never will.
///
/// # Examples
///
/// The best way to demonstrate this is using an example implementation of `Error`'s `provide` trait
/// method:
///
/// ```
/// #![feature(error_generic_member_access)]
/// use core::fmt;
/// use core::error::Request;
/// use core::error::request_ref;
///
/// #[derive(Debug)]
/// enum MyLittleTeaPot {
///     Empty,
/// }
///
/// #[derive(Debug)]
/// struct MyBacktrace {
///     // ...
/// }
///
/// impl MyBacktrace {
///     fn new() -> MyBacktrace {
///         // ...
///         # MyBacktrace {}
///     }
/// }
///
/// #[derive(Debug)]
/// struct Error {
///     backtrace: MyBacktrace,
/// }
///
/// impl fmt::Display for Error {
///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
///         write!(f, "Example Error")
///     }
/// }
///
/// impl std::error::Error for Error {
///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
///         request
///             .provide_ref::<MyBacktrace>(&self.backtrace);
///     }
/// }
///
/// fn main() {
///     let backtrace = MyBacktrace::new();
///     let error = Error { backtrace };
///     let dyn_error = &error as &dyn std::error::Error;
///     let backtrace_ref = request_ref::<MyBacktrace>(dyn_error).unwrap();
///
///     assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
///     assert!(request_ref::<MyLittleTeaPot>(dyn_error).is_none());
/// }
/// ```
///
#[unstable(feature = "error_generic_member_access", issue = "99301")]
#[repr(transparent)]
pub struct Request<'a>(Tagged<dyn Erased<'a> + 'a>);

impl<'a> Request<'a> {
    /// Provides a value or other type with only static lifetimes.
    ///
    /// # Examples
    ///
    /// Provides an `u8`.
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    ///
    /// use core::error::Request;
    ///
    /// #[derive(Debug)]
    /// struct SomeConcreteType { field: u8 }
    ///
    /// impl std::fmt::Display for SomeConcreteType {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "{} failed", self.field)
    ///     }
    /// }
    ///
    /// impl std::error::Error for SomeConcreteType {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         request.provide_value::<u8>(self.field);
    ///     }
    /// }
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    pub fn provide_value<T>(&mut self, value: T) -> &mut Self
    where
        T: 'static,
    {
        self.provide::<tags::Value<T>>(value)
    }

    /// Provides a value or other type with only static lifetimes computed using a closure.
    ///
    /// # Examples
    ///
    /// Provides a `String` by cloning.
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    ///
    /// use core::error::Request;
    ///
    /// #[derive(Debug)]
    /// struct SomeConcreteType { field: String }
    ///
    /// impl std::fmt::Display for SomeConcreteType {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "{} failed", self.field)
    ///     }
    /// }
    ///
    /// impl std::error::Error for SomeConcreteType {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         request.provide_value_with::<String>(|| self.field.clone());
    ///     }
    /// }
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    pub fn provide_value_with<T>(&mut self, fulfil: impl FnOnce() -> T) -> &mut Self
    where
        T: 'static,
    {
        self.provide_with::<tags::Value<T>>(fulfil)
    }

    /// Provides a reference. The referee type must be bounded by `'static`,
    /// but may be unsized.
    ///
    /// # Examples
    ///
    /// Provides a reference to a field as a `&str`.
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    ///
    /// use core::error::Request;
    ///
    /// #[derive(Debug)]
    /// struct SomeConcreteType { field: String }
    ///
    /// impl std::fmt::Display for SomeConcreteType {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "{} failed", self.field)
    ///     }
    /// }
    ///
    /// impl std::error::Error for SomeConcreteType {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         request.provide_ref::<str>(&self.field);
    ///     }
    /// }
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    pub fn provide_ref<T: ?Sized + 'static>(&mut self, value: &'a T) -> &mut Self {
        self.provide::<tags::Ref<tags::MaybeSizedValue<T>>>(value)
    }

    /// Provides a reference computed using a closure. The referee type
    /// must be bounded by `'static`, but may be unsized.
    ///
    /// # Examples
    ///
    /// Provides a reference to a field as a `&str`.
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    ///
    /// use core::error::Request;
    ///
    /// #[derive(Debug)]
    /// struct SomeConcreteType { business: String, party: String }
    /// fn today_is_a_weekday() -> bool { true }
    ///
    /// impl std::fmt::Display for SomeConcreteType {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "{} failed", self.business)
    ///     }
    /// }
    ///
    /// impl std::error::Error for SomeConcreteType {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         request.provide_ref_with::<str>(|| {
    ///             if today_is_a_weekday() {
    ///                 &self.business
    ///             } else {
    ///                 &self.party
    ///             }
    ///         });
    ///     }
    /// }
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    pub fn provide_ref_with<T: ?Sized + 'static>(
        &mut self,
        fulfil: impl FnOnce() -> &'a T,
    ) -> &mut Self {
        self.provide_with::<tags::Ref<tags::MaybeSizedValue<T>>>(fulfil)
    }

    /// Provides a value with the given `Type` tag.
    fn provide<I>(&mut self, value: I::Reified) -> &mut Self
    where
        I: tags::Type<'a>,
    {
        if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
            res.0 = Some(value);
        }
        self
    }

    /// Provides a value with the given `Type` tag, using a closure to prevent unnecessary work.
    fn provide_with<I>(&mut self, fulfil: impl FnOnce() -> I::Reified) -> &mut Self
    where
        I: tags::Type<'a>,
    {
        if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
            res.0 = Some(fulfil());
        }
        self
    }

    /// Checks if the `Request` would be satisfied if provided with a
    /// value of the specified type. If the type does not match or has
    /// already been provided, returns false.
    ///
    /// # Examples
    ///
    /// Checks if a `u8` still needs to be provided and then provides
    /// it.
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    ///
    /// use core::error::Request;
    /// use core::error::request_value;
    ///
    /// #[derive(Debug)]
    /// struct Parent(Option<u8>);
    ///
    /// impl std::fmt::Display for Parent {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "a parent failed")
    ///     }
    /// }
    ///
    /// impl std::error::Error for Parent {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         if let Some(v) = self.0 {
    ///             request.provide_value::<u8>(v);
    ///         }
    ///     }
    /// }
    ///
    /// #[derive(Debug)]
    /// struct Child {
    ///     parent: Parent,
    /// }
    ///
    /// impl Child {
    ///     // Pretend that this takes a lot of resources to evaluate.
    ///     fn an_expensive_computation(&self) -> Option<u8> {
    ///         Some(99)
    ///     }
    /// }
    ///
    /// impl std::fmt::Display for Child {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "child failed: \n  because of parent: {}", self.parent)
    ///     }
    /// }
    ///
    /// impl std::error::Error for Child {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         // In general, we don't know if this call will provide
    ///         // an `u8` value or not...
    ///         self.parent.provide(request);
    ///
    ///         // ...so we check to see if the `u8` is needed before
    ///         // we run our expensive computation.
    ///         if request.would_be_satisfied_by_value_of::<u8>() {
    ///             if let Some(v) = self.an_expensive_computation() {
    ///                 request.provide_value::<u8>(v);
    ///             }
    ///         }
    ///
    ///         // The request will be satisfied now, regardless of if
    ///         // the parent provided the value or we did.
    ///         assert!(!request.would_be_satisfied_by_value_of::<u8>());
    ///     }
    /// }
    ///
    /// let parent = Parent(Some(42));
    /// let child = Child { parent };
    /// assert_eq!(Some(42), request_value::<u8>(&child));
    ///
    /// let parent = Parent(None);
    /// let child = Child { parent };
    /// assert_eq!(Some(99), request_value::<u8>(&child));
    ///
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    pub fn would_be_satisfied_by_value_of<T>(&self) -> bool
    where
        T: 'static,
    {
        self.would_be_satisfied_by::<tags::Value<T>>()
    }

    /// Checks if the `Request` would be satisfied if provided with a
    /// reference to a value of the specified type.
    ///
    /// If the type does not match or has already been provided, returns false.
    ///
    /// # Examples
    ///
    /// Checks if a `&str` still needs to be provided and then provides
    /// it.
    ///
    /// ```rust
    /// #![feature(error_generic_member_access)]
    ///
    /// use core::error::Request;
    /// use core::error::request_ref;
    ///
    /// #[derive(Debug)]
    /// struct Parent(Option<String>);
    ///
    /// impl std::fmt::Display for Parent {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "a parent failed")
    ///     }
    /// }
    ///
    /// impl std::error::Error for Parent {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         if let Some(v) = &self.0 {
    ///             request.provide_ref::<str>(v);
    ///         }
    ///     }
    /// }
    ///
    /// #[derive(Debug)]
    /// struct Child {
    ///     parent: Parent,
    ///     name: String,
    /// }
    ///
    /// impl Child {
    ///     // Pretend that this takes a lot of resources to evaluate.
    ///     fn an_expensive_computation(&self) -> Option<&str> {
    ///         Some(&self.name)
    ///     }
    /// }
    ///
    /// impl std::fmt::Display for Child {
    ///     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    ///         write!(f, "{} failed: \n  {}", self.name, self.parent)
    ///     }
    /// }
    ///
    /// impl std::error::Error for Child {
    ///     fn provide<'a>(&'a self, request: &mut Request<'a>) {
    ///         // In general, we don't know if this call will provide
    ///         // a `str` reference or not...
    ///         self.parent.provide(request);
    ///
    ///         // ...so we check to see if the `&str` is needed before
    ///         // we run our expensive computation.
    ///         if request.would_be_satisfied_by_ref_of::<str>() {
    ///             if let Some(v) = self.an_expensive_computation() {
    ///                 request.provide_ref::<str>(v);
    ///             }
    ///         }
    ///
    ///         // The request will be satisfied now, regardless of if
    ///         // the parent provided the reference or we did.
    ///         assert!(!request.would_be_satisfied_by_ref_of::<str>());
    ///     }
    /// }
    ///
    /// let parent = Parent(Some("parent".into()));
    /// let child = Child { parent, name: "child".into() };
    /// assert_eq!(Some("parent"), request_ref::<str>(&child));
    ///
    /// let parent = Parent(None);
    /// let child = Child { parent, name: "child".into() };
    /// assert_eq!(Some("child"), request_ref::<str>(&child));
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    pub fn would_be_satisfied_by_ref_of<T>(&self) -> bool
    where
        T: ?Sized + 'static,
    {
        self.would_be_satisfied_by::<tags::Ref<tags::MaybeSizedValue<T>>>()
    }

    fn would_be_satisfied_by<I>(&self) -> bool
    where
        I: tags::Type<'a>,
    {
        matches!(self.0.downcast::<I>(), Some(TaggedOption(None)))
    }
}

#[unstable(feature = "error_generic_member_access", issue = "99301")]
impl<'a> Debug for Request<'a> {
    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
        f.debug_struct("Request").finish_non_exhaustive()
    }
}

///////////////////////////////////////////////////////////////////////////////
// Type tags
///////////////////////////////////////////////////////////////////////////////

pub(crate) mod tags {
    //! Type tags are used to identify a type using a separate value. This module includes type tags
    //! for some very common types.
    //!
    //! Currently type tags are not exposed to the user. But in the future, if you want to use the
    //! Request API with more complex types (typically those including lifetime parameters), you
    //! will need to write your own tags.

    use crate::marker::PhantomData;

    /// This trait is implemented by specific tag types in order to allow
    /// describing a type which can be requested for a given lifetime `'a`.
    ///
    /// A few example implementations for type-driven tags can be found in this
    /// module, although crates may also implement their own tags for more
    /// complex types with internal lifetimes.
    pub(crate) trait Type<'a>: Sized + 'static {
        /// The type of values which may be tagged by this tag for the given
        /// lifetime.
        type Reified: 'a;
    }

    /// Similar to the [`Type`] trait, but represents a type which may be unsized (i.e., has a
    /// `?Sized` bound). E.g., `str`.
    pub(crate) trait MaybeSizedType<'a>: Sized + 'static {
        type Reified: 'a + ?Sized;
    }

    impl<'a, T: Type<'a>> MaybeSizedType<'a> for T {
        type Reified = T::Reified;
    }

    /// Type-based tag for types bounded by `'static`, i.e., with no borrowed elements.
    #[derive(Debug)]
    pub(crate) struct Value<T: 'static>(PhantomData<T>);

    impl<'a, T: 'static> Type<'a> for Value<T> {
        type Reified = T;
    }

    /// Type-based tag similar to [`Value`] but which may be unsized (i.e., has a `?Sized` bound).
    #[derive(Debug)]
    pub(crate) struct MaybeSizedValue<T: ?Sized + 'static>(PhantomData<T>);

    impl<'a, T: ?Sized + 'static> MaybeSizedType<'a> for MaybeSizedValue<T> {
        type Reified = T;
    }

    /// Type-based tag for reference types (`&'a T`, where T is represented by
    /// `<I as MaybeSizedType<'a>>::Reified`.
    #[derive(Debug)]
    pub(crate) struct Ref<I>(PhantomData<I>);

    impl<'a, I: MaybeSizedType<'a>> Type<'a> for Ref<I> {
        type Reified = &'a I::Reified;
    }
}

/// An `Option` with a type tag `I`.
///
/// Since this struct implements `Erased`, the type can be erased to make a dynamically typed
/// option. The type can be checked dynamically using `Tagged::tag_id` and since this is statically
/// checked for the concrete type, there is some degree of type safety.
#[repr(transparent)]
pub(crate) struct TaggedOption<'a, I: tags::Type<'a>>(pub Option<I::Reified>);

impl<'a, I: tags::Type<'a>> Tagged<TaggedOption<'a, I>> {
    pub(crate) fn as_request(&mut self) -> &mut Request<'a> {
        let erased = self as &mut Tagged<dyn Erased<'a> + 'a>;
        // SAFETY: transmuting `&mut Tagged<dyn Erased<'a> + 'a>` to `&mut Request<'a>` is safe since
        // `Request` is repr(transparent).
        unsafe { &mut *(erased as *mut Tagged<dyn Erased<'a>> as *mut Request<'a>) }
    }
}

/// Represents a type-erased but identifiable object.
///
/// This trait is exclusively implemented by the `TaggedOption` type.
unsafe trait Erased<'a>: 'a {}

unsafe impl<'a, I: tags::Type<'a>> Erased<'a> for TaggedOption<'a, I> {}

struct Tagged<E: ?Sized> {
    tag_id: TypeId,
    value: E,
}

impl<'a> Tagged<dyn Erased<'a> + 'a> {
    /// Returns some reference to the dynamic value if it is tagged with `I`,
    /// or `None` otherwise.
    #[inline]
    fn downcast<I>(&self) -> Option<&TaggedOption<'a, I>>
    where
        I: tags::Type<'a>,
    {
        if self.tag_id == TypeId::of::<I>() {
            // SAFETY: Just checked whether we're pointing to an I.
            Some(&unsafe { &*(self as *const Self).cast::<Tagged<TaggedOption<'a, I>>>() }.value)
        } else {
            None
        }
    }

    /// Returns some mutable reference to the dynamic value if it is tagged with `I`,
    /// or `None` otherwise.
    #[inline]
    fn downcast_mut<I>(&mut self) -> Option<&mut TaggedOption<'a, I>>
    where
        I: tags::Type<'a>,
    {
        if self.tag_id == TypeId::of::<I>() {
            Some(
                // SAFETY: Just checked whether we're pointing to an I.
                &mut unsafe { &mut *(self as *mut Self).cast::<Tagged<TaggedOption<'a, I>>>() }
                    .value,
            )
        } else {
            None
        }
    }
}

/// An iterator over an [`Error`] and its sources.
///
/// If you want to omit the initial error and only process
/// its sources, use `skip(1)`.
#[unstable(feature = "error_iter", issue = "58520")]
#[derive(Clone, Debug)]
pub struct Source<'a> {
    current: Option<&'a (dyn Error + 'static)>,
}

#[unstable(feature = "error_iter", issue = "58520")]
impl<'a> Iterator for Source<'a> {
    type Item = &'a (dyn Error + 'static);

    fn next(&mut self) -> Option<Self::Item> {
        let current = self.current;
        self.current = self.current.and_then(Error::source);
        current
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        if self.current.is_some() { (1, None) } else { (0, Some(0)) }
    }
}

#[unstable(feature = "error_iter", issue = "58520")]
impl<'a> crate::iter::FusedIterator for Source<'a> {}

#[stable(feature = "error_by_ref", since = "1.51.0")]
impl<'a, T: Error + ?Sized> Error for &'a T {
    #[allow(deprecated, deprecated_in_future)]
    fn description(&self) -> &str {
        Error::description(&**self)
    }

    #[allow(deprecated)]
    fn cause(&self) -> Option<&dyn Error> {
        Error::cause(&**self)
    }

    fn source(&self) -> Option<&(dyn Error + 'static)> {
        Error::source(&**self)
    }

    fn provide<'b>(&'b self, request: &mut Request<'b>) {
        Error::provide(&**self, request);
    }
}

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

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

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

#[stable(feature = "try_from", since = "1.34.0")]
impl Error for crate::char::CharTryFromError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        "converted integer out of range for `char`"
    }
}

#[stable(feature = "duration_checked_float", since = "1.66.0")]
impl Error for crate::time::TryFromFloatSecsError {}

#[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
impl Error for crate::ffi::FromBytesUntilNulError {}

#[unstable(feature = "get_many_mut", issue = "104642")]
impl<const N: usize> Error for crate::slice::GetManyMutError<N> {}