summaryrefslogtreecommitdiffstats
path: root/vendor/yoke/src/yoke.rs
blob: 7468b4d99ecc0d7bff2d8ed92f0cad3ef872b507 (plain)
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
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

use crate::either::EitherCart;
#[cfg(feature = "alloc")]
use crate::erased::{ErasedArcCart, ErasedBoxCart, ErasedRcCart};
use crate::trait_hack::YokeTraitHack;
use crate::Yokeable;
use core::marker::PhantomData;
use core::ops::Deref;
use stable_deref_trait::StableDeref;

#[cfg(feature = "alloc")]
use alloc::boxed::Box;
#[cfg(feature = "alloc")]
use alloc::rc::Rc;
#[cfg(feature = "alloc")]
use alloc::sync::Arc;

/// A Cow-like borrowed object "yoked" to its backing data.
///
/// This allows things like zero copy deserialized data to carry around
/// shared references to their backing buffer, by "erasing" their static lifetime
/// and turning it into a dynamically managed one.
///
/// `Y` (the [`Yokeable`]) is the object containing the references,
/// and will typically be of the form `Foo<'static>`. The `'static` is
/// not the actual lifetime of the data, rather it is a convenient way to mark the
/// erased lifetime and make it dynamic.
///
/// `C` is the "cart", which `Y` may contain references to. After the yoke is constructed,
/// the cart serves little purpose except to guarantee that `Y`'s references remain valid
/// for as long as the yoke remains in memory (by calling the destructor at the appropriate moment).
///
/// The primary constructor for [`Yoke`] is [`Yoke::attach_to_cart()`]. Several variants of that
/// constructor are provided to serve numerous types of call sites and `Yoke` signatures.
///
/// The key behind this type is [`Yoke::get()`], where calling [`.get()`][Yoke::get] on a type like
/// `Yoke<Cow<'static, str>, _>` will get you a short-lived `&'a Cow<'a, str>`, restricted to the
/// lifetime of the borrow used during `.get()`. This is entirely safe since the `Cow` borrows from
/// the cart type `C`, which cannot be interfered with as long as the `Yoke` is borrowed by `.get
/// ()`. `.get()` protects access by essentially reifying the erased lifetime to a safe local one
/// when necessary.
///
/// Furthermore, there are various [`.map_project()`][Yoke::map_project] methods that allow turning a `Yoke`
/// into another `Yoke` containing a different type that may contain elements of the original yoked
/// value. See the [`Yoke::map_project()`] docs for more details.
///
/// In general, `C` is a concrete type, but it is also possible for it to be a trait object.
///
/// # Example
///
/// For example, we can use this to store zero-copy deserialized data in a cache:
///
/// ```rust
/// # use yoke::{Yoke, Yokeable};
/// # use std::rc::Rc;
/// # use std::borrow::Cow;
/// # fn load_from_cache(_filename: &str) -> Rc<[u8]> {
/// #     // dummy implementation
/// #     Rc::new([0x5, 0, 0, 0, 0, 0, 0, 0, 0x68, 0x65, 0x6c, 0x6c, 0x6f])
/// # }
///
/// fn load_object(filename: &str) -> Yoke<Cow<'static, str>, Rc<[u8]>> {
///     let rc: Rc<[u8]> = load_from_cache(filename);
///     Yoke::<Cow<'static, str>, Rc<[u8]>>::attach_to_cart(rc, |data: &[u8]| {
///         // essentially forcing a #[serde(borrow)]
///         Cow::Borrowed(bincode::deserialize(data).unwrap())
///     })
/// }
///
/// let yoke = load_object("filename.bincode");
/// assert_eq!(&**yoke.get(), "hello");
/// assert!(matches!(yoke.get(), &Cow::Borrowed(_)));
/// ```
pub struct Yoke<Y: for<'a> Yokeable<'a>, C> {
    // must be the first field for drop order
    // this will have a 'static lifetime parameter, that parameter is a lie
    yokeable: Y,
    cart: C,
}

impl<Y: for<'a> Yokeable<'a>, C: StableDeref> Yoke<Y, C>
where
    <C as Deref>::Target: 'static,
{
    /// Construct a [`Yoke`] by yokeing an object to a cart in a closure.
    ///
    /// See also [`Yoke::try_attach_to_cart()`] to return a `Result` from the closure.
    ///
    /// Call sites for this function may not compile pre-1.61; if this still happens, use
    /// [`Yoke::attach_to_cart_badly()`] and file a bug.
    ///
    /// # Examples
    ///
    /// ```
    /// # use yoke::{Yoke, Yokeable};
    /// # use std::rc::Rc;
    /// # use std::borrow::Cow;
    /// # fn load_from_cache(_filename: &str) -> Rc<[u8]> {
    /// #     // dummy implementation
    /// #     Rc::new([0x5, 0, 0, 0, 0, 0, 0, 0, 0x68, 0x65, 0x6c, 0x6c, 0x6f])
    /// # }
    ///
    /// fn load_object(filename: &str) -> Yoke<Cow<'static, str>, Rc<[u8]>> {
    ///     let rc: Rc<[u8]> = load_from_cache(filename);
    ///     Yoke::<Cow<'static, str>, Rc<[u8]>>::attach_to_cart(rc, |data: &[u8]| {
    ///         // essentially forcing a #[serde(borrow)]
    ///         Cow::Borrowed(bincode::deserialize(data).unwrap())
    ///     })
    /// }
    ///
    /// let yoke: Yoke<Cow<str>, _> = load_object("filename.bincode");
    /// assert_eq!(&**yoke.get(), "hello");
    /// assert!(matches!(yoke.get(), &Cow::Borrowed(_)));
    /// ```
    pub fn attach_to_cart<F>(cart: C, f: F) -> Self
    where
        // safety note: This works by enforcing that the *only* place the return value of F
        // can borrow from is the cart, since `F` must be valid for all lifetimes `'de`
        //
        // The <C as Deref>::Target: 'static on the impl is crucial for safety as well
        //
        // See safety docs at the bottom of this file for more information
        F: for<'de> FnOnce(&'de <C as Deref>::Target) -> <Y as Yokeable<'de>>::Output,
        <C as Deref>::Target: 'static,
    {
        let deserialized = f(cart.deref());
        Self {
            yokeable: unsafe { Y::make(deserialized) },
            cart,
        }
    }

    /// Construct a [`Yoke`] by yokeing an object to a cart. If an error occurs in the
    /// deserializer function, the error is passed up to the caller.
    ///
    /// Call sites for this function may not compile pre-1.61; if this still happens, use
    /// [`Yoke::try_attach_to_cart_badly()`] and file a bug.
    pub fn try_attach_to_cart<E, F>(cart: C, f: F) -> Result<Self, E>
    where
        F: for<'de> FnOnce(&'de <C as Deref>::Target) -> Result<<Y as Yokeable<'de>>::Output, E>,
    {
        let deserialized = f(cart.deref())?;
        Ok(Self {
            yokeable: unsafe { Y::make(deserialized) },
            cart,
        })
    }

    /// Use [`Yoke::attach_to_cart()`].
    ///
    /// This was needed because the pre-1.61 compiler couldn't always handle the FnOnce trait bound.
    #[deprecated]
    pub fn attach_to_cart_badly(
        cart: C,
        f: for<'de> fn(&'de <C as Deref>::Target) -> <Y as Yokeable<'de>>::Output,
    ) -> Self {
        Self::attach_to_cart(cart, f)
    }

    /// Use [`Yoke::try_attach_to_cart()`].
    ///
    /// This was needed because the pre-1.61 compiler couldn't always handle the FnOnce trait bound.
    #[deprecated]
    pub fn try_attach_to_cart_badly<E>(
        cart: C,
        f: for<'de> fn(&'de <C as Deref>::Target) -> Result<<Y as Yokeable<'de>>::Output, E>,
    ) -> Result<Self, E> {
        Self::try_attach_to_cart(cart, f)
    }
}

impl<Y: for<'a> Yokeable<'a>, C> Yoke<Y, C> {
    /// Obtain a valid reference to the yokeable data
    ///
    /// This essentially transforms the lifetime of the internal yokeable data to
    /// be valid.
    /// For example, if you're working with a `Yoke<Cow<'static, T>, C>`, this
    /// will return an `&'a Cow<'a, T>`
    ///
    /// # Example
    ///
    /// ```rust
    /// # use yoke::{Yoke, Yokeable};
    /// # use std::rc::Rc;
    /// # use std::borrow::Cow;
    /// # fn load_from_cache(_filename: &str) -> Rc<[u8]> {
    /// #     // dummy implementation
    /// #     Rc::new([0x5, 0, 0, 0, 0, 0, 0, 0, 0x68, 0x65, 0x6c, 0x6c, 0x6f])
    /// # }
    /// #
    /// # fn load_object(filename: &str) -> Yoke<Cow<'static, str>, Rc<[u8]>> {
    /// #     let rc: Rc<[u8]> = load_from_cache(filename);
    /// #     Yoke::<Cow<'static, str>, Rc<[u8]>>::attach_to_cart(rc, |data: &[u8]| {
    /// #         Cow::Borrowed(bincode::deserialize(data).unwrap())
    /// #     })
    /// # }
    ///
    /// // load_object() defined in the example at the top of this page
    /// let yoke: Yoke<Cow<str>, _> = load_object("filename.bincode");
    /// assert_eq!(yoke.get(), "hello");
    /// ```
    #[inline]
    pub fn get<'a>(&'a self) -> &'a <Y as Yokeable<'a>>::Output {
        self.yokeable.transform()
    }

    /// Get a reference to the backing cart.
    ///
    /// This can be useful when building caches, etc. However, if you plan to store the cart
    /// separately from the yoke, read the note of caution below in [`Yoke::into_backing_cart`].
    pub fn backing_cart(&self) -> &C {
        &self.cart
    }

    /// Get the backing cart by value, dropping the yokeable object.
    ///
    /// **Caution:** Calling this method could cause information saved in the yokeable object but
    /// not the cart to be lost. Use this method only if the yokeable object cannot contain its
    /// own information.
    ///
    /// # Example
    ///
    /// Good example: the yokeable object is only a reference, so no information can be lost.
    ///
    /// ```
    /// use yoke::Yoke;
    ///
    /// let local_data = "foo".to_owned();
    /// let yoke = Yoke::<&'static str, Box<String>>::attach_to_zero_copy_cart(
    ///     Box::new(local_data),
    /// );
    /// assert_eq!(*yoke.get(), "foo");
    ///
    /// // Get back the cart
    /// let cart = yoke.into_backing_cart();
    /// assert_eq!(&*cart, "foo");
    /// ```
    ///
    /// Bad example: information specified in `.with_mut()` is lost.
    ///
    /// ```
    /// use std::borrow::Cow;
    /// use yoke::Yoke;
    ///
    /// let local_data = "foo".to_owned();
    /// let mut yoke =
    ///     Yoke::<Cow<'static, str>, Box<String>>::attach_to_zero_copy_cart(
    ///         Box::new(local_data),
    ///     );
    /// assert_eq!(yoke.get(), "foo");
    ///
    /// // Override data in the cart
    /// yoke.with_mut(|cow| {
    ///     let mut_str = cow.to_mut();
    ///     mut_str.clear();
    ///     mut_str.push_str("bar");
    /// });
    /// assert_eq!(yoke.get(), "bar");
    ///
    /// // Get back the cart
    /// let cart = yoke.into_backing_cart();
    /// assert_eq!(&*cart, "foo"); // WHOOPS!
    /// ```
    pub fn into_backing_cart(self) -> C {
        self.cart
    }

    /// Unsafe function for replacing the cart with another
    ///
    /// This can be used for type-erasing the cart, for example.
    ///
    /// # Safety
    ///
    /// - `f()` must not panic
    /// - References from the yokeable `Y` should still be valid for the lifetime of the
    ///   returned cart type `C`.
    /// - Lifetimes inside C must not be lengthened, even if they are themselves contravariant.
    ///   I.e., if C contains an `fn(&'a u8)`, it cannot be replaced with `fn(&'static u8),
    ///   even though that is typically safe.
    ///
    /// Typically, this means implementing `f` as something which _wraps_ the inner cart type `C`.
    /// `Yoke` only really cares about destructors for its carts so it's fine to erase other
    /// information about the cart, as long as the backing data will still be destroyed at the
    /// same time.
    #[inline]
    pub unsafe fn replace_cart<C2>(self, f: impl FnOnce(C) -> C2) -> Yoke<Y, C2> {
        Yoke {
            yokeable: self.yokeable,
            cart: f(self.cart),
        }
    }

    /// Mutate the stored [`Yokeable`] data.
    ///
    /// See [`Yokeable::transform_mut()`] for why this operation is safe.
    ///
    /// # Example
    ///
    /// This can be used to partially mutate the stored data, provided
    /// no _new_ borrowed data is introduced.
    ///
    /// ```rust
    /// # use yoke::{Yoke, Yokeable};
    /// # use std::rc::Rc;
    /// # use std::borrow::Cow;
    /// # use std::mem;
    /// # fn load_from_cache(_filename: &str) -> Rc<[u8]> {
    /// #     // dummy implementation
    /// #     Rc::new([0x5, 0, 0, 0, 0, 0, 0, 0, 0x68, 0x65, 0x6c, 0x6c, 0x6f])
    /// # }
    /// #
    /// # fn load_object(filename: &str) -> Yoke<Bar<'static>, Rc<[u8]>> {
    /// #     let rc: Rc<[u8]> = load_from_cache(filename);
    /// #     Yoke::<Bar<'static>, Rc<[u8]>>::attach_to_cart(rc, |data: &[u8]| {
    /// #         // A real implementation would properly deserialize `Bar` as a whole
    /// #         Bar {
    /// #             numbers: Cow::Borrowed(bincode::deserialize(data).unwrap()),
    /// #             string: Cow::Borrowed(bincode::deserialize(data).unwrap()),
    /// #             owned: Vec::new(),
    /// #         }
    /// #     })
    /// # }
    ///
    /// // also implements Yokeable
    /// struct Bar<'a> {
    ///     numbers: Cow<'a, [u8]>,
    ///     string: Cow<'a, str>,
    ///     owned: Vec<u8>,
    /// }
    ///
    /// // `load_object()` deserializes an object from a file
    /// let mut bar: Yoke<Bar, _> = load_object("filename.bincode");
    /// assert_eq!(bar.get().string, "hello");
    /// assert!(matches!(bar.get().string, Cow::Borrowed(_)));
    /// assert_eq!(&*bar.get().numbers, &[0x68, 0x65, 0x6c, 0x6c, 0x6f]);
    /// assert!(matches!(bar.get().numbers, Cow::Borrowed(_)));
    /// assert_eq!(&*bar.get().owned, &[]);
    ///
    /// bar.with_mut(|bar| {
    ///     bar.string.to_mut().push_str(" world");
    ///     bar.owned.extend_from_slice(&[1, 4, 1, 5, 9]);
    /// });
    ///
    /// assert_eq!(bar.get().string, "hello world");
    /// assert!(matches!(bar.get().string, Cow::Owned(_)));
    /// assert_eq!(&*bar.get().owned, &[1, 4, 1, 5, 9]);
    /// // Unchanged and still Cow::Borrowed
    /// assert_eq!(&*bar.get().numbers, &[0x68, 0x65, 0x6c, 0x6c, 0x6f]);
    /// assert!(matches!(bar.get().numbers, Cow::Borrowed(_)));
    ///
    /// # unsafe impl<'a> Yokeable<'a> for Bar<'static> {
    /// #     type Output = Bar<'a>;
    /// #     fn transform(&'a self) -> &'a Bar<'a> {
    /// #         self
    /// #     }
    /// #
    /// #     fn transform_owned(self) -> Bar<'a> {
    /// #         // covariant lifetime cast, can be done safely
    /// #         self
    /// #     }
    /// #
    /// #     unsafe fn make(from: Bar<'a>) -> Self {
    /// #         let ret = mem::transmute_copy(&from);
    /// #         mem::forget(from);
    /// #         ret
    /// #     }
    /// #
    /// #     fn transform_mut<F>(&'a mut self, f: F)
    /// #     where
    /// #         F: 'static + FnOnce(&'a mut Self::Output),
    /// #     {
    /// #         unsafe { f(mem::transmute(self)) }
    /// #     }
    /// # }
    /// ```
    pub fn with_mut<'a, F>(&'a mut self, f: F)
    where
        F: 'static + for<'b> FnOnce(&'b mut <Y as Yokeable<'a>>::Output),
    {
        self.yokeable.transform_mut(f)
    }

    /// Helper function allowing one to wrap the cart type `C` in an `Option<T>`.
    #[inline]
    pub fn wrap_cart_in_option(self) -> Yoke<Y, Option<C>> {
        unsafe {
            // safe because the cart is preserved, just wrapped
            self.replace_cart(Some)
        }
    }
}

impl<Y: for<'a> Yokeable<'a>> Yoke<Y, ()> {
    /// Construct a new [`Yoke`] from static data. There will be no
    /// references to `cart` here since [`Yokeable`]s are `'static`,
    /// this is good for e.g. constructing fully owned
    /// [`Yoke`]s with no internal borrowing.
    ///
    /// This is similar to [`Yoke::new_owned()`] but it does not allow you to
    /// mix the [`Yoke`] with borrowed data. This is primarily useful
    /// for using [`Yoke`] in generic scenarios.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use yoke::Yoke;
    /// # use std::borrow::Cow;
    /// # use std::rc::Rc;
    ///
    /// let owned: Cow<str> = "hello".to_owned().into();
    /// // this yoke can be intermingled with actually-borrowed Yokes
    /// let yoke: Yoke<Cow<str>, ()> = Yoke::new_always_owned(owned);
    ///
    /// assert_eq!(yoke.get(), "hello");
    /// ```
    pub fn new_always_owned(yokeable: Y) -> Self {
        Self { yokeable, cart: () }
    }

    /// Obtain the yokeable out of a `Yoke<Y, ()>`
    ///
    /// For most `Yoke` types this would be unsafe but it's
    /// fine for `Yoke<Y, ()>` since there are no actual internal
    /// references
    pub fn into_yokeable(self) -> Y {
        self.yokeable
    }
}

impl<Y: for<'a> Yokeable<'a>, C: StableDeref> Yoke<Y, Option<C>> {
    /// Construct a new [`Yoke`] from static data. There will be no
    /// references to `cart` here since [`Yokeable`]s are `'static`,
    /// this is good for e.g. constructing fully owned
    /// [`Yoke`]s with no internal borrowing.
    ///
    /// This can be paired with [`Yoke:: wrap_cart_in_option()`] to mix owned
    /// and borrowed data.
    ///
    /// If you do not wish to pair this with borrowed data, [`Yoke::new_always_owned()`] can
    /// be used to get a [`Yoke`] API on always-owned data.
    ///
    /// # Example
    ///
    /// ```rust
    /// # use yoke::Yoke;
    /// # use std::borrow::Cow;
    /// # use std::rc::Rc;
    ///
    /// let owned: Cow<str> = "hello".to_owned().into();
    /// // this yoke can be intermingled with actually-borrowed Yokes
    /// let yoke: Yoke<Cow<str>, Option<Rc<[u8]>>> = Yoke::new_owned(owned);
    ///
    /// assert_eq!(yoke.get(), "hello");
    /// ```
    pub fn new_owned(yokeable: Y) -> Self {
        Self {
            yokeable,
            cart: None,
        }
    }

    /// Obtain the yokeable out of a `Yoke<Y, Option<C>>` if possible.
    ///
    /// If the cart is `None`, this returns `Some`, but if the cart is `Some`,
    /// this returns `self` as an error.
    pub fn try_into_yokeable(self) -> Result<Y, Self> {
        match self.cart {
            Some(_) => Err(self),
            None => Ok(self.yokeable),
        }
    }
}

/// This trait marks cart types that do not change source on cloning
///
/// This is conceptually similar to [`stable_deref_trait::CloneStableDeref`],
/// however [`stable_deref_trait::CloneStableDeref`] is not (and should not) be
/// implemented on [`Option`] (since it's not [`Deref`]). [`CloneableCart`] essentially is
/// "if there _is_ data to borrow from here, cloning the cart gives you an additional
/// handle to the same data".
///
/// # Safety
/// This trait is safe to implement `StableDeref` types which, once `Clone`d, point to the same underlying data.
///
/// (This trait is also implemented on `Option<T>` and `()`, which are the two non-`StableDeref` cart types that
/// Yokes can be constructed for)
pub unsafe trait CloneableCart: Clone {}

#[cfg(feature = "alloc")]
unsafe impl<T: ?Sized> CloneableCart for Rc<T> {}
#[cfg(feature = "alloc")]
unsafe impl<T: ?Sized> CloneableCart for Arc<T> {}
unsafe impl<T: CloneableCart> CloneableCart for Option<T> {}
unsafe impl<'a, T: ?Sized> CloneableCart for &'a T {}
unsafe impl CloneableCart for () {}

/// Clone requires that the cart type `C` derefs to the same address after it is cloned. This works for
/// Rc, Arc, and &'a T.
///
/// For other cart types, clone `.backing_cart()` and re-use `.attach_to_cart()`; however, doing
/// so may lose mutations performed via `.with_mut()`.
///
/// Cloning a `Yoke` is often a cheap operation requiring no heap allocations, in much the same
/// way that cloning an `Rc` is a cheap operation. However, if the `yokeable` contains owned data
/// (e.g., from `.with_mut()`), that data will need to be cloned.
impl<Y: for<'a> Yokeable<'a>, C: CloneableCart> Clone for Yoke<Y, C>
where
    for<'a> YokeTraitHack<<Y as Yokeable<'a>>::Output>: Clone,
{
    fn clone(&self) -> Self {
        let this: &Y::Output = self.get();
        // We have an &T not a T, and we can clone YokeTraitHack<T>
        let this_hack = YokeTraitHack(this).into_ref();
        Yoke {
            yokeable: unsafe { Y::make(this_hack.clone().0) },
            cart: self.cart.clone(),
        }
    }
}

#[test]
fn test_clone() {
    let local_data = "foo".to_owned();
    let y1 = Yoke::<alloc::borrow::Cow<'static, str>, Rc<String>>::attach_to_zero_copy_cart(
        Rc::new(local_data),
    );

    // Test basic clone
    let y2 = y1.clone();
    assert_eq!(y1.get(), "foo");
    assert_eq!(y2.get(), "foo");

    // Test clone with mutation on target
    let mut y3 = y1.clone();
    y3.with_mut(|y| {
        y.to_mut().push_str("bar");
    });
    assert_eq!(y1.get(), "foo");
    assert_eq!(y2.get(), "foo");
    assert_eq!(y3.get(), "foobar");

    // Test that mutations on source do not affect target
    let y4 = y3.clone();
    y3.with_mut(|y| {
        y.to_mut().push_str("baz");
    });
    assert_eq!(y1.get(), "foo");
    assert_eq!(y2.get(), "foo");
    assert_eq!(y3.get(), "foobarbaz");
    assert_eq!(y4.get(), "foobar");
}

impl<Y: for<'a> Yokeable<'a>, C> Yoke<Y, C> {
    /// Allows one to "project" a yoke to perform a transformation on the data, potentially
    /// looking at a subfield, and producing a new yoke. This will move cart, and the provided
    /// transformation is only allowed to use data known to be borrowed from the cart.
    ///
    /// The callback takes an additional `PhantomData<&()>` parameter to anchor lifetimes
    /// (see [#86702](https://github.com/rust-lang/rust/issues/86702)) This parameter
    /// should just be ignored in the callback.
    ///
    /// This can be used, for example, to transform data from one format to another:
    ///
    /// ```
    /// # use std::rc::Rc;
    /// # use yoke::Yoke;
    /// #
    /// fn slice(y: Yoke<&'static str, Rc<[u8]>>) -> Yoke<&'static [u8], Rc<[u8]>> {
    ///     y.map_project(move |yk, _| yk.as_bytes())
    /// }
    /// ```
    ///
    /// This can also be used to create a yoke for a subfield
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// # use yoke::{Yoke, Yokeable};
    /// # use std::mem;
    /// # use std::rc::Rc;
    /// #
    /// // also safely implements Yokeable<'a>
    /// struct Bar<'a> {
    ///     string_1: &'a str,
    ///     string_2: &'a str,
    /// }
    ///
    /// fn map_project_string_1(
    ///     bar: Yoke<Bar<'static>, Rc<[u8]>>,
    /// ) -> Yoke<&'static str, Rc<[u8]>> {
    ///     bar.map_project(|bar, _| bar.string_1)
    /// }
    ///
    /// #
    /// # unsafe impl<'a> Yokeable<'a> for Bar<'static> {
    /// #     type Output = Bar<'a>;
    /// #     fn transform(&'a self) -> &'a Bar<'a> {
    /// #         self
    /// #     }
    /// #
    /// #     fn transform_owned(self) -> Bar<'a> {
    /// #         // covariant lifetime cast, can be done safely
    /// #         self
    /// #     }
    /// #
    /// #     unsafe fn make(from: Bar<'a>) -> Self {
    /// #         let ret = mem::transmute_copy(&from);
    /// #         mem::forget(from);
    /// #         ret
    /// #     }
    /// #
    /// #     fn transform_mut<F>(&'a mut self, f: F)
    /// #     where
    /// #         F: 'static + FnOnce(&'a mut Self::Output),
    /// #     {
    /// #         unsafe { f(mem::transmute(self)) }
    /// #     }
    /// # }
    /// ```
    //
    // Safety docs can be found below on `__project_safety_docs()`
    pub fn map_project<P, F>(self, f: F) -> Yoke<P, C>
    where
        P: for<'a> Yokeable<'a>,
        F: for<'a> FnOnce(
            <Y as Yokeable<'a>>::Output,
            PhantomData<&'a ()>,
        ) -> <P as Yokeable<'a>>::Output,
    {
        let p = f(self.yokeable.transform_owned(), PhantomData);
        Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart,
        }
    }

    /// This is similar to [`Yoke::map_project`], however it does not move
    /// [`Self`] and instead clones the cart (only if the cart is a [`CloneableCart`])
    ///
    /// This is a bit more efficient than cloning the [`Yoke`] and then calling [`Yoke::map_project`]
    /// because then it will not clone fields that are going to be discarded.
    pub fn map_project_cloned<'this, P, F>(&'this self, f: F) -> Yoke<P, C>
    where
        P: for<'a> Yokeable<'a>,
        C: CloneableCart,
        F: for<'a> FnOnce(
            &'this <Y as Yokeable<'a>>::Output,
            PhantomData<&'a ()>,
        ) -> <P as Yokeable<'a>>::Output,
    {
        let p = f(self.get(), PhantomData);
        Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart.clone(),
        }
    }

    /// This is similar to [`Yoke::map_project`], however it can also bubble up an error
    /// from the callback.
    ///
    /// ```
    /// # use std::rc::Rc;
    /// # use yoke::Yoke;
    /// # use std::str::{self, Utf8Error};
    /// #
    /// fn slice(
    ///     y: Yoke<&'static [u8], Rc<[u8]>>,
    /// ) -> Result<Yoke<&'static str, Rc<[u8]>>, Utf8Error> {
    ///     y.try_map_project(move |bytes, _| str::from_utf8(bytes))
    /// }
    /// ```
    ///
    /// This can also be used to create a yoke for a subfield
    ///
    /// ```
    /// # use std::borrow::Cow;
    /// # use yoke::{Yoke, Yokeable};
    /// # use std::mem;
    /// # use std::rc::Rc;
    /// # use std::str::{self, Utf8Error};
    /// #
    /// // also safely implements Yokeable<'a>
    /// struct Bar<'a> {
    ///     bytes_1: &'a [u8],
    ///     string_2: &'a str,
    /// }
    ///
    /// fn map_project_string_1(
    ///     bar: Yoke<Bar<'static>, Rc<[u8]>>,
    /// ) -> Result<Yoke<&'static str, Rc<[u8]>>, Utf8Error> {
    ///     bar.try_map_project(|bar, _| str::from_utf8(bar.bytes_1))
    /// }
    ///
    /// #
    /// # unsafe impl<'a> Yokeable<'a> for Bar<'static> {
    /// #     type Output = Bar<'a>;
    /// #     fn transform(&'a self) -> &'a Bar<'a> {
    /// #         self
    /// #     }
    /// #
    /// #     fn transform_owned(self) -> Bar<'a> {
    /// #         // covariant lifetime cast, can be done safely
    /// #         self
    /// #     }
    /// #
    /// #     unsafe fn make(from: Bar<'a>) -> Self {
    /// #         let ret = mem::transmute_copy(&from);
    /// #         mem::forget(from);
    /// #         ret
    /// #     }
    /// #
    /// #     fn transform_mut<F>(&'a mut self, f: F)
    /// #     where
    /// #         F: 'static + FnOnce(&'a mut Self::Output),
    /// #     {
    /// #         unsafe { f(mem::transmute(self)) }
    /// #     }
    /// # }
    /// ```
    pub fn try_map_project<P, F, E>(self, f: F) -> Result<Yoke<P, C>, E>
    where
        P: for<'a> Yokeable<'a>,
        F: for<'a> FnOnce(
            <Y as Yokeable<'a>>::Output,
            PhantomData<&'a ()>,
        ) -> Result<<P as Yokeable<'a>>::Output, E>,
    {
        let p = f(self.yokeable.transform_owned(), PhantomData)?;
        Ok(Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart,
        })
    }

    /// This is similar to [`Yoke::try_map_project`], however it does not move
    /// [`Self`] and instead clones the cart (only if the cart is a [`CloneableCart`])
    ///
    /// This is a bit more efficient than cloning the [`Yoke`] and then calling [`Yoke::map_project`]
    /// because then it will not clone fields that are going to be discarded.
    pub fn try_map_project_cloned<'this, P, F, E>(&'this self, f: F) -> Result<Yoke<P, C>, E>
    where
        P: for<'a> Yokeable<'a>,
        C: CloneableCart,
        F: for<'a> FnOnce(
            &'this <Y as Yokeable<'a>>::Output,
            PhantomData<&'a ()>,
        ) -> Result<<P as Yokeable<'a>>::Output, E>,
    {
        let p = f(self.get(), PhantomData)?;
        Ok(Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart.clone(),
        })
    }
    /// This is similar to [`Yoke::map_project`], but it works around older versions
    /// of Rust not being able to use `FnOnce` by using an explicit capture input.
    /// See [#1061](https://github.com/unicode-org/icu4x/issues/1061).
    ///
    /// See the docs of [`Yoke::map_project`] for how this works.
    pub fn map_project_with_explicit_capture<P, T>(
        self,
        capture: T,
        f: for<'a> fn(
            <Y as Yokeable<'a>>::Output,
            capture: T,
            PhantomData<&'a ()>,
        ) -> <P as Yokeable<'a>>::Output,
    ) -> Yoke<P, C>
    where
        P: for<'a> Yokeable<'a>,
    {
        let p = f(self.yokeable.transform_owned(), capture, PhantomData);
        Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart,
        }
    }

    /// This is similar to [`Yoke::map_project_cloned`], but it works around older versions
    /// of Rust not being able to use `FnOnce` by using an explicit capture input.
    /// See [#1061](https://github.com/unicode-org/icu4x/issues/1061).
    ///
    /// See the docs of [`Yoke::map_project_cloned`] for how this works.
    pub fn map_project_cloned_with_explicit_capture<'this, P, T>(
        &'this self,
        capture: T,
        f: for<'a> fn(
            &'this <Y as Yokeable<'a>>::Output,
            capture: T,
            PhantomData<&'a ()>,
        ) -> <P as Yokeable<'a>>::Output,
    ) -> Yoke<P, C>
    where
        P: for<'a> Yokeable<'a>,
        C: CloneableCart,
    {
        let p = f(self.get(), capture, PhantomData);
        Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart.clone(),
        }
    }

    /// This is similar to [`Yoke::try_map_project`], but it works around older versions
    /// of Rust not being able to use `FnOnce` by using an explicit capture input.
    /// See [#1061](https://github.com/unicode-org/icu4x/issues/1061).
    ///
    /// See the docs of [`Yoke::try_map_project`] for how this works.
    #[allow(clippy::type_complexity)]
    pub fn try_map_project_with_explicit_capture<P, T, E>(
        self,
        capture: T,
        f: for<'a> fn(
            <Y as Yokeable<'a>>::Output,
            capture: T,
            PhantomData<&'a ()>,
        ) -> Result<<P as Yokeable<'a>>::Output, E>,
    ) -> Result<Yoke<P, C>, E>
    where
        P: for<'a> Yokeable<'a>,
    {
        let p = f(self.yokeable.transform_owned(), capture, PhantomData)?;
        Ok(Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart,
        })
    }

    /// This is similar to [`Yoke::try_map_project_cloned`], but it works around older versions
    /// of Rust not being able to use `FnOnce` by using an explicit capture input.
    /// See [#1061](https://github.com/unicode-org/icu4x/issues/1061).
    ///
    /// See the docs of [`Yoke::try_map_project_cloned`] for how this works.
    #[allow(clippy::type_complexity)]
    pub fn try_map_project_cloned_with_explicit_capture<'this, P, T, E>(
        &'this self,
        capture: T,
        f: for<'a> fn(
            &'this <Y as Yokeable<'a>>::Output,
            capture: T,
            PhantomData<&'a ()>,
        ) -> Result<<P as Yokeable<'a>>::Output, E>,
    ) -> Result<Yoke<P, C>, E>
    where
        P: for<'a> Yokeable<'a>,
        C: CloneableCart,
    {
        let p = f(self.get(), capture, PhantomData)?;
        Ok(Yoke {
            yokeable: unsafe { P::make(p) },
            cart: self.cart.clone(),
        })
    }
}

#[cfg(feature = "alloc")]
impl<Y: for<'a> Yokeable<'a>, C: 'static + Sized> Yoke<Y, Rc<C>> {
    /// Allows type-erasing the cart in a `Yoke<Y, Rc<C>>`.
    ///
    /// The yoke only carries around a cart type `C` for its destructor,
    /// since it needs to be able to guarantee that its internal references
    /// are valid for the lifetime of the Yoke. As such, the actual type of the
    /// Cart is not very useful unless you wish to extract data out of it
    /// via [`Yoke::backing_cart()`]. Erasing the cart allows for one to mix
    /// [`Yoke`]s obtained from different sources.
    ///
    /// In case the cart type `C` is not already an `Rc<T>`, you can use
    /// [`Yoke::wrap_cart_in_rc()`] to wrap it.
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::rc::Rc;
    /// use yoke::erased::ErasedRcCart;
    /// use yoke::Yoke;
    ///
    /// let buffer1: Rc<String> = Rc::new("   foo bar baz  ".into());
    /// let buffer2: Box<String> = Box::new("  baz quux  ".into());
    ///
    /// let yoke1 =
    ///     Yoke::<&'static str, _>::attach_to_cart(buffer1, |rc| rc.trim());
    /// let yoke2 = Yoke::<&'static str, _>::attach_to_cart(buffer2, |b| b.trim());
    ///
    /// let erased1: Yoke<_, ErasedRcCart> = yoke1.erase_rc_cart();
    /// // Wrap the Box in an Rc to make it compatible
    /// let erased2: Yoke<_, ErasedRcCart> =
    ///     yoke2.wrap_cart_in_rc().erase_rc_cart();
    ///
    /// // Now erased1 and erased2 have the same type!
    /// ```
    ///
    /// Available with the `"alloc"` Cargo feature enabled.
    pub fn erase_rc_cart(self) -> Yoke<Y, ErasedRcCart> {
        unsafe {
            // safe because the cart is preserved, just
            // type-erased
            self.replace_cart(|c| c as ErasedRcCart)
        }
    }
}

#[cfg(feature = "alloc")]
impl<Y: for<'a> Yokeable<'a>, C: 'static + Sized + Send + Sync> Yoke<Y, Arc<C>> {
    /// Allows type-erasing the cart in a `Yoke<Y, Arc<C>>`.
    ///
    /// The yoke only carries around a cart type `C` for its destructor,
    /// since it needs to be able to guarantee that its internal references
    /// are valid for the lifetime of the Yoke. As such, the actual type of the
    /// Cart is not very useful unless you wish to extract data out of it
    /// via [`Yoke::backing_cart()`]. Erasing the cart allows for one to mix
    /// [`Yoke`]s obtained from different sources.
    ///
    /// In case the cart type `C` is not already an `Arc<T>`, you can use
    /// [`Yoke::wrap_cart_in_arc()`] to wrap it.
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::sync::Arc;
    /// use yoke::erased::ErasedArcCart;
    /// use yoke::Yoke;
    ///
    /// let buffer1: Arc<String> = Arc::new("   foo bar baz  ".into());
    /// let buffer2: Box<String> = Box::new("  baz quux  ".into());
    ///
    /// let yoke1 =
    ///     Yoke::<&'static str, _>::attach_to_cart(buffer1, |arc| arc.trim());
    /// let yoke2 = Yoke::<&'static str, _>::attach_to_cart(buffer2, |b| b.trim());
    ///
    /// let erased1: Yoke<_, ErasedArcCart> = yoke1.erase_arc_cart();
    /// // Wrap the Box in an Rc to make it compatible
    /// let erased2: Yoke<_, ErasedArcCart> =
    ///     yoke2.wrap_cart_in_arc().erase_arc_cart();
    ///
    /// // Now erased1 and erased2 have the same type!
    /// ```
    ///
    /// Available with the `"alloc"` Cargo feature enabled.
    pub fn erase_arc_cart(self) -> Yoke<Y, ErasedArcCart> {
        unsafe {
            // safe because the cart is preserved, just
            // type-erased
            self.replace_cart(|c| c as ErasedArcCart)
        }
    }
}

#[cfg(feature = "alloc")]
impl<Y: for<'a> Yokeable<'a>, C: 'static + Sized> Yoke<Y, Box<C>> {
    /// Allows type-erasing the cart in a `Yoke<Y, Box<C>>`.
    ///
    /// The yoke only carries around a cart type `C` for its destructor,
    /// since it needs to be able to guarantee that its internal references
    /// are valid for the lifetime of the Yoke. As such, the actual type of the
    /// Cart is not very useful unless you wish to extract data out of it
    /// via [`Yoke::backing_cart()`]. Erasing the cart allows for one to mix
    /// [`Yoke`]s obtained from different sources.
    ///
    /// In case the cart type `C` is not already `Box<T>`, you can use
    /// [`Yoke::wrap_cart_in_box()`] to wrap it.
    ///
    /// # Example
    ///
    /// ```rust
    /// use std::rc::Rc;
    /// use yoke::erased::ErasedBoxCart;
    /// use yoke::Yoke;
    ///
    /// let buffer1: Rc<String> = Rc::new("   foo bar baz  ".into());
    /// let buffer2: Box<String> = Box::new("  baz quux  ".into());
    ///
    /// let yoke1 =
    ///     Yoke::<&'static str, _>::attach_to_cart(buffer1, |rc| rc.trim());
    /// let yoke2 = Yoke::<&'static str, _>::attach_to_cart(buffer2, |b| b.trim());
    ///
    /// // Wrap the Rc in an Box to make it compatible
    /// let erased1: Yoke<_, ErasedBoxCart> =
    ///     yoke1.wrap_cart_in_box().erase_box_cart();
    /// let erased2: Yoke<_, ErasedBoxCart> = yoke2.erase_box_cart();
    ///
    /// // Now erased1 and erased2 have the same type!
    /// ```
    ///
    /// Available with the `"alloc"` Cargo feature enabled.
    pub fn erase_box_cart(self) -> Yoke<Y, ErasedBoxCart> {
        unsafe {
            // safe because the cart is preserved, just
            // type-erased
            self.replace_cart(|c| c as ErasedBoxCart)
        }
    }
}

#[cfg(feature = "alloc")]
impl<Y: for<'a> Yokeable<'a>, C> Yoke<Y, C> {
    /// Helper function allowing one to wrap the cart type `C` in a `Box<T>`.
    /// Can be paired with [`Yoke::erase_box_cart()`]
    ///
    /// Available with the `"alloc"` Cargo feature enabled.
    #[inline]
    pub fn wrap_cart_in_box(self) -> Yoke<Y, Box<C>> {
        unsafe {
            // safe because the cart is preserved, just wrapped
            self.replace_cart(Box::new)
        }
    }
    /// Helper function allowing one to wrap the cart type `C` in an `Rc<T>`.
    /// Can be paired with [`Yoke::erase_rc_cart()`], or generally used
    /// to make the [`Yoke`] cloneable.
    ///
    /// Available with the `"alloc"` Cargo feature enabled.
    #[inline]
    pub fn wrap_cart_in_rc(self) -> Yoke<Y, Rc<C>> {
        unsafe {
            // safe because the cart is preserved, just wrapped
            self.replace_cart(Rc::new)
        }
    }
    /// Helper function allowing one to wrap the cart type `C` in an `Rc<T>`.
    /// Can be paired with [`Yoke::erase_arc_cart()`], or generally used
    /// to make the [`Yoke`] cloneable.
    ///
    /// Available with the `"alloc"` Cargo feature enabled.
    #[inline]
    pub fn wrap_cart_in_arc(self) -> Yoke<Y, Arc<C>> {
        unsafe {
            // safe because the cart is preserved, just wrapped
            self.replace_cart(Arc::new)
        }
    }
}

impl<Y: for<'a> Yokeable<'a>, C> Yoke<Y, C> {
    /// Helper function allowing one to wrap the cart type `C` in an [`EitherCart`].
    ///
    /// This function wraps the cart into the `A` variant. To wrap it into the
    /// `B` variant, use [`Self::wrap_cart_in_either_b()`].
    ///
    /// For an example, see [`EitherCart`].
    #[inline]
    pub fn wrap_cart_in_either_a<B>(self) -> Yoke<Y, EitherCart<C, B>> {
        unsafe {
            // safe because the cart is preserved, just wrapped
            self.replace_cart(EitherCart::A)
        }
    }
    /// Helper function allowing one to wrap the cart type `C` in an [`EitherCart`].
    ///
    /// This function wraps the cart into the `B` variant. To wrap it into the
    /// `A` variant, use [`Self::wrap_cart_in_either_a()`].
    ///
    /// For an example, see [`EitherCart`].
    #[inline]
    pub fn wrap_cart_in_either_b<A>(self) -> Yoke<Y, EitherCart<A, C>> {
        unsafe {
            // safe because the cart is preserved, just wrapped
            self.replace_cart(EitherCart::B)
        }
    }
}

/// # Safety docs for project()
///
/// (Docs are on a private const to allow the use of compile_fail doctests)
///
/// This is safe to perform because of the choice of lifetimes on `f`, that is,
/// `for<a> fn(<Y as Yokeable<'a>>::Output, &'a ()) -> <P as Yokeable<'a>>::Output`.
///
/// What we want this function to do is take a Yokeable (`Y`) that is borrowing from the cart, and
/// produce another Yokeable (`P`) that also borrows from the same cart. There are a couple potential
/// hazards here:
///
/// - `P` ends up borrowing data from `Y` (or elsewhere) that did _not_ come from the cart,
///   for example `P` could borrow owned data from a `Cow`. This would make the `Yoke<P>` dependent
///   on data owned only by the `Yoke<Y>`.
/// - Borrowed data from `Y` escapes with the wrong lifetime
///
/// Let's walk through these and see how they're prevented.
///
/// ```rust, compile_fail
/// # use std::rc::Rc;
/// # use yoke::Yoke;
/// # use std::borrow::Cow;
/// fn borrow_potentially_owned(y: &Yoke<Cow<'static, str>, Rc<[u8]>>) -> Yoke<&'static str, Rc<[u8]>> {
///    y.map_project_cloned(|cow, _| &*cow)   
/// }
/// ```
///
/// In this case, the lifetime of `&*cow` is `&'this str`, however the function needs to be able to return
/// `&'a str` _for all `'a`_, which isn't possible.
///
///
/// ```rust, compile_fail
/// # use std::rc::Rc;
/// # use yoke::Yoke;
/// # use std::borrow::Cow;
/// fn borrow_potentially_owned(y: Yoke<Cow<'static, str>, Rc<[u8]>>) -> Yoke<&'static str, Rc<[u8]>> {
///    y.map_project(|cow, _| &*cow)   
/// }
/// ```
///
/// This has the same issue, `&*cow` is borrowing for a local lifetime.
///
/// Similarly, trying to project an owned field of a struct will produce similar errors:
///
/// ```rust,compile_fail
/// # use std::borrow::Cow;
/// # use yoke::{Yoke, Yokeable};
/// # use std::mem;
/// # use std::rc::Rc;
/// #
/// // also safely implements Yokeable<'a>
/// struct Bar<'a> {
///     owned: String,
///     string_2: &'a str,
/// }
///
/// fn map_project_owned(bar: &Yoke<Bar<'static>, Rc<[u8]>>) -> Yoke<&'static str, Rc<[u8]>> {
///     // ERROR (but works if you replace owned with string_2)
///     bar.map_project_cloned(|bar, _| &*bar.owned)   
/// }
///
/// #
/// # unsafe impl<'a> Yokeable<'a> for Bar<'static> {
/// #     type Output = Bar<'a>;
/// #     fn transform(&'a self) -> &'a Bar<'a> {
/// #         self
/// #     }
/// #
/// #     fn transform_owned(self) -> Bar<'a> {
/// #         // covariant lifetime cast, can be done safely
/// #         self
/// #     }
/// #
/// #     unsafe fn make(from: Bar<'a>) -> Self {
/// #         let ret = mem::transmute_copy(&from);
/// #         mem::forget(from);
/// #         ret
/// #     }
/// #
/// #     fn transform_mut<F>(&'a mut self, f: F)
/// #     where
/// #         F: 'static + FnOnce(&'a mut Self::Output),
/// #     {
/// #         unsafe { f(mem::transmute(self)) }
/// #     }
/// # }
/// ```
///
/// Borrowed data from `Y` similarly cannot escape with the wrong lifetime because of the `for<'a>`, since
/// it will never be valid for the borrowed data to escape for all lifetimes of 'a. Internally, `.project()`
/// uses `.get()`, however the signature forces the callers to be able to handle every lifetime.
///
///  `'a` is the only lifetime that matters here; `Yokeable`s must be `'static` and since
/// `Output` is an associated type it can only have one lifetime, `'a` (there's nowhere for it to get another from).
/// `Yoke`s can get additional lifetimes via the cart, and indeed, `project()` can operate on `Yoke<_, &'b [u8]>`,
/// however this lifetime is inaccessible to the closure, and even if it were accessible the `for<'a>` would force
/// it out of the output. All external lifetimes (from other found outside the yoke/closures
/// are similarly constrained here.
///
/// Essentially, safety is achieved by using `for<'a> fn(...)` with `'a` used in both `Yokeable`s to ensure that
/// the output yokeable can _only_ have borrowed data flow in to it from the input. All paths of unsoundness require the
/// unification of an existential and universal lifetime, which isn't possible.
const _: () = ();

/// # Safety docs for attach_to_cart()'s signature
///
/// The `attach_to_cart()` family of methods get by by using the following bound:
///
/// ```rust,ignore
/// F: for<'de> FnOnce(&'de <C as Deref>::Target) -> <Y as Yokeable<'de>>::Output,
/// C::Target: 'static
/// ```
///
/// to enforce that the yoking closure produces a yokeable that is *only* allowed to borrow from the cart.
/// A way to be sure of this is as follows: imagine if `F` *did* borrow data of lifetime `'a` and stuff it in
/// its output. Then that lifetime `'a` would have to live at least as long as `'de` *for all `'de`*.
/// The only lifetime that satisfies that is `'static` (since at least one of the potential `'de`s is `'static`),
/// and we're fine with that.
///
/// ## Implied bounds and variance
///
/// The `C::Target: 'static` bound is tricky, however. Let's imagine a situation where we *didn't* have that bound.
///
/// One thing to remember is that we are okay with the cart itself borrowing from places,
/// e.g. `&[u8]` is a valid cart, as is `Box<&[u8]>`. `C` is not `'static`.
///
/// (I'm going to use `CT` in prose to refer to `C::Target` here, since almost everything here has to do
/// with C::Target and not C itself.)
///
/// Unfortunately, there's a sneaky additional bound inside `F`. The signature of `F` is *actually*
///
/// ```rust,ignore
/// F: for<'de> where<C::Target: 'de> FnOnce(&'de C::Target) -> <Y as Yokeable<'de>>::Output
/// ```
///
/// using made-up "where clause inside HRTB" syntax to represent a type that can be represented inside the compiler
/// and type system but not in Rust code. The `CT: 'de` bond comes from the `&'de C::Target`: any time you
/// write `&'a T`, an implied bound of `T: 'a` materializes and is stored alongside it, since references cannot refer
/// to data that itself refers to data of shorter lifetimes. If a reference is valid, its referent must be valid for
/// the duration of the reference's lifetime, so every reference *inside* its referent must also be valid, giving us `T: 'a`.
/// This kind of constraint is often called a "well formedness" constraint: `&'a T` is not "well formed" without that
/// bound, and rustc is being helpful by giving it to us for free.
///
/// Unfortunately, this messes with our universal quantification. The `for<'de>` is no longer "For all lifetimes `'de`",
/// it is "for all lifetimes `'de` *where `CT: 'de`*". And if `CT` borrows from somewhere (with lifetime `'ct`), then we get a
/// `'ct: 'de` bound, and `'de` candidates that live longer than `'ct` won't actually be considered.
/// The neat little logic at the beginning stops working.
///
/// `attach_to_cart()` will instead enforce that the produced yokeable *either* borrows from the cart (fine), or from
/// data that has a lifetime that is at least `'ct`. Which means that `attach_to_cart()` will allow us to borrow locals
/// provided they live at least as long as `'ct`.
///
/// Is this a problem?
///
/// This is totally fine if CT's lifetime is covariant: if C is something like `Box<&'ct [u8]>`, even if our
/// yoked object borrows from locals outliving `'ct`, our Yoke can't outlive that
/// lifetime `'ct` anyway (since it's a part of the cart type), so we're fine.
///
/// However it's completely broken for contravariant carts (e.g. `Box<fn(&'ct u8)>`). In that case
/// we still get `'ct: 'de`, and we still end up being able to
/// borrow from locals that outlive `'ct`. However, our Yoke _can_ outlive
/// that lifetime, because Yoke shares its variance over `'ct`
/// with the cart type, and the cart type is contravariant over `'ct`.
/// So the Yoke can be upcast to having a longer lifetime than `'ct`, and *that* Yoke
/// can outlive `'ct`.
///
/// We fix this by forcing `C::Target: 'static` in `attach_to_cart()`, which would make it work
/// for fewer types, but would also allow Yoke to continue to be covariant over cart lifetimes if necessary.
///
/// An alternate fix would be to not allowing yoke to ever be upcast over lifetimes contained in the cart
/// by forcing them to be invariant. This is a bit more restrictive and affects *all* `Yoke` users, not just
/// those using `attach_to_cart()`.
///
/// See https://github.com/unicode-org/icu4x/issues/2926
/// See also https://github.com/rust-lang/rust/issues/106431 for potentially fixing this upstream by
/// changing how the bound works.
///
/// # Tests
///
/// Here's a broken `attach_to_cart()` that attempts to borrow from a local:
///
/// ```rust,compile_fail
/// use yoke::{Yoke, Yokeable};
///
/// let cart = vec![1, 2, 3, 4].into_boxed_slice();
/// let local = vec![4, 5, 6, 7];
/// let yoke: Yoke<&[u8], Box<[u8]>> = Yoke::attach_to_cart(cart, |_| &*local);
/// ```
///
/// Fails as expected.
///
/// And here's a working one with a local borrowed cart that does not do any sneaky borrows whilst attaching.
///
/// ```rust
/// use yoke::{Yoke, Yokeable};
///
/// let cart = vec![1, 2, 3, 4].into_boxed_slice();
/// let local = vec![4, 5, 6, 7];
/// let yoke: Yoke<&[u8], &[u8]> = Yoke::attach_to_cart(&cart, |c| &*c);
/// ```
///
/// Here's an `attach_to_cart()` that attempts to borrow from a longer-lived local due to
/// the cart being covariant. It fails, but would not if the alternate fix of forcing Yoke to be invariant
/// were implemented. It is technically a safe operation:
///
/// ```rust,compile_fail
/// use yoke::{Yoke, Yokeable};
/// // longer lived
/// let local = vec![4, 5, 6, 7];
///
/// let backing = vec![1, 2, 3, 4];
/// let cart = Box::new(&*backing);
///
/// let yoke: Yoke<&[u8], Box<&[u8]>> = Yoke::attach_to_cart(cart, |_| &*local);
/// println!("{:?}", yoke.get());
/// ```
///
/// Finally, here's an `attach_to_cart()` that attempts to borrow from a longer lived local
/// in the case of a contravariant lifetime. It does not compile, but in and of itself is not dangerous:
///
/// ```rust,compile_fail
/// use yoke::Yoke;
///
/// type Contra<'a> = fn(&'a ());
///
/// let local = String::from("Hello World!");
/// let yoke: Yoke<&'static str, Box<Contra<'_>>> = Yoke::attach_to_cart(Box::new((|_| {}) as _), |_| &local[..]);
/// println!("{:?}", yoke.get());
/// ```
///
/// It is dangerous if allowed to transform (testcase from #2926)
///
/// ```rust,compile_fail
/// use yoke::Yoke;
///
/// type Contra<'a> = fn(&'a ());
///
///
/// let local = String::from("Hello World!");
/// let yoke: Yoke<&'static str, Box<Contra<'_>>> = Yoke::attach_to_cart(Box::new((|_| {}) as _), |_| &local[..]);
/// println!("{:?}", yoke.get());
/// let yoke_longer: Yoke<&'static str, Box<Contra<'static>>> = yoke;
/// let leaked: &'static Yoke<&'static str, Box<Contra<'static>>> = Box::leak(Box::new(yoke_longer));
/// let reference: &'static str = leaked.get();
///
/// println!("pre-drop: {reference}");
/// drop(local);
/// println!("post-drop: {reference}");
///
/// ```
const _: () = ();