summaryrefslogtreecommitdiffstats
path: root/third_party/rust/bumpalo/src/lib.rs
blob: 74dfcd4361f81e1fda2cef80a15181511e50744b (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
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
#![doc = include_str!("../README.md")]
#![deny(missing_debug_implementations)]
#![deny(missing_docs)]
#![no_std]
#![cfg_attr(
    feature = "allocator_api",
    feature(allocator_api, nonnull_slice_from_raw_parts)
)]

#[doc(hidden)]
pub extern crate alloc as core_alloc;

#[cfg(feature = "boxed")]
pub mod boxed;
#[cfg(feature = "collections")]
pub mod collections;

mod alloc;

use core::cell::Cell;
use core::fmt::Display;
use core::iter;
use core::marker::PhantomData;
use core::mem;
use core::ptr::{self, NonNull};
use core::slice;
use core::str;
use core_alloc::alloc::{alloc, dealloc, Layout};
#[cfg(feature = "allocator_api")]
use core_alloc::alloc::{AllocError, Allocator};

pub use alloc::AllocErr;

/// An error returned from [`Bump::try_alloc_try_with`].
#[derive(Clone, PartialEq, Eq, Debug)]
pub enum AllocOrInitError<E> {
    /// Indicates that the initial allocation failed.
    Alloc(AllocErr),
    /// Indicates that the initializer failed with the contained error after
    /// allocation.
    ///
    /// It is possible but not guaranteed that the allocated memory has been
    /// released back to the allocator at this point.
    Init(E),
}
impl<E> From<AllocErr> for AllocOrInitError<E> {
    fn from(e: AllocErr) -> Self {
        Self::Alloc(e)
    }
}
impl<E: Display> Display for AllocOrInitError<E> {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            AllocOrInitError::Alloc(err) => err.fmt(f),
            AllocOrInitError::Init(err) => write!(f, "initialization failed: {}", err),
        }
    }
}

/// An arena to bump allocate into.
///
/// ## No `Drop`s
///
/// Objects that are bump-allocated will never have their [`Drop`] implementation
/// called &mdash; unless you do it manually yourself. This makes it relatively
/// easy to leak memory or other resources.
///
/// If you have a type which internally manages
///
/// * an allocation from the global heap (e.g. [`Vec<T>`]),
/// * open file descriptors (e.g. [`std::fs::File`]), or
/// * any other resource that must be cleaned up (e.g. an `mmap`)
///
/// and relies on its `Drop` implementation to clean up the internal resource,
/// then if you allocate that type with a `Bump`, you need to find a new way to
/// clean up after it yourself.
///
/// Potential solutions are:
///
/// * Using [`bumpalo::boxed::Box::new_in`] instead of [`Bump::alloc`], that
///   will drop wrapped values similarly to [`std::boxed::Box`]. Note that this
///   requires enabling the `"boxed"` Cargo feature for this crate. **This is
///   often the easiest solution.**
///
/// * Calling [`drop_in_place`][drop_in_place] or using
///   [`std::mem::ManuallyDrop`][manuallydrop] to manually drop these types.
///
/// * Using [`bumpalo::collections::Vec`] instead of [`std::vec::Vec`].
///
/// * Avoiding allocating these problematic types within a `Bump`.
///
/// Note that not calling `Drop` is memory safe! Destructors are never
/// guaranteed to run in Rust, you can't rely on them for enforcing memory
/// safety.
///
/// [`Drop`]: https://doc.rust-lang.org/std/ops/trait.Drop.html
/// [`Vec<T>`]: https://doc.rust-lang.org/std/vec/struct.Vec.html
/// [`std::fs::File`]: https://doc.rust-lang.org/std/fs/struct.File.html
/// [drop_in_place]: https://doc.rust-lang.org/std/ptr/fn.drop_in_place.html
/// [manuallydrop]: https://doc.rust-lang.org/std/mem/struct.ManuallyDrop.html
/// [`bumpalo::collections::Vec`]: collections/vec/struct.Vec.html
/// [`std::vec::Vec`]: https://doc.rust-lang.org/std/vec/struct.Vec.html
/// [`bumpalo::boxed::Box::new_in`]: boxed/struct.Box.html#method.new_in
/// [`std::boxed::Box`]: https://doc.rust-lang.org/std/boxed/struct.Box.html
///
/// ## Example
///
/// ```
/// use bumpalo::Bump;
///
/// // Create a new bump arena.
/// let bump = Bump::new();
///
/// // Allocate values into the arena.
/// let forty_two = bump.alloc(42);
/// assert_eq!(*forty_two, 42);
///
/// // Mutable references are returned from allocation.
/// let mut s = bump.alloc("bumpalo");
/// *s = "the bump allocator; and also is a buffalo";
/// ```
///
/// ## Allocation Methods Come in Many Flavors
///
/// There are various allocation methods on `Bump`, the simplest being
/// [`alloc`][Bump::alloc]. The others exist to satisfy some combination of
/// fallible allocation and initialization. The allocation methods are
/// summarized in the following table:
///
/// <table>
///   <thead>
///     <tr>
///       <th></th>
///       <th>Infallible Allocation</th>
///       <th>Fallible Allocation</th>
///     </tr>
///   </thead>
///     <tr>
///       <th>By Value</th>
///       <td><a href="#method.alloc"><code>alloc</code></a></td>
///       <td><a href="#method.try_alloc"><code>try_alloc</code></a></td>
///     </tr>
///     <tr>
///       <th>Infallible Initializer Function</th>
///       <td><a href="#method.alloc_with"><code>alloc_with</code></a></td>
///       <td><a href="#method.try_alloc_with"><code>try_alloc_with</code></a></td>
///     </tr>
///     <tr>
///       <th>Fallible Initializer Function</th>
///       <td><a href="#method.alloc_try_with"><code>alloc_try_with</code></a></td>
///       <td><a href="#method.try_alloc_try_with"><code>try_alloc_try_with</code></a></td>
///     </tr>
///   <tbody>
///   </tbody>
/// </table>
///
/// ### Fallible Allocation: The `try_alloc_` Method Prefix
///
/// These allocation methods let you recover from out-of-memory (OOM)
/// scenarioes, rather than raising a panic on OOM.
///
/// ```
/// use bumpalo::Bump;
///
/// let bump = Bump::new();
///
/// match bump.try_alloc(MyStruct {
///     // ...
/// }) {
///     Ok(my_struct) => {
///         // Allocation succeeded.
///     }
///     Err(e) => {
///         // Out of memory.
///     }
/// }
///
/// struct MyStruct {
///     // ...
/// }
/// ```
///
/// ### Initializer Functions: The `_with` Method Suffix
///
/// Calling one of the generic `…alloc(x)` methods is essentially equivalent to
/// the matching [`…alloc_with(|| x)`](?search=alloc_with). However if you use
/// `…alloc_with`, then the closure will not be invoked until after allocating
/// space for storing `x` on the heap.
///
/// This can be useful in certain edge-cases related to compiler optimizations.
/// When evaluating for example `bump.alloc(x)`, semantically `x` is first put
/// on the stack and then moved onto the heap. In some cases, the compiler is
/// able to optimize this into constructing `x` directly on the heap, however
/// in many cases it does not.
///
/// The `…alloc_with` functions try to help the compiler be smarter. In most
/// cases doing for example `bump.try_alloc_with(|| x)` on release mode will be
/// enough to help the compiler realize that this optimization is valid and
/// to construct `x` directly onto the heap.
///
/// #### Warning
///
/// These functions critically depend on compiler optimizations to achieve their
/// desired effect. This means that it is not an effective tool when compiling
/// without optimizations on.
///
/// Even when optimizations are on, these functions do not **guarantee** that
/// the value is constructed on the heap. To the best of our knowledge no such
/// guarantee can be made in stable Rust as of 1.54.
///
/// ### Fallible Initialization: The `_try_with` Method Suffix
///
/// The generic [`…alloc_try_with(|| x)`](?search=_try_with) methods behave
/// like the purely `_with` suffixed methods explained above. However, they
/// allow for fallible initialization by accepting a closure that returns a
/// [`Result`] and will attempt to undo the initial allocation if this closure
/// returns [`Err`].
///
/// #### Warning
///
/// If the inner closure returns [`Ok`], space for the entire [`Result`] remains
/// allocated inside `self`. This can be a problem especially if the [`Err`]
/// variant is larger, but even otherwise there may be overhead for the
/// [`Result`]'s discriminant.
///
/// <p><details><summary>Undoing the allocation in the <code>Err</code> case
/// always fails if <code>f</code> successfully made any additional allocations
/// in <code>self</code>.</summary>
///
/// For example, the following will always leak also space for the [`Result`]
/// into this `Bump`, even though the inner reference isn't kept and the [`Err`]
/// payload is returned semantically by value:
///
/// ```rust
/// let bump = bumpalo::Bump::new();
///
/// let r: Result<&mut [u8; 1000], ()> = bump.alloc_try_with(|| {
///     let _ = bump.alloc(0_u8);
///     Err(())
/// });
///
/// assert!(r.is_err());
/// ```
///
///</details></p>
///
/// Since [`Err`] payloads are first placed on the heap and then moved to the
/// stack, `bump.…alloc_try_with(|| x)?` is likely to execute more slowly than
/// the matching `bump.…alloc(x?)` in case of initialization failure. If this
/// happens frequently, using the plain un-suffixed method may perform better.
///
/// [`Result`]: https://doc.rust-lang.org/std/result/enum.Result.html
/// [`Ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Ok
/// [`Err`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Err
///
/// ### `Bump` Allocation Limits
///
/// `bumpalo` supports setting a limit on the maximum bytes of memory that can
/// be allocated for use in a particular `Bump` arena. This limit can be set and removed with
/// [`set_allocation_limit`][Bump::set_allocation_limit].
/// The allocation limit is only enforced when allocating new backing chunks for
/// a `Bump`. Updating the allocation limit will not affect existing allocations
/// or any future allocations within the `Bump`'s current chunk.
///
/// #### Example
///
/// ```
/// let bump = bumpalo::Bump::new();
///
/// assert_eq!(bump.allocation_limit(), None);
/// bump.set_allocation_limit(Some(0));
///
/// assert!(bump.try_alloc(5).is_err());
///
/// bump.set_allocation_limit(Some(6));
///
/// assert_eq!(bump.allocation_limit(), Some(6));
///
/// bump.set_allocation_limit(None);
///
/// assert_eq!(bump.allocation_limit(), None);
/// ```
///
/// #### Warning
///
/// Because of backwards compatibility, allocations that fail
/// due to allocation limits will not present differently than
/// errors due to resource exhaustion.

#[derive(Debug)]
pub struct Bump {
    // The current chunk we are bump allocating within.
    current_chunk_footer: Cell<NonNull<ChunkFooter>>,
    allocation_limit: Cell<Option<usize>>,
}

#[repr(C)]
#[derive(Debug)]
struct ChunkFooter {
    // Pointer to the start of this chunk allocation. This footer is always at
    // the end of the chunk.
    data: NonNull<u8>,

    // The layout of this chunk's allocation.
    layout: Layout,

    // Link to the previous chunk.
    //
    // Note that the last node in the `prev` linked list is the canonical empty
    // chunk, whose `prev` link points to itself.
    prev: Cell<NonNull<ChunkFooter>>,

    // Bump allocation finger that is always in the range `self.data..=self`.
    ptr: Cell<NonNull<u8>>,

    // The bytes allocated in all chunks so far, the canonical empty chunk has
    // a size of 0 and for all other chunks, `allocated_bytes` will be
    // the allocated_bytes of the current chunk plus the allocated bytes
    // of the `prev` chunk.
    allocated_bytes: usize,
}

/// A wrapper type for the canonical, statically allocated empty chunk.
///
/// For the canonical empty chunk to be `static`, its type must be `Sync`, which
/// is the purpose of this wrapper type. This is safe because the empty chunk is
/// immutable and never actually modified.
#[repr(transparent)]
struct EmptyChunkFooter(ChunkFooter);

unsafe impl Sync for EmptyChunkFooter {}

static EMPTY_CHUNK: EmptyChunkFooter = EmptyChunkFooter(ChunkFooter {
    // This chunk is empty (except the foot itself).
    layout: Layout::new::<ChunkFooter>(),

    // The start of the (empty) allocatable region for this chunk is itself.
    data: unsafe { NonNull::new_unchecked(&EMPTY_CHUNK as *const EmptyChunkFooter as *mut u8) },

    // The end of the (empty) allocatable region for this chunk is also itself.
    ptr: Cell::new(unsafe {
        NonNull::new_unchecked(&EMPTY_CHUNK as *const EmptyChunkFooter as *mut u8)
    }),

    // Invariant: the last chunk footer in all `ChunkFooter::prev` linked lists
    // is the empty chunk footer, whose `prev` points to itself.
    prev: Cell::new(unsafe {
        NonNull::new_unchecked(&EMPTY_CHUNK as *const EmptyChunkFooter as *mut ChunkFooter)
    }),

    // Empty chunks count as 0 allocated bytes in an arena.
    allocated_bytes: 0,
});

impl EmptyChunkFooter {
    fn get(&'static self) -> NonNull<ChunkFooter> {
        unsafe { NonNull::new_unchecked(&self.0 as *const ChunkFooter as *mut ChunkFooter) }
    }
}

impl ChunkFooter {
    // Returns the start and length of the currently allocated region of this
    // chunk.
    fn as_raw_parts(&self) -> (*const u8, usize) {
        let data = self.data.as_ptr() as *const u8;
        let ptr = self.ptr.get().as_ptr() as *const u8;
        debug_assert!(data <= ptr);
        debug_assert!(ptr <= self as *const ChunkFooter as *const u8);
        let len = unsafe { (self as *const ChunkFooter as *const u8).offset_from(ptr) as usize };
        (ptr, len)
    }

    /// Is this chunk the last empty chunk?
    fn is_empty(&self) -> bool {
        ptr::eq(self, EMPTY_CHUNK.get().as_ptr())
    }
}

impl Default for Bump {
    fn default() -> Bump {
        Bump::new()
    }
}

impl Drop for Bump {
    fn drop(&mut self) {
        unsafe {
            dealloc_chunk_list(self.current_chunk_footer.get());
        }
    }
}

#[inline]
unsafe fn dealloc_chunk_list(mut footer: NonNull<ChunkFooter>) {
    while !footer.as_ref().is_empty() {
        let f = footer;
        footer = f.as_ref().prev.get();
        dealloc(f.as_ref().data.as_ptr(), f.as_ref().layout);
    }
}

// `Bump`s are safe to send between threads because nothing aliases its owned
// chunks until you start allocating from it. But by the time you allocate from
// it, the returned references to allocations borrow the `Bump` and therefore
// prevent sending the `Bump` across threads until the borrows end.
unsafe impl Send for Bump {}

#[inline]
pub(crate) fn round_up_to(n: usize, divisor: usize) -> Option<usize> {
    debug_assert!(divisor > 0);
    debug_assert!(divisor.is_power_of_two());
    Some(n.checked_add(divisor - 1)? & !(divisor - 1))
}

#[inline]
pub(crate) fn round_down_to(n: usize, divisor: usize) -> usize {
    debug_assert!(divisor > 0);
    debug_assert!(divisor.is_power_of_two());
    n & !(divisor - 1)
}

// After this point, we try to hit page boundaries instead of powers of 2
const PAGE_STRATEGY_CUTOFF: usize = 0x1000;

// We only support alignments of up to 16 bytes for iter_allocated_chunks.
const SUPPORTED_ITER_ALIGNMENT: usize = 16;
const CHUNK_ALIGN: usize = SUPPORTED_ITER_ALIGNMENT;
const FOOTER_SIZE: usize = mem::size_of::<ChunkFooter>();

// Assert that ChunkFooter is at most the supported alignment. This will give a compile time error if it is not the case
const _FOOTER_ALIGN_ASSERTION: bool = mem::align_of::<ChunkFooter>() <= CHUNK_ALIGN;
const _: [(); _FOOTER_ALIGN_ASSERTION as usize] = [()];

// Maximum typical overhead per allocation imposed by allocators.
const MALLOC_OVERHEAD: usize = 16;

// This is the overhead from malloc, footer and alignment. For instance, if
// we want to request a chunk of memory that has at least X bytes usable for
// allocations (where X is aligned to CHUNK_ALIGN), then we expect that the
// after adding a footer, malloc overhead and alignment, the chunk of memory
// the allocator actually sets aside for us is X+OVERHEAD rounded up to the
// nearest suitable size boundary.
const OVERHEAD: usize = (MALLOC_OVERHEAD + FOOTER_SIZE + (CHUNK_ALIGN - 1)) & !(CHUNK_ALIGN - 1);

// Choose a relatively small default initial chunk size, since we double chunk
// sizes as we grow bump arenas to amortize costs of hitting the global
// allocator.
const FIRST_ALLOCATION_GOAL: usize = 1 << 9;

// The actual size of the first allocation is going to be a bit smaller
// than the goal. We need to make room for the footer, and we also need
// take the alignment into account.
const DEFAULT_CHUNK_SIZE_WITHOUT_FOOTER: usize = FIRST_ALLOCATION_GOAL - OVERHEAD;

/// The memory size and alignment details for a potential new chunk
/// allocation.
#[derive(Debug, Clone, Copy)]
struct NewChunkMemoryDetails {
    new_size_without_footer: usize,
    align: usize,
    size: usize,
}

/// Wrapper around `Layout::from_size_align` that adds debug assertions.
#[inline]
unsafe fn layout_from_size_align(size: usize, align: usize) -> Layout {
    if cfg!(debug_assertions) {
        Layout::from_size_align(size, align).unwrap()
    } else {
        Layout::from_size_align_unchecked(size, align)
    }
}

#[inline(never)]
fn allocation_size_overflow<T>() -> T {
    panic!("requested allocation size overflowed")
}

// This can be migrated to directly use `usize::abs_diff` when the MSRV
// reaches `1.60`
fn abs_diff(a: usize, b: usize) -> usize {
    usize::max(a, b) - usize::min(a, b)
}

impl Bump {
    /// Construct a new arena to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// # let _ = bump;
    /// ```
    pub fn new() -> Bump {
        Self::with_capacity(0)
    }

    /// Attempt to construct a new arena to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::try_new();
    /// # let _ = bump.unwrap();
    /// ```
    pub fn try_new() -> Result<Bump, AllocErr> {
        Bump::try_with_capacity(0)
    }

    /// Construct a new arena with the specified byte capacity to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::with_capacity(100);
    /// # let _ = bump;
    /// ```
    pub fn with_capacity(capacity: usize) -> Bump {
        Bump::try_with_capacity(capacity).unwrap_or_else(|_| oom())
    }

    /// Attempt to construct a new arena with the specified byte capacity to bump allocate into.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::try_with_capacity(100);
    /// # let _ = bump.unwrap();
    /// ```
    pub fn try_with_capacity(capacity: usize) -> Result<Self, AllocErr> {
        if capacity == 0 {
            return Ok(Bump {
                current_chunk_footer: Cell::new(EMPTY_CHUNK.get()),
                allocation_limit: Cell::new(None),
            });
        }

        let layout = unsafe { layout_from_size_align(capacity, 1) };

        let chunk_footer = unsafe {
            Self::new_chunk(
                Bump::new_chunk_memory_details(None, layout).ok_or(AllocErr)?,
                layout,
                EMPTY_CHUNK.get(),
            )
            .ok_or(AllocErr)?
        };

        Ok(Bump {
            current_chunk_footer: Cell::new(chunk_footer),
            allocation_limit: Cell::new(None),
        })
    }

    /// The allocation limit for this arena in bytes.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::with_capacity(0);
    ///
    /// assert_eq!(bump.allocation_limit(), None);
    ///
    /// bump.set_allocation_limit(Some(6));
    ///
    /// assert_eq!(bump.allocation_limit(), Some(6));
    ///
    /// bump.set_allocation_limit(None);
    ///
    /// assert_eq!(bump.allocation_limit(), None);
    /// ```
    pub fn allocation_limit(&self) -> Option<usize> {
        self.allocation_limit.get()
    }

    /// Set the allocation limit in bytes for this arena.
    ///
    /// The allocation limit is only enforced when allocating new backing chunks for
    /// a `Bump`. Updating the allocation limit will not affect existing allocations
    /// or any future allocations within the `Bump`'s current chunk.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::with_capacity(0);
    ///
    /// bump.set_allocation_limit(Some(0));
    ///
    /// assert!(bump.try_alloc(5).is_err());
    /// ```
    pub fn set_allocation_limit(&self, limit: Option<usize>) {
        self.allocation_limit.set(limit)
    }

    /// How much headroom an arena has before it hits its allocation
    /// limit.
    fn allocation_limit_remaining(&self) -> Option<usize> {
        self.allocation_limit.get().and_then(|allocation_limit| {
            let allocated_bytes = self.allocated_bytes();
            if allocated_bytes > allocation_limit {
                None
            } else {
                Some(abs_diff(allocation_limit, allocated_bytes))
            }
        })
    }

    /// Whether a request to allocate a new chunk with a given size for a given
    /// requested layout will fit under the allocation limit set on a `Bump`.
    fn chunk_fits_under_limit(
        allocation_limit_remaining: Option<usize>,
        new_chunk_memory_details: NewChunkMemoryDetails,
    ) -> bool {
        allocation_limit_remaining
            .map(|allocation_limit_left| {
                allocation_limit_left >= new_chunk_memory_details.new_size_without_footer
            })
            .unwrap_or(true)
    }

    /// Determine the memory details including final size, alignment and
    /// final size without footer for a new chunk that would be allocated
    /// to fulfill an allocation request.
    fn new_chunk_memory_details(
        new_size_without_footer: Option<usize>,
        requested_layout: Layout,
    ) -> Option<NewChunkMemoryDetails> {
        let mut new_size_without_footer =
            new_size_without_footer.unwrap_or(DEFAULT_CHUNK_SIZE_WITHOUT_FOOTER);

        // We want to have CHUNK_ALIGN or better alignment
        let mut align = CHUNK_ALIGN;

        // If we already know we need to fulfill some request,
        // make sure we allocate at least enough to satisfy it
        align = align.max(requested_layout.align());
        let requested_size =
            round_up_to(requested_layout.size(), align).unwrap_or_else(allocation_size_overflow);
        new_size_without_footer = new_size_without_footer.max(requested_size);

        // We want our allocations to play nice with the memory allocator,
        // and waste as little memory as possible.
        // For small allocations, this means that the entire allocation
        // including the chunk footer and mallocs internal overhead is
        // as close to a power of two as we can go without going over.
        // For larger allocations, we only need to get close to a page
        // boundary without going over.
        if new_size_without_footer < PAGE_STRATEGY_CUTOFF {
            new_size_without_footer =
                (new_size_without_footer + OVERHEAD).next_power_of_two() - OVERHEAD;
        } else {
            new_size_without_footer =
                round_up_to(new_size_without_footer + OVERHEAD, 0x1000)? - OVERHEAD;
        }

        debug_assert_eq!(align % CHUNK_ALIGN, 0);
        debug_assert_eq!(new_size_without_footer % CHUNK_ALIGN, 0);
        let size = new_size_without_footer
            .checked_add(FOOTER_SIZE)
            .unwrap_or_else(allocation_size_overflow);

        Some(NewChunkMemoryDetails {
            new_size_without_footer,
            size,
            align,
        })
    }

    /// Allocate a new chunk and return its initialized footer.
    ///
    /// If given, `layouts` is a tuple of the current chunk size and the
    /// layout of the allocation request that triggered us to fall back to
    /// allocating a new chunk of memory.
    unsafe fn new_chunk(
        new_chunk_memory_details: NewChunkMemoryDetails,
        requested_layout: Layout,
        prev: NonNull<ChunkFooter>,
    ) -> Option<NonNull<ChunkFooter>> {
        let NewChunkMemoryDetails {
            new_size_without_footer,
            align,
            size,
        } = new_chunk_memory_details;

        let layout = layout_from_size_align(size, align);

        debug_assert!(size >= requested_layout.size());

        let data = alloc(layout);
        let data = NonNull::new(data)?;

        // The `ChunkFooter` is at the end of the chunk.
        let footer_ptr = data.as_ptr().add(new_size_without_footer);
        debug_assert_eq!((data.as_ptr() as usize) % align, 0);
        debug_assert_eq!(footer_ptr as usize % CHUNK_ALIGN, 0);
        let footer_ptr = footer_ptr as *mut ChunkFooter;

        // The bump pointer is initialized to the end of the range we will
        // bump out of.
        let ptr = Cell::new(NonNull::new_unchecked(footer_ptr as *mut u8));

        // The `allocated_bytes` of a new chunk counts the total size
        // of the chunks, not how much of the chunks are used.
        let allocated_bytes = prev.as_ref().allocated_bytes + new_size_without_footer;

        ptr::write(
            footer_ptr,
            ChunkFooter {
                data,
                layout,
                prev: Cell::new(prev),
                ptr,
                allocated_bytes,
            },
        );

        Some(NonNull::new_unchecked(footer_ptr))
    }

    /// Reset this bump allocator.
    ///
    /// Performs mass deallocation on everything allocated in this arena by
    /// resetting the pointer into the underlying chunk of memory to the start
    /// of the chunk. Does not run any `Drop` implementations on deallocated
    /// objects; see [the top-level documentation](struct.Bump.html) for details.
    ///
    /// If this arena has allocated multiple chunks to bump allocate into, then
    /// the excess chunks are returned to the global allocator.
    ///
    /// ## Example
    ///
    /// ```
    /// let mut bump = bumpalo::Bump::new();
    ///
    /// // Allocate a bunch of things.
    /// {
    ///     for i in 0..100 {
    ///         bump.alloc(i);
    ///     }
    /// }
    ///
    /// // Reset the arena.
    /// bump.reset();
    ///
    /// // Allocate some new things in the space previously occupied by the
    /// // original things.
    /// for j in 200..400 {
    ///     bump.alloc(j);
    /// }
    ///```
    pub fn reset(&mut self) {
        // Takes `&mut self` so `self` must be unique and there can't be any
        // borrows active that would get invalidated by resetting.
        unsafe {
            if self.current_chunk_footer.get().as_ref().is_empty() {
                return;
            }

            let mut cur_chunk = self.current_chunk_footer.get();

            // Deallocate all chunks except the current one
            let prev_chunk = cur_chunk.as_ref().prev.replace(EMPTY_CHUNK.get());
            dealloc_chunk_list(prev_chunk);

            // Reset the bump finger to the end of the chunk.
            cur_chunk.as_ref().ptr.set(cur_chunk.cast());

            // Reset the allocated size of the chunk.
            cur_chunk.as_mut().allocated_bytes = cur_chunk.as_ref().layout.size();

            debug_assert!(
                self.current_chunk_footer
                    .get()
                    .as_ref()
                    .prev
                    .get()
                    .as_ref()
                    .is_empty(),
                "We should only have a single chunk"
            );
            debug_assert_eq!(
                self.current_chunk_footer.get().as_ref().ptr.get(),
                self.current_chunk_footer.get().cast(),
                "Our chunk's bump finger should be reset to the start of its allocation"
            );
        }
    }

    /// Allocate an object in this `Bump` and return an exclusive reference to
    /// it.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc("hello");
    /// assert_eq!(*x, "hello");
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc<T>(&self, val: T) -> &mut T {
        self.alloc_with(|| val)
    }

    /// Try to allocate an object in this `Bump` and return an exclusive
    /// reference to it.
    ///
    /// ## Errors
    ///
    /// Errors if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.try_alloc("hello");
    /// assert_eq!(x, Ok(&mut "hello"));
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn try_alloc<T>(&self, val: T) -> Result<&mut T, AllocErr> {
        self.try_alloc_with(|| val)
    }

    /// Pre-allocate space for an object in this `Bump`, initializes it using
    /// the closure, then returns an exclusive reference to it.
    ///
    /// See [The `_with` Method Suffix](#initializer-functions-the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_with(|| "hello");
    /// assert_eq!(*x, "hello");
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_with<F, T>(&self, f: F) -> &mut T
    where
        F: FnOnce() -> T,
    {
        #[inline(always)]
        unsafe fn inner_writer<T, F>(ptr: *mut T, f: F)
        where
            F: FnOnce() -> T,
        {
            // This function is translated as:
            // - allocate space for a T on the stack
            // - call f() with the return value being put onto this stack space
            // - memcpy from the stack to the heap
            //
            // Ideally we want LLVM to always realize that doing a stack
            // allocation is unnecessary and optimize the code so it writes
            // directly into the heap instead. It seems we get it to realize
            // this most consistently if we put this critical line into it's
            // own function instead of inlining it into the surrounding code.
            ptr::write(ptr, f())
        }

        let layout = Layout::new::<T>();

        unsafe {
            let p = self.alloc_layout(layout);
            let p = p.as_ptr() as *mut T;
            inner_writer(p, f);
            &mut *p
        }
    }

    /// Tries to pre-allocate space for an object in this `Bump`, initializes
    /// it using the closure, then returns an exclusive reference to it.
    ///
    /// See [The `_with` Method Suffix](#initializer-functions-the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// ## Errors
    ///
    /// Errors if reserving space for `T` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.try_alloc_with(|| "hello");
    /// assert_eq!(x, Ok(&mut "hello"));
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn try_alloc_with<F, T>(&self, f: F) -> Result<&mut T, AllocErr>
    where
        F: FnOnce() -> T,
    {
        #[inline(always)]
        unsafe fn inner_writer<T, F>(ptr: *mut T, f: F)
        where
            F: FnOnce() -> T,
        {
            // This function is translated as:
            // - allocate space for a T on the stack
            // - call f() with the return value being put onto this stack space
            // - memcpy from the stack to the heap
            //
            // Ideally we want LLVM to always realize that doing a stack
            // allocation is unnecessary and optimize the code so it writes
            // directly into the heap instead. It seems we get it to realize
            // this most consistently if we put this critical line into it's
            // own function instead of inlining it into the surrounding code.
            ptr::write(ptr, f())
        }

        //SAFETY: Self-contained:
        // `p` is allocated for `T` and then a `T` is written.
        let layout = Layout::new::<T>();
        let p = self.try_alloc_layout(layout)?;
        let p = p.as_ptr() as *mut T;

        unsafe {
            inner_writer(p, f);
            Ok(&mut *p)
        }
    }

    /// Pre-allocates space for a [`Result`] in this `Bump`, initializes it using
    /// the closure, then returns an exclusive reference to its `T` if [`Ok`].
    ///
    /// Iff the allocation fails, the closure is not run.
    ///
    /// Iff [`Err`], an allocator rewind is *attempted* and the `E` instance is
    /// moved out of the allocator to be consumed or dropped as normal.
    ///
    /// See [The `_with` Method Suffix](#initializer-functions-the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// For caveats specific to fallible initialization, see
    /// [The `_try_with` Method Suffix](#fallible-initialization-the-_try_with-method-suffix).
    ///
    /// [`Result`]: https://doc.rust-lang.org/std/result/enum.Result.html
    /// [`Ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Ok
    /// [`Err`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Err
    ///
    /// ## Errors
    ///
    /// Iff the allocation succeeds but `f` fails, that error is forwarded by value.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for `Result<T, E>` fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_try_with(|| Ok("hello"))?;
    /// assert_eq!(*x, "hello");
    /// # Result::<_, ()>::Ok(())
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_try_with<F, T, E>(&self, f: F) -> Result<&mut T, E>
    where
        F: FnOnce() -> Result<T, E>,
    {
        let rewind_footer = self.current_chunk_footer.get();
        let rewind_ptr = unsafe { rewind_footer.as_ref() }.ptr.get();
        let mut inner_result_ptr = NonNull::from(self.alloc_with(f));
        match unsafe { inner_result_ptr.as_mut() } {
            Ok(t) => Ok(unsafe {
                //SAFETY:
                // The `&mut Result<T, E>` returned by `alloc_with` may be
                // lifetime-limited by `E`, but the derived `&mut T` still has
                // the same validity as in `alloc_with` since the error variant
                // is already ruled out here.

                // We could conditionally truncate the allocation here, but
                // since it grows backwards, it seems unlikely that we'd get
                // any more than the `Result`'s discriminant this way, if
                // anything at all.
                &mut *(t as *mut _)
            }),
            Err(e) => unsafe {
                // If this result was the last allocation in this arena, we can
                // reclaim its space. In fact, sometimes we can do even better
                // than simply calling `dealloc` on the result pointer: we can
                // reclaim any alignment padding we might have added (which
                // `dealloc` cannot do) if we didn't allocate a new chunk for
                // this result.
                if self.is_last_allocation(inner_result_ptr.cast()) {
                    let current_footer_p = self.current_chunk_footer.get();
                    let current_ptr = &current_footer_p.as_ref().ptr;
                    if current_footer_p == rewind_footer {
                        // It's still the same chunk, so reset the bump pointer
                        // to its original value upon entry to this method
                        // (reclaiming any alignment padding we may have
                        // added).
                        current_ptr.set(rewind_ptr);
                    } else {
                        // We allocated a new chunk for this result.
                        //
                        // We know the result is the only allocation in this
                        // chunk: Any additional allocations since the start of
                        // this method could only have happened when running
                        // the initializer function, which is called *after*
                        // reserving space for this result. Therefore, since we
                        // already determined via the check above that this
                        // result was the last allocation, there must not have
                        // been any other allocations, and this result is the
                        // only allocation in this chunk.
                        //
                        // Because this is the only allocation in this chunk,
                        // we can reset the chunk's bump finger to the start of
                        // the chunk.
                        current_ptr.set(current_footer_p.as_ref().data);
                    }
                }
                //SAFETY:
                // As we received `E` semantically by value from `f`, we can
                // just copy that value here as long as we avoid a double-drop
                // (which can't happen as any specific references to the `E`'s
                // data in `self` are destroyed when this function returns).
                //
                // The order between this and the deallocation doesn't matter
                // because `Self: !Sync`.
                Err(ptr::read(e as *const _))
            },
        }
    }

    /// Tries to pre-allocates space for a [`Result`] in this `Bump`,
    /// initializes it using the closure, then returns an exclusive reference
    /// to its `T` if all [`Ok`].
    ///
    /// Iff the allocation fails, the closure is not run.
    ///
    /// Iff the closure returns [`Err`], an allocator rewind is *attempted* and
    /// the `E` instance is moved out of the allocator to be consumed or dropped
    /// as normal.
    ///
    /// See [The `_with` Method Suffix](#initializer-functions-the-_with-method-suffix) for a
    /// discussion on the differences between the `_with` suffixed methods and
    /// those methods without it, their performance characteristics, and when
    /// you might or might not choose a `_with` suffixed method.
    ///
    /// For caveats specific to fallible initialization, see
    /// [The `_try_with` Method Suffix](#fallible-initialization-the-_try_with-method-suffix).
    ///
    /// [`Result`]: https://doc.rust-lang.org/std/result/enum.Result.html
    /// [`Ok`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Ok
    /// [`Err`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Err
    ///
    /// ## Errors
    ///
    /// Errors with the [`Alloc`](`AllocOrInitError::Alloc`) variant iff
    /// reserving space for `Result<T, E>` fails.
    ///
    /// Iff the allocation succeeds but `f` fails, that error is forwarded by
    /// value inside the [`Init`](`AllocOrInitError::Init`) variant.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.try_alloc_try_with(|| Ok("hello"))?;
    /// assert_eq!(*x, "hello");
    /// # Result::<_, bumpalo::AllocOrInitError<()>>::Ok(())
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn try_alloc_try_with<F, T, E>(&self, f: F) -> Result<&mut T, AllocOrInitError<E>>
    where
        F: FnOnce() -> Result<T, E>,
    {
        let rewind_footer = self.current_chunk_footer.get();
        let rewind_ptr = unsafe { rewind_footer.as_ref() }.ptr.get();
        let mut inner_result_ptr = NonNull::from(self.try_alloc_with(f)?);
        match unsafe { inner_result_ptr.as_mut() } {
            Ok(t) => Ok(unsafe {
                //SAFETY:
                // The `&mut Result<T, E>` returned by `alloc_with` may be
                // lifetime-limited by `E`, but the derived `&mut T` still has
                // the same validity as in `alloc_with` since the error variant
                // is already ruled out here.

                // We could conditionally truncate the allocation here, but
                // since it grows backwards, it seems unlikely that we'd get
                // any more than the `Result`'s discriminant this way, if
                // anything at all.
                &mut *(t as *mut _)
            }),
            Err(e) => unsafe {
                // If this result was the last allocation in this arena, we can
                // reclaim its space. In fact, sometimes we can do even better
                // than simply calling `dealloc` on the result pointer: we can
                // reclaim any alignment padding we might have added (which
                // `dealloc` cannot do) if we didn't allocate a new chunk for
                // this result.
                if self.is_last_allocation(inner_result_ptr.cast()) {
                    let current_footer_p = self.current_chunk_footer.get();
                    let current_ptr = &current_footer_p.as_ref().ptr;
                    if current_footer_p == rewind_footer {
                        // It's still the same chunk, so reset the bump pointer
                        // to its original value upon entry to this method
                        // (reclaiming any alignment padding we may have
                        // added).
                        current_ptr.set(rewind_ptr);
                    } else {
                        // We allocated a new chunk for this result.
                        //
                        // We know the result is the only allocation in this
                        // chunk: Any additional allocations since the start of
                        // this method could only have happened when running
                        // the initializer function, which is called *after*
                        // reserving space for this result. Therefore, since we
                        // already determined via the check above that this
                        // result was the last allocation, there must not have
                        // been any other allocations, and this result is the
                        // only allocation in this chunk.
                        //
                        // Because this is the only allocation in this chunk,
                        // we can reset the chunk's bump finger to the start of
                        // the chunk.
                        current_ptr.set(current_footer_p.as_ref().data);
                    }
                }
                //SAFETY:
                // As we received `E` semantically by value from `f`, we can
                // just copy that value here as long as we avoid a double-drop
                // (which can't happen as any specific references to the `E`'s
                // data in `self` are destroyed when this function returns).
                //
                // The order between this and the deallocation doesn't matter
                // because `Self: !Sync`.
                Err(AllocOrInitError::Init(ptr::read(e as *const _)))
            },
        }
    }

    /// `Copy` a slice into this `Bump` and return an exclusive reference to
    /// the copy.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_copy(&[1, 2, 3]);
    /// assert_eq!(x, &[1, 2, 3]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_copy<T>(&self, src: &[T]) -> &mut [T]
    where
        T: Copy,
    {
        let layout = Layout::for_value(src);
        let dst = self.alloc_layout(layout).cast::<T>();

        unsafe {
            ptr::copy_nonoverlapping(src.as_ptr(), dst.as_ptr(), src.len());
            slice::from_raw_parts_mut(dst.as_ptr(), src.len())
        }
    }

    /// `Clone` a slice into this `Bump` and return an exclusive reference to
    /// the clone. Prefer [`alloc_slice_copy`](#method.alloc_slice_copy) if `T` is `Copy`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// #[derive(Clone, Debug, Eq, PartialEq)]
    /// struct Sheep {
    ///     name: String,
    /// }
    ///
    /// let originals = [
    ///     Sheep { name: "Alice".into() },
    ///     Sheep { name: "Bob".into() },
    ///     Sheep { name: "Cathy".into() },
    /// ];
    ///
    /// let bump = bumpalo::Bump::new();
    /// let clones = bump.alloc_slice_clone(&originals);
    /// assert_eq!(originals, clones);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_clone<T>(&self, src: &[T]) -> &mut [T]
    where
        T: Clone,
    {
        let layout = Layout::for_value(src);
        let dst = self.alloc_layout(layout).cast::<T>();

        unsafe {
            for (i, val) in src.iter().cloned().enumerate() {
                ptr::write(dst.as_ptr().add(i), val);
            }

            slice::from_raw_parts_mut(dst.as_ptr(), src.len())
        }
    }

    /// `Copy` a string slice into this `Bump` and return an exclusive reference to it.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the string fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let hello = bump.alloc_str("hello world");
    /// assert_eq!("hello world", hello);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_str(&self, src: &str) -> &mut str {
        let buffer = self.alloc_slice_copy(src.as_bytes());
        unsafe {
            // This is OK, because it already came in as str, so it is guaranteed to be utf8
            str::from_utf8_unchecked_mut(buffer)
        }
    }

    /// Allocates a new slice of size `len` into this `Bump` and returns an
    /// exclusive reference to the copy.
    ///
    /// The elements of the slice are initialized using the supplied closure.
    /// The closure argument is the position in the slice.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_fill_with(5, |i| 5 * (i + 1));
    /// assert_eq!(x, &[5, 10, 15, 20, 25]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_with<T, F>(&self, len: usize, mut f: F) -> &mut [T]
    where
        F: FnMut(usize) -> T,
    {
        let layout = Layout::array::<T>(len).unwrap_or_else(|_| oom());
        let dst = self.alloc_layout(layout).cast::<T>();

        unsafe {
            for i in 0..len {
                ptr::write(dst.as_ptr().add(i), f(i));
            }

            let result = slice::from_raw_parts_mut(dst.as_ptr(), len);
            debug_assert_eq!(Layout::for_value(result), layout);
            result
        }
    }

    /// Allocates a new slice of size `len` into this `Bump` and returns an
    /// exclusive reference to the copy.
    ///
    /// All elements of the slice are initialized to `value`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_fill_copy(5, 42);
    /// assert_eq!(x, &[42, 42, 42, 42, 42]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_copy<T: Copy>(&self, len: usize, value: T) -> &mut [T] {
        self.alloc_slice_fill_with(len, |_| value)
    }

    /// Allocates a new slice of size `len` slice into this `Bump` and return an
    /// exclusive reference to the copy.
    ///
    /// All elements of the slice are initialized to `value.clone()`.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let s: String = "Hello Bump!".to_string();
    /// let x: &[String] = bump.alloc_slice_fill_clone(2, &s);
    /// assert_eq!(x.len(), 2);
    /// assert_eq!(&x[0], &s);
    /// assert_eq!(&x[1], &s);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_clone<T: Clone>(&self, len: usize, value: &T) -> &mut [T] {
        self.alloc_slice_fill_with(len, |_| value.clone())
    }

    /// Allocates a new slice of size `len` slice into this `Bump` and return an
    /// exclusive reference to the copy.
    ///
    /// The elements are initialized using the supplied iterator.
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails, or if the supplied
    /// iterator returns fewer elements than it promised.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x: &[i32] = bump.alloc_slice_fill_iter([2, 3, 5].iter().cloned().map(|i| i * i));
    /// assert_eq!(x, [4, 9, 25]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_iter<T, I>(&self, iter: I) -> &mut [T]
    where
        I: IntoIterator<Item = T>,
        I::IntoIter: ExactSizeIterator,
    {
        let mut iter = iter.into_iter();
        self.alloc_slice_fill_with(iter.len(), |_| {
            iter.next().expect("Iterator supplied too few elements")
        })
    }

    /// Allocates a new slice of size `len` slice into this `Bump` and return an
    /// exclusive reference to the copy.
    ///
    /// All elements of the slice are initialized to [`T::default()`].
    ///
    /// [`T::default()`]: https://doc.rust-lang.org/std/default/trait.Default.html#tymethod.default
    ///
    /// ## Panics
    ///
    /// Panics if reserving space for the slice fails.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let x = bump.alloc_slice_fill_default::<u32>(5);
    /// assert_eq!(x, &[0, 0, 0, 0, 0]);
    /// ```
    #[inline(always)]
    #[allow(clippy::mut_from_ref)]
    pub fn alloc_slice_fill_default<T: Default>(&self, len: usize) -> &mut [T] {
        self.alloc_slice_fill_with(len, |_| T::default())
    }

    /// Allocate space for an object with the given `Layout`.
    ///
    /// The returned pointer points at uninitialized memory, and should be
    /// initialized with
    /// [`std::ptr::write`](https://doc.rust-lang.org/std/ptr/fn.write.html).
    ///
    /// # Panics
    ///
    /// Panics if reserving space matching `layout` fails.
    #[inline(always)]
    pub fn alloc_layout(&self, layout: Layout) -> NonNull<u8> {
        self.try_alloc_layout(layout).unwrap_or_else(|_| oom())
    }

    /// Attempts to allocate space for an object with the given `Layout` or else returns
    /// an `Err`.
    ///
    /// The returned pointer points at uninitialized memory, and should be
    /// initialized with
    /// [`std::ptr::write`](https://doc.rust-lang.org/std/ptr/fn.write.html).
    ///
    /// # Errors
    ///
    /// Errors if reserving space matching `layout` fails.
    #[inline(always)]
    pub fn try_alloc_layout(&self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
        if let Some(p) = self.try_alloc_layout_fast(layout) {
            Ok(p)
        } else {
            self.alloc_layout_slow(layout).ok_or(AllocErr)
        }
    }

    #[inline(always)]
    fn try_alloc_layout_fast(&self, layout: Layout) -> Option<NonNull<u8>> {
        // We don't need to check for ZSTs here since they will automatically
        // be handled properly: the pointer will be bumped by zero bytes,
        // modulo alignment. This keeps the fast path optimized for non-ZSTs,
        // which are much more common.
        unsafe {
            let footer = self.current_chunk_footer.get();
            let footer = footer.as_ref();
            let ptr = footer.ptr.get().as_ptr();
            let start = footer.data.as_ptr();
            debug_assert!(start <= ptr);
            debug_assert!(ptr as *const u8 <= footer as *const _ as *const u8);

            if (ptr as usize) < layout.size() {
                return None;
            }

            let ptr = ptr.wrapping_sub(layout.size());
            let rem = ptr as usize % layout.align();
            let aligned_ptr = ptr.wrapping_sub(rem);

            if aligned_ptr >= start {
                let aligned_ptr = NonNull::new_unchecked(aligned_ptr as *mut u8);
                footer.ptr.set(aligned_ptr);
                Some(aligned_ptr)
            } else {
                None
            }
        }
    }

    /// Gets the remaining capacity in the current chunk (in bytes).
    ///
    /// ## Example
    ///
    /// ```
    /// use bumpalo::Bump;
    ///
    /// let bump = Bump::with_capacity(100);
    ///
    /// let capacity = bump.chunk_capacity();
    /// assert!(capacity >= 100);
    /// ```
    pub fn chunk_capacity(&self) -> usize {
        let current_footer = self.current_chunk_footer.get();
        let current_footer = unsafe { current_footer.as_ref() };

        current_footer as *const _ as usize - current_footer.data.as_ptr() as usize
    }

    /// Slow path allocation for when we need to allocate a new chunk from the
    /// parent bump set because there isn't enough room in our current chunk.
    #[inline(never)]
    fn alloc_layout_slow(&self, layout: Layout) -> Option<NonNull<u8>> {
        unsafe {
            let size = layout.size();
            let allocation_limit_remaining = self.allocation_limit_remaining();

            // Get a new chunk from the global allocator.
            let current_footer = self.current_chunk_footer.get();
            let current_layout = current_footer.as_ref().layout;

            // By default, we want our new chunk to be about twice as big
            // as the previous chunk. If the global allocator refuses it,
            // we try to divide it by half until it works or the requested
            // size is smaller than the default footer size.
            let min_new_chunk_size = layout.size().max(DEFAULT_CHUNK_SIZE_WITHOUT_FOOTER);
            let mut base_size = (current_layout.size() - FOOTER_SIZE)
                .checked_mul(2)?
                .max(min_new_chunk_size);
            let chunk_memory_details = iter::from_fn(|| {
                let bypass_min_chunk_size_for_small_limits = match self.allocation_limit() {
                    Some(limit)
                        if layout.size() < limit
                            && base_size >= layout.size()
                            && limit < DEFAULT_CHUNK_SIZE_WITHOUT_FOOTER
                            && self.allocated_bytes() == 0 =>
                    {
                        true
                    }
                    _ => false,
                };

                if base_size >= min_new_chunk_size || bypass_min_chunk_size_for_small_limits {
                    let size = base_size;
                    base_size = base_size / 2;
                    Bump::new_chunk_memory_details(Some(size), layout)
                } else {
                    None
                }
            });

            let new_footer = chunk_memory_details
                .filter_map(|chunk_memory_details| {
                    if Bump::chunk_fits_under_limit(
                        allocation_limit_remaining,
                        chunk_memory_details,
                    ) {
                        Bump::new_chunk(chunk_memory_details, layout, current_footer)
                    } else {
                        None
                    }
                })
                .next()?;

            debug_assert_eq!(
                new_footer.as_ref().data.as_ptr() as usize % layout.align(),
                0
            );

            // Set the new chunk as our new current chunk.
            self.current_chunk_footer.set(new_footer);

            let new_footer = new_footer.as_ref();

            // Move the bump ptr finger down to allocate room for `val`. We know
            // this can't overflow because we successfully allocated a chunk of
            // at least the requested size.
            let mut ptr = new_footer.ptr.get().as_ptr().sub(size);
            // Round the pointer down to the requested alignment.
            ptr = ptr.sub(ptr as usize % layout.align());
            debug_assert!(
                ptr as *const _ <= new_footer,
                "{:p} <= {:p}",
                ptr,
                new_footer
            );
            let ptr = NonNull::new_unchecked(ptr as *mut u8);
            new_footer.ptr.set(ptr);

            // Return a pointer to the freshly allocated region in this chunk.
            Some(ptr)
        }
    }

    /// Returns an iterator over each chunk of allocated memory that
    /// this arena has bump allocated into.
    ///
    /// The chunks are returned ordered by allocation time, with the most
    /// recently allocated chunk being returned first, and the least recently
    /// allocated chunk being returned last.
    ///
    /// The values inside each chunk are also ordered by allocation time, with
    /// the most recent allocation being earlier in the slice, and the least
    /// recent allocation being towards the end of the slice.
    ///
    /// ## Safety
    ///
    /// Because this method takes `&mut self`, we know that the bump arena
    /// reference is unique and therefore there aren't any active references to
    /// any of the objects we've allocated in it either. This potential aliasing
    /// of exclusive references is one common footgun for unsafe code that we
    /// don't need to worry about here.
    ///
    /// However, there could be regions of uninitialized memory used as padding
    /// between allocations, which is why this iterator has items of type
    /// `[MaybeUninit<u8>]`, instead of simply `[u8]`.
    ///
    /// The only way to guarantee that there is no padding between allocations
    /// or within allocated objects is if all of these properties hold:
    ///
    /// 1. Every object allocated in this arena has the same alignment,
    ///    and that alignment is at most 16.
    /// 2. Every object's size is a multiple of its alignment.
    /// 3. None of the objects allocated in this arena contain any internal
    ///    padding.
    ///
    /// If you want to use this `iter_allocated_chunks` method, it is *your*
    /// responsibility to ensure that these properties hold before calling
    /// `MaybeUninit::assume_init` or otherwise reading the returned values.
    ///
    /// Finally, you must also ensure that any values allocated into the bump
    /// arena have not had their `Drop` implementations called on them,
    /// e.g. after dropping a [`bumpalo::boxed::Box<T>`][crate::boxed::Box].
    ///
    /// ## Example
    ///
    /// ```
    /// let mut bump = bumpalo::Bump::new();
    ///
    /// // Allocate a bunch of `i32`s in this bump arena, potentially causing
    /// // additional memory chunks to be reserved.
    /// for i in 0..10000 {
    ///     bump.alloc(i);
    /// }
    ///
    /// // Iterate over each chunk we've bump allocated into. This is safe
    /// // because we have only allocated `i32`s in this arena, which fulfills
    /// // the above requirements.
    /// for ch in bump.iter_allocated_chunks() {
    ///     println!("Used a chunk that is {} bytes long", ch.len());
    ///     println!("The first byte is {:?}", unsafe {
    ///         ch[0].assume_init()
    ///     });
    /// }
    ///
    /// // Within a chunk, allocations are ordered from most recent to least
    /// // recent. If we allocated 'a', then 'b', then 'c', when we iterate
    /// // through the chunk's data, we get them in the order 'c', then 'b',
    /// // then 'a'.
    ///
    /// bump.reset();
    /// bump.alloc(b'a');
    /// bump.alloc(b'b');
    /// bump.alloc(b'c');
    ///
    /// assert_eq!(bump.iter_allocated_chunks().count(), 1);
    /// let chunk = bump.iter_allocated_chunks().nth(0).unwrap();
    /// assert_eq!(chunk.len(), 3);
    ///
    /// // Safe because we've only allocated `u8`s in this arena, which
    /// // fulfills the above requirements.
    /// unsafe {
    ///     assert_eq!(chunk[0].assume_init(), b'c');
    ///     assert_eq!(chunk[1].assume_init(), b'b');
    ///     assert_eq!(chunk[2].assume_init(), b'a');
    /// }
    /// ```
    pub fn iter_allocated_chunks(&mut self) -> ChunkIter<'_> {
        // SAFE: Ensured by mutable borrow of `self`.
        let raw = unsafe { self.iter_allocated_chunks_raw() };
        ChunkIter {
            raw,
            bump: PhantomData,
        }
    }

    /// Returns an iterator over raw pointers to chunks of allocated memory that
    /// this arena has bump allocated into.
    ///
    /// This is an unsafe version of [`iter_allocated_chunks()`](Bump::iter_allocated_chunks),
    /// with the caller responsible for safe usage of the returned pointers as
    /// well as ensuring that the iterator is not invalidated by new
    /// allocations.
    ///
    /// ## Safety
    ///
    /// Allocations from this arena must not be performed while the returned
    /// iterator is alive. If reading the chunk data (or casting to a reference)
    /// the caller must ensure that there exist no mutable references to
    /// previously allocated data.
    ///
    /// In addition, all of the caveats when reading the chunk data from
    /// [`iter_allocated_chunks()`](Bump::iter_allocated_chunks) still apply.
    pub unsafe fn iter_allocated_chunks_raw(&self) -> ChunkRawIter<'_> {
        ChunkRawIter {
            footer: self.current_chunk_footer.get(),
            bump: PhantomData,
        }
    }

    /// Calculates the number of bytes currently allocated across all chunks in
    /// this bump arena.
    ///
    /// If you allocate types of different alignments or types with
    /// larger-than-typical alignment in the same arena, some padding
    /// bytes might get allocated in the bump arena. Note that those padding
    /// bytes will add to this method's resulting sum, so you cannot rely
    /// on it only counting the sum of the sizes of the things
    /// you've allocated in the arena.
    ///
    /// The allocated bytes do not include the size of bumpalo's metadata,
    /// so the amount of memory requested from the Rust allocator is higher
    /// than the returned value.
    ///
    /// ## Example
    ///
    /// ```
    /// let bump = bumpalo::Bump::new();
    /// let _x = bump.alloc_slice_fill_default::<u32>(5);
    /// let bytes = bump.allocated_bytes();
    /// assert!(bytes >= core::mem::size_of::<u32>() * 5);
    /// ```
    pub fn allocated_bytes(&self) -> usize {
        let footer = self.current_chunk_footer.get();

        unsafe { footer.as_ref().allocated_bytes }
    }

    #[inline]
    unsafe fn is_last_allocation(&self, ptr: NonNull<u8>) -> bool {
        let footer = self.current_chunk_footer.get();
        let footer = footer.as_ref();
        footer.ptr.get() == ptr
    }

    #[inline]
    unsafe fn dealloc(&self, ptr: NonNull<u8>, layout: Layout) {
        // If the pointer is the last allocation we made, we can reuse the bytes,
        // otherwise they are simply leaked -- at least until somebody calls reset().
        if self.is_last_allocation(ptr) {
            let ptr = NonNull::new_unchecked(ptr.as_ptr().add(layout.size()));
            self.current_chunk_footer.get().as_ref().ptr.set(ptr);
        }
    }

    #[inline]
    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<u8>, AllocErr> {
        let old_size = old_layout.size();
        let new_size = new_layout.size();
        let align_is_compatible = old_layout.align() >= new_layout.align();

        if !align_is_compatible {
            return Err(AllocErr);
        }

        // This is how much space we would *actually* reclaim while satisfying
        // the requested alignment.
        let delta = round_down_to(old_size - new_size, new_layout.align());

        if self.is_last_allocation(ptr)
                // Only reclaim the excess space (which requires a copy) if it
                // is worth it: we are actually going to recover "enough" space
                // and we can do a non-overlapping copy.
                && delta >= old_size / 2
        {
            let footer = self.current_chunk_footer.get();
            let footer = footer.as_ref();

            // NB: new_ptr is aligned, because ptr *has to* be aligned, and we
            // made sure delta is aligned.
            let new_ptr = NonNull::new_unchecked(footer.ptr.get().as_ptr().add(delta));
            footer.ptr.set(new_ptr);

            // NB: we know it is non-overlapping because of the size check
            // in the `if` condition.
            ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), new_size);

            return Ok(new_ptr);
        } else {
            return Ok(ptr);
        }
    }

    #[inline]
    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<u8>, AllocErr> {
        let old_size = old_layout.size();
        let new_size = new_layout.size();
        let align_is_compatible = old_layout.align() >= new_layout.align();

        if align_is_compatible && self.is_last_allocation(ptr) {
            // Try to allocate the delta size within this same block so we can
            // reuse the currently allocated space.
            let delta = new_size - old_size;
            if let Some(p) =
                self.try_alloc_layout_fast(layout_from_size_align(delta, old_layout.align()))
            {
                ptr::copy(ptr.as_ptr(), p.as_ptr(), old_size);
                return Ok(p);
            }
        }

        // Fallback: do a fresh allocation and copy the existing data into it.
        let new_ptr = self.try_alloc_layout(new_layout)?;
        ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr(), old_size);
        Ok(new_ptr)
    }
}

/// An iterator over each chunk of allocated memory that
/// an arena has bump allocated into.
///
/// The chunks are returned ordered by allocation time, with the most recently
/// allocated chunk being returned first.
///
/// The values inside each chunk are also ordered by allocation time, with the most
/// recent allocation being earlier in the slice.
///
/// This struct is created by the [`iter_allocated_chunks`] method on
/// [`Bump`]. See that function for a safety description regarding reading from the returned items.
///
/// [`Bump`]: struct.Bump.html
/// [`iter_allocated_chunks`]: struct.Bump.html#method.iter_allocated_chunks
#[derive(Debug)]
pub struct ChunkIter<'a> {
    raw: ChunkRawIter<'a>,
    bump: PhantomData<&'a mut Bump>,
}

impl<'a> Iterator for ChunkIter<'a> {
    type Item = &'a [mem::MaybeUninit<u8>];
    fn next(&mut self) -> Option<&'a [mem::MaybeUninit<u8>]> {
        unsafe {
            let (ptr, len) = self.raw.next()?;
            let slice = slice::from_raw_parts(ptr as *const mem::MaybeUninit<u8>, len);
            Some(slice)
        }
    }
}

impl<'a> iter::FusedIterator for ChunkIter<'a> {}

/// An iterator over raw pointers to chunks of allocated memory that this
/// arena has bump allocated into.
///
/// See [`ChunkIter`] for details regarding the returned chunks.
///
/// This struct is created by the [`iter_allocated_chunks_raw`] method on
/// [`Bump`]. See that function for a safety description regarding reading from
/// the returned items.
///
/// [`Bump`]: struct.Bump.html
/// [`iter_allocated_chunks_raw`]: struct.Bump.html#method.iter_allocated_chunks_raw
#[derive(Debug)]
pub struct ChunkRawIter<'a> {
    footer: NonNull<ChunkFooter>,
    bump: PhantomData<&'a Bump>,
}

impl Iterator for ChunkRawIter<'_> {
    type Item = (*mut u8, usize);
    fn next(&mut self) -> Option<(*mut u8, usize)> {
        unsafe {
            let foot = self.footer.as_ref();
            if foot.is_empty() {
                return None;
            }
            let (ptr, len) = foot.as_raw_parts();
            self.footer = foot.prev.get();
            Some((ptr as *mut u8, len))
        }
    }
}

impl iter::FusedIterator for ChunkRawIter<'_> {}

#[inline(never)]
#[cold]
fn oom() -> ! {
    panic!("out of memory")
}

unsafe impl<'a> alloc::Alloc for &'a Bump {
    #[inline(always)]
    unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
        self.try_alloc_layout(layout)
    }

    #[inline]
    unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout) {
        Bump::dealloc(self, ptr, layout)
    }

    #[inline]
    unsafe fn realloc(
        &mut self,
        ptr: NonNull<u8>,
        layout: Layout,
        new_size: usize,
    ) -> Result<NonNull<u8>, AllocErr> {
        let old_size = layout.size();

        if old_size == 0 {
            return self.try_alloc_layout(layout);
        }

        let new_layout = layout_from_size_align(new_size, layout.align());
        if new_size <= old_size {
            self.shrink(ptr, layout, new_layout)
        } else {
            self.grow(ptr, layout, new_layout)
        }
    }
}

#[cfg(feature = "allocator_api")]
unsafe impl<'a> Allocator for &'a Bump {
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        self.try_alloc_layout(layout)
            .map(|p| NonNull::slice_from_raw_parts(p, layout.size()))
            .map_err(|_| AllocError)
    }

    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
        Bump::dealloc(self, ptr, layout)
    }

    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        Bump::shrink(self, ptr, old_layout, new_layout)
            .map(|p| NonNull::slice_from_raw_parts(p, new_layout.size()))
            .map_err(|_| AllocError)
    }

    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        Bump::grow(self, ptr, old_layout, new_layout)
            .map(|p| NonNull::slice_from_raw_parts(p, new_layout.size()))
            .map_err(|_| AllocError)
    }

    unsafe fn grow_zeroed(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        let mut ptr = self.grow(ptr, old_layout, new_layout)?;
        ptr.as_mut()[old_layout.size()..].fill(0);
        Ok(ptr)
    }
}

// NB: Only tests which require private types, fields, or methods should be in
// here. Anything that can just be tested via public API surface should be in
// `bumpalo/tests/all/*`.
#[cfg(test)]
mod tests {
    use super::*;

    // Uses private type `ChunkFooter`.
    #[test]
    fn chunk_footer_is_five_words() {
        assert_eq!(mem::size_of::<ChunkFooter>(), mem::size_of::<usize>() * 6);
    }

    // Uses private `alloc` module.
    #[test]
    #[allow(clippy::cognitive_complexity)]
    fn test_realloc() {
        use crate::alloc::Alloc;

        unsafe {
            const CAPACITY: usize = 1024 - OVERHEAD;
            let mut b = Bump::with_capacity(CAPACITY);

            // `realloc` doesn't shrink allocations that aren't "worth it".
            let layout = Layout::from_size_align(100, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 51).unwrap();
            assert_eq!(p, q);
            b.reset();

            // `realloc` will shrink allocations that are "worth it".
            let layout = Layout::from_size_align(100, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 50).unwrap();
            assert!(p != q);
            b.reset();

            // `realloc` will reuse the last allocation when growing.
            let layout = Layout::from_size_align(10, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 11).unwrap();
            assert_eq!(q.as_ptr() as usize, p.as_ptr() as usize - 1);
            b.reset();

            // `realloc` will allocate a new chunk when growing the last
            // allocation, if need be.
            let layout = Layout::from_size_align(1, 1).unwrap();
            let p = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, CAPACITY + 1).unwrap();
            assert!(q.as_ptr() as usize != p.as_ptr() as usize - CAPACITY);
            b = Bump::with_capacity(CAPACITY);

            // `realloc` will allocate and copy when reallocating anything that
            // wasn't the last allocation.
            let layout = Layout::from_size_align(1, 1).unwrap();
            let p = b.alloc_layout(layout);
            let _ = b.alloc_layout(layout);
            let q = (&b).realloc(p, layout, 2).unwrap();
            assert!(q.as_ptr() as usize != p.as_ptr() as usize - 1);
            b.reset();
        }
    }

    // Uses our private `alloc` module.
    #[test]
    fn invalid_read() {
        use alloc::Alloc;

        let mut b = &Bump::new();

        unsafe {
            let l1 = Layout::from_size_align(12000, 4).unwrap();
            let p1 = Alloc::alloc(&mut b, l1).unwrap();

            let l2 = Layout::from_size_align(1000, 4).unwrap();
            Alloc::alloc(&mut b, l2).unwrap();

            let p1 = b.realloc(p1, l1, 24000).unwrap();
            let l3 = Layout::from_size_align(24000, 4).unwrap();
            b.realloc(p1, l3, 48000).unwrap();
        }
    }
}