alloc.rs.html -- source
Description: Source to the Rust file `src/libcore/alloc.rs`.
Keywords: rust, rustlang, rust-lang

       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

    //! Memory allocation APIs

    // ignore-tidy-undocumented-unsafe

    #![stable(feature = "alloc_module", since = "1.28.0")]

    use crate::cmp;
    use crate::fmt;
    use crate::mem;
    use crate::usize;
    use crate::ptr::{self, NonNull};
    use crate::num::NonZeroUsize;

    /// Represents the combination of a starting address and
    /// a total capacity of the returned block.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[derive(Debug)]
    pub struct Excess(pub NonNull<u8>, pub usize);

    fn size_align<T>() -> (usize, usize) {
        (mem::size_of::<T>(), mem::align_of::<T>())
    }

    /// Layout of a block of memory.
    ///
    /// An instance of `Layout` describes a particular layout of memory.
    /// You build a `Layout` up as an input to give to an allocator.
    ///
    /// All layouts have an associated non-negative size and a
    /// power-of-two alignment.
    ///
    /// (Note however that layouts are *not* required to have positive
    /// size, even though many allocators require that all memory
    /// requests have positive size. A caller to the `Alloc::alloc`
    /// method must either ensure that conditions like this are met, or
    /// use specific allocators with looser requirements.)
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    #[lang = "alloc_layout"]
    pub struct Layout {
        // size of the requested block of memory, measured in bytes.
        size_: usize,

        // alignment of the requested block of memory, measured in bytes.
        // we ensure that this is always a power-of-two, because API's
        // like `posix_memalign` require it and it is a reasonable
        // constraint to impose on Layout constructors.
        //
        // (However, we do not analogously require `align >= sizeof(void*)`,
        //  even though that is *also* a requirement of `posix_memalign`.)
        align_: NonZeroUsize,
    }

    impl Layout {
        /// Constructs a `Layout` from a given `size` and `align`,
        /// or returns `LayoutErr` if any of the following conditions
        /// are not met:
        ///
        /// * `align` must not be zero,
        ///
        /// * `align` must be a power of two,
        ///
        /// * `size`, when rounded up to the nearest multiple of `align`,
        ///    must not overflow (i.e., the rounded value must be less than
        ///    `usize::MAX`).
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
            if !align.is_power_of_two() {
                return Err(LayoutErr { private: () });
            }

            // (power-of-two implies align != 0.)

            // Rounded up size is:
            //   size_rounded_up = (size + align - 1) & !(align - 1);
            //
            // We know from above that align != 0. If adding (align - 1)
            // does not overflow, then rounding up will be fine.
            //
            // Conversely, &-masking with !(align - 1) will subtract off
            // only low-order-bits. Thus if overflow occurs with the sum,
            // the &-mask cannot subtract enough to undo that overflow.
            //
            // Above implies that checking for summation overflow is both
            // necessary and sufficient.
            if size > usize::MAX - (align - 1) {
                return Err(LayoutErr { private: () });
            }

            unsafe {
                Ok(Layout::from_size_align_unchecked(size, align))
            }
        }

        /// Creates a layout, bypassing all checks.
        ///
        /// # Safety
        ///
        /// This function is unsafe as it does not verify the preconditions from
        /// [`Layout::from_size_align`](#method.from_size_align).
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "alloc_layout", since = "1.28.0"))]
        #[inline]
        pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
            Layout { size_: size, align_: NonZeroUsize::new_unchecked(align) }
        }

        /// The minimum size in bytes for a memory block of this layout.
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn size(&self) -> usize { self.size_ }

        /// The minimum byte alignment for a memory block of this layout.
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn align(&self) -> usize { self.align_.get() }

        /// Constructs a `Layout` suitable for holding a value of type `T`.
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn new<T>() -> Self {
            let (size, align) = size_align::<T>();
            // Note that the align is guaranteed by rustc to be a power of two and
            // the size+align combo is guaranteed to fit in our address space. As a
            // result use the unchecked constructor here to avoid inserting code
            // that panics if it isn't optimized well enough.
            debug_assert!(Layout::from_size_align(size, align).is_ok());
            unsafe {
                Layout::from_size_align_unchecked(size, align)
            }
        }

        /// Produces layout describing a record that could be used to
        /// allocate backing structure for `T` (which could be a trait
        /// or other unsized type like a slice).
        #[stable(feature = "alloc_layout", since = "1.28.0")]
        #[inline]
        pub fn for_value<T: ?Sized>(t: &T) -> Self {
            let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
            // See rationale in `new` for why this is using an unsafe variant below
            debug_assert!(Layout::from_size_align(size, align).is_ok());
            unsafe {
                Layout::from_size_align_unchecked(size, align)
            }
        }

        /// Creates a layout describing the record that can hold a value
        /// of the same layout as `self`, but that also is aligned to
        /// alignment `align` (measured in bytes).
        ///
        /// If `self` already meets the prescribed alignment, then returns
        /// `self`.
        ///
        /// Note that this method does not add any padding to the overall
        /// size, regardless of whether the returned layout has a different
        /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
        /// will *still* have size 16.
        ///
        /// Returns an error if the combination of `self.size()` and the given
        /// `align` violates the conditions listed in
        /// [`Layout::from_size_align`](#method.from_size_align).
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
            Layout::from_size_align(self.size(), cmp::max(self.align(), align))
        }

        /// Returns the amount of padding we must insert after `self`
        /// to ensure that the following address will satisfy `align`
        /// (measured in bytes).
        ///
        /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
        /// returns 3, because that is the minimum number of bytes of
        /// padding required to get a 4-aligned address (assuming that the
        /// corresponding memory block starts at a 4-aligned address).
        ///
        /// The return value of this function has no meaning if `align` is
        /// not a power-of-two.
        ///
        /// Note that the utility of the returned value requires `align`
        /// to be less than or equal to the alignment of the starting
        /// address for the whole allocated block of memory. One way to
        /// satisfy this constraint is to ensure `align <= self.align()`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn padding_needed_for(&self, align: usize) -> usize {
            let len = self.size();

            // Rounded up value is:
            //   len_rounded_up = (len + align - 1) & !(align - 1);
            // and then we return the padding difference: `len_rounded_up - len`.
            //
            // We use modular arithmetic throughout:
            //
            // 1. align is guaranteed to be > 0, so align - 1 is always
            //    valid.
            //
            // 2. `len + align - 1` can overflow by at most `align - 1`,
            //    so the &-mask with `!(align - 1)` will ensure that in the
            //    case of overflow, `len_rounded_up` will itself be 0.
            //    Thus the returned padding, when added to `len`, yields 0,
            //    which trivially satisfies the alignment `align`.
            //
            // (Of course, attempts to allocate blocks of memory whose
            // size and padding overflow in the above manner should cause
            // the allocator to yield an error anyway.)

            let len_rounded_up = len.wrapping_add(align).wrapping_sub(1)
                & !align.wrapping_sub(1);
            len_rounded_up.wrapping_sub(len)
        }

        /// Creates a layout by rounding the size of this layout up to a multiple
        /// of the layout's alignment.
        ///
        /// This is equivalent to adding the result of `padding_needed_for`
        /// to the layout's current size.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn pad_to_align(&self) -> Layout {
            let pad = self.padding_needed_for(self.align());
            // This cannot overflow. Quoting from the invariant of Layout:
            // > `size`, when rounded up to the nearest multiple of `align`,
            // > must not overflow (i.e., the rounded value must be less than
            // > `usize::MAX`)
            let new_size = self.size() + pad;

            Layout::from_size_align(new_size, self.align()).unwrap()
        }

        /// Creates a layout describing the record for `n` instances of
        /// `self`, with a suitable amount of padding between each to
        /// ensure that each instance is given its requested size and
        /// alignment. On success, returns `(k, offs)` where `k` is the
        /// layout of the array and `offs` is the distance between the start
        /// of each element in the array.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
            // This cannot overflow. Quoting from the invariant of Layout:
            // > `size`, when rounded up to the nearest multiple of `align`,
            // > must not overflow (i.e., the rounded value must be less than
            // > `usize::MAX`)
            let padded_size = self.size() + self.padding_needed_for(self.align());
            let alloc_size = padded_size.checked_mul(n)
                .ok_or(LayoutErr { private: () })?;

            unsafe {
                // self.align is already known to be valid and alloc_size has been
                // padded already.
                Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size))
            }
        }

        /// Creates a layout describing the record for `self` followed by
        /// `next`, including any necessary padding to ensure that `next`
        /// will be properly aligned. Note that the resulting layout will
        /// satisfy the alignment properties of both `self` and `next`.
        ///
        /// The resulting layout will be the same as that of a C struct containing
        /// two fields with the layouts of `self` and `next`, in that order.
        ///
        /// Returns `Some((k, offset))`, where `k` is layout of the concatenated
        /// record and `offset` is the relative location, in bytes, of the
        /// start of the `next` embedded within the concatenated record
        /// (assuming that the record itself starts at offset 0).
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
            let new_align = cmp::max(self.align(), next.align());
            let pad = self.padding_needed_for(next.align());

            let offset = self.size().checked_add(pad)
                .ok_or(LayoutErr { private: () })?;
            let new_size = offset.checked_add(next.size())
                .ok_or(LayoutErr { private: () })?;

            let layout = Layout::from_size_align(new_size, new_align)?;
            Ok((layout, offset))
        }

        /// Creates a layout describing the record for `n` instances of
        /// `self`, with no padding between each instance.
        ///
        /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
        /// that the repeated instances of `self` will be properly
        /// aligned, even if a given instance of `self` is properly
        /// aligned. In other words, if the layout returned by
        /// `repeat_packed` is used to allocate an array, it is not
        /// guaranteed that all elements in the array will be properly
        /// aligned.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
            let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
            Layout::from_size_align(size, self.align())
        }

        /// Creates a layout describing the record for `self` followed by
        /// `next` with no additional padding between the two. Since no
        /// padding is inserted, the alignment of `next` is irrelevant,
        /// and is not incorporated *at all* into the resulting layout.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
            let new_size = self.size().checked_add(next.size())
                .ok_or(LayoutErr { private: () })?;
            Layout::from_size_align(new_size, self.align())
        }

        /// Creates a layout describing the record for a `[T; n]`.
        ///
        /// On arithmetic overflow, returns `LayoutErr`.
        #[unstable(feature = "alloc_layout_extra", issue = "55724")]
        #[inline]
        pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
            Layout::new::<T>()
                .repeat(n)
                .map(|(k, offs)| {
                    debug_assert!(offs == mem::size_of::<T>());
                    k
                })
        }
    }

    /// The parameters given to `Layout::from_size_align`
    /// or some other `Layout` constructor
    /// do not satisfy its documented constraints.
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    #[derive(Clone, PartialEq, Eq, Debug)]
    pub struct LayoutErr {
        private: ()
    }

    // (we need this for downstream impl of trait Error)
    #[stable(feature = "alloc_layout", since = "1.28.0")]
    impl fmt::Display for LayoutErr {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("invalid parameters to Layout::from_size_align")
        }
    }

    /// The `AllocErr` error indicates an allocation failure
    /// that may be due to resource exhaustion or to
    /// something wrong when combining the given input arguments with this
    /// allocator.
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[derive(Clone, PartialEq, Eq, Debug)]
    pub struct AllocErr;

    // (we need this for downstream impl of trait Error)
    #[unstable(feature = "allocator_api", issue = "32838")]
    impl fmt::Display for AllocErr {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("memory allocation failed")
        }
    }

    /// The `CannotReallocInPlace` error is used when [`grow_in_place`] or
    /// [`shrink_in_place`] were unable to reuse the given memory block for
    /// a requested layout.
    ///
    /// [`grow_in_place`]: ./trait.Alloc.html#method.grow_in_place
    /// [`shrink_in_place`]: ./trait.Alloc.html#method.shrink_in_place
    #[unstable(feature = "allocator_api", issue = "32838")]
    #[derive(Clone, PartialEq, Eq, Debug)]
    pub struct CannotReallocInPlace;

    #[unstable(feature = "allocator_api", issue = "32838")]
    impl CannotReallocInPlace {
        pub fn description(&self) -> &str {
            "cannot reallocate allocator's memory in place"
        }
    }

    // (we need this for downstream impl of trait Error)
    #[unstable(feature = "allocator_api", issue = "32838")]
    impl fmt::Display for CannotReallocInPlace {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "{}", self.description())
        }
    }

    /// A memory allocator that can be registered as the standard library’s default
    /// through the `#[global_allocator]` attribute.
    ///
    /// Some of the methods require that a memory block be *currently
    /// allocated* via an allocator. This means that:
    ///
    /// * the starting address for that memory block was previously
    ///   returned by a previous call to an allocation method
    ///   such as `alloc`, and
    ///
    /// * the memory block has not been subsequently deallocated, where
    ///   blocks are deallocated either by being passed to a deallocation
    ///   method such as `dealloc` or by being
    ///   passed to a reallocation method that returns a non-null pointer.
    ///
    ///
    /// # Example
    ///
    /// ```no_run
    /// use std::alloc::{GlobalAlloc, Layout, alloc};
    /// use std::ptr::null_mut;
    ///
    /// struct MyAllocator;
    ///
    /// unsafe impl GlobalAlloc for MyAllocator {
    ///     unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
    ///     unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
    /// }
    ///
    /// #[global_allocator]
    /// static A: MyAllocator = MyAllocator;
    ///
    /// fn main() {
    ///     unsafe {
    ///         assert!(alloc(Layout::new::<u32>()).is_null())
    ///     }
    /// }
    /// ```
    ///
    /// # Safety
    ///
    /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
    /// implementors must ensure that they adhere to these contracts:
    ///
    /// * It's undefined behavior if global allocators unwind. This restriction may
    ///   be lifted in the future, but currently a panic from any of these
    ///   functions may lead to memory unsafety.
    ///
    /// * `Layout` queries and calculations in general must be correct. Callers of
    ///   this trait are allowed to rely on the contracts defined on each method,
    ///   and implementors must ensure such contracts remain true.
    #[stable(feature = "global_alloc", since = "1.28.0")]
    pub unsafe trait GlobalAlloc {
        /// Allocate memory as described by the given `layout`.
        ///
        /// Returns a pointer to newly-allocated memory,
        /// or null to indicate allocation failure.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure that `layout` has non-zero size.
        ///
        /// (Extension subtraits might provide more specific bounds on
        /// behavior, e.g., guarantee a sentinel address or a null pointer
        /// in response to a zero-size allocation request.)
        ///
        /// The allocated block of memory may or may not be initialized.
        ///
        /// # Errors
        ///
        /// Returning a null pointer indicates that either memory is exhausted
        /// or `layout` does not meet this allocator's size or alignment constraints.
        ///
        /// Implementations are encouraged to return null on memory
        /// exhaustion rather than aborting, but this is not
        /// a strict requirement. (Specifically: it is *legal* to
        /// implement this trait atop an underlying native allocation
        /// library that aborts on memory exhaustion.)
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn alloc(&self, layout: Layout) -> *mut u8;

        /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must denote a block of memory currently allocated via
        ///   this allocator,
        ///
        /// * `layout` must be the same layout that was used
        ///   to allocate that block of memory,
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);

        /// Behaves like `alloc`, but also ensures that the contents
        /// are set to zero before being returned.
        ///
        /// # Safety
        ///
        /// This function is unsafe for the same reasons that `alloc` is.
        /// However the allocated block of memory is guaranteed to be initialized.
        ///
        /// # Errors
        ///
        /// Returning a null pointer indicates that either memory is exhausted
        /// or `layout` does not meet allocator's size or alignment constraints,
        /// just as in `alloc`.
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
            let size = layout.size();
            let ptr = self.alloc(layout);
            if !ptr.is_null() {
                ptr::write_bytes(ptr, 0, size);
            }
            ptr
        }

        /// Shrink or grow a block of memory to the given `new_size`.
        /// The block is described by the given `ptr` pointer and `layout`.
        ///
        /// If this returns a non-null pointer, then ownership of the memory block
        /// referenced by `ptr` has been transferred to this allocator.
        /// The memory may or may not have been deallocated,
        /// and should be considered unusable (unless of course it was
        /// transferred back to the caller again via the return value of
        /// this method).
        ///
        /// If this method returns null, then ownership of the memory
        /// block has not been transferred to this allocator, and the
        /// contents of the memory block are unaltered.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must be currently allocated via this allocator,
        ///
        /// * `layout` must be the same layout that was used
        ///   to allocate that block of memory,
        ///
        /// * `new_size` must be greater than zero.
        ///
        /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
        ///   must not overflow (i.e., the rounded value must be less than `usize::MAX`).
        ///
        /// (Extension subtraits might provide more specific bounds on
        /// behavior, e.g., guarantee a sentinel address or a null pointer
        /// in response to a zero-size allocation request.)
        ///
        /// # Errors
        ///
        /// Returns null if the new layout does not meet the size
        /// and alignment constraints of the allocator, or if reallocation
        /// otherwise fails.
        ///
        /// Implementations are encouraged to return null on memory
        /// exhaustion rather than panicking or aborting, but this is not
        /// a strict requirement. (Specifically: it is *legal* to
        /// implement this trait atop an underlying native allocation
        /// library that aborts on memory exhaustion.)
        ///
        /// Clients wishing to abort computation in response to a
        /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        #[stable(feature = "global_alloc", since = "1.28.0")]
        unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
            let new_ptr = self.alloc(new_layout);
            if !new_ptr.is_null() {
                ptr::copy_nonoverlapping(
                    ptr,
                    new_ptr,
                    cmp::min(layout.size(), new_size),
                );
                self.dealloc(ptr, layout);
            }
            new_ptr
        }
    }

    /// An implementation of `Alloc` can allocate, reallocate, and
    /// deallocate arbitrary blocks of data described via `Layout`.
    ///
    /// Some of the methods require that a memory block be *currently
    /// allocated* via an allocator. This means that:
    ///
    /// * the starting address for that memory block was previously
    ///   returned by a previous call to an allocation method (`alloc`,
    ///   `alloc_zeroed`, `alloc_excess`, `alloc_one`, `alloc_array`) or
    ///   reallocation method (`realloc`, `realloc_excess`, or
    ///   `realloc_array`), and
    ///
    /// * the memory block has not been subsequently deallocated, where
    ///   blocks are deallocated either by being passed to a deallocation
    ///   method (`dealloc`, `dealloc_one`, `dealloc_array`) or by being
    ///   passed to a reallocation method (see above) that returns `Ok`.
    ///
    /// A note regarding zero-sized types and zero-sized layouts: many
    /// methods in the `Alloc` trait state that allocation requests
    /// must be non-zero size, or else undefined behavior can result.
    ///
    /// * However, some higher-level allocation methods (`alloc_one`,
    ///   `alloc_array`) are well-defined on zero-sized types and can
    ///   optionally support them: it is left up to the implementor
    ///   whether to return `Err`, or to return `Ok` with some pointer.
    ///
    /// * If an `Alloc` implementation chooses to return `Ok` in this
    ///   case (i.e., the pointer denotes a zero-sized inaccessible block)
    ///   then that returned pointer must be considered "currently
    ///   allocated". On such an allocator, *all* methods that take
    ///   currently-allocated pointers as inputs must accept these
    ///   zero-sized pointers, *without* causing undefined behavior.
    ///
    /// * In other words, if a zero-sized pointer can flow out of an
    ///   allocator, then that allocator must likewise accept that pointer
    ///   flowing back into its deallocation and reallocation methods.
    ///
    /// Some of the methods require that a layout *fit* a memory block.
    /// What it means for a layout to "fit" a memory block means (or
    /// equivalently, for a memory block to "fit" a layout) is that the
    /// following two conditions must hold:
    ///
    /// 1. The block's starting address must be aligned to `layout.align()`.
    ///
    /// 2. The block's size must fall in the range `[use_min, use_max]`, where:
    ///
    ///    * `use_min` is `self.usable_size(layout).0`, and
    ///
    ///    * `use_max` is the capacity that was (or would have been)
    ///      returned when (if) the block was allocated via a call to
    ///      `alloc_excess` or `realloc_excess`.
    ///
    /// Note that:
    ///
    ///  * the size of the layout most recently used to allocate the block
    ///    is guaranteed to be in the range `[use_min, use_max]`, and
    ///
    ///  * a lower-bound on `use_max` can be safely approximated by a call to
    ///    `usable_size`.
    ///
    ///  * if a layout `k` fits a memory block (denoted by `ptr`)
    ///    currently allocated via an allocator `a`, then it is legal to
    ///    use that layout to deallocate it, i.e., `a.dealloc(ptr, k);`.
    ///
    /// # Safety
    ///
    /// The `Alloc` trait is an `unsafe` trait for a number of reasons, and
    /// implementors must ensure that they adhere to these contracts:
    ///
    /// * Pointers returned from allocation functions must point to valid memory and
    ///   retain their validity until at least the instance of `Alloc` is dropped
    ///   itself.
    ///
    /// * `Layout` queries and calculations in general must be correct. Callers of
    ///   this trait are allowed to rely on the contracts defined on each method,
    ///   and implementors must ensure such contracts remain true.
    ///
    /// Note that this list may get tweaked over time as clarifications are made in
    /// the future.
    #[unstable(feature = "allocator_api", issue = "32838")]
    pub unsafe trait Alloc {

        // (Note: some existing allocators have unspecified but well-defined
        // behavior in response to a zero size allocation request ;
        // e.g., in C, `malloc` of 0 will either return a null pointer or a
        // unique pointer, but will not have arbitrary undefined
        // behavior.
        // However in jemalloc for example,
        // `mallocx(0)` is documented as undefined behavior.)

        /// Returns a pointer meeting the size and alignment guarantees of
        /// `layout`.
        ///
        /// If this method returns an `Ok(addr)`, then the `addr` returned
        /// will be non-null address pointing to a block of storage
        /// suitable for holding an instance of `layout`.
        ///
        /// The returned block of storage may or may not have its contents
        /// initialized. (Extension subtraits might restrict this
        /// behavior, e.g., to ensure initialization to particular sets of
        /// bit patterns.)
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure that `layout` has non-zero size.
        ///
        /// (Extension subtraits might provide more specific bounds on
        /// behavior, e.g., guarantee a sentinel address or a null pointer
        /// in response to a zero-size allocation request.)
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `layout` does not meet allocator's size or alignment
        /// constraints.
        ///
        /// Implementations are encouraged to return `Err` on memory
        /// exhaustion rather than panicking or aborting, but this is not
        /// a strict requirement. (Specifically: it is *legal* to
        /// implement this trait atop an underlying native allocation
        /// library that aborts on memory exhaustion.)
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;

        /// Deallocate the memory referenced by `ptr`.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must denote a block of memory currently allocated via
        ///   this allocator,
        ///
        /// * `layout` must *fit* that block of memory,
        ///
        /// * In addition to fitting the block of memory `layout`, the
        ///   alignment of the `layout` must match the alignment used
        ///   to allocate that block of memory.
        unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);

        // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
        // usable_size

        /// Returns bounds on the guaranteed usable size of a successful
        /// allocation created with the specified `layout`.
        ///
        /// In particular, if one has a memory block allocated via a given
        /// allocator `a` and layout `k` where `a.usable_size(k)` returns
        /// `(l, u)`, then one can pass that block to `a.dealloc()` with a
        /// layout in the size range [l, u].
        ///
        /// (All implementors of `usable_size` must ensure that
        /// `l <= k.size() <= u`)
        ///
        /// Both the lower- and upper-bounds (`l` and `u` respectively)
        /// are provided, because an allocator based on size classes could
        /// misbehave if one attempts to deallocate a block without
        /// providing a correct value for its size (i.e., one within the
        /// range `[l, u]`).
        ///
        /// Clients who wish to make use of excess capacity are encouraged
        /// to use the `alloc_excess` and `realloc_excess` instead, as
        /// this method is constrained to report conservative values that
        /// serve as valid bounds for *all possible* allocation method
        /// calls.
        ///
        /// However, for clients that do not wish to track the capacity
        /// returned by `alloc_excess` locally, this method is likely to
        /// produce useful results.
        #[inline]
        fn usable_size(&self, layout: &Layout) -> (usize, usize) {
            (layout.size(), layout.size())
        }

        // == METHODS FOR MEMORY REUSE ==
        // realloc. alloc_excess, realloc_excess

        /// Returns a pointer suitable for holding data described by
        /// a new layout with `layout`’s alignment and a size given
        /// by `new_size`. To
        /// accomplish this, this may extend or shrink the allocation
        /// referenced by `ptr` to fit the new layout.
        ///
        /// If this returns `Ok`, then ownership of the memory block
        /// referenced by `ptr` has been transferred to this
        /// allocator. The memory may or may not have been freed, and
        /// should be considered unusable (unless of course it was
        /// transferred back to the caller again via the return value of
        /// this method).
        ///
        /// If this method returns `Err`, then ownership of the memory
        /// block has not been transferred to this allocator, and the
        /// contents of the memory block are unaltered.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must be currently allocated via this allocator,
        ///
        /// * `layout` must *fit* the `ptr` (see above). (The `new_size`
        ///   argument need not fit it.)
        ///
        /// * `new_size` must be greater than zero.
        ///
        /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
        ///   must not overflow (i.e., the rounded value must be less than `usize::MAX`).
        ///
        /// (Extension subtraits might provide more specific bounds on
        /// behavior, e.g., guarantee a sentinel address or a null pointer
        /// in response to a zero-size allocation request.)
        ///
        /// # Errors
        ///
        /// Returns `Err` only if the new layout
        /// does not meet the allocator's size
        /// and alignment constraints of the allocator, or if reallocation
        /// otherwise fails.
        ///
        /// Implementations are encouraged to return `Err` on memory
        /// exhaustion rather than panicking or aborting, but this is not
        /// a strict requirement. (Specifically: it is *legal* to
        /// implement this trait atop an underlying native allocation
        /// library that aborts on memory exhaustion.)
        ///
        /// Clients wishing to abort computation in response to a
        /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        unsafe fn realloc(&mut self,
                          ptr: NonNull<u8>,
                          layout: Layout,
                          new_size: usize) -> Result<NonNull<u8>, AllocErr> {
            let old_size = layout.size();

            if new_size >= old_size {
                if let Ok(()) = self.grow_in_place(ptr, layout, new_size) {
                    return Ok(ptr);
                }
            } else if new_size < old_size {
                if let Ok(()) = self.shrink_in_place(ptr, layout, new_size) {
                    return Ok(ptr);
                }
            }

            // otherwise, fall back on alloc + copy + dealloc.
            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
            let result = self.alloc(new_layout);
            if let Ok(new_ptr) = result {
                ptr::copy_nonoverlapping(ptr.as_ptr(),
                                         new_ptr.as_ptr(),
                                         cmp::min(old_size, new_size));
                self.dealloc(ptr, layout);
            }
            result
        }

        /// Behaves like `alloc`, but also ensures that the contents
        /// are set to zero before being returned.
        ///
        /// # Safety
        ///
        /// This function is unsafe for the same reasons that `alloc` is.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `layout` does not meet allocator's size or alignment
        /// constraints, just as in `alloc`.
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
            let size = layout.size();
            let p = self.alloc(layout);
            if let Ok(p) = p {
                ptr::write_bytes(p.as_ptr(), 0, size);
            }
            p
        }

        /// Behaves like `alloc`, but also returns the whole size of
        /// the returned block. For some `layout` inputs, like arrays, this
        /// may include extra storage usable for additional data.
        ///
        /// # Safety
        ///
        /// This function is unsafe for the same reasons that `alloc` is.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `layout` does not meet allocator's size or alignment
        /// constraints, just as in `alloc`.
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
            let usable_size = self.usable_size(&layout);
            self.alloc(layout).map(|p| Excess(p, usable_size.1))
        }

        /// Behaves like `realloc`, but also returns the whole size of
        /// the returned block. For some `layout` inputs, like arrays, this
        /// may include extra storage usable for additional data.
        ///
        /// # Safety
        ///
        /// This function is unsafe for the same reasons that `realloc` is.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `layout` does not meet allocator's size or alignment
        /// constraints, just as in `realloc`.
        ///
        /// Clients wishing to abort computation in response to a
        /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        unsafe fn realloc_excess(&mut self,
                                 ptr: NonNull<u8>,
                                 layout: Layout,
                                 new_size: usize) -> Result<Excess, AllocErr> {
            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
            let usable_size = self.usable_size(&new_layout);
            self.realloc(ptr, layout, new_size)
                .map(|p| Excess(p, usable_size.1))
        }

        /// Attempts to extend the allocation referenced by `ptr` to fit `new_size`.
        ///
        /// If this returns `Ok`, then the allocator has asserted that the
        /// memory block referenced by `ptr` now fits `new_size`, and thus can
        /// be used to carry data of a layout of that size and same alignment as
        /// `layout`. (The allocator is allowed to
        /// expend effort to accomplish this, such as extending the memory block to
        /// include successor blocks, or virtual memory tricks.)
        ///
        /// Regardless of what this method returns, ownership of the
        /// memory block referenced by `ptr` has not been transferred, and
        /// the contents of the memory block are unaltered.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must be currently allocated via this allocator,
        ///
        /// * `layout` must *fit* the `ptr` (see above); note the
        ///   `new_size` argument need not fit it,
        ///
        /// * `new_size` must not be less than `layout.size()`,
        ///
        /// # Errors
        ///
        /// Returns `Err(CannotReallocInPlace)` when the allocator is
        /// unable to assert that the memory block referenced by `ptr`
        /// could fit `layout`.
        ///
        /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
        /// function; clients are expected either to be able to recover from
        /// `grow_in_place` failures without aborting, or to fall back on
        /// another reallocation method before resorting to an abort.
        unsafe fn grow_in_place(&mut self,
                                ptr: NonNull<u8>,
                                layout: Layout,
                                new_size: usize) -> Result<(), CannotReallocInPlace> {
            let _ = ptr; // this default implementation doesn't care about the actual address.
            debug_assert!(new_size >= layout.size());
            let (_l, u) = self.usable_size(&layout);
            // _l <= layout.size()                       [guaranteed by usable_size()]
            //       layout.size() <= new_layout.size()  [required by this method]
            if new_size <= u {
                Ok(())
            } else {
                Err(CannotReallocInPlace)
            }
        }

        /// Attempts to shrink the allocation referenced by `ptr` to fit `new_size`.
        ///
        /// If this returns `Ok`, then the allocator has asserted that the
        /// memory block referenced by `ptr` now fits `new_size`, and
        /// thus can only be used to carry data of that smaller
        /// layout. (The allocator is allowed to take advantage of this,
        /// carving off portions of the block for reuse elsewhere.) The
        /// truncated contents of the block within the smaller layout are
        /// unaltered, and ownership of block has not been transferred.
        ///
        /// If this returns `Err`, then the memory block is considered to
        /// still represent the original (larger) `layout`. None of the
        /// block has been carved off for reuse elsewhere, ownership of
        /// the memory block has not been transferred, and the contents of
        /// the memory block are unaltered.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must be currently allocated via this allocator,
        ///
        /// * `layout` must *fit* the `ptr` (see above); note the
        ///   `new_size` argument need not fit it,
        ///
        /// * `new_size` must not be greater than `layout.size()`
        ///   (and must be greater than zero),
        ///
        /// # Errors
        ///
        /// Returns `Err(CannotReallocInPlace)` when the allocator is
        /// unable to assert that the memory block referenced by `ptr`
        /// could fit `layout`.
        ///
        /// Note that one cannot pass `CannotReallocInPlace` to the `handle_alloc_error`
        /// function; clients are expected either to be able to recover from
        /// `shrink_in_place` failures without aborting, or to fall back
        /// on another reallocation method before resorting to an abort.
        unsafe fn shrink_in_place(&mut self,
                                  ptr: NonNull<u8>,
                                  layout: Layout,
                                  new_size: usize) -> Result<(), CannotReallocInPlace> {
            let _ = ptr; // this default implementation doesn't care about the actual address.
            debug_assert!(new_size <= layout.size());
            let (l, _u) = self.usable_size(&layout);
            //                      layout.size() <= _u  [guaranteed by usable_size()]
            // new_layout.size() <= layout.size()        [required by this method]
            if l <= new_size {
                Ok(())
            } else {
                Err(CannotReallocInPlace)
            }
        }

        // == COMMON USAGE PATTERNS ==
        // alloc_one, dealloc_one, alloc_array, realloc_array. dealloc_array

        /// Allocates a block suitable for holding an instance of `T`.
        ///
        /// Captures a common usage pattern for allocators.
        ///
        /// The returned block is suitable for passing to the
        /// `realloc`/`dealloc` methods of this allocator.
        ///
        /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
        /// must be considered "currently allocated" and must be
        /// acceptable input to methods such as `realloc` or `dealloc`,
        /// *even if* `T` is a zero-sized type. In other words, if your
        /// `Alloc` implementation overrides this method in a manner
        /// that can return a zero-sized `ptr`, then all reallocation and
        /// deallocation methods need to be similarly overridden to accept
        /// such values as input.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `T` does not meet allocator's size or alignment constraints.
        ///
        /// For zero-sized `T`, may return either of `Ok` or `Err`, but
        /// will *not* yield undefined behavior.
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
            where Self: Sized
        {
            let k = Layout::new::<T>();
            if k.size() > 0 {
                unsafe { self.alloc(k).map(|p| p.cast()) }
            } else {
                Err(AllocErr)
            }
        }

        /// Deallocates a block suitable for holding an instance of `T`.
        ///
        /// The given block must have been produced by this allocator,
        /// and must be suitable for storing a `T` (in terms of alignment
        /// as well as minimum and maximum size); otherwise yields
        /// undefined behavior.
        ///
        /// Captures a common usage pattern for allocators.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure both:
        ///
        /// * `ptr` must denote a block of memory currently allocated via this allocator
        ///
        /// * the layout of `T` must *fit* that block of memory.
        unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)
            where Self: Sized
        {
            let k = Layout::new::<T>();
            if k.size() > 0 {
                self.dealloc(ptr.cast(), k);
            }
        }

        /// Allocates a block suitable for holding `n` instances of `T`.
        ///
        /// Captures a common usage pattern for allocators.
        ///
        /// The returned block is suitable for passing to the
        /// `realloc`/`dealloc` methods of this allocator.
        ///
        /// Note to implementors: If this returns `Ok(ptr)`, then `ptr`
        /// must be considered "currently allocated" and must be
        /// acceptable input to methods such as `realloc` or `dealloc`,
        /// *even if* `T` is a zero-sized type. In other words, if your
        /// `Alloc` implementation overrides this method in a manner
        /// that can return a zero-sized `ptr`, then all reallocation and
        /// deallocation methods need to be similarly overridden to accept
        /// such values as input.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `[T; n]` does not meet allocator's size or alignment
        /// constraints.
        ///
        /// For zero-sized `T` or `n == 0`, may return either of `Ok` or
        /// `Err`, but will *not* yield undefined behavior.
        ///
        /// Always returns `Err` on arithmetic overflow.
        ///
        /// Clients wishing to abort computation in response to an
        /// allocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
            where Self: Sized
        {
            match Layout::array::<T>(n) {
                Ok(layout) if layout.size() > 0 => {
                    unsafe {
                        self.alloc(layout).map(|p| p.cast())
                    }
                }
                _ => Err(AllocErr),
            }
        }

        /// Reallocates a block previously suitable for holding `n_old`
        /// instances of `T`, returning a block suitable for holding
        /// `n_new` instances of `T`.
        ///
        /// Captures a common usage pattern for allocators.
        ///
        /// The returned block is suitable for passing to the
        /// `realloc`/`dealloc` methods of this allocator.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure all of the following:
        ///
        /// * `ptr` must be currently allocated via this allocator,
        ///
        /// * the layout of `[T; n_old]` must *fit* that block of memory.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either memory is exhausted or
        /// `[T; n_new]` does not meet allocator's size or alignment
        /// constraints.
        ///
        /// For zero-sized `T` or `n_new == 0`, may return either of `Ok` or
        /// `Err`, but will *not* yield undefined behavior.
        ///
        /// Always returns `Err` on arithmetic overflow.
        ///
        /// Clients wishing to abort computation in response to a
        /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
        /// rather than directly invoking `panic!` or similar.
        ///
        /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
        unsafe fn realloc_array<T>(&mut self,
                                   ptr: NonNull<T>,
                                   n_old: usize,
                                   n_new: usize) -> Result<NonNull<T>, AllocErr>
            where Self: Sized
        {
            match (Layout::array::<T>(n_old), Layout::array::<T>(n_new)) {
                (Ok(k_old), Ok(k_new)) if k_old.size() > 0 && k_new.size() > 0 => {
                    debug_assert!(k_old.align() == k_new.align());
                    self.realloc(ptr.cast(), k_old, k_new.size()).map(NonNull::cast)
                }
                _ => {
                    Err(AllocErr)
                }
            }
        }

        /// Deallocates a block suitable for holding `n` instances of `T`.
        ///
        /// Captures a common usage pattern for allocators.
        ///
        /// # Safety
        ///
        /// This function is unsafe because undefined behavior can result
        /// if the caller does not ensure both:
        ///
        /// * `ptr` must denote a block of memory currently allocated via this allocator
        ///
        /// * the layout of `[T; n]` must *fit* that block of memory.
        ///
        /// # Errors
        ///
        /// Returning `Err` indicates that either `[T; n]` or the given
        /// memory block does not meet allocator's size or alignment
        /// constraints.
        ///
        /// Always returns `Err` on arithmetic overflow.
        unsafe fn dealloc_array<T>(&mut self, ptr: NonNull<T>, n: usize) -> Result<(), AllocErr>
            where Self: Sized
        {
            match Layout::array::<T>(n) {
                Ok(k) if k.size() > 0 => {
                    Ok(self.dealloc(ptr.cast(), k))
                }
                _ => {
                    Err(AllocErr)
                }
            }
        }
    }