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
use std::borrow::{Borrow, Cow};
use std::cmp;
use std::fmt::{self, Write};
use std::iter;
use std::ops::Bound;
use std::ops::Deref;

use rustc_index::Idx;
use tracing::debug;

use crate::{
    Abi, AbiAndPrefAlign, Align, FieldsShape, IndexSlice, IndexVec, Integer, LayoutS, Niche,
    NonZeroUsize, Primitive, ReprOptions, Scalar, Size, StructKind, TagEncoding, TargetDataLayout,
    Variants, WrappingRange,
};

// A variant is absent if it's uninhabited and only has ZST fields.
// Present uninhabited variants only require space for their fields,
// but *not* an encoding of the discriminant (e.g., a tag value).
// See issue #49298 for more details on the need to leave space
// for non-ZST uninhabited data (mostly partial initialization).
fn absent<'a, FieldIdx, VariantIdx, F>(fields: &IndexSlice<FieldIdx, F>) -> bool
where
    FieldIdx: Idx,
    VariantIdx: Idx,
    F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
{
    let uninhabited = fields.iter().any(|f| f.abi.is_uninhabited());
    // We cannot ignore alignment; that might lead us to entirely discard a variant and
    // produce an enum that is less aligned than it should be!
    let is_1zst = fields.iter().all(|f| f.is_1zst());
    uninhabited && is_1zst
}

pub trait LayoutCalculator {
    type TargetDataLayoutRef: Borrow<TargetDataLayout>;

    fn delayed_bug(&self, txt: impl Into<Cow<'static, str>>);
    fn current_data_layout(&self) -> Self::TargetDataLayoutRef;

    fn scalar_pair<FieldIdx: Idx, VariantIdx: Idx>(
        &self,
        a: Scalar,
        b: Scalar,
    ) -> LayoutS<FieldIdx, VariantIdx> {
        let dl = self.current_data_layout();
        let dl = dl.borrow();
        let b_align = b.align(dl);
        let align = a.align(dl).max(b_align).max(dl.aggregate_align);
        let b_offset = a.size(dl).align_to(b_align.abi);
        let size = (b_offset + b.size(dl)).align_to(align.abi);

        // HACK(nox): We iter on `b` and then `a` because `max_by_key`
        // returns the last maximum.
        let largest_niche = Niche::from_scalar(dl, b_offset, b)
            .into_iter()
            .chain(Niche::from_scalar(dl, Size::ZERO, a))
            .max_by_key(|niche| niche.available(dl));

        LayoutS {
            variants: Variants::Single { index: VariantIdx::new(0) },
            fields: FieldsShape::Arbitrary {
                offsets: [Size::ZERO, b_offset].into(),
                memory_index: [0, 1].into(),
            },
            abi: Abi::ScalarPair(a, b),
            largest_niche,
            align,
            size,
            max_repr_align: None,
            unadjusted_abi_align: align.abi,
        }
    }

    fn univariant<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
    >(
        &self,
        dl: &TargetDataLayout,
        fields: &IndexSlice<FieldIdx, F>,
        repr: &ReprOptions,
        kind: StructKind,
    ) -> Option<LayoutS<FieldIdx, VariantIdx>> {
        let layout = univariant(self, dl, fields, repr, kind, NicheBias::Start);
        // Enums prefer niches close to the beginning or the end of the variants so that other
        // (smaller) data-carrying variants can be packed into the space after/before the niche.
        // If the default field ordering does not give us a niche at the front then we do a second
        // run and bias niches to the right and then check which one is closer to one of the
        // struct's edges.
        if let Some(layout) = &layout {
            // Don't try to calculate an end-biased layout for unsizable structs,
            // otherwise we could end up with different layouts for
            // Foo<Type> and Foo<dyn Trait> which would break unsizing.
            if !matches!(kind, StructKind::MaybeUnsized) {
                if let Some(niche) = layout.largest_niche {
                    let head_space = niche.offset.bytes();
                    let niche_len = niche.value.size(dl).bytes();
                    let tail_space = layout.size.bytes() - head_space - niche_len;

                    // This may end up doing redundant work if the niche is already in the last
                    // field (e.g. a trailing bool) and there is tail padding. But it's non-trivial
                    // to get the unpadded size so we try anyway.
                    if fields.len() > 1 && head_space != 0 && tail_space > 0 {
                        let alt_layout = univariant(self, dl, fields, repr, kind, NicheBias::End)
                            .expect("alt layout should always work");
                        let alt_niche = alt_layout
                            .largest_niche
                            .expect("alt layout should have a niche like the regular one");
                        let alt_head_space = alt_niche.offset.bytes();
                        let alt_niche_len = alt_niche.value.size(dl).bytes();
                        let alt_tail_space =
                            alt_layout.size.bytes() - alt_head_space - alt_niche_len;

                        debug_assert_eq!(layout.size.bytes(), alt_layout.size.bytes());

                        let prefer_alt_layout =
                            alt_head_space > head_space && alt_head_space > tail_space;

                        debug!(
                            "sz: {}, default_niche_at: {}+{}, default_tail_space: {}, alt_niche_at/head_space: {}+{}, alt_tail: {}, num_fields: {}, better: {}\n\
                            layout: {}\n\
                            alt_layout: {}\n",
                            layout.size.bytes(),
                            head_space,
                            niche_len,
                            tail_space,
                            alt_head_space,
                            alt_niche_len,
                            alt_tail_space,
                            layout.fields.count(),
                            prefer_alt_layout,
                            format_field_niches(layout, fields, dl),
                            format_field_niches(&alt_layout, fields, dl),
                        );

                        if prefer_alt_layout {
                            return Some(alt_layout);
                        }
                    }
                }
            }
        }
        layout
    }

    fn layout_of_never_type<FieldIdx: Idx, VariantIdx: Idx>(
        &self,
    ) -> LayoutS<FieldIdx, VariantIdx> {
        let dl = self.current_data_layout();
        let dl = dl.borrow();
        LayoutS {
            variants: Variants::Single { index: VariantIdx::new(0) },
            fields: FieldsShape::Primitive,
            abi: Abi::Uninhabited,
            largest_niche: None,
            align: dl.i8_align,
            size: Size::ZERO,
            max_repr_align: None,
            unadjusted_abi_align: dl.i8_align.abi,
        }
    }

    fn layout_of_struct_or_enum<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
    >(
        &self,
        repr: &ReprOptions,
        variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
        is_enum: bool,
        is_unsafe_cell: bool,
        scalar_valid_range: (Bound<u128>, Bound<u128>),
        discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
        discriminants: impl Iterator<Item = (VariantIdx, i128)>,
        dont_niche_optimize_enum: bool,
        always_sized: bool,
    ) -> Option<LayoutS<FieldIdx, VariantIdx>> {
        let dl = self.current_data_layout();
        let dl = dl.borrow();

        let (present_first, present_second) = {
            let mut present_variants = variants
                .iter_enumerated()
                .filter_map(|(i, v)| if absent(v) { None } else { Some(i) });
            (present_variants.next(), present_variants.next())
        };
        let present_first = match present_first {
            Some(present_first) => present_first,
            // Uninhabited because it has no variants, or only absent ones.
            None if is_enum => {
                return Some(self.layout_of_never_type());
            }
            // If it's a struct, still compute a layout so that we can still compute the
            // field offsets.
            None => VariantIdx::new(0),
        };

        // take the struct path if it is an actual struct
        if !is_enum ||
            // or for optimizing univariant enums
            (present_second.is_none() && !repr.inhibit_enum_layout_opt())
        {
            layout_of_struct(
                self,
                repr,
                variants,
                is_enum,
                is_unsafe_cell,
                scalar_valid_range,
                always_sized,
                dl,
                present_first,
            )
        } else {
            // At this point, we have handled all unions and
            // structs. (We have also handled univariant enums
            // that allow representation optimization.)
            assert!(is_enum);
            layout_of_enum(
                self,
                repr,
                variants,
                discr_range_of_repr,
                discriminants,
                dont_niche_optimize_enum,
                dl,
            )
        }
    }

    fn layout_of_union<
        'a,
        FieldIdx: Idx,
        VariantIdx: Idx,
        F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
    >(
        &self,
        repr: &ReprOptions,
        variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
    ) -> Option<LayoutS<FieldIdx, VariantIdx>> {
        let dl = self.current_data_layout();
        let dl = dl.borrow();
        let mut align = if repr.pack.is_some() { dl.i8_align } else { dl.aggregate_align };
        let mut max_repr_align = repr.align;

        // If all the non-ZST fields have the same ABI and union ABI optimizations aren't
        // disabled, we can use that common ABI for the union as a whole.
        struct AbiMismatch;
        let mut common_non_zst_abi_and_align = if repr.inhibit_union_abi_opt() {
            // Can't optimize
            Err(AbiMismatch)
        } else {
            Ok(None)
        };

        let mut size = Size::ZERO;
        let only_variant = &variants[VariantIdx::new(0)];
        for field in only_variant {
            if field.is_unsized() {
                self.delayed_bug("unsized field in union".to_string());
            }

            align = align.max(field.align);
            max_repr_align = max_repr_align.max(field.max_repr_align);
            size = cmp::max(size, field.size);

            if field.is_zst() {
                // Nothing more to do for ZST fields
                continue;
            }

            if let Ok(common) = common_non_zst_abi_and_align {
                // Discard valid range information and allow undef
                let field_abi = field.abi.to_union();

                if let Some((common_abi, common_align)) = common {
                    if common_abi != field_abi {
                        // Different fields have different ABI: disable opt
                        common_non_zst_abi_and_align = Err(AbiMismatch);
                    } else {
                        // Fields with the same non-Aggregate ABI should also
                        // have the same alignment
                        if !matches!(common_abi, Abi::Aggregate { .. }) {
                            assert_eq!(
                                common_align, field.align.abi,
                                "non-Aggregate field with matching ABI but differing alignment"
                            );
                        }
                    }
                } else {
                    // First non-ZST field: record its ABI and alignment
                    common_non_zst_abi_and_align = Ok(Some((field_abi, field.align.abi)));
                }
            }
        }

        if let Some(pack) = repr.pack {
            align = align.min(AbiAndPrefAlign::new(pack));
        }
        // The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
        // See documentation on `LayoutS::unadjusted_abi_align`.
        let unadjusted_abi_align = align.abi;
        if let Some(repr_align) = repr.align {
            align = align.max(AbiAndPrefAlign::new(repr_align));
        }
        // `align` must not be modified after this, or `unadjusted_abi_align` could be inaccurate.
        let align = align;

        // If all non-ZST fields have the same ABI, we may forward that ABI
        // for the union as a whole, unless otherwise inhibited.
        let abi = match common_non_zst_abi_and_align {
            Err(AbiMismatch) | Ok(None) => Abi::Aggregate { sized: true },
            Ok(Some((abi, _))) => {
                if abi.inherent_align(dl).map(|a| a.abi) != Some(align.abi) {
                    // Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
                    Abi::Aggregate { sized: true }
                } else {
                    abi
                }
            }
        };

        Some(LayoutS {
            variants: Variants::Single { index: VariantIdx::new(0) },
            fields: FieldsShape::Union(NonZeroUsize::new(only_variant.len())?),
            abi,
            largest_niche: None,
            align,
            size: size.align_to(align.abi),
            max_repr_align,
            unadjusted_abi_align,
        })
    }
}

/// single-variant enums are just structs, if you think about it
fn layout_of_struct<'a, LC, FieldIdx: Idx, VariantIdx: Idx, F>(
    layout_calc: &LC,
    repr: &ReprOptions,
    variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
    is_enum: bool,
    is_unsafe_cell: bool,
    scalar_valid_range: (Bound<u128>, Bound<u128>),
    always_sized: bool,
    dl: &TargetDataLayout,
    present_first: VariantIdx,
) -> Option<LayoutS<FieldIdx, VariantIdx>>
where
    LC: LayoutCalculator + ?Sized,
    F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
{
    // Struct, or univariant enum equivalent to a struct.
    // (Typechecking will reject discriminant-sizing attrs.)

    let v = present_first;
    let kind = if is_enum || variants[v].is_empty() || always_sized {
        StructKind::AlwaysSized
    } else {
        StructKind::MaybeUnsized
    };

    let mut st = layout_calc.univariant(dl, &variants[v], repr, kind)?;
    st.variants = Variants::Single { index: v };

    if is_unsafe_cell {
        let hide_niches = |scalar: &mut _| match scalar {
            Scalar::Initialized { value, valid_range } => {
                *valid_range = WrappingRange::full(value.size(dl))
            }
            // Already doesn't have any niches
            Scalar::Union { .. } => {}
        };
        match &mut st.abi {
            Abi::Uninhabited => {}
            Abi::Scalar(scalar) => hide_niches(scalar),
            Abi::ScalarPair(a, b) => {
                hide_niches(a);
                hide_niches(b);
            }
            Abi::Vector { element, count: _ } => hide_niches(element),
            Abi::Aggregate { sized: _ } => {}
        }
        st.largest_niche = None;
        return Some(st);
    }

    let (start, end) = scalar_valid_range;
    match st.abi {
        Abi::Scalar(ref mut scalar) | Abi::ScalarPair(ref mut scalar, _) => {
            // Enlarging validity ranges would result in missed
            // optimizations, *not* wrongly assuming the inner
            // value is valid. e.g. unions already enlarge validity ranges,
            // because the values may be uninitialized.
            //
            // Because of that we only check that the start and end
            // of the range is representable with this scalar type.

            let max_value = scalar.size(dl).unsigned_int_max();
            if let Bound::Included(start) = start {
                // FIXME(eddyb) this might be incorrect - it doesn't
                // account for wrap-around (end < start) ranges.
                assert!(start <= max_value, "{start} > {max_value}");
                scalar.valid_range_mut().start = start;
            }
            if let Bound::Included(end) = end {
                // FIXME(eddyb) this might be incorrect - it doesn't
                // account for wrap-around (end < start) ranges.
                assert!(end <= max_value, "{end} > {max_value}");
                scalar.valid_range_mut().end = end;
            }

            // Update `largest_niche` if we have introduced a larger niche.
            let niche = Niche::from_scalar(dl, Size::ZERO, *scalar);
            if let Some(niche) = niche {
                match st.largest_niche {
                    Some(largest_niche) => {
                        // Replace the existing niche even if they're equal,
                        // because this one is at a lower offset.
                        if largest_niche.available(dl) <= niche.available(dl) {
                            st.largest_niche = Some(niche);
                        }
                    }
                    None => st.largest_niche = Some(niche),
                }
            }
        }
        _ => assert!(
            start == Bound::Unbounded && end == Bound::Unbounded,
            "nonscalar layout for layout_scalar_valid_range type: {st:#?}",
        ),
    }

    Some(st)
}

fn layout_of_enum<'a, LC, FieldIdx: Idx, VariantIdx: Idx, F>(
    layout_calc: &LC,
    repr: &ReprOptions,
    variants: &IndexSlice<VariantIdx, IndexVec<FieldIdx, F>>,
    discr_range_of_repr: impl Fn(i128, i128) -> (Integer, bool),
    discriminants: impl Iterator<Item = (VariantIdx, i128)>,
    dont_niche_optimize_enum: bool,
    dl: &TargetDataLayout,
) -> Option<LayoutS<FieldIdx, VariantIdx>>
where
    LC: LayoutCalculator + ?Sized,
    F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
{
    // Until we've decided whether to use the tagged or
    // niche filling LayoutS, we don't want to intern the
    // variant layouts, so we can't store them in the
    // overall LayoutS. Store the overall LayoutS
    // and the variant LayoutSs here until then.
    struct TmpLayout<FieldIdx: Idx, VariantIdx: Idx> {
        layout: LayoutS<FieldIdx, VariantIdx>,
        variants: IndexVec<VariantIdx, LayoutS<FieldIdx, VariantIdx>>,
    }

    let calculate_niche_filling_layout = || -> Option<TmpLayout<FieldIdx, VariantIdx>> {
        if dont_niche_optimize_enum {
            return None;
        }

        if variants.len() < 2 {
            return None;
        }

        let mut align = dl.aggregate_align;
        let mut max_repr_align = repr.align;
        let mut unadjusted_abi_align = align.abi;

        let mut variant_layouts = variants
            .iter_enumerated()
            .map(|(j, v)| {
                let mut st = layout_calc.univariant(dl, v, repr, StructKind::AlwaysSized)?;
                st.variants = Variants::Single { index: j };

                align = align.max(st.align);
                max_repr_align = max_repr_align.max(st.max_repr_align);
                unadjusted_abi_align = unadjusted_abi_align.max(st.unadjusted_abi_align);

                Some(st)
            })
            .collect::<Option<IndexVec<VariantIdx, _>>>()?;

        let largest_variant_index = variant_layouts
            .iter_enumerated()
            .max_by_key(|(_i, layout)| layout.size.bytes())
            .map(|(i, _layout)| i)?;

        let all_indices = variants.indices();
        let needs_disc =
            |index: VariantIdx| index != largest_variant_index && !absent(&variants[index]);
        let niche_variants = all_indices.clone().find(|v| needs_disc(*v)).unwrap()
            ..=all_indices.rev().find(|v| needs_disc(*v)).unwrap();

        let count =
            (niche_variants.end().index() as u128 - niche_variants.start().index() as u128) + 1;

        // Find the field with the largest niche
        let (field_index, niche, (niche_start, niche_scalar)) = variants[largest_variant_index]
            .iter()
            .enumerate()
            .filter_map(|(j, field)| Some((j, field.largest_niche?)))
            .max_by_key(|(_, niche)| niche.available(dl))
            .and_then(|(j, niche)| Some((j, niche, niche.reserve(dl, count)?)))?;
        let niche_offset =
            niche.offset + variant_layouts[largest_variant_index].fields.offset(field_index);
        let niche_size = niche.value.size(dl);
        let size = variant_layouts[largest_variant_index].size.align_to(align.abi);

        let all_variants_fit = variant_layouts.iter_enumerated_mut().all(|(i, layout)| {
            if i == largest_variant_index {
                return true;
            }

            layout.largest_niche = None;

            if layout.size <= niche_offset {
                // This variant will fit before the niche.
                return true;
            }

            // Determine if it'll fit after the niche.
            let this_align = layout.align.abi;
            let this_offset = (niche_offset + niche_size).align_to(this_align);

            if this_offset + layout.size > size {
                return false;
            }

            // It'll fit, but we need to make some adjustments.
            match layout.fields {
                FieldsShape::Arbitrary { ref mut offsets, .. } => {
                    for offset in offsets.iter_mut() {
                        *offset += this_offset;
                    }
                }
                FieldsShape::Primitive | FieldsShape::Array { .. } | FieldsShape::Union(..) => {
                    panic!("Layout of fields should be Arbitrary for variants")
                }
            }

            // It can't be a Scalar or ScalarPair because the offset isn't 0.
            if !layout.abi.is_uninhabited() {
                layout.abi = Abi::Aggregate { sized: true };
            }
            layout.size += this_offset;

            true
        });

        if !all_variants_fit {
            return None;
        }

        let largest_niche = Niche::from_scalar(dl, niche_offset, niche_scalar);

        let others_zst = variant_layouts
            .iter_enumerated()
            .all(|(i, layout)| i == largest_variant_index || layout.size == Size::ZERO);
        let same_size = size == variant_layouts[largest_variant_index].size;
        let same_align = align == variant_layouts[largest_variant_index].align;

        let abi = if variant_layouts.iter().all(|v| v.abi.is_uninhabited()) {
            Abi::Uninhabited
        } else if same_size && same_align && others_zst {
            match variant_layouts[largest_variant_index].abi {
                // When the total alignment and size match, we can use the
                // same ABI as the scalar variant with the reserved niche.
                Abi::Scalar(_) => Abi::Scalar(niche_scalar),
                Abi::ScalarPair(first, second) => {
                    // Only the niche is guaranteed to be initialised,
                    // so use union layouts for the other primitive.
                    if niche_offset == Size::ZERO {
                        Abi::ScalarPair(niche_scalar, second.to_union())
                    } else {
                        Abi::ScalarPair(first.to_union(), niche_scalar)
                    }
                }
                _ => Abi::Aggregate { sized: true },
            }
        } else {
            Abi::Aggregate { sized: true }
        };

        let layout = LayoutS {
            variants: Variants::Multiple {
                tag: niche_scalar,
                tag_encoding: TagEncoding::Niche {
                    untagged_variant: largest_variant_index,
                    niche_variants,
                    niche_start,
                },
                tag_field: 0,
                variants: IndexVec::new(),
            },
            fields: FieldsShape::Arbitrary {
                offsets: [niche_offset].into(),
                memory_index: [0].into(),
            },
            abi,
            largest_niche,
            size,
            align,
            max_repr_align,
            unadjusted_abi_align,
        };

        Some(TmpLayout { layout, variants: variant_layouts })
    };

    let niche_filling_layout = calculate_niche_filling_layout();

    let (mut min, mut max) = (i128::MAX, i128::MIN);
    let discr_type = repr.discr_type();
    let bits = Integer::from_attr(dl, discr_type).size().bits();
    for (i, mut val) in discriminants {
        if variants[i].iter().any(|f| f.abi.is_uninhabited()) {
            continue;
        }
        if discr_type.is_signed() {
            // sign extend the raw representation to be an i128
            val = (val << (128 - bits)) >> (128 - bits);
        }
        if val < min {
            min = val;
        }
        if val > max {
            max = val;
        }
    }
    // We might have no inhabited variants, so pretend there's at least one.
    if (min, max) == (i128::MAX, i128::MIN) {
        min = 0;
        max = 0;
    }
    assert!(min <= max, "discriminant range is {min}...{max}");
    let (min_ity, signed) = discr_range_of_repr(min, max); //Integer::repr_discr(tcx, ty, &repr, min, max);

    let mut align = dl.aggregate_align;
    let mut max_repr_align = repr.align;
    let mut unadjusted_abi_align = align.abi;

    let mut size = Size::ZERO;

    // We're interested in the smallest alignment, so start large.
    let mut start_align = Align::from_bytes(256).unwrap();
    assert_eq!(Integer::for_align(dl, start_align), None);

    // repr(C) on an enum tells us to make a (tag, union) layout,
    // so we need to grow the prefix alignment to be at least
    // the alignment of the union. (This value is used both for
    // determining the alignment of the overall enum, and the
    // determining the alignment of the payload after the tag.)
    let mut prefix_align = min_ity.align(dl).abi;
    if repr.c() {
        for fields in variants {
            for field in fields {
                prefix_align = prefix_align.max(field.align.abi);
            }
        }
    }

    // Create the set of structs that represent each variant.
    let mut layout_variants = variants
        .iter_enumerated()
        .map(|(i, field_layouts)| {
            let mut st = layout_calc.univariant(
                dl,
                field_layouts,
                repr,
                StructKind::Prefixed(min_ity.size(), prefix_align),
            )?;
            st.variants = Variants::Single { index: i };
            // Find the first field we can't move later
            // to make room for a larger discriminant.
            for field_idx in st.fields.index_by_increasing_offset() {
                let field = &field_layouts[FieldIdx::new(field_idx)];
                if !field.is_1zst() {
                    start_align = start_align.min(field.align.abi);
                    break;
                }
            }
            size = cmp::max(size, st.size);
            align = align.max(st.align);
            max_repr_align = max_repr_align.max(st.max_repr_align);
            unadjusted_abi_align = unadjusted_abi_align.max(st.unadjusted_abi_align);
            Some(st)
        })
        .collect::<Option<IndexVec<VariantIdx, _>>>()?;

    // Align the maximum variant size to the largest alignment.
    size = size.align_to(align.abi);

    // FIXME(oli-obk): deduplicate and harden these checks
    if size.bytes() >= dl.obj_size_bound() {
        return None;
    }

    let typeck_ity = Integer::from_attr(dl, repr.discr_type());
    if typeck_ity < min_ity {
        // It is a bug if Layout decided on a greater discriminant size than typeck for
        // some reason at this point (based on values discriminant can take on). Mostly
        // because this discriminant will be loaded, and then stored into variable of
        // type calculated by typeck. Consider such case (a bug): typeck decided on
        // byte-sized discriminant, but layout thinks we need a 16-bit to store all
        // discriminant values. That would be a bug, because then, in codegen, in order
        // to store this 16-bit discriminant into 8-bit sized temporary some of the
        // space necessary to represent would have to be discarded (or layout is wrong
        // on thinking it needs 16 bits)
        panic!(
            "layout decided on a larger discriminant type ({min_ity:?}) than typeck ({typeck_ity:?})"
        );
        // However, it is fine to make discr type however large (as an optimisation)
        // after this point – we’ll just truncate the value we load in codegen.
    }

    // Check to see if we should use a different type for the
    // discriminant. We can safely use a type with the same size
    // as the alignment of the first field of each variant.
    // We increase the size of the discriminant to avoid LLVM copying
    // padding when it doesn't need to. This normally causes unaligned
    // load/stores and excessive memcpy/memset operations. By using a
    // bigger integer size, LLVM can be sure about its contents and
    // won't be so conservative.

    // Use the initial field alignment
    let mut ity = if repr.c() || repr.int.is_some() {
        min_ity
    } else {
        Integer::for_align(dl, start_align).unwrap_or(min_ity)
    };

    // If the alignment is not larger than the chosen discriminant size,
    // don't use the alignment as the final size.
    if ity <= min_ity {
        ity = min_ity;
    } else {
        // Patch up the variants' first few fields.
        let old_ity_size = min_ity.size();
        let new_ity_size = ity.size();
        for variant in &mut layout_variants {
            match variant.fields {
                FieldsShape::Arbitrary { ref mut offsets, .. } => {
                    for i in offsets {
                        if *i <= old_ity_size {
                            assert_eq!(*i, old_ity_size);
                            *i = new_ity_size;
                        }
                    }
                    // We might be making the struct larger.
                    if variant.size <= old_ity_size {
                        variant.size = new_ity_size;
                    }
                }
                FieldsShape::Primitive | FieldsShape::Array { .. } | FieldsShape::Union(..) => {
                    panic!("encountered a non-arbitrary layout during enum layout")
                }
            }
        }
    }

    let tag_mask = ity.size().unsigned_int_max();
    let tag = Scalar::Initialized {
        value: Primitive::Int(ity, signed),
        valid_range: WrappingRange {
            start: (min as u128 & tag_mask),
            end: (max as u128 & tag_mask),
        },
    };
    let mut abi = Abi::Aggregate { sized: true };

    if layout_variants.iter().all(|v| v.abi.is_uninhabited()) {
        abi = Abi::Uninhabited;
    } else if tag.size(dl) == size {
        // Make sure we only use scalar layout when the enum is entirely its
        // own tag (i.e. it has no padding nor any non-ZST variant fields).
        abi = Abi::Scalar(tag);
    } else {
        // Try to use a ScalarPair for all tagged enums.
        // That's possible only if we can find a common primitive type for all variants.
        let mut common_prim = None;
        let mut common_prim_initialized_in_all_variants = true;
        for (field_layouts, layout_variant) in iter::zip(variants, &layout_variants) {
            let FieldsShape::Arbitrary { ref offsets, .. } = layout_variant.fields else {
                panic!("encountered a non-arbitrary layout during enum layout");
            };
            // We skip *all* ZST here and later check if we are good in terms of alignment.
            // This lets us handle some cases involving aligned ZST.
            let mut fields = iter::zip(field_layouts, offsets).filter(|p| !p.0.is_zst());
            let (field, offset) = match (fields.next(), fields.next()) {
                (None, None) => {
                    common_prim_initialized_in_all_variants = false;
                    continue;
                }
                (Some(pair), None) => pair,
                _ => {
                    common_prim = None;
                    break;
                }
            };
            let prim = match field.abi {
                Abi::Scalar(scalar) => {
                    common_prim_initialized_in_all_variants &=
                        matches!(scalar, Scalar::Initialized { .. });
                    scalar.primitive()
                }
                _ => {
                    common_prim = None;
                    break;
                }
            };
            if let Some((old_prim, common_offset)) = common_prim {
                // All variants must be at the same offset
                if offset != common_offset {
                    common_prim = None;
                    break;
                }
                // This is pretty conservative. We could go fancier
                // by realising that (u8, u8) could just cohabit with
                // u16 or even u32.
                let new_prim = match (old_prim, prim) {
                    // Allow all identical primitives.
                    (x, y) if x == y => x,
                    // Allow integers of the same size with differing signedness.
                    // We arbitrarily choose the signedness of the first variant.
                    (p @ Primitive::Int(x, _), Primitive::Int(y, _)) if x == y => p,
                    // Allow integers mixed with pointers of the same layout.
                    // We must represent this using a pointer, to avoid
                    // roundtripping pointers through ptrtoint/inttoptr.
                    (p @ Primitive::Pointer(_), i @ Primitive::Int(..))
                    | (i @ Primitive::Int(..), p @ Primitive::Pointer(_))
                        if p.size(dl) == i.size(dl) && p.align(dl) == i.align(dl) =>
                    {
                        p
                    }
                    _ => {
                        common_prim = None;
                        break;
                    }
                };
                // We may be updating the primitive here, for example from int->ptr.
                common_prim = Some((new_prim, common_offset));
            } else {
                common_prim = Some((prim, offset));
            }
        }
        if let Some((prim, offset)) = common_prim {
            let prim_scalar = if common_prim_initialized_in_all_variants {
                let size = prim.size(dl);
                assert!(size.bits() <= 128);
                Scalar::Initialized { value: prim, valid_range: WrappingRange::full(size) }
            } else {
                // Common prim might be uninit.
                Scalar::Union { value: prim }
            };
            let pair = layout_calc.scalar_pair::<FieldIdx, VariantIdx>(tag, prim_scalar);
            let pair_offsets = match pair.fields {
                FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
                    assert_eq!(memory_index.raw, [0, 1]);
                    offsets
                }
                _ => panic!("encountered a non-arbitrary layout during enum layout"),
            };
            if pair_offsets[FieldIdx::new(0)] == Size::ZERO
                && pair_offsets[FieldIdx::new(1)] == *offset
                && align == pair.align
                && size == pair.size
            {
                // We can use `ScalarPair` only when it matches our
                // already computed layout (including `#[repr(C)]`).
                abi = pair.abi;
            }
        }
    }

    // If we pick a "clever" (by-value) ABI, we might have to adjust the ABI of the
    // variants to ensure they are consistent. This is because a downcast is
    // semantically a NOP, and thus should not affect layout.
    if matches!(abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
        for variant in &mut layout_variants {
            // We only do this for variants with fields; the others are not accessed anyway.
            // Also do not overwrite any already existing "clever" ABIs.
            if variant.fields.count() > 0 && matches!(variant.abi, Abi::Aggregate { .. }) {
                variant.abi = abi;
                // Also need to bump up the size and alignment, so that the entire value fits
                // in here.
                variant.size = cmp::max(variant.size, size);
                variant.align.abi = cmp::max(variant.align.abi, align.abi);
            }
        }
    }

    let largest_niche = Niche::from_scalar(dl, Size::ZERO, tag);

    let tagged_layout = LayoutS {
        variants: Variants::Multiple {
            tag,
            tag_encoding: TagEncoding::Direct,
            tag_field: 0,
            variants: IndexVec::new(),
        },
        fields: FieldsShape::Arbitrary { offsets: [Size::ZERO].into(), memory_index: [0].into() },
        largest_niche,
        abi,
        align,
        size,
        max_repr_align,
        unadjusted_abi_align,
    };

    let tagged_layout = TmpLayout { layout: tagged_layout, variants: layout_variants };

    let mut best_layout = match (tagged_layout, niche_filling_layout) {
        (tl, Some(nl)) => {
            // Pick the smaller layout; otherwise,
            // pick the layout with the larger niche; otherwise,
            // pick tagged as it has simpler codegen.
            use cmp::Ordering::*;
            let niche_size = |tmp_l: &TmpLayout<FieldIdx, VariantIdx>| {
                tmp_l.layout.largest_niche.map_or(0, |n| n.available(dl))
            };
            match (tl.layout.size.cmp(&nl.layout.size), niche_size(&tl).cmp(&niche_size(&nl))) {
                (Greater, _) => nl,
                (Equal, Less) => nl,
                _ => tl,
            }
        }
        (tl, None) => tl,
    };

    // Now we can intern the variant layouts and store them in the enum layout.
    best_layout.layout.variants = match best_layout.layout.variants {
        Variants::Multiple { tag, tag_encoding, tag_field, .. } => {
            Variants::Multiple { tag, tag_encoding, tag_field, variants: best_layout.variants }
        }
        Variants::Single { .. } => {
            panic!("encountered a single-variant enum during multi-variant layout")
        }
    };
    Some(best_layout.layout)
}

/// Determines towards which end of a struct layout optimizations will try to place the best niches.
enum NicheBias {
    Start,
    End,
}

fn univariant<
    'a,
    FieldIdx: Idx,
    VariantIdx: Idx,
    F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
>(
    this: &(impl LayoutCalculator + ?Sized),
    dl: &TargetDataLayout,
    fields: &IndexSlice<FieldIdx, F>,
    repr: &ReprOptions,
    kind: StructKind,
    niche_bias: NicheBias,
) -> Option<LayoutS<FieldIdx, VariantIdx>> {
    let pack = repr.pack;
    let mut align = if pack.is_some() { dl.i8_align } else { dl.aggregate_align };
    let mut max_repr_align = repr.align;
    let mut inverse_memory_index: IndexVec<u32, FieldIdx> = fields.indices().collect();
    let optimize = !repr.inhibit_struct_field_reordering_opt();
    if optimize && fields.len() > 1 {
        let end = if let StructKind::MaybeUnsized = kind { fields.len() - 1 } else { fields.len() };
        let optimizing = &mut inverse_memory_index.raw[..end];
        let fields_excluding_tail = &fields.raw[..end];

        // If `-Z randomize-layout` was enabled for the type definition we can shuffle
        // the field ordering to try and catch some code making assumptions about layouts
        // we don't guarantee.
        if repr.can_randomize_type_layout() && cfg!(feature = "randomize") {
            #[cfg(feature = "randomize")]
            {
                use rand::{seq::SliceRandom, SeedableRng};
                // `ReprOptions.field_shuffle_seed` is a deterministic seed we can use to randomize field
                // ordering.
                let mut rng =
                    rand_xoshiro::Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);

                // Shuffle the ordering of the fields.
                optimizing.shuffle(&mut rng);
            }
            // Otherwise we just leave things alone and actually optimize the type's fields
        } else {
            // To allow unsizing `&Foo<Type>` -> `&Foo<dyn Trait>`, the layout of the struct must
            // not depend on the layout of the tail.
            let max_field_align =
                fields_excluding_tail.iter().map(|f| f.align.abi.bytes()).max().unwrap_or(1);
            let largest_niche_size = fields_excluding_tail
                .iter()
                .filter_map(|f| f.largest_niche)
                .map(|n| n.available(dl))
                .max()
                .unwrap_or(0);

            // Calculates a sort key to group fields by their alignment or possibly some
            // size-derived pseudo-alignment.
            let alignment_group_key = |layout: &F| {
                if let Some(pack) = pack {
                    // Return the packed alignment in bytes.
                    layout.align.abi.min(pack).bytes()
                } else {
                    // Returns `log2(effective-align)`. This is ok since `pack` applies to all
                    // fields equally. The calculation assumes that size is an integer multiple of
                    // align, except for ZSTs.
                    let align = layout.align.abi.bytes();
                    let size = layout.size.bytes();
                    let niche_size = layout.largest_niche.map(|n| n.available(dl)).unwrap_or(0);
                    // Group [u8; 4] with align-4 or [u8; 6] with align-2 fields.
                    let size_as_align = align.max(size).trailing_zeros();
                    let size_as_align = if largest_niche_size > 0 {
                        match niche_bias {
                            // Given `A(u8, [u8; 16])` and `B(bool, [u8; 16])` we want to bump the
                            // array to the front in the first case (for aligned loads) but keep
                            // the bool in front in the second case for its niches.
                            NicheBias::Start => max_field_align.trailing_zeros().min(size_as_align),
                            // When moving niches towards the end of the struct then for
                            // A((u8, u8, u8, bool), (u8, bool, u8)) we want to keep the first tuple
                            // in the align-1 group because its bool can be moved closer to the end.
                            NicheBias::End if niche_size == largest_niche_size => {
                                align.trailing_zeros()
                            }
                            NicheBias::End => size_as_align,
                        }
                    } else {
                        size_as_align
                    };
                    size_as_align as u64
                }
            };

            match kind {
                StructKind::AlwaysSized | StructKind::MaybeUnsized => {
                    // Currently `LayoutS` only exposes a single niche so sorting is usually
                    // sufficient to get one niche into the preferred position. If it ever
                    // supported multiple niches then a more advanced pick-and-pack approach could
                    // provide better results. But even for the single-niche cache it's not
                    // optimal. E.g. for A(u32, (bool, u8), u16) it would be possible to move the
                    // bool to the front but it would require packing the tuple together with the
                    // u16 to build a 4-byte group so that the u32 can be placed after it without
                    // padding. This kind of packing can't be achieved by sorting.
                    optimizing.sort_by_key(|&x| {
                        let f = &fields[x];
                        let field_size = f.size.bytes();
                        let niche_size = f.largest_niche.map_or(0, |n| n.available(dl));
                        let niche_size_key = match niche_bias {
                            // large niche first
                            NicheBias::Start => !niche_size,
                            // large niche last
                            NicheBias::End => niche_size,
                        };
                        let inner_niche_offset_key = match niche_bias {
                            NicheBias::Start => f.largest_niche.map_or(0, |n| n.offset.bytes()),
                            NicheBias::End => f.largest_niche.map_or(0, |n| {
                                !(field_size - n.value.size(dl).bytes() - n.offset.bytes())
                            }),
                        };

                        (
                            // Then place largest alignments first.
                            cmp::Reverse(alignment_group_key(f)),
                            // Then prioritize niche placement within alignment group according to
                            // `niche_bias_start`.
                            niche_size_key,
                            // Then among fields with equally-sized niches prefer the ones
                            // closer to the start/end of the field.
                            inner_niche_offset_key,
                        )
                    });
                }

                StructKind::Prefixed(..) => {
                    // Sort in ascending alignment so that the layout stays optimal
                    // regardless of the prefix.
                    // And put the largest niche in an alignment group at the end
                    // so it can be used as discriminant in jagged enums
                    optimizing.sort_by_key(|&x| {
                        let f = &fields[x];
                        let niche_size = f.largest_niche.map_or(0, |n| n.available(dl));
                        (alignment_group_key(f), niche_size)
                    });
                }
            }

            // FIXME(Kixiron): We can always shuffle fields within a given alignment class
            //                 regardless of the status of `-Z randomize-layout`
        }
    }
    // inverse_memory_index holds field indices by increasing memory offset.
    // That is, if field 5 has offset 0, the first element of inverse_memory_index is 5.
    // We now write field offsets to the corresponding offset slot;
    // field 5 with offset 0 puts 0 in offsets[5].
    // At the bottom of this function, we invert `inverse_memory_index` to
    // produce `memory_index` (see `invert_mapping`).
    let mut sized = true;
    let mut offsets = IndexVec::from_elem(Size::ZERO, fields);
    let mut offset = Size::ZERO;
    let mut largest_niche = None;
    let mut largest_niche_available = 0;
    if let StructKind::Prefixed(prefix_size, prefix_align) = kind {
        let prefix_align =
            if let Some(pack) = pack { prefix_align.min(pack) } else { prefix_align };
        align = align.max(AbiAndPrefAlign::new(prefix_align));
        offset = prefix_size.align_to(prefix_align);
    }
    for &i in &inverse_memory_index {
        let field = &fields[i];
        if !sized {
            this.delayed_bug(format!(
                "univariant: field #{} comes after unsized field",
                offsets.len(),
            ));
        }

        if field.is_unsized() {
            sized = false;
        }

        // Invariant: offset < dl.obj_size_bound() <= 1<<61
        let field_align = if let Some(pack) = pack {
            field.align.min(AbiAndPrefAlign::new(pack))
        } else {
            field.align
        };
        offset = offset.align_to(field_align.abi);
        align = align.max(field_align);
        max_repr_align = max_repr_align.max(field.max_repr_align);

        debug!("univariant offset: {:?} field: {:#?}", offset, field);
        offsets[i] = offset;

        if let Some(mut niche) = field.largest_niche {
            let available = niche.available(dl);
            // Pick up larger niches.
            let prefer_new_niche = match niche_bias {
                NicheBias::Start => available > largest_niche_available,
                // if there are several niches of the same size then pick the last one
                NicheBias::End => available >= largest_niche_available,
            };
            if prefer_new_niche {
                largest_niche_available = available;
                niche.offset += offset;
                largest_niche = Some(niche);
            }
        }

        offset = offset.checked_add(field.size, dl)?;
    }

    // The unadjusted ABI alignment does not include repr(align), but does include repr(pack).
    // See documentation on `LayoutS::unadjusted_abi_align`.
    let unadjusted_abi_align = align.abi;
    if let Some(repr_align) = repr.align {
        align = align.max(AbiAndPrefAlign::new(repr_align));
    }
    // `align` must not be modified after this point, or `unadjusted_abi_align` could be inaccurate.
    let align = align;

    debug!("univariant min_size: {:?}", offset);
    let min_size = offset;
    // As stated above, inverse_memory_index holds field indices by increasing offset.
    // This makes it an already-sorted view of the offsets vec.
    // To invert it, consider:
    // If field 5 has offset 0, offsets[0] is 5, and memory_index[5] should be 0.
    // Field 5 would be the first element, so memory_index is i:
    // Note: if we didn't optimize, it's already right.
    let memory_index = if optimize {
        inverse_memory_index.invert_bijective_mapping()
    } else {
        debug_assert!(inverse_memory_index.iter().copied().eq(fields.indices()));
        inverse_memory_index.into_iter().map(|it| it.index() as u32).collect()
    };
    let size = min_size.align_to(align.abi);
    // FIXME(oli-obk): deduplicate and harden these checks
    if size.bytes() >= dl.obj_size_bound() {
        return None;
    }
    let mut layout_of_single_non_zst_field = None;
    let mut abi = Abi::Aggregate { sized };
    // Try to make this a Scalar/ScalarPair.
    if sized && size.bytes() > 0 {
        // We skip *all* ZST here and later check if we are good in terms of alignment.
        // This lets us handle some cases involving aligned ZST.
        let mut non_zst_fields = fields.iter_enumerated().filter(|&(_, f)| !f.is_zst());

        match (non_zst_fields.next(), non_zst_fields.next(), non_zst_fields.next()) {
            // We have exactly one non-ZST field.
            (Some((i, field)), None, None) => {
                layout_of_single_non_zst_field = Some(field);

                // Field fills the struct and it has a scalar or scalar pair ABI.
                if offsets[i].bytes() == 0 && align.abi == field.align.abi && size == field.size {
                    match field.abi {
                        // For plain scalars, or vectors of them, we can't unpack
                        // newtypes for `#[repr(C)]`, as that affects C ABIs.
                        Abi::Scalar(_) | Abi::Vector { .. } if optimize => {
                            abi = field.abi;
                        }
                        // But scalar pairs are Rust-specific and get
                        // treated as aggregates by C ABIs anyway.
                        Abi::ScalarPair(..) => {
                            abi = field.abi;
                        }
                        _ => {}
                    }
                }
            }

            // Two non-ZST fields, and they're both scalars.
            (Some((i, a)), Some((j, b)), None) => {
                match (a.abi, b.abi) {
                    (Abi::Scalar(a), Abi::Scalar(b)) => {
                        // Order by the memory placement, not source order.
                        let ((i, a), (j, b)) = if offsets[i] < offsets[j] {
                            ((i, a), (j, b))
                        } else {
                            ((j, b), (i, a))
                        };
                        let pair = this.scalar_pair::<FieldIdx, VariantIdx>(a, b);
                        let pair_offsets = match pair.fields {
                            FieldsShape::Arbitrary { ref offsets, ref memory_index } => {
                                assert_eq!(memory_index.raw, [0, 1]);
                                offsets
                            }
                            FieldsShape::Primitive
                            | FieldsShape::Array { .. }
                            | FieldsShape::Union(..) => {
                                panic!("encountered a non-arbitrary layout during enum layout")
                            }
                        };
                        if offsets[i] == pair_offsets[FieldIdx::new(0)]
                            && offsets[j] == pair_offsets[FieldIdx::new(1)]
                            && align == pair.align
                            && size == pair.size
                        {
                            // We can use `ScalarPair` only when it matches our
                            // already computed layout (including `#[repr(C)]`).
                            abi = pair.abi;
                        }
                    }
                    _ => {}
                }
            }

            _ => {}
        }
    }
    if fields.iter().any(|f| f.abi.is_uninhabited()) {
        abi = Abi::Uninhabited;
    }

    let unadjusted_abi_align = if repr.transparent() {
        match layout_of_single_non_zst_field {
            Some(l) => l.unadjusted_abi_align,
            None => {
                // `repr(transparent)` with all ZST fields.
                align.abi
            }
        }
    } else {
        unadjusted_abi_align
    };

    Some(LayoutS {
        variants: Variants::Single { index: VariantIdx::new(0) },
        fields: FieldsShape::Arbitrary { offsets, memory_index },
        abi,
        largest_niche,
        align,
        size,
        max_repr_align,
        unadjusted_abi_align,
    })
}

fn format_field_niches<
    'a,
    FieldIdx: Idx,
    VariantIdx: Idx,
    F: Deref<Target = &'a LayoutS<FieldIdx, VariantIdx>> + fmt::Debug,
>(
    layout: &LayoutS<FieldIdx, VariantIdx>,
    fields: &IndexSlice<FieldIdx, F>,
    dl: &TargetDataLayout,
) -> String {
    let mut s = String::new();
    for i in layout.fields.index_by_increasing_offset() {
        let offset = layout.fields.offset(i);
        let f = &fields[FieldIdx::new(i)];
        write!(s, "[o{}a{}s{}", offset.bytes(), f.align.abi.bytes(), f.size.bytes()).unwrap();
        if let Some(n) = f.largest_niche {
            write!(
                s,
                " n{}b{}s{}",
                n.offset.bytes(),
                n.available(dl).ilog2(),
                n.value.size(dl).bytes()
            )
            .unwrap();
        }
        write!(s, "] ").unwrap();
    }
    s
}