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
//! Lexical region resolution.

use crate::infer::region_constraints::Constraint;
use crate::infer::region_constraints::GenericKind;
use crate::infer::region_constraints::RegionConstraintData;
use crate::infer::region_constraints::VarInfos;
use crate::infer::region_constraints::VerifyBound;
use crate::infer::RegionRelations;
use crate::infer::RegionVariableOrigin;
use crate::infer::SubregionOrigin;
use rustc_data_structures::fx::FxHashSet;
use rustc_data_structures::graph::implementation::{
    Direction, Graph, NodeIndex, INCOMING, OUTGOING,
};
use rustc_data_structures::intern::Interned;
use rustc_index::{IndexSlice, IndexVec};
use rustc_middle::ty::fold::TypeFoldable;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{ReBound, RePlaceholder, ReVar};
use rustc_middle::ty::{ReEarlyParam, ReErased, ReError, ReLateParam, ReStatic};
use rustc_middle::ty::{Region, RegionVid};
use rustc_span::Span;
use std::fmt;

use super::outlives::test_type_match;

/// This function performs lexical region resolution given a complete
/// set of constraints and variable origins. It performs a fixed-point
/// iteration to find region values which satisfy all constraints,
/// assuming such values can be found. It returns the final values of
/// all the variables as well as a set of errors that must be reported.
#[instrument(level = "debug", skip(region_rels, var_infos, data))]
pub(crate) fn resolve<'tcx>(
    region_rels: &RegionRelations<'_, 'tcx>,
    var_infos: VarInfos,
    data: RegionConstraintData<'tcx>,
) -> (LexicalRegionResolutions<'tcx>, Vec<RegionResolutionError<'tcx>>) {
    let mut errors = vec![];
    let mut resolver = LexicalResolver { region_rels, var_infos, data };
    let values = resolver.infer_variable_values(&mut errors);
    (values, errors)
}

/// Contains the result of lexical region resolution. Offers methods
/// to lookup up the final value of a region variable.
#[derive(Clone)]
pub struct LexicalRegionResolutions<'tcx> {
    pub(crate) values: IndexVec<RegionVid, VarValue<'tcx>>,
}

#[derive(Copy, Clone, Debug)]
pub(crate) enum VarValue<'tcx> {
    /// Empty lifetime is for data that is never accessed. We tag the
    /// empty lifetime with a universe -- the idea is that we don't
    /// want `exists<'a> { forall<'b> { 'b: 'a } }` to be satisfiable.
    /// Therefore, the `'empty` in a universe `U` is less than all
    /// regions visible from `U`, but not less than regions not visible
    /// from `U`.
    Empty(ty::UniverseIndex),
    Value(Region<'tcx>),
    ErrorValue,
}

#[derive(Clone, Debug)]
pub enum RegionResolutionError<'tcx> {
    /// `ConcreteFailure(o, a, b)`:
    ///
    /// `o` requires that `a <= b`, but this does not hold
    ConcreteFailure(SubregionOrigin<'tcx>, Region<'tcx>, Region<'tcx>),

    /// `GenericBoundFailure(p, s, a)`:
    ///
    /// The parameter/associated-type `p` must be known to outlive the lifetime
    /// `a` (but none of the known bounds are sufficient).
    GenericBoundFailure(SubregionOrigin<'tcx>, GenericKind<'tcx>, Region<'tcx>),

    /// `SubSupConflict(v, v_origin, sub_origin, sub_r, sup_origin, sup_r)`:
    ///
    /// Could not infer a value for `v` (which has origin `v_origin`)
    /// because `sub_r <= v` (due to `sub_origin`) but `v <= sup_r` (due to `sup_origin`) and
    /// `sub_r <= sup_r` does not hold.
    SubSupConflict(
        RegionVid,
        RegionVariableOrigin,
        SubregionOrigin<'tcx>,
        Region<'tcx>,
        SubregionOrigin<'tcx>,
        Region<'tcx>,
        Vec<Span>, // All the influences on a given value that didn't meet its constraints.
    ),

    /// Indicates a `'b: 'a` constraint where `'a` is in a universe that
    /// cannot name the placeholder `'b`.
    UpperBoundUniverseConflict(
        RegionVid,
        RegionVariableOrigin,
        ty::UniverseIndex,     // the universe index of the region variable
        SubregionOrigin<'tcx>, // cause of the constraint
        Region<'tcx>,          // the placeholder `'b`
    ),

    CannotNormalize(ty::PolyTypeOutlivesPredicate<'tcx>, SubregionOrigin<'tcx>),
}

impl<'tcx> RegionResolutionError<'tcx> {
    pub fn origin(&self) -> &SubregionOrigin<'tcx> {
        match self {
            RegionResolutionError::ConcreteFailure(origin, _, _)
            | RegionResolutionError::GenericBoundFailure(origin, _, _)
            | RegionResolutionError::SubSupConflict(_, _, origin, _, _, _, _)
            | RegionResolutionError::UpperBoundUniverseConflict(_, _, _, origin, _)
            | RegionResolutionError::CannotNormalize(_, origin) => origin,
        }
    }
}

struct RegionAndOrigin<'tcx> {
    region: Region<'tcx>,
    origin: SubregionOrigin<'tcx>,
}

type RegionGraph<'tcx> = Graph<(), Constraint<'tcx>>;

struct LexicalResolver<'cx, 'tcx> {
    region_rels: &'cx RegionRelations<'cx, 'tcx>,
    var_infos: VarInfos,
    data: RegionConstraintData<'tcx>,
}

impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.region_rels.tcx
    }

    fn infer_variable_values(
        &mut self,
        errors: &mut Vec<RegionResolutionError<'tcx>>,
    ) -> LexicalRegionResolutions<'tcx> {
        let mut var_data = self.construct_var_data();

        // Deduplicating constraints is shown to have a positive perf impact.
        self.data.constraints.sort_by_key(|(constraint, _)| *constraint);
        self.data.constraints.dedup_by_key(|(constraint, _)| *constraint);

        if cfg!(debug_assertions) {
            self.dump_constraints();
        }

        self.expansion(&mut var_data);
        self.collect_errors(&mut var_data, errors);
        self.collect_var_errors(&var_data, errors);
        var_data
    }

    fn num_vars(&self) -> usize {
        self.var_infos.len()
    }

    /// Initially, the value for all variables is set to `'empty`, the
    /// empty region. The `expansion` phase will grow this larger.
    fn construct_var_data(&self) -> LexicalRegionResolutions<'tcx> {
        LexicalRegionResolutions {
            values: IndexVec::from_fn_n(
                |vid| {
                    let vid_universe = self.var_infos[vid].universe;
                    VarValue::Empty(vid_universe)
                },
                self.num_vars(),
            ),
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn dump_constraints(&self) {
        for (idx, (constraint, _)) in self.data.constraints.iter().enumerate() {
            debug!("Constraint {} => {:?}", idx, constraint);
        }
    }

    fn expansion(&self, var_values: &mut LexicalRegionResolutions<'tcx>) {
        // In the first pass, we expand region vids according to constraints we
        // have previously found. In the second pass, we loop through the region
        // vids we expanded and expand *across* region vids (effectively
        // "expanding" new `RegSubVar` constraints).

        // Tracks the `VarSubVar` constraints generated for each region vid. We
        // later use this to expand across vids.
        let mut constraints = IndexVec::from_elem(Vec::new(), &var_values.values);
        // Tracks the changed region vids.
        let mut changes = Vec::new();
        for (constraint, _) in &self.data.constraints {
            match *constraint {
                Constraint::RegSubVar(a_region, b_vid) => {
                    let b_data = var_values.value_mut(b_vid);

                    if self.expand_node(a_region, b_vid, b_data) {
                        changes.push(b_vid);
                    }
                }
                Constraint::VarSubVar(a_vid, b_vid) => match *var_values.value(a_vid) {
                    VarValue::ErrorValue => continue,
                    VarValue::Empty(a_universe) => {
                        let b_data = var_values.value_mut(b_vid);

                        let changed = match *b_data {
                            VarValue::Empty(b_universe) => {
                                // Empty regions are ordered according to the universe
                                // they are associated with.
                                let ui = a_universe.min(b_universe);

                                debug!(
                                    "Expanding value of {:?} \
                                    from empty lifetime with universe {:?} \
                                    to empty lifetime with universe {:?}",
                                    b_vid, b_universe, ui
                                );

                                *b_data = VarValue::Empty(ui);
                                true
                            }
                            VarValue::Value(cur_region) => {
                                match *cur_region {
                                    // If this empty region is from a universe that can name the
                                    // placeholder universe, then the LUB is the Placeholder region
                                    // (which is the cur_region). Otherwise, the LUB is the Static
                                    // lifetime.
                                    RePlaceholder(placeholder)
                                        if !a_universe.can_name(placeholder.universe) =>
                                    {
                                        let lub = self.tcx().lifetimes.re_static;
                                        debug!(
                                            "Expanding value of {:?} from {:?} to {:?}",
                                            b_vid, cur_region, lub
                                        );

                                        *b_data = VarValue::Value(lub);
                                        true
                                    }

                                    _ => false,
                                }
                            }

                            VarValue::ErrorValue => false,
                        };

                        if changed {
                            changes.push(b_vid);
                        }
                        match b_data {
                            VarValue::Value(Region(Interned(ReStatic, _)))
                            | VarValue::ErrorValue => (),
                            _ => {
                                constraints[a_vid].push((a_vid, b_vid));
                                constraints[b_vid].push((a_vid, b_vid));
                            }
                        }
                    }
                    VarValue::Value(a_region) => {
                        let b_data = var_values.value_mut(b_vid);

                        if self.expand_node(a_region, b_vid, b_data) {
                            changes.push(b_vid);
                        }
                        match b_data {
                            VarValue::Value(Region(Interned(ReStatic, _)))
                            | VarValue::ErrorValue => (),
                            _ => {
                                constraints[a_vid].push((a_vid, b_vid));
                                constraints[b_vid].push((a_vid, b_vid));
                            }
                        }
                    }
                },
                Constraint::RegSubReg(..) | Constraint::VarSubReg(..) => {
                    // These constraints are checked after expansion
                    // is done, in `collect_errors`.
                    continue;
                }
            }
        }

        while let Some(vid) = changes.pop() {
            constraints[vid].retain(|&(a_vid, b_vid)| {
                let VarValue::Value(a_region) = *var_values.value(a_vid) else {
                    return false;
                };
                let b_data = var_values.value_mut(b_vid);
                if self.expand_node(a_region, b_vid, b_data) {
                    changes.push(b_vid);
                }
                !matches!(
                    b_data,
                    VarValue::Value(Region(Interned(ReStatic, _))) | VarValue::ErrorValue
                )
            });
        }
    }

    /// Expands the value of the region represented with `b_vid` with current
    /// value `b_data` to the lub of `b_data` and `a_region`. The corresponds
    /// with the constraint `'?b: 'a` (`'a <: '?b`), where `'a` is some known
    /// region and `'?b` is some region variable.
    fn expand_node(
        &self,
        a_region: Region<'tcx>,
        b_vid: RegionVid,
        b_data: &mut VarValue<'tcx>,
    ) -> bool {
        debug!("expand_node({:?}, {:?} == {:?})", a_region, b_vid, b_data);

        match *b_data {
            VarValue::Empty(empty_ui) => {
                let lub = match *a_region {
                    RePlaceholder(placeholder) => {
                        // If this empty region is from a universe that can
                        // name the placeholder, then the placeholder is
                        // larger; otherwise, the only ancestor is `'static`.
                        if empty_ui.can_name(placeholder.universe) {
                            ty::Region::new_placeholder(self.tcx(), placeholder)
                        } else {
                            self.tcx().lifetimes.re_static
                        }
                    }

                    _ => a_region,
                };

                debug!("Expanding value of {:?} from empty lifetime to {:?}", b_vid, lub);

                *b_data = VarValue::Value(lub);
                true
            }
            VarValue::Value(cur_region) => {
                // This is a specialized version of the `lub_concrete_regions`
                // check below for a common case, here purely as an
                // optimization.
                let b_universe = self.var_infos[b_vid].universe;

                let mut lub = self.lub_concrete_regions(a_region, cur_region);
                if lub == cur_region {
                    return false;
                }

                // Watch out for `'b: !1` relationships, where the
                // universe of `'b` can't name the placeholder `!1`. In
                // that case, we have to grow `'b` to be `'static` for the
                // relationship to hold. This is obviously a kind of sub-optimal
                // choice -- in the future, when we incorporate a knowledge
                // of the parameter environment, we might be able to find a
                // tighter bound than `'static`.
                //
                // (This might e.g. arise from being asked to prove `for<'a> { 'b: 'a }`.)
                if let ty::RePlaceholder(p) = *lub
                    && b_universe.cannot_name(p.universe)
                {
                    lub = self.tcx().lifetimes.re_static;
                }

                debug!("Expanding value of {:?} from {:?} to {:?}", b_vid, cur_region, lub);

                *b_data = VarValue::Value(lub);
                true
            }

            VarValue::ErrorValue => false,
        }
    }

    /// True if `a <= b`.
    fn sub_region_values(&self, a: VarValue<'tcx>, b: VarValue<'tcx>) -> bool {
        match (a, b) {
            // Error region is `'static`
            (VarValue::ErrorValue, _) | (_, VarValue::ErrorValue) => return true,
            (VarValue::Empty(a_ui), VarValue::Empty(b_ui)) => {
                // Empty regions are ordered according to the universe
                // they are associated with.
                a_ui.min(b_ui) == b_ui
            }
            (VarValue::Value(a), VarValue::Empty(_)) => {
                match *a {
                    // this is always on an error path,
                    // so it doesn't really matter if it's shorter or longer than an empty region
                    ReError(_) => false,

                    ReBound(..) | ReErased => {
                        bug!("cannot relate region: {:?}", a);
                    }

                    ReVar(v_id) => {
                        span_bug!(
                            self.var_infos[v_id].origin.span(),
                            "lub_concrete_regions invoked with non-concrete region: {:?}",
                            a
                        );
                    }

                    ReStatic | ReEarlyParam(_) | ReLateParam(_) => {
                        // nothing lives longer than `'static`

                        // All empty regions are less than early-bound, free,
                        // and scope regions.

                        false
                    }

                    RePlaceholder(_) => {
                        // The LUB is either `a` or `'static`
                        false
                    }
                }
            }
            (VarValue::Empty(a_ui), VarValue::Value(b)) => {
                match *b {
                    // this is always on an error path,
                    // so it doesn't really matter if it's shorter or longer than an empty region
                    ReError(_) => false,

                    ReBound(..) | ReErased => {
                        bug!("cannot relate region: {:?}", b);
                    }

                    ReVar(v_id) => {
                        span_bug!(
                            self.var_infos[v_id].origin.span(),
                            "lub_concrete_regions invoked with non-concrete regions: {:?}",
                            b
                        );
                    }

                    ReStatic | ReEarlyParam(_) | ReLateParam(_) => {
                        // nothing lives longer than `'static`
                        // All empty regions are less than early-bound, late-bound,
                        // and scope regions.
                        true
                    }

                    RePlaceholder(placeholder) => {
                        // If this empty region is from a universe that can
                        // name the placeholder, then the placeholder is
                        // larger; otherwise, the only ancestor is `'static`.
                        return a_ui.can_name(placeholder.universe);
                    }
                }
            }
            (VarValue::Value(a), VarValue::Value(b)) => self.sub_concrete_regions(a, b),
        }
    }

    /// True if `a <= b`, but not defined over inference variables.
    #[instrument(level = "trace", skip(self))]
    fn sub_concrete_regions(&self, a: Region<'tcx>, b: Region<'tcx>) -> bool {
        let tcx = self.tcx();
        let sub_free_regions = |r1, r2| self.region_rels.free_regions.sub_free_regions(tcx, r1, r2);

        // Check for the case where we know that `'b: 'static` -- in that case,
        // `a <= b` for all `a`.
        if b.is_free() && sub_free_regions(tcx.lifetimes.re_static, b) {
            return true;
        }

        // If both `a` and `b` are free, consult the declared
        // relationships. Note that this can be more precise than the
        // `lub` relationship defined below, since sometimes the "lub"
        // is actually the `postdom_upper_bound` (see
        // `TransitiveRelation` for more details).
        if a.is_free() && b.is_free() {
            return sub_free_regions(a, b);
        }

        // For other cases, leverage the LUB code to find the LUB and
        // check if it is equal to `b`.
        self.lub_concrete_regions(a, b) == b
    }

    /// Returns the least-upper-bound of `a` and `b`; i.e., the
    /// smallest region `c` such that `a <= c` and `b <= c`.
    ///
    /// Neither `a` nor `b` may be an inference variable (hence the
    /// term "concrete regions").
    #[instrument(level = "trace", skip(self), ret)]
    fn lub_concrete_regions(&self, a: Region<'tcx>, b: Region<'tcx>) -> Region<'tcx> {
        match (*a, *b) {
            (ReBound(..), _) | (_, ReBound(..)) | (ReErased, _) | (_, ReErased) => {
                bug!("cannot relate region: LUB({:?}, {:?})", a, b);
            }

            (ReVar(v_id), _) | (_, ReVar(v_id)) => {
                span_bug!(
                    self.var_infos[v_id].origin.span(),
                    "lub_concrete_regions invoked with non-concrete \
                     regions: {:?}, {:?}",
                    a,
                    b
                );
            }

            (ReError(_), _) => a,

            (_, ReError(_)) => b,

            (ReStatic, _) | (_, ReStatic) => {
                // nothing lives longer than `'static`
                self.tcx().lifetimes.re_static
            }

            (ReEarlyParam(_) | ReLateParam(_), ReEarlyParam(_) | ReLateParam(_)) => {
                self.region_rels.lub_param_regions(a, b)
            }

            // For these types, we cannot define any additional
            // relationship:
            (RePlaceholder(..), _) | (_, RePlaceholder(..)) => {
                if a == b {
                    a
                } else {
                    self.tcx().lifetimes.re_static
                }
            }
        }
    }

    /// After expansion is complete, go and check upper bounds (i.e.,
    /// cases where the region cannot grow larger than a fixed point)
    /// and check that they are satisfied.
    #[instrument(skip(self, var_data, errors))]
    fn collect_errors(
        &self,
        var_data: &mut LexicalRegionResolutions<'tcx>,
        errors: &mut Vec<RegionResolutionError<'tcx>>,
    ) {
        for (constraint, origin) in &self.data.constraints {
            debug!(?constraint, ?origin);
            match *constraint {
                Constraint::RegSubVar(..) | Constraint::VarSubVar(..) => {
                    // Expansion will ensure that these constraints hold. Ignore.
                }

                Constraint::RegSubReg(sub, sup) => {
                    if self.sub_concrete_regions(sub, sup) {
                        continue;
                    }

                    debug!(
                        "region error at {:?}: \
                         cannot verify that {:?} <= {:?}",
                        origin, sub, sup
                    );

                    errors.push(RegionResolutionError::ConcreteFailure(
                        (*origin).clone(),
                        sub,
                        sup,
                    ));
                }

                Constraint::VarSubReg(a_vid, b_region) => {
                    let a_data = var_data.value_mut(a_vid);
                    debug!("contraction: {:?} == {:?}, {:?}", a_vid, a_data, b_region);

                    let VarValue::Value(a_region) = *a_data else {
                        continue;
                    };

                    // Do not report these errors immediately:
                    // instead, set the variable value to error and
                    // collect them later.
                    if !self.sub_concrete_regions(a_region, b_region) {
                        debug!(
                            "region error at {:?}: \
                            cannot verify that {:?}={:?} <= {:?}",
                            origin, a_vid, a_region, b_region
                        );
                        *a_data = VarValue::ErrorValue;
                    }
                }
            }
        }

        for verify in &self.data.verifys {
            debug!("collect_errors: verify={:?}", verify);
            let sub = var_data.normalize(self.tcx(), verify.region);

            let verify_kind_ty = verify.kind.to_ty(self.tcx());
            let verify_kind_ty = var_data.normalize(self.tcx(), verify_kind_ty);
            if self.bound_is_met(&verify.bound, var_data, verify_kind_ty, sub) {
                continue;
            }

            debug!(
                "collect_errors: region error at {:?}: \
                 cannot verify that {:?} <= {:?}",
                verify.origin, verify.region, verify.bound
            );

            errors.push(RegionResolutionError::GenericBoundFailure(
                verify.origin.clone(),
                verify.kind,
                sub,
            ));
        }
    }

    /// Go over the variables that were declared to be error variables
    /// and create a `RegionResolutionError` for each of them.
    fn collect_var_errors(
        &self,
        var_data: &LexicalRegionResolutions<'tcx>,
        errors: &mut Vec<RegionResolutionError<'tcx>>,
    ) {
        debug!("collect_var_errors, var_data = {:#?}", var_data.values);

        // This is the best way that I have found to suppress
        // duplicate and related errors. Basically we keep a set of
        // flags for every node. Whenever an error occurs, we will
        // walk some portion of the graph looking to find pairs of
        // conflicting regions to report to the user. As we walk, we
        // trip the flags from false to true, and if we find that
        // we've already reported an error involving any particular
        // node we just stop and don't report the current error. The
        // idea is to report errors that derive from independent
        // regions of the graph, but not those that derive from
        // overlapping locations.
        let mut dup_vec = IndexVec::from_elem_n(None, self.num_vars());

        // Only construct the graph when necessary, because it's moderately
        // expensive.
        let mut graph = None;

        for (node_vid, value) in var_data.values.iter_enumerated() {
            match *value {
                VarValue::Empty(_) | VarValue::Value(_) => { /* Inference successful */ }
                VarValue::ErrorValue => {
                    // Inference impossible: this value contains
                    // inconsistent constraints.
                    //
                    // I think that in this case we should report an
                    // error now -- unlike the case above, we can't
                    // wait to see whether the user needs the result
                    // of this variable. The reason is that the mere
                    // existence of this variable implies that the
                    // region graph is inconsistent, whether or not it
                    // is used.
                    //
                    // For example, we may have created a region
                    // variable that is the GLB of two other regions
                    // which do not have a GLB. Even if that variable
                    // is not used, it implies that those two regions
                    // *should* have a GLB.
                    //
                    // At least I think this is true. It may be that
                    // the mere existence of a conflict in a region
                    // variable that is not used is not a problem, so
                    // if this rule starts to create problems we'll
                    // have to revisit this portion of the code and
                    // think hard about it. =) -- nikomatsakis

                    // Obtain the spans for all the places that can
                    // influence the constraints on this value for
                    // richer diagnostics in `static_impl_trait`.

                    let g = graph.get_or_insert_with(|| self.construct_graph());
                    self.collect_error_for_expanding_node(g, &mut dup_vec, node_vid, errors);
                }
            }
        }
    }

    fn construct_graph(&self) -> RegionGraph<'tcx> {
        let num_vars = self.num_vars();

        let mut graph = Graph::new();

        for _ in 0..num_vars {
            graph.add_node(());
        }

        // Issue #30438: two distinct dummy nodes, one for incoming
        // edges (dummy_source) and another for outgoing edges
        // (dummy_sink). In `dummy -> a -> b -> dummy`, using one
        // dummy node leads one to think (erroneously) there exists a
        // path from `b` to `a`. Two dummy nodes sidesteps the issue.
        let dummy_source = graph.add_node(());
        let dummy_sink = graph.add_node(());

        for (constraint, _) in &self.data.constraints {
            match *constraint {
                Constraint::VarSubVar(a_id, b_id) => {
                    graph.add_edge(NodeIndex(a_id.index()), NodeIndex(b_id.index()), *constraint);
                }
                Constraint::RegSubVar(_, b_id) => {
                    graph.add_edge(dummy_source, NodeIndex(b_id.index()), *constraint);
                }
                Constraint::VarSubReg(a_id, _) => {
                    graph.add_edge(NodeIndex(a_id.index()), dummy_sink, *constraint);
                }
                Constraint::RegSubReg(..) => {
                    // this would be an edge from `dummy_source` to
                    // `dummy_sink`; just ignore it.
                }
            }
        }

        graph
    }

    fn collect_error_for_expanding_node(
        &self,
        graph: &RegionGraph<'tcx>,
        dup_vec: &mut IndexSlice<RegionVid, Option<RegionVid>>,
        node_idx: RegionVid,
        errors: &mut Vec<RegionResolutionError<'tcx>>,
    ) {
        // Errors in expanding nodes result from a lower-bound that is
        // not contained by an upper-bound.
        let (mut lower_bounds, lower_vid_bounds, lower_dup) =
            self.collect_bounding_regions(graph, node_idx, INCOMING, Some(dup_vec));
        let (mut upper_bounds, _, upper_dup) =
            self.collect_bounding_regions(graph, node_idx, OUTGOING, Some(dup_vec));

        if lower_dup || upper_dup {
            return;
        }

        // We place late-bound regions first because we are special casing
        // SubSupConflict(ReLateParam, ReLateParam) when reporting error, and so
        // the user will more likely get a specific suggestion.
        fn region_order_key(x: &RegionAndOrigin<'_>) -> u8 {
            match *x.region {
                ReEarlyParam(_) => 0,
                ReLateParam(_) => 1,
                _ => 2,
            }
        }
        lower_bounds.sort_by_key(region_order_key);
        upper_bounds.sort_by_key(region_order_key);

        let node_universe = self.var_infos[node_idx].universe;

        for lower_bound in &lower_bounds {
            let effective_lower_bound = if let ty::RePlaceholder(p) = *lower_bound.region {
                if node_universe.cannot_name(p.universe) {
                    self.tcx().lifetimes.re_static
                } else {
                    lower_bound.region
                }
            } else {
                lower_bound.region
            };

            for upper_bound in &upper_bounds {
                if !self.sub_concrete_regions(effective_lower_bound, upper_bound.region) {
                    let origin = self.var_infos[node_idx].origin;
                    debug!(
                        "region inference error at {:?} for {:?}: SubSupConflict sub: {:?} \
                         sup: {:?}",
                        origin, node_idx, lower_bound.region, upper_bound.region
                    );

                    errors.push(RegionResolutionError::SubSupConflict(
                        node_idx,
                        origin,
                        lower_bound.origin.clone(),
                        lower_bound.region,
                        upper_bound.origin.clone(),
                        upper_bound.region,
                        vec![],
                    ));
                    return;
                }
            }
        }

        // If we have a scenario like `exists<'a> { forall<'b> { 'b:
        // 'a } }`, we wind up without any lower-bound -- all we have
        // are placeholders as upper bounds, but the universe of the
        // variable `'a`, or some variable that `'a` has to outlive, doesn't
        // permit those placeholders.
        //
        // We only iterate to find the min, which means it doesn't cause reproducibility issues
        #[allow(rustc::potential_query_instability)]
        let min_universe = lower_vid_bounds
            .into_iter()
            .map(|vid| self.var_infos[vid].universe)
            .min()
            .expect("lower_vid_bounds should at least include `node_idx`");

        for upper_bound in &upper_bounds {
            if let ty::RePlaceholder(p) = *upper_bound.region {
                if min_universe.cannot_name(p.universe) {
                    let origin = self.var_infos[node_idx].origin;
                    errors.push(RegionResolutionError::UpperBoundUniverseConflict(
                        node_idx,
                        origin,
                        min_universe,
                        upper_bound.origin.clone(),
                        upper_bound.region,
                    ));
                    return;
                }
            }
        }

        // Errors in earlier passes can yield error variables without
        // resolution errors here; ICE if no errors have been emitted yet.
        assert!(
            self.tcx().dcx().has_errors().is_some(),
            "collect_error_for_expanding_node() could not find error for var {node_idx:?} in \
            universe {node_universe:?}, lower_bounds={lower_bounds:#?}, \
            upper_bounds={upper_bounds:#?}",
        );
    }

    /// Collects all regions that "bound" the variable `orig_node_idx` in the
    /// given direction.
    ///
    /// If `dup_vec` is `Some` it's used to track duplicates between successive
    /// calls of this function.
    ///
    /// The return tuple fields are:
    /// - a list of all concrete regions bounding the given region.
    /// - the set of all region variables bounding the given region.
    /// - a `bool` that's true if the returned region variables overlap with
    ///   those returned by a previous call for another region.
    fn collect_bounding_regions(
        &self,
        graph: &RegionGraph<'tcx>,
        orig_node_idx: RegionVid,
        dir: Direction,
        mut dup_vec: Option<&mut IndexSlice<RegionVid, Option<RegionVid>>>,
    ) -> (Vec<RegionAndOrigin<'tcx>>, FxHashSet<RegionVid>, bool) {
        struct WalkState<'tcx> {
            set: FxHashSet<RegionVid>,
            stack: Vec<RegionVid>,
            result: Vec<RegionAndOrigin<'tcx>>,
            dup_found: bool,
        }
        let mut state = WalkState {
            set: Default::default(),
            stack: vec![orig_node_idx],
            result: Vec::new(),
            dup_found: false,
        };
        state.set.insert(orig_node_idx);

        // to start off the process, walk the source node in the
        // direction specified
        process_edges(&self.data, &mut state, graph, orig_node_idx, dir);

        while let Some(node_idx) = state.stack.pop() {
            // check whether we've visited this node on some previous walk
            if let Some(dup_vec) = &mut dup_vec {
                if dup_vec[node_idx].is_none() {
                    dup_vec[node_idx] = Some(orig_node_idx);
                } else if dup_vec[node_idx] != Some(orig_node_idx) {
                    state.dup_found = true;
                }

                debug!(
                    "collect_concrete_regions(orig_node_idx={:?}, node_idx={:?})",
                    orig_node_idx, node_idx
                );
            }

            process_edges(&self.data, &mut state, graph, node_idx, dir);
        }

        let WalkState { result, dup_found, set, .. } = state;
        return (result, set, dup_found);

        fn process_edges<'tcx>(
            this: &RegionConstraintData<'tcx>,
            state: &mut WalkState<'tcx>,
            graph: &RegionGraph<'tcx>,
            source_vid: RegionVid,
            dir: Direction,
        ) {
            debug!("process_edges(source_vid={:?}, dir={:?})", source_vid, dir);

            let source_node_index = NodeIndex(source_vid.index());
            for (_, edge) in graph.adjacent_edges(source_node_index, dir) {
                match edge.data {
                    Constraint::VarSubVar(from_vid, to_vid) => {
                        let opp_vid = if from_vid == source_vid { to_vid } else { from_vid };
                        if state.set.insert(opp_vid) {
                            state.stack.push(opp_vid);
                        }
                    }

                    Constraint::RegSubVar(region, _) | Constraint::VarSubReg(_, region) => {
                        let constraint_idx =
                            this.constraints.binary_search_by(|(c, _)| c.cmp(&edge.data)).unwrap();
                        state.result.push(RegionAndOrigin {
                            region,
                            origin: this.constraints[constraint_idx].1.clone(),
                        });
                    }

                    Constraint::RegSubReg(..) => panic!(
                        "cannot reach reg-sub-reg edge in region inference \
                         post-processing"
                    ),
                }
            }
        }
    }

    fn bound_is_met(
        &self,
        bound: &VerifyBound<'tcx>,
        var_values: &LexicalRegionResolutions<'tcx>,
        generic_ty: Ty<'tcx>,
        min: ty::Region<'tcx>,
    ) -> bool {
        if let ty::ReError(_) = *min {
            return true;
        }

        match bound {
            VerifyBound::IfEq(verify_if_eq_b) => {
                let verify_if_eq_b = var_values.normalize(self.region_rels.tcx, *verify_if_eq_b);
                match test_type_match::extract_verify_if_eq(self.tcx(), &verify_if_eq_b, generic_ty)
                {
                    Some(r) => {
                        self.bound_is_met(&VerifyBound::OutlivedBy(r), var_values, generic_ty, min)
                    }

                    None => false,
                }
            }

            VerifyBound::OutlivedBy(r) => {
                let a = match *min {
                    ty::ReVar(rid) => var_values.values[rid],
                    _ => VarValue::Value(min),
                };
                let b = match **r {
                    ty::ReVar(rid) => var_values.values[rid],
                    _ => VarValue::Value(*r),
                };
                self.sub_region_values(a, b)
            }

            VerifyBound::IsEmpty => match *min {
                ty::ReVar(rid) => match var_values.values[rid] {
                    VarValue::ErrorValue => false,
                    VarValue::Empty(_) => true,
                    VarValue::Value(_) => false,
                },
                _ => false,
            },

            VerifyBound::AnyBound(bs) => {
                bs.iter().any(|b| self.bound_is_met(b, var_values, generic_ty, min))
            }

            VerifyBound::AllBounds(bs) => {
                bs.iter().all(|b| self.bound_is_met(b, var_values, generic_ty, min))
            }
        }
    }
}

impl<'tcx> fmt::Debug for RegionAndOrigin<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "RegionAndOrigin({:?},{:?})", self.region, self.origin)
    }
}

impl<'tcx> LexicalRegionResolutions<'tcx> {
    fn normalize<T>(&self, tcx: TyCtxt<'tcx>, value: T) -> T
    where
        T: TypeFoldable<TyCtxt<'tcx>>,
    {
        tcx.fold_regions(value, |r, _db| self.resolve_region(tcx, r))
    }

    fn value(&self, rid: RegionVid) -> &VarValue<'tcx> {
        &self.values[rid]
    }

    fn value_mut(&mut self, rid: RegionVid) -> &mut VarValue<'tcx> {
        &mut self.values[rid]
    }

    pub(crate) fn resolve_region(
        &self,
        tcx: TyCtxt<'tcx>,
        r: ty::Region<'tcx>,
    ) -> ty::Region<'tcx> {
        let result = match *r {
            ty::ReVar(rid) => match self.values[rid] {
                VarValue::Empty(_) => r,
                VarValue::Value(r) => r,
                VarValue::ErrorValue => tcx.lifetimes.re_static,
            },
            _ => r,
        };
        debug!("resolve_region({:?}) = {:?}", r, result);
        result
    }
}