summaryrefslogtreecommitdiffstats
path: root/compiler/rustc_trait_selection/src/traits/wf.rs
blob: 5ea28fb47cf4fbd1f29dd8a4038535a66751b6b9 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
use crate::infer::InferCtxt;
use crate::traits;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::lang_items::LangItem;
use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeVisitable};
use rustc_span::Span;

use std::iter;
/// Returns the set of obligations needed to make `arg` well-formed.
/// If `arg` contains unresolved inference variables, this may include
/// further WF obligations. However, if `arg` IS an unresolved
/// inference variable, returns `None`, because we are not able to
/// make any progress at all. This is to prevent "livelock" where we
/// say "$0 is WF if $0 is WF".
pub fn obligations<'a, 'tcx>(
    infcx: &InferCtxt<'a, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    body_id: hir::HirId,
    recursion_depth: usize,
    arg: GenericArg<'tcx>,
    span: Span,
) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
    // Handle the "livelock" case (see comment above) by bailing out if necessary.
    let arg = match arg.unpack() {
        GenericArgKind::Type(ty) => {
            match ty.kind() {
                ty::Infer(ty::TyVar(_)) => {
                    let resolved_ty = infcx.shallow_resolve(ty);
                    if resolved_ty == ty {
                        // No progress, bail out to prevent "livelock".
                        return None;
                    } else {
                        resolved_ty
                    }
                }
                _ => ty,
            }
            .into()
        }
        GenericArgKind::Const(ct) => {
            match ct.kind() {
                ty::ConstKind::Infer(_) => {
                    let resolved = infcx.shallow_resolve(ct);
                    if resolved == ct {
                        // No progress.
                        return None;
                    } else {
                        resolved
                    }
                }
                _ => ct,
            }
            .into()
        }
        // There is nothing we have to do for lifetimes.
        GenericArgKind::Lifetime(..) => return Some(Vec::new()),
    };

    let mut wf = WfPredicates {
        tcx: infcx.tcx,
        param_env,
        body_id,
        span,
        out: vec![],
        recursion_depth,
        item: None,
    };
    wf.compute(arg);
    debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);

    let result = wf.normalize(infcx);
    debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
    Some(result)
}

/// Returns the obligations that make this trait reference
/// well-formed.  For example, if there is a trait `Set` defined like
/// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
/// if `Bar: Eq`.
pub fn trait_obligations<'a, 'tcx>(
    infcx: &InferCtxt<'a, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    body_id: hir::HirId,
    trait_pred: &ty::TraitPredicate<'tcx>,
    span: Span,
    item: &'tcx hir::Item<'tcx>,
) -> Vec<traits::PredicateObligation<'tcx>> {
    let mut wf = WfPredicates {
        tcx: infcx.tcx,
        param_env,
        body_id,
        span,
        out: vec![],
        recursion_depth: 0,
        item: Some(item),
    };
    wf.compute_trait_pred(trait_pred, Elaborate::All);
    debug!(obligations = ?wf.out);
    wf.normalize(infcx)
}

#[instrument(skip(infcx), ret)]
pub fn predicate_obligations<'a, 'tcx>(
    infcx: &InferCtxt<'a, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    body_id: hir::HirId,
    predicate: ty::Predicate<'tcx>,
    span: Span,
) -> Vec<traits::PredicateObligation<'tcx>> {
    let mut wf = WfPredicates {
        tcx: infcx.tcx,
        param_env,
        body_id,
        span,
        out: vec![],
        recursion_depth: 0,
        item: None,
    };

    // It's ok to skip the binder here because wf code is prepared for it
    match predicate.kind().skip_binder() {
        ty::PredicateKind::Trait(t) => {
            wf.compute_trait_pred(&t, Elaborate::None);
        }
        ty::PredicateKind::RegionOutlives(..) => {}
        ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
            wf.compute(ty.into());
        }
        ty::PredicateKind::Projection(t) => {
            wf.compute_projection(t.projection_ty);
            wf.compute(match t.term.unpack() {
                ty::TermKind::Ty(ty) => ty.into(),
                ty::TermKind::Const(c) => c.into(),
            })
        }
        ty::PredicateKind::WellFormed(arg) => {
            wf.compute(arg);
        }
        ty::PredicateKind::ObjectSafe(_) => {}
        ty::PredicateKind::ClosureKind(..) => {}
        ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
            wf.compute(a.into());
            wf.compute(b.into());
        }
        ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
            wf.compute(a.into());
            wf.compute(b.into());
        }
        ty::PredicateKind::ConstEvaluatable(uv) => {
            let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
            wf.out.extend(obligations);

            for arg in uv.substs.iter() {
                wf.compute(arg);
            }
        }
        ty::PredicateKind::ConstEquate(c1, c2) => {
            wf.compute(c1.into());
            wf.compute(c2.into());
        }
        ty::PredicateKind::TypeWellFormedFromEnv(..) => {
            bug!("TypeWellFormedFromEnv is only used for Chalk")
        }
    }

    wf.normalize(infcx)
}

struct WfPredicates<'tcx> {
    tcx: TyCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    body_id: hir::HirId,
    span: Span,
    out: Vec<traits::PredicateObligation<'tcx>>,
    recursion_depth: usize,
    item: Option<&'tcx hir::Item<'tcx>>,
}

/// Controls whether we "elaborate" supertraits and so forth on the WF
/// predicates. This is a kind of hack to address #43784. The
/// underlying problem in that issue was a trait structure like:
///
/// ```ignore (illustrative)
/// trait Foo: Copy { }
/// trait Bar: Foo { }
/// impl<T: Bar> Foo for T { }
/// impl<T> Bar for T { }
/// ```
///
/// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
/// we decide that this is true because `T: Bar` is in the
/// where-clauses (and we can elaborate that to include `T:
/// Copy`). This wouldn't be a problem, except that when we check the
/// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
/// impl. And so nowhere did we check that `T: Copy` holds!
///
/// To resolve this, we elaborate the WF requirements that must be
/// proven when checking impls. This means that (e.g.) the `impl Bar
/// for T` will be forced to prove not only that `T: Foo` but also `T:
/// Copy` (which it won't be able to do, because there is no `Copy`
/// impl for `T`).
#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum Elaborate {
    All,
    None,
}

fn extend_cause_with_original_assoc_item_obligation<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_ref: &ty::TraitRef<'tcx>,
    item: Option<&hir::Item<'tcx>>,
    cause: &mut traits::ObligationCause<'tcx>,
    pred: ty::Predicate<'tcx>,
) {
    debug!(
        "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
        trait_ref, item, cause, pred
    );
    let (items, impl_def_id) = match item {
        Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
        _ => return,
    };
    let fix_span =
        |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
            hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
            _ => impl_item_ref.span,
        };

    // It is fine to skip the binder as we don't care about regions here.
    match pred.kind().skip_binder() {
        ty::PredicateKind::Projection(proj) => {
            // The obligation comes not from the current `impl` nor the `trait` being implemented,
            // but rather from a "second order" obligation, where an associated type has a
            // projection coming from another associated type. See
            // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
            // `traits-assoc-type-in-supertrait-bad.rs`.
            if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
                && let Some(&impl_item_id) =
                    tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
                && let Some(impl_item_span) = items
                    .iter()
                    .find(|item| item.id.def_id.to_def_id() == impl_item_id)
                    .map(fix_span)
            {
                cause.span = impl_item_span;
            }
        }
        ty::PredicateKind::Trait(pred) => {
            // An associated item obligation born out of the `trait` failed to be met. An example
            // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
            debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
            if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
                && let Some(&impl_item_id) =
                    tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
                && let Some(impl_item_span) = items
                    .iter()
                    .find(|item| item.id.def_id.to_def_id() == impl_item_id)
                    .map(fix_span)
            {
                cause.span = impl_item_span;
            }
        }
        _ => {}
    }
}

impl<'tcx> WfPredicates<'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
        traits::ObligationCause::new(self.span, self.body_id, code)
    }

    fn normalize(self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
        let cause = self.cause(traits::WellFormed(None));
        let param_env = self.param_env;
        let mut obligations = Vec::with_capacity(self.out.len());
        for mut obligation in self.out {
            assert!(!obligation.has_escaping_bound_vars());
            let mut selcx = traits::SelectionContext::new(infcx);
            // Don't normalize the whole obligation, the param env is either
            // already normalized, or we're currently normalizing the
            // param_env. Either way we should only normalize the predicate.
            let normalized_predicate = traits::project::normalize_with_depth_to(
                &mut selcx,
                param_env,
                cause.clone(),
                self.recursion_depth,
                obligation.predicate,
                &mut obligations,
            );
            obligation.predicate = normalized_predicate;
            obligations.push(obligation);
        }
        obligations
    }

    /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
    fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
        let tcx = self.tcx;
        let trait_ref = &trait_pred.trait_ref;

        // if the trait predicate is not const, the wf obligations should not be const as well.
        let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
            self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
        } else {
            self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
        };

        debug!("compute_trait_pred obligations {:?}", obligations);
        let param_env = self.param_env;
        let depth = self.recursion_depth;

        let item = self.item;

        let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
            if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
                cause = cause.derived_cause(
                    parent_trait_pred,
                    traits::ObligationCauseCode::DerivedObligation,
                );
            }
            extend_cause_with_original_assoc_item_obligation(
                tcx, trait_ref, item, &mut cause, predicate,
            );
            traits::Obligation::with_depth(cause, depth, param_env, predicate)
        };

        if let Elaborate::All = elaborate {
            let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
            let implied_obligations = implied_obligations.map(extend);
            self.out.extend(implied_obligations);
        } else {
            self.out.extend(obligations);
        }

        let tcx = self.tcx();
        self.out.extend(
            trait_ref
                .substs
                .iter()
                .enumerate()
                .filter(|(_, arg)| {
                    matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
                })
                .filter(|(_, arg)| !arg.has_escaping_bound_vars())
                .map(|(i, arg)| {
                    let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
                    // The first subst is the self ty - use the correct span for it.
                    if i == 0 {
                        if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
                            item.map(|i| &i.kind)
                        {
                            cause.span = self_ty.span;
                        }
                    }
                    traits::Obligation::with_depth(
                        cause,
                        depth,
                        param_env,
                        ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
                    )
                }),
        );
    }

    /// Pushes the obligations required for `trait_ref::Item` to be WF
    /// into `self.out`.
    fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
        // A projection is well-formed if
        //
        // (a) its predicates hold (*)
        // (b) its substs are wf
        //
        // (*) The predicates of an associated type include the predicates of
        //     the trait that it's contained in. For example, given
        //
        // trait A<T>: Clone {
        //     type X where T: Copy;
        // }
        //
        // The predicates of `<() as A<i32>>::X` are:
        // [
        //     `(): Sized`
        //     `(): Clone`
        //     `(): A<i32>`
        //     `i32: Sized`
        //     `i32: Clone`
        //     `i32: Copy`
        // ]
        let obligations = self.nominal_obligations(data.item_def_id, data.substs);
        self.out.extend(obligations);

        let tcx = self.tcx();
        let cause = self.cause(traits::WellFormed(None));
        let param_env = self.param_env;
        let depth = self.recursion_depth;

        self.out.extend(
            data.substs
                .iter()
                .filter(|arg| {
                    matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
                })
                .filter(|arg| !arg.has_escaping_bound_vars())
                .map(|arg| {
                    traits::Obligation::with_depth(
                        cause.clone(),
                        depth,
                        param_env,
                        ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
                    )
                }),
        );
    }

    fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
        if !subty.has_escaping_bound_vars() {
            let cause = self.cause(cause);
            let trait_ref = ty::TraitRef {
                def_id: self.tcx.require_lang_item(LangItem::Sized, None),
                substs: self.tcx.mk_substs_trait(subty, &[]),
            };
            self.out.push(traits::Obligation::with_depth(
                cause,
                self.recursion_depth,
                self.param_env,
                ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx),
            ));
        }
    }

    /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
    #[instrument(level = "debug", skip(self))]
    fn compute(&mut self, arg: GenericArg<'tcx>) {
        let mut walker = arg.walk();
        let param_env = self.param_env;
        let depth = self.recursion_depth;
        while let Some(arg) = walker.next() {
            debug!(?arg, ?self.out);
            let ty = match arg.unpack() {
                GenericArgKind::Type(ty) => ty,

                // No WF constraints for lifetimes being present, any outlives
                // obligations are handled by the parent (e.g. `ty::Ref`).
                GenericArgKind::Lifetime(_) => continue,

                GenericArgKind::Const(constant) => {
                    match constant.kind() {
                        ty::ConstKind::Unevaluated(uv) => {
                            let obligations = self.nominal_obligations(uv.def.did, uv.substs);
                            self.out.extend(obligations);

                            let predicate =
                                ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv))
                                    .to_predicate(self.tcx());
                            let cause = self.cause(traits::WellFormed(None));
                            self.out.push(traits::Obligation::with_depth(
                                cause,
                                self.recursion_depth,
                                self.param_env,
                                predicate,
                            ));
                        }
                        ty::ConstKind::Infer(_) => {
                            let cause = self.cause(traits::WellFormed(None));

                            self.out.push(traits::Obligation::with_depth(
                                cause,
                                self.recursion_depth,
                                self.param_env,
                                ty::Binder::dummy(ty::PredicateKind::WellFormed(constant.into()))
                                    .to_predicate(self.tcx()),
                            ));
                        }
                        ty::ConstKind::Error(_)
                        | ty::ConstKind::Param(_)
                        | ty::ConstKind::Bound(..)
                        | ty::ConstKind::Placeholder(..) => {
                            // These variants are trivially WF, so nothing to do here.
                        }
                        ty::ConstKind::Value(..) => {
                            // FIXME: Enforce that values are structurally-matchable.
                        }
                    }
                    continue;
                }
            };

            debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());

            match *ty.kind() {
                ty::Bool
                | ty::Char
                | ty::Int(..)
                | ty::Uint(..)
                | ty::Float(..)
                | ty::Error(_)
                | ty::Str
                | ty::GeneratorWitness(..)
                | ty::Never
                | ty::Param(_)
                | ty::Bound(..)
                | ty::Placeholder(..)
                | ty::Foreign(..) => {
                    // WfScalar, WfParameter, etc
                }

                // Can only infer to `ty::Int(_) | ty::Uint(_)`.
                ty::Infer(ty::IntVar(_)) => {}

                // Can only infer to `ty::Float(_)`.
                ty::Infer(ty::FloatVar(_)) => {}

                ty::Slice(subty) => {
                    self.require_sized(subty, traits::SliceOrArrayElem);
                }

                ty::Array(subty, _) => {
                    self.require_sized(subty, traits::SliceOrArrayElem);
                    // Note that we handle the len is implicitly checked while walking `arg`.
                }

                ty::Tuple(ref tys) => {
                    if let Some((_last, rest)) = tys.split_last() {
                        for &elem in rest {
                            self.require_sized(elem, traits::TupleElem);
                        }
                    }
                }

                ty::RawPtr(_) => {
                    // Simple cases that are WF if their type args are WF.
                }

                ty::Projection(data) => {
                    walker.skip_current_subtree(); // Subtree handled by compute_projection.
                    self.compute_projection(data);
                }

                ty::Adt(def, substs) => {
                    // WfNominalType
                    let obligations = self.nominal_obligations(def.did(), substs);
                    self.out.extend(obligations);
                }

                ty::FnDef(did, substs) => {
                    let obligations = self.nominal_obligations(did, substs);
                    self.out.extend(obligations);
                }

                ty::Ref(r, rty, _) => {
                    // WfReference
                    if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
                        let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
                        self.out.push(traits::Obligation::with_depth(
                            cause,
                            depth,
                            param_env,
                            ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
                                ty::OutlivesPredicate(rty, r),
                            ))
                            .to_predicate(self.tcx()),
                        ));
                    }
                }

                ty::Generator(did, substs, ..) => {
                    // Walk ALL the types in the generator: this will
                    // include the upvar types as well as the yield
                    // type. Note that this is mildly distinct from
                    // the closure case, where we have to be careful
                    // about the signature of the closure. We don't
                    // have the problem of implied bounds here since
                    // generators don't take arguments.
                    let obligations = self.nominal_obligations(did, substs);
                    self.out.extend(obligations);
                }

                ty::Closure(did, substs) => {
                    // Only check the upvar types for WF, not the rest
                    // of the types within. This is needed because we
                    // capture the signature and it may not be WF
                    // without the implied bounds. Consider a closure
                    // like `|x: &'a T|` -- it may be that `T: 'a` is
                    // not known to hold in the creator's context (and
                    // indeed the closure may not be invoked by its
                    // creator, but rather turned to someone who *can*
                    // verify that).
                    //
                    // The special treatment of closures here really
                    // ought not to be necessary either; the problem
                    // is related to #25860 -- there is no way for us
                    // to express a fn type complete with the implied
                    // bounds that it is assuming. I think in reality
                    // the WF rules around fn are a bit messed up, and
                    // that is the rot problem: `fn(&'a T)` should
                    // probably always be WF, because it should be
                    // shorthand for something like `where(T: 'a) {
                    // fn(&'a T) }`, as discussed in #25860.
                    walker.skip_current_subtree(); // subtree handled below
                    // FIXME(eddyb) add the type to `walker` instead of recursing.
                    self.compute(substs.as_closure().tupled_upvars_ty().into());
                    // Note that we cannot skip the generic types
                    // types. Normally, within the fn
                    // body where they are created, the generics will
                    // always be WF, and outside of that fn body we
                    // are not directly inspecting closure types
                    // anyway, except via auto trait matching (which
                    // only inspects the upvar types).
                    // But when a closure is part of a type-alias-impl-trait
                    // then the function that created the defining site may
                    // have had more bounds available than the type alias
                    // specifies. This may cause us to have a closure in the
                    // hidden type that is not actually well formed and
                    // can cause compiler crashes when the user abuses unsafe
                    // code to procure such a closure.
                    // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
                    let obligations = self.nominal_obligations(did, substs);
                    self.out.extend(obligations);
                }

                ty::FnPtr(_) => {
                    // let the loop iterate into the argument/return
                    // types appearing in the fn signature
                }

                ty::Opaque(did, substs) => {
                    // All of the requirements on type parameters
                    // have already been checked for `impl Trait` in
                    // return position. We do need to check type-alias-impl-trait though.
                    if ty::is_impl_trait_defn(self.tcx, did).is_none() {
                        let obligations = self.nominal_obligations(did, substs);
                        self.out.extend(obligations);
                    }
                }

                ty::Dynamic(data, r, _) => {
                    // WfObject
                    //
                    // Here, we defer WF checking due to higher-ranked
                    // regions. This is perhaps not ideal.
                    self.from_object_ty(ty, data, r);

                    // FIXME(#27579) RFC also considers adding trait
                    // obligations that don't refer to Self and
                    // checking those

                    let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;

                    if !defer_to_coercion {
                        let cause = self.cause(traits::WellFormed(None));
                        let component_traits = data.auto_traits().chain(data.principal_def_id());
                        let tcx = self.tcx();
                        self.out.extend(component_traits.map(|did| {
                            traits::Obligation::with_depth(
                                cause.clone(),
                                depth,
                                param_env,
                                ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
                                    .to_predicate(tcx),
                            )
                        }));
                    }
                }

                // Inference variables are the complicated case, since we don't
                // know what type they are. We do two things:
                //
                // 1. Check if they have been resolved, and if so proceed with
                //    THAT type.
                // 2. If not, we've at least simplified things (e.g., we went
                //    from `Vec<$0>: WF` to `$0: WF`), so we can
                //    register a pending obligation and keep
                //    moving. (Goal is that an "inductive hypothesis"
                //    is satisfied to ensure termination.)
                // See also the comment on `fn obligations`, describing "livelock"
                // prevention, which happens before this can be reached.
                ty::Infer(_) => {
                    let cause = self.cause(traits::WellFormed(None));
                    self.out.push(traits::Obligation::with_depth(
                        cause,
                        self.recursion_depth,
                        param_env,
                        ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
                            .to_predicate(self.tcx()),
                    ));
                }
            }

            debug!(?self.out);
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn nominal_obligations_inner(
        &mut self,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
        remap_constness: bool,
    ) -> Vec<traits::PredicateObligation<'tcx>> {
        let predicates = self.tcx.predicates_of(def_id);
        let mut origins = vec![def_id; predicates.predicates.len()];
        let mut head = predicates;
        while let Some(parent) = head.parent {
            head = self.tcx.predicates_of(parent);
            origins.extend(iter::repeat(parent).take(head.predicates.len()));
        }

        let predicates = predicates.instantiate(self.tcx, substs);
        trace!("{:#?}", predicates);
        debug_assert_eq!(predicates.predicates.len(), origins.len());

        iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
            .map(|((mut pred, span), origin_def_id)| {
                let code = if span.is_dummy() {
                    traits::ItemObligation(origin_def_id)
                } else {
                    traits::BindingObligation(origin_def_id, span)
                };
                let cause = self.cause(code);
                if remap_constness {
                    pred = pred.without_const(self.tcx);
                }
                traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
            })
            .filter(|pred| !pred.has_escaping_bound_vars())
            .collect()
    }

    fn nominal_obligations(
        &mut self,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
    ) -> Vec<traits::PredicateObligation<'tcx>> {
        self.nominal_obligations_inner(def_id, substs, false)
    }

    fn nominal_obligations_without_const(
        &mut self,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
    ) -> Vec<traits::PredicateObligation<'tcx>> {
        self.nominal_obligations_inner(def_id, substs, true)
    }

    fn from_object_ty(
        &mut self,
        ty: Ty<'tcx>,
        data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
        region: ty::Region<'tcx>,
    ) {
        // Imagine a type like this:
        //
        //     trait Foo { }
        //     trait Bar<'c> : 'c { }
        //
        //     &'b (Foo+'c+Bar<'d>)
        //         ^
        //
        // In this case, the following relationships must hold:
        //
        //     'b <= 'c
        //     'd <= 'c
        //
        // The first conditions is due to the normal region pointer
        // rules, which say that a reference cannot outlive its
        // referent.
        //
        // The final condition may be a bit surprising. In particular,
        // you may expect that it would have been `'c <= 'd`, since
        // usually lifetimes of outer things are conservative
        // approximations for inner things. However, it works somewhat
        // differently with trait objects: here the idea is that if the
        // user specifies a region bound (`'c`, in this case) it is the
        // "master bound" that *implies* that bounds from other traits are
        // all met. (Remember that *all bounds* in a type like
        // `Foo+Bar+Zed` must be met, not just one, hence if we write
        // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
        // 'y.)
        //
        // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
        // am looking forward to the future here.
        if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
            let implicit_bounds = object_region_bounds(self.tcx, data);

            let explicit_bound = region;

            self.out.reserve(implicit_bounds.len());
            for implicit_bound in implicit_bounds {
                let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
                let outlives =
                    ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
                self.out.push(traits::Obligation::with_depth(
                    cause,
                    self.recursion_depth,
                    self.param_env,
                    outlives.to_predicate(self.tcx),
                ));
            }
        }
    }
}

/// Given an object type like `SomeTrait + Send`, computes the lifetime
/// bounds that must hold on the elided self type. These are derived
/// from the declarations of `SomeTrait`, `Send`, and friends -- if
/// they declare `trait SomeTrait : 'static`, for example, then
/// `'static` would appear in the list. The hard work is done by
/// `infer::required_region_bounds`, see that for more information.
pub fn object_region_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
) -> Vec<ty::Region<'tcx>> {
    // Since we don't actually *know* the self type for an object,
    // this "open(err)" serves as a kind of dummy standin -- basically
    // a placeholder type.
    let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));

    let predicates = existential_predicates.iter().filter_map(|predicate| {
        if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
            None
        } else {
            Some(predicate.with_self_ty(tcx, open_ty))
        }
    });

    required_region_bounds(tcx, open_ty, predicates)
}

/// Given a set of predicates that apply to an object type, returns
/// the region bounds that the (erased) `Self` type must
/// outlive. Precisely *because* the `Self` type is erased, the
/// parameter `erased_self_ty` must be supplied to indicate what type
/// has been used to represent `Self` in the predicates
/// themselves. This should really be a unique type; `FreshTy(0)` is a
/// popular choice.
///
/// N.B., in some cases, particularly around higher-ranked bounds,
/// this function returns a kind of conservative approximation.
/// That is, all regions returned by this function are definitely
/// required, but there may be other region bounds that are not
/// returned, as well as requirements like `for<'a> T: 'a`.
///
/// Requires that trait definitions have been processed so that we can
/// elaborate predicates and walk supertraits.
#[instrument(skip(tcx, predicates), level = "debug", ret)]
pub(crate) fn required_region_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    erased_self_ty: Ty<'tcx>,
    predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
) -> Vec<ty::Region<'tcx>> {
    assert!(!erased_self_ty.has_escaping_bound_vars());

    traits::elaborate_predicates(tcx, predicates)
        .filter_map(|obligation| {
            debug!(?obligation);
            match obligation.predicate.kind().skip_binder() {
                ty::PredicateKind::Projection(..)
                | ty::PredicateKind::Trait(..)
                | ty::PredicateKind::Subtype(..)
                | ty::PredicateKind::Coerce(..)
                | ty::PredicateKind::WellFormed(..)
                | ty::PredicateKind::ObjectSafe(..)
                | ty::PredicateKind::ClosureKind(..)
                | ty::PredicateKind::RegionOutlives(..)
                | ty::PredicateKind::ConstEvaluatable(..)
                | ty::PredicateKind::ConstEquate(..)
                | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
                ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
                    // Search for a bound of the form `erased_self_ty
                    // : 'a`, but be wary of something like `for<'a>
                    // erased_self_ty : 'a` (we interpret a
                    // higher-ranked bound like that as 'static,
                    // though at present the code in `fulfill.rs`
                    // considers such bounds to be unsatisfiable, so
                    // it's kind of a moot point since you could never
                    // construct such an object, but this seems
                    // correct even if that code changes).
                    if t == &erased_self_ty && !r.has_escaping_bound_vars() {
                        Some(*r)
                    } else {
                        None
                    }
                }
            }
        })
        .collect()
}