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
//! Code shared by trait and projection goals for candidate assembly.

pub(super) mod structural_traits;

use rustc_type_ir::elaborate;
use rustc_type_ir::fold::TypeFoldable;
use rustc_type_ir::inherent::*;
use rustc_type_ir::lang_items::TraitSolverLangItem;
use rustc_type_ir::visit::TypeVisitableExt as _;
use rustc_type_ir::{self as ty, Interner, Upcast as _};
use tracing::{debug, instrument};

use crate::delegate::SolverDelegate;
use crate::solve::inspect::ProbeKind;
use crate::solve::{
    BuiltinImplSource, CandidateSource, CanonicalResponse, Certainty, EvalCtxt, Goal, GoalSource,
    MaybeCause, NoSolution, QueryResult, SolverMode,
};

/// A candidate is a possible way to prove a goal.
///
/// It consists of both the `source`, which describes how that goal would be proven,
/// and the `result` when using the given `source`.
#[derive(derivative::Derivative)]
#[derivative(Debug(bound = ""), Clone(bound = ""))]
pub(super) struct Candidate<I: Interner> {
    pub(super) source: CandidateSource<I>,
    pub(super) result: CanonicalResponse<I>,
}

/// Methods used to assemble candidates for either trait or projection goals.
pub(super) trait GoalKind<D, I = <D as SolverDelegate>::Interner>:
    TypeFoldable<I> + Copy + Eq + std::fmt::Display
where
    D: SolverDelegate<Interner = I>,
    I: Interner,
{
    fn self_ty(self) -> I::Ty;

    fn trait_ref(self, cx: I) -> ty::TraitRef<I>;

    fn with_self_ty(self, cx: I, self_ty: I::Ty) -> Self;

    fn trait_def_id(self, cx: I) -> I::DefId;

    /// Try equating an assumption predicate against a goal's predicate. If it
    /// holds, then execute the `then` callback, which should do any additional
    /// work, then produce a response (typically by executing
    /// [`EvalCtxt::evaluate_added_goals_and_make_canonical_response`]).
    fn probe_and_match_goal_against_assumption(
        ecx: &mut EvalCtxt<'_, D>,
        source: CandidateSource<I>,
        goal: Goal<I, Self>,
        assumption: I::Clause,
        then: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// Consider a clause, which consists of a "assumption" and some "requirements",
    /// to satisfy a goal. If the requirements hold, then attempt to satisfy our
    /// goal by equating it with the assumption.
    fn probe_and_consider_implied_clause(
        ecx: &mut EvalCtxt<'_, D>,
        parent_source: CandidateSource<I>,
        goal: Goal<I, Self>,
        assumption: I::Clause,
        requirements: impl IntoIterator<Item = (GoalSource, Goal<I, I::Predicate>)>,
    ) -> Result<Candidate<I>, NoSolution> {
        Self::probe_and_match_goal_against_assumption(ecx, parent_source, goal, assumption, |ecx| {
            for (nested_source, goal) in requirements {
                ecx.add_goal(nested_source, goal);
            }
            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
        })
    }

    /// Consider a clause specifically for a `dyn Trait` self type. This requires
    /// additionally checking all of the supertraits and object bounds to hold,
    /// since they're not implied by the well-formedness of the object type.
    fn probe_and_consider_object_bound_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        source: CandidateSource<I>,
        goal: Goal<I, Self>,
        assumption: I::Clause,
    ) -> Result<Candidate<I>, NoSolution> {
        Self::probe_and_match_goal_against_assumption(ecx, source, goal, assumption, |ecx| {
            let cx = ecx.cx();
            let ty::Dynamic(bounds, _, _) = goal.predicate.self_ty().kind() else {
                panic!("expected object type in `probe_and_consider_object_bound_candidate`");
            };
            ecx.add_goals(
                GoalSource::ImplWhereBound,
                structural_traits::predicates_for_object_candidate(
                    ecx,
                    goal.param_env,
                    goal.predicate.trait_ref(cx),
                    bounds,
                ),
            );
            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
        })
    }

    fn consider_impl_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
        impl_def_id: I::DefId,
    ) -> Result<Candidate<I>, NoSolution>;

    /// If the predicate contained an error, we want to avoid emitting unnecessary trait
    /// errors but still want to emit errors for other trait goals. We have some special
    /// handling for this case.
    ///
    /// Trait goals always hold while projection goals never do. This is a bit arbitrary
    /// but prevents incorrect normalization while hiding any trait errors.
    fn consider_error_guaranteed_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        guar: I::ErrorGuaranteed,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A type implements an `auto trait` if its components do as well.
    ///
    /// These components are given by built-in rules from
    /// [`structural_traits::instantiate_constituent_tys_for_auto_trait`].
    fn consider_auto_trait_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A trait alias holds if the RHS traits and `where` clauses hold.
    fn consider_trait_alias_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A type is `Sized` if its tail component is `Sized`.
    ///
    /// These components are given by built-in rules from
    /// [`structural_traits::instantiate_constituent_tys_for_sized_trait`].
    fn consider_builtin_sized_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A type is `Copy` or `Clone` if its components are `Copy` or `Clone`.
    ///
    /// These components are given by built-in rules from
    /// [`structural_traits::instantiate_constituent_tys_for_copy_clone_trait`].
    fn consider_builtin_copy_clone_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A type is `PointerLike` if we can compute its layout, and that layout
    /// matches the layout of `usize`.
    fn consider_builtin_pointer_like_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A type is a `FnPtr` if it is of `FnPtr` type.
    fn consider_builtin_fn_ptr_trait_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A callable type (a closure, fn def, or fn ptr) is known to implement the `Fn<A>`
    /// family of traits where `A` is given by the signature of the type.
    fn consider_builtin_fn_trait_candidates(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
        kind: ty::ClosureKind,
    ) -> Result<Candidate<I>, NoSolution>;

    /// An async closure is known to implement the `AsyncFn<A>` family of traits
    /// where `A` is given by the signature of the type.
    fn consider_builtin_async_fn_trait_candidates(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
        kind: ty::ClosureKind,
    ) -> Result<Candidate<I>, NoSolution>;

    /// Compute the built-in logic of the `AsyncFnKindHelper` helper trait, which
    /// is used internally to delay computation for async closures until after
    /// upvar analysis is performed in HIR typeck.
    fn consider_builtin_async_fn_kind_helper_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// `Tuple` is implemented if the `Self` type is a tuple.
    fn consider_builtin_tuple_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// `Pointee` is always implemented.
    ///
    /// See the projection implementation for the `Metadata` types for all of
    /// the built-in types. For structs, the metadata type is given by the struct
    /// tail.
    fn consider_builtin_pointee_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A coroutine (that comes from an `async` desugaring) is known to implement
    /// `Future<Output = O>`, where `O` is given by the coroutine's return type
    /// that was computed during type-checking.
    fn consider_builtin_future_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A coroutine (that comes from a `gen` desugaring) is known to implement
    /// `Iterator<Item = O>`, where `O` is given by the generator's yield type
    /// that was computed during type-checking.
    fn consider_builtin_iterator_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A coroutine (that comes from a `gen` desugaring) is known to implement
    /// `FusedIterator`
    fn consider_builtin_fused_iterator_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    fn consider_builtin_async_iterator_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// A coroutine (that doesn't come from an `async` or `gen` desugaring) is known to
    /// implement `Coroutine<R, Yield = Y, Return = O>`, given the resume, yield,
    /// and return types of the coroutine computed during type-checking.
    fn consider_builtin_coroutine_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    fn consider_builtin_discriminant_kind_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    fn consider_builtin_async_destruct_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    fn consider_builtin_destruct_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    fn consider_builtin_transmute_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;

    /// Consider (possibly several) candidates to upcast or unsize a type to another
    /// type, excluding the coercion of a sized type into a `dyn Trait`.
    ///
    /// We return the `BuiltinImplSource` for each candidate as it is needed
    /// for unsize coercion in hir typeck and because it is difficult to
    /// otherwise recompute this for codegen. This is a bit of a mess but the
    /// easiest way to maintain the existing behavior for now.
    fn consider_structural_builtin_unsize_candidates(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Vec<Candidate<I>>;

    fn consider_builtin_effects_intersection_candidate(
        ecx: &mut EvalCtxt<'_, D>,
        goal: Goal<I, Self>,
    ) -> Result<Candidate<I>, NoSolution>;
}

impl<D, I> EvalCtxt<'_, D>
where
    D: SolverDelegate<Interner = I>,
    I: Interner,
{
    pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
    ) -> Vec<Candidate<I>> {
        let Ok(normalized_self_ty) =
            self.structurally_normalize_ty(goal.param_env, goal.predicate.self_ty())
        else {
            return vec![];
        };

        if normalized_self_ty.is_ty_var() {
            debug!("self type has been normalized to infer");
            return self.forced_ambiguity(MaybeCause::Ambiguity).into_iter().collect();
        }

        let goal: Goal<I, G> =
            goal.with(self.cx(), goal.predicate.with_self_ty(self.cx(), normalized_self_ty));
        // Vars that show up in the rest of the goal substs may have been constrained by
        // normalizing the self type as well, since type variables are not uniquified.
        let goal = self.resolve_vars_if_possible(goal);

        let mut candidates = vec![];

        self.assemble_impl_candidates(goal, &mut candidates);

        self.assemble_builtin_impl_candidates(goal, &mut candidates);

        self.assemble_alias_bound_candidates(goal, &mut candidates);

        self.assemble_object_bound_candidates(goal, &mut candidates);

        self.assemble_param_env_candidates(goal, &mut candidates);

        match self.solver_mode() {
            SolverMode::Normal => self.discard_impls_shadowed_by_env(goal, &mut candidates),
            SolverMode::Coherence => {
                self.assemble_coherence_unknowable_candidates(goal, &mut candidates)
            }
        }

        candidates
    }

    pub(super) fn forced_ambiguity(
        &mut self,
        cause: MaybeCause,
    ) -> Result<Candidate<I>, NoSolution> {
        // This may fail if `try_evaluate_added_goals` overflows because it
        // fails to reach a fixpoint but ends up getting an error after
        // running for some additional step.
        //
        // cc trait-system-refactor-initiative#105
        let source = CandidateSource::BuiltinImpl(BuiltinImplSource::Misc);
        let certainty = Certainty::Maybe(cause);
        self.probe_trait_candidate(source)
            .enter(|this| this.evaluate_added_goals_and_make_canonical_response(certainty))
    }

    #[instrument(level = "trace", skip_all)]
    fn assemble_impl_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let cx = self.cx();
        cx.for_each_relevant_impl(
            goal.predicate.trait_def_id(cx),
            goal.predicate.self_ty(),
            |impl_def_id| {
                // For every `default impl`, there's always a non-default `impl`
                // that will *also* apply. There's no reason to register a candidate
                // for this impl, since it is *not* proof that the trait goal holds.
                if cx.impl_is_default(impl_def_id) {
                    return;
                }

                match G::consider_impl_candidate(self, goal, impl_def_id) {
                    Ok(candidate) => candidates.push(candidate),
                    Err(NoSolution) => (),
                }
            },
        );
    }

    #[instrument(level = "trace", skip_all)]
    fn assemble_builtin_impl_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let cx = self.cx();
        let trait_def_id = goal.predicate.trait_def_id(cx);

        // N.B. When assembling built-in candidates for lang items that are also
        // `auto` traits, then the auto trait candidate that is assembled in
        // `consider_auto_trait_candidate` MUST be disqualified to remain sound.
        //
        // Instead of adding the logic here, it's a better idea to add it in
        // `EvalCtxt::disqualify_auto_trait_candidate_due_to_possible_impl` in
        // `solve::trait_goals` instead.
        let result = if let Err(guar) = goal.predicate.error_reported() {
            G::consider_error_guaranteed_candidate(self, guar)
        } else if cx.trait_is_auto(trait_def_id) {
            G::consider_auto_trait_candidate(self, goal)
        } else if cx.trait_is_alias(trait_def_id) {
            G::consider_trait_alias_candidate(self, goal)
        } else {
            match cx.as_lang_item(trait_def_id) {
                Some(TraitSolverLangItem::Sized) => G::consider_builtin_sized_candidate(self, goal),
                Some(TraitSolverLangItem::Copy | TraitSolverLangItem::Clone) => {
                    G::consider_builtin_copy_clone_candidate(self, goal)
                }
                Some(TraitSolverLangItem::Fn) => {
                    G::consider_builtin_fn_trait_candidates(self, goal, ty::ClosureKind::Fn)
                }
                Some(TraitSolverLangItem::FnMut) => {
                    G::consider_builtin_fn_trait_candidates(self, goal, ty::ClosureKind::FnMut)
                }
                Some(TraitSolverLangItem::FnOnce) => {
                    G::consider_builtin_fn_trait_candidates(self, goal, ty::ClosureKind::FnOnce)
                }
                Some(TraitSolverLangItem::AsyncFn) => {
                    G::consider_builtin_async_fn_trait_candidates(self, goal, ty::ClosureKind::Fn)
                }
                Some(TraitSolverLangItem::AsyncFnMut) => {
                    G::consider_builtin_async_fn_trait_candidates(
                        self,
                        goal,
                        ty::ClosureKind::FnMut,
                    )
                }
                Some(TraitSolverLangItem::AsyncFnOnce) => {
                    G::consider_builtin_async_fn_trait_candidates(
                        self,
                        goal,
                        ty::ClosureKind::FnOnce,
                    )
                }
                Some(TraitSolverLangItem::PointerLike) => {
                    G::consider_builtin_pointer_like_candidate(self, goal)
                }
                Some(TraitSolverLangItem::FnPtrTrait) => {
                    G::consider_builtin_fn_ptr_trait_candidate(self, goal)
                }
                Some(TraitSolverLangItem::AsyncFnKindHelper) => {
                    G::consider_builtin_async_fn_kind_helper_candidate(self, goal)
                }
                Some(TraitSolverLangItem::Tuple) => G::consider_builtin_tuple_candidate(self, goal),
                Some(TraitSolverLangItem::PointeeTrait) => {
                    G::consider_builtin_pointee_candidate(self, goal)
                }
                Some(TraitSolverLangItem::Future) => {
                    G::consider_builtin_future_candidate(self, goal)
                }
                Some(TraitSolverLangItem::Iterator) => {
                    G::consider_builtin_iterator_candidate(self, goal)
                }
                Some(TraitSolverLangItem::FusedIterator) => {
                    G::consider_builtin_fused_iterator_candidate(self, goal)
                }
                Some(TraitSolverLangItem::AsyncIterator) => {
                    G::consider_builtin_async_iterator_candidate(self, goal)
                }
                Some(TraitSolverLangItem::Coroutine) => {
                    G::consider_builtin_coroutine_candidate(self, goal)
                }
                Some(TraitSolverLangItem::DiscriminantKind) => {
                    G::consider_builtin_discriminant_kind_candidate(self, goal)
                }
                Some(TraitSolverLangItem::AsyncDestruct) => {
                    G::consider_builtin_async_destruct_candidate(self, goal)
                }
                Some(TraitSolverLangItem::Destruct) => {
                    G::consider_builtin_destruct_candidate(self, goal)
                }
                Some(TraitSolverLangItem::TransmuteTrait) => {
                    G::consider_builtin_transmute_candidate(self, goal)
                }
                Some(TraitSolverLangItem::EffectsIntersection) => {
                    G::consider_builtin_effects_intersection_candidate(self, goal)
                }
                _ => Err(NoSolution),
            }
        };

        candidates.extend(result);

        // There may be multiple unsize candidates for a trait with several supertraits:
        // `trait Foo: Bar<A> + Bar<B>` and `dyn Foo: Unsize<dyn Bar<_>>`
        if cx.is_lang_item(trait_def_id, TraitSolverLangItem::Unsize) {
            candidates.extend(G::consider_structural_builtin_unsize_candidates(self, goal));
        }
    }

    #[instrument(level = "trace", skip_all)]
    fn assemble_param_env_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        for (i, assumption) in goal.param_env.caller_bounds().into_iter().enumerate() {
            candidates.extend(G::probe_and_consider_implied_clause(
                self,
                CandidateSource::ParamEnv(i),
                goal,
                assumption,
                [],
            ));
        }
    }

    #[instrument(level = "trace", skip_all)]
    fn assemble_alias_bound_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let () = self.probe(|_| ProbeKind::NormalizedSelfTyAssembly).enter(|ecx| {
            ecx.assemble_alias_bound_candidates_recur(goal.predicate.self_ty(), goal, candidates);
        });
    }

    /// For some deeply nested `<T>::A::B::C::D` rigid associated type,
    /// we should explore the item bounds for all levels, since the
    /// `associated_type_bounds` feature means that a parent associated
    /// type may carry bounds for a nested associated type.
    ///
    /// If we have a projection, check that its self type is a rigid projection.
    /// If so, continue searching by recursively calling after normalization.
    // FIXME: This may recurse infinitely, but I can't seem to trigger it without
    // hitting another overflow error something. Add a depth parameter needed later.
    fn assemble_alias_bound_candidates_recur<G: GoalKind<D>>(
        &mut self,
        self_ty: I::Ty,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let (kind, alias_ty) = match self_ty.kind() {
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Adt(_, _)
            | ty::Foreign(_)
            | ty::Str
            | ty::Array(_, _)
            | ty::Pat(_, _)
            | ty::Slice(_)
            | ty::RawPtr(_, _)
            | ty::Ref(_, _, _)
            | ty::FnDef(_, _)
            | ty::FnPtr(_)
            | ty::Dynamic(..)
            | ty::Closure(..)
            | ty::CoroutineClosure(..)
            | ty::Coroutine(..)
            | ty::CoroutineWitness(..)
            | ty::Never
            | ty::Tuple(_)
            | ty::Param(_)
            | ty::Placeholder(..)
            | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
            | ty::Error(_) => return,
            ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) | ty::Bound(..) => {
                panic!("unexpected self type for `{goal:?}`")
            }

            ty::Infer(ty::TyVar(_)) => {
                // If we hit infer when normalizing the self type of an alias,
                // then bail with ambiguity. We should never encounter this on
                // the *first* iteration of this recursive function.
                if let Ok(result) =
                    self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                {
                    candidates.push(Candidate { source: CandidateSource::AliasBound, result });
                }
                return;
            }

            ty::Alias(kind @ (ty::Projection | ty::Opaque), alias_ty) => (kind, alias_ty),
            ty::Alias(ty::Inherent | ty::Weak, _) => {
                self.cx().delay_bug(format!("could not normalize {self_ty:?}, it is not WF"));
                return;
            }
        };

        for assumption in
            self.cx().item_bounds(alias_ty.def_id).iter_instantiated(self.cx(), alias_ty.args)
        {
            candidates.extend(G::probe_and_consider_implied_clause(
                self,
                CandidateSource::AliasBound,
                goal,
                assumption,
                [],
            ));
        }

        if kind != ty::Projection {
            return;
        }

        // Recurse on the self type of the projection.
        match self.structurally_normalize_ty(goal.param_env, alias_ty.self_ty()) {
            Ok(next_self_ty) => {
                self.assemble_alias_bound_candidates_recur(next_self_ty, goal, candidates)
            }
            Err(NoSolution) => {}
        }
    }

    #[instrument(level = "trace", skip_all)]
    fn assemble_object_bound_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let cx = self.cx();
        if !cx.trait_may_be_implemented_via_object(goal.predicate.trait_def_id(cx)) {
            return;
        }

        let self_ty = goal.predicate.self_ty();
        let bounds = match self_ty.kind() {
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Adt(_, _)
            | ty::Foreign(_)
            | ty::Str
            | ty::Array(_, _)
            | ty::Pat(_, _)
            | ty::Slice(_)
            | ty::RawPtr(_, _)
            | ty::Ref(_, _, _)
            | ty::FnDef(_, _)
            | ty::FnPtr(_)
            | ty::Alias(..)
            | ty::Closure(..)
            | ty::CoroutineClosure(..)
            | ty::Coroutine(..)
            | ty::CoroutineWitness(..)
            | ty::Never
            | ty::Tuple(_)
            | ty::Param(_)
            | ty::Placeholder(..)
            | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
            | ty::Error(_) => return,
            ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
            | ty::Bound(..) => panic!("unexpected self type for `{goal:?}`"),
            ty::Dynamic(bounds, ..) => bounds,
        };

        // Do not consider built-in object impls for non-object-safe types.
        if bounds.principal_def_id().is_some_and(|def_id| !cx.trait_is_object_safe(def_id)) {
            return;
        }

        // Consider all of the auto-trait and projection bounds, which don't
        // need to be recorded as a `BuiltinImplSource::Object` since they don't
        // really have a vtable base...
        for bound in bounds.iter() {
            match bound.skip_binder() {
                ty::ExistentialPredicate::Trait(_) => {
                    // Skip principal
                }
                ty::ExistentialPredicate::Projection(_)
                | ty::ExistentialPredicate::AutoTrait(_) => {
                    candidates.extend(G::probe_and_consider_object_bound_candidate(
                        self,
                        CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
                        goal,
                        bound.with_self_ty(cx, self_ty),
                    ));
                }
            }
        }

        // FIXME: We only need to do *any* of this if we're considering a trait goal,
        // since we don't need to look at any supertrait or anything if we are doing
        // a projection goal.
        if let Some(principal) = bounds.principal() {
            let principal_trait_ref = principal.with_self_ty(cx, self_ty);
            for (idx, assumption) in elaborate::supertraits(cx, principal_trait_ref).enumerate() {
                candidates.extend(G::probe_and_consider_object_bound_candidate(
                    self,
                    CandidateSource::BuiltinImpl(BuiltinImplSource::Object(idx)),
                    goal,
                    assumption.upcast(cx),
                ));
            }
        }
    }

    /// In coherence we have to not only care about all impls we know about, but
    /// also consider impls which may get added in a downstream or sibling crate
    /// or which an upstream impl may add in a minor release.
    ///
    /// To do so we add an ambiguous candidate in case such an unknown impl could
    /// apply to the current goal.
    #[instrument(level = "trace", skip_all)]
    fn assemble_coherence_unknowable_candidates<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let cx = self.cx();

        candidates.extend(self.probe_trait_candidate(CandidateSource::CoherenceUnknowable).enter(
            |ecx| {
                let trait_ref = goal.predicate.trait_ref(cx);
                if ecx.trait_ref_is_knowable(goal.param_env, trait_ref)? {
                    Err(NoSolution)
                } else {
                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                }
            },
        ))
    }

    /// If there's a where-bound for the current goal, do not use any impl candidates
    /// to prove the current goal. Most importantly, if there is a where-bound which does
    /// not specify any associated types, we do not allow normalizing the associated type
    /// by using an impl, even if it would apply.
    ///
    ///  <https://github.com/rust-lang/trait-system-refactor-initiative/issues/76>
    // FIXME(@lcnr): The current structure here makes me unhappy and feels ugly. idk how
    // to improve this however. However, this should make it fairly straightforward to refine
    // the filtering going forward, so it seems alright-ish for now.
    #[instrument(level = "debug", skip(self, goal))]
    fn discard_impls_shadowed_by_env<G: GoalKind<D>>(
        &mut self,
        goal: Goal<I, G>,
        candidates: &mut Vec<Candidate<I>>,
    ) {
        let cx = self.cx();
        let trait_goal: Goal<I, ty::TraitPredicate<I>> =
            goal.with(cx, goal.predicate.trait_ref(cx));

        let mut trait_candidates_from_env = vec![];
        self.probe(|_| ProbeKind::ShadowedEnvProbing).enter(|ecx| {
            ecx.assemble_param_env_candidates(trait_goal, &mut trait_candidates_from_env);
            ecx.assemble_alias_bound_candidates(trait_goal, &mut trait_candidates_from_env);
        });

        if !trait_candidates_from_env.is_empty() {
            let trait_env_result = self.merge_candidates(trait_candidates_from_env);
            match trait_env_result.unwrap().value.certainty {
                // If proving the trait goal succeeds by using the env,
                // we freely drop all impl candidates.
                //
                // FIXME(@lcnr): It feels like this could easily hide
                // a forced ambiguity candidate added earlier.
                // This feels dangerous.
                Certainty::Yes => {
                    candidates.retain(|c| match c.source {
                        CandidateSource::Impl(_) | CandidateSource::BuiltinImpl(_) => {
                            debug!(?c, "discard impl candidate");
                            false
                        }
                        CandidateSource::ParamEnv(_) | CandidateSource::AliasBound => true,
                        CandidateSource::CoherenceUnknowable => panic!("uh oh"),
                    });
                }
                // If it is still ambiguous we instead just force the whole goal
                // to be ambig and wait for inference constraints. See
                // tests/ui/traits/next-solver/env-shadows-impls/ambig-env-no-shadow.rs
                Certainty::Maybe(cause) => {
                    debug!(?cause, "force ambiguity");
                    *candidates = self.forced_ambiguity(cause).into_iter().collect();
                }
            }
        }
    }

    /// If there are multiple ways to prove a trait or projection goal, we have
    /// to somehow try to merge the candidates into one. If that fails, we return
    /// ambiguity.
    #[instrument(level = "debug", skip(self), ret)]
    pub(super) fn merge_candidates(&mut self, candidates: Vec<Candidate<I>>) -> QueryResult<I> {
        // First try merging all candidates. This is complete and fully sound.
        let responses = candidates.iter().map(|c| c.result).collect::<Vec<_>>();
        if let Some(result) = self.try_merge_responses(&responses) {
            return Ok(result);
        } else {
            self.flounder(&responses)
        }
    }
}