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
//! See Rustc Dev Guide chapters on [trait-resolution] and [trait-specialization] for more info on
//! how this works.
//!
//! [trait-resolution]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
//! [trait-specialization]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html

use std::fmt::Debug;

use rustc_data_structures::fx::FxIndexSet;
use rustc_errors::{Diag, EmissionGuarantee};
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, TyCtxtInferExt};
use rustc_middle::bug;
use rustc_middle::traits::query::NoSolution;
use rustc_middle::traits::solve::{CandidateSource, Certainty, Goal};
use rustc_middle::traits::specialization_graph::OverlapMode;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
use rustc_middle::ty::{self, Ty, TyCtxt};
pub use rustc_next_trait_solver::coherence::*;
use rustc_span::symbol::sym;
use rustc_span::{Span, DUMMY_SP};
use tracing::{debug, instrument, warn};

use super::ObligationCtxt;
use crate::error_reporting::traits::suggest_new_overflow_limit;
use crate::infer::outlives::env::OutlivesEnvironment;
use crate::infer::InferOk;
use crate::solve::inspect::{InspectGoal, ProofTreeInferCtxtExt, ProofTreeVisitor};
use crate::solve::{deeply_normalize_for_diagnostics, inspect};
use crate::traits::select::IntercrateAmbiguityCause;
use crate::traits::{
    util, FulfillmentErrorCode, NormalizeExt, Obligation, ObligationCause, PredicateObligation,
    SelectionContext, SkipLeakCheck,
};

pub struct OverlapResult<'tcx> {
    pub impl_header: ty::ImplHeader<'tcx>,
    pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause<'tcx>>,

    /// `true` if the overlap might've been permitted before the shift
    /// to universes.
    pub involves_placeholder: bool,

    /// Used in the new solver to suggest increasing the recursion limit.
    pub overflowing_predicates: Vec<ty::Predicate<'tcx>>,
}

pub fn add_placeholder_note<G: EmissionGuarantee>(err: &mut Diag<'_, G>) {
    err.note(
        "this behavior recently changed as a result of a bug fix; \
         see rust-lang/rust#56105 for details",
    );
}

pub fn suggest_increasing_recursion_limit<'tcx, G: EmissionGuarantee>(
    tcx: TyCtxt<'tcx>,
    err: &mut Diag<'_, G>,
    overflowing_predicates: &[ty::Predicate<'tcx>],
) {
    for pred in overflowing_predicates {
        err.note(format!("overflow evaluating the requirement `{}`", pred));
    }

    suggest_new_overflow_limit(tcx, err);
}

#[derive(Debug, Clone, Copy)]
enum TrackAmbiguityCauses {
    Yes,
    No,
}

impl TrackAmbiguityCauses {
    fn is_yes(self) -> bool {
        match self {
            TrackAmbiguityCauses::Yes => true,
            TrackAmbiguityCauses::No => false,
        }
    }
}

/// If there are types that satisfy both impls, returns `Some`
/// with a suitably-freshened `ImplHeader` with those types
/// instantiated. Otherwise, returns `None`.
#[instrument(skip(tcx, skip_leak_check), level = "debug")]
pub fn overlapping_impls(
    tcx: TyCtxt<'_>,
    impl1_def_id: DefId,
    impl2_def_id: DefId,
    skip_leak_check: SkipLeakCheck,
    overlap_mode: OverlapMode,
) -> Option<OverlapResult<'_>> {
    // Before doing expensive operations like entering an inference context, do
    // a quick check via fast_reject to tell if the impl headers could possibly
    // unify.
    let drcx = DeepRejectCtxt::new(tcx, TreatParams::AsCandidateKey);
    let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
    let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
    let may_overlap = match (impl1_ref, impl2_ref) {
        (Some(a), Some(b)) => drcx.args_may_unify(a.skip_binder().args, b.skip_binder().args),
        (None, None) => {
            let self_ty1 = tcx.type_of(impl1_def_id).skip_binder();
            let self_ty2 = tcx.type_of(impl2_def_id).skip_binder();
            drcx.types_may_unify(self_ty1, self_ty2)
        }
        _ => bug!("unexpected impls: {impl1_def_id:?} {impl2_def_id:?}"),
    };

    if !may_overlap {
        // Some types involved are definitely different, so the impls couldn't possibly overlap.
        debug!("overlapping_impls: fast_reject early-exit");
        return None;
    }

    let _overlap_with_bad_diagnostics = overlap(
        tcx,
        TrackAmbiguityCauses::No,
        skip_leak_check,
        impl1_def_id,
        impl2_def_id,
        overlap_mode,
    )?;

    // In the case where we detect an error, run the check again, but
    // this time tracking intercrate ambiguity causes for better
    // diagnostics. (These take time and can lead to false errors.)
    let overlap = overlap(
        tcx,
        TrackAmbiguityCauses::Yes,
        skip_leak_check,
        impl1_def_id,
        impl2_def_id,
        overlap_mode,
    )
    .unwrap();
    Some(overlap)
}

fn fresh_impl_header<'tcx>(infcx: &InferCtxt<'tcx>, impl_def_id: DefId) -> ty::ImplHeader<'tcx> {
    let tcx = infcx.tcx;
    let impl_args = infcx.fresh_args_for_item(DUMMY_SP, impl_def_id);

    ty::ImplHeader {
        impl_def_id,
        impl_args,
        self_ty: tcx.type_of(impl_def_id).instantiate(tcx, impl_args),
        trait_ref: tcx.impl_trait_ref(impl_def_id).map(|i| i.instantiate(tcx, impl_args)),
        predicates: tcx
            .predicates_of(impl_def_id)
            .instantiate(tcx, impl_args)
            .iter()
            .map(|(c, _)| c.as_predicate())
            .collect(),
    }
}

fn fresh_impl_header_normalized<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    impl_def_id: DefId,
) -> ty::ImplHeader<'tcx> {
    let header = fresh_impl_header(infcx, impl_def_id);

    let InferOk { value: mut header, obligations } =
        infcx.at(&ObligationCause::dummy(), param_env).normalize(header);

    header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
    header
}

/// Can both impl `a` and impl `b` be satisfied by a common type (including
/// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
#[instrument(level = "debug", skip(tcx))]
fn overlap<'tcx>(
    tcx: TyCtxt<'tcx>,
    track_ambiguity_causes: TrackAmbiguityCauses,
    skip_leak_check: SkipLeakCheck,
    impl1_def_id: DefId,
    impl2_def_id: DefId,
    overlap_mode: OverlapMode,
) -> Option<OverlapResult<'tcx>> {
    if overlap_mode.use_negative_impl() {
        if impl_intersection_has_negative_obligation(tcx, impl1_def_id, impl2_def_id)
            || impl_intersection_has_negative_obligation(tcx, impl2_def_id, impl1_def_id)
        {
            return None;
        }
    }

    let infcx = tcx
        .infer_ctxt()
        .skip_leak_check(skip_leak_check.is_yes())
        .intercrate(true)
        .with_next_trait_solver(tcx.next_trait_solver_in_coherence())
        .build();
    let selcx = &mut SelectionContext::new(&infcx);
    if track_ambiguity_causes.is_yes() {
        selcx.enable_tracking_intercrate_ambiguity_causes();
    }

    // For the purposes of this check, we don't bring any placeholder
    // types into scope; instead, we replace the generic types with
    // fresh type variables, and hence we do our evaluations in an
    // empty environment.
    let param_env = ty::ParamEnv::empty();

    let impl1_header = fresh_impl_header_normalized(selcx.infcx, param_env, impl1_def_id);
    let impl2_header = fresh_impl_header_normalized(selcx.infcx, param_env, impl2_def_id);

    // Equate the headers to find their intersection (the general type, with infer vars,
    // that may apply both impls).
    let mut obligations =
        equate_impl_headers(selcx.infcx, param_env, &impl1_header, &impl2_header)?;
    debug!("overlap: unification check succeeded");

    obligations.extend(
        [&impl1_header.predicates, &impl2_header.predicates].into_iter().flatten().map(
            |&predicate| Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, predicate),
        ),
    );

    let mut overflowing_predicates = Vec::new();
    if overlap_mode.use_implicit_negative() {
        match impl_intersection_has_impossible_obligation(selcx, &obligations) {
            IntersectionHasImpossibleObligations::Yes => return None,
            IntersectionHasImpossibleObligations::No { overflowing_predicates: p } => {
                overflowing_predicates = p
            }
        }
    }

    // We toggle the `leak_check` by using `skip_leak_check` when constructing the
    // inference context, so this may be a noop.
    if infcx.leak_check(ty::UniverseIndex::ROOT, None).is_err() {
        debug!("overlap: leak check failed");
        return None;
    }

    let intercrate_ambiguity_causes = if !overlap_mode.use_implicit_negative() {
        Default::default()
    } else if infcx.next_trait_solver() {
        compute_intercrate_ambiguity_causes(&infcx, &obligations)
    } else {
        selcx.take_intercrate_ambiguity_causes()
    };

    debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
    let involves_placeholder = infcx
        .inner
        .borrow_mut()
        .unwrap_region_constraints()
        .data()
        .constraints
        .iter()
        .any(|c| c.0.involves_placeholders());

    let mut impl_header = infcx.resolve_vars_if_possible(impl1_header);

    // Deeply normalize the impl header for diagnostics, ignoring any errors if this fails.
    if infcx.next_trait_solver() {
        impl_header = deeply_normalize_for_diagnostics(&infcx, param_env, impl_header);
    }

    Some(OverlapResult {
        impl_header,
        intercrate_ambiguity_causes,
        involves_placeholder,
        overflowing_predicates,
    })
}

#[instrument(level = "debug", skip(infcx), ret)]
fn equate_impl_headers<'tcx>(
    infcx: &InferCtxt<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    impl1: &ty::ImplHeader<'tcx>,
    impl2: &ty::ImplHeader<'tcx>,
) -> Option<Vec<PredicateObligation<'tcx>>> {
    let result =
        match (impl1.trait_ref, impl2.trait_ref) {
            (Some(impl1_ref), Some(impl2_ref)) => infcx
                .at(&ObligationCause::dummy(), param_env)
                .eq(DefineOpaqueTypes::Yes, impl1_ref, impl2_ref),
            (None, None) => infcx.at(&ObligationCause::dummy(), param_env).eq(
                DefineOpaqueTypes::Yes,
                impl1.self_ty,
                impl2.self_ty,
            ),
            _ => bug!("equate_impl_headers given mismatched impl kinds"),
        };

    result.map(|infer_ok| infer_ok.obligations).ok()
}

/// The result of [fn impl_intersection_has_impossible_obligation].
enum IntersectionHasImpossibleObligations<'tcx> {
    Yes,
    No {
        /// With `-Znext-solver=coherence`, some obligations may
        /// fail if only the user increased the recursion limit.
        ///
        /// We return those obligations here and mention them in the
        /// error message.
        overflowing_predicates: Vec<ty::Predicate<'tcx>>,
    },
}

/// Check if both impls can be satisfied by a common type by considering whether
/// any of either impl's obligations is not known to hold.
///
/// For example, given these two impls:
///     `impl From<MyLocalType> for Box<dyn Error>` (in my crate)
///     `impl<E> From<E> for Box<dyn Error> where E: Error` (in libstd)
///
/// After replacing both impl headers with inference vars (which happens before
/// this function is called), we get:
///     `Box<dyn Error>: From<MyLocalType>`
///     `Box<dyn Error>: From<?E>`
///
/// This gives us `?E = MyLocalType`. We then certainly know that `MyLocalType: Error`
/// never holds in intercrate mode since a local impl does not exist, and a
/// downstream impl cannot be added -- therefore can consider the intersection
/// of the two impls above to be empty.
///
/// Importantly, this works even if there isn't a `impl !Error for MyLocalType`.
fn impl_intersection_has_impossible_obligation<'a, 'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>,
    obligations: &'a [PredicateObligation<'tcx>],
) -> IntersectionHasImpossibleObligations<'tcx> {
    let infcx = selcx.infcx;

    if infcx.next_trait_solver() {
        let ocx = ObligationCtxt::new_with_diagnostics(infcx);
        ocx.register_obligations(obligations.iter().cloned());
        let errors_and_ambiguities = ocx.select_all_or_error();
        // We only care about the obligations that are *definitely* true errors.
        // Ambiguities do not prove the disjointness of two impls.
        let (errors, ambiguities): (Vec<_>, Vec<_>) =
            errors_and_ambiguities.into_iter().partition(|error| error.is_true_error());

        if errors.is_empty() {
            IntersectionHasImpossibleObligations::No {
                overflowing_predicates: ambiguities
                    .into_iter()
                    .filter(|error| {
                        matches!(
                            error.code,
                            FulfillmentErrorCode::Ambiguity { overflow: Some(true) }
                        )
                    })
                    .map(|e| infcx.resolve_vars_if_possible(e.obligation.predicate))
                    .collect(),
            }
        } else {
            IntersectionHasImpossibleObligations::Yes
        }
    } else {
        for obligation in obligations {
            // We use `evaluate_root_obligation` to correctly track intercrate
            // ambiguity clauses.
            let evaluation_result = selcx.evaluate_root_obligation(obligation);

            match evaluation_result {
                Ok(result) => {
                    if !result.may_apply() {
                        return IntersectionHasImpossibleObligations::Yes;
                    }
                }
                // If overflow occurs, we need to conservatively treat the goal as possibly holding,
                // since there can be instantiations of this goal that don't overflow and result in
                // success. While this isn't much of a problem in the old solver, since we treat overflow
                // fatally, this still can be encountered: <https://github.com/rust-lang/rust/issues/105231>.
                Err(_overflow) => {}
            }
        }

        IntersectionHasImpossibleObligations::No { overflowing_predicates: Vec::new() }
    }
}

/// Check if both impls can be satisfied by a common type by considering whether
/// any of first impl's obligations is known not to hold *via a negative predicate*.
///
/// For example, given these two impls:
///     `struct MyCustomBox<T: ?Sized>(Box<T>);`
///     `impl From<&str> for MyCustomBox<dyn Error>` (in my crate)
///     `impl<E> From<E> for MyCustomBox<dyn Error> where E: Error` (in my crate)
///
/// After replacing the second impl's header with inference vars, we get:
///     `MyCustomBox<dyn Error>: From<&str>`
///     `MyCustomBox<dyn Error>: From<?E>`
///
/// This gives us `?E = &str`. We then try to prove the first impl's predicates
/// after negating, giving us `&str: !Error`. This is a negative impl provided by
/// libstd, and therefore we can guarantee for certain that libstd will never add
/// a positive impl for `&str: Error` (without it being a breaking change).
fn impl_intersection_has_negative_obligation(
    tcx: TyCtxt<'_>,
    impl1_def_id: DefId,
    impl2_def_id: DefId,
) -> bool {
    debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);

    // N.B. We need to unify impl headers *with* intercrate mode, even if proving negative predicates
    // do not need intercrate mode enabled.
    let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
    let root_universe = infcx.universe();
    assert_eq!(root_universe, ty::UniverseIndex::ROOT);

    let impl1_header = fresh_impl_header(infcx, impl1_def_id);
    let param_env =
        ty::EarlyBinder::bind(tcx.param_env(impl1_def_id)).instantiate(tcx, impl1_header.impl_args);

    let impl2_header = fresh_impl_header(infcx, impl2_def_id);

    // Equate the headers to find their intersection (the general type, with infer vars,
    // that may apply both impls).
    let Some(equate_obligations) =
        equate_impl_headers(infcx, param_env, &impl1_header, &impl2_header)
    else {
        return false;
    };

    // FIXME(with_negative_coherence): the infcx has constraints from equating
    // the impl headers. We should use these constraints as assumptions, not as
    // requirements, when proving the negated where clauses below.
    drop(equate_obligations);
    drop(infcx.take_registered_region_obligations());
    drop(infcx.take_and_reset_region_constraints());

    plug_infer_with_placeholders(
        infcx,
        root_universe,
        (impl1_header.impl_args, impl2_header.impl_args),
    );
    let param_env = infcx.resolve_vars_if_possible(param_env);

    util::elaborate(tcx, tcx.predicates_of(impl2_def_id).instantiate(tcx, impl2_header.impl_args))
        .any(|(clause, _)| try_prove_negated_where_clause(infcx, clause, param_env))
}

fn plug_infer_with_placeholders<'tcx>(
    infcx: &InferCtxt<'tcx>,
    universe: ty::UniverseIndex,
    value: impl TypeVisitable<TyCtxt<'tcx>>,
) {
    struct PlugInferWithPlaceholder<'a, 'tcx> {
        infcx: &'a InferCtxt<'tcx>,
        universe: ty::UniverseIndex,
        var: ty::BoundVar,
    }

    impl<'tcx> PlugInferWithPlaceholder<'_, 'tcx> {
        fn next_var(&mut self) -> ty::BoundVar {
            let var = self.var;
            self.var = self.var + 1;
            var
        }
    }

    impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for PlugInferWithPlaceholder<'_, 'tcx> {
        fn visit_ty(&mut self, ty: Ty<'tcx>) {
            let ty = self.infcx.shallow_resolve(ty);
            if ty.is_ty_var() {
                let Ok(InferOk { value: (), obligations }) =
                    self.infcx.at(&ObligationCause::dummy(), ty::ParamEnv::empty()).eq(
                        // Comparing against a type variable never registers hidden types anyway
                        DefineOpaqueTypes::Yes,
                        ty,
                        Ty::new_placeholder(
                            self.infcx.tcx,
                            ty::Placeholder {
                                universe: self.universe,
                                bound: ty::BoundTy {
                                    var: self.next_var(),
                                    kind: ty::BoundTyKind::Anon,
                                },
                            },
                        ),
                    )
                else {
                    bug!("we always expect to be able to plug an infer var with placeholder")
                };
                assert_eq!(obligations, &[]);
            } else {
                ty.super_visit_with(self);
            }
        }

        fn visit_const(&mut self, ct: ty::Const<'tcx>) {
            let ct = self.infcx.shallow_resolve_const(ct);
            if ct.is_ct_infer() {
                let Ok(InferOk { value: (), obligations }) =
                    self.infcx.at(&ObligationCause::dummy(), ty::ParamEnv::empty()).eq(
                        // The types of the constants are the same, so there is no hidden type
                        // registration happening anyway.
                        DefineOpaqueTypes::Yes,
                        ct,
                        ty::Const::new_placeholder(
                            self.infcx.tcx,
                            ty::Placeholder { universe: self.universe, bound: self.next_var() },
                        ),
                    )
                else {
                    bug!("we always expect to be able to plug an infer var with placeholder")
                };
                assert_eq!(obligations, &[]);
            } else {
                ct.super_visit_with(self);
            }
        }

        fn visit_region(&mut self, r: ty::Region<'tcx>) {
            if let ty::ReVar(vid) = *r {
                let r = self
                    .infcx
                    .inner
                    .borrow_mut()
                    .unwrap_region_constraints()
                    .opportunistic_resolve_var(self.infcx.tcx, vid);
                if r.is_var() {
                    let Ok(InferOk { value: (), obligations }) =
                        self.infcx.at(&ObligationCause::dummy(), ty::ParamEnv::empty()).eq(
                            // Lifetimes don't contain opaque types (or any types for that matter).
                            DefineOpaqueTypes::Yes,
                            r,
                            ty::Region::new_placeholder(
                                self.infcx.tcx,
                                ty::Placeholder {
                                    universe: self.universe,
                                    bound: ty::BoundRegion {
                                        var: self.next_var(),
                                        kind: ty::BoundRegionKind::BrAnon,
                                    },
                                },
                            ),
                        )
                    else {
                        bug!("we always expect to be able to plug an infer var with placeholder")
                    };
                    assert_eq!(obligations, &[]);
                }
            }
        }
    }

    value.visit_with(&mut PlugInferWithPlaceholder { infcx, universe, var: ty::BoundVar::ZERO });
}

fn try_prove_negated_where_clause<'tcx>(
    root_infcx: &InferCtxt<'tcx>,
    clause: ty::Clause<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
) -> bool {
    let Some(negative_predicate) = clause.as_predicate().flip_polarity(root_infcx.tcx) else {
        return false;
    };

    // N.B. We don't need to use intercrate mode here because we're trying to prove
    // the *existence* of a negative goal, not the non-existence of a positive goal.
    // Without this, we over-eagerly register coherence ambiguity candidates when
    // impl candidates do exist.
    let ref infcx = root_infcx.fork_with_intercrate(false);
    let ocx = ObligationCtxt::new(infcx);
    ocx.register_obligation(Obligation::new(
        infcx.tcx,
        ObligationCause::dummy(),
        param_env,
        negative_predicate,
    ));
    if !ocx.select_all_or_error().is_empty() {
        return false;
    }

    // FIXME: We could use the assumed_wf_types from both impls, I think,
    // if that wasn't implemented just for LocalDefId, and we'd need to do
    // the normalization ourselves since this is totally fallible...
    let outlives_env = OutlivesEnvironment::new(param_env);
    let errors = ocx.resolve_regions(&outlives_env);
    if !errors.is_empty() {
        return false;
    }

    true
}

/// Compute the `intercrate_ambiguity_causes` for the new solver using
/// "proof trees".
///
/// This is a bit scuffed but seems to be good enough, at least
/// when looking at UI tests. Given that it is only used to improve
/// diagnostics this is good enough. We can always improve it once there
/// are test cases where it is currently not enough.
fn compute_intercrate_ambiguity_causes<'tcx>(
    infcx: &InferCtxt<'tcx>,
    obligations: &[PredicateObligation<'tcx>],
) -> FxIndexSet<IntercrateAmbiguityCause<'tcx>> {
    let mut causes: FxIndexSet<IntercrateAmbiguityCause<'tcx>> = Default::default();

    for obligation in obligations {
        search_ambiguity_causes(infcx, obligation.clone().into(), &mut causes);
    }

    causes
}

struct AmbiguityCausesVisitor<'a, 'tcx> {
    causes: &'a mut FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
}

impl<'a, 'tcx> ProofTreeVisitor<'tcx> for AmbiguityCausesVisitor<'a, 'tcx> {
    fn span(&self) -> Span {
        DUMMY_SP
    }

    fn visit_goal(&mut self, goal: &InspectGoal<'_, 'tcx>) {
        let infcx = goal.infcx();
        for cand in goal.candidates() {
            cand.visit_nested_in_probe(self);
        }
        // When searching for intercrate ambiguity causes, we only need to look
        // at ambiguous goals, as for others the coherence unknowable candidate
        // was irrelevant.
        match goal.result() {
            Ok(Certainty::Maybe(_)) => {}
            Ok(Certainty::Yes) | Err(NoSolution) => return,
        }

        let Goal { param_env, predicate } = goal.goal();

        // For bound predicates we simply call `infcx.enter_forall`
        // and then prove the resulting predicate as a nested goal.
        let trait_ref = match predicate.kind().no_bound_vars() {
            Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(tr))) => tr.trait_ref,
            Some(ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)))
                if matches!(
                    infcx.tcx.def_kind(proj.projection_term.def_id),
                    DefKind::AssocTy | DefKind::AssocConst
                ) =>
            {
                proj.projection_term.trait_ref(infcx.tcx)
            }
            _ => return,
        };

        // Add ambiguity causes for reservation impls.
        for cand in goal.candidates() {
            if let inspect::ProbeKind::TraitCandidate {
                source: CandidateSource::Impl(def_id),
                result: Ok(_),
            } = cand.kind()
            {
                if let ty::ImplPolarity::Reservation = infcx.tcx.impl_polarity(def_id) {
                    let message = infcx
                        .tcx
                        .get_attr(def_id, sym::rustc_reservation_impl)
                        .and_then(|a| a.value_str());
                    if let Some(message) = message {
                        self.causes.insert(IntercrateAmbiguityCause::ReservationImpl { message });
                    }
                }
            }
        }

        // Add ambiguity causes for unknowable goals.
        let mut ambiguity_cause = None;
        for cand in goal.candidates() {
            if let inspect::ProbeKind::TraitCandidate {
                source: CandidateSource::CoherenceUnknowable,
                result: Ok(_),
            } = cand.kind()
            {
                let lazily_normalize_ty = |mut ty: Ty<'tcx>| {
                    if matches!(ty.kind(), ty::Alias(..)) {
                        let ocx = ObligationCtxt::new(infcx);
                        ty = ocx
                            .structurally_normalize(&ObligationCause::dummy(), param_env, ty)
                            .map_err(|_| ())?;
                        if !ocx.select_where_possible().is_empty() {
                            return Err(());
                        }
                    }
                    Ok(ty)
                };

                infcx.probe(|_| {
                    match trait_ref_is_knowable(infcx, trait_ref, lazily_normalize_ty) {
                        Err(()) => {}
                        Ok(Ok(())) => warn!("expected an unknowable trait ref: {trait_ref:?}"),
                        Ok(Err(conflict)) => {
                            if !trait_ref.references_error() {
                                // Normalize the trait ref for diagnostics, ignoring any errors if this fails.
                                let trait_ref =
                                    deeply_normalize_for_diagnostics(infcx, param_env, trait_ref);

                                let self_ty = trait_ref.self_ty();
                                let self_ty = self_ty.has_concrete_skeleton().then(|| self_ty);
                                ambiguity_cause = Some(match conflict {
                                    Conflict::Upstream => {
                                        IntercrateAmbiguityCause::UpstreamCrateUpdate {
                                            trait_ref,
                                            self_ty,
                                        }
                                    }
                                    Conflict::Downstream => {
                                        IntercrateAmbiguityCause::DownstreamCrate {
                                            trait_ref,
                                            self_ty,
                                        }
                                    }
                                });
                            }
                        }
                    }
                })
            } else {
                match cand.result() {
                    // We only add an ambiguity cause if the goal would otherwise
                    // result in an error.
                    //
                    // FIXME: While this matches the behavior of the
                    // old solver, it is not the only way in which the unknowable
                    // candidates *weaken* coherence, they can also force otherwise
                    // sucessful normalization to be ambiguous.
                    Ok(Certainty::Maybe(_) | Certainty::Yes) => {
                        ambiguity_cause = None;
                        break;
                    }
                    Err(NoSolution) => continue,
                }
            }
        }

        if let Some(ambiguity_cause) = ambiguity_cause {
            self.causes.insert(ambiguity_cause);
        }
    }
}

fn search_ambiguity_causes<'tcx>(
    infcx: &InferCtxt<'tcx>,
    goal: Goal<'tcx, ty::Predicate<'tcx>>,
    causes: &mut FxIndexSet<IntercrateAmbiguityCause<'tcx>>,
) {
    infcx.probe(|_| infcx.visit_proof_tree(goal, &mut AmbiguityCausesVisitor { causes }));
}