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
use std::marker::PhantomData;
use std::mem;
use std::ops::ControlFlow;

use rustc_infer::infer::InferCtxt;
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::solve::{CandidateSource, GoalSource, MaybeCause};
use rustc_infer::traits::{
    self, FromSolverError, MismatchedProjectionTypes, Obligation, ObligationCause,
    ObligationCauseCode, PredicateObligation, SelectionError, TraitEngine,
};
use rustc_middle::bug;
use rustc_middle::ty::error::{ExpectedFound, TypeError};
use rustc_middle::ty::{self, TyCtxt};
use rustc_next_trait_solver::solve::{GenerateProofTree, SolverDelegateEvalExt as _};
use tracing::instrument;

use super::delegate::SolverDelegate;
use super::inspect::{self, ProofTreeInferCtxtExt, ProofTreeVisitor};
use super::Certainty;
use crate::traits::{FulfillmentError, FulfillmentErrorCode, ScrubbedTraitError};

/// A trait engine using the new trait solver.
///
/// This is mostly identical to how `evaluate_all` works inside of the
/// solver, except that the requirements are slightly different.
///
/// Unlike `evaluate_all` it is possible to add new obligations later on
/// and we also have to track diagnostics information by using `Obligation`
/// instead of `Goal`.
///
/// It is also likely that we want to use slightly different datastructures
/// here as this will have to deal with far more root goals than `evaluate_all`.
pub struct FulfillmentCtxt<'tcx, E: 'tcx> {
    obligations: ObligationStorage<'tcx>,

    /// The snapshot in which this context was created. Using the context
    /// outside of this snapshot leads to subtle bugs if the snapshot
    /// gets rolled back. Because of this we explicitly check that we only
    /// use the context in exactly this snapshot.
    usable_in_snapshot: usize,
    _errors: PhantomData<E>,
}

#[derive(Default)]
struct ObligationStorage<'tcx> {
    /// Obligations which resulted in an overflow in fulfillment itself.
    ///
    /// We cannot eagerly return these as error so we instead store them here
    /// to avoid recomputing them each time `select_where_possible` is called.
    /// This also allows us to return the correct `FulfillmentError` for them.
    overflowed: Vec<PredicateObligation<'tcx>>,
    pending: Vec<PredicateObligation<'tcx>>,
}

impl<'tcx> ObligationStorage<'tcx> {
    fn register(&mut self, obligation: PredicateObligation<'tcx>) {
        self.pending.push(obligation);
    }

    fn clone_pending(&self) -> Vec<PredicateObligation<'tcx>> {
        let mut obligations = self.pending.clone();
        obligations.extend(self.overflowed.iter().cloned());
        obligations
    }

    fn take_pending(&mut self) -> Vec<PredicateObligation<'tcx>> {
        let mut obligations = mem::take(&mut self.pending);
        obligations.append(&mut self.overflowed);
        obligations
    }

    fn unstalled_for_select(&mut self) -> impl Iterator<Item = PredicateObligation<'tcx>> {
        mem::take(&mut self.pending).into_iter()
    }

    fn on_fulfillment_overflow(&mut self, infcx: &InferCtxt<'tcx>) {
        infcx.probe(|_| {
            // IMPORTANT: we must not use solve any inference variables in the obligations
            // as this is all happening inside of a probe. We use a probe to make sure
            // we get all obligations involved in the overflow. We pretty much check: if
            // we were to do another step of `select_where_possible`, which goals would
            // change.
            self.overflowed.extend(self.pending.extract_if(|o| {
                let goal = o.clone().into();
                let result = <&SolverDelegate<'tcx>>::from(infcx)
                    .evaluate_root_goal(goal, GenerateProofTree::No)
                    .0;
                match result {
                    Ok((has_changed, _)) => has_changed,
                    _ => false,
                }
            }));
        })
    }
}

impl<'tcx, E: 'tcx> FulfillmentCtxt<'tcx, E> {
    pub fn new(infcx: &InferCtxt<'tcx>) -> FulfillmentCtxt<'tcx, E> {
        assert!(
            infcx.next_trait_solver(),
            "new trait solver fulfillment context created when \
            infcx is set up for old trait solver"
        );
        FulfillmentCtxt {
            obligations: Default::default(),
            usable_in_snapshot: infcx.num_open_snapshots(),
            _errors: PhantomData,
        }
    }

    fn inspect_evaluated_obligation(
        &self,
        infcx: &InferCtxt<'tcx>,
        obligation: &PredicateObligation<'tcx>,
        result: &Result<(bool, Certainty), NoSolution>,
    ) {
        if let Some(inspector) = infcx.obligation_inspector.get() {
            let result = match result {
                Ok((_, c)) => Ok(*c),
                Err(NoSolution) => Err(NoSolution),
            };
            (inspector)(infcx, &obligation, result);
        }
    }
}

impl<'tcx, E> TraitEngine<'tcx, E> for FulfillmentCtxt<'tcx, E>
where
    E: FromSolverError<'tcx, NextSolverError<'tcx>>,
{
    #[instrument(level = "trace", skip(self, infcx))]
    fn register_predicate_obligation(
        &mut self,
        infcx: &InferCtxt<'tcx>,
        obligation: PredicateObligation<'tcx>,
    ) {
        assert_eq!(self.usable_in_snapshot, infcx.num_open_snapshots());
        self.obligations.register(obligation);
    }

    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
        self.obligations
            .pending
            .drain(..)
            .map(|obligation| NextSolverError::Ambiguity(obligation))
            .chain(
                self.obligations
                    .overflowed
                    .drain(..)
                    .map(|obligation| NextSolverError::Overflow(obligation)),
            )
            .map(|e| E::from_solver_error(infcx, e))
            .collect()
    }

    fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<E> {
        assert_eq!(self.usable_in_snapshot, infcx.num_open_snapshots());
        let mut errors = Vec::new();
        for i in 0.. {
            if !infcx.tcx.recursion_limit().value_within_limit(i) {
                self.obligations.on_fulfillment_overflow(infcx);
                // Only return true errors that we have accumulated while processing.
                return errors;
            }

            let mut has_changed = false;
            for obligation in self.obligations.unstalled_for_select() {
                let goal = obligation.clone().into();
                let result = <&SolverDelegate<'tcx>>::from(infcx)
                    .evaluate_root_goal(goal, GenerateProofTree::No)
                    .0;
                self.inspect_evaluated_obligation(infcx, &obligation, &result);
                let (changed, certainty) = match result {
                    Ok(result) => result,
                    Err(NoSolution) => {
                        errors.push(E::from_solver_error(
                            infcx,
                            NextSolverError::TrueError(obligation),
                        ));
                        continue;
                    }
                };
                has_changed |= changed;
                match certainty {
                    Certainty::Yes => {}
                    Certainty::Maybe(_) => self.obligations.register(obligation),
                }
            }

            if !has_changed {
                break;
            }
        }

        errors
    }

    fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
        self.obligations.clone_pending()
    }

    fn drain_unstalled_obligations(
        &mut self,
        _: &InferCtxt<'tcx>,
    ) -> Vec<PredicateObligation<'tcx>> {
        self.obligations.take_pending()
    }
}

pub enum NextSolverError<'tcx> {
    TrueError(PredicateObligation<'tcx>),
    Ambiguity(PredicateObligation<'tcx>),
    Overflow(PredicateObligation<'tcx>),
}

impl<'tcx> FromSolverError<'tcx, NextSolverError<'tcx>> for FulfillmentError<'tcx> {
    fn from_solver_error(infcx: &InferCtxt<'tcx>, error: NextSolverError<'tcx>) -> Self {
        match error {
            NextSolverError::TrueError(obligation) => {
                fulfillment_error_for_no_solution(infcx, obligation)
            }
            NextSolverError::Ambiguity(obligation) => {
                fulfillment_error_for_stalled(infcx, obligation)
            }
            NextSolverError::Overflow(obligation) => {
                fulfillment_error_for_overflow(infcx, obligation)
            }
        }
    }
}

impl<'tcx> FromSolverError<'tcx, NextSolverError<'tcx>> for ScrubbedTraitError<'tcx> {
    fn from_solver_error(_infcx: &InferCtxt<'tcx>, error: NextSolverError<'tcx>) -> Self {
        match error {
            NextSolverError::TrueError(_) => ScrubbedTraitError::TrueError,
            NextSolverError::Ambiguity(_) | NextSolverError::Overflow(_) => {
                ScrubbedTraitError::Ambiguity
            }
        }
    }
}

fn fulfillment_error_for_no_solution<'tcx>(
    infcx: &InferCtxt<'tcx>,
    root_obligation: PredicateObligation<'tcx>,
) -> FulfillmentError<'tcx> {
    let obligation = find_best_leaf_obligation(infcx, &root_obligation, false);

    let code = match obligation.predicate.kind().skip_binder() {
        ty::PredicateKind::Clause(ty::ClauseKind::Projection(_)) => {
            FulfillmentErrorCode::Project(
                // FIXME: This could be a `Sorts` if the term is a type
                MismatchedProjectionTypes { err: TypeError::Mismatch },
            )
        }
        ty::PredicateKind::NormalizesTo(..) => {
            FulfillmentErrorCode::Project(MismatchedProjectionTypes { err: TypeError::Mismatch })
        }
        ty::PredicateKind::AliasRelate(_, _, _) => {
            FulfillmentErrorCode::Project(MismatchedProjectionTypes { err: TypeError::Mismatch })
        }
        ty::PredicateKind::Subtype(pred) => {
            let (a, b) = infcx.enter_forall_and_leak_universe(
                obligation.predicate.kind().rebind((pred.a, pred.b)),
            );
            let expected_found = ExpectedFound::new(true, a, b);
            FulfillmentErrorCode::Subtype(expected_found, TypeError::Sorts(expected_found))
        }
        ty::PredicateKind::Coerce(pred) => {
            let (a, b) = infcx.enter_forall_and_leak_universe(
                obligation.predicate.kind().rebind((pred.a, pred.b)),
            );
            let expected_found = ExpectedFound::new(false, a, b);
            FulfillmentErrorCode::Subtype(expected_found, TypeError::Sorts(expected_found))
        }
        ty::PredicateKind::Clause(_)
        | ty::PredicateKind::ObjectSafe(_)
        | ty::PredicateKind::Ambiguous => {
            FulfillmentErrorCode::Select(SelectionError::Unimplemented)
        }
        ty::PredicateKind::ConstEquate(..) => {
            bug!("unexpected goal: {obligation:?}")
        }
    };

    FulfillmentError { obligation, code, root_obligation }
}

fn fulfillment_error_for_stalled<'tcx>(
    infcx: &InferCtxt<'tcx>,
    root_obligation: PredicateObligation<'tcx>,
) -> FulfillmentError<'tcx> {
    let (code, refine_obligation) = infcx.probe(|_| {
        match <&SolverDelegate<'tcx>>::from(infcx)
            .evaluate_root_goal(root_obligation.clone().into(), GenerateProofTree::No)
            .0
        {
            Ok((_, Certainty::Maybe(MaybeCause::Ambiguity))) => {
                (FulfillmentErrorCode::Ambiguity { overflow: None }, true)
            }
            Ok((_, Certainty::Maybe(MaybeCause::Overflow { suggest_increasing_limit }))) => (
                FulfillmentErrorCode::Ambiguity { overflow: Some(suggest_increasing_limit) },
                // Don't look into overflows because we treat overflows weirdly anyways.
                // In `instantiate_response_discarding_overflow` we set `has_changed = false`,
                // recomputing the goal again during `find_best_leaf_obligation` may apply
                // inference guidance that makes other goals go from ambig -> pass, for example.
                //
                // FIXME: We should probably just look into overflows here.
                false,
            ),
            Ok((_, Certainty::Yes)) => {
                bug!("did not expect successful goal when collecting ambiguity errors")
            }
            Err(_) => {
                bug!("did not expect selection error when collecting ambiguity errors")
            }
        }
    });

    FulfillmentError {
        obligation: if refine_obligation {
            find_best_leaf_obligation(infcx, &root_obligation, true)
        } else {
            root_obligation.clone()
        },
        code,
        root_obligation,
    }
}

fn fulfillment_error_for_overflow<'tcx>(
    infcx: &InferCtxt<'tcx>,
    root_obligation: PredicateObligation<'tcx>,
) -> FulfillmentError<'tcx> {
    FulfillmentError {
        obligation: find_best_leaf_obligation(infcx, &root_obligation, true),
        code: FulfillmentErrorCode::Ambiguity { overflow: Some(true) },
        root_obligation,
    }
}

fn find_best_leaf_obligation<'tcx>(
    infcx: &InferCtxt<'tcx>,
    obligation: &PredicateObligation<'tcx>,
    consider_ambiguities: bool,
) -> PredicateObligation<'tcx> {
    let obligation = infcx.resolve_vars_if_possible(obligation.clone());
    infcx
        .visit_proof_tree(
            obligation.clone().into(),
            &mut BestObligation { obligation: obligation.clone(), consider_ambiguities },
        )
        .break_value()
        .unwrap_or(obligation)
}

struct BestObligation<'tcx> {
    obligation: PredicateObligation<'tcx>,
    consider_ambiguities: bool,
}

impl<'tcx> BestObligation<'tcx> {
    fn with_derived_obligation(
        &mut self,
        derived_obligation: PredicateObligation<'tcx>,
        and_then: impl FnOnce(&mut Self) -> <Self as ProofTreeVisitor<'tcx>>::Result,
    ) -> <Self as ProofTreeVisitor<'tcx>>::Result {
        let old_obligation = std::mem::replace(&mut self.obligation, derived_obligation);
        let res = and_then(self);
        self.obligation = old_obligation;
        res
    }

    /// Filter out the candidates that aren't interesting to visit for the
    /// purposes of reporting errors. For ambiguities, we only consider
    /// candidates that may hold. For errors, we only consider candidates that
    /// *don't* hold and which have impl-where clauses that also don't hold.
    fn non_trivial_candidates<'a>(
        &self,
        goal: &'a inspect::InspectGoal<'a, 'tcx>,
    ) -> Vec<inspect::InspectCandidate<'a, 'tcx>> {
        let mut candidates = goal.candidates();
        match self.consider_ambiguities {
            true => {
                // If we have an ambiguous obligation, we must consider *all* candidates
                // that hold, or else we may guide inference causing other goals to go
                // from ambig -> pass/fail.
                candidates.retain(|candidate| candidate.result().is_ok());
            }
            false => {
                // If we have >1 candidate, one may still be due to "boring" reasons, like
                // an alias-relate that failed to hold when deeply evaluated. We really
                // don't care about reasons like this.
                if candidates.len() > 1 {
                    candidates.retain(|candidate| {
                        goal.infcx().probe(|_| {
                            candidate.instantiate_nested_goals(self.span()).iter().any(
                                |nested_goal| {
                                    matches!(
                                        nested_goal.source(),
                                        GoalSource::ImplWhereBound
                                            | GoalSource::InstantiateHigherRanked
                                    ) && match self.consider_ambiguities {
                                        true => {
                                            matches!(
                                                nested_goal.result(),
                                                Ok(Certainty::Maybe(MaybeCause::Ambiguity))
                                            )
                                        }
                                        false => matches!(nested_goal.result(), Err(_)),
                                    }
                                },
                            )
                        })
                    });
                }
            }
        }

        candidates
    }
}

impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
    type Result = ControlFlow<PredicateObligation<'tcx>>;

    fn span(&self) -> rustc_span::Span {
        self.obligation.cause.span
    }

    fn visit_goal(&mut self, goal: &inspect::InspectGoal<'_, 'tcx>) -> Self::Result {
        let candidates = self.non_trivial_candidates(goal);
        let [candidate] = candidates.as_slice() else {
            return ControlFlow::Break(self.obligation.clone());
        };

        // Don't walk into impls that have `do_not_recommend`.
        if let inspect::ProbeKind::TraitCandidate {
            source: CandidateSource::Impl(impl_def_id),
            result: _,
        } = candidate.kind()
            && goal.infcx().tcx.do_not_recommend_impl(impl_def_id)
        {
            return ControlFlow::Break(self.obligation.clone());
        }

        let tcx = goal.infcx().tcx;
        // FIXME: Also, what about considering >1 layer up the stack? May be necessary
        // for normalizes-to.
        let pred_kind = goal.goal().predicate.kind();
        let child_mode = match pred_kind.skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::Trait(parent_trait_pred)) => {
                ChildMode::Trait(pred_kind.rebind(parent_trait_pred))
            }
            ty::PredicateKind::NormalizesTo(normalizes_to)
                if matches!(
                    normalizes_to.alias.kind(tcx),
                    ty::AliasTermKind::ProjectionTy | ty::AliasTermKind::ProjectionConst
                ) =>
            {
                ChildMode::Trait(pred_kind.rebind(ty::TraitPredicate {
                    trait_ref: normalizes_to.alias.trait_ref(tcx),
                    polarity: ty::PredicatePolarity::Positive,
                }))
            }
            ty::PredicateKind::Clause(
                ty::ClauseKind::WellFormed(_) | ty::ClauseKind::Projection(..),
            )
            | ty::PredicateKind::AliasRelate(..) => ChildMode::PassThrough,
            _ => {
                return ControlFlow::Break(self.obligation.clone());
            }
        };

        let mut impl_where_bound_count = 0;
        for nested_goal in candidate.instantiate_nested_goals(self.span()) {
            let make_obligation = |cause| Obligation {
                cause,
                param_env: nested_goal.goal().param_env,
                predicate: nested_goal.goal().predicate,
                recursion_depth: self.obligation.recursion_depth + 1,
            };

            let obligation;
            match (child_mode, nested_goal.source()) {
                (ChildMode::Trait(_), GoalSource::Misc) => {
                    continue;
                }
                (ChildMode::Trait(parent_trait_pred), GoalSource::ImplWhereBound) => {
                    obligation = make_obligation(derive_cause(
                        tcx,
                        candidate.kind(),
                        self.obligation.cause.clone(),
                        impl_where_bound_count,
                        parent_trait_pred,
                    ));
                    impl_where_bound_count += 1;
                }
                // Skip over a higher-ranked predicate.
                (_, GoalSource::InstantiateHigherRanked) => {
                    obligation = self.obligation.clone();
                }
                (ChildMode::PassThrough, _) => {
                    obligation = make_obligation(self.obligation.cause.clone());
                }
            }

            // Skip nested goals that aren't the *reason* for our goal's failure.
            match self.consider_ambiguities {
                true if matches!(
                    nested_goal.result(),
                    Ok(Certainty::Maybe(MaybeCause::Ambiguity))
                ) => {}
                false if matches!(nested_goal.result(), Err(_)) => {}
                _ => continue,
            }

            self.with_derived_obligation(obligation, |this| nested_goal.visit_with(this))?;
        }

        // alias-relate may fail because the lhs or rhs can't be normalized,
        // and therefore is treated as rigid.
        if let Some(ty::PredicateKind::AliasRelate(lhs, rhs, _)) = pred_kind.no_bound_vars() {
            if let Some(obligation) = goal
                .infcx()
                .visit_proof_tree_at_depth(
                    goal.goal().with(goal.infcx().tcx, ty::ClauseKind::WellFormed(lhs.into())),
                    goal.depth() + 1,
                    self,
                )
                .break_value()
            {
                return ControlFlow::Break(obligation);
            } else if let Some(obligation) = goal
                .infcx()
                .visit_proof_tree_at_depth(
                    goal.goal().with(goal.infcx().tcx, ty::ClauseKind::WellFormed(rhs.into())),
                    goal.depth() + 1,
                    self,
                )
                .break_value()
            {
                return ControlFlow::Break(obligation);
            }
        }

        ControlFlow::Break(self.obligation.clone())
    }
}

#[derive(Copy, Clone)]
enum ChildMode<'tcx> {
    // Try to derive an `ObligationCause::{ImplDerived,BuiltinDerived}`,
    // and skip all `GoalSource::Misc`, which represent useless obligations
    // such as alias-eq which may not hold.
    Trait(ty::PolyTraitPredicate<'tcx>),
    // Skip trying to derive an `ObligationCause` from this obligation, and
    // report *all* sub-obligations as if they came directly from the parent
    // obligation.
    PassThrough,
}

fn derive_cause<'tcx>(
    tcx: TyCtxt<'tcx>,
    candidate_kind: inspect::ProbeKind<TyCtxt<'tcx>>,
    mut cause: ObligationCause<'tcx>,
    idx: usize,
    parent_trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> ObligationCause<'tcx> {
    match candidate_kind {
        inspect::ProbeKind::TraitCandidate {
            source: CandidateSource::Impl(impl_def_id),
            result: _,
        } => {
            if let Some((_, span)) =
                tcx.predicates_of(impl_def_id).instantiate_identity(tcx).iter().nth(idx)
            {
                cause = cause.derived_cause(parent_trait_pred, |derived| {
                    ObligationCauseCode::ImplDerived(Box::new(traits::ImplDerivedCause {
                        derived,
                        impl_or_alias_def_id: impl_def_id,
                        impl_def_predicate_index: Some(idx),
                        span,
                    }))
                })
            }
        }
        inspect::ProbeKind::TraitCandidate {
            source: CandidateSource::BuiltinImpl(..),
            result: _,
        } => {
            cause = cause.derived_cause(parent_trait_pred, ObligationCauseCode::BuiltinDerived);
        }
        _ => {}
    };
    cause
}