1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
//! "Object safety" refers to the ability for a trait to be converted
//! to an object. In general, traits may only be converted to an
//! object if all of their methods meet certain criteria. In particular,
//! they must:
//!
//!   - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version
//!     that doesn't contain the vtable;
//!   - not reference the erased type `Self` except for in this receiver;
//!   - not have generic type parameters.

use super::elaborate;

use crate::infer::TyCtxtInferExt;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{self, Obligation, ObligationCause};
use rustc_errors::FatalError;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::query::Providers;
use rustc_middle::ty::{
    self, EarlyBinder, ExistentialPredicateStableCmpExt as _, Ty, TyCtxt, TypeSuperVisitable,
    TypeVisitable, TypeVisitor,
};
use rustc_middle::ty::{GenericArg, GenericArgs};
use rustc_middle::ty::{TypeVisitableExt, Upcast};
use rustc_span::symbol::Symbol;
use rustc_span::Span;
use rustc_target::abi::Abi;
use smallvec::SmallVec;

use std::iter;
use std::ops::ControlFlow;

pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};

/// Returns the object safety violations that affect HIR ty lowering.
///
/// Currently that is `Self` in supertraits. This is needed
/// because `object_safety_violations` can't be used during
/// type collection.
#[instrument(level = "debug", skip(tcx))]
pub fn hir_ty_lowering_object_safety_violations(
    tcx: TyCtxt<'_>,
    trait_def_id: DefId,
) -> Vec<ObjectSafetyViolation> {
    debug_assert!(tcx.generics_of(trait_def_id).has_self);
    let violations = tcx
        .supertrait_def_ids(trait_def_id)
        .map(|def_id| predicates_reference_self(tcx, def_id, true))
        .filter(|spans| !spans.is_empty())
        .map(ObjectSafetyViolation::SupertraitSelf)
        .collect();
    debug!(?violations);
    violations
}

fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [ObjectSafetyViolation] {
    debug_assert!(tcx.generics_of(trait_def_id).has_self);
    debug!("object_safety_violations: {:?}", trait_def_id);

    tcx.arena.alloc_from_iter(
        tcx.supertrait_def_ids(trait_def_id)
            .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
    )
}

fn is_object_safe(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
    tcx.object_safety_violations(trait_def_id).is_empty()
}

/// We say a method is *vtable safe* if it can be invoked on a trait
/// object. Note that object-safe traits can have some
/// non-vtable-safe methods, so long as they require `Self: Sized` or
/// otherwise ensure that they cannot be used when `Self = Trait`.
pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: ty::AssocItem) -> bool {
    debug_assert!(tcx.generics_of(trait_def_id).has_self);
    debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
    // Any method that has a `Self: Sized` bound cannot be called.
    if tcx.generics_require_sized_self(method.def_id) {
        return false;
    }

    virtual_call_violations_for_method(tcx, trait_def_id, method).is_empty()
}

fn object_safety_violations_for_trait(
    tcx: TyCtxt<'_>,
    trait_def_id: DefId,
) -> Vec<ObjectSafetyViolation> {
    // Check assoc items for violations.
    let mut violations: Vec<_> = tcx
        .associated_items(trait_def_id)
        .in_definition_order()
        .flat_map(|&item| object_safety_violations_for_assoc_item(tcx, trait_def_id, item))
        .collect();

    // Check the trait itself.
    if trait_has_sized_self(tcx, trait_def_id) {
        // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
        let spans = get_sized_bounds(tcx, trait_def_id);
        violations.push(ObjectSafetyViolation::SizedSelf(spans));
    }
    let spans = predicates_reference_self(tcx, trait_def_id, false);
    if !spans.is_empty() {
        violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
    }
    let spans = bounds_reference_self(tcx, trait_def_id);
    if !spans.is_empty() {
        violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
    }
    let spans = super_predicates_have_non_lifetime_binders(tcx, trait_def_id);
    if !spans.is_empty() {
        violations.push(ObjectSafetyViolation::SupertraitNonLifetimeBinder(spans));
    }

    if violations.is_empty() {
        for item in tcx.associated_items(trait_def_id).in_definition_order() {
            if let ty::AssocKind::Fn = item.kind {
                check_receiver_correct(tcx, trait_def_id, *item);
            }
        }
    }

    debug!(
        "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
        trait_def_id, violations
    );

    violations
}

fn sized_trait_bound_spans<'tcx>(
    tcx: TyCtxt<'tcx>,
    bounds: hir::GenericBounds<'tcx>,
) -> impl 'tcx + Iterator<Item = Span> {
    bounds.iter().filter_map(move |b| match b {
        hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
            if trait_has_sized_self(
                tcx,
                trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
            ) =>
        {
            // Fetch spans for supertraits that are `Sized`: `trait T: Super`
            Some(trait_ref.span)
        }
        _ => None,
    })
}

fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
    tcx.hir()
        .get_if_local(trait_def_id)
        .and_then(|node| match node {
            hir::Node::Item(hir::Item {
                kind: hir::ItemKind::Trait(.., generics, bounds, _),
                ..
            }) => Some(
                generics
                    .predicates
                    .iter()
                    .filter_map(|pred| {
                        match pred {
                            hir::WherePredicate::BoundPredicate(pred)
                                if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
                            {
                                // Fetch spans for trait bounds that are Sized:
                                // `trait T where Self: Pred`
                                Some(sized_trait_bound_spans(tcx, pred.bounds))
                            }
                            _ => None,
                        }
                    })
                    .flatten()
                    // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
                    .chain(sized_trait_bound_spans(tcx, bounds))
                    .collect::<SmallVec<[Span; 1]>>(),
            ),
            _ => None,
        })
        .unwrap_or_else(SmallVec::new)
}

fn predicates_reference_self(
    tcx: TyCtxt<'_>,
    trait_def_id: DefId,
    supertraits_only: bool,
) -> SmallVec<[Span; 1]> {
    let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
    let predicates = if supertraits_only {
        tcx.explicit_super_predicates_of(trait_def_id)
    } else {
        tcx.predicates_of(trait_def_id)
    };
    predicates
        .predicates
        .iter()
        .map(|&(predicate, sp)| (predicate.instantiate_supertrait(tcx, trait_ref), sp))
        .filter_map(|predicate| predicate_references_self(tcx, predicate))
        .collect()
}

fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
    tcx.associated_items(trait_def_id)
        .in_definition_order()
        .filter(|item| item.kind == ty::AssocKind::Type)
        .flat_map(|item| tcx.explicit_item_bounds(item.def_id).iter_identity_copied())
        .filter_map(|c| predicate_references_self(tcx, c))
        .collect()
}

fn predicate_references_self<'tcx>(
    tcx: TyCtxt<'tcx>,
    (predicate, sp): (ty::Clause<'tcx>, Span),
) -> Option<Span> {
    let self_ty = tcx.types.self_param;
    let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
    match predicate.kind().skip_binder() {
        ty::ClauseKind::Trait(ref data) => {
            // In the case of a trait predicate, we can skip the "self" type.
            data.trait_ref.args[1..].iter().any(has_self_ty).then_some(sp)
        }
        ty::ClauseKind::Projection(ref data) => {
            // And similarly for projections. This should be redundant with
            // the previous check because any projection should have a
            // matching `Trait` predicate with the same inputs, but we do
            // the check to be safe.
            //
            // It's also won't be redundant if we allow type-generic associated
            // types for trait objects.
            //
            // Note that we *do* allow projection *outputs* to contain
            // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
            // we just require the user to specify *both* outputs
            // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
            //
            // This is ALT2 in issue #56288, see that for discussion of the
            // possible alternatives.
            data.projection_term.args[1..].iter().any(has_self_ty).then_some(sp)
        }
        ty::ClauseKind::ConstArgHasType(_ct, ty) => has_self_ty(&ty.into()).then_some(sp),

        ty::ClauseKind::WellFormed(..)
        | ty::ClauseKind::TypeOutlives(..)
        | ty::ClauseKind::RegionOutlives(..)
        // FIXME(generic_const_exprs): this can mention `Self`
        | ty::ClauseKind::ConstEvaluatable(..)
         => None,
    }
}

fn super_predicates_have_non_lifetime_binders(
    tcx: TyCtxt<'_>,
    trait_def_id: DefId,
) -> SmallVec<[Span; 1]> {
    // If non_lifetime_binders is disabled, then exit early
    if !tcx.features().non_lifetime_binders {
        return SmallVec::new();
    }
    tcx.explicit_super_predicates_of(trait_def_id)
        .predicates
        .iter()
        .filter_map(|(pred, span)| pred.has_non_region_bound_vars().then_some(*span))
        .collect()
}

fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
    tcx.generics_require_sized_self(trait_def_id)
}

fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
        return false; /* No Sized trait, can't require it! */
    };

    // Search for a predicate like `Self : Sized` amongst the trait bounds.
    let predicates = tcx.predicates_of(def_id);
    let predicates = predicates.instantiate_identity(tcx).predicates;
    elaborate(tcx, predicates).any(|pred| match pred.kind().skip_binder() {
        ty::ClauseKind::Trait(ref trait_pred) => {
            trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
        }
        ty::ClauseKind::RegionOutlives(_)
        | ty::ClauseKind::TypeOutlives(_)
        | ty::ClauseKind::Projection(_)
        | ty::ClauseKind::ConstArgHasType(_, _)
        | ty::ClauseKind::WellFormed(_)
        | ty::ClauseKind::ConstEvaluatable(_) => false,
    })
}

/// Returns `Some(_)` if this item makes the containing trait not object safe.
#[instrument(level = "debug", skip(tcx), ret)]
pub fn object_safety_violations_for_assoc_item(
    tcx: TyCtxt<'_>,
    trait_def_id: DefId,
    item: ty::AssocItem,
) -> Vec<ObjectSafetyViolation> {
    // Any item that has a `Self : Sized` requisite is otherwise
    // exempt from the regulations.
    if tcx.generics_require_sized_self(item.def_id) {
        return Vec::new();
    }

    match item.kind {
        // Associated consts are never object safe, as they can't have `where` bounds yet at all,
        // and associated const bounds in trait objects aren't a thing yet either.
        ty::AssocKind::Const => {
            vec![ObjectSafetyViolation::AssocConst(item.name, item.ident(tcx).span)]
        }
        ty::AssocKind::Fn => virtual_call_violations_for_method(tcx, trait_def_id, item)
            .into_iter()
            .map(|v| {
                let node = tcx.hir().get_if_local(item.def_id);
                // Get an accurate span depending on the violation.
                let span = match (&v, node) {
                    (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
                    (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
                    (MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
                    (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
                        node.fn_decl().map_or(item.ident(tcx).span, |decl| decl.output.span())
                    }
                    _ => item.ident(tcx).span,
                };

                ObjectSafetyViolation::Method(item.name, v, span)
            })
            .collect(),
        // Associated types can only be object safe if they have `Self: Sized` bounds.
        ty::AssocKind::Type => {
            if !tcx.features().generic_associated_types_extended
                && !tcx.generics_of(item.def_id).is_own_empty()
                && !item.is_impl_trait_in_trait()
            {
                vec![ObjectSafetyViolation::GAT(item.name, item.ident(tcx).span)]
            } else {
                // We will permit associated types if they are explicitly mentioned in the trait object.
                // We can't check this here, as here we only check if it is guaranteed to not be possible.
                Vec::new()
            }
        }
    }
}

/// Returns `Some(_)` if this method cannot be called on a trait
/// object; this does not necessarily imply that the enclosing trait
/// is not object safe, because the method might have a where clause
/// `Self:Sized`.
fn virtual_call_violations_for_method<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_def_id: DefId,
    method: ty::AssocItem,
) -> Vec<MethodViolationCode> {
    let sig = tcx.fn_sig(method.def_id).instantiate_identity();

    // The method's first parameter must be named `self`
    if !method.fn_has_self_parameter {
        let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
            generics,
            kind: hir::TraitItemKind::Fn(sig, _),
            ..
        })) = tcx.hir().get_if_local(method.def_id).as_ref()
        {
            let sm = tcx.sess.source_map();
            Some((
                (
                    format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
                    sm.span_through_char(sig.span, '(').shrink_to_hi(),
                ),
                (
                    format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
                    generics.tail_span_for_predicate_suggestion(),
                ),
            ))
        } else {
            None
        };

        // Not having `self` parameter messes up the later checks,
        // so we need to return instead of pushing
        return vec![MethodViolationCode::StaticMethod(sugg)];
    }

    let mut errors = Vec::new();

    for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
        if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
            let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
                kind: hir::TraitItemKind::Fn(sig, _),
                ..
            })) = tcx.hir().get_if_local(method.def_id).as_ref()
            {
                Some(sig.decl.inputs[i].span)
            } else {
                None
            };
            errors.push(MethodViolationCode::ReferencesSelfInput(span));
        }
    }
    if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
        errors.push(MethodViolationCode::ReferencesSelfOutput);
    }
    if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
        errors.push(code);
    }

    // We can't monomorphize things like `fn foo<A>(...)`.
    let own_counts = tcx.generics_of(method.def_id).own_counts();
    if own_counts.types > 0 || own_counts.consts > 0 {
        errors.push(MethodViolationCode::Generic);
    }

    let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));

    // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
    // However, this is already considered object-safe. We allow it as a special case here.
    // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
    // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
    if receiver_ty != tcx.types.self_param {
        if !receiver_is_dispatchable(tcx, method, receiver_ty) {
            let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
                kind: hir::TraitItemKind::Fn(sig, _),
                ..
            })) = tcx.hir().get_if_local(method.def_id).as_ref()
            {
                Some(sig.decl.inputs[0].span)
            } else {
                None
            };
            errors.push(MethodViolationCode::UndispatchableReceiver(span));
        } else {
            // We confirm that the `receiver_is_dispatchable` is accurate later,
            // see `check_receiver_correct`. It should be kept in sync with this code.
        }
    }

    // NOTE: This check happens last, because it results in a lint, and not a
    // hard error.
    if tcx.predicates_of(method.def_id).predicates.iter().any(|&(pred, _span)| {
        // dyn Trait is okay:
        //
        //     trait Trait {
        //         fn f(&self) where Self: 'static;
        //     }
        //
        // because a trait object can't claim to live longer than the concrete
        // type. If the lifetime bound holds on dyn Trait then it's guaranteed
        // to hold as well on the concrete type.
        if pred.as_type_outlives_clause().is_some() {
            return false;
        }

        // dyn Trait is okay:
        //
        //     auto trait AutoTrait {}
        //
        //     trait Trait {
        //         fn f(&self) where Self: AutoTrait;
        //     }
        //
        // because `impl AutoTrait for dyn Trait` is disallowed by coherence.
        // Traits with a default impl are implemented for a trait object if and
        // only if the autotrait is one of the trait object's trait bounds, like
        // in `dyn Trait + AutoTrait`. This guarantees that trait objects only
        // implement auto traits if the underlying type does as well.
        if let ty::ClauseKind::Trait(ty::TraitPredicate {
            trait_ref: pred_trait_ref,
            polarity: ty::PredicatePolarity::Positive,
        }) = pred.kind().skip_binder()
            && pred_trait_ref.self_ty() == tcx.types.self_param
            && tcx.trait_is_auto(pred_trait_ref.def_id)
        {
            // Consider bounds like `Self: Bound<Self>`. Auto traits are not
            // allowed to have generic parameters so `auto trait Bound<T> {}`
            // would already have reported an error at the definition of the
            // auto trait.
            if pred_trait_ref.args.len() != 1 {
                assert!(
                    tcx.dcx().has_errors().is_some(),
                    "auto traits cannot have generic parameters"
                );
            }
            return false;
        }

        contains_illegal_self_type_reference(tcx, trait_def_id, pred)
    }) {
        errors.push(MethodViolationCode::WhereClauseReferencesSelf);
    }

    errors
}

/// This code checks that `receiver_is_dispatchable` is correctly implemented.
///
/// This check is outlined from the object safety check to avoid cycles with
/// layout computation, which relies on knowing whether methods are object safe.
pub fn check_receiver_correct<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId, method: ty::AssocItem) {
    if !is_vtable_safe_method(tcx, trait_def_id, method) {
        return;
    }

    let method_def_id = method.def_id;
    let sig = tcx.fn_sig(method_def_id).instantiate_identity();
    let param_env = tcx.param_env(method_def_id);
    let receiver_ty = tcx.liberate_late_bound_regions(method_def_id, sig.input(0));

    if receiver_ty == tcx.types.self_param {
        // Assumed OK, may change later if unsized_locals permits `self: Self` as dispatchable.
        return;
    }

    // e.g., `Rc<()>`
    let unit_receiver_ty = receiver_for_self_ty(tcx, receiver_ty, tcx.types.unit, method_def_id);
    match tcx.layout_of(param_env.and(unit_receiver_ty)).map(|l| l.abi) {
        Ok(Abi::Scalar(..)) => (),
        abi => {
            tcx.dcx().span_delayed_bug(
                tcx.def_span(method_def_id),
                format!("receiver {unit_receiver_ty:?} when `Self = ()` should have a Scalar ABI; found {abi:?}"),
            );
        }
    }

    let trait_object_ty = object_ty_for_trait(tcx, trait_def_id, tcx.lifetimes.re_static);

    // e.g., `Rc<dyn Trait>`
    let trait_object_receiver =
        receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method_def_id);
    match tcx.layout_of(param_env.and(trait_object_receiver)).map(|l| l.abi) {
        Ok(Abi::ScalarPair(..)) => (),
        abi => {
            tcx.dcx().span_delayed_bug(
                tcx.def_span(method_def_id),
                format!(
                    "receiver {trait_object_receiver:?} when `Self = {trait_object_ty}` should have a ScalarPair ABI; found {abi:?}"
                ),
            );
        }
    }
}

/// Performs a type instantiation to produce the version of `receiver_ty` when `Self = self_ty`.
/// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
fn receiver_for_self_ty<'tcx>(
    tcx: TyCtxt<'tcx>,
    receiver_ty: Ty<'tcx>,
    self_ty: Ty<'tcx>,
    method_def_id: DefId,
) -> Ty<'tcx> {
    debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
    let args = GenericArgs::for_item(tcx, method_def_id, |param, _| {
        if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
    });

    let result = EarlyBinder::bind(receiver_ty).instantiate(tcx, args);
    debug!(
        "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
        receiver_ty, self_ty, method_def_id, result
    );
    result
}

/// Creates the object type for the current trait. For example,
/// if the current trait is `Deref`, then this will be
/// `dyn Deref<Target = Self::Target> + 'static`.
#[instrument(level = "trace", skip(tcx), ret)]
fn object_ty_for_trait<'tcx>(
    tcx: TyCtxt<'tcx>,
    trait_def_id: DefId,
    lifetime: ty::Region<'tcx>,
) -> Ty<'tcx> {
    let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
    debug!(?trait_ref);

    let trait_predicate = ty::Binder::dummy(ty::ExistentialPredicate::Trait(
        ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref),
    ));
    debug!(?trait_predicate);

    let pred: ty::Predicate<'tcx> = trait_ref.upcast(tcx);
    let mut elaborated_predicates: Vec<_> = elaborate(tcx, [pred])
        .filter_map(|pred| {
            debug!(?pred);
            let pred = pred.as_projection_clause()?;
            Some(pred.map_bound(|p| {
                ty::ExistentialPredicate::Projection(ty::ExistentialProjection::erase_self_ty(
                    tcx, p,
                ))
            }))
        })
        .collect();
    // NOTE: Since #37965, the existential predicates list has depended on the
    // list of predicates to be sorted. This is mostly to enforce that the primary
    // predicate comes first.
    elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
    elaborated_predicates.dedup();

    let existential_predicates = tcx.mk_poly_existential_predicates_from_iter(
        iter::once(trait_predicate).chain(elaborated_predicates),
    );
    debug!(?existential_predicates);

    Ty::new_dynamic(tcx, existential_predicates, lifetime, ty::Dyn)
}

/// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
/// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
/// in the following way:
/// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
/// - require the following bound:
///
///   ```ignore (not-rust)
///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
///   ```
///
///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
///   (instantiation notation).
///
/// Some examples of receiver types and their required obligation:
/// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
/// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
/// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
///
/// The only case where the receiver is not dispatchable, but is still a valid receiver
/// type (just not object-safe), is when there is more than one level of pointer indirection.
/// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
/// is no way, or at least no inexpensive way, to coerce the receiver from the version where
/// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
/// contained by the trait object, because the object that needs to be coerced is behind
/// a pointer.
///
/// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
/// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
/// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
/// Instead, we fudge a little by introducing a new type parameter `U` such that
/// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
/// Written as a chalk-style query:
/// ```ignore (not-rust)
/// forall (U: Trait + ?Sized) {
///     if (Self: Unsize<U>) {
///         Receiver: DispatchFromDyn<Receiver[Self => U]>
///     }
/// }
/// ```
/// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
/// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
/// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
//
// FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
// fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
// `self: Wrapper<Self>`.
fn receiver_is_dispatchable<'tcx>(
    tcx: TyCtxt<'tcx>,
    method: ty::AssocItem,
    receiver_ty: Ty<'tcx>,
) -> bool {
    debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);

    let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
    let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
        debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
        return false;
    };

    // the type `U` in the query
    // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
    // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
    // replace this with `dyn Trait`
    let unsized_self_ty: Ty<'tcx> =
        Ty::new_param(tcx, u32::MAX, Symbol::intern("RustaceansAreAwesome"));

    // `Receiver[Self => U]`
    let unsized_receiver_ty =
        receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);

    // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
    // `U: ?Sized` is already implied here
    let param_env = {
        let param_env = tcx.param_env(method.def_id);

        // Self: Unsize<U>
        let unsize_predicate =
            ty::TraitRef::new(tcx, unsize_did, [tcx.types.self_param, unsized_self_ty]).upcast(tcx);

        // U: Trait<Arg1, ..., ArgN>
        let trait_predicate = {
            let trait_def_id = method.trait_container(tcx).unwrap();
            let args = GenericArgs::for_item(tcx, trait_def_id, |param, _| {
                if param.index == 0 { unsized_self_ty.into() } else { tcx.mk_param_from_def(param) }
            });

            ty::TraitRef::new_from_args(tcx, trait_def_id, args).upcast(tcx)
        };

        let caller_bounds =
            param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]);

        ty::ParamEnv::new(tcx.mk_clauses_from_iter(caller_bounds), param_env.reveal())
    };

    // Receiver: DispatchFromDyn<Receiver[Self => U]>
    let obligation = {
        let predicate =
            ty::TraitRef::new(tcx, dispatch_from_dyn_did, [receiver_ty, unsized_receiver_ty]);

        Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
    };

    let infcx = tcx.infer_ctxt().build();
    // the receiver is dispatchable iff the obligation holds
    infcx.predicate_must_hold_modulo_regions(&obligation)
}

fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
    tcx: TyCtxt<'tcx>,
    trait_def_id: DefId,
    value: T,
) -> bool {
    // This is somewhat subtle. In general, we want to forbid
    // references to `Self` in the argument and return types,
    // since the value of `Self` is erased. However, there is one
    // exception: it is ok to reference `Self` in order to access
    // an associated type of the current trait, since we retain
    // the value of those associated types in the object type
    // itself.
    //
    // ```rust
    // trait SuperTrait {
    //     type X;
    // }
    //
    // trait Trait : SuperTrait {
    //     type Y;
    //     fn foo(&self, x: Self) // bad
    //     fn foo(&self) -> Self // bad
    //     fn foo(&self) -> Option<Self> // bad
    //     fn foo(&self) -> Self::Y // OK, desugars to next example
    //     fn foo(&self) -> <Self as Trait>::Y // OK
    //     fn foo(&self) -> Self::X // OK, desugars to next example
    //     fn foo(&self) -> <Self as SuperTrait>::X // OK
    // }
    // ```
    //
    // However, it is not as simple as allowing `Self` in a projected
    // type, because there are illegal ways to use `Self` as well:
    //
    // ```rust
    // trait Trait : SuperTrait {
    //     ...
    //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
    // }
    // ```
    //
    // Here we will not have the type of `X` recorded in the
    // object type, and we cannot resolve `Self as SomeOtherTrait`
    // without knowing what `Self` is.

    struct IllegalSelfTypeVisitor<'tcx> {
        tcx: TyCtxt<'tcx>,
        trait_def_id: DefId,
        supertraits: Option<Vec<DefId>>,
    }

    impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalSelfTypeVisitor<'tcx> {
        type Result = ControlFlow<()>;

        fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
            match t.kind() {
                ty::Param(_) => {
                    if t == self.tcx.types.self_param {
                        ControlFlow::Break(())
                    } else {
                        ControlFlow::Continue(())
                    }
                }
                ty::Alias(ty::Projection, ref data)
                    if self.tcx.is_impl_trait_in_trait(data.def_id) =>
                {
                    // We'll deny these later in their own pass
                    ControlFlow::Continue(())
                }
                ty::Alias(ty::Projection, ref data) => {
                    // This is a projected type `<Foo as SomeTrait>::X`.

                    // Compute supertraits of current trait lazily.
                    if self.supertraits.is_none() {
                        let trait_ref =
                            ty::Binder::dummy(ty::TraitRef::identity(self.tcx, self.trait_def_id));
                        self.supertraits = Some(
                            traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
                        );
                    }

                    // Determine whether the trait reference `Foo as
                    // SomeTrait` is in fact a supertrait of the
                    // current trait. In that case, this type is
                    // legal, because the type `X` will be specified
                    // in the object type. Note that we can just use
                    // direct equality here because all of these types
                    // are part of the formal parameter listing, and
                    // hence there should be no inference variables.
                    let is_supertrait_of_current_trait = self
                        .supertraits
                        .as_ref()
                        .unwrap()
                        .contains(&data.trait_ref(self.tcx).def_id);

                    // only walk contained types if it's not a super trait
                    if is_supertrait_of_current_trait {
                        ControlFlow::Continue(())
                    } else {
                        t.super_visit_with(self) // POSSIBLY reporting an error
                    }
                }
                _ => t.super_visit_with(self), // walk contained types, if any
            }
        }

        fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
            // Constants can only influence object safety if they are generic and reference `Self`.
            // This is only possible for unevaluated constants, so we walk these here.
            self.tcx.expand_abstract_consts(ct).super_visit_with(self)
        }
    }

    value
        .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
        .is_break()
}

pub fn contains_illegal_impl_trait_in_trait<'tcx>(
    tcx: TyCtxt<'tcx>,
    fn_def_id: DefId,
    ty: ty::Binder<'tcx, Ty<'tcx>>,
) -> Option<MethodViolationCode> {
    // This would be caught below, but rendering the error as a separate
    // `async-specific` message is better.
    if tcx.asyncness(fn_def_id).is_async() {
        return Some(MethodViolationCode::AsyncFn);
    }

    // FIXME(RPITIT): Perhaps we should use a visitor here?
    ty.skip_binder().walk().find_map(|arg| {
        if let ty::GenericArgKind::Type(ty) = arg.unpack()
            && let ty::Alias(ty::Projection, proj) = ty.kind()
            && tcx.is_impl_trait_in_trait(proj.def_id)
        {
            Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.def_id)))
        } else {
            None
        }
    })
}

pub fn provide(providers: &mut Providers) {
    *providers = Providers {
        object_safety_violations,
        is_object_safe,
        generics_require_sized_self,
        ..*providers
    };
}