compiler/rustc_trait_selection/src/traits/object_safety.rs

   1 //! "Object safety" refers to the ability for a trait to be converted
   2 //! to an object. In general, traits may only be converted to an
   3 //! object if all of their methods meet certain criteria. In particular,
   4 //! they must:
   5 //!
   6 //!   - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version
   7 //!     that doesn't contain the vtable;
   8 //!   - not reference the erased type `Self` except for in this receiver;
   9 //!   - not have generic type parameters.
  10
  11 use super::elaborate_predicates;
  12
  13 use crate::infer::TyCtxtInferExt;
  14 use crate::traits::const_evaluatable::{self, AbstractConst};
  15 use crate::traits::query::evaluate_obligation::InferCtxtExt;
  16 use crate::traits::{self, Obligation, ObligationCause};
  17 use rustc_errors::{FatalError, MultiSpan};
  18 use rustc_hir as hir;
  19 use rustc_hir::def_id::DefId;
  20 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
  21 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor};
  22 use rustc_middle::ty::{Predicate, ToPredicate};
  23 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
  24 use rustc_span::symbol::Symbol;
  25 use rustc_span::Span;
  26 use smallvec::SmallVec;
  27
  28 use std::iter;
  29 use std::ops::ControlFlow;
  30
  31 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
  32
  33 /// Returns the object safety violations that affect
  34 /// astconv -- currently, `Self` in supertraits. This is needed
  35 /// because `object_safety_violations` can't be used during
  36 /// type collection.
  37 pub fn astconv_object_safety_violations(
  38     tcx: TyCtxt<'_>,
  39     trait_def_id: DefId,
  40 ) -> Vec<ObjectSafetyViolation> {
  41     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  42     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
  43         .map(|def_id| predicates_reference_self(tcx, def_id, true))
  44         .filter(|spans| !spans.is_empty())
  45         .map(ObjectSafetyViolation::SupertraitSelf)
  46         .collect();
  47
  48     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
  49
  50     violations
  51 }
  52
  53 fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [ObjectSafetyViolation] {
  54     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  55     debug!("object_safety_violations: {:?}", trait_def_id);
  56
  57     tcx.arena.alloc_from_iter(
  58         traits::supertrait_def_ids(tcx, trait_def_id)
  59             .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
  60     )
  61 }
  62
  63 /// We say a method is *vtable safe* if it can be invoked on a trait
  64 /// object. Note that object-safe traits can have some
  65 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  66 /// otherwise ensure that they cannot be used when `Self = Trait`.
  67 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
  68     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  69     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
  70     // Any method that has a `Self: Sized` bound cannot be called.
  71     if generics_require_sized_self(tcx, method.def_id) {
  72         return false;
  73     }
  74
  75     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
  76         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
  77         Some(_) => false,
  78     }
  79 }
  80
  81 fn object_safety_violations_for_trait(
  82     tcx: TyCtxt<'_>,
  83     trait_def_id: DefId,
  84 ) -> Vec<ObjectSafetyViolation> {
  85     // Check methods for violations.
  86     let mut violations: Vec<_> = tcx
  87         .associated_items(trait_def_id)
  88         .in_definition_order()
  89         .filter(|item| item.kind == ty::AssocKind::Fn)
  90         .filter_map(|item| {
  91             object_safety_violation_for_method(tcx, trait_def_id, &item)
  92                 .map(|(code, span)| ObjectSafetyViolation::Method(item.name, code, span))
  93         })
  94         .filter(|violation| {
  95             if let ObjectSafetyViolation::Method(
  96                 _,
  97                 MethodViolationCode::WhereClauseReferencesSelf,
  98                 span,
  99             ) = violation
 100             {
 101                 lint_object_unsafe_trait(tcx, *span, trait_def_id, violation);
 102                 false
 103             } else {
 104                 true
 105             }
 106         })
 107         .collect();
 108
 109     // Check the trait itself.
 110     if trait_has_sized_self(tcx, trait_def_id) {
 111         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 112         let spans = get_sized_bounds(tcx, trait_def_id);
 113         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 114     }
 115     let spans = predicates_reference_self(tcx, trait_def_id, false);
 116     if !spans.is_empty() {
 117         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 118     }
 119     let spans = bounds_reference_self(tcx, trait_def_id);
 120     if !spans.is_empty() {
 121         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 122     }
 123
 124     violations.extend(
 125         tcx.associated_items(trait_def_id)
 126             .in_definition_order()
 127             .filter(|item| item.kind == ty::AssocKind::Const)
 128             .map(|item| {
 129                 let ident = item.ident(tcx);
 130                 ObjectSafetyViolation::AssocConst(ident.name, ident.span)
 131             }),
 132     );
 133
 134     if !tcx.features().generic_associated_types_extended {
 135         violations.extend(
 136             tcx.associated_items(trait_def_id)
 137                 .in_definition_order()
 138                 .filter(|item| item.kind == ty::AssocKind::Type)
 139                 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
 140                 .map(|item| {
 141                     let ident = item.ident(tcx);
 142                     ObjectSafetyViolation::GAT(ident.name, ident.span)
 143                 }),
 144         );
 145     }
 146
 147     debug!(
 148         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 149         trait_def_id, violations
 150     );
 151
 152     violations
 153 }
 154
 155 /// Lint object-unsafe trait.
 156 fn lint_object_unsafe_trait(
 157     tcx: TyCtxt<'_>,
 158     span: Span,
 159     trait_def_id: DefId,
 160     violation: &ObjectSafetyViolation,
 161 ) {
 162     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 163     // It's also hard to get a use site span, so we use the method definition span.
 164     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 165         let mut err = lint.build(&format!(
 166             "the trait `{}` cannot be made into an object",
 167             tcx.def_path_str(trait_def_id)
 168         ));
 169         let node = tcx.hir().get_if_local(trait_def_id);
 170         let mut spans = MultiSpan::from_span(span);
 171         if let Some(hir::Node::Item(item)) = node {
 172             spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
 173             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 174         } else {
 175             spans.push_span_label(
 176                 span,
 177                 format!(
 178                     "the trait cannot be made into an object because {}",
 179                     violation.error_msg()
 180                 ),
 181             );
 182         };
 183         err.span_note(
 184             spans,
 185             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 186              call to be resolvable dynamically; for more information visit \
 187              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 188         );
 189         if node.is_some() {
 190             // Only provide the help if its a local trait, otherwise it's not
 191             violation.solution(&mut err);
 192         }
 193         err.emit();
 194     });
 195 }
 196
 197 fn sized_trait_bound_spans<'tcx>(
 198     tcx: TyCtxt<'tcx>,
 199     bounds: hir::GenericBounds<'tcx>,
 200 ) -> impl 'tcx + Iterator<Item = Span> {
 201     bounds.iter().filter_map(move |b| match b {
 202         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 203             if trait_has_sized_self(
 204                 tcx,
 205                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 206             ) =>
 207         {
 208             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 209             Some(trait_ref.span)
 210         }
 211         _ => None,
 212     })
 213 }
 214
 215 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 216     tcx.hir()
 217         .get_if_local(trait_def_id)
 218         .and_then(|node| match node {
 219             hir::Node::Item(hir::Item {
 220                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 221                 ..
 222             }) => Some(
 223                 generics
 224                     .predicates
 225                     .iter()
 226                     .filter_map(|pred| {
 227                         match pred {
 228                             hir::WherePredicate::BoundPredicate(pred)
 229                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 230                             {
 231                                 // Fetch spans for trait bounds that are Sized:
 232                                 // `trait T where Self: Pred`
 233                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 234                             }
 235                             _ => None,
 236                         }
 237                     })
 238                     .flatten()
 239                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 240                     .chain(sized_trait_bound_spans(tcx, bounds))
 241                     .collect::<SmallVec<[Span; 1]>>(),
 242             ),
 243             _ => None,
 244         })
 245         .unwrap_or_else(SmallVec::new)
 246 }
 247
 248 fn predicates_reference_self(
 249     tcx: TyCtxt<'_>,
 250     trait_def_id: DefId,
 251     supertraits_only: bool,
 252 ) -> SmallVec<[Span; 1]> {
 253     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 254     let predicates = if supertraits_only {
 255         tcx.super_predicates_of(trait_def_id)
 256     } else {
 257         tcx.predicates_of(trait_def_id)
 258     };
 259     predicates
 260         .predicates
 261         .iter()
 262         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 263         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 264         .collect()
 265 }
 266
 267 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 268     tcx.associated_items(trait_def_id)
 269         .in_definition_order()
 270         .filter(|item| item.kind == ty::AssocKind::Type)
 271         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 272         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 273         .collect()
 274 }
 275
 276 fn predicate_references_self<'tcx>(
 277     tcx: TyCtxt<'tcx>,
 278     (predicate, sp): (ty::Predicate<'tcx>, Span),
 279 ) -> Option<Span> {
 280     let self_ty = tcx.types.self_param;
 281     let has_self_ty = |arg: &GenericArg<'_>| arg.walk().any(|arg| arg == self_ty.into());
 282     match predicate.kind().skip_binder() {
 283         ty::PredicateKind::Trait(ref data) => {
 284             // In the case of a trait predicate, we can skip the "self" type.
 285             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 286         }
 287         ty::PredicateKind::Projection(ref data) => {
 288             // And similarly for projections. This should be redundant with
 289             // the previous check because any projection should have a
 290             // matching `Trait` predicate with the same inputs, but we do
 291             // the check to be safe.
 292             //
 293             // It's also won't be redundant if we allow type-generic associated
 294             // types for trait objects.
 295             //
 296             // Note that we *do* allow projection *outputs* to contain
 297             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 298             // we just require the user to specify *both* outputs
 299             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 300             //
 301             // This is ALT2 in issue #56288, see that for discussion of the
 302             // possible alternatives.
 303             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 304         }
 305         ty::PredicateKind::WellFormed(..)
 306         | ty::PredicateKind::ObjectSafe(..)
 307         | ty::PredicateKind::TypeOutlives(..)
 308         | ty::PredicateKind::RegionOutlives(..)
 309         | ty::PredicateKind::ClosureKind(..)
 310         | ty::PredicateKind::Subtype(..)
 311         | ty::PredicateKind::Coerce(..)
 312         | ty::PredicateKind::ConstEvaluatable(..)
 313         | ty::PredicateKind::ConstEquate(..)
 314         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 315     }
 316 }
 317
 318 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 319     generics_require_sized_self(tcx, trait_def_id)
 320 }
 321
 322 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 323     let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
 324         return false; /* No Sized trait, can't require it! */
 325     };
 326
 327     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 328     let predicates = tcx.predicates_of(def_id);
 329     let predicates = predicates.instantiate_identity(tcx).predicates;
 330     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 331         match obligation.predicate.kind().skip_binder() {
 332             ty::PredicateKind::Trait(ref trait_pred) => {
 333                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 334             }
 335             ty::PredicateKind::Projection(..)
 336             | ty::PredicateKind::Subtype(..)
 337             | ty::PredicateKind::Coerce(..)
 338             | ty::PredicateKind::RegionOutlives(..)
 339             | ty::PredicateKind::WellFormed(..)
 340             | ty::PredicateKind::ObjectSafe(..)
 341             | ty::PredicateKind::ClosureKind(..)
 342             | ty::PredicateKind::TypeOutlives(..)
 343             | ty::PredicateKind::ConstEvaluatable(..)
 344             | ty::PredicateKind::ConstEquate(..)
 345             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 346         }
 347     })
 348 }
 349
 350 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 351 fn object_safety_violation_for_method(
 352     tcx: TyCtxt<'_>,
 353     trait_def_id: DefId,
 354     method: &ty::AssocItem,
 355 ) -> Option<(MethodViolationCode, Span)> {
 356     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 357     // Any method that has a `Self : Sized` requisite is otherwise
 358     // exempt from the regulations.
 359     if generics_require_sized_self(tcx, method.def_id) {
 360         return None;
 361     }
 362
 363     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 364     // Get an accurate span depending on the violation.
 365     violation.map(|v| {
 366         let node = tcx.hir().get_if_local(method.def_id);
 367         let span = match (v, node) {
 368             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 369                 .fn_decl()
 370                 .and_then(|decl| decl.inputs.get(arg + 1))
 371                 .map_or(method.ident(tcx).span, |arg| arg.span),
 372             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 373                 .fn_decl()
 374                 .and_then(|decl| decl.inputs.get(0))
 375                 .map_or(method.ident(tcx).span, |arg| arg.span),
 376             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 377                 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
 378             }
 379             _ => method.ident(tcx).span,
 380         };
 381         (v, span)
 382     })
 383 }
 384
 385 /// Returns `Some(_)` if this method cannot be called on a trait
 386 /// object; this does not necessarily imply that the enclosing trait
 387 /// is not object safe, because the method might have a where clause
 388 /// `Self:Sized`.
 389 fn virtual_call_violation_for_method<'tcx>(
 390     tcx: TyCtxt<'tcx>,
 391     trait_def_id: DefId,
 392     method: &ty::AssocItem,
 393 ) -> Option<MethodViolationCode> {
 394     let sig = tcx.fn_sig(method.def_id);
 395
 396     // The method's first parameter must be named `self`
 397     if !method.fn_has_self_parameter {
 398         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 399         let sugg =
 400             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 401                 |generics| match generics.predicates {
 402                     [] => (" where Self: Sized", generics.where_clause_span),
 403                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 404                 },
 405             );
 406         // Get the span pointing at where the `self` receiver should be.
 407         let sm = tcx.sess.source_map();
 408         let self_span = method.ident(tcx).span.to(tcx
 409             .hir()
 410             .span_if_local(method.def_id)
 411             .unwrap_or_else(|| sm.next_point(method.ident(tcx).span))
 412             .shrink_to_hi());
 413         let self_span = sm.span_through_char(self_span, '(').shrink_to_hi();
 414         return Some(MethodViolationCode::StaticMethod(
 415             sugg,
 416             self_span,
 417             !sig.inputs().skip_binder().is_empty(),
 418         ));
 419     }
 420
 421     for (i, &input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 422         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 423             return Some(MethodViolationCode::ReferencesSelfInput(i));
 424         }
 425     }
 426     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 427         return Some(MethodViolationCode::ReferencesSelfOutput);
 428     }
 429
 430     // We can't monomorphize things like `fn foo<A>(...)`.
 431     let own_counts = tcx.generics_of(method.def_id).own_counts();
 432     if own_counts.types + own_counts.consts != 0 {
 433         return Some(MethodViolationCode::Generic);
 434     }
 435
 436     if tcx
 437         .predicates_of(method.def_id)
 438         .predicates
 439         .iter()
 440         // A trait object can't claim to live more than the concrete type,
 441         // so outlives predicates will always hold.
 442         .cloned()
 443         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 444         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 445     {
 446         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 447     }
 448
 449     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 450
 451     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 452     // However, this is already considered object-safe. We allow it as a special case here.
 453     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 454     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 455     if receiver_ty != tcx.types.self_param {
 456         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 457             return Some(MethodViolationCode::UndispatchableReceiver);
 458         } else {
 459             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 460
 461             use rustc_target::abi::Abi;
 462
 463             let param_env = tcx.param_env(method.def_id);
 464
 465             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 466                 match tcx.layout_of(param_env.and(ty)) {
 467                     Ok(layout) => Some(layout.abi),
 468                     Err(err) => {
 469                         // #78372
 470                         tcx.sess.delay_span_bug(
 471                             tcx.def_span(method.def_id),
 472                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 473                         );
 474                         None
 475                     }
 476                 }
 477             };
 478
 479             // e.g., `Rc<()>`
 480             let unit_receiver_ty =
 481                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 482
 483             match abi_of_ty(unit_receiver_ty) {
 484                 Some(Abi::Scalar(..)) => (),
 485                 abi => {
 486                     tcx.sess.delay_span_bug(
 487                         tcx.def_span(method.def_id),
 488                         &format!(
 489                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 490                             abi
 491                         ),
 492                     );
 493                 }
 494             }
 495
 496             let trait_object_ty =
 497                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 498
 499             // e.g., `Rc<dyn Trait>`
 500             let trait_object_receiver =
 501                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 502
 503             match abi_of_ty(trait_object_receiver) {
 504                 Some(Abi::ScalarPair(..)) => (),
 505                 abi => {
 506                     tcx.sess.delay_span_bug(
 507                         tcx.def_span(method.def_id),
 508                         &format!(
 509                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 510                             trait_object_ty, abi
 511                         ),
 512                     );
 513                 }
 514             }
 515         }
 516     }
 517
 518     None
 519 }
 520
 521 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 522 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 523 fn receiver_for_self_ty<'tcx>(
 524     tcx: TyCtxt<'tcx>,
 525     receiver_ty: Ty<'tcx>,
 526     self_ty: Ty<'tcx>,
 527     method_def_id: DefId,
 528 ) -> Ty<'tcx> {
 529     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 530     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 531         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 532     });
 533
 534     let result = receiver_ty.subst(tcx, substs);
 535     debug!(
 536         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 537         receiver_ty, self_ty, method_def_id, result
 538     );
 539     result
 540 }
 541
 542 /// Creates the object type for the current trait. For example,
 543 /// if the current trait is `Deref`, then this will be
 544 /// `dyn Deref<Target = Self::Target> + 'static`.
 545 fn object_ty_for_trait<'tcx>(
 546     tcx: TyCtxt<'tcx>,
 547     trait_def_id: DefId,
 548     lifetime: ty::Region<'tcx>,
 549 ) -> Ty<'tcx> {
 550     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 551
 552     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 553
 554     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 555         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 556     });
 557
 558     let mut associated_types = traits::supertraits(tcx, trait_ref)
 559         .flat_map(|super_trait_ref| {
 560             tcx.associated_items(super_trait_ref.def_id())
 561                 .in_definition_order()
 562                 .map(move |item| (super_trait_ref, item))
 563         })
 564         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 565         .collect::<Vec<_>>();
 566
 567     // existential predicates need to be in a specific order
 568     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 569
 570     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 571         // We *can* get bound lifetimes here in cases like
 572         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 573         super_trait_ref.map_bound(|super_trait_ref| {
 574             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 575                 term: tcx.mk_projection(item.def_id, super_trait_ref.substs).into(),
 576                 item_def_id: item.def_id,
 577                 substs: super_trait_ref.substs,
 578             })
 579         })
 580     });
 581
 582     let existential_predicates = tcx
 583         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 584
 585     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
 586
 587     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 588
 589     object_ty
 590 }
 591
 592 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 593 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 594 /// in the following way:
 595 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 596 /// - require the following bound:
 597 ///
 598 ///   ```ignore (not-rust)
 599 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 600 ///   ```
 601 ///
 602 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 603 ///   (substitution notation).
 604 ///
 605 /// Some examples of receiver types and their required obligation:
 606 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 607 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 608 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 609 ///
 610 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 611 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 612 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 613 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 614 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 615 /// contained by the trait object, because the object that needs to be coerced is behind
 616 /// a pointer.
 617 ///
 618 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 619 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 620 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 621 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 622 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 623 /// Written as a chalk-style query:
 624 /// ```ignore (not-rust)
 625 /// forall (U: Trait + ?Sized) {
 626 ///     if (Self: Unsize<U>) {
 627 ///         Receiver: DispatchFromDyn<Receiver[Self => U]>
 628 ///     }
 629 /// }
 630 /// ```
 631 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 632 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 633 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 634 //
 635 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 636 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 637 // `self: Wrapper<Self>`.
 638 #[allow(dead_code)]
 639 fn receiver_is_dispatchable<'tcx>(
 640     tcx: TyCtxt<'tcx>,
 641     method: &ty::AssocItem,
 642     receiver_ty: Ty<'tcx>,
 643 ) -> bool {
 644     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 645
 646     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 647     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 648         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 649         return false;
 650     };
 651
 652     // the type `U` in the query
 653     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 654     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 655     // replace this with `dyn Trait`
 656     let unsized_self_ty: Ty<'tcx> =
 657         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 658
 659     // `Receiver[Self => U]`
 660     let unsized_receiver_ty =
 661         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 662
 663     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 664     // `U: ?Sized` is already implied here
 665     let param_env = {
 666         let param_env = tcx.param_env(method.def_id);
 667
 668         // Self: Unsize<U>
 669         let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
 670             def_id: unsize_did,
 671             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 672         })
 673         .without_const()
 674         .to_predicate(tcx);
 675
 676         // U: Trait<Arg1, ..., ArgN>
 677         let trait_predicate = {
 678             let substs =
 679                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 680                     if param.index == 0 {
 681                         unsized_self_ty.into()
 682                     } else {
 683                         tcx.mk_param_from_def(param)
 684                     }
 685                 });
 686
 687             ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
 688                 .without_const()
 689                 .to_predicate(tcx)
 690         };
 691
 692         let caller_bounds: Vec<Predicate<'tcx>> =
 693             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 694
 695         ty::ParamEnv::new(
 696             tcx.intern_predicates(&caller_bounds),
 697             param_env.reveal(),
 698             param_env.constness(),
 699         )
 700     };
 701
 702     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 703     let obligation = {
 704         let predicate = ty::Binder::dummy(ty::TraitRef {
 705             def_id: dispatch_from_dyn_did,
 706             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 707         })
 708         .without_const()
 709         .to_predicate(tcx);
 710
 711         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 712     };
 713
 714     tcx.infer_ctxt().enter(|ref infcx| {
 715         // the receiver is dispatchable iff the obligation holds
 716         infcx.predicate_must_hold_modulo_regions(&obligation)
 717     })
 718 }
 719
 720 fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
 721     tcx: TyCtxt<'tcx>,
 722     trait_def_id: DefId,
 723     value: T,
 724 ) -> bool {
 725     // This is somewhat subtle. In general, we want to forbid
 726     // references to `Self` in the argument and return types,
 727     // since the value of `Self` is erased. However, there is one
 728     // exception: it is ok to reference `Self` in order to access
 729     // an associated type of the current trait, since we retain
 730     // the value of those associated types in the object type
 731     // itself.
 732     //
 733     // ```rust
 734     // trait SuperTrait {
 735     //     type X;
 736     // }
 737     //
 738     // trait Trait : SuperTrait {
 739     //     type Y;
 740     //     fn foo(&self, x: Self) // bad
 741     //     fn foo(&self) -> Self // bad
 742     //     fn foo(&self) -> Option<Self> // bad
 743     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 744     //     fn foo(&self) -> <Self as Trait>::Y // OK
 745     //     fn foo(&self) -> Self::X // OK, desugars to next example
 746     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 747     // }
 748     // ```
 749     //
 750     // However, it is not as simple as allowing `Self` in a projected
 751     // type, because there are illegal ways to use `Self` as well:
 752     //
 753     // ```rust
 754     // trait Trait : SuperTrait {
 755     //     ...
 756     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 757     // }
 758     // ```
 759     //
 760     // Here we will not have the type of `X` recorded in the
 761     // object type, and we cannot resolve `Self as SomeOtherTrait`
 762     // without knowing what `Self` is.
 763
 764     struct IllegalSelfTypeVisitor<'tcx> {
 765         tcx: TyCtxt<'tcx>,
 766         trait_def_id: DefId,
 767         supertraits: Option<Vec<DefId>>,
 768     }
 769
 770     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 771         type BreakTy = ();
 772
 773         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 774             match t.kind() {
 775                 ty::Param(_) => {
 776                     if t == self.tcx.types.self_param {
 777                         ControlFlow::BREAK
 778                     } else {
 779                         ControlFlow::CONTINUE
 780                     }
 781                 }
 782                 ty::Projection(ref data) => {
 783                     // This is a projected type `<Foo as SomeTrait>::X`.
 784
 785                     // Compute supertraits of current trait lazily.
 786                     if self.supertraits.is_none() {
 787                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 788                         self.supertraits = Some(
 789                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 790                         );
 791                     }
 792
 793                     // Determine whether the trait reference `Foo as
 794                     // SomeTrait` is in fact a supertrait of the
 795                     // current trait. In that case, this type is
 796                     // legal, because the type `X` will be specified
 797                     // in the object type.  Note that we can just use
 798                     // direct equality here because all of these types
 799                     // are part of the formal parameter listing, and
 800                     // hence there should be no inference variables.
 801                     let is_supertrait_of_current_trait = self
 802                         .supertraits
 803                         .as_ref()
 804                         .unwrap()
 805                         .contains(&data.trait_ref(self.tcx).def_id);
 806
 807                     if is_supertrait_of_current_trait {
 808                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 809                     } else {
 810                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 811                     }
 812                 }
 813                 _ => t.super_visit_with(self), // walk contained types, if any
 814             }
 815         }
 816
 817         fn visit_unevaluated_const(
 818             &mut self,
 819             uv: ty::Unevaluated<'tcx>,
 820         ) -> ControlFlow<Self::BreakTy> {
 821             // Constants can only influence object safety if they reference `Self`.
 822             // This is only possible for unevaluated constants, so we walk these here.
 823             //
 824             // If `AbstractConst::new` returned an error we already failed compilation
 825             // so we don't have to emit an additional error here.
 826             //
 827             // We currently recurse into abstract consts here but do not recurse in
 828             // `is_const_evaluatable`. This means that the object safety check is more
 829             // liberal than the const eval check.
 830             //
 831             // This shouldn't really matter though as we can't really use any
 832             // constants which are not considered const evaluatable.
 833             use rustc_middle::thir::abstract_const::Node;
 834             if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
 835                 const_evaluatable::walk_abstract_const(self.tcx, ct, |node| {
 836                     match node.root(self.tcx) {
 837                         Node::Leaf(leaf) => self.visit_const(leaf),
 838                         Node::Cast(_, _, ty) => self.visit_ty(ty),
 839                         Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 840                             ControlFlow::CONTINUE
 841                         }
 842                     }
 843                 })
 844             } else {
 845                 ControlFlow::CONTINUE
 846             }
 847         }
 848     }
 849
 850     value
 851         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 852         .is_break()
 853 }
 854
 855 pub fn provide(providers: &mut ty::query::Providers) {
 856     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 857 }