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;
  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, WithConstness};
  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::{MultiSpan, Span};
  26 use smallvec::SmallVec;
  27
  28 use std::array;
  29 use std::iter;
  30 use std::ops::ControlFlow;
  31
  32 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
  33
  34 /// Returns the object safety violations that affect
  35 /// astconv -- currently, `Self` in supertraits. This is needed
  36 /// because `object_safety_violations` can't be used during
  37 /// type collection.
  38 pub fn astconv_object_safety_violations(
  39     tcx: TyCtxt<'_>,
  40     trait_def_id: DefId,
  41 ) -> Vec<ObjectSafetyViolation> {
  42     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  43     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
  44         .map(|def_id| predicates_reference_self(tcx, def_id, true))
  45         .filter(|spans| !spans.is_empty())
  46         .map(ObjectSafetyViolation::SupertraitSelf)
  47         .collect();
  48
  49     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
  50
  51     violations
  52 }
  53
  54 fn object_safety_violations(
  55     tcx: TyCtxt<'tcx>,
  56     trait_def_id: DefId,
  57 ) -> &'tcx [ObjectSafetyViolation] {
  58     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  59     debug!("object_safety_violations: {:?}", trait_def_id);
  60
  61     tcx.arena.alloc_from_iter(
  62         traits::supertrait_def_ids(tcx, trait_def_id)
  63             .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
  64     )
  65 }
  66
  67 /// We say a method is *vtable safe* if it can be invoked on a trait
  68 /// object. Note that object-safe traits can have some
  69 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  70 /// otherwise ensure that they cannot be used when `Self = Trait`.
  71 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
  72     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  73     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
  74     // Any method that has a `Self: Sized` bound cannot be called.
  75     if generics_require_sized_self(tcx, method.def_id) {
  76         return false;
  77     }
  78
  79     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
  80         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
  81         Some(_) => false,
  82     }
  83 }
  84
  85 fn object_safety_violations_for_trait(
  86     tcx: TyCtxt<'_>,
  87     trait_def_id: DefId,
  88 ) -> Vec<ObjectSafetyViolation> {
  89     // Check methods for violations.
  90     let mut violations: Vec<_> = tcx
  91         .associated_items(trait_def_id)
  92         .in_definition_order()
  93         .filter(|item| item.kind == ty::AssocKind::Fn)
  94         .filter_map(|item| {
  95             object_safety_violation_for_method(tcx, trait_def_id, &item)
  96                 .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span))
  97         })
  98         .filter(|violation| {
  99             if let ObjectSafetyViolation::Method(
 100                 _,
 101                 MethodViolationCode::WhereClauseReferencesSelf,
 102                 span,
 103             ) = violation
 104             {
 105                 lint_object_unsafe_trait(tcx, *span, trait_def_id, violation);
 106                 false
 107             } else {
 108                 true
 109             }
 110         })
 111         .collect();
 112
 113     // Check the trait itself.
 114     if trait_has_sized_self(tcx, trait_def_id) {
 115         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 116         let spans = get_sized_bounds(tcx, trait_def_id);
 117         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 118     }
 119     let spans = predicates_reference_self(tcx, trait_def_id, false);
 120     if !spans.is_empty() {
 121         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 122     }
 123     let spans = bounds_reference_self(tcx, trait_def_id);
 124     if !spans.is_empty() {
 125         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 126     }
 127
 128     violations.extend(
 129         tcx.associated_items(trait_def_id)
 130             .in_definition_order()
 131             .filter(|item| item.kind == ty::AssocKind::Const)
 132             .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 133     );
 134
 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| ObjectSafetyViolation::GAT(item.ident.name, item.ident.span)),
 141     );
 142
 143     debug!(
 144         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 145         trait_def_id, violations
 146     );
 147
 148     violations
 149 }
 150
 151 /// Lint object-unsafe trait.
 152 fn lint_object_unsafe_trait(
 153     tcx: TyCtxt<'_>,
 154     span: Span,
 155     trait_def_id: DefId,
 156     violation: &ObjectSafetyViolation,
 157 ) {
 158     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 159     // It's also hard to get a use site span, so we use the method definition span.
 160     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 161         let mut err = lint.build(&format!(
 162             "the trait `{}` cannot be made into an object",
 163             tcx.def_path_str(trait_def_id)
 164         ));
 165         let node = tcx.hir().get_if_local(trait_def_id);
 166         let mut spans = MultiSpan::from_span(span);
 167         if let Some(hir::Node::Item(item)) = node {
 168             spans.push_span_label(
 169                 item.ident.span,
 170                 "this trait cannot be made into an object...".into(),
 171             );
 172             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 173         } else {
 174             spans.push_span_label(
 175                 span,
 176                 format!(
 177                     "the trait cannot be made into an object because {}",
 178                     violation.error_msg()
 179                 ),
 180             );
 181         };
 182         err.span_note(
 183             spans,
 184             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 185              call to be resolvable dynamically; for more information visit \
 186              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 187         );
 188         if node.is_some() {
 189             // Only provide the help if its a local trait, otherwise it's not
 190             violation.solution(&mut err);
 191         }
 192         err.emit();
 193     });
 194 }
 195
 196 fn sized_trait_bound_spans<'tcx>(
 197     tcx: TyCtxt<'tcx>,
 198     bounds: hir::GenericBounds<'tcx>,
 199 ) -> impl 'tcx + Iterator<Item = Span> {
 200     bounds.iter().filter_map(move |b| match b {
 201         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 202             if trait_has_sized_self(
 203                 tcx,
 204                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 205             ) =>
 206         {
 207             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 208             Some(trait_ref.span)
 209         }
 210         _ => None,
 211     })
 212 }
 213
 214 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 215     tcx.hir()
 216         .get_if_local(trait_def_id)
 217         .and_then(|node| match node {
 218             hir::Node::Item(hir::Item {
 219                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 220                 ..
 221             }) => Some(
 222                 generics
 223                     .where_clause
 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(
 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<'tcx>| arg.walk(tcx).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 sized_def_id = match tcx.lang_items().sized_trait() {
 324         Some(def_id) => def_id,
 325         None => {
 326             return false; /* No Sized trait, can't require it! */
 327         }
 328     };
 329
 330     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 331     let predicates = tcx.predicates_of(def_id);
 332     let predicates = predicates.instantiate_identity(tcx).predicates;
 333     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 334         match obligation.predicate.kind().skip_binder() {
 335             ty::PredicateKind::Trait(ref trait_pred) => {
 336                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 337             }
 338             ty::PredicateKind::Projection(..)
 339             | ty::PredicateKind::Subtype(..)
 340             | ty::PredicateKind::Coerce(..)
 341             | ty::PredicateKind::RegionOutlives(..)
 342             | ty::PredicateKind::WellFormed(..)
 343             | ty::PredicateKind::ObjectSafe(..)
 344             | ty::PredicateKind::ClosureKind(..)
 345             | ty::PredicateKind::TypeOutlives(..)
 346             | ty::PredicateKind::ConstEvaluatable(..)
 347             | ty::PredicateKind::ConstEquate(..)
 348             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 349         }
 350     })
 351 }
 352
 353 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 354 fn object_safety_violation_for_method(
 355     tcx: TyCtxt<'_>,
 356     trait_def_id: DefId,
 357     method: &ty::AssocItem,
 358 ) -> Option<(MethodViolationCode, Span)> {
 359     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 360     // Any method that has a `Self : Sized` requisite is otherwise
 361     // exempt from the regulations.
 362     if generics_require_sized_self(tcx, method.def_id) {
 363         return None;
 364     }
 365
 366     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 367     // Get an accurate span depending on the violation.
 368     violation.map(|v| {
 369         let node = tcx.hir().get_if_local(method.def_id);
 370         let span = match (v, node) {
 371             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 372                 .fn_decl()
 373                 .and_then(|decl| decl.inputs.get(arg + 1))
 374                 .map_or(method.ident.span, |arg| arg.span),
 375             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 376                 .fn_decl()
 377                 .and_then(|decl| decl.inputs.get(0))
 378                 .map_or(method.ident.span, |arg| arg.span),
 379             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 380                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 381             }
 382             _ => method.ident.span,
 383         };
 384         (v, span)
 385     })
 386 }
 387
 388 /// Returns `Some(_)` if this method cannot be called on a trait
 389 /// object; this does not necessarily imply that the enclosing trait
 390 /// is not object safe, because the method might have a where clause
 391 /// `Self:Sized`.
 392 fn virtual_call_violation_for_method<'tcx>(
 393     tcx: TyCtxt<'tcx>,
 394     trait_def_id: DefId,
 395     method: &ty::AssocItem,
 396 ) -> Option<MethodViolationCode> {
 397     let sig = tcx.fn_sig(method.def_id);
 398
 399     // The method's first parameter must be named `self`
 400     if !method.fn_has_self_parameter {
 401         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 402         let sugg =
 403             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 404                 |generics| match generics.where_clause.predicates {
 405                     [] => (" where Self: Sized", generics.where_clause.span),
 406                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 407                 },
 408             );
 409         // Get the span pointing at where the `self` receiver should be.
 410         let sm = tcx.sess.source_map();
 411         let self_span = method.ident.span.to(tcx
 412             .hir()
 413             .span_if_local(method.def_id)
 414             .unwrap_or_else(|| sm.next_point(method.ident.span))
 415             .shrink_to_hi());
 416         let self_span = sm.span_through_char(self_span, '(').shrink_to_hi();
 417         return Some(MethodViolationCode::StaticMethod(
 418             sugg,
 419             self_span,
 420             !sig.inputs().skip_binder().is_empty(),
 421         ));
 422     }
 423
 424     for (i, &input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 425         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 426             return Some(MethodViolationCode::ReferencesSelfInput(i));
 427         }
 428     }
 429     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 430         return Some(MethodViolationCode::ReferencesSelfOutput);
 431     }
 432
 433     // We can't monomorphize things like `fn foo<A>(...)`.
 434     let own_counts = tcx.generics_of(method.def_id).own_counts();
 435     if own_counts.types + own_counts.consts != 0 {
 436         return Some(MethodViolationCode::Generic);
 437     }
 438
 439     if tcx
 440         .predicates_of(method.def_id)
 441         .predicates
 442         .iter()
 443         // A trait object can't claim to live more than the concrete type,
 444         // so outlives predicates will always hold.
 445         .cloned()
 446         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 447         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 448     {
 449         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 450     }
 451
 452     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 453
 454     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 455     // However, this is already considered object-safe. We allow it as a special case here.
 456     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 457     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 458     if receiver_ty != tcx.types.self_param {
 459         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 460             return Some(MethodViolationCode::UndispatchableReceiver);
 461         } else {
 462             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 463
 464             use rustc_target::abi::Abi;
 465
 466             let param_env = tcx.param_env(method.def_id);
 467
 468             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 469                 match tcx.layout_of(param_env.and(ty)) {
 470                     Ok(layout) => Some(layout.abi),
 471                     Err(err) => {
 472                         // #78372
 473                         tcx.sess.delay_span_bug(
 474                             tcx.def_span(method.def_id),
 475                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 476                         );
 477                         None
 478                     }
 479                 }
 480             };
 481
 482             // e.g., `Rc<()>`
 483             let unit_receiver_ty =
 484                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 485
 486             match abi_of_ty(unit_receiver_ty) {
 487                 Some(Abi::Scalar(..)) => (),
 488                 abi => {
 489                     tcx.sess.delay_span_bug(
 490                         tcx.def_span(method.def_id),
 491                         &format!(
 492                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 493                             abi
 494                         ),
 495                     );
 496                 }
 497             }
 498
 499             let trait_object_ty =
 500                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 501
 502             // e.g., `Rc<dyn Trait>`
 503             let trait_object_receiver =
 504                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 505
 506             match abi_of_ty(trait_object_receiver) {
 507                 Some(Abi::ScalarPair(..)) => (),
 508                 abi => {
 509                     tcx.sess.delay_span_bug(
 510                         tcx.def_span(method.def_id),
 511                         &format!(
 512                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 513                             trait_object_ty, abi
 514                         ),
 515                     );
 516                 }
 517             }
 518         }
 519     }
 520
 521     None
 522 }
 523
 524 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 525 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 526 fn receiver_for_self_ty<'tcx>(
 527     tcx: TyCtxt<'tcx>,
 528     receiver_ty: Ty<'tcx>,
 529     self_ty: Ty<'tcx>,
 530     method_def_id: DefId,
 531 ) -> Ty<'tcx> {
 532     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 533     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 534         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 535     });
 536
 537     let result = receiver_ty.subst(tcx, substs);
 538     debug!(
 539         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 540         receiver_ty, self_ty, method_def_id, result
 541     );
 542     result
 543 }
 544
 545 /// Creates the object type for the current trait. For example,
 546 /// if the current trait is `Deref`, then this will be
 547 /// `dyn Deref<Target = Self::Target> + 'static`.
 548 fn object_ty_for_trait<'tcx>(
 549     tcx: TyCtxt<'tcx>,
 550     trait_def_id: DefId,
 551     lifetime: ty::Region<'tcx>,
 552 ) -> Ty<'tcx> {
 553     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 554
 555     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 556
 557     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 558         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 559     });
 560
 561     let mut associated_types = traits::supertraits(tcx, trait_ref)
 562         .flat_map(|super_trait_ref| {
 563             tcx.associated_items(super_trait_ref.def_id())
 564                 .in_definition_order()
 565                 .map(move |item| (super_trait_ref, item))
 566         })
 567         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 568         .collect::<Vec<_>>();
 569
 570     // existential predicates need to be in a specific order
 571     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 572
 573     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 574         // We *can* get bound lifetimes here in cases like
 575         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 576         super_trait_ref.map_bound(|super_trait_ref| {
 577             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 578                 ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 579                 item_def_id: item.def_id,
 580                 substs: super_trait_ref.substs,
 581             })
 582         })
 583     });
 584
 585     let existential_predicates = tcx
 586         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 587
 588     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
 589
 590     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 591
 592     object_ty
 593 }
 594
 595 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 596 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 597 /// in the following way:
 598 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 599 /// - require the following bound:
 600 ///
 601 ///   ```
 602 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 603 ///   ```
 604 ///
 605 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 606 ///   (substitution notation).
 607 ///
 608 /// Some examples of receiver types and their required obligation:
 609 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 610 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 611 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 612 ///
 613 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 614 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 615 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 616 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 617 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 618 /// contained by the trait object, because the object that needs to be coerced is behind
 619 /// a pointer.
 620 ///
 621 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 622 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 623 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 624 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 625 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 626 /// Written as a chalk-style query:
 627 ///
 628 ///     forall (U: Trait + ?Sized) {
 629 ///         if (Self: Unsize<U>) {
 630 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 631 ///         }
 632 ///     }
 633 ///
 634 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 635 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 636 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 637 //
 638 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 639 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 640 // `self: Wrapper<Self>`.
 641 #[allow(dead_code)]
 642 fn receiver_is_dispatchable<'tcx>(
 643     tcx: TyCtxt<'tcx>,
 644     method: &ty::AssocItem,
 645     receiver_ty: Ty<'tcx>,
 646 ) -> bool {
 647     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 648
 649     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 650     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 651         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 652         return false;
 653     };
 654
 655     // the type `U` in the query
 656     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 657     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 658     // replace this with `dyn Trait`
 659     let unsized_self_ty: Ty<'tcx> =
 660         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 661
 662     // `Receiver[Self => U]`
 663     let unsized_receiver_ty =
 664         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 665
 666     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 667     // `U: ?Sized` is already implied here
 668     let param_env = {
 669         let param_env = tcx.param_env(method.def_id);
 670
 671         // Self: Unsize<U>
 672         let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
 673             def_id: unsize_did,
 674             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 675         })
 676         .without_const()
 677         .to_predicate(tcx);
 678
 679         // U: Trait<Arg1, ..., ArgN>
 680         let trait_predicate = {
 681             let substs =
 682                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 683                     if param.index == 0 {
 684                         unsized_self_ty.into()
 685                     } else {
 686                         tcx.mk_param_from_def(param)
 687                     }
 688                 });
 689
 690             ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
 691                 .without_const()
 692                 .to_predicate(tcx)
 693         };
 694
 695         let caller_bounds: Vec<Predicate<'tcx>> = param_env
 696             .caller_bounds()
 697             .iter()
 698             .chain(array::IntoIter::new([unsize_predicate, trait_predicate]))
 699             .collect();
 700
 701         ty::ParamEnv::new(tcx.intern_predicates(&caller_bounds), param_env.reveal())
 702     };
 703
 704     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 705     let obligation = {
 706         let predicate = ty::Binder::dummy(ty::TraitRef {
 707             def_id: dispatch_from_dyn_did,
 708             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 709         })
 710         .without_const()
 711         .to_predicate(tcx);
 712
 713         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 714     };
 715
 716     tcx.infer_ctxt().enter(|ref infcx| {
 717         // the receiver is dispatchable iff the obligation holds
 718         infcx.predicate_must_hold_modulo_regions(&obligation)
 719     })
 720 }
 721
 722 fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
 723     tcx: TyCtxt<'tcx>,
 724     trait_def_id: DefId,
 725     value: T,
 726 ) -> bool {
 727     // This is somewhat subtle. In general, we want to forbid
 728     // references to `Self` in the argument and return types,
 729     // since the value of `Self` is erased. However, there is one
 730     // exception: it is ok to reference `Self` in order to access
 731     // an associated type of the current trait, since we retain
 732     // the value of those associated types in the object type
 733     // itself.
 734     //
 735     // ```rust
 736     // trait SuperTrait {
 737     //     type X;
 738     // }
 739     //
 740     // trait Trait : SuperTrait {
 741     //     type Y;
 742     //     fn foo(&self, x: Self) // bad
 743     //     fn foo(&self) -> Self // bad
 744     //     fn foo(&self) -> Option<Self> // bad
 745     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 746     //     fn foo(&self) -> <Self as Trait>::Y // OK
 747     //     fn foo(&self) -> Self::X // OK, desugars to next example
 748     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 749     // }
 750     // ```
 751     //
 752     // However, it is not as simple as allowing `Self` in a projected
 753     // type, because there are illegal ways to use `Self` as well:
 754     //
 755     // ```rust
 756     // trait Trait : SuperTrait {
 757     //     ...
 758     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 759     // }
 760     // ```
 761     //
 762     // Here we will not have the type of `X` recorded in the
 763     // object type, and we cannot resolve `Self as SomeOtherTrait`
 764     // without knowing what `Self` is.
 765
 766     struct IllegalSelfTypeVisitor<'tcx> {
 767         tcx: TyCtxt<'tcx>,
 768         trait_def_id: DefId,
 769         supertraits: Option<Vec<DefId>>,
 770     }
 771
 772     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 773         type BreakTy = ();
 774         fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
 775             Some(self.tcx)
 776         }
 777
 778         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 779             match t.kind() {
 780                 ty::Param(_) => {
 781                     if t == self.tcx.types.self_param {
 782                         ControlFlow::BREAK
 783                     } else {
 784                         ControlFlow::CONTINUE
 785                     }
 786                 }
 787                 ty::Projection(ref data) => {
 788                     // This is a projected type `<Foo as SomeTrait>::X`.
 789
 790                     // Compute supertraits of current trait lazily.
 791                     if self.supertraits.is_none() {
 792                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 793                         self.supertraits = Some(
 794                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 795                         );
 796                     }
 797
 798                     // Determine whether the trait reference `Foo as
 799                     // SomeTrait` is in fact a supertrait of the
 800                     // current trait. In that case, this type is
 801                     // legal, because the type `X` will be specified
 802                     // in the object type.  Note that we can just use
 803                     // direct equality here because all of these types
 804                     // are part of the formal parameter listing, and
 805                     // hence there should be no inference variables.
 806                     let is_supertrait_of_current_trait = self
 807                         .supertraits
 808                         .as_ref()
 809                         .unwrap()
 810                         .contains(&data.trait_ref(self.tcx).def_id);
 811
 812                     if is_supertrait_of_current_trait {
 813                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 814                     } else {
 815                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 816                     }
 817                 }
 818                 _ => t.super_visit_with(self), // walk contained types, if any
 819             }
 820         }
 821
 822         fn visit_unevaluated_const(
 823             &mut self,
 824             uv: ty::Unevaluated<'tcx>,
 825         ) -> ControlFlow<Self::BreakTy> {
 826             // Constants can only influence object safety if they reference `Self`.
 827             // This is only possible for unevaluated constants, so we walk these here.
 828             //
 829             // If `AbstractConst::new` returned an error we already failed compilation
 830             // so we don't have to emit an additional error here.
 831             //
 832             // We currently recurse into abstract consts here but do not recurse in
 833             // `is_const_evaluatable`. This means that the object safety check is more
 834             // liberal than the const eval check.
 835             //
 836             // This shouldn't really matter though as we can't really use any
 837             // constants which are not considered const evaluatable.
 838             use rustc_middle::thir::abstract_const::Node;
 839             if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
 840                 const_evaluatable::walk_abstract_const(self.tcx, ct, |node| {
 841                     match node.root(self.tcx) {
 842                         Node::Leaf(leaf) => self.visit_const(leaf),
 843                         Node::Cast(_, _, ty) => self.visit_ty(ty),
 844                         Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 845                             ControlFlow::CONTINUE
 846                         }
 847                     }
 848                 })
 849             } else {
 850                 ControlFlow::CONTINUE
 851             }
 852         }
 853     }
 854
 855     value
 856         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 857         .is_break()
 858 }
 859
 860 pub fn provide(providers: &mut ty::query::Providers) {
 861     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 862 }