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};
  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::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.ident.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| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 129     );
 130
 131     violations.extend(
 132         tcx.associated_items(trait_def_id)
 133             .in_definition_order()
 134             .filter(|item| item.kind == ty::AssocKind::Type)
 135             .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
 136             .map(|item| ObjectSafetyViolation::GAT(item.ident.name, item.ident.span)),
 137     );
 138
 139     debug!(
 140         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 141         trait_def_id, violations
 142     );
 143
 144     violations
 145 }
 146
 147 /// Lint object-unsafe trait.
 148 fn lint_object_unsafe_trait(
 149     tcx: TyCtxt<'_>,
 150     span: Span,
 151     trait_def_id: DefId,
 152     violation: &ObjectSafetyViolation,
 153 ) {
 154     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 155     // It's also hard to get a use site span, so we use the method definition span.
 156     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 157         let mut err = lint.build(&format!(
 158             "the trait `{}` cannot be made into an object",
 159             tcx.def_path_str(trait_def_id)
 160         ));
 161         let node = tcx.hir().get_if_local(trait_def_id);
 162         let mut spans = MultiSpan::from_span(span);
 163         if let Some(hir::Node::Item(item)) = node {
 164             spans.push_span_label(
 165                 item.ident.span,
 166                 "this trait cannot be made into an object...".into(),
 167             );
 168             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 169         } else {
 170             spans.push_span_label(
 171                 span,
 172                 format!(
 173                     "the trait cannot be made into an object because {}",
 174                     violation.error_msg()
 175                 ),
 176             );
 177         };
 178         err.span_note(
 179             spans,
 180             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 181              call to be resolvable dynamically; for more information visit \
 182              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 183         );
 184         if node.is_some() {
 185             // Only provide the help if its a local trait, otherwise it's not
 186             violation.solution(&mut err);
 187         }
 188         err.emit();
 189     });
 190 }
 191
 192 fn sized_trait_bound_spans<'tcx>(
 193     tcx: TyCtxt<'tcx>,
 194     bounds: hir::GenericBounds<'tcx>,
 195 ) -> impl 'tcx + Iterator<Item = Span> {
 196     bounds.iter().filter_map(move |b| match b {
 197         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 198             if trait_has_sized_self(
 199                 tcx,
 200                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 201             ) =>
 202         {
 203             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 204             Some(trait_ref.span)
 205         }
 206         _ => None,
 207     })
 208 }
 209
 210 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 211     tcx.hir()
 212         .get_if_local(trait_def_id)
 213         .and_then(|node| match node {
 214             hir::Node::Item(hir::Item {
 215                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 216                 ..
 217             }) => Some(
 218                 generics
 219                     .where_clause
 220                     .predicates
 221                     .iter()
 222                     .filter_map(|pred| {
 223                         match pred {
 224                             hir::WherePredicate::BoundPredicate(pred)
 225                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 226                             {
 227                                 // Fetch spans for trait bounds that are Sized:
 228                                 // `trait T where Self: Pred`
 229                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 230                             }
 231                             _ => None,
 232                         }
 233                     })
 234                     .flatten()
 235                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 236                     .chain(sized_trait_bound_spans(tcx, bounds))
 237                     .collect::<SmallVec<[Span; 1]>>(),
 238             ),
 239             _ => None,
 240         })
 241         .unwrap_or_else(SmallVec::new)
 242 }
 243
 244 fn predicates_reference_self(
 245     tcx: TyCtxt<'_>,
 246     trait_def_id: DefId,
 247     supertraits_only: bool,
 248 ) -> SmallVec<[Span; 1]> {
 249     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 250     let predicates = if supertraits_only {
 251         tcx.super_predicates_of(trait_def_id)
 252     } else {
 253         tcx.predicates_of(trait_def_id)
 254     };
 255     predicates
 256         .predicates
 257         .iter()
 258         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 259         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 260         .collect()
 261 }
 262
 263 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 264     tcx.associated_items(trait_def_id)
 265         .in_definition_order()
 266         .filter(|item| item.kind == ty::AssocKind::Type)
 267         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 268         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 269         .collect()
 270 }
 271
 272 fn predicate_references_self<'tcx>(
 273     tcx: TyCtxt<'tcx>,
 274     (predicate, sp): (ty::Predicate<'tcx>, Span),
 275 ) -> Option<Span> {
 276     let self_ty = tcx.types.self_param;
 277     let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk(tcx).any(|arg| arg == self_ty.into());
 278     match predicate.kind().skip_binder() {
 279         ty::PredicateKind::Trait(ref data) => {
 280             // In the case of a trait predicate, we can skip the "self" type.
 281             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 282         }
 283         ty::PredicateKind::Projection(ref data) => {
 284             // And similarly for projections. This should be redundant with
 285             // the previous check because any projection should have a
 286             // matching `Trait` predicate with the same inputs, but we do
 287             // the check to be safe.
 288             //
 289             // It's also won't be redundant if we allow type-generic associated
 290             // types for trait objects.
 291             //
 292             // Note that we *do* allow projection *outputs* to contain
 293             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 294             // we just require the user to specify *both* outputs
 295             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 296             //
 297             // This is ALT2 in issue #56288, see that for discussion of the
 298             // possible alternatives.
 299             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 300         }
 301         ty::PredicateKind::WellFormed(..)
 302         | ty::PredicateKind::ObjectSafe(..)
 303         | ty::PredicateKind::TypeOutlives(..)
 304         | ty::PredicateKind::RegionOutlives(..)
 305         | ty::PredicateKind::ClosureKind(..)
 306         | ty::PredicateKind::Subtype(..)
 307         | ty::PredicateKind::Coerce(..)
 308         | ty::PredicateKind::ConstEvaluatable(..)
 309         | ty::PredicateKind::ConstEquate(..)
 310         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 311     }
 312 }
 313
 314 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 315     generics_require_sized_self(tcx, trait_def_id)
 316 }
 317
 318 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 319     let sized_def_id = match tcx.lang_items().sized_trait() {
 320         Some(def_id) => def_id,
 321         None => {
 322             return false; /* No Sized trait, can't require it! */
 323         }
 324     };
 325
 326     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 327     let predicates = tcx.predicates_of(def_id);
 328     let predicates = predicates.instantiate_identity(tcx).predicates;
 329     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 330         match obligation.predicate.kind().skip_binder() {
 331             ty::PredicateKind::Trait(ref trait_pred) => {
 332                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 333             }
 334             ty::PredicateKind::Projection(..)
 335             | ty::PredicateKind::Subtype(..)
 336             | ty::PredicateKind::Coerce(..)
 337             | ty::PredicateKind::RegionOutlives(..)
 338             | ty::PredicateKind::WellFormed(..)
 339             | ty::PredicateKind::ObjectSafe(..)
 340             | ty::PredicateKind::ClosureKind(..)
 341             | ty::PredicateKind::TypeOutlives(..)
 342             | ty::PredicateKind::ConstEvaluatable(..)
 343             | ty::PredicateKind::ConstEquate(..)
 344             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 345         }
 346     })
 347 }
 348
 349 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 350 fn object_safety_violation_for_method(
 351     tcx: TyCtxt<'_>,
 352     trait_def_id: DefId,
 353     method: &ty::AssocItem,
 354 ) -> Option<(MethodViolationCode, Span)> {
 355     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 356     // Any method that has a `Self : Sized` requisite is otherwise
 357     // exempt from the regulations.
 358     if generics_require_sized_self(tcx, method.def_id) {
 359         return None;
 360     }
 361
 362     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 363     // Get an accurate span depending on the violation.
 364     violation.map(|v| {
 365         let node = tcx.hir().get_if_local(method.def_id);
 366         let span = match (v, node) {
 367             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 368                 .fn_decl()
 369                 .and_then(|decl| decl.inputs.get(arg + 1))
 370                 .map_or(method.ident.span, |arg| arg.span),
 371             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 372                 .fn_decl()
 373                 .and_then(|decl| decl.inputs.get(0))
 374                 .map_or(method.ident.span, |arg| arg.span),
 375             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 376                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 377             }
 378             _ => method.ident.span,
 379         };
 380         (v, span)
 381     })
 382 }
 383
 384 /// Returns `Some(_)` if this method cannot be called on a trait
 385 /// object; this does not necessarily imply that the enclosing trait
 386 /// is not object safe, because the method might have a where clause
 387 /// `Self:Sized`.
 388 fn virtual_call_violation_for_method<'tcx>(
 389     tcx: TyCtxt<'tcx>,
 390     trait_def_id: DefId,
 391     method: &ty::AssocItem,
 392 ) -> Option<MethodViolationCode> {
 393     let sig = tcx.fn_sig(method.def_id);
 394
 395     // The method's first parameter must be named `self`
 396     if !method.fn_has_self_parameter {
 397         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 398         let sugg =
 399             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 400                 |generics| match generics.where_clause.predicates {
 401                     [] => (" where Self: Sized", generics.where_clause.span),
 402                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 403                 },
 404             );
 405         // Get the span pointing at where the `self` receiver should be.
 406         let sm = tcx.sess.source_map();
 407         let self_span = method.ident.span.to(tcx
 408             .hir()
 409             .span_if_local(method.def_id)
 410             .unwrap_or_else(|| sm.next_point(method.ident.span))
 411             .shrink_to_hi());
 412         let self_span = sm.span_through_char(self_span, '(').shrink_to_hi();
 413         return Some(MethodViolationCode::StaticMethod(
 414             sugg,
 415             self_span,
 416             !sig.inputs().skip_binder().is_empty(),
 417         ));
 418     }
 419
 420     for (i, &input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 421         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 422             return Some(MethodViolationCode::ReferencesSelfInput(i));
 423         }
 424     }
 425     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 426         return Some(MethodViolationCode::ReferencesSelfOutput);
 427     }
 428
 429     // We can't monomorphize things like `fn foo<A>(...)`.
 430     let own_counts = tcx.generics_of(method.def_id).own_counts();
 431     if own_counts.types + own_counts.consts != 0 {
 432         return Some(MethodViolationCode::Generic);
 433     }
 434
 435     if tcx
 436         .predicates_of(method.def_id)
 437         .predicates
 438         .iter()
 439         // A trait object can't claim to live more than the concrete type,
 440         // so outlives predicates will always hold.
 441         .cloned()
 442         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 443         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 444     {
 445         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 446     }
 447
 448     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 449
 450     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 451     // However, this is already considered object-safe. We allow it as a special case here.
 452     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 453     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 454     if receiver_ty != tcx.types.self_param {
 455         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 456             return Some(MethodViolationCode::UndispatchableReceiver);
 457         } else {
 458             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 459
 460             use rustc_target::abi::Abi;
 461
 462             let param_env = tcx.param_env(method.def_id);
 463
 464             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 465                 match tcx.layout_of(param_env.and(ty)) {
 466                     Ok(layout) => Some(layout.abi),
 467                     Err(err) => {
 468                         // #78372
 469                         tcx.sess.delay_span_bug(
 470                             tcx.def_span(method.def_id),
 471                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 472                         );
 473                         None
 474                     }
 475                 }
 476             };
 477
 478             // e.g., `Rc<()>`
 479             let unit_receiver_ty =
 480                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 481
 482             match abi_of_ty(unit_receiver_ty) {
 483                 Some(Abi::Scalar(..)) => (),
 484                 abi => {
 485                     tcx.sess.delay_span_bug(
 486                         tcx.def_span(method.def_id),
 487                         &format!(
 488                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 489                             abi
 490                         ),
 491                     );
 492                 }
 493             }
 494
 495             let trait_object_ty =
 496                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 497
 498             // e.g., `Rc<dyn Trait>`
 499             let trait_object_receiver =
 500                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 501
 502             match abi_of_ty(trait_object_receiver) {
 503                 Some(Abi::ScalarPair(..)) => (),
 504                 abi => {
 505                     tcx.sess.delay_span_bug(
 506                         tcx.def_span(method.def_id),
 507                         &format!(
 508                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 509                             trait_object_ty, abi
 510                         ),
 511                     );
 512                 }
 513             }
 514         }
 515     }
 516
 517     None
 518 }
 519
 520 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 521 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 522 fn receiver_for_self_ty<'tcx>(
 523     tcx: TyCtxt<'tcx>,
 524     receiver_ty: Ty<'tcx>,
 525     self_ty: Ty<'tcx>,
 526     method_def_id: DefId,
 527 ) -> Ty<'tcx> {
 528     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 529     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 530         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 531     });
 532
 533     let result = receiver_ty.subst(tcx, substs);
 534     debug!(
 535         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 536         receiver_ty, self_ty, method_def_id, result
 537     );
 538     result
 539 }
 540
 541 /// Creates the object type for the current trait. For example,
 542 /// if the current trait is `Deref`, then this will be
 543 /// `dyn Deref<Target = Self::Target> + 'static`.
 544 fn object_ty_for_trait<'tcx>(
 545     tcx: TyCtxt<'tcx>,
 546     trait_def_id: DefId,
 547     lifetime: ty::Region<'tcx>,
 548 ) -> Ty<'tcx> {
 549     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 550
 551     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 552
 553     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 554         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 555     });
 556
 557     let mut associated_types = traits::supertraits(tcx, trait_ref)
 558         .flat_map(|super_trait_ref| {
 559             tcx.associated_items(super_trait_ref.def_id())
 560                 .in_definition_order()
 561                 .map(move |item| (super_trait_ref, item))
 562         })
 563         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 564         .collect::<Vec<_>>();
 565
 566     // existential predicates need to be in a specific order
 567     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 568
 569     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 570         // We *can* get bound lifetimes here in cases like
 571         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 572         super_trait_ref.map_bound(|super_trait_ref| {
 573             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 574                 ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 575                 item_def_id: item.def_id,
 576                 substs: super_trait_ref.substs,
 577             })
 578         })
 579     });
 580
 581     let existential_predicates = tcx
 582         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 583
 584     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
 585
 586     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 587
 588     object_ty
 589 }
 590
 591 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 592 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 593 /// in the following way:
 594 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 595 /// - require the following bound:
 596 ///
 597 ///   ```
 598 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 599 ///   ```
 600 ///
 601 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 602 ///   (substitution notation).
 603 ///
 604 /// Some examples of receiver types and their required obligation:
 605 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 606 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 607 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 608 ///
 609 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 610 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 611 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 612 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 613 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 614 /// contained by the trait object, because the object that needs to be coerced is behind
 615 /// a pointer.
 616 ///
 617 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 618 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 619 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 620 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 621 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 622 /// Written as a chalk-style query:
 623 ///
 624 ///     forall (U: Trait + ?Sized) {
 625 ///         if (Self: Unsize<U>) {
 626 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 627 ///         }
 628 ///     }
 629 ///
 630 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 631 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 632 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 633 //
 634 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 635 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 636 // `self: Wrapper<Self>`.
 637 #[allow(dead_code)]
 638 fn receiver_is_dispatchable<'tcx>(
 639     tcx: TyCtxt<'tcx>,
 640     method: &ty::AssocItem,
 641     receiver_ty: Ty<'tcx>,
 642 ) -> bool {
 643     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 644
 645     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 646     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 647         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 648         return false;
 649     };
 650
 651     // the type `U` in the query
 652     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 653     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 654     // replace this with `dyn Trait`
 655     let unsized_self_ty: Ty<'tcx> =
 656         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 657
 658     // `Receiver[Self => U]`
 659     let unsized_receiver_ty =
 660         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 661
 662     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 663     // `U: ?Sized` is already implied here
 664     let param_env = {
 665         let param_env = tcx.param_env(method.def_id);
 666
 667         // Self: Unsize<U>
 668         let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
 669             def_id: unsize_did,
 670             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 671         })
 672         .without_const()
 673         .to_predicate(tcx);
 674
 675         // U: Trait<Arg1, ..., ArgN>
 676         let trait_predicate = {
 677             let substs =
 678                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 679                     if param.index == 0 {
 680                         unsized_self_ty.into()
 681                     } else {
 682                         tcx.mk_param_from_def(param)
 683                     }
 684                 });
 685
 686             ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
 687                 .without_const()
 688                 .to_predicate(tcx)
 689         };
 690
 691         let caller_bounds: Vec<Predicate<'tcx>> =
 692             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 693
 694         ty::ParamEnv::new(
 695             tcx.intern_predicates(&caller_bounds),
 696             param_env.reveal(),
 697             param_env.constness(),
 698         )
 699     };
 700
 701     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 702     let obligation = {
 703         let predicate = ty::Binder::dummy(ty::TraitRef {
 704             def_id: dispatch_from_dyn_did,
 705             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 706         })
 707         .without_const()
 708         .to_predicate(tcx);
 709
 710         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 711     };
 712
 713     tcx.infer_ctxt().enter(|ref infcx| {
 714         // the receiver is dispatchable iff the obligation holds
 715         infcx.predicate_must_hold_modulo_regions(&obligation)
 716     })
 717 }
 718
 719 fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
 720     tcx: TyCtxt<'tcx>,
 721     trait_def_id: DefId,
 722     value: T,
 723 ) -> bool {
 724     // This is somewhat subtle. In general, we want to forbid
 725     // references to `Self` in the argument and return types,
 726     // since the value of `Self` is erased. However, there is one
 727     // exception: it is ok to reference `Self` in order to access
 728     // an associated type of the current trait, since we retain
 729     // the value of those associated types in the object type
 730     // itself.
 731     //
 732     // ```rust
 733     // trait SuperTrait {
 734     //     type X;
 735     // }
 736     //
 737     // trait Trait : SuperTrait {
 738     //     type Y;
 739     //     fn foo(&self, x: Self) // bad
 740     //     fn foo(&self) -> Self // bad
 741     //     fn foo(&self) -> Option<Self> // bad
 742     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 743     //     fn foo(&self) -> <Self as Trait>::Y // OK
 744     //     fn foo(&self) -> Self::X // OK, desugars to next example
 745     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 746     // }
 747     // ```
 748     //
 749     // However, it is not as simple as allowing `Self` in a projected
 750     // type, because there are illegal ways to use `Self` as well:
 751     //
 752     // ```rust
 753     // trait Trait : SuperTrait {
 754     //     ...
 755     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 756     // }
 757     // ```
 758     //
 759     // Here we will not have the type of `X` recorded in the
 760     // object type, and we cannot resolve `Self as SomeOtherTrait`
 761     // without knowing what `Self` is.
 762
 763     struct IllegalSelfTypeVisitor<'tcx> {
 764         tcx: TyCtxt<'tcx>,
 765         trait_def_id: DefId,
 766         supertraits: Option<Vec<DefId>>,
 767     }
 768
 769     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 770         type BreakTy = ();
 771         fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
 772             Some(self.tcx)
 773         }
 774
 775         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 776             match t.kind() {
 777                 ty::Param(_) => {
 778                     if t == self.tcx.types.self_param {
 779                         ControlFlow::BREAK
 780                     } else {
 781                         ControlFlow::CONTINUE
 782                     }
 783                 }
 784                 ty::Projection(ref data) => {
 785                     // This is a projected type `<Foo as SomeTrait>::X`.
 786
 787                     // Compute supertraits of current trait lazily.
 788                     if self.supertraits.is_none() {
 789                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 790                         self.supertraits = Some(
 791                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 792                         );
 793                     }
 794
 795                     // Determine whether the trait reference `Foo as
 796                     // SomeTrait` is in fact a supertrait of the
 797                     // current trait. In that case, this type is
 798                     // legal, because the type `X` will be specified
 799                     // in the object type.  Note that we can just use
 800                     // direct equality here because all of these types
 801                     // are part of the formal parameter listing, and
 802                     // hence there should be no inference variables.
 803                     let is_supertrait_of_current_trait = self
 804                         .supertraits
 805                         .as_ref()
 806                         .unwrap()
 807                         .contains(&data.trait_ref(self.tcx).def_id);
 808
 809                     if is_supertrait_of_current_trait {
 810                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 811                     } else {
 812                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 813                     }
 814                 }
 815                 _ => t.super_visit_with(self), // walk contained types, if any
 816             }
 817         }
 818
 819         fn visit_unevaluated_const(
 820             &mut self,
 821             uv: ty::Unevaluated<'tcx>,
 822         ) -> ControlFlow<Self::BreakTy> {
 823             // Constants can only influence object safety if they reference `Self`.
 824             // This is only possible for unevaluated constants, so we walk these here.
 825             //
 826             // If `AbstractConst::new` returned an error we already failed compilation
 827             // so we don't have to emit an additional error here.
 828             //
 829             // We currently recurse into abstract consts here but do not recurse in
 830             // `is_const_evaluatable`. This means that the object safety check is more
 831             // liberal than the const eval check.
 832             //
 833             // This shouldn't really matter though as we can't really use any
 834             // constants which are not considered const evaluatable.
 835             use rustc_middle::thir::abstract_const::Node;
 836             if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
 837                 const_evaluatable::walk_abstract_const(self.tcx, ct, |node| {
 838                     match node.root(self.tcx) {
 839                         Node::Leaf(leaf) => self.visit_const(leaf),
 840                         Node::Cast(_, _, ty) => self.visit_ty(ty),
 841                         Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 842                             ControlFlow::CONTINUE
 843                         }
 844                     }
 845                 })
 846             } else {
 847                 ControlFlow::CONTINUE
 848             }
 849         }
 850     }
 851
 852     value
 853         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 854         .is_break()
 855 }
 856
 857 pub fn provide(providers: &mut ty::query::Providers) {
 858     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 859 }