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(
  54     tcx: TyCtxt<'tcx>,
  55     trait_def_id: DefId,
  56 ) -> &'tcx [ObjectSafetyViolation] {
  57     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  58     debug!("object_safety_violations: {:?}", trait_def_id);
  59
  60     tcx.arena.alloc_from_iter(
  61         traits::supertrait_def_ids(tcx, trait_def_id)
  62             .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
  63     )
  64 }
  65
  66 /// We say a method is *vtable safe* if it can be invoked on a trait
  67 /// object. Note that object-safe traits can have some
  68 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  69 /// otherwise ensure that they cannot be used when `Self = Trait`.
  70 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
  71     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  72     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
  73     // Any method that has a `Self: Sized` bound cannot be called.
  74     if generics_require_sized_self(tcx, method.def_id) {
  75         return false;
  76     }
  77
  78     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
  79         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
  80         Some(_) => false,
  81     }
  82 }
  83
  84 fn object_safety_violations_for_trait(
  85     tcx: TyCtxt<'_>,
  86     trait_def_id: DefId,
  87 ) -> Vec<ObjectSafetyViolation> {
  88     // Check methods for violations.
  89     let mut violations: Vec<_> = tcx
  90         .associated_items(trait_def_id)
  91         .in_definition_order()
  92         .filter(|item| item.kind == ty::AssocKind::Fn)
  93         .filter_map(|item| {
  94             object_safety_violation_for_method(tcx, trait_def_id, &item)
  95                 .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span))
  96         })
  97         .filter(|violation| {
  98             if let ObjectSafetyViolation::Method(
  99                 _,
 100                 MethodViolationCode::WhereClauseReferencesSelf,
 101                 span,
 102             ) = violation
 103             {
 104                 lint_object_unsafe_trait(tcx, *span, trait_def_id, violation);
 105                 false
 106             } else {
 107                 true
 108             }
 109         })
 110         .collect();
 111
 112     // Check the trait itself.
 113     if trait_has_sized_self(tcx, trait_def_id) {
 114         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 115         let spans = get_sized_bounds(tcx, trait_def_id);
 116         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 117     }
 118     let spans = predicates_reference_self(tcx, trait_def_id, false);
 119     if !spans.is_empty() {
 120         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 121     }
 122     let spans = bounds_reference_self(tcx, trait_def_id);
 123     if !spans.is_empty() {
 124         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 125     }
 126
 127     violations.extend(
 128         tcx.associated_items(trait_def_id)
 129             .in_definition_order()
 130             .filter(|item| item.kind == ty::AssocKind::Const)
 131             .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 132     );
 133
 134     violations.extend(
 135         tcx.associated_items(trait_def_id)
 136             .in_definition_order()
 137             .filter(|item| item.kind == ty::AssocKind::Type)
 138             .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
 139             .map(|item| ObjectSafetyViolation::GAT(item.ident.name, item.ident.span)),
 140     );
 141
 142     debug!(
 143         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 144         trait_def_id, violations
 145     );
 146
 147     violations
 148 }
 149
 150 /// Lint object-unsafe trait.
 151 fn lint_object_unsafe_trait(
 152     tcx: TyCtxt<'_>,
 153     span: Span,
 154     trait_def_id: DefId,
 155     violation: &ObjectSafetyViolation,
 156 ) {
 157     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 158     // It's also hard to get a use site span, so we use the method definition span.
 159     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 160         let mut err = lint.build(&format!(
 161             "the trait `{}` cannot be made into an object",
 162             tcx.def_path_str(trait_def_id)
 163         ));
 164         let node = tcx.hir().get_if_local(trait_def_id);
 165         let mut spans = MultiSpan::from_span(span);
 166         if let Some(hir::Node::Item(item)) = node {
 167             spans.push_span_label(
 168                 item.ident.span,
 169                 "this trait cannot be made into an object...".into(),
 170             );
 171             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 172         } else {
 173             spans.push_span_label(
 174                 span,
 175                 format!(
 176                     "the trait cannot be made into an object because {}",
 177                     violation.error_msg()
 178                 ),
 179             );
 180         };
 181         err.span_note(
 182             spans,
 183             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 184              call to be resolvable dynamically; for more information visit \
 185              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 186         );
 187         if node.is_some() {
 188             // Only provide the help if its a local trait, otherwise it's not
 189             violation.solution(&mut err);
 190         }
 191         err.emit();
 192     });
 193 }
 194
 195 fn sized_trait_bound_spans<'tcx>(
 196     tcx: TyCtxt<'tcx>,
 197     bounds: hir::GenericBounds<'tcx>,
 198 ) -> impl 'tcx + Iterator<Item = Span> {
 199     bounds.iter().filter_map(move |b| match b {
 200         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 201             if trait_has_sized_self(
 202                 tcx,
 203                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 204             ) =>
 205         {
 206             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 207             Some(trait_ref.span)
 208         }
 209         _ => None,
 210     })
 211 }
 212
 213 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 214     tcx.hir()
 215         .get_if_local(trait_def_id)
 216         .and_then(|node| match node {
 217             hir::Node::Item(hir::Item {
 218                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 219                 ..
 220             }) => Some(
 221                 generics
 222                     .where_clause
 223                     .predicates
 224                     .iter()
 225                     .filter_map(|pred| {
 226                         match pred {
 227                             hir::WherePredicate::BoundPredicate(pred)
 228                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 229                             {
 230                                 // Fetch spans for trait bounds that are Sized:
 231                                 // `trait T where Self: Pred`
 232                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 233                             }
 234                             _ => None,
 235                         }
 236                     })
 237                     .flatten()
 238                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 239                     .chain(sized_trait_bound_spans(tcx, bounds))
 240                     .collect::<SmallVec<[Span; 1]>>(),
 241             ),
 242             _ => None,
 243         })
 244         .unwrap_or_else(SmallVec::new)
 245 }
 246
 247 fn predicates_reference_self(
 248     tcx: TyCtxt<'_>,
 249     trait_def_id: DefId,
 250     supertraits_only: bool,
 251 ) -> SmallVec<[Span; 1]> {
 252     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 253     let predicates = if supertraits_only {
 254         tcx.super_predicates_of(trait_def_id)
 255     } else {
 256         tcx.predicates_of(trait_def_id)
 257     };
 258     predicates
 259         .predicates
 260         .iter()
 261         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 262         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 263         .collect()
 264 }
 265
 266 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 267     tcx.associated_items(trait_def_id)
 268         .in_definition_order()
 269         .filter(|item| item.kind == ty::AssocKind::Type)
 270         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 271         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 272         .collect()
 273 }
 274
 275 fn predicate_references_self(
 276     tcx: TyCtxt<'tcx>,
 277     (predicate, sp): (ty::Predicate<'tcx>, Span),
 278 ) -> Option<Span> {
 279     let self_ty = tcx.types.self_param;
 280     let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk(tcx).any(|arg| arg == self_ty.into());
 281     match predicate.kind().skip_binder() {
 282         ty::PredicateKind::Trait(ref data) => {
 283             // In the case of a trait predicate, we can skip the "self" type.
 284             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 285         }
 286         ty::PredicateKind::Projection(ref data) => {
 287             // And similarly for projections. This should be redundant with
 288             // the previous check because any projection should have a
 289             // matching `Trait` predicate with the same inputs, but we do
 290             // the check to be safe.
 291             //
 292             // It's also won't be redundant if we allow type-generic associated
 293             // types for trait objects.
 294             //
 295             // Note that we *do* allow projection *outputs* to contain
 296             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 297             // we just require the user to specify *both* outputs
 298             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 299             //
 300             // This is ALT2 in issue #56288, see that for discussion of the
 301             // possible alternatives.
 302             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 303         }
 304         ty::PredicateKind::WellFormed(..)
 305         | ty::PredicateKind::ObjectSafe(..)
 306         | ty::PredicateKind::TypeOutlives(..)
 307         | ty::PredicateKind::RegionOutlives(..)
 308         | ty::PredicateKind::ClosureKind(..)
 309         | ty::PredicateKind::Subtype(..)
 310         | ty::PredicateKind::Coerce(..)
 311         | ty::PredicateKind::ConstEvaluatable(..)
 312         | ty::PredicateKind::ConstEquate(..)
 313         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 314     }
 315 }
 316
 317 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 318     generics_require_sized_self(tcx, trait_def_id)
 319 }
 320
 321 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 322     let sized_def_id = match tcx.lang_items().sized_trait() {
 323         Some(def_id) => def_id,
 324         None => {
 325             return false; /* No Sized trait, can't require it! */
 326         }
 327     };
 328
 329     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 330     let predicates = tcx.predicates_of(def_id);
 331     let predicates = predicates.instantiate_identity(tcx).predicates;
 332     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 333         match obligation.predicate.kind().skip_binder() {
 334             ty::PredicateKind::Trait(ref trait_pred) => {
 335                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 336             }
 337             ty::PredicateKind::Projection(..)
 338             | ty::PredicateKind::Subtype(..)
 339             | ty::PredicateKind::Coerce(..)
 340             | ty::PredicateKind::RegionOutlives(..)
 341             | ty::PredicateKind::WellFormed(..)
 342             | ty::PredicateKind::ObjectSafe(..)
 343             | ty::PredicateKind::ClosureKind(..)
 344             | ty::PredicateKind::TypeOutlives(..)
 345             | ty::PredicateKind::ConstEvaluatable(..)
 346             | ty::PredicateKind::ConstEquate(..)
 347             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 348         }
 349     })
 350 }
 351
 352 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 353 fn object_safety_violation_for_method(
 354     tcx: TyCtxt<'_>,
 355     trait_def_id: DefId,
 356     method: &ty::AssocItem,
 357 ) -> Option<(MethodViolationCode, Span)> {
 358     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 359     // Any method that has a `Self : Sized` requisite is otherwise
 360     // exempt from the regulations.
 361     if generics_require_sized_self(tcx, method.def_id) {
 362         return None;
 363     }
 364
 365     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 366     // Get an accurate span depending on the violation.
 367     violation.map(|v| {
 368         let node = tcx.hir().get_if_local(method.def_id);
 369         let span = match (v, node) {
 370             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 371                 .fn_decl()
 372                 .and_then(|decl| decl.inputs.get(arg + 1))
 373                 .map_or(method.ident.span, |arg| arg.span),
 374             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 375                 .fn_decl()
 376                 .and_then(|decl| decl.inputs.get(0))
 377                 .map_or(method.ident.span, |arg| arg.span),
 378             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 379                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 380             }
 381             _ => method.ident.span,
 382         };
 383         (v, span)
 384     })
 385 }
 386
 387 /// Returns `Some(_)` if this method cannot be called on a trait
 388 /// object; this does not necessarily imply that the enclosing trait
 389 /// is not object safe, because the method might have a where clause
 390 /// `Self:Sized`.
 391 fn virtual_call_violation_for_method<'tcx>(
 392     tcx: TyCtxt<'tcx>,
 393     trait_def_id: DefId,
 394     method: &ty::AssocItem,
 395 ) -> Option<MethodViolationCode> {
 396     let sig = tcx.fn_sig(method.def_id);
 397
 398     // The method's first parameter must be named `self`
 399     if !method.fn_has_self_parameter {
 400         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 401         let sugg =
 402             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 403                 |generics| match generics.where_clause.predicates {
 404                     [] => (" where Self: Sized", generics.where_clause.span),
 405                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 406                 },
 407             );
 408         // Get the span pointing at where the `self` receiver should be.
 409         let sm = tcx.sess.source_map();
 410         let self_span = method.ident.span.to(tcx
 411             .hir()
 412             .span_if_local(method.def_id)
 413             .unwrap_or_else(|| sm.next_point(method.ident.span))
 414             .shrink_to_hi());
 415         let self_span = sm.span_through_char(self_span, '(').shrink_to_hi();
 416         return Some(MethodViolationCode::StaticMethod(
 417             sugg,
 418             self_span,
 419             !sig.inputs().skip_binder().is_empty(),
 420         ));
 421     }
 422
 423     for (i, &input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 424         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 425             return Some(MethodViolationCode::ReferencesSelfInput(i));
 426         }
 427     }
 428     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 429         return Some(MethodViolationCode::ReferencesSelfOutput);
 430     }
 431
 432     // We can't monomorphize things like `fn foo<A>(...)`.
 433     let own_counts = tcx.generics_of(method.def_id).own_counts();
 434     if own_counts.types + own_counts.consts != 0 {
 435         return Some(MethodViolationCode::Generic);
 436     }
 437
 438     if tcx
 439         .predicates_of(method.def_id)
 440         .predicates
 441         .iter()
 442         // A trait object can't claim to live more than the concrete type,
 443         // so outlives predicates will always hold.
 444         .cloned()
 445         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 446         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 447     {
 448         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 449     }
 450
 451     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 452
 453     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 454     // However, this is already considered object-safe. We allow it as a special case here.
 455     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 456     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 457     if receiver_ty != tcx.types.self_param {
 458         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 459             return Some(MethodViolationCode::UndispatchableReceiver);
 460         } else {
 461             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 462
 463             use rustc_target::abi::Abi;
 464
 465             let param_env = tcx.param_env(method.def_id);
 466
 467             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 468                 match tcx.layout_of(param_env.and(ty)) {
 469                     Ok(layout) => Some(layout.abi),
 470                     Err(err) => {
 471                         // #78372
 472                         tcx.sess.delay_span_bug(
 473                             tcx.def_span(method.def_id),
 474                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 475                         );
 476                         None
 477                     }
 478                 }
 479             };
 480
 481             // e.g., `Rc<()>`
 482             let unit_receiver_ty =
 483                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 484
 485             match abi_of_ty(unit_receiver_ty) {
 486                 Some(Abi::Scalar(..)) => (),
 487                 abi => {
 488                     tcx.sess.delay_span_bug(
 489                         tcx.def_span(method.def_id),
 490                         &format!(
 491                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 492                             abi
 493                         ),
 494                     );
 495                 }
 496             }
 497
 498             let trait_object_ty =
 499                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 500
 501             // e.g., `Rc<dyn Trait>`
 502             let trait_object_receiver =
 503                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 504
 505             match abi_of_ty(trait_object_receiver) {
 506                 Some(Abi::ScalarPair(..)) => (),
 507                 abi => {
 508                     tcx.sess.delay_span_bug(
 509                         tcx.def_span(method.def_id),
 510                         &format!(
 511                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 512                             trait_object_ty, abi
 513                         ),
 514                     );
 515                 }
 516             }
 517         }
 518     }
 519
 520     None
 521 }
 522
 523 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 524 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 525 fn receiver_for_self_ty<'tcx>(
 526     tcx: TyCtxt<'tcx>,
 527     receiver_ty: Ty<'tcx>,
 528     self_ty: Ty<'tcx>,
 529     method_def_id: DefId,
 530 ) -> Ty<'tcx> {
 531     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 532     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 533         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 534     });
 535
 536     let result = receiver_ty.subst(tcx, substs);
 537     debug!(
 538         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 539         receiver_ty, self_ty, method_def_id, result
 540     );
 541     result
 542 }
 543
 544 /// Creates the object type for the current trait. For example,
 545 /// if the current trait is `Deref`, then this will be
 546 /// `dyn Deref<Target = Self::Target> + 'static`.
 547 fn object_ty_for_trait<'tcx>(
 548     tcx: TyCtxt<'tcx>,
 549     trait_def_id: DefId,
 550     lifetime: ty::Region<'tcx>,
 551 ) -> Ty<'tcx> {
 552     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 553
 554     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 555
 556     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 557         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 558     });
 559
 560     let mut associated_types = traits::supertraits(tcx, trait_ref)
 561         .flat_map(|super_trait_ref| {
 562             tcx.associated_items(super_trait_ref.def_id())
 563                 .in_definition_order()
 564                 .map(move |item| (super_trait_ref, item))
 565         })
 566         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 567         .collect::<Vec<_>>();
 568
 569     // existential predicates need to be in a specific order
 570     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 571
 572     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 573         // We *can* get bound lifetimes here in cases like
 574         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 575         super_trait_ref.map_bound(|super_trait_ref| {
 576             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 577                 ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 578                 item_def_id: item.def_id,
 579                 substs: super_trait_ref.substs,
 580             })
 581         })
 582     });
 583
 584     let existential_predicates = tcx
 585         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 586
 587     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
 588
 589     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 590
 591     object_ty
 592 }
 593
 594 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 595 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 596 /// in the following way:
 597 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 598 /// - require the following bound:
 599 ///
 600 ///   ```
 601 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 602 ///   ```
 603 ///
 604 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 605 ///   (substitution notation).
 606 ///
 607 /// Some examples of receiver types and their required obligation:
 608 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 609 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 610 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 611 ///
 612 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 613 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 614 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 615 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 616 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 617 /// contained by the trait object, because the object that needs to be coerced is behind
 618 /// a pointer.
 619 ///
 620 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 621 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 622 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 623 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 624 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 625 /// Written as a chalk-style query:
 626 ///
 627 ///     forall (U: Trait + ?Sized) {
 628 ///         if (Self: Unsize<U>) {
 629 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 630 ///         }
 631 ///     }
 632 ///
 633 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 634 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 635 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 636 //
 637 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 638 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 639 // `self: Wrapper<Self>`.
 640 #[allow(dead_code)]
 641 fn receiver_is_dispatchable<'tcx>(
 642     tcx: TyCtxt<'tcx>,
 643     method: &ty::AssocItem,
 644     receiver_ty: Ty<'tcx>,
 645 ) -> bool {
 646     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 647
 648     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 649     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 650         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 651         return false;
 652     };
 653
 654     // the type `U` in the query
 655     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 656     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 657     // replace this with `dyn Trait`
 658     let unsized_self_ty: Ty<'tcx> =
 659         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 660
 661     // `Receiver[Self => U]`
 662     let unsized_receiver_ty =
 663         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 664
 665     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 666     // `U: ?Sized` is already implied here
 667     let param_env = {
 668         let param_env = tcx.param_env(method.def_id);
 669
 670         // Self: Unsize<U>
 671         let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
 672             def_id: unsize_did,
 673             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 674         })
 675         .without_const()
 676         .to_predicate(tcx);
 677
 678         // U: Trait<Arg1, ..., ArgN>
 679         let trait_predicate = {
 680             let substs =
 681                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 682                     if param.index == 0 {
 683                         unsized_self_ty.into()
 684                     } else {
 685                         tcx.mk_param_from_def(param)
 686                     }
 687                 });
 688
 689             ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
 690                 .without_const()
 691                 .to_predicate(tcx)
 692         };
 693
 694         let caller_bounds: Vec<Predicate<'tcx>> =
 695             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 696
 697         ty::ParamEnv::new(
 698             tcx.intern_predicates(&caller_bounds),
 699             param_env.reveal(),
 700             param_env.constness(),
 701         )
 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 }