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     debug!(
 136         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 137         trait_def_id, violations
 138     );
 139
 140     violations
 141 }
 142
 143 /// Lint object-unsafe trait.
 144 fn lint_object_unsafe_trait(
 145     tcx: TyCtxt<'_>,
 146     span: Span,
 147     trait_def_id: DefId,
 148     violation: &ObjectSafetyViolation,
 149 ) {
 150     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 151     // It's also hard to get a use site span, so we use the method definition span.
 152     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 153         let mut err = lint.build(&format!(
 154             "the trait `{}` cannot be made into an object",
 155             tcx.def_path_str(trait_def_id)
 156         ));
 157         let node = tcx.hir().get_if_local(trait_def_id);
 158         let mut spans = MultiSpan::from_span(span);
 159         if let Some(hir::Node::Item(item)) = node {
 160             spans.push_span_label(
 161                 item.ident.span,
 162                 "this trait cannot be made into an object...".into(),
 163             );
 164             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 165         } else {
 166             spans.push_span_label(
 167                 span,
 168                 format!(
 169                     "the trait cannot be made into an object because {}",
 170                     violation.error_msg()
 171                 ),
 172             );
 173         };
 174         err.span_note(
 175             spans,
 176             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 177              call to be resolvable dynamically; for more information visit \
 178              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 179         );
 180         if node.is_some() {
 181             // Only provide the help if its a local trait, otherwise it's not
 182             violation.solution(&mut err);
 183         }
 184         err.emit();
 185     });
 186 }
 187
 188 fn sized_trait_bound_spans<'tcx>(
 189     tcx: TyCtxt<'tcx>,
 190     bounds: hir::GenericBounds<'tcx>,
 191 ) -> impl 'tcx + Iterator<Item = Span> {
 192     bounds.iter().filter_map(move |b| match b {
 193         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 194             if trait_has_sized_self(
 195                 tcx,
 196                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 197             ) =>
 198         {
 199             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 200             Some(trait_ref.span)
 201         }
 202         _ => None,
 203     })
 204 }
 205
 206 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 207     tcx.hir()
 208         .get_if_local(trait_def_id)
 209         .and_then(|node| match node {
 210             hir::Node::Item(hir::Item {
 211                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 212                 ..
 213             }) => Some(
 214                 generics
 215                     .where_clause
 216                     .predicates
 217                     .iter()
 218                     .filter_map(|pred| {
 219                         match pred {
 220                             hir::WherePredicate::BoundPredicate(pred)
 221                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 222                             {
 223                                 // Fetch spans for trait bounds that are Sized:
 224                                 // `trait T where Self: Pred`
 225                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 226                             }
 227                             _ => None,
 228                         }
 229                     })
 230                     .flatten()
 231                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 232                     .chain(sized_trait_bound_spans(tcx, bounds))
 233                     .collect::<SmallVec<[Span; 1]>>(),
 234             ),
 235             _ => None,
 236         })
 237         .unwrap_or_else(SmallVec::new)
 238 }
 239
 240 fn predicates_reference_self(
 241     tcx: TyCtxt<'_>,
 242     trait_def_id: DefId,
 243     supertraits_only: bool,
 244 ) -> SmallVec<[Span; 1]> {
 245     let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
 246     let predicates = if supertraits_only {
 247         tcx.super_predicates_of(trait_def_id)
 248     } else {
 249         tcx.predicates_of(trait_def_id)
 250     };
 251     predicates
 252         .predicates
 253         .iter()
 254         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 255         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 256         .collect()
 257 }
 258
 259 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 260     tcx.associated_items(trait_def_id)
 261         .in_definition_order()
 262         .filter(|item| item.kind == ty::AssocKind::Type)
 263         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 264         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 265         .collect()
 266 }
 267
 268 fn predicate_references_self(
 269     tcx: TyCtxt<'tcx>,
 270     (predicate, sp): (ty::Predicate<'tcx>, Span),
 271 ) -> Option<Span> {
 272     let self_ty = tcx.types.self_param;
 273     let has_self_ty = |arg: &GenericArg<'_>| arg.walk().any(|arg| arg == self_ty.into());
 274     match predicate.kind().skip_binder() {
 275         ty::PredicateKind::Trait(ref data, _) => {
 276             // In the case of a trait predicate, we can skip the "self" type.
 277             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 278         }
 279         ty::PredicateKind::Projection(ref data) => {
 280             // And similarly for projections. This should be redundant with
 281             // the previous check because any projection should have a
 282             // matching `Trait` predicate with the same inputs, but we do
 283             // the check to be safe.
 284             //
 285             // It's also won't be redundant if we allow type-generic associated
 286             // types for trait objects.
 287             //
 288             // Note that we *do* allow projection *outputs* to contain
 289             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 290             // we just require the user to specify *both* outputs
 291             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 292             //
 293             // This is ALT2 in issue #56288, see that for discussion of the
 294             // possible alternatives.
 295             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 296         }
 297         ty::PredicateKind::WellFormed(..)
 298         | ty::PredicateKind::ObjectSafe(..)
 299         | ty::PredicateKind::TypeOutlives(..)
 300         | ty::PredicateKind::RegionOutlives(..)
 301         | ty::PredicateKind::ClosureKind(..)
 302         | ty::PredicateKind::Subtype(..)
 303         | ty::PredicateKind::ConstEvaluatable(..)
 304         | ty::PredicateKind::ConstEquate(..)
 305         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 306     }
 307 }
 308
 309 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 310     generics_require_sized_self(tcx, trait_def_id)
 311 }
 312
 313 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 314     let sized_def_id = match tcx.lang_items().sized_trait() {
 315         Some(def_id) => def_id,
 316         None => {
 317             return false; /* No Sized trait, can't require it! */
 318         }
 319     };
 320
 321     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 322     let predicates = tcx.predicates_of(def_id);
 323     let predicates = predicates.instantiate_identity(tcx).predicates;
 324     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 325         match obligation.predicate.kind().skip_binder() {
 326             ty::PredicateKind::Trait(ref trait_pred, _) => {
 327                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 328             }
 329             ty::PredicateKind::Projection(..)
 330             | ty::PredicateKind::Subtype(..)
 331             | ty::PredicateKind::RegionOutlives(..)
 332             | ty::PredicateKind::WellFormed(..)
 333             | ty::PredicateKind::ObjectSafe(..)
 334             | ty::PredicateKind::ClosureKind(..)
 335             | ty::PredicateKind::TypeOutlives(..)
 336             | ty::PredicateKind::ConstEvaluatable(..)
 337             | ty::PredicateKind::ConstEquate(..)
 338             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 339         }
 340     })
 341 }
 342
 343 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 344 fn object_safety_violation_for_method(
 345     tcx: TyCtxt<'_>,
 346     trait_def_id: DefId,
 347     method: &ty::AssocItem,
 348 ) -> Option<(MethodViolationCode, Span)> {
 349     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 350     // Any method that has a `Self : Sized` requisite is otherwise
 351     // exempt from the regulations.
 352     if generics_require_sized_self(tcx, method.def_id) {
 353         return None;
 354     }
 355
 356     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 357     // Get an accurate span depending on the violation.
 358     violation.map(|v| {
 359         let node = tcx.hir().get_if_local(method.def_id);
 360         let span = match (v, node) {
 361             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 362                 .fn_decl()
 363                 .and_then(|decl| decl.inputs.get(arg + 1))
 364                 .map_or(method.ident.span, |arg| arg.span),
 365             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 366                 .fn_decl()
 367                 .and_then(|decl| decl.inputs.get(0))
 368                 .map_or(method.ident.span, |arg| arg.span),
 369             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 370                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 371             }
 372             _ => method.ident.span,
 373         };
 374         (v, span)
 375     })
 376 }
 377
 378 /// Returns `Some(_)` if this method cannot be called on a trait
 379 /// object; this does not necessarily imply that the enclosing trait
 380 /// is not object safe, because the method might have a where clause
 381 /// `Self:Sized`.
 382 fn virtual_call_violation_for_method<'tcx>(
 383     tcx: TyCtxt<'tcx>,
 384     trait_def_id: DefId,
 385     method: &ty::AssocItem,
 386 ) -> Option<MethodViolationCode> {
 387     let sig = tcx.fn_sig(method.def_id);
 388
 389     // The method's first parameter must be named `self`
 390     if !method.fn_has_self_parameter {
 391         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 392         let sugg =
 393             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 394                 |generics| match generics.where_clause.predicates {
 395                     [] => (" where Self: Sized", generics.where_clause.span),
 396                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 397                 },
 398             );
 399         // Get the span pointing at where the `self` receiver should be.
 400         let sm = tcx.sess.source_map();
 401         let self_span = method.ident.span.to(tcx
 402             .hir()
 403             .span_if_local(method.def_id)
 404             .unwrap_or_else(|| sm.next_point(method.ident.span))
 405             .shrink_to_hi());
 406         let self_span = sm.span_through_char(self_span, '(').shrink_to_hi();
 407         return Some(MethodViolationCode::StaticMethod(
 408             sugg,
 409             self_span,
 410             !sig.inputs().skip_binder().is_empty(),
 411         ));
 412     }
 413
 414     for (i, &input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 415         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 416             return Some(MethodViolationCode::ReferencesSelfInput(i));
 417         }
 418     }
 419     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 420         return Some(MethodViolationCode::ReferencesSelfOutput);
 421     }
 422
 423     // We can't monomorphize things like `fn foo<A>(...)`.
 424     let own_counts = tcx.generics_of(method.def_id).own_counts();
 425     if own_counts.types + own_counts.consts != 0 {
 426         return Some(MethodViolationCode::Generic);
 427     }
 428
 429     if tcx
 430         .predicates_of(method.def_id)
 431         .predicates
 432         .iter()
 433         // A trait object can't claim to live more than the concrete type,
 434         // so outlives predicates will always hold.
 435         .cloned()
 436         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 437         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 438     {
 439         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 440     }
 441
 442     let receiver_ty =
 443         tcx.liberate_late_bound_regions(method.def_id, sig.map_bound(|sig| sig.inputs()[0]));
 444
 445     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 446     // However, this is already considered object-safe. We allow it as a special case here.
 447     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 448     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 449     if receiver_ty != tcx.types.self_param {
 450         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 451             return Some(MethodViolationCode::UndispatchableReceiver);
 452         } else {
 453             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 454
 455             use rustc_target::abi::Abi;
 456
 457             let param_env = tcx.param_env(method.def_id);
 458
 459             let abi_of_ty = |ty: Ty<'tcx>| -> Option<&Abi> {
 460                 match tcx.layout_of(param_env.and(ty)) {
 461                     Ok(layout) => Some(&layout.abi),
 462                     Err(err) => {
 463                         // #78372
 464                         tcx.sess.delay_span_bug(
 465                             tcx.def_span(method.def_id),
 466                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 467                         );
 468                         None
 469                     }
 470                 }
 471             };
 472
 473             // e.g., `Rc<()>`
 474             let unit_receiver_ty =
 475                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 476
 477             match abi_of_ty(unit_receiver_ty) {
 478                 Some(Abi::Scalar(..)) => (),
 479                 abi => {
 480                     tcx.sess.delay_span_bug(
 481                         tcx.def_span(method.def_id),
 482                         &format!(
 483                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 484                             abi
 485                         ),
 486                     );
 487                 }
 488             }
 489
 490             let trait_object_ty =
 491                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 492
 493             // e.g., `Rc<dyn Trait>`
 494             let trait_object_receiver =
 495                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 496
 497             match abi_of_ty(trait_object_receiver) {
 498                 Some(Abi::ScalarPair(..)) => (),
 499                 abi => {
 500                     tcx.sess.delay_span_bug(
 501                         tcx.def_span(method.def_id),
 502                         &format!(
 503                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 504                             trait_object_ty, abi
 505                         ),
 506                     );
 507                 }
 508             }
 509         }
 510     }
 511
 512     None
 513 }
 514
 515 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 516 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 517 fn receiver_for_self_ty<'tcx>(
 518     tcx: TyCtxt<'tcx>,
 519     receiver_ty: Ty<'tcx>,
 520     self_ty: Ty<'tcx>,
 521     method_def_id: DefId,
 522 ) -> Ty<'tcx> {
 523     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 524     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 525         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 526     });
 527
 528     let result = receiver_ty.subst(tcx, substs);
 529     debug!(
 530         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 531         receiver_ty, self_ty, method_def_id, result
 532     );
 533     result
 534 }
 535
 536 /// Creates the object type for the current trait. For example,
 537 /// if the current trait is `Deref`, then this will be
 538 /// `dyn Deref<Target = Self::Target> + 'static`.
 539 fn object_ty_for_trait<'tcx>(
 540     tcx: TyCtxt<'tcx>,
 541     trait_def_id: DefId,
 542     lifetime: ty::Region<'tcx>,
 543 ) -> Ty<'tcx> {
 544     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 545
 546     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 547
 548     let trait_predicate = ty::Binder::dummy(ty::ExistentialPredicate::Trait(
 549         ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref),
 550     ));
 551
 552     let mut associated_types = traits::supertraits(tcx, ty::Binder::dummy(trait_ref))
 553         .flat_map(|super_trait_ref| {
 554             tcx.associated_items(super_trait_ref.def_id())
 555                 .in_definition_order()
 556                 .map(move |item| (super_trait_ref, item))
 557         })
 558         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 559         .collect::<Vec<_>>();
 560
 561     // existential predicates need to be in a specific order
 562     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 563
 564     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 565         // We *can* get bound lifetimes here in cases like
 566         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 567         super_trait_ref.map_bound(|super_trait_ref| {
 568             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 569                 ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 570                 item_def_id: item.def_id,
 571                 substs: super_trait_ref.substs,
 572             })
 573         })
 574     });
 575
 576     let existential_predicates = tcx
 577         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 578
 579     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
 580
 581     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 582
 583     object_ty
 584 }
 585
 586 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 587 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 588 /// in the following way:
 589 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 590 /// - require the following bound:
 591 ///
 592 ///   ```
 593 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 594 ///   ```
 595 ///
 596 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 597 ///   (substitution notation).
 598 ///
 599 /// Some examples of receiver types and their required obligation:
 600 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 601 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 602 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 603 ///
 604 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 605 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 606 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 607 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 608 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 609 /// contained by the trait object, because the object that needs to be coerced is behind
 610 /// a pointer.
 611 ///
 612 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 613 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 614 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 615 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 616 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 617 /// Written as a chalk-style query:
 618 ///
 619 ///     forall (U: Trait + ?Sized) {
 620 ///         if (Self: Unsize<U>) {
 621 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 622 ///         }
 623 ///     }
 624 ///
 625 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 626 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 627 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 628 //
 629 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 630 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 631 // `self: Wrapper<Self>`.
 632 #[allow(dead_code)]
 633 fn receiver_is_dispatchable<'tcx>(
 634     tcx: TyCtxt<'tcx>,
 635     method: &ty::AssocItem,
 636     receiver_ty: Ty<'tcx>,
 637 ) -> bool {
 638     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 639
 640     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 641     let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
 642         (u, cu)
 643     } else {
 644         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 645         return false;
 646     };
 647
 648     // the type `U` in the query
 649     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 650     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 651     // replace this with `dyn Trait`
 652     let unsized_self_ty: Ty<'tcx> =
 653         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 654
 655     // `Receiver[Self => U]`
 656     let unsized_receiver_ty =
 657         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 658
 659     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 660     // `U: ?Sized` is already implied here
 661     let param_env = {
 662         let param_env = tcx.param_env(method.def_id);
 663
 664         // Self: Unsize<U>
 665         let unsize_predicate = ty::TraitRef {
 666             def_id: unsize_did,
 667             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 668         }
 669         .without_const()
 670         .to_predicate(tcx);
 671
 672         // U: Trait<Arg1, ..., ArgN>
 673         let trait_predicate = {
 674             let substs =
 675                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 676                     if param.index == 0 {
 677                         unsized_self_ty.into()
 678                     } else {
 679                         tcx.mk_param_from_def(param)
 680                     }
 681                 });
 682
 683             ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate(tcx)
 684         };
 685
 686         let caller_bounds: Vec<Predicate<'tcx>> = param_env
 687             .caller_bounds()
 688             .iter()
 689             .chain(array::IntoIter::new([unsize_predicate, trait_predicate]))
 690             .collect();
 691
 692         ty::ParamEnv::new(tcx.intern_predicates(&caller_bounds), param_env.reveal())
 693     };
 694
 695     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 696     let obligation = {
 697         let predicate = ty::TraitRef {
 698             def_id: dispatch_from_dyn_did,
 699             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 700         }
 701         .without_const()
 702         .to_predicate(tcx);
 703
 704         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 705     };
 706
 707     tcx.infer_ctxt().enter(|ref infcx| {
 708         // the receiver is dispatchable iff the obligation holds
 709         infcx.predicate_must_hold_modulo_regions(&obligation)
 710     })
 711 }
 712
 713 fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
 714     tcx: TyCtxt<'tcx>,
 715     trait_def_id: DefId,
 716     value: T,
 717 ) -> bool {
 718     // This is somewhat subtle. In general, we want to forbid
 719     // references to `Self` in the argument and return types,
 720     // since the value of `Self` is erased. However, there is one
 721     // exception: it is ok to reference `Self` in order to access
 722     // an associated type of the current trait, since we retain
 723     // the value of those associated types in the object type
 724     // itself.
 725     //
 726     // ```rust
 727     // trait SuperTrait {
 728     //     type X;
 729     // }
 730     //
 731     // trait Trait : SuperTrait {
 732     //     type Y;
 733     //     fn foo(&self, x: Self) // bad
 734     //     fn foo(&self) -> Self // bad
 735     //     fn foo(&self) -> Option<Self> // bad
 736     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 737     //     fn foo(&self) -> <Self as Trait>::Y // OK
 738     //     fn foo(&self) -> Self::X // OK, desugars to next example
 739     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 740     // }
 741     // ```
 742     //
 743     // However, it is not as simple as allowing `Self` in a projected
 744     // type, because there are illegal ways to use `Self` as well:
 745     //
 746     // ```rust
 747     // trait Trait : SuperTrait {
 748     //     ...
 749     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 750     // }
 751     // ```
 752     //
 753     // Here we will not have the type of `X` recorded in the
 754     // object type, and we cannot resolve `Self as SomeOtherTrait`
 755     // without knowing what `Self` is.
 756
 757     struct IllegalSelfTypeVisitor<'tcx> {
 758         tcx: TyCtxt<'tcx>,
 759         trait_def_id: DefId,
 760         supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>>,
 761     }
 762
 763     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 764         type BreakTy = ();
 765
 766         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 767             match t.kind() {
 768                 ty::Param(_) => {
 769                     if t == self.tcx.types.self_param {
 770                         ControlFlow::BREAK
 771                     } else {
 772                         ControlFlow::CONTINUE
 773                     }
 774                 }
 775                 ty::Projection(ref data) => {
 776                     // This is a projected type `<Foo as SomeTrait>::X`.
 777
 778                     // Compute supertraits of current trait lazily.
 779                     if self.supertraits.is_none() {
 780                         let trait_ref =
 781                             ty::Binder::bind(ty::TraitRef::identity(self.tcx, self.trait_def_id));
 782                         self.supertraits = Some(traits::supertraits(self.tcx, trait_ref).collect());
 783                     }
 784
 785                     // Determine whether the trait reference `Foo as
 786                     // SomeTrait` is in fact a supertrait of the
 787                     // current trait. In that case, this type is
 788                     // legal, because the type `X` will be specified
 789                     // in the object type.  Note that we can just use
 790                     // direct equality here because all of these types
 791                     // are part of the formal parameter listing, and
 792                     // hence there should be no inference variables.
 793                     let projection_trait_ref = ty::Binder::bind(data.trait_ref(self.tcx));
 794                     let is_supertrait_of_current_trait =
 795                         self.supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 796
 797                     if is_supertrait_of_current_trait {
 798                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 799                     } else {
 800                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 801                     }
 802                 }
 803                 _ => t.super_visit_with(self), // walk contained types, if any
 804             }
 805         }
 806
 807         fn visit_const(&mut self, ct: &ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 808             // First check if the type of this constant references `Self`.
 809             self.visit_ty(ct.ty)?;
 810
 811             // Constants can only influence object safety if they reference `Self`.
 812             // This is only possible for unevaluated constants, so we walk these here.
 813             //
 814             // If `AbstractConst::new` returned an error we already failed compilation
 815             // so we don't have to emit an additional error here.
 816             //
 817             // We currently recurse into abstract consts here but do not recurse in
 818             // `is_const_evaluatable`. This means that the object safety check is more
 819             // liberal than the const eval check.
 820             //
 821             // This shouldn't really matter though as we can't really use any
 822             // constants which are not considered const evaluatable.
 823             use rustc_middle::mir::abstract_const::Node;
 824             if let Ok(Some(ct)) = AbstractConst::from_const(self.tcx, ct) {
 825                 const_evaluatable::walk_abstract_const(self.tcx, ct, |node| match node.root() {
 826                     Node::Leaf(leaf) => {
 827                         let leaf = leaf.subst(self.tcx, ct.substs);
 828                         self.visit_const(leaf)
 829                     }
 830                     Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 831                         ControlFlow::CONTINUE
 832                     }
 833                 })
 834             } else {
 835                 ControlFlow::CONTINUE
 836             }
 837         }
 838
 839         fn visit_predicate(&mut self, pred: ty::Predicate<'tcx>) -> ControlFlow<Self::BreakTy> {
 840             if let ty::PredicateKind::ConstEvaluatable(def, substs) = pred.kind().skip_binder() {
 841                 // FIXME(const_evaluatable_checked): We should probably deduplicate the logic for
 842                 // `AbstractConst`s here, it might make sense to change `ConstEvaluatable` to
 843                 // take a `ty::Const` instead.
 844                 use rustc_middle::mir::abstract_const::Node;
 845                 if let Ok(Some(ct)) = AbstractConst::new(self.tcx, def, substs) {
 846                     const_evaluatable::walk_abstract_const(self.tcx, ct, |node| match node.root() {
 847                         Node::Leaf(leaf) => {
 848                             let leaf = leaf.subst(self.tcx, ct.substs);
 849                             self.visit_const(leaf)
 850                         }
 851                         Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 852                             ControlFlow::CONTINUE
 853                         }
 854                     })
 855                 } else {
 856                     ControlFlow::CONTINUE
 857                 }
 858             } else {
 859                 pred.super_visit_with(self)
 860             }
 861         }
 862     }
 863
 864     value
 865         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 866         .is_break()
 867 }
 868
 869 pub fn provide(providers: &mut ty::query::Providers) {
 870     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 871 }