src/librustc_trait_selection/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::query::evaluate_obligation::InferCtxtExt;
  15 use crate::traits::{self, Obligation, ObligationCause};
  16 use rustc_errors::{Applicability, FatalError};
  17 use rustc_hir as hir;
  18 use rustc_hir::def_id::DefId;
  19 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
  20 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor, WithConstness};
  21 use rustc_middle::ty::{Predicate, ToPredicate};
  22 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
  23 use rustc_span::symbol::Symbol;
  24 use rustc_span::Span;
  25 use smallvec::SmallVec;
  26
  27 use std::iter;
  28
  29 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
  30
  31 /// Returns the object safety violations that affect
  32 /// astconv -- currently, `Self` in supertraits. This is needed
  33 /// because `object_safety_violations` can't be used during
  34 /// type collection.
  35 pub fn astconv_object_safety_violations(
  36     tcx: TyCtxt<'_>,
  37     trait_def_id: DefId,
  38 ) -> Vec<ObjectSafetyViolation> {
  39     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  40     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
  41         .map(|def_id| predicates_reference_self(tcx, def_id, true))
  42         .filter(|spans| !spans.is_empty())
  43         .map(ObjectSafetyViolation::SupertraitSelf)
  44         .collect();
  45
  46     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
  47
  48     violations
  49 }
  50
  51 fn object_safety_violations(
  52     tcx: TyCtxt<'tcx>,
  53     trait_def_id: DefId,
  54 ) -> &'tcx [ObjectSafetyViolation] {
  55     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  56     debug!("object_safety_violations: {:?}", trait_def_id);
  57
  58     tcx.arena.alloc_from_iter(
  59         traits::supertrait_def_ids(tcx, trait_def_id)
  60             .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
  61     )
  62 }
  63
  64 /// We say a method is *vtable safe* if it can be invoked on a trait
  65 /// object. Note that object-safe traits can have some
  66 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  67 /// otherwise ensure that they cannot be used when `Self = Trait`.
  68 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
  69     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  70     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
  71     // Any method that has a `Self: Sized` bound cannot be called.
  72     if generics_require_sized_self(tcx, method.def_id) {
  73         return false;
  74     }
  75
  76     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
  77         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
  78         Some(_) => false,
  79     }
  80 }
  81
  82 fn object_safety_violations_for_trait(
  83     tcx: TyCtxt<'_>,
  84     trait_def_id: DefId,
  85 ) -> Vec<ObjectSafetyViolation> {
  86     // Check methods for violations.
  87     let mut violations: Vec<_> = tcx
  88         .associated_items(trait_def_id)
  89         .in_definition_order()
  90         .filter(|item| item.kind == ty::AssocKind::Fn)
  91         .filter_map(|item| {
  92             object_safety_violation_for_method(tcx, trait_def_id, &item)
  93                 .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span))
  94         })
  95         .filter(|violation| {
  96             if let ObjectSafetyViolation::Method(
  97                 _,
  98                 MethodViolationCode::WhereClauseReferencesSelf,
  99                 span,
 100             ) = violation
 101             {
 102                 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 103                 // It's also hard to get a use site span, so we use the method definition span.
 104                 tcx.struct_span_lint_hir(
 105                     WHERE_CLAUSES_OBJECT_SAFETY,
 106                     hir::CRATE_HIR_ID,
 107                     *span,
 108                     |lint| {
 109                         let mut err = lint.build(&format!(
 110                             "the trait `{}` cannot be made into an object",
 111                             tcx.def_path_str(trait_def_id)
 112                         ));
 113                         let node = tcx.hir().get_if_local(trait_def_id);
 114                         let msg = if let Some(hir::Node::Item(item)) = node {
 115                             err.span_label(
 116                                 item.ident.span,
 117                                 "this trait cannot be made into an object...",
 118                             );
 119                             format!("...because {}", violation.error_msg())
 120                         } else {
 121                             format!(
 122                                 "the trait cannot be made into an object because {}",
 123                                 violation.error_msg()
 124                             )
 125                         };
 126                         err.span_label(*span, &msg);
 127                         match (node, violation.solution()) {
 128                             (Some(_), Some((note, None))) => {
 129                                 err.help(&note);
 130                             }
 131                             (Some(_), Some((note, Some((sugg, span))))) => {
 132                                 err.span_suggestion(
 133                                     span,
 134                                     &note,
 135                                     sugg,
 136                                     Applicability::MachineApplicable,
 137                                 );
 138                             }
 139                             // Only provide the help if its a local trait, otherwise it's not actionable.
 140                             _ => {}
 141                         }
 142                         err.emit();
 143                     },
 144                 );
 145                 false
 146             } else {
 147                 true
 148             }
 149         })
 150         .collect();
 151
 152     // Check the trait itself.
 153     if trait_has_sized_self(tcx, trait_def_id) {
 154         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 155         let spans = get_sized_bounds(tcx, trait_def_id);
 156         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 157     }
 158     let spans = predicates_reference_self(tcx, trait_def_id, false);
 159     if !spans.is_empty() {
 160         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 161     }
 162
 163     violations.extend(
 164         tcx.associated_items(trait_def_id)
 165             .in_definition_order()
 166             .filter(|item| item.kind == ty::AssocKind::Const)
 167             .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 168     );
 169
 170     debug!(
 171         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 172         trait_def_id, violations
 173     );
 174
 175     violations
 176 }
 177
 178 fn sized_trait_bound_spans<'tcx>(
 179     tcx: TyCtxt<'tcx>,
 180     bounds: hir::GenericBounds<'tcx>,
 181 ) -> impl 'tcx + Iterator<Item = Span> {
 182     bounds.iter().filter_map(move |b| match b {
 183         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 184             if trait_has_sized_self(
 185                 tcx,
 186                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 187             ) =>
 188         {
 189             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 190             Some(trait_ref.span)
 191         }
 192         _ => None,
 193     })
 194 }
 195
 196 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 197     tcx.hir()
 198         .get_if_local(trait_def_id)
 199         .and_then(|node| match node {
 200             hir::Node::Item(hir::Item {
 201                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 202                 ..
 203             }) => Some(
 204                 generics
 205                     .where_clause
 206                     .predicates
 207                     .iter()
 208                     .filter_map(|pred| {
 209                         match pred {
 210                             hir::WherePredicate::BoundPredicate(pred)
 211                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 212                             {
 213                                 // Fetch spans for trait bounds that are Sized:
 214                                 // `trait T where Self: Pred`
 215                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 216                             }
 217                             _ => None,
 218                         }
 219                     })
 220                     .flatten()
 221                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 222                     .chain(sized_trait_bound_spans(tcx, bounds))
 223                     .collect::<SmallVec<[Span; 1]>>(),
 224             ),
 225             _ => None,
 226         })
 227         .unwrap_or_else(SmallVec::new)
 228 }
 229
 230 fn predicates_reference_self(
 231     tcx: TyCtxt<'_>,
 232     trait_def_id: DefId,
 233     supertraits_only: bool,
 234 ) -> SmallVec<[Span; 1]> {
 235     let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
 236     let predicates = if supertraits_only {
 237         tcx.super_predicates_of(trait_def_id)
 238     } else {
 239         tcx.predicates_of(trait_def_id)
 240     };
 241     let self_ty = tcx.types.self_param;
 242     let has_self_ty = |arg: &GenericArg<'_>| arg.walk().any(|arg| arg == self_ty.into());
 243     predicates
 244         .predicates
 245         .iter()
 246         .map(|(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 247         .filter_map(|(predicate, &sp)| {
 248             match predicate.skip_binders() {
 249                 ty::PredicateAtom::Trait(ref data, _) => {
 250                     // In the case of a trait predicate, we can skip the "self" type.
 251                     if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 252                 }
 253                 ty::PredicateAtom::Projection(ref data) => {
 254                     // And similarly for projections. This should be redundant with
 255                     // the previous check because any projection should have a
 256                     // matching `Trait` predicate with the same inputs, but we do
 257                     // the check to be safe.
 258                     //
 259                     // Note that we *do* allow projection *outputs* to contain
 260                     // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 261                     // we just require the user to specify *both* outputs
 262                     // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 263                     //
 264                     // This is ALT2 in issue #56288, see that for discussion of the
 265                     // possible alternatives.
 266                     if data.projection_ty.trait_ref(tcx).substs[1..].iter().any(has_self_ty) {
 267                         Some(sp)
 268                     } else {
 269                         None
 270                     }
 271                 }
 272                 ty::PredicateAtom::WellFormed(..)
 273                 | ty::PredicateAtom::ObjectSafe(..)
 274                 | ty::PredicateAtom::TypeOutlives(..)
 275                 | ty::PredicateAtom::RegionOutlives(..)
 276                 | ty::PredicateAtom::ClosureKind(..)
 277                 | ty::PredicateAtom::Subtype(..)
 278                 | ty::PredicateAtom::ConstEvaluatable(..)
 279                 | ty::PredicateAtom::ConstEquate(..) => None,
 280             }
 281         })
 282         .collect()
 283 }
 284
 285 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 286     generics_require_sized_self(tcx, trait_def_id)
 287 }
 288
 289 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 290     let sized_def_id = match tcx.lang_items().sized_trait() {
 291         Some(def_id) => def_id,
 292         None => {
 293             return false; /* No Sized trait, can't require it! */
 294         }
 295     };
 296
 297     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 298     let predicates = tcx.predicates_of(def_id);
 299     let predicates = predicates.instantiate_identity(tcx).predicates;
 300     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 301         match obligation.predicate.skip_binders() {
 302             ty::PredicateAtom::Trait(ref trait_pred, _) => {
 303                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 304             }
 305             ty::PredicateAtom::Projection(..)
 306             | ty::PredicateAtom::Subtype(..)
 307             | ty::PredicateAtom::RegionOutlives(..)
 308             | ty::PredicateAtom::WellFormed(..)
 309             | ty::PredicateAtom::ObjectSafe(..)
 310             | ty::PredicateAtom::ClosureKind(..)
 311             | ty::PredicateAtom::TypeOutlives(..)
 312             | ty::PredicateAtom::ConstEvaluatable(..)
 313             | ty::PredicateAtom::ConstEquate(..) => false,
 314         }
 315     })
 316 }
 317
 318 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 319 fn object_safety_violation_for_method(
 320     tcx: TyCtxt<'_>,
 321     trait_def_id: DefId,
 322     method: &ty::AssocItem,
 323 ) -> Option<(MethodViolationCode, Span)> {
 324     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 325     // Any method that has a `Self : Sized` requisite is otherwise
 326     // exempt from the regulations.
 327     if generics_require_sized_self(tcx, method.def_id) {
 328         return None;
 329     }
 330
 331     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 332     // Get an accurate span depending on the violation.
 333     violation.map(|v| {
 334         let node = tcx.hir().get_if_local(method.def_id);
 335         let span = match (v, node) {
 336             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 337                 .fn_decl()
 338                 .and_then(|decl| decl.inputs.get(arg + 1))
 339                 .map_or(method.ident.span, |arg| arg.span),
 340             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 341                 .fn_decl()
 342                 .and_then(|decl| decl.inputs.get(0))
 343                 .map_or(method.ident.span, |arg| arg.span),
 344             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 345                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 346             }
 347             _ => method.ident.span,
 348         };
 349         (v, span)
 350     })
 351 }
 352
 353 /// Returns `Some(_)` if this method cannot be called on a trait
 354 /// object; this does not necessarily imply that the enclosing trait
 355 /// is not object safe, because the method might have a where clause
 356 /// `Self:Sized`.
 357 fn virtual_call_violation_for_method<'tcx>(
 358     tcx: TyCtxt<'tcx>,
 359     trait_def_id: DefId,
 360     method: &ty::AssocItem,
 361 ) -> Option<MethodViolationCode> {
 362     // The method's first parameter must be named `self`
 363     if !method.fn_has_self_parameter {
 364         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 365         let sugg =
 366             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 367                 |generics| match generics.where_clause.predicates {
 368                     [] => (" where Self: Sized", generics.where_clause.span),
 369                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 370                 },
 371             );
 372         return Some(MethodViolationCode::StaticMethod(sugg));
 373     }
 374
 375     let sig = tcx.fn_sig(method.def_id);
 376
 377     for (i, input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 378         if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) {
 379             return Some(MethodViolationCode::ReferencesSelfInput(i));
 380         }
 381     }
 382     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output().skip_binder()) {
 383         return Some(MethodViolationCode::ReferencesSelfOutput);
 384     }
 385
 386     // We can't monomorphize things like `fn foo<A>(...)`.
 387     let own_counts = tcx.generics_of(method.def_id).own_counts();
 388     if own_counts.types + own_counts.consts != 0 {
 389         return Some(MethodViolationCode::Generic);
 390     }
 391
 392     if tcx
 393         .predicates_of(method.def_id)
 394         .predicates
 395         .iter()
 396         // A trait object can't claim to live more than the concrete type,
 397         // so outlives predicates will always hold.
 398         .cloned()
 399         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 400         .collect::<Vec<_>>()
 401         // Do a shallow visit so that `contains_illegal_self_type_reference`
 402         // may apply it's custom visiting.
 403         .visit_tys_shallow(|t| contains_illegal_self_type_reference(tcx, trait_def_id, t))
 404     {
 405         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 406     }
 407
 408     let receiver_ty =
 409         tcx.liberate_late_bound_regions(method.def_id, &sig.map_bound(|sig| sig.inputs()[0]));
 410
 411     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 412     // However, this is already considered object-safe. We allow it as a special case here.
 413     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 414     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 415     if receiver_ty != tcx.types.self_param {
 416         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 417             return Some(MethodViolationCode::UndispatchableReceiver);
 418         } else {
 419             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 420
 421             use rustc_target::abi::Abi;
 422
 423             let param_env = tcx.param_env(method.def_id);
 424
 425             let abi_of_ty = |ty: Ty<'tcx>| -> &Abi {
 426                 match tcx.layout_of(param_env.and(ty)) {
 427                     Ok(layout) => &layout.abi,
 428                     Err(err) => bug!("error: {}\n while computing layout for type {:?}", err, ty),
 429                 }
 430             };
 431
 432             // e.g., `Rc<()>`
 433             let unit_receiver_ty =
 434                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 435
 436             match abi_of_ty(unit_receiver_ty) {
 437                 &Abi::Scalar(..) => (),
 438                 abi => {
 439                     tcx.sess.delay_span_bug(
 440                         tcx.def_span(method.def_id),
 441                         &format!(
 442                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 443                             abi
 444                         ),
 445                     );
 446                 }
 447             }
 448
 449             let trait_object_ty =
 450                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 451
 452             // e.g., `Rc<dyn Trait>`
 453             let trait_object_receiver =
 454                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 455
 456             match abi_of_ty(trait_object_receiver) {
 457                 &Abi::ScalarPair(..) => (),
 458                 abi => {
 459                     tcx.sess.delay_span_bug(
 460                         tcx.def_span(method.def_id),
 461                         &format!(
 462                             "receiver when `Self = {}` should have a ScalarPair ABI; \
 463                                  found {:?}",
 464                             trait_object_ty, abi
 465                         ),
 466                     );
 467                 }
 468             }
 469         }
 470     }
 471
 472     None
 473 }
 474
 475 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 476 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 477 fn receiver_for_self_ty<'tcx>(
 478     tcx: TyCtxt<'tcx>,
 479     receiver_ty: Ty<'tcx>,
 480     self_ty: Ty<'tcx>,
 481     method_def_id: DefId,
 482 ) -> Ty<'tcx> {
 483     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 484     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 485         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 486     });
 487
 488     let result = receiver_ty.subst(tcx, substs);
 489     debug!(
 490         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 491         receiver_ty, self_ty, method_def_id, result
 492     );
 493     result
 494 }
 495
 496 /// Creates the object type for the current trait. For example,
 497 /// if the current trait is `Deref`, then this will be
 498 /// `dyn Deref<Target = Self::Target> + 'static`.
 499 fn object_ty_for_trait<'tcx>(
 500     tcx: TyCtxt<'tcx>,
 501     trait_def_id: DefId,
 502     lifetime: ty::Region<'tcx>,
 503 ) -> Ty<'tcx> {
 504     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 505
 506     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 507
 508     let trait_predicate =
 509         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 510
 511     let mut associated_types = traits::supertraits(tcx, ty::Binder::dummy(trait_ref))
 512         .flat_map(|super_trait_ref| {
 513             tcx.associated_items(super_trait_ref.def_id())
 514                 .in_definition_order()
 515                 .map(move |item| (super_trait_ref, item))
 516         })
 517         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 518         .collect::<Vec<_>>();
 519
 520     // existential predicates need to be in a specific order
 521     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 522
 523     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 524         // We *can* get bound lifetimes here in cases like
 525         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 526         //
 527         // binder moved to (*)...
 528         let super_trait_ref = super_trait_ref.skip_binder();
 529         ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 530             ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 531             item_def_id: item.def_id,
 532             substs: super_trait_ref.substs,
 533         })
 534     });
 535
 536     let existential_predicates =
 537         tcx.mk_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 538
 539     let object_ty = tcx.mk_dynamic(
 540         // (*) ... binder re-introduced here
 541         ty::Binder::bind(existential_predicates),
 542         lifetime,
 543     );
 544
 545     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 546
 547     object_ty
 548 }
 549
 550 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 551 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 552 /// in the following way:
 553 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 554 /// - require the following bound:
 555 ///
 556 ///   ```
 557 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 558 ///   ```
 559 ///
 560 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 561 ///   (substitution notation).
 562 ///
 563 /// Some examples of receiver types and their required obligation:
 564 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 565 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 566 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 567 ///
 568 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 569 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 570 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 571 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 572 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 573 /// contained by the trait object, because the object that needs to be coerced is behind
 574 /// a pointer.
 575 ///
 576 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 577 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 578 /// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
 579 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 580 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 581 /// Written as a chalk-style query:
 582 ///
 583 ///     forall (U: Trait + ?Sized) {
 584 ///         if (Self: Unsize<U>) {
 585 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 586 ///         }
 587 ///     }
 588 ///
 589 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 590 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 591 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 592 //
 593 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 594 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 595 // `self: Wrapper<Self>`.
 596 #[allow(dead_code)]
 597 fn receiver_is_dispatchable<'tcx>(
 598     tcx: TyCtxt<'tcx>,
 599     method: &ty::AssocItem,
 600     receiver_ty: Ty<'tcx>,
 601 ) -> bool {
 602     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 603
 604     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 605     let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
 606         (u, cu)
 607     } else {
 608         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 609         return false;
 610     };
 611
 612     // the type `U` in the query
 613     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 614     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 615     // replace this with `dyn Trait`
 616     let unsized_self_ty: Ty<'tcx> =
 617         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 618
 619     // `Receiver[Self => U]`
 620     let unsized_receiver_ty =
 621         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 622
 623     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 624     // `U: ?Sized` is already implied here
 625     let param_env = {
 626         let param_env = tcx.param_env(method.def_id);
 627
 628         // Self: Unsize<U>
 629         let unsize_predicate = ty::TraitRef {
 630             def_id: unsize_did,
 631             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 632         }
 633         .without_const()
 634         .to_predicate(tcx);
 635
 636         // U: Trait<Arg1, ..., ArgN>
 637         let trait_predicate = {
 638             let substs =
 639                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 640                     if param.index == 0 {
 641                         unsized_self_ty.into()
 642                     } else {
 643                         tcx.mk_param_from_def(param)
 644                     }
 645                 });
 646
 647             ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate(tcx)
 648         };
 649
 650         let caller_bounds: Vec<Predicate<'tcx>> = param_env
 651             .caller_bounds()
 652             .iter()
 653             .chain(iter::once(unsize_predicate))
 654             .chain(iter::once(trait_predicate))
 655             .collect();
 656
 657         ty::ParamEnv::new(
 658             tcx.intern_predicates(&caller_bounds),
 659             param_env.reveal(),
 660             param_env.def_id,
 661         )
 662     };
 663
 664     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 665     let obligation = {
 666         let predicate = ty::TraitRef {
 667             def_id: dispatch_from_dyn_did,
 668             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 669         }
 670         .without_const()
 671         .to_predicate(tcx);
 672
 673         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 674     };
 675
 676     tcx.infer_ctxt().enter(|ref infcx| {
 677         // the receiver is dispatchable iff the obligation holds
 678         infcx.predicate_must_hold_modulo_regions(&obligation)
 679     })
 680 }
 681
 682 fn contains_illegal_self_type_reference<'tcx>(
 683     tcx: TyCtxt<'tcx>,
 684     trait_def_id: DefId,
 685     ty: Ty<'tcx>,
 686 ) -> bool {
 687     // This is somewhat subtle. In general, we want to forbid
 688     // references to `Self` in the argument and return types,
 689     // since the value of `Self` is erased. However, there is one
 690     // exception: it is ok to reference `Self` in order to access
 691     // an associated type of the current trait, since we retain
 692     // the value of those associated types in the object type
 693     // itself.
 694     //
 695     // ```rust
 696     // trait SuperTrait {
 697     //     type X;
 698     // }
 699     //
 700     // trait Trait : SuperTrait {
 701     //     type Y;
 702     //     fn foo(&self, x: Self) // bad
 703     //     fn foo(&self) -> Self // bad
 704     //     fn foo(&self) -> Option<Self> // bad
 705     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 706     //     fn foo(&self) -> <Self as Trait>::Y // OK
 707     //     fn foo(&self) -> Self::X // OK, desugars to next example
 708     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 709     // }
 710     // ```
 711     //
 712     // However, it is not as simple as allowing `Self` in a projected
 713     // type, because there are illegal ways to use `Self` as well:
 714     //
 715     // ```rust
 716     // trait Trait : SuperTrait {
 717     //     ...
 718     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 719     // }
 720     // ```
 721     //
 722     // Here we will not have the type of `X` recorded in the
 723     // object type, and we cannot resolve `Self as SomeOtherTrait`
 724     // without knowing what `Self` is.
 725
 726     struct IllegalSelfTypeVisitor<'tcx> {
 727         tcx: TyCtxt<'tcx>,
 728         self_ty: Ty<'tcx>,
 729         trait_def_id: DefId,
 730         supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>>,
 731     }
 732
 733     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 734         fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 735             match t.kind {
 736                 ty::Param(_) => t == self.self_ty,
 737                 ty::Projection(ref data) => {
 738                     // This is a projected type `<Foo as SomeTrait>::X`.
 739
 740                     // Compute supertraits of current trait lazily.
 741                     if self.supertraits.is_none() {
 742                         let trait_ref =
 743                             ty::Binder::bind(ty::TraitRef::identity(self.tcx, self.trait_def_id));
 744                         self.supertraits = Some(traits::supertraits(self.tcx, trait_ref).collect());
 745                     }
 746
 747                     // Determine whether the trait reference `Foo as
 748                     // SomeTrait` is in fact a supertrait of the
 749                     // current trait. In that case, this type is
 750                     // legal, because the type `X` will be specified
 751                     // in the object type.  Note that we can just use
 752                     // direct equality here because all of these types
 753                     // are part of the formal parameter listing, and
 754                     // hence there should be no inference variables.
 755                     let projection_trait_ref = ty::Binder::bind(data.trait_ref(self.tcx));
 756                     let is_supertrait_of_current_trait =
 757                         self.supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 758
 759                     if is_supertrait_of_current_trait {
 760                         false // do not walk contained types, do not report error, do collect $200
 761                     } else {
 762                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 763                     }
 764                 }
 765                 _ => t.super_visit_with(self), // walk contained types, if any
 766             }
 767         }
 768
 769         fn visit_const(&mut self, _c: &ty::Const<'tcx>) -> bool {
 770             // FIXME(#72219) Look into the unevaluated constants for object safety violations.
 771             // Do not walk substitutions of unevaluated consts, as they contain `Self`, even
 772             // though the const expression doesn't necessary use it. Currently type variables
 773             // inside array length expressions are forbidden, so they can't break the above
 774             // rules.
 775             false
 776         }
 777     }
 778
 779     ty.visit_with(&mut IllegalSelfTypeVisitor {
 780         tcx,
 781         self_ty: tcx.types.self_param,
 782         trait_def_id,
 783         supertraits: None,
 784     })
 785 }
 786
 787 pub fn provide(providers: &mut ty::query::Providers) {
 788     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 789 }