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::query::evaluate_obligation::InferCtxtExt;
  15 use crate::traits::{self, Obligation, ObligationCause};
  16 use hir::def::DefKind;
  17 use rustc_errors::{FatalError, MultiSpan};
  18 use rustc_hir as hir;
  19 use rustc_hir::def_id::DefId;
  20 use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
  21 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
  22 use rustc_middle::ty::{
  23     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
  24 };
  25 use rustc_middle::ty::{Predicate, ToPredicate};
  26 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
  27 use rustc_span::symbol::Symbol;
  28 use rustc_span::Span;
  29 use smallvec::SmallVec;
  30
  31 use std::iter;
  32 use std::ops::ControlFlow;
  33
  34 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
  35
  36 /// Returns the object safety violations that affect
  37 /// astconv -- currently, `Self` in supertraits. This is needed
  38 /// because `object_safety_violations` can't be used during
  39 /// type collection.
  40 pub fn astconv_object_safety_violations(
  41     tcx: TyCtxt<'_>,
  42     trait_def_id: DefId,
  43 ) -> Vec<ObjectSafetyViolation> {
  44     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  45     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
  46         .map(|def_id| predicates_reference_self(tcx, def_id, true))
  47         .filter(|spans| !spans.is_empty())
  48         .map(ObjectSafetyViolation::SupertraitSelf)
  49         .collect();
  50
  51     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
  52
  53     violations
  54 }
  55
  56 fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [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.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| {
 132                 let ident = item.ident(tcx);
 133                 ObjectSafetyViolation::AssocConst(ident.name, ident.span)
 134             }),
 135     );
 136
 137     if !tcx.features().generic_associated_types_extended {
 138         violations.extend(
 139             tcx.associated_items(trait_def_id)
 140                 .in_definition_order()
 141                 .filter(|item| item.kind == ty::AssocKind::Type)
 142                 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
 143                 .map(|item| {
 144                     let ident = item.ident(tcx);
 145                     ObjectSafetyViolation::GAT(ident.name, ident.span)
 146                 }),
 147         );
 148     }
 149
 150     debug!(
 151         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 152         trait_def_id, violations
 153     );
 154
 155     violations
 156 }
 157
 158 /// Lint object-unsafe trait.
 159 fn lint_object_unsafe_trait(
 160     tcx: TyCtxt<'_>,
 161     span: Span,
 162     trait_def_id: DefId,
 163     violation: &ObjectSafetyViolation,
 164 ) {
 165     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 166     // It's also hard to get a use site span, so we use the method definition span.
 167     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 168         let mut err = lint.build(&format!(
 169             "the trait `{}` cannot be made into an object",
 170             tcx.def_path_str(trait_def_id)
 171         ));
 172         let node = tcx.hir().get_if_local(trait_def_id);
 173         let mut spans = MultiSpan::from_span(span);
 174         if let Some(hir::Node::Item(item)) = node {
 175             spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
 176             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 177         } else {
 178             spans.push_span_label(
 179                 span,
 180                 format!(
 181                     "the trait cannot be made into an object because {}",
 182                     violation.error_msg()
 183                 ),
 184             );
 185         };
 186         err.span_note(
 187             spans,
 188             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 189              call to be resolvable dynamically; for more information visit \
 190              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 191         );
 192         if node.is_some() {
 193             // Only provide the help if its a local trait, otherwise it's not
 194             violation.solution(&mut err);
 195         }
 196         err.emit();
 197     });
 198 }
 199
 200 fn sized_trait_bound_spans<'tcx>(
 201     tcx: TyCtxt<'tcx>,
 202     bounds: hir::GenericBounds<'tcx>,
 203 ) -> impl 'tcx + Iterator<Item = Span> {
 204     bounds.iter().filter_map(move |b| match b {
 205         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 206             if trait_has_sized_self(
 207                 tcx,
 208                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 209             ) =>
 210         {
 211             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 212             Some(trait_ref.span)
 213         }
 214         _ => None,
 215     })
 216 }
 217
 218 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 219     tcx.hir()
 220         .get_if_local(trait_def_id)
 221         .and_then(|node| match node {
 222             hir::Node::Item(hir::Item {
 223                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 224                 ..
 225             }) => Some(
 226                 generics
 227                     .predicates
 228                     .iter()
 229                     .filter_map(|pred| {
 230                         match pred {
 231                             hir::WherePredicate::BoundPredicate(pred)
 232                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 233                             {
 234                                 // Fetch spans for trait bounds that are Sized:
 235                                 // `trait T where Self: Pred`
 236                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 237                             }
 238                             _ => None,
 239                         }
 240                     })
 241                     .flatten()
 242                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 243                     .chain(sized_trait_bound_spans(tcx, bounds))
 244                     .collect::<SmallVec<[Span; 1]>>(),
 245             ),
 246             _ => None,
 247         })
 248         .unwrap_or_else(SmallVec::new)
 249 }
 250
 251 fn predicates_reference_self(
 252     tcx: TyCtxt<'_>,
 253     trait_def_id: DefId,
 254     supertraits_only: bool,
 255 ) -> SmallVec<[Span; 1]> {
 256     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 257     let predicates = if supertraits_only {
 258         tcx.super_predicates_of(trait_def_id)
 259     } else {
 260         tcx.predicates_of(trait_def_id)
 261     };
 262     predicates
 263         .predicates
 264         .iter()
 265         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 266         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 267         .collect()
 268 }
 269
 270 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 271     tcx.associated_items(trait_def_id)
 272         .in_definition_order()
 273         .filter(|item| item.kind == ty::AssocKind::Type)
 274         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 275         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 276         .collect()
 277 }
 278
 279 fn predicate_references_self<'tcx>(
 280     tcx: TyCtxt<'tcx>,
 281     (predicate, sp): (ty::Predicate<'tcx>, Span),
 282 ) -> Option<Span> {
 283     let self_ty = tcx.types.self_param;
 284     let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
 285     match predicate.kind().skip_binder() {
 286         ty::PredicateKind::Trait(ref data) => {
 287             // In the case of a trait predicate, we can skip the "self" type.
 288             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 289         }
 290         ty::PredicateKind::Projection(ref data) => {
 291             // And similarly for projections. This should be redundant with
 292             // the previous check because any projection should have a
 293             // matching `Trait` predicate with the same inputs, but we do
 294             // the check to be safe.
 295             //
 296             // It's also won't be redundant if we allow type-generic associated
 297             // types for trait objects.
 298             //
 299             // Note that we *do* allow projection *outputs* to contain
 300             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 301             // we just require the user to specify *both* outputs
 302             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 303             //
 304             // This is ALT2 in issue #56288, see that for discussion of the
 305             // possible alternatives.
 306             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 307         }
 308         ty::PredicateKind::WellFormed(..)
 309         | ty::PredicateKind::ObjectSafe(..)
 310         | ty::PredicateKind::TypeOutlives(..)
 311         | ty::PredicateKind::RegionOutlives(..)
 312         | ty::PredicateKind::ClosureKind(..)
 313         | ty::PredicateKind::Subtype(..)
 314         | ty::PredicateKind::Coerce(..)
 315         | ty::PredicateKind::ConstEvaluatable(..)
 316         | ty::PredicateKind::ConstEquate(..)
 317         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 318     }
 319 }
 320
 321 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 322     generics_require_sized_self(tcx, trait_def_id)
 323 }
 324
 325 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 326     let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
 327         return false; /* No Sized trait, can't require it! */
 328     };
 329
 330     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 331     let predicates = tcx.predicates_of(def_id);
 332     let predicates = predicates.instantiate_identity(tcx).predicates;
 333     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 334         match obligation.predicate.kind().skip_binder() {
 335             ty::PredicateKind::Trait(ref trait_pred) => {
 336                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 337             }
 338             ty::PredicateKind::Projection(..)
 339             | ty::PredicateKind::Subtype(..)
 340             | ty::PredicateKind::Coerce(..)
 341             | ty::PredicateKind::RegionOutlives(..)
 342             | ty::PredicateKind::WellFormed(..)
 343             | ty::PredicateKind::ObjectSafe(..)
 344             | ty::PredicateKind::ClosureKind(..)
 345             | ty::PredicateKind::TypeOutlives(..)
 346             | ty::PredicateKind::ConstEvaluatable(..)
 347             | ty::PredicateKind::ConstEquate(..)
 348             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 349         }
 350     })
 351 }
 352
 353 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 354 fn object_safety_violation_for_method(
 355     tcx: TyCtxt<'_>,
 356     trait_def_id: DefId,
 357     method: &ty::AssocItem,
 358 ) -> Option<(MethodViolationCode, Span)> {
 359     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 360     // Any method that has a `Self : Sized` requisite is otherwise
 361     // exempt from the regulations.
 362     if generics_require_sized_self(tcx, method.def_id) {
 363         return None;
 364     }
 365
 366     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 367     // Get an accurate span depending on the violation.
 368     violation.map(|v| {
 369         let node = tcx.hir().get_if_local(method.def_id);
 370         let span = match (&v, node) {
 371             (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
 372             (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
 373             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 374                 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
 375             }
 376             _ => method.ident(tcx).span,
 377         };
 378         (v, span)
 379     })
 380 }
 381
 382 /// Returns `Some(_)` if this method cannot be called on a trait
 383 /// object; this does not necessarily imply that the enclosing trait
 384 /// is not object safe, because the method might have a where clause
 385 /// `Self:Sized`.
 386 fn virtual_call_violation_for_method<'tcx>(
 387     tcx: TyCtxt<'tcx>,
 388     trait_def_id: DefId,
 389     method: &ty::AssocItem,
 390 ) -> Option<MethodViolationCode> {
 391     let sig = tcx.fn_sig(method.def_id);
 392
 393     // The method's first parameter must be named `self`
 394     if !method.fn_has_self_parameter {
 395         let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
 396             generics,
 397             kind: hir::TraitItemKind::Fn(sig, _),
 398             ..
 399         })) = tcx.hir().get_if_local(method.def_id).as_ref()
 400         {
 401             let sm = tcx.sess.source_map();
 402             Some((
 403                 (
 404                     format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
 405                     sm.span_through_char(sig.span, '(').shrink_to_hi(),
 406                 ),
 407                 (
 408                     format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
 409                     generics.tail_span_for_predicate_suggestion(),
 410                 ),
 411             ))
 412         } else {
 413             None
 414         };
 415         return Some(MethodViolationCode::StaticMethod(sugg));
 416     }
 417
 418     for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
 419         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 420             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 421                 kind: hir::TraitItemKind::Fn(sig, _),
 422                 ..
 423             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 424             {
 425                 Some(sig.decl.inputs[i].span)
 426             } else {
 427                 None
 428             };
 429             return Some(MethodViolationCode::ReferencesSelfInput(span));
 430         }
 431     }
 432     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 433         return Some(MethodViolationCode::ReferencesSelfOutput);
 434     }
 435     if contains_illegal_impl_trait_in_trait(tcx, sig.output()) {
 436         return Some(MethodViolationCode::ReferencesImplTraitInTrait);
 437     }
 438
 439     // We can't monomorphize things like `fn foo<A>(...)`.
 440     let own_counts = tcx.generics_of(method.def_id).own_counts();
 441     if own_counts.types + own_counts.consts != 0 {
 442         return Some(MethodViolationCode::Generic);
 443     }
 444
 445     if tcx
 446         .predicates_of(method.def_id)
 447         .predicates
 448         .iter()
 449         // A trait object can't claim to live more than the concrete type,
 450         // so outlives predicates will always hold.
 451         .cloned()
 452         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 453         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 454     {
 455         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 456     }
 457
 458     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 459
 460     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 461     // However, this is already considered object-safe. We allow it as a special case here.
 462     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 463     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 464     if receiver_ty != tcx.types.self_param {
 465         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 466             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 467                 kind: hir::TraitItemKind::Fn(sig, _),
 468                 ..
 469             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 470             {
 471                 Some(sig.decl.inputs[0].span)
 472             } else {
 473                 None
 474             };
 475             return Some(MethodViolationCode::UndispatchableReceiver(span));
 476         } else {
 477             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 478
 479             use rustc_target::abi::Abi;
 480
 481             let param_env = tcx.param_env(method.def_id);
 482
 483             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 484                 match tcx.layout_of(param_env.and(ty)) {
 485                     Ok(layout) => Some(layout.abi),
 486                     Err(err) => {
 487                         // #78372
 488                         tcx.sess.delay_span_bug(
 489                             tcx.def_span(method.def_id),
 490                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 491                         );
 492                         None
 493                     }
 494                 }
 495             };
 496
 497             // e.g., `Rc<()>`
 498             let unit_receiver_ty =
 499                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 500
 501             match abi_of_ty(unit_receiver_ty) {
 502                 Some(Abi::Scalar(..)) => (),
 503                 abi => {
 504                     tcx.sess.delay_span_bug(
 505                         tcx.def_span(method.def_id),
 506                         &format!(
 507                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 508                             abi
 509                         ),
 510                     );
 511                 }
 512             }
 513
 514             let trait_object_ty =
 515                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 516
 517             // e.g., `Rc<dyn Trait>`
 518             let trait_object_receiver =
 519                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 520
 521             match abi_of_ty(trait_object_receiver) {
 522                 Some(Abi::ScalarPair(..)) => (),
 523                 abi => {
 524                     tcx.sess.delay_span_bug(
 525                         tcx.def_span(method.def_id),
 526                         &format!(
 527                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 528                             trait_object_ty, abi
 529                         ),
 530                     );
 531                 }
 532             }
 533         }
 534     }
 535
 536     None
 537 }
 538
 539 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 540 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 541 fn receiver_for_self_ty<'tcx>(
 542     tcx: TyCtxt<'tcx>,
 543     receiver_ty: Ty<'tcx>,
 544     self_ty: Ty<'tcx>,
 545     method_def_id: DefId,
 546 ) -> Ty<'tcx> {
 547     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 548     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 549         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 550     });
 551
 552     let result = EarlyBinder(receiver_ty).subst(tcx, substs);
 553     debug!(
 554         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 555         receiver_ty, self_ty, method_def_id, result
 556     );
 557     result
 558 }
 559
 560 /// Creates the object type for the current trait. For example,
 561 /// if the current trait is `Deref`, then this will be
 562 /// `dyn Deref<Target = Self::Target> + 'static`.
 563 fn object_ty_for_trait<'tcx>(
 564     tcx: TyCtxt<'tcx>,
 565     trait_def_id: DefId,
 566     lifetime: ty::Region<'tcx>,
 567 ) -> Ty<'tcx> {
 568     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 569
 570     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 571
 572     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 573         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 574     });
 575
 576     let mut associated_types = traits::supertraits(tcx, trait_ref)
 577         .flat_map(|super_trait_ref| {
 578             tcx.associated_items(super_trait_ref.def_id())
 579                 .in_definition_order()
 580                 .map(move |item| (super_trait_ref, item))
 581         })
 582         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 583         .collect::<Vec<_>>();
 584
 585     // existential predicates need to be in a specific order
 586     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 587
 588     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 589         // We *can* get bound lifetimes here in cases like
 590         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 591         super_trait_ref.map_bound(|super_trait_ref| {
 592             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 593                 term: tcx.mk_projection(item.def_id, super_trait_ref.substs).into(),
 594                 item_def_id: item.def_id,
 595                 substs: super_trait_ref.substs,
 596             })
 597         })
 598     });
 599
 600     let existential_predicates = tcx
 601         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 602
 603     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime, ty::Dyn);
 604
 605     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 606
 607     object_ty
 608 }
 609
 610 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 611 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 612 /// in the following way:
 613 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 614 /// - require the following bound:
 615 ///
 616 ///   ```ignore (not-rust)
 617 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 618 ///   ```
 619 ///
 620 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 621 ///   (substitution notation).
 622 ///
 623 /// Some examples of receiver types and their required obligation:
 624 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 625 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 626 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 627 ///
 628 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 629 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 630 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 631 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 632 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 633 /// contained by the trait object, because the object that needs to be coerced is behind
 634 /// a pointer.
 635 ///
 636 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 637 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 638 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 639 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 640 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 641 /// Written as a chalk-style query:
 642 /// ```ignore (not-rust)
 643 /// forall (U: Trait + ?Sized) {
 644 ///     if (Self: Unsize<U>) {
 645 ///         Receiver: DispatchFromDyn<Receiver[Self => U]>
 646 ///     }
 647 /// }
 648 /// ```
 649 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 650 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 651 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 652 //
 653 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 654 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 655 // `self: Wrapper<Self>`.
 656 #[allow(dead_code)]
 657 fn receiver_is_dispatchable<'tcx>(
 658     tcx: TyCtxt<'tcx>,
 659     method: &ty::AssocItem,
 660     receiver_ty: Ty<'tcx>,
 661 ) -> bool {
 662     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 663
 664     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 665     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 666         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 667         return false;
 668     };
 669
 670     // the type `U` in the query
 671     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 672     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 673     // replace this with `dyn Trait`
 674     let unsized_self_ty: Ty<'tcx> =
 675         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 676
 677     // `Receiver[Self => U]`
 678     let unsized_receiver_ty =
 679         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 680
 681     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 682     // `U: ?Sized` is already implied here
 683     let param_env = {
 684         let param_env = tcx.param_env(method.def_id);
 685
 686         // Self: Unsize<U>
 687         let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
 688             def_id: unsize_did,
 689             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 690         })
 691         .without_const()
 692         .to_predicate(tcx);
 693
 694         // U: Trait<Arg1, ..., ArgN>
 695         let trait_predicate = {
 696             let substs =
 697                 InternalSubsts::for_item(tcx, method.trait_container(tcx).unwrap(), |param, _| {
 698                     if param.index == 0 {
 699                         unsized_self_ty.into()
 700                     } else {
 701                         tcx.mk_param_from_def(param)
 702                     }
 703                 });
 704
 705             ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
 706                 .without_const()
 707                 .to_predicate(tcx)
 708         };
 709
 710         let caller_bounds: Vec<Predicate<'tcx>> =
 711             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 712
 713         ty::ParamEnv::new(
 714             tcx.intern_predicates(&caller_bounds),
 715             param_env.reveal(),
 716             param_env.constness(),
 717         )
 718     };
 719
 720     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 721     let obligation = {
 722         let predicate = ty::Binder::dummy(ty::TraitRef {
 723             def_id: dispatch_from_dyn_did,
 724             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 725         })
 726         .without_const()
 727         .to_predicate(tcx);
 728
 729         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 730     };
 731
 732     tcx.infer_ctxt().enter(|ref infcx| {
 733         // the receiver is dispatchable iff the obligation holds
 734         infcx.predicate_must_hold_modulo_regions(&obligation)
 735     })
 736 }
 737
 738 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
 739     tcx: TyCtxt<'tcx>,
 740     trait_def_id: DefId,
 741     value: T,
 742 ) -> bool {
 743     // This is somewhat subtle. In general, we want to forbid
 744     // references to `Self` in the argument and return types,
 745     // since the value of `Self` is erased. However, there is one
 746     // exception: it is ok to reference `Self` in order to access
 747     // an associated type of the current trait, since we retain
 748     // the value of those associated types in the object type
 749     // itself.
 750     //
 751     // ```rust
 752     // trait SuperTrait {
 753     //     type X;
 754     // }
 755     //
 756     // trait Trait : SuperTrait {
 757     //     type Y;
 758     //     fn foo(&self, x: Self) // bad
 759     //     fn foo(&self) -> Self // bad
 760     //     fn foo(&self) -> Option<Self> // bad
 761     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 762     //     fn foo(&self) -> <Self as Trait>::Y // OK
 763     //     fn foo(&self) -> Self::X // OK, desugars to next example
 764     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 765     // }
 766     // ```
 767     //
 768     // However, it is not as simple as allowing `Self` in a projected
 769     // type, because there are illegal ways to use `Self` as well:
 770     //
 771     // ```rust
 772     // trait Trait : SuperTrait {
 773     //     ...
 774     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 775     // }
 776     // ```
 777     //
 778     // Here we will not have the type of `X` recorded in the
 779     // object type, and we cannot resolve `Self as SomeOtherTrait`
 780     // without knowing what `Self` is.
 781
 782     struct IllegalSelfTypeVisitor<'tcx> {
 783         tcx: TyCtxt<'tcx>,
 784         trait_def_id: DefId,
 785         supertraits: Option<Vec<DefId>>,
 786     }
 787
 788     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 789         type BreakTy = ();
 790
 791         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 792             match t.kind() {
 793                 ty::Param(_) => {
 794                     if t == self.tcx.types.self_param {
 795                         ControlFlow::BREAK
 796                     } else {
 797                         ControlFlow::CONTINUE
 798                     }
 799                 }
 800                 ty::Projection(ref data)
 801                     if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
 802                 {
 803                     // We'll deny these later in their own pass
 804                     ControlFlow::CONTINUE
 805                 }
 806                 ty::Projection(ref data) => {
 807                     // This is a projected type `<Foo as SomeTrait>::X`.
 808
 809                     // Compute supertraits of current trait lazily.
 810                     if self.supertraits.is_none() {
 811                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 812                         self.supertraits = Some(
 813                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 814                         );
 815                     }
 816
 817                     // Determine whether the trait reference `Foo as
 818                     // SomeTrait` is in fact a supertrait of the
 819                     // current trait. In that case, this type is
 820                     // legal, because the type `X` will be specified
 821                     // in the object type.  Note that we can just use
 822                     // direct equality here because all of these types
 823                     // are part of the formal parameter listing, and
 824                     // hence there should be no inference variables.
 825                     let is_supertrait_of_current_trait = self
 826                         .supertraits
 827                         .as_ref()
 828                         .unwrap()
 829                         .contains(&data.trait_ref(self.tcx).def_id);
 830
 831                     if is_supertrait_of_current_trait {
 832                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 833                     } else {
 834                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 835                     }
 836                 }
 837                 _ => t.super_visit_with(self), // walk contained types, if any
 838             }
 839         }
 840
 841         fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
 842             // Constants can only influence object safety if they reference `Self`.
 843             // This is only possible for unevaluated constants, so we walk these here.
 844             //
 845             // If `AbstractConst::new` returned an error we already failed compilation
 846             // so we don't have to emit an additional error here.
 847             //
 848             // We currently recurse into abstract consts here but do not recurse in
 849             // `is_const_evaluatable`. This means that the object safety check is more
 850             // liberal than the const eval check.
 851             //
 852             // This shouldn't really matter though as we can't really use any
 853             // constants which are not considered const evaluatable.
 854             use rustc_middle::ty::abstract_const::Node;
 855             if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
 856                 walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
 857                     Node::Leaf(leaf) => self.visit_const(leaf),
 858                     Node::Cast(_, _, ty) => self.visit_ty(ty),
 859                     Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 860                         ControlFlow::CONTINUE
 861                     }
 862                 })
 863             } else {
 864                 ControlFlow::CONTINUE
 865             }
 866         }
 867     }
 868
 869     value
 870         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 871         .is_break()
 872 }
 873
 874 pub fn contains_illegal_impl_trait_in_trait<'tcx>(
 875     tcx: TyCtxt<'tcx>,
 876     ty: ty::Binder<'tcx, Ty<'tcx>>,
 877 ) -> bool {
 878     // FIXME(RPITIT): Perhaps we should use a visitor here?
 879     ty.skip_binder().walk().any(|arg| {
 880         if let ty::GenericArgKind::Type(ty) = arg.unpack()
 881             && let ty::Projection(proj) = ty.kind()
 882         {
 883             tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
 884         } else {
 885             false
 886         }
 887     })
 888 }
 889
 890 pub fn provide(providers: &mut ty::query::Providers) {
 891     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 892 }