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 rustc_errors::{FatalError, MultiSpan};
  17 use rustc_hir as hir;
  18 use rustc_hir::def_id::DefId;
  19 use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
  20 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
  21 use rustc_middle::ty::{
  22     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
  23 };
  24 use rustc_middle::ty::{Predicate, ToPredicate};
  25 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
  26 use rustc_span::symbol::Symbol;
  27 use rustc_span::Span;
  28 use smallvec::SmallVec;
  29
  30 use std::iter;
  31 use std::ops::ControlFlow;
  32
  33 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
  34
  35 /// Returns the object safety violations that affect
  36 /// astconv -- currently, `Self` in supertraits. This is needed
  37 /// because `object_safety_violations` can't be used during
  38 /// type collection.
  39 pub fn astconv_object_safety_violations(
  40     tcx: TyCtxt<'_>,
  41     trait_def_id: DefId,
  42 ) -> Vec<ObjectSafetyViolation> {
  43     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  44     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
  45         .map(|def_id| predicates_reference_self(tcx, def_id, true))
  46         .filter(|spans| !spans.is_empty())
  47         .map(ObjectSafetyViolation::SupertraitSelf)
  48         .collect();
  49
  50     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
  51
  52     violations
  53 }
  54
  55 fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [ObjectSafetyViolation] {
  56     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  57     debug!("object_safety_violations: {:?}", trait_def_id);
  58
  59     tcx.arena.alloc_from_iter(
  60         traits::supertrait_def_ids(tcx, trait_def_id)
  61             .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
  62     )
  63 }
  64
  65 /// We say a method is *vtable safe* if it can be invoked on a trait
  66 /// object. Note that object-safe traits can have some
  67 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  68 /// otherwise ensure that they cannot be used when `Self = Trait`.
  69 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
  70     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  71     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
  72     // Any method that has a `Self: Sized` bound cannot be called.
  73     if generics_require_sized_self(tcx, method.def_id) {
  74         return false;
  75     }
  76
  77     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
  78         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
  79         Some(_) => false,
  80     }
  81 }
  82
  83 fn object_safety_violations_for_trait(
  84     tcx: TyCtxt<'_>,
  85     trait_def_id: DefId,
  86 ) -> Vec<ObjectSafetyViolation> {
  87     // Check methods for violations.
  88     let mut violations: Vec<_> = tcx
  89         .associated_items(trait_def_id)
  90         .in_definition_order()
  91         .filter(|item| item.kind == ty::AssocKind::Fn)
  92         .filter_map(|item| {
  93             object_safety_violation_for_method(tcx, trait_def_id, &item)
  94                 .map(|(code, span)| ObjectSafetyViolation::Method(item.name, code, span))
  95         })
  96         .filter(|violation| {
  97             if let ObjectSafetyViolation::Method(
  98                 _,
  99                 MethodViolationCode::WhereClauseReferencesSelf,
 100                 span,
 101             ) = violation
 102             {
 103                 lint_object_unsafe_trait(tcx, *span, trait_def_id, &violation);
 104                 false
 105             } else {
 106                 true
 107             }
 108         })
 109         .collect();
 110
 111     // Check the trait itself.
 112     if trait_has_sized_self(tcx, trait_def_id) {
 113         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 114         let spans = get_sized_bounds(tcx, trait_def_id);
 115         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 116     }
 117     let spans = predicates_reference_self(tcx, trait_def_id, false);
 118     if !spans.is_empty() {
 119         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 120     }
 121     let spans = bounds_reference_self(tcx, trait_def_id);
 122     if !spans.is_empty() {
 123         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 124     }
 125
 126     violations.extend(
 127         tcx.associated_items(trait_def_id)
 128             .in_definition_order()
 129             .filter(|item| item.kind == ty::AssocKind::Const)
 130             .map(|item| {
 131                 let ident = item.ident(tcx);
 132                 ObjectSafetyViolation::AssocConst(ident.name, ident.span)
 133             }),
 134     );
 135
 136     if !tcx.features().generic_associated_types_extended {
 137         violations.extend(
 138             tcx.associated_items(trait_def_id)
 139                 .in_definition_order()
 140                 .filter(|item| item.kind == ty::AssocKind::Type)
 141                 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
 142                 .map(|item| {
 143                     let ident = item.ident(tcx);
 144                     ObjectSafetyViolation::GAT(ident.name, ident.span)
 145                 }),
 146         );
 147     }
 148
 149     debug!(
 150         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 151         trait_def_id, violations
 152     );
 153
 154     violations
 155 }
 156
 157 /// Lint object-unsafe trait.
 158 fn lint_object_unsafe_trait(
 159     tcx: TyCtxt<'_>,
 160     span: Span,
 161     trait_def_id: DefId,
 162     violation: &ObjectSafetyViolation,
 163 ) {
 164     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 165     // It's also hard to get a use site span, so we use the method definition span.
 166     tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
 167         let mut err = lint.build(&format!(
 168             "the trait `{}` cannot be made into an object",
 169             tcx.def_path_str(trait_def_id)
 170         ));
 171         let node = tcx.hir().get_if_local(trait_def_id);
 172         let mut spans = MultiSpan::from_span(span);
 173         if let Some(hir::Node::Item(item)) = node {
 174             spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
 175             spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 176         } else {
 177             spans.push_span_label(
 178                 span,
 179                 format!(
 180                     "the trait cannot be made into an object because {}",
 181                     violation.error_msg()
 182                 ),
 183             );
 184         };
 185         err.span_note(
 186             spans,
 187             "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 188              call to be resolvable dynamically; for more information visit \
 189              <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 190         );
 191         if node.is_some() {
 192             // Only provide the help if its a local trait, otherwise it's not
 193             violation.solution(&mut err);
 194         }
 195         err.emit();
 196     });
 197 }
 198
 199 fn sized_trait_bound_spans<'tcx>(
 200     tcx: TyCtxt<'tcx>,
 201     bounds: hir::GenericBounds<'tcx>,
 202 ) -> impl 'tcx + Iterator<Item = Span> {
 203     bounds.iter().filter_map(move |b| match b {
 204         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 205             if trait_has_sized_self(
 206                 tcx,
 207                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 208             ) =>
 209         {
 210             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 211             Some(trait_ref.span)
 212         }
 213         _ => None,
 214     })
 215 }
 216
 217 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 218     tcx.hir()
 219         .get_if_local(trait_def_id)
 220         .and_then(|node| match node {
 221             hir::Node::Item(hir::Item {
 222                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 223                 ..
 224             }) => Some(
 225                 generics
 226                     .predicates
 227                     .iter()
 228                     .filter_map(|pred| {
 229                         match pred {
 230                             hir::WherePredicate::BoundPredicate(pred)
 231                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 232                             {
 233                                 // Fetch spans for trait bounds that are Sized:
 234                                 // `trait T where Self: Pred`
 235                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 236                             }
 237                             _ => None,
 238                         }
 239                     })
 240                     .flatten()
 241                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 242                     .chain(sized_trait_bound_spans(tcx, bounds))
 243                     .collect::<SmallVec<[Span; 1]>>(),
 244             ),
 245             _ => None,
 246         })
 247         .unwrap_or_else(SmallVec::new)
 248 }
 249
 250 fn predicates_reference_self(
 251     tcx: TyCtxt<'_>,
 252     trait_def_id: DefId,
 253     supertraits_only: bool,
 254 ) -> SmallVec<[Span; 1]> {
 255     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 256     let predicates = if supertraits_only {
 257         tcx.super_predicates_of(trait_def_id)
 258     } else {
 259         tcx.predicates_of(trait_def_id)
 260     };
 261     predicates
 262         .predicates
 263         .iter()
 264         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 265         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 266         .collect()
 267 }
 268
 269 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 270     tcx.associated_items(trait_def_id)
 271         .in_definition_order()
 272         .filter(|item| item.kind == ty::AssocKind::Type)
 273         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 274         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 275         .collect()
 276 }
 277
 278 fn predicate_references_self<'tcx>(
 279     tcx: TyCtxt<'tcx>,
 280     (predicate, sp): (ty::Predicate<'tcx>, Span),
 281 ) -> Option<Span> {
 282     let self_ty = tcx.types.self_param;
 283     let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
 284     match predicate.kind().skip_binder() {
 285         ty::PredicateKind::Trait(ref data) => {
 286             // In the case of a trait predicate, we can skip the "self" type.
 287             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 288         }
 289         ty::PredicateKind::Projection(ref data) => {
 290             // And similarly for projections. This should be redundant with
 291             // the previous check because any projection should have a
 292             // matching `Trait` predicate with the same inputs, but we do
 293             // the check to be safe.
 294             //
 295             // It's also won't be redundant if we allow type-generic associated
 296             // types for trait objects.
 297             //
 298             // Note that we *do* allow projection *outputs* to contain
 299             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 300             // we just require the user to specify *both* outputs
 301             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 302             //
 303             // This is ALT2 in issue #56288, see that for discussion of the
 304             // possible alternatives.
 305             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 306         }
 307         ty::PredicateKind::WellFormed(..)
 308         | ty::PredicateKind::ObjectSafe(..)
 309         | ty::PredicateKind::TypeOutlives(..)
 310         | ty::PredicateKind::RegionOutlives(..)
 311         | ty::PredicateKind::ClosureKind(..)
 312         | ty::PredicateKind::Subtype(..)
 313         | ty::PredicateKind::Coerce(..)
 314         | ty::PredicateKind::ConstEvaluatable(..)
 315         | ty::PredicateKind::ConstEquate(..)
 316         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 317     }
 318 }
 319
 320 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 321     generics_require_sized_self(tcx, trait_def_id)
 322 }
 323
 324 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 325     let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
 326         return false; /* No Sized trait, can't require it! */
 327     };
 328
 329     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 330     let predicates = tcx.predicates_of(def_id);
 331     let predicates = predicates.instantiate_identity(tcx).predicates;
 332     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 333         match obligation.predicate.kind().skip_binder() {
 334             ty::PredicateKind::Trait(ref trait_pred) => {
 335                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 336             }
 337             ty::PredicateKind::Projection(..)
 338             | ty::PredicateKind::Subtype(..)
 339             | ty::PredicateKind::Coerce(..)
 340             | ty::PredicateKind::RegionOutlives(..)
 341             | ty::PredicateKind::WellFormed(..)
 342             | ty::PredicateKind::ObjectSafe(..)
 343             | ty::PredicateKind::ClosureKind(..)
 344             | ty::PredicateKind::TypeOutlives(..)
 345             | ty::PredicateKind::ConstEvaluatable(..)
 346             | ty::PredicateKind::ConstEquate(..)
 347             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 348         }
 349     })
 350 }
 351
 352 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 353 fn object_safety_violation_for_method(
 354     tcx: TyCtxt<'_>,
 355     trait_def_id: DefId,
 356     method: &ty::AssocItem,
 357 ) -> Option<(MethodViolationCode, Span)> {
 358     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 359     // Any method that has a `Self : Sized` requisite is otherwise
 360     // exempt from the regulations.
 361     if generics_require_sized_self(tcx, method.def_id) {
 362         return None;
 363     }
 364
 365     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 366     // Get an accurate span depending on the violation.
 367     violation.map(|v| {
 368         let node = tcx.hir().get_if_local(method.def_id);
 369         let span = match (&v, node) {
 370             (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
 371             (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
 372             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 373                 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
 374             }
 375             _ => method.ident(tcx).span,
 376         };
 377         (v, span)
 378     })
 379 }
 380
 381 /// Returns `Some(_)` if this method cannot be called on a trait
 382 /// object; this does not necessarily imply that the enclosing trait
 383 /// is not object safe, because the method might have a where clause
 384 /// `Self:Sized`.
 385 fn virtual_call_violation_for_method<'tcx>(
 386     tcx: TyCtxt<'tcx>,
 387     trait_def_id: DefId,
 388     method: &ty::AssocItem,
 389 ) -> Option<MethodViolationCode> {
 390     let sig = tcx.fn_sig(method.def_id);
 391
 392     // The method's first parameter must be named `self`
 393     if !method.fn_has_self_parameter {
 394         let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
 395             generics,
 396             kind: hir::TraitItemKind::Fn(sig, _),
 397             ..
 398         })) = tcx.hir().get_if_local(method.def_id).as_ref()
 399         {
 400             let sm = tcx.sess.source_map();
 401             Some((
 402                 (
 403                     format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
 404                     sm.span_through_char(sig.span, '(').shrink_to_hi(),
 405                 ),
 406                 (
 407                     format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
 408                     generics.tail_span_for_predicate_suggestion(),
 409                 ),
 410             ))
 411         } else {
 412             None
 413         };
 414         return Some(MethodViolationCode::StaticMethod(sugg));
 415     }
 416
 417     for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
 418         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 419             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 420                 kind: hir::TraitItemKind::Fn(sig, _),
 421                 ..
 422             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 423             {
 424                 Some(sig.decl.inputs[i].span)
 425             } else {
 426                 None
 427             };
 428             return Some(MethodViolationCode::ReferencesSelfInput(span));
 429         }
 430     }
 431     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 432         return Some(MethodViolationCode::ReferencesSelfOutput);
 433     }
 434
 435     // We can't monomorphize things like `fn foo<A>(...)`.
 436     let own_counts = tcx.generics_of(method.def_id).own_counts();
 437     if own_counts.types + own_counts.consts != 0 {
 438         return Some(MethodViolationCode::Generic);
 439     }
 440
 441     if tcx
 442         .predicates_of(method.def_id)
 443         .predicates
 444         .iter()
 445         // A trait object can't claim to live more than the concrete type,
 446         // so outlives predicates will always hold.
 447         .cloned()
 448         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 449         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 450     {
 451         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 452     }
 453
 454     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 455
 456     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 457     // However, this is already considered object-safe. We allow it as a special case here.
 458     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 459     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 460     if receiver_ty != tcx.types.self_param {
 461         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 462             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 463                 kind: hir::TraitItemKind::Fn(sig, _),
 464                 ..
 465             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 466             {
 467                 Some(sig.decl.inputs[0].span)
 468             } else {
 469                 None
 470             };
 471             return Some(MethodViolationCode::UndispatchableReceiver(span));
 472         } else {
 473             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 474
 475             use rustc_target::abi::Abi;
 476
 477             let param_env = tcx.param_env(method.def_id);
 478
 479             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 480                 match tcx.layout_of(param_env.and(ty)) {
 481                     Ok(layout) => Some(layout.abi),
 482                     Err(err) => {
 483                         // #78372
 484                         tcx.sess.delay_span_bug(
 485                             tcx.def_span(method.def_id),
 486                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 487                         );
 488                         None
 489                     }
 490                 }
 491             };
 492
 493             // e.g., `Rc<()>`
 494             let unit_receiver_ty =
 495                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 496
 497             match abi_of_ty(unit_receiver_ty) {
 498                 Some(Abi::Scalar(..)) => (),
 499                 abi => {
 500                     tcx.sess.delay_span_bug(
 501                         tcx.def_span(method.def_id),
 502                         &format!(
 503                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 504                             abi
 505                         ),
 506                     );
 507                 }
 508             }
 509
 510             let trait_object_ty =
 511                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 512
 513             // e.g., `Rc<dyn Trait>`
 514             let trait_object_receiver =
 515                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 516
 517             match abi_of_ty(trait_object_receiver) {
 518                 Some(Abi::ScalarPair(..)) => (),
 519                 abi => {
 520                     tcx.sess.delay_span_bug(
 521                         tcx.def_span(method.def_id),
 522                         &format!(
 523                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 524                             trait_object_ty, abi
 525                         ),
 526                     );
 527                 }
 528             }
 529         }
 530     }
 531
 532     None
 533 }
 534
 535 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 536 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 537 fn receiver_for_self_ty<'tcx>(
 538     tcx: TyCtxt<'tcx>,
 539     receiver_ty: Ty<'tcx>,
 540     self_ty: Ty<'tcx>,
 541     method_def_id: DefId,
 542 ) -> Ty<'tcx> {
 543     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 544     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 545         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 546     });
 547
 548     let result = EarlyBinder(receiver_ty).subst(tcx, substs);
 549     debug!(
 550         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 551         receiver_ty, self_ty, method_def_id, result
 552     );
 553     result
 554 }
 555
 556 /// Creates the object type for the current trait. For example,
 557 /// if the current trait is `Deref`, then this will be
 558 /// `dyn Deref<Target = Self::Target> + 'static`.
 559 fn object_ty_for_trait<'tcx>(
 560     tcx: TyCtxt<'tcx>,
 561     trait_def_id: DefId,
 562     lifetime: ty::Region<'tcx>,
 563 ) -> Ty<'tcx> {
 564     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 565
 566     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 567
 568     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 569         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 570     });
 571
 572     let mut associated_types = traits::supertraits(tcx, trait_ref)
 573         .flat_map(|super_trait_ref| {
 574             tcx.associated_items(super_trait_ref.def_id())
 575                 .in_definition_order()
 576                 .map(move |item| (super_trait_ref, item))
 577         })
 578         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 579         .collect::<Vec<_>>();
 580
 581     // existential predicates need to be in a specific order
 582     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 583
 584     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 585         // We *can* get bound lifetimes here in cases like
 586         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 587         super_trait_ref.map_bound(|super_trait_ref| {
 588             ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 589                 term: tcx.mk_projection(item.def_id, super_trait_ref.substs).into(),
 590                 item_def_id: item.def_id,
 591                 substs: super_trait_ref.substs,
 592             })
 593         })
 594     });
 595
 596     let existential_predicates = tcx
 597         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 598
 599     let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
 600
 601     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 602
 603     object_ty
 604 }
 605
 606 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 607 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 608 /// in the following way:
 609 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 610 /// - require the following bound:
 611 ///
 612 ///   ```ignore (not-rust)
 613 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 614 ///   ```
 615 ///
 616 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 617 ///   (substitution notation).
 618 ///
 619 /// Some examples of receiver types and their required obligation:
 620 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 621 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 622 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 623 ///
 624 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 625 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 626 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 627 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 628 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 629 /// contained by the trait object, because the object that needs to be coerced is behind
 630 /// a pointer.
 631 ///
 632 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 633 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 634 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 635 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 636 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 637 /// Written as a chalk-style query:
 638 /// ```ignore (not-rust)
 639 /// forall (U: Trait + ?Sized) {
 640 ///     if (Self: Unsize<U>) {
 641 ///         Receiver: DispatchFromDyn<Receiver[Self => U]>
 642 ///     }
 643 /// }
 644 /// ```
 645 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 646 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 647 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 648 //
 649 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 650 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 651 // `self: Wrapper<Self>`.
 652 #[allow(dead_code)]
 653 fn receiver_is_dispatchable<'tcx>(
 654     tcx: TyCtxt<'tcx>,
 655     method: &ty::AssocItem,
 656     receiver_ty: Ty<'tcx>,
 657 ) -> bool {
 658     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 659
 660     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 661     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 662         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 663         return false;
 664     };
 665
 666     // the type `U` in the query
 667     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 668     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 669     // replace this with `dyn Trait`
 670     let unsized_self_ty: Ty<'tcx> =
 671         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 672
 673     // `Receiver[Self => U]`
 674     let unsized_receiver_ty =
 675         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 676
 677     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 678     // `U: ?Sized` is already implied here
 679     let param_env = {
 680         let param_env = tcx.param_env(method.def_id);
 681
 682         // Self: Unsize<U>
 683         let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
 684             def_id: unsize_did,
 685             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 686         })
 687         .without_const()
 688         .to_predicate(tcx);
 689
 690         // U: Trait<Arg1, ..., ArgN>
 691         let trait_predicate = {
 692             let substs =
 693                 InternalSubsts::for_item(tcx, method.trait_container(tcx).unwrap(), |param, _| {
 694                     if param.index == 0 {
 695                         unsized_self_ty.into()
 696                     } else {
 697                         tcx.mk_param_from_def(param)
 698                     }
 699                 });
 700
 701             ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
 702                 .without_const()
 703                 .to_predicate(tcx)
 704         };
 705
 706         let caller_bounds: Vec<Predicate<'tcx>> =
 707             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 708
 709         ty::ParamEnv::new(
 710             tcx.intern_predicates(&caller_bounds),
 711             param_env.reveal(),
 712             param_env.constness(),
 713         )
 714     };
 715
 716     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 717     let obligation = {
 718         let predicate = ty::Binder::dummy(ty::TraitRef {
 719             def_id: dispatch_from_dyn_did,
 720             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 721         })
 722         .without_const()
 723         .to_predicate(tcx);
 724
 725         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 726     };
 727
 728     tcx.infer_ctxt().enter(|ref infcx| {
 729         // the receiver is dispatchable iff the obligation holds
 730         infcx.predicate_must_hold_modulo_regions(&obligation)
 731     })
 732 }
 733
 734 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
 735     tcx: TyCtxt<'tcx>,
 736     trait_def_id: DefId,
 737     value: T,
 738 ) -> bool {
 739     // This is somewhat subtle. In general, we want to forbid
 740     // references to `Self` in the argument and return types,
 741     // since the value of `Self` is erased. However, there is one
 742     // exception: it is ok to reference `Self` in order to access
 743     // an associated type of the current trait, since we retain
 744     // the value of those associated types in the object type
 745     // itself.
 746     //
 747     // ```rust
 748     // trait SuperTrait {
 749     //     type X;
 750     // }
 751     //
 752     // trait Trait : SuperTrait {
 753     //     type Y;
 754     //     fn foo(&self, x: Self) // bad
 755     //     fn foo(&self) -> Self // bad
 756     //     fn foo(&self) -> Option<Self> // bad
 757     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 758     //     fn foo(&self) -> <Self as Trait>::Y // OK
 759     //     fn foo(&self) -> Self::X // OK, desugars to next example
 760     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 761     // }
 762     // ```
 763     //
 764     // However, it is not as simple as allowing `Self` in a projected
 765     // type, because there are illegal ways to use `Self` as well:
 766     //
 767     // ```rust
 768     // trait Trait : SuperTrait {
 769     //     ...
 770     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 771     // }
 772     // ```
 773     //
 774     // Here we will not have the type of `X` recorded in the
 775     // object type, and we cannot resolve `Self as SomeOtherTrait`
 776     // without knowing what `Self` is.
 777
 778     struct IllegalSelfTypeVisitor<'tcx> {
 779         tcx: TyCtxt<'tcx>,
 780         trait_def_id: DefId,
 781         supertraits: Option<Vec<DefId>>,
 782     }
 783
 784     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 785         type BreakTy = ();
 786
 787         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 788             match t.kind() {
 789                 ty::Param(_) => {
 790                     if t == self.tcx.types.self_param {
 791                         ControlFlow::BREAK
 792                     } else {
 793                         ControlFlow::CONTINUE
 794                     }
 795                 }
 796                 ty::Projection(ref data) => {
 797                     // This is a projected type `<Foo as SomeTrait>::X`.
 798
 799                     // Compute supertraits of current trait lazily.
 800                     if self.supertraits.is_none() {
 801                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 802                         self.supertraits = Some(
 803                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 804                         );
 805                     }
 806
 807                     // Determine whether the trait reference `Foo as
 808                     // SomeTrait` is in fact a supertrait of the
 809                     // current trait. In that case, this type is
 810                     // legal, because the type `X` will be specified
 811                     // in the object type.  Note that we can just use
 812                     // direct equality here because all of these types
 813                     // are part of the formal parameter listing, and
 814                     // hence there should be no inference variables.
 815                     let is_supertrait_of_current_trait = self
 816                         .supertraits
 817                         .as_ref()
 818                         .unwrap()
 819                         .contains(&data.trait_ref(self.tcx).def_id);
 820
 821                     if is_supertrait_of_current_trait {
 822                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 823                     } else {
 824                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 825                     }
 826                 }
 827                 _ => t.super_visit_with(self), // walk contained types, if any
 828             }
 829         }
 830
 831         fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
 832             // Constants can only influence object safety if they reference `Self`.
 833             // This is only possible for unevaluated constants, so we walk these here.
 834             //
 835             // If `AbstractConst::new` returned an error we already failed compilation
 836             // so we don't have to emit an additional error here.
 837             //
 838             // We currently recurse into abstract consts here but do not recurse in
 839             // `is_const_evaluatable`. This means that the object safety check is more
 840             // liberal than the const eval check.
 841             //
 842             // This shouldn't really matter though as we can't really use any
 843             // constants which are not considered const evaluatable.
 844             use rustc_middle::ty::abstract_const::Node;
 845             if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
 846                 walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
 847                     Node::Leaf(leaf) => self.visit_const(leaf),
 848                     Node::Cast(_, _, ty) => self.visit_ty(ty),
 849                     Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 850                         ControlFlow::CONTINUE
 851                     }
 852                 })
 853             } else {
 854                 ControlFlow::CONTINUE
 855             }
 856         }
 857     }
 858
 859     value
 860         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 861         .is_break()
 862 }
 863
 864 pub fn provide(providers: &mut ty::query::Providers) {
 865     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 866 }