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, elaborate_trait_ref};
  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::{DelayDm, FatalError, MultiSpan};
  18 use rustc_hir as hir;
  19 use rustc_hir::def_id::DefId;
  20 use rustc_middle::ty::subst::{GenericArg, InternalSubsts};
  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 fn check_is_object_safe(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
  66     let violations = tcx.object_safety_violations(trait_def_id);
  67
  68     if violations.is_empty() {
  69         return true;
  70     }
  71
  72     // If the trait contains any other violations, then let the error reporting path
  73     // report it instead of emitting a warning here.
  74     if violations.iter().all(|violation| {
  75         matches!(
  76             violation,
  77             ObjectSafetyViolation::Method(_, MethodViolationCode::WhereClauseReferencesSelf, _)
  78         )
  79     }) {
  80         for violation in violations {
  81             if let ObjectSafetyViolation::Method(
  82                 _,
  83                 MethodViolationCode::WhereClauseReferencesSelf,
  84                 span,
  85             ) = violation
  86             {
  87                 lint_object_unsafe_trait(tcx, *span, trait_def_id, &violation);
  88             }
  89         }
  90         return true;
  91     }
  92
  93     false
  94 }
  95
  96 /// We say a method is *vtable safe* if it can be invoked on a trait
  97 /// object. Note that object-safe traits can have some
  98 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  99 /// otherwise ensure that they cannot be used when `Self = Trait`.
 100 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
 101     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 102     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
 103     // Any method that has a `Self: Sized` bound cannot be called.
 104     if generics_require_sized_self(tcx, method.def_id) {
 105         return false;
 106     }
 107
 108     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
 109         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
 110         Some(_) => false,
 111     }
 112 }
 113
 114 fn object_safety_violations_for_trait(
 115     tcx: TyCtxt<'_>,
 116     trait_def_id: DefId,
 117 ) -> Vec<ObjectSafetyViolation> {
 118     // Check methods for violations.
 119     let mut violations: Vec<_> = tcx
 120         .associated_items(trait_def_id)
 121         .in_definition_order()
 122         .filter(|item| item.kind == ty::AssocKind::Fn)
 123         .filter_map(|item| {
 124             object_safety_violation_for_method(tcx, trait_def_id, &item)
 125                 .map(|(code, span)| ObjectSafetyViolation::Method(item.name, code, span))
 126         })
 127         .collect();
 128
 129     // Check the trait itself.
 130     if trait_has_sized_self(tcx, trait_def_id) {
 131         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 132         let spans = get_sized_bounds(tcx, trait_def_id);
 133         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 134     }
 135     let spans = predicates_reference_self(tcx, trait_def_id, false);
 136     if !spans.is_empty() {
 137         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 138     }
 139     let spans = bounds_reference_self(tcx, trait_def_id);
 140     if !spans.is_empty() {
 141         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 142     }
 143
 144     violations.extend(
 145         tcx.associated_items(trait_def_id)
 146             .in_definition_order()
 147             .filter(|item| item.kind == ty::AssocKind::Const)
 148             .map(|item| {
 149                 let ident = item.ident(tcx);
 150                 ObjectSafetyViolation::AssocConst(ident.name, ident.span)
 151             }),
 152     );
 153
 154     if !tcx.features().generic_associated_types_extended {
 155         violations.extend(
 156             tcx.associated_items(trait_def_id)
 157                 .in_definition_order()
 158                 .filter(|item| item.kind == ty::AssocKind::Type)
 159                 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
 160                 .map(|item| {
 161                     let ident = item.ident(tcx);
 162                     ObjectSafetyViolation::GAT(ident.name, ident.span)
 163                 }),
 164         );
 165     }
 166
 167     debug!(
 168         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 169         trait_def_id, violations
 170     );
 171
 172     violations
 173 }
 174
 175 /// Lint object-unsafe trait.
 176 fn lint_object_unsafe_trait(
 177     tcx: TyCtxt<'_>,
 178     span: Span,
 179     trait_def_id: DefId,
 180     violation: &ObjectSafetyViolation,
 181 ) {
 182     // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 183     // It's also hard to get a use site span, so we use the method definition span.
 184     tcx.struct_span_lint_hir(
 185         WHERE_CLAUSES_OBJECT_SAFETY,
 186         hir::CRATE_HIR_ID,
 187         span,
 188         DelayDm(|| format!("the trait `{}` cannot be made into an object", tcx.def_path_str(trait_def_id))),
 189         |err| {
 190             let node = tcx.hir().get_if_local(trait_def_id);
 191             let mut spans = MultiSpan::from_span(span);
 192             if let Some(hir::Node::Item(item)) = node {
 193                 spans.push_span_label(
 194                     item.ident.span,
 195                     "this trait cannot be made into an object...",
 196                 );
 197                 spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 198             } else {
 199                 spans.push_span_label(
 200                     span,
 201                     format!(
 202                         "the trait cannot be made into an object because {}",
 203                         violation.error_msg()
 204                     ),
 205                 );
 206             };
 207             err.span_note(
 208                 spans,
 209                 "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 210                 call to be resolvable dynamically; for more information visit \
 211                 <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 212             );
 213             if node.is_some() {
 214                 // Only provide the help if its a local trait, otherwise it's not
 215                 violation.solution(err);
 216             }
 217             err
 218         },
 219     );
 220 }
 221
 222 fn sized_trait_bound_spans<'tcx>(
 223     tcx: TyCtxt<'tcx>,
 224     bounds: hir::GenericBounds<'tcx>,
 225 ) -> impl 'tcx + Iterator<Item = Span> {
 226     bounds.iter().filter_map(move |b| match b {
 227         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 228             if trait_has_sized_self(
 229                 tcx,
 230                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 231             ) =>
 232         {
 233             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 234             Some(trait_ref.span)
 235         }
 236         _ => None,
 237     })
 238 }
 239
 240 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 241     tcx.hir()
 242         .get_if_local(trait_def_id)
 243         .and_then(|node| match node {
 244             hir::Node::Item(hir::Item {
 245                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 246                 ..
 247             }) => Some(
 248                 generics
 249                     .predicates
 250                     .iter()
 251                     .filter_map(|pred| {
 252                         match pred {
 253                             hir::WherePredicate::BoundPredicate(pred)
 254                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 255                             {
 256                                 // Fetch spans for trait bounds that are Sized:
 257                                 // `trait T where Self: Pred`
 258                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 259                             }
 260                             _ => None,
 261                         }
 262                     })
 263                     .flatten()
 264                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 265                     .chain(sized_trait_bound_spans(tcx, bounds))
 266                     .collect::<SmallVec<[Span; 1]>>(),
 267             ),
 268             _ => None,
 269         })
 270         .unwrap_or_else(SmallVec::new)
 271 }
 272
 273 fn predicates_reference_self(
 274     tcx: TyCtxt<'_>,
 275     trait_def_id: DefId,
 276     supertraits_only: bool,
 277 ) -> SmallVec<[Span; 1]> {
 278     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 279     let predicates = if supertraits_only {
 280         tcx.super_predicates_of(trait_def_id)
 281     } else {
 282         tcx.predicates_of(trait_def_id)
 283     };
 284     predicates
 285         .predicates
 286         .iter()
 287         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 288         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 289         .collect()
 290 }
 291
 292 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 293     tcx.associated_items(trait_def_id)
 294         .in_definition_order()
 295         .filter(|item| item.kind == ty::AssocKind::Type)
 296         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 297         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 298         .collect()
 299 }
 300
 301 fn predicate_references_self<'tcx>(
 302     tcx: TyCtxt<'tcx>,
 303     (predicate, sp): (ty::Predicate<'tcx>, Span),
 304 ) -> Option<Span> {
 305     let self_ty = tcx.types.self_param;
 306     let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
 307     match predicate.kind().skip_binder() {
 308         ty::PredicateKind::Clause(ty::Clause::Trait(ref data)) => {
 309             // In the case of a trait predicate, we can skip the "self" type.
 310             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 311         }
 312         ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
 313             // And similarly for projections. This should be redundant with
 314             // the previous check because any projection should have a
 315             // matching `Trait` predicate with the same inputs, but we do
 316             // the check to be safe.
 317             //
 318             // It's also won't be redundant if we allow type-generic associated
 319             // types for trait objects.
 320             //
 321             // Note that we *do* allow projection *outputs* to contain
 322             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 323             // we just require the user to specify *both* outputs
 324             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 325             //
 326             // This is ALT2 in issue #56288, see that for discussion of the
 327             // possible alternatives.
 328             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 329         }
 330         ty::PredicateKind::WellFormed(..)
 331         | ty::PredicateKind::ObjectSafe(..)
 332         | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
 333         | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
 334         | ty::PredicateKind::ClosureKind(..)
 335         | ty::PredicateKind::Subtype(..)
 336         | ty::PredicateKind::Coerce(..)
 337         | ty::PredicateKind::ConstEvaluatable(..)
 338         | ty::PredicateKind::ConstEquate(..)
 339         | ty::PredicateKind::Ambiguous
 340         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 341     }
 342 }
 343
 344 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 345     generics_require_sized_self(tcx, trait_def_id)
 346 }
 347
 348 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 349     let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
 350         return false; /* No Sized trait, can't require it! */
 351     };
 352
 353     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 354     let predicates = tcx.predicates_of(def_id);
 355     let predicates = predicates.instantiate_identity(tcx).predicates;
 356     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 357         match obligation.predicate.kind().skip_binder() {
 358             ty::PredicateKind::Clause(ty::Clause::Trait(ref trait_pred)) => {
 359                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 360             }
 361             ty::PredicateKind::Clause(ty::Clause::Projection(..))
 362             | ty::PredicateKind::Subtype(..)
 363             | ty::PredicateKind::Coerce(..)
 364             | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
 365             | ty::PredicateKind::WellFormed(..)
 366             | ty::PredicateKind::ObjectSafe(..)
 367             | ty::PredicateKind::ClosureKind(..)
 368             | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
 369             | ty::PredicateKind::ConstEvaluatable(..)
 370             | ty::PredicateKind::ConstEquate(..)
 371             | ty::PredicateKind::Ambiguous
 372             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 373         }
 374     })
 375 }
 376
 377 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 378 fn object_safety_violation_for_method(
 379     tcx: TyCtxt<'_>,
 380     trait_def_id: DefId,
 381     method: &ty::AssocItem,
 382 ) -> Option<(MethodViolationCode, Span)> {
 383     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 384     // Any method that has a `Self : Sized` requisite is otherwise
 385     // exempt from the regulations.
 386     if generics_require_sized_self(tcx, method.def_id) {
 387         return None;
 388     }
 389
 390     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 391     // Get an accurate span depending on the violation.
 392     violation.map(|v| {
 393         let node = tcx.hir().get_if_local(method.def_id);
 394         let span = match (&v, node) {
 395             (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
 396             (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
 397             (MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
 398             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 399                 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
 400             }
 401             _ => method.ident(tcx).span,
 402         };
 403         (v, span)
 404     })
 405 }
 406
 407 /// Returns `Some(_)` if this method cannot be called on a trait
 408 /// object; this does not necessarily imply that the enclosing trait
 409 /// is not object safe, because the method might have a where clause
 410 /// `Self:Sized`.
 411 fn virtual_call_violation_for_method<'tcx>(
 412     tcx: TyCtxt<'tcx>,
 413     trait_def_id: DefId,
 414     method: &ty::AssocItem,
 415 ) -> Option<MethodViolationCode> {
 416     let sig = tcx.fn_sig(method.def_id).subst_identity();
 417
 418     // The method's first parameter must be named `self`
 419     if !method.fn_has_self_parameter {
 420         let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
 421             generics,
 422             kind: hir::TraitItemKind::Fn(sig, _),
 423             ..
 424         })) = tcx.hir().get_if_local(method.def_id).as_ref()
 425         {
 426             let sm = tcx.sess.source_map();
 427             Some((
 428                 (
 429                     format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
 430                     sm.span_through_char(sig.span, '(').shrink_to_hi(),
 431                 ),
 432                 (
 433                     format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
 434                     generics.tail_span_for_predicate_suggestion(),
 435                 ),
 436             ))
 437         } else {
 438             None
 439         };
 440         return Some(MethodViolationCode::StaticMethod(sugg));
 441     }
 442
 443     for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
 444         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 445             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 446                 kind: hir::TraitItemKind::Fn(sig, _),
 447                 ..
 448             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 449             {
 450                 Some(sig.decl.inputs[i].span)
 451             } else {
 452                 None
 453             };
 454             return Some(MethodViolationCode::ReferencesSelfInput(span));
 455         }
 456     }
 457     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 458         return Some(MethodViolationCode::ReferencesSelfOutput);
 459     }
 460     if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
 461         return Some(code);
 462     }
 463
 464     // We can't monomorphize things like `fn foo<A>(...)`.
 465     let own_counts = tcx.generics_of(method.def_id).own_counts();
 466     if own_counts.types + own_counts.consts != 0 {
 467         return Some(MethodViolationCode::Generic);
 468     }
 469
 470     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 471
 472     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 473     // However, this is already considered object-safe. We allow it as a special case here.
 474     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 475     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 476     if receiver_ty != tcx.types.self_param {
 477         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 478             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 479                 kind: hir::TraitItemKind::Fn(sig, _),
 480                 ..
 481             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 482             {
 483                 Some(sig.decl.inputs[0].span)
 484             } else {
 485                 None
 486             };
 487             return Some(MethodViolationCode::UndispatchableReceiver(span));
 488         } else {
 489             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 490
 491             use rustc_target::abi::Abi;
 492
 493             let param_env = tcx.param_env(method.def_id);
 494
 495             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 496                 match tcx.layout_of(param_env.and(ty)) {
 497                     Ok(layout) => Some(layout.abi),
 498                     Err(err) => {
 499                         // #78372
 500                         tcx.sess.delay_span_bug(
 501                             tcx.def_span(method.def_id),
 502                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 503                         );
 504                         None
 505                     }
 506                 }
 507             };
 508
 509             // e.g., `Rc<()>`
 510             let unit_receiver_ty =
 511                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 512
 513             match abi_of_ty(unit_receiver_ty) {
 514                 Some(Abi::Scalar(..)) => (),
 515                 abi => {
 516                     tcx.sess.delay_span_bug(
 517                         tcx.def_span(method.def_id),
 518                         &format!(
 519                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 520                             abi
 521                         ),
 522                     );
 523                 }
 524             }
 525
 526             let trait_object_ty =
 527                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 528
 529             // e.g., `Rc<dyn Trait>`
 530             let trait_object_receiver =
 531                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 532
 533             match abi_of_ty(trait_object_receiver) {
 534                 Some(Abi::ScalarPair(..)) => (),
 535                 abi => {
 536                     tcx.sess.delay_span_bug(
 537                         tcx.def_span(method.def_id),
 538                         &format!(
 539                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 540                             trait_object_ty, abi
 541                         ),
 542                     );
 543                 }
 544             }
 545         }
 546     }
 547
 548     // NOTE: This check happens last, because it results in a lint, and not a
 549     // hard error.
 550     if tcx
 551         .predicates_of(method.def_id)
 552         .predicates
 553         .iter()
 554         // A trait object can't claim to live more than the concrete type,
 555         // so outlives predicates will always hold.
 556         .cloned()
 557         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 558         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 559     {
 560         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 561     }
 562
 563     None
 564 }
 565
 566 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 567 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 568 fn receiver_for_self_ty<'tcx>(
 569     tcx: TyCtxt<'tcx>,
 570     receiver_ty: Ty<'tcx>,
 571     self_ty: Ty<'tcx>,
 572     method_def_id: DefId,
 573 ) -> Ty<'tcx> {
 574     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 575     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 576         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 577     });
 578
 579     let result = EarlyBinder(receiver_ty).subst(tcx, substs);
 580     debug!(
 581         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 582         receiver_ty, self_ty, method_def_id, result
 583     );
 584     result
 585 }
 586
 587 /// Creates the object type for the current trait. For example,
 588 /// if the current trait is `Deref`, then this will be
 589 /// `dyn Deref<Target = Self::Target> + 'static`.
 590 #[instrument(level = "trace", skip(tcx), ret)]
 591 fn object_ty_for_trait<'tcx>(
 592     tcx: TyCtxt<'tcx>,
 593     trait_def_id: DefId,
 594     lifetime: ty::Region<'tcx>,
 595 ) -> Ty<'tcx> {
 596     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 597     debug!(?trait_ref);
 598
 599     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 600         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 601     });
 602     debug!(?trait_predicate);
 603
 604     let mut elaborated_predicates: Vec<_> = elaborate_trait_ref(tcx, trait_ref)
 605         .filter_map(|obligation| {
 606             debug!(?obligation);
 607             let pred = obligation.predicate.to_opt_poly_projection_pred()?;
 608             Some(pred.map_bound(|p| {
 609                 ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 610                     def_id: p.projection_ty.def_id,
 611                     substs: p.projection_ty.substs,
 612                     term: p.term,
 613                 })
 614             }))
 615         })
 616         .collect();
 617     // NOTE: Since #37965, the existential predicates list has depended on the
 618     // list of predicates to be sorted. This is mostly to enforce that the primary
 619     // predicate comes first.
 620     elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
 621     elaborated_predicates.dedup();
 622
 623     let existential_predicates = tcx
 624         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(elaborated_predicates));
 625     debug!(?existential_predicates);
 626
 627     tcx.mk_dynamic(existential_predicates, lifetime, ty::Dyn)
 628 }
 629
 630 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 631 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 632 /// in the following way:
 633 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 634 /// - require the following bound:
 635 ///
 636 ///   ```ignore (not-rust)
 637 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 638 ///   ```
 639 ///
 640 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 641 ///   (substitution notation).
 642 ///
 643 /// Some examples of receiver types and their required obligation:
 644 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 645 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 646 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 647 ///
 648 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 649 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 650 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 651 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 652 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 653 /// contained by the trait object, because the object that needs to be coerced is behind
 654 /// a pointer.
 655 ///
 656 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 657 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 658 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
 659 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 660 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 661 /// Written as a chalk-style query:
 662 /// ```ignore (not-rust)
 663 /// forall (U: Trait + ?Sized) {
 664 ///     if (Self: Unsize<U>) {
 665 ///         Receiver: DispatchFromDyn<Receiver[Self => U]>
 666 ///     }
 667 /// }
 668 /// ```
 669 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 670 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 671 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 672 //
 673 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 674 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 675 // `self: Wrapper<Self>`.
 676 #[allow(dead_code)]
 677 fn receiver_is_dispatchable<'tcx>(
 678     tcx: TyCtxt<'tcx>,
 679     method: &ty::AssocItem,
 680     receiver_ty: Ty<'tcx>,
 681 ) -> bool {
 682     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 683
 684     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 685     let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
 686         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 687         return false;
 688     };
 689
 690     // the type `U` in the query
 691     // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
 692     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 693     // replace this with `dyn Trait`
 694     let unsized_self_ty: Ty<'tcx> =
 695         tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 696
 697     // `Receiver[Self => U]`
 698     let unsized_receiver_ty =
 699         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 700
 701     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 702     // `U: ?Sized` is already implied here
 703     let param_env = {
 704         let param_env = tcx.param_env(method.def_id);
 705
 706         // Self: Unsize<U>
 707         let unsize_predicate = ty::Binder::dummy(
 708             tcx.mk_trait_ref(unsize_did, [tcx.types.self_param, unsized_self_ty]),
 709         )
 710         .without_const()
 711         .to_predicate(tcx);
 712
 713         // U: Trait<Arg1, ..., ArgN>
 714         let trait_predicate = {
 715             let trait_def_id = method.trait_container(tcx).unwrap();
 716             let substs = InternalSubsts::for_item(tcx, trait_def_id, |param, _| {
 717                 if param.index == 0 { unsized_self_ty.into() } else { tcx.mk_param_from_def(param) }
 718             });
 719
 720             ty::Binder::dummy(tcx.mk_trait_ref(trait_def_id, substs)).to_predicate(tcx)
 721         };
 722
 723         let caller_bounds: Vec<Predicate<'tcx>> =
 724             param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
 725
 726         ty::ParamEnv::new(
 727             tcx.intern_predicates(&caller_bounds),
 728             param_env.reveal(),
 729             param_env.constness(),
 730         )
 731     };
 732
 733     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 734     let obligation = {
 735         let predicate = ty::Binder::dummy(
 736             tcx.mk_trait_ref(dispatch_from_dyn_did, [receiver_ty, unsized_receiver_ty]),
 737         );
 738
 739         Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
 740     };
 741
 742     let infcx = tcx.infer_ctxt().build();
 743     // the receiver is dispatchable iff the obligation holds
 744     infcx.predicate_must_hold_modulo_regions(&obligation)
 745 }
 746
 747 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
 748     tcx: TyCtxt<'tcx>,
 749     trait_def_id: DefId,
 750     value: T,
 751 ) -> bool {
 752     // This is somewhat subtle. In general, we want to forbid
 753     // references to `Self` in the argument and return types,
 754     // since the value of `Self` is erased. However, there is one
 755     // exception: it is ok to reference `Self` in order to access
 756     // an associated type of the current trait, since we retain
 757     // the value of those associated types in the object type
 758     // itself.
 759     //
 760     // ```rust
 761     // trait SuperTrait {
 762     //     type X;
 763     // }
 764     //
 765     // trait Trait : SuperTrait {
 766     //     type Y;
 767     //     fn foo(&self, x: Self) // bad
 768     //     fn foo(&self) -> Self // bad
 769     //     fn foo(&self) -> Option<Self> // bad
 770     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 771     //     fn foo(&self) -> <Self as Trait>::Y // OK
 772     //     fn foo(&self) -> Self::X // OK, desugars to next example
 773     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 774     // }
 775     // ```
 776     //
 777     // However, it is not as simple as allowing `Self` in a projected
 778     // type, because there are illegal ways to use `Self` as well:
 779     //
 780     // ```rust
 781     // trait Trait : SuperTrait {
 782     //     ...
 783     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 784     // }
 785     // ```
 786     //
 787     // Here we will not have the type of `X` recorded in the
 788     // object type, and we cannot resolve `Self as SomeOtherTrait`
 789     // without knowing what `Self` is.
 790
 791     struct IllegalSelfTypeVisitor<'tcx> {
 792         tcx: TyCtxt<'tcx>,
 793         trait_def_id: DefId,
 794         supertraits: Option<Vec<DefId>>,
 795     }
 796
 797     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 798         type BreakTy = ();
 799
 800         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 801             match t.kind() {
 802                 ty::Param(_) => {
 803                     if t == self.tcx.types.self_param {
 804                         ControlFlow::Break(())
 805                     } else {
 806                         ControlFlow::Continue(())
 807                     }
 808                 }
 809                 ty::Alias(ty::Projection, ref data)
 810                     if self.tcx.def_kind(data.def_id) == DefKind::ImplTraitPlaceholder =>
 811                 {
 812                     // We'll deny these later in their own pass
 813                     ControlFlow::Continue(())
 814                 }
 815                 ty::Alias(ty::Projection, ref data) => {
 816                     // This is a projected type `<Foo as SomeTrait>::X`.
 817
 818                     // Compute supertraits of current trait lazily.
 819                     if self.supertraits.is_none() {
 820                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 821                         self.supertraits = Some(
 822                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 823                         );
 824                     }
 825
 826                     // Determine whether the trait reference `Foo as
 827                     // SomeTrait` is in fact a supertrait of the
 828                     // current trait. In that case, this type is
 829                     // legal, because the type `X` will be specified
 830                     // in the object type. Note that we can just use
 831                     // direct equality here because all of these types
 832                     // are part of the formal parameter listing, and
 833                     // hence there should be no inference variables.
 834                     let is_supertrait_of_current_trait = self
 835                         .supertraits
 836                         .as_ref()
 837                         .unwrap()
 838                         .contains(&data.trait_ref(self.tcx).def_id);
 839
 840                     if is_supertrait_of_current_trait {
 841                         ControlFlow::Continue(()) // do not walk contained types, do not report error, do collect $200
 842                     } else {
 843                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 844                     }
 845                 }
 846                 _ => t.super_visit_with(self), // walk contained types, if any
 847             }
 848         }
 849
 850         fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 851             // Constants can only influence object safety if they are generic and reference `Self`.
 852             // This is only possible for unevaluated constants, so we walk these here.
 853             self.tcx.expand_abstract_consts(ct).super_visit_with(self)
 854         }
 855     }
 856
 857     value
 858         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 859         .is_break()
 860 }
 861
 862 pub fn contains_illegal_impl_trait_in_trait<'tcx>(
 863     tcx: TyCtxt<'tcx>,
 864     fn_def_id: DefId,
 865     ty: ty::Binder<'tcx, Ty<'tcx>>,
 866 ) -> Option<MethodViolationCode> {
 867     // This would be caught below, but rendering the error as a separate
 868     // `async-specific` message is better.
 869     if tcx.asyncness(fn_def_id).is_async() {
 870         return Some(MethodViolationCode::AsyncFn);
 871     }
 872
 873     // FIXME(RPITIT): Perhaps we should use a visitor here?
 874     ty.skip_binder().walk().find_map(|arg| {
 875         if let ty::GenericArgKind::Type(ty) = arg.unpack()
 876             && let ty::Alias(ty::Projection, proj) = ty.kind()
 877             && tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
 878         {
 879             Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.def_id)))
 880         } else {
 881             None
 882         }
 883     })
 884 }
 885
 886 pub fn provide(providers: &mut ty::query::Providers) {
 887     *providers =
 888         ty::query::Providers { object_safety_violations, check_is_object_safe, ..*providers };
 889 }