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::abstract_const::{walk_abstract_const, AbstractConst};
  21 use rustc_middle::ty::{
  22     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
  23 };
  24 use rustc_middle::ty::{GenericArg, InternalSubsts};
  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(
 168         WHERE_CLAUSES_OBJECT_SAFETY,
 169         hir::CRATE_HIR_ID,
 170         span,
 171         DelayDm(|| format!("the trait `{}` cannot be made into an object", tcx.def_path_str(trait_def_id))),
 172         |err| {
 173             let node = tcx.hir().get_if_local(trait_def_id);
 174             let mut spans = MultiSpan::from_span(span);
 175             if let Some(hir::Node::Item(item)) = node {
 176                 spans.push_span_label(
 177                     item.ident.span,
 178                     "this trait cannot be made into an object...",
 179                 );
 180                 spans.push_span_label(span, format!("...because {}", violation.error_msg()));
 181             } else {
 182                 spans.push_span_label(
 183                     span,
 184                     format!(
 185                         "the trait cannot be made into an object because {}",
 186                         violation.error_msg()
 187                     ),
 188                 );
 189             };
 190             err.span_note(
 191                 spans,
 192                 "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
 193                 call to be resolvable dynamically; for more information visit \
 194                 <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
 195             );
 196             if node.is_some() {
 197                 // Only provide the help if its a local trait, otherwise it's not
 198                 violation.solution(err);
 199             }
 200             err
 201         },
 202     );
 203 }
 204
 205 fn sized_trait_bound_spans<'tcx>(
 206     tcx: TyCtxt<'tcx>,
 207     bounds: hir::GenericBounds<'tcx>,
 208 ) -> impl 'tcx + Iterator<Item = Span> {
 209     bounds.iter().filter_map(move |b| match b {
 210         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 211             if trait_has_sized_self(
 212                 tcx,
 213                 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 214             ) =>
 215         {
 216             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 217             Some(trait_ref.span)
 218         }
 219         _ => None,
 220     })
 221 }
 222
 223 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 224     tcx.hir()
 225         .get_if_local(trait_def_id)
 226         .and_then(|node| match node {
 227             hir::Node::Item(hir::Item {
 228                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 229                 ..
 230             }) => Some(
 231                 generics
 232                     .predicates
 233                     .iter()
 234                     .filter_map(|pred| {
 235                         match pred {
 236                             hir::WherePredicate::BoundPredicate(pred)
 237                                 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
 238                             {
 239                                 // Fetch spans for trait bounds that are Sized:
 240                                 // `trait T where Self: Pred`
 241                                 Some(sized_trait_bound_spans(tcx, pred.bounds))
 242                             }
 243                             _ => None,
 244                         }
 245                     })
 246                     .flatten()
 247                     // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
 248                     .chain(sized_trait_bound_spans(tcx, bounds))
 249                     .collect::<SmallVec<[Span; 1]>>(),
 250             ),
 251             _ => None,
 252         })
 253         .unwrap_or_else(SmallVec::new)
 254 }
 255
 256 fn predicates_reference_self(
 257     tcx: TyCtxt<'_>,
 258     trait_def_id: DefId,
 259     supertraits_only: bool,
 260 ) -> SmallVec<[Span; 1]> {
 261     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 262     let predicates = if supertraits_only {
 263         tcx.super_predicates_of(trait_def_id)
 264     } else {
 265         tcx.predicates_of(trait_def_id)
 266     };
 267     predicates
 268         .predicates
 269         .iter()
 270         .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 271         .filter_map(|predicate| predicate_references_self(tcx, predicate))
 272         .collect()
 273 }
 274
 275 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 276     tcx.associated_items(trait_def_id)
 277         .in_definition_order()
 278         .filter(|item| item.kind == ty::AssocKind::Type)
 279         .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
 280         .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
 281         .collect()
 282 }
 283
 284 fn predicate_references_self<'tcx>(
 285     tcx: TyCtxt<'tcx>,
 286     (predicate, sp): (ty::Predicate<'tcx>, Span),
 287 ) -> Option<Span> {
 288     let self_ty = tcx.types.self_param;
 289     let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
 290     match predicate.kind().skip_binder() {
 291         ty::PredicateKind::Trait(ref data) => {
 292             // In the case of a trait predicate, we can skip the "self" type.
 293             if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 294         }
 295         ty::PredicateKind::Projection(ref data) => {
 296             // And similarly for projections. This should be redundant with
 297             // the previous check because any projection should have a
 298             // matching `Trait` predicate with the same inputs, but we do
 299             // the check to be safe.
 300             //
 301             // It's also won't be redundant if we allow type-generic associated
 302             // types for trait objects.
 303             //
 304             // Note that we *do* allow projection *outputs* to contain
 305             // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 306             // we just require the user to specify *both* outputs
 307             // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 308             //
 309             // This is ALT2 in issue #56288, see that for discussion of the
 310             // possible alternatives.
 311             if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
 312         }
 313         ty::PredicateKind::WellFormed(..)
 314         | ty::PredicateKind::ObjectSafe(..)
 315         | ty::PredicateKind::TypeOutlives(..)
 316         | ty::PredicateKind::RegionOutlives(..)
 317         | ty::PredicateKind::ClosureKind(..)
 318         | ty::PredicateKind::Subtype(..)
 319         | ty::PredicateKind::Coerce(..)
 320         | ty::PredicateKind::ConstEvaluatable(..)
 321         | ty::PredicateKind::ConstEquate(..)
 322         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 323     }
 324 }
 325
 326 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 327     generics_require_sized_self(tcx, trait_def_id)
 328 }
 329
 330 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 331     let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
 332         return false; /* No Sized trait, can't require it! */
 333     };
 334
 335     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 336     let predicates = tcx.predicates_of(def_id);
 337     let predicates = predicates.instantiate_identity(tcx).predicates;
 338     elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
 339         match obligation.predicate.kind().skip_binder() {
 340             ty::PredicateKind::Trait(ref trait_pred) => {
 341                 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
 342             }
 343             ty::PredicateKind::Projection(..)
 344             | ty::PredicateKind::Subtype(..)
 345             | ty::PredicateKind::Coerce(..)
 346             | ty::PredicateKind::RegionOutlives(..)
 347             | ty::PredicateKind::WellFormed(..)
 348             | ty::PredicateKind::ObjectSafe(..)
 349             | ty::PredicateKind::ClosureKind(..)
 350             | ty::PredicateKind::TypeOutlives(..)
 351             | ty::PredicateKind::ConstEvaluatable(..)
 352             | ty::PredicateKind::ConstEquate(..)
 353             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
 354         }
 355     })
 356 }
 357
 358 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 359 fn object_safety_violation_for_method(
 360     tcx: TyCtxt<'_>,
 361     trait_def_id: DefId,
 362     method: &ty::AssocItem,
 363 ) -> Option<(MethodViolationCode, Span)> {
 364     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 365     // Any method that has a `Self : Sized` requisite is otherwise
 366     // exempt from the regulations.
 367     if generics_require_sized_self(tcx, method.def_id) {
 368         return None;
 369     }
 370
 371     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 372     // Get an accurate span depending on the violation.
 373     violation.map(|v| {
 374         let node = tcx.hir().get_if_local(method.def_id);
 375         let span = match (&v, node) {
 376             (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
 377             (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
 378             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 379                 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
 380             }
 381             _ => method.ident(tcx).span,
 382         };
 383         (v, span)
 384     })
 385 }
 386
 387 /// Returns `Some(_)` if this method cannot be called on a trait
 388 /// object; this does not necessarily imply that the enclosing trait
 389 /// is not object safe, because the method might have a where clause
 390 /// `Self:Sized`.
 391 fn virtual_call_violation_for_method<'tcx>(
 392     tcx: TyCtxt<'tcx>,
 393     trait_def_id: DefId,
 394     method: &ty::AssocItem,
 395 ) -> Option<MethodViolationCode> {
 396     let sig = tcx.fn_sig(method.def_id);
 397
 398     // The method's first parameter must be named `self`
 399     if !method.fn_has_self_parameter {
 400         let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
 401             generics,
 402             kind: hir::TraitItemKind::Fn(sig, _),
 403             ..
 404         })) = tcx.hir().get_if_local(method.def_id).as_ref()
 405         {
 406             let sm = tcx.sess.source_map();
 407             Some((
 408                 (
 409                     format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
 410                     sm.span_through_char(sig.span, '(').shrink_to_hi(),
 411                 ),
 412                 (
 413                     format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
 414                     generics.tail_span_for_predicate_suggestion(),
 415                 ),
 416             ))
 417         } else {
 418             None
 419         };
 420         return Some(MethodViolationCode::StaticMethod(sugg));
 421     }
 422
 423     for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
 424         if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
 425             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 426                 kind: hir::TraitItemKind::Fn(sig, _),
 427                 ..
 428             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 429             {
 430                 Some(sig.decl.inputs[i].span)
 431             } else {
 432                 None
 433             };
 434             return Some(MethodViolationCode::ReferencesSelfInput(span));
 435         }
 436     }
 437     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
 438         return Some(MethodViolationCode::ReferencesSelfOutput);
 439     }
 440     if contains_illegal_impl_trait_in_trait(tcx, sig.output()) {
 441         return Some(MethodViolationCode::ReferencesImplTraitInTrait);
 442     }
 443
 444     // We can't monomorphize things like `fn foo<A>(...)`.
 445     let own_counts = tcx.generics_of(method.def_id).own_counts();
 446     if own_counts.types + own_counts.consts != 0 {
 447         return Some(MethodViolationCode::Generic);
 448     }
 449
 450     let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
 451
 452     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 453     // However, this is already considered object-safe. We allow it as a special case here.
 454     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 455     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 456     if receiver_ty != tcx.types.self_param {
 457         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 458             let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
 459                 kind: hir::TraitItemKind::Fn(sig, _),
 460                 ..
 461             })) = tcx.hir().get_if_local(method.def_id).as_ref()
 462             {
 463                 Some(sig.decl.inputs[0].span)
 464             } else {
 465                 None
 466             };
 467             return Some(MethodViolationCode::UndispatchableReceiver(span));
 468         } else {
 469             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 470
 471             use rustc_target::abi::Abi;
 472
 473             let param_env = tcx.param_env(method.def_id);
 474
 475             let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
 476                 match tcx.layout_of(param_env.and(ty)) {
 477                     Ok(layout) => Some(layout.abi),
 478                     Err(err) => {
 479                         // #78372
 480                         tcx.sess.delay_span_bug(
 481                             tcx.def_span(method.def_id),
 482                             &format!("error: {}\n while computing layout for type {:?}", err, ty),
 483                         );
 484                         None
 485                     }
 486                 }
 487             };
 488
 489             // e.g., `Rc<()>`
 490             let unit_receiver_ty =
 491                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 492
 493             match abi_of_ty(unit_receiver_ty) {
 494                 Some(Abi::Scalar(..)) => (),
 495                 abi => {
 496                     tcx.sess.delay_span_bug(
 497                         tcx.def_span(method.def_id),
 498                         &format!(
 499                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 500                             abi
 501                         ),
 502                     );
 503                 }
 504             }
 505
 506             let trait_object_ty =
 507                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 508
 509             // e.g., `Rc<dyn Trait>`
 510             let trait_object_receiver =
 511                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 512
 513             match abi_of_ty(trait_object_receiver) {
 514                 Some(Abi::ScalarPair(..)) => (),
 515                 abi => {
 516                     tcx.sess.delay_span_bug(
 517                         tcx.def_span(method.def_id),
 518                         &format!(
 519                             "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
 520                             trait_object_ty, abi
 521                         ),
 522                     );
 523                 }
 524             }
 525         }
 526     }
 527
 528     // NOTE: This check happens last, because it results in a lint, and not a
 529     // hard error.
 530     if tcx
 531         .predicates_of(method.def_id)
 532         .predicates
 533         .iter()
 534         // A trait object can't claim to live more than the concrete type,
 535         // so outlives predicates will always hold.
 536         .cloned()
 537         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 538         .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
 539     {
 540         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 541     }
 542
 543     None
 544 }
 545
 546 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 547 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 548 fn receiver_for_self_ty<'tcx>(
 549     tcx: TyCtxt<'tcx>,
 550     receiver_ty: Ty<'tcx>,
 551     self_ty: Ty<'tcx>,
 552     method_def_id: DefId,
 553 ) -> Ty<'tcx> {
 554     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 555     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 556         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 557     });
 558
 559     let result = EarlyBinder(receiver_ty).subst(tcx, substs);
 560     debug!(
 561         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 562         receiver_ty, self_ty, method_def_id, result
 563     );
 564     result
 565 }
 566
 567 /// Creates the object type for the current trait. For example,
 568 /// if the current trait is `Deref`, then this will be
 569 /// `dyn Deref<Target = Self::Target> + 'static`.
 570 #[instrument(level = "trace", skip(tcx), ret)]
 571 fn object_ty_for_trait<'tcx>(
 572     tcx: TyCtxt<'tcx>,
 573     trait_def_id: DefId,
 574     lifetime: ty::Region<'tcx>,
 575 ) -> Ty<'tcx> {
 576     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 577     debug!(?trait_ref);
 578
 579     let trait_predicate = trait_ref.map_bound(|trait_ref| {
 580         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
 581     });
 582     debug!(?trait_predicate);
 583
 584     let mut elaborated_predicates: Vec<_> = elaborate_trait_ref(tcx, trait_ref)
 585         .filter_map(|obligation| {
 586             debug!(?obligation);
 587             let pred = obligation.predicate.to_opt_poly_projection_pred()?;
 588             Some(pred.map_bound(|p| {
 589                 ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 590                     item_def_id: p.projection_ty.item_def_id,
 591                     substs: p.projection_ty.substs,
 592                     term: p.term,
 593                 })
 594             }))
 595         })
 596         .collect();
 597     // NOTE: Since #37965, the existential predicates list has depended on the
 598     // list of predicates to be sorted. This is mostly to enforce that the primary
 599     // predicate comes first.
 600     elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
 601     elaborated_predicates.dedup();
 602
 603     let existential_predicates = tcx
 604         .mk_poly_existential_predicates(iter::once(trait_predicate).chain(elaborated_predicates));
 605     debug!(?existential_predicates);
 606
 607     tcx.mk_dynamic(existential_predicates, lifetime, ty::Dyn)
 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
 728         Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
 729     };
 730
 731     let infcx = tcx.infer_ctxt().build();
 732     // the receiver is dispatchable iff the obligation holds
 733     infcx.predicate_must_hold_modulo_regions(&obligation)
 734 }
 735
 736 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
 737     tcx: TyCtxt<'tcx>,
 738     trait_def_id: DefId,
 739     value: T,
 740 ) -> bool {
 741     // This is somewhat subtle. In general, we want to forbid
 742     // references to `Self` in the argument and return types,
 743     // since the value of `Self` is erased. However, there is one
 744     // exception: it is ok to reference `Self` in order to access
 745     // an associated type of the current trait, since we retain
 746     // the value of those associated types in the object type
 747     // itself.
 748     //
 749     // ```rust
 750     // trait SuperTrait {
 751     //     type X;
 752     // }
 753     //
 754     // trait Trait : SuperTrait {
 755     //     type Y;
 756     //     fn foo(&self, x: Self) // bad
 757     //     fn foo(&self) -> Self // bad
 758     //     fn foo(&self) -> Option<Self> // bad
 759     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 760     //     fn foo(&self) -> <Self as Trait>::Y // OK
 761     //     fn foo(&self) -> Self::X // OK, desugars to next example
 762     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 763     // }
 764     // ```
 765     //
 766     // However, it is not as simple as allowing `Self` in a projected
 767     // type, because there are illegal ways to use `Self` as well:
 768     //
 769     // ```rust
 770     // trait Trait : SuperTrait {
 771     //     ...
 772     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 773     // }
 774     // ```
 775     //
 776     // Here we will not have the type of `X` recorded in the
 777     // object type, and we cannot resolve `Self as SomeOtherTrait`
 778     // without knowing what `Self` is.
 779
 780     struct IllegalSelfTypeVisitor<'tcx> {
 781         tcx: TyCtxt<'tcx>,
 782         trait_def_id: DefId,
 783         supertraits: Option<Vec<DefId>>,
 784     }
 785
 786     impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
 787         type BreakTy = ();
 788
 789         fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 790             match t.kind() {
 791                 ty::Param(_) => {
 792                     if t == self.tcx.types.self_param {
 793                         ControlFlow::BREAK
 794                     } else {
 795                         ControlFlow::CONTINUE
 796                     }
 797                 }
 798                 ty::Projection(ref data)
 799                     if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
 800                 {
 801                     // We'll deny these later in their own pass
 802                     ControlFlow::CONTINUE
 803                 }
 804                 ty::Projection(ref data) => {
 805                     // This is a projected type `<Foo as SomeTrait>::X`.
 806
 807                     // Compute supertraits of current trait lazily.
 808                     if self.supertraits.is_none() {
 809                         let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
 810                         self.supertraits = Some(
 811                             traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
 812                         );
 813                     }
 814
 815                     // Determine whether the trait reference `Foo as
 816                     // SomeTrait` is in fact a supertrait of the
 817                     // current trait. In that case, this type is
 818                     // legal, because the type `X` will be specified
 819                     // in the object type.  Note that we can just use
 820                     // direct equality here because all of these types
 821                     // are part of the formal parameter listing, and
 822                     // hence there should be no inference variables.
 823                     let is_supertrait_of_current_trait = self
 824                         .supertraits
 825                         .as_ref()
 826                         .unwrap()
 827                         .contains(&data.trait_ref(self.tcx).def_id);
 828
 829                     if is_supertrait_of_current_trait {
 830                         ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
 831                     } else {
 832                         t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
 833                     }
 834                 }
 835                 _ => t.super_visit_with(self), // walk contained types, if any
 836             }
 837         }
 838
 839         fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 840             // Constants can only influence object safety if they reference `Self`.
 841             // This is only possible for unevaluated constants, so we walk these here.
 842             //
 843             // If `AbstractConst::from_const` returned an error we already failed compilation
 844             // so we don't have to emit an additional error here.
 845             use rustc_middle::ty::abstract_const::Node;
 846             if let Ok(Some(ct)) = AbstractConst::from_const(self.tcx, ct) {
 847                 walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
 848                     Node::Leaf(leaf) => self.visit_const(leaf),
 849                     Node::Cast(_, _, ty) => self.visit_ty(ty),
 850                     Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
 851                         ControlFlow::CONTINUE
 852                     }
 853                 })
 854             } else {
 855                 ct.super_visit_with(self)
 856             }
 857         }
 858     }
 859
 860     value
 861         .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
 862         .is_break()
 863 }
 864
 865 pub fn contains_illegal_impl_trait_in_trait<'tcx>(
 866     tcx: TyCtxt<'tcx>,
 867     ty: ty::Binder<'tcx, Ty<'tcx>>,
 868 ) -> bool {
 869     // FIXME(RPITIT): Perhaps we should use a visitor here?
 870     ty.skip_binder().walk().any(|arg| {
 871         if let ty::GenericArgKind::Type(ty) = arg.unpack()
 872             && let ty::Projection(proj) = ty.kind()
 873         {
 874             tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
 875         } else {
 876             false
 877         }
 878     })
 879 }
 880
 881 pub fn provide(providers: &mut ty::query::Providers) {
 882     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 883 }