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