src/librustc_infer/traits/object_safety.rs

   1 //! "Object safety" refers to the ability for a trait to be converted
   2 //! to an object. In general, traits may only be converted to an
   3 //! object if all of their methods meet certain criteria. In particular,
   4 //! they must:
   5 //!
   6 //!   - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version
   7 //!     that doesn't contain the vtable;
   8 //!   - not reference the erased type `Self` except for in this receiver;
   9 //!   - not have generic type parameters.
  10
  11 use super::elaborate_predicates;
  12
  13 use crate::infer::TyCtxtInferExt;
  14 use crate::traits::{self, Obligation, ObligationCause};
  15 use rustc::ty::subst::{InternalSubsts, Subst};
  16 use rustc::ty::{self, Predicate, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
  17 use rustc_errors::Applicability;
  18 use rustc_hir as hir;
  19 use rustc_hir::def_id::DefId;
  20 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
  21 use rustc_span::symbol::Symbol;
  22 use rustc_span::Span;
  23 use smallvec::SmallVec;
  24
  25 use std::iter;
  26
  27 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
  28
  29 /// Returns the object safety violations that affect
  30 /// astconv -- currently, `Self` in supertraits. This is needed
  31 /// because `object_safety_violations` can't be used during
  32 /// type collection.
  33 pub fn astconv_object_safety_violations(
  34     tcx: TyCtxt<'_>,
  35     trait_def_id: DefId,
  36 ) -> Vec<ObjectSafetyViolation> {
  37     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  38     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
  39         .map(|def_id| predicates_reference_self(tcx, def_id, true))
  40         .filter(|spans| !spans.is_empty())
  41         .map(|spans| ObjectSafetyViolation::SupertraitSelf(spans))
  42         .collect();
  43
  44     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
  45
  46     violations
  47 }
  48
  49 fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> Vec<ObjectSafetyViolation> {
  50     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  51     debug!("object_safety_violations: {:?}", trait_def_id);
  52
  53     traits::supertrait_def_ids(tcx, trait_def_id)
  54         .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id))
  55         .collect()
  56 }
  57
  58 /// We say a method is *vtable safe* if it can be invoked on a trait
  59 /// object. Note that object-safe traits can have some
  60 /// non-vtable-safe methods, so long as they require `Self: Sized` or
  61 /// otherwise ensure that they cannot be used when `Self = Trait`.
  62 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
  63     debug_assert!(tcx.generics_of(trait_def_id).has_self);
  64     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
  65     // Any method that has a `Self: Sized` bound cannot be called.
  66     if generics_require_sized_self(tcx, method.def_id) {
  67         return false;
  68     }
  69
  70     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
  71         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
  72         Some(_) => false,
  73     }
  74 }
  75
  76 fn object_safety_violations_for_trait(
  77     tcx: TyCtxt<'_>,
  78     trait_def_id: DefId,
  79 ) -> Vec<ObjectSafetyViolation> {
  80     // Check methods for violations.
  81     let mut violations: Vec<_> = tcx
  82         .associated_items(trait_def_id)
  83         .in_definition_order()
  84         .filter(|item| item.kind == ty::AssocKind::Method)
  85         .filter_map(|item| {
  86             object_safety_violation_for_method(tcx, trait_def_id, &item)
  87                 .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span))
  88         })
  89         .filter(|violation| {
  90             if let ObjectSafetyViolation::Method(
  91                 _,
  92                 MethodViolationCode::WhereClauseReferencesSelf,
  93                 span,
  94             ) = violation
  95             {
  96                 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
  97                 // It's also hard to get a use site span, so we use the method definition span.
  98                 tcx.struct_span_lint_hir(
  99                     WHERE_CLAUSES_OBJECT_SAFETY,
 100                     hir::CRATE_HIR_ID,
 101                     *span,
 102                     |lint| {
 103                         let mut err = lint.build(&format!(
 104                             "the trait `{}` cannot be made into an object",
 105                             tcx.def_path_str(trait_def_id)
 106                         ));
 107                         let node = tcx.hir().get_if_local(trait_def_id);
 108                         let msg = if let Some(hir::Node::Item(item)) = node {
 109                             err.span_label(
 110                                 item.ident.span,
 111                                 "this trait cannot be made into an object...",
 112                             );
 113                             format!("...because {}", violation.error_msg())
 114                         } else {
 115                             format!(
 116                                 "the trait cannot be made into an object because {}",
 117                                 violation.error_msg()
 118                             )
 119                         };
 120                         err.span_label(*span, &msg);
 121                         match (node, violation.solution()) {
 122                             (Some(_), Some((note, None))) => {
 123                                 err.help(&note);
 124                             }
 125                             (Some(_), Some((note, Some((sugg, span))))) => {
 126                                 err.span_suggestion(
 127                                     span,
 128                                     &note,
 129                                     sugg,
 130                                     Applicability::MachineApplicable,
 131                                 );
 132                             }
 133                             // Only provide the help if its a local trait, otherwise it's not actionable.
 134                             _ => {}
 135                         }
 136                         err.emit();
 137                     },
 138                 );
 139                 false
 140             } else {
 141                 true
 142             }
 143         })
 144         .collect();
 145
 146     // Check the trait itself.
 147     if trait_has_sized_self(tcx, trait_def_id) {
 148         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 149         let spans = get_sized_bounds(tcx, trait_def_id);
 150         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 151     }
 152     let spans = predicates_reference_self(tcx, trait_def_id, false);
 153     if !spans.is_empty() {
 154         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 155     }
 156
 157     violations.extend(
 158         tcx.associated_items(trait_def_id)
 159             .in_definition_order()
 160             .filter(|item| item.kind == ty::AssocKind::Const)
 161             .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 162     );
 163
 164     debug!(
 165         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 166         trait_def_id, violations
 167     );
 168
 169     violations
 170 }
 171
 172 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 173     tcx.hir()
 174         .get_if_local(trait_def_id)
 175         .and_then(|node| match node {
 176             hir::Node::Item(hir::Item {
 177                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 178                 ..
 179             }) => Some(
 180                 generics
 181                     .where_clause
 182                     .predicates
 183                     .iter()
 184                     .filter_map(|pred| {
 185                         match pred {
 186                             hir::WherePredicate::BoundPredicate(pred)
 187                                 if pred.bounded_ty.hir_id.owner_def_id() == trait_def_id =>
 188                             {
 189                                 // Fetch spans for trait bounds that are Sized:
 190                                 // `trait T where Self: Pred`
 191                                 Some(pred.bounds.iter().filter_map(|b| match b {
 192                                     hir::GenericBound::Trait(
 193                                         trait_ref,
 194                                         hir::TraitBoundModifier::None,
 195                                     ) if trait_has_sized_self(
 196                                         tcx,
 197                                         trait_ref.trait_ref.trait_def_id(),
 198                                     ) =>
 199                                     {
 200                                         Some(trait_ref.span)
 201                                     }
 202                                     _ => None,
 203                                 }))
 204                             }
 205                             _ => None,
 206                         }
 207                     })
 208                     .flatten()
 209                     .chain(bounds.iter().filter_map(|b| match b {
 210                         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 211                             if trait_has_sized_self(tcx, trait_ref.trait_ref.trait_def_id()) =>
 212                         {
 213                             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 214                             Some(trait_ref.span)
 215                         }
 216                         _ => None,
 217                     }))
 218                     .collect::<SmallVec<[Span; 1]>>(),
 219             ),
 220             _ => None,
 221         })
 222         .unwrap_or_else(SmallVec::new)
 223 }
 224
 225 fn predicates_reference_self(
 226     tcx: TyCtxt<'_>,
 227     trait_def_id: DefId,
 228     supertraits_only: bool,
 229 ) -> SmallVec<[Span; 1]> {
 230     let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
 231     let predicates = if supertraits_only {
 232         tcx.super_predicates_of(trait_def_id)
 233     } else {
 234         tcx.predicates_of(trait_def_id)
 235     };
 236     let self_ty = tcx.types.self_param;
 237     let has_self_ty = |t: Ty<'_>| t.walk().any(|t| t == self_ty);
 238     predicates
 239         .predicates
 240         .iter()
 241         .map(|(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 242         .filter_map(|(predicate, &sp)| {
 243             match predicate {
 244                 ty::Predicate::Trait(ref data, _) => {
 245                     // In the case of a trait predicate, we can skip the "self" type.
 246                     if data.skip_binder().input_types().skip(1).any(has_self_ty) {
 247                         Some(sp)
 248                     } else {
 249                         None
 250                     }
 251                 }
 252                 ty::Predicate::Projection(ref data) => {
 253                     // And similarly for projections. This should be redundant with
 254                     // the previous check because any projection should have a
 255                     // matching `Trait` predicate with the same inputs, but we do
 256                     // the check to be safe.
 257                     //
 258                     // Note that we *do* allow projection *outputs* to contain
 259                     // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 260                     // we just require the user to specify *both* outputs
 261                     // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 262                     //
 263                     // This is ALT2 in issue #56288, see that for discussion of the
 264                     // possible alternatives.
 265                     if data
 266                         .skip_binder()
 267                         .projection_ty
 268                         .trait_ref(tcx)
 269                         .input_types()
 270                         .skip(1)
 271                         .any(has_self_ty)
 272                     {
 273                         Some(sp)
 274                     } else {
 275                         None
 276                     }
 277                 }
 278                 ty::Predicate::WellFormed(..)
 279                 | ty::Predicate::ObjectSafe(..)
 280                 | ty::Predicate::TypeOutlives(..)
 281                 | ty::Predicate::RegionOutlives(..)
 282                 | ty::Predicate::ClosureKind(..)
 283                 | ty::Predicate::Subtype(..)
 284                 | ty::Predicate::ConstEvaluatable(..) => None,
 285             }
 286         })
 287         .collect()
 288 }
 289
 290 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 291     generics_require_sized_self(tcx, trait_def_id)
 292 }
 293
 294 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 295     let sized_def_id = match tcx.lang_items().sized_trait() {
 296         Some(def_id) => def_id,
 297         None => {
 298             return false; /* No Sized trait, can't require it! */
 299         }
 300     };
 301
 302     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 303     let predicates = tcx.predicates_of(def_id);
 304     let predicates = predicates.instantiate_identity(tcx).predicates;
 305     elaborate_predicates(tcx, predicates).any(|predicate| match predicate {
 306         ty::Predicate::Trait(ref trait_pred, _) => {
 307             trait_pred.def_id() == sized_def_id && trait_pred.skip_binder().self_ty().is_param(0)
 308         }
 309         ty::Predicate::Projection(..)
 310         | ty::Predicate::Subtype(..)
 311         | ty::Predicate::RegionOutlives(..)
 312         | ty::Predicate::WellFormed(..)
 313         | ty::Predicate::ObjectSafe(..)
 314         | ty::Predicate::ClosureKind(..)
 315         | ty::Predicate::TypeOutlives(..)
 316         | ty::Predicate::ConstEvaluatable(..) => false,
 317     })
 318 }
 319
 320 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 321 fn object_safety_violation_for_method(
 322     tcx: TyCtxt<'_>,
 323     trait_def_id: DefId,
 324     method: &ty::AssocItem,
 325 ) -> Option<(MethodViolationCode, Span)> {
 326     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 327     // Any method that has a `Self : Sized` requisite is otherwise
 328     // exempt from the regulations.
 329     if generics_require_sized_self(tcx, method.def_id) {
 330         return None;
 331     }
 332
 333     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 334     // Get an accurate span depending on the violation.
 335     violation.map(|v| {
 336         let node = tcx.hir().get_if_local(method.def_id);
 337         let span = match (v, node) {
 338             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 339                 .fn_decl()
 340                 .and_then(|decl| decl.inputs.get(arg + 1))
 341                 .map_or(method.ident.span, |arg| arg.span),
 342             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 343                 .fn_decl()
 344                 .and_then(|decl| decl.inputs.get(0))
 345                 .map_or(method.ident.span, |arg| arg.span),
 346             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 347                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 348             }
 349             _ => method.ident.span,
 350         };
 351         (v, span)
 352     })
 353 }
 354
 355 /// Returns `Some(_)` if this method cannot be called on a trait
 356 /// object; this does not necessarily imply that the enclosing trait
 357 /// is not object safe, because the method might have a where clause
 358 /// `Self:Sized`.
 359 fn virtual_call_violation_for_method<'tcx>(
 360     tcx: TyCtxt<'tcx>,
 361     trait_def_id: DefId,
 362     method: &ty::AssocItem,
 363 ) -> Option<MethodViolationCode> {
 364     // The method's first parameter must be named `self`
 365     if !method.method_has_self_argument {
 366         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 367         let sugg =
 368             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 369                 |generics| match generics.where_clause.predicates {
 370                     [] => (" where Self: Sized", generics.where_clause.span),
 371                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 372                 },
 373             );
 374         return Some(MethodViolationCode::StaticMethod(sugg));
 375     }
 376
 377     let sig = tcx.fn_sig(method.def_id);
 378
 379     for (i, input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 380         if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) {
 381             return Some(MethodViolationCode::ReferencesSelfInput(i));
 382         }
 383     }
 384     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output().skip_binder()) {
 385         return Some(MethodViolationCode::ReferencesSelfOutput);
 386     }
 387
 388     // We can't monomorphize things like `fn foo<A>(...)`.
 389     let own_counts = tcx.generics_of(method.def_id).own_counts();
 390     if own_counts.types + own_counts.consts != 0 {
 391         return Some(MethodViolationCode::Generic);
 392     }
 393
 394     if tcx
 395         .predicates_of(method.def_id)
 396         .predicates
 397         .iter()
 398         // A trait object can't claim to live more than the concrete type,
 399         // so outlives predicates will always hold.
 400         .cloned()
 401         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 402         .collect::<Vec<_>>()
 403         // Do a shallow visit so that `contains_illegal_self_type_reference`
 404         // may apply it's custom visiting.
 405         .visit_tys_shallow(|t| contains_illegal_self_type_reference(tcx, trait_def_id, t))
 406     {
 407         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 408     }
 409
 410     let receiver_ty =
 411         tcx.liberate_late_bound_regions(method.def_id, &sig.map_bound(|sig| sig.inputs()[0]));
 412
 413     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 414     // However, this is already considered object-safe. We allow it as a special case here.
 415     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 416     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 417     if receiver_ty != tcx.types.self_param {
 418         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 419             return Some(MethodViolationCode::UndispatchableReceiver);
 420         } else {
 421             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 422
 423             use rustc::ty::layout::Abi;
 424
 425             let param_env = tcx.param_env(method.def_id);
 426
 427             let abi_of_ty = |ty: Ty<'tcx>| -> &Abi {
 428                 match tcx.layout_of(param_env.and(ty)) {
 429                     Ok(layout) => &layout.abi,
 430                     Err(err) => bug!("error: {}\n while computing layout for type {:?}", err, ty),
 431                 }
 432             };
 433
 434             // e.g., `Rc<()>`
 435             let unit_receiver_ty =
 436                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 437
 438             match abi_of_ty(unit_receiver_ty) {
 439                 &Abi::Scalar(..) => (),
 440                 abi => {
 441                     tcx.sess.delay_span_bug(
 442                         tcx.def_span(method.def_id),
 443                         &format!(
 444                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 445                             abi
 446                         ),
 447                     );
 448                 }
 449             }
 450
 451             let trait_object_ty =
 452                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 453
 454             // e.g., `Rc<dyn Trait>`
 455             let trait_object_receiver =
 456                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 457
 458             match abi_of_ty(trait_object_receiver) {
 459                 &Abi::ScalarPair(..) => (),
 460                 abi => {
 461                     tcx.sess.delay_span_bug(
 462                         tcx.def_span(method.def_id),
 463                         &format!(
 464                             "receiver when `Self = {}` should have a ScalarPair ABI; \
 465                                  found {:?}",
 466                             trait_object_ty, abi
 467                         ),
 468                     );
 469                 }
 470             }
 471         }
 472     }
 473
 474     None
 475 }
 476
 477 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 478 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 479 fn receiver_for_self_ty<'tcx>(
 480     tcx: TyCtxt<'tcx>,
 481     receiver_ty: Ty<'tcx>,
 482     self_ty: Ty<'tcx>,
 483     method_def_id: DefId,
 484 ) -> Ty<'tcx> {
 485     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 486     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 487         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 488     });
 489
 490     let result = receiver_ty.subst(tcx, substs);
 491     debug!(
 492         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 493         receiver_ty, self_ty, method_def_id, result
 494     );
 495     result
 496 }
 497
 498 /// Creates the object type for the current trait. For example,
 499 /// if the current trait is `Deref`, then this will be
 500 /// `dyn Deref<Target = Self::Target> + 'static`.
 501 fn object_ty_for_trait<'tcx>(
 502     tcx: TyCtxt<'tcx>,
 503     trait_def_id: DefId,
 504     lifetime: ty::Region<'tcx>,
 505 ) -> Ty<'tcx> {
 506     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 507
 508     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 509
 510     let trait_predicate =
 511         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 512
 513     let mut associated_types = traits::supertraits(tcx, ty::Binder::dummy(trait_ref))
 514         .flat_map(|super_trait_ref| {
 515             tcx.associated_items(super_trait_ref.def_id())
 516                 .in_definition_order()
 517                 .map(move |item| (super_trait_ref, item))
 518         })
 519         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 520         .collect::<Vec<_>>();
 521
 522     // existential predicates need to be in a specific order
 523     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 524
 525     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 526         // We *can* get bound lifetimes here in cases like
 527         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 528         //
 529         // binder moved to (*)...
 530         let super_trait_ref = super_trait_ref.skip_binder();
 531         ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 532             ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 533             item_def_id: item.def_id,
 534             substs: super_trait_ref.substs,
 535         })
 536     });
 537
 538     let existential_predicates =
 539         tcx.mk_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 540
 541     let object_ty = tcx.mk_dynamic(
 542         // (*) ... binder re-introduced here
 543         ty::Binder::bind(existential_predicates),
 544         lifetime,
 545     );
 546
 547     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 548
 549     object_ty
 550 }
 551
 552 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 553 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 554 /// in the following way:
 555 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 556 /// - require the following bound:
 557 ///
 558 ///   ```
 559 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 560 ///   ```
 561 ///
 562 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 563 ///   (substitution notation).
 564 ///
 565 /// Some examples of receiver types and their required obligation:
 566 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 567 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 568 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 569 ///
 570 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 571 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 572 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 573 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 574 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 575 /// contained by the trait object, because the object that needs to be coerced is behind
 576 /// a pointer.
 577 ///
 578 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 579 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 580 /// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
 581 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 582 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 583 /// Written as a chalk-style query:
 584 ///
 585 ///     forall (U: Trait + ?Sized) {
 586 ///         if (Self: Unsize<U>) {
 587 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 588 ///         }
 589 ///     }
 590 ///
 591 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 592 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 593 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 594 //
 595 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 596 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 597 // `self: Wrapper<Self>`.
 598 #[allow(dead_code)]
 599 fn receiver_is_dispatchable<'tcx>(
 600     tcx: TyCtxt<'tcx>,
 601     method: &ty::AssocItem,
 602     receiver_ty: Ty<'tcx>,
 603 ) -> bool {
 604     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 605
 606     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 607     let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
 608         (u, cu)
 609     } else {
 610         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 611         return false;
 612     };
 613
 614     // the type `U` in the query
 615     // use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
 616     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 617     // replace this with `dyn Trait`
 618     let unsized_self_ty: Ty<'tcx> =
 619         tcx.mk_ty_param(::std::u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 620
 621     // `Receiver[Self => U]`
 622     let unsized_receiver_ty =
 623         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 624
 625     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 626     // `U: ?Sized` is already implied here
 627     let param_env = {
 628         let mut param_env = tcx.param_env(method.def_id);
 629
 630         // Self: Unsize<U>
 631         let unsize_predicate = ty::TraitRef {
 632             def_id: unsize_did,
 633             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 634         }
 635         .without_const()
 636         .to_predicate();
 637
 638         // U: Trait<Arg1, ..., ArgN>
 639         let trait_predicate = {
 640             let substs =
 641                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 642                     if param.index == 0 {
 643                         unsized_self_ty.into()
 644                     } else {
 645                         tcx.mk_param_from_def(param)
 646                     }
 647                 });
 648
 649             ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate()
 650         };
 651
 652         let caller_bounds: Vec<Predicate<'tcx>> = param_env
 653             .caller_bounds
 654             .iter()
 655             .cloned()
 656             .chain(iter::once(unsize_predicate))
 657             .chain(iter::once(trait_predicate))
 658             .collect();
 659
 660         param_env.caller_bounds = tcx.intern_predicates(&caller_bounds);
 661
 662         param_env
 663     };
 664
 665     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 666     let obligation = {
 667         let predicate = ty::TraitRef {
 668             def_id: dispatch_from_dyn_did,
 669             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 670         }
 671         .without_const()
 672         .to_predicate();
 673
 674         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 675     };
 676
 677     tcx.infer_ctxt().enter(|ref infcx| {
 678         // the receiver is dispatchable iff the obligation holds
 679         infcx.predicate_must_hold_modulo_regions(&obligation)
 680     })
 681 }
 682
 683 fn contains_illegal_self_type_reference<'tcx>(
 684     tcx: TyCtxt<'tcx>,
 685     trait_def_id: DefId,
 686     ty: Ty<'tcx>,
 687 ) -> bool {
 688     // This is somewhat subtle. In general, we want to forbid
 689     // references to `Self` in the argument and return types,
 690     // since the value of `Self` is erased. However, there is one
 691     // exception: it is ok to reference `Self` in order to access
 692     // an associated type of the current trait, since we retain
 693     // the value of those associated types in the object type
 694     // itself.
 695     //
 696     // ```rust
 697     // trait SuperTrait {
 698     //     type X;
 699     // }
 700     //
 701     // trait Trait : SuperTrait {
 702     //     type Y;
 703     //     fn foo(&self, x: Self) // bad
 704     //     fn foo(&self) -> Self // bad
 705     //     fn foo(&self) -> Option<Self> // bad
 706     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 707     //     fn foo(&self) -> <Self as Trait>::Y // OK
 708     //     fn foo(&self) -> Self::X // OK, desugars to next example
 709     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 710     // }
 711     // ```
 712     //
 713     // However, it is not as simple as allowing `Self` in a projected
 714     // type, because there are illegal ways to use `Self` as well:
 715     //
 716     // ```rust
 717     // trait Trait : SuperTrait {
 718     //     ...
 719     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 720     // }
 721     // ```
 722     //
 723     // Here we will not have the type of `X` recorded in the
 724     // object type, and we cannot resolve `Self as SomeOtherTrait`
 725     // without knowing what `Self` is.
 726
 727     let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
 728     let mut error = false;
 729     let self_ty = tcx.types.self_param;
 730     ty.maybe_walk(|ty| {
 731         match ty.kind {
 732             ty::Param(_) => {
 733                 if ty == self_ty {
 734                     error = true;
 735                 }
 736
 737                 false // no contained types to walk
 738             }
 739
 740             ty::Projection(ref data) => {
 741                 // This is a projected type `<Foo as SomeTrait>::X`.
 742
 743                 // Compute supertraits of current trait lazily.
 744                 if supertraits.is_none() {
 745                     let trait_ref = ty::Binder::bind(ty::TraitRef::identity(tcx, trait_def_id));
 746                     supertraits = Some(traits::supertraits(tcx, trait_ref).collect());
 747                 }
 748
 749                 // Determine whether the trait reference `Foo as
 750                 // SomeTrait` is in fact a supertrait of the
 751                 // current trait. In that case, this type is
 752                 // legal, because the type `X` will be specified
 753                 // in the object type.  Note that we can just use
 754                 // direct equality here because all of these types
 755                 // are part of the formal parameter listing, and
 756                 // hence there should be no inference variables.
 757                 let projection_trait_ref = ty::Binder::bind(data.trait_ref(tcx));
 758                 let is_supertrait_of_current_trait =
 759                     supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 760
 761                 if is_supertrait_of_current_trait {
 762                     false // do not walk contained types, do not report error, do collect $200
 763                 } else {
 764                     true // DO walk contained types, POSSIBLY reporting an error
 765                 }
 766             }
 767
 768             _ => true, // walk contained types, if any
 769         }
 770     });
 771
 772     error
 773 }
 774
 775 pub fn provide(providers: &mut ty::query::Providers<'_>) {
 776     *providers = ty::query::Providers { object_safety_violations, ..*providers };
 777 }