src/librustc/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::traits::{self, Obligation, ObligationCause};
  14 use crate::ty::subst::{InternalSubsts, Subst};
  15 use crate::ty::{self, Predicate, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
  16 use rustc_errors::Applicability;
  17 use rustc_hir as hir;
  18 use rustc_hir::def_id::DefId;
  19 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
  20 use rustc_span::symbol::Symbol;
  21 use rustc_span::{Span, DUMMY_SP};
  22 use smallvec::{smallvec, SmallVec};
  23 use syntax::ast;
  24
  25 use std::borrow::Cow;
  26 use std::iter::{self};
  27
  28 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
  29 pub enum ObjectSafetyViolation {
  30     /// `Self: Sized` declared on the trait.
  31     SizedSelf(SmallVec<[Span; 1]>),
  32
  33     /// Supertrait reference references `Self` an in illegal location
  34     /// (e.g., `trait Foo : Bar<Self>`).
  35     SupertraitSelf,
  36
  37     /// Method has something illegal.
  38     Method(ast::Name, MethodViolationCode, Span),
  39
  40     /// Associated const.
  41     AssocConst(ast::Name, Span),
  42 }
  43
  44 impl ObjectSafetyViolation {
  45     pub fn error_msg(&self) -> Cow<'static, str> {
  46         match *self {
  47             ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
  48             ObjectSafetyViolation::SupertraitSelf => {
  49                 "it cannot use `Self` as a type parameter in the supertraits or `where`-clauses"
  50                     .into()
  51             }
  52             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
  53                 format!("associated function `{}` has no `self` parameter", name).into()
  54             }
  55             ObjectSafetyViolation::Method(
  56                 name,
  57                 MethodViolationCode::ReferencesSelfInput(_),
  58                 DUMMY_SP,
  59             ) => format!("method `{}` references the `Self` type in its parameters", name).into(),
  60             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfInput(_), _) => {
  61                 format!("method `{}` references the `Self` type in this parameter", name).into()
  62             }
  63             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
  64                 format!("method `{}` references the `Self` type in its return type", name).into()
  65             }
  66             ObjectSafetyViolation::Method(
  67                 name,
  68                 MethodViolationCode::WhereClauseReferencesSelf,
  69                 _,
  70             ) => {
  71                 format!("method `{}` references the `Self` type in its `where` clause", name).into()
  72             }
  73             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
  74                 format!("method `{}` has generic type parameters", name).into()
  75             }
  76             ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) => {
  77                 format!("method `{}`'s `self` parameter cannot be dispatched on", name).into()
  78             }
  79             ObjectSafetyViolation::AssocConst(_, DUMMY_SP) => {
  80                 "it cannot contain associated consts".into()
  81             }
  82             ObjectSafetyViolation::AssocConst(name, _) => {
  83                 format!("it cannot contain associated consts like `{}`", name).into()
  84             }
  85         }
  86     }
  87
  88     pub fn solution(&self) -> Option<(String, Option<(String, Span)>)> {
  89         Some(match *self {
  90             ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf => {
  91                 return None;
  92             }
  93             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(sugg), _) => (
  94                 format!(
  95                     "consider turning `{}` into a method by giving it a `&self` argument or \
  96                      constraining it so it does not apply to trait objects",
  97                     name
  98                 ),
  99                 sugg.map(|(sugg, sp)| (sugg.to_string(), sp)),
 100             ),
 101             ObjectSafetyViolation::Method(
 102                 name,
 103                 MethodViolationCode::UndispatchableReceiver,
 104                 span,
 105             ) => (
 106                 format!("consider changing method `{}`'s `self` parameter to be `&self`", name)
 107                     .into(),
 108                 Some(("&Self".to_string(), span)),
 109             ),
 110             ObjectSafetyViolation::AssocConst(name, _)
 111             | ObjectSafetyViolation::Method(name, ..) => {
 112                 (format!("consider moving `{}` to another trait", name), None)
 113             }
 114         })
 115     }
 116
 117     pub fn spans(&self) -> SmallVec<[Span; 1]> {
 118         // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
 119         // diagnostics use a `note` instead of a `span_label`.
 120         match self {
 121             ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
 122             ObjectSafetyViolation::AssocConst(_, span)
 123             | ObjectSafetyViolation::Method(_, _, span)
 124                 if *span != DUMMY_SP =>
 125             {
 126                 smallvec![*span]
 127             }
 128             _ => smallvec![],
 129         }
 130     }
 131 }
 132
 133 /// Reasons a method might not be object-safe.
 134 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 135 pub enum MethodViolationCode {
 136     /// e.g., `fn foo()`
 137     StaticMethod(Option<(&'static str, Span)>),
 138
 139     /// e.g., `fn foo(&self, x: Self)`
 140     ReferencesSelfInput(usize),
 141
 142     /// e.g., `fn foo(&self) -> Self`
 143     ReferencesSelfOutput,
 144
 145     /// e.g., `fn foo(&self) where Self: Clone`
 146     WhereClauseReferencesSelf,
 147
 148     /// e.g., `fn foo<A>()`
 149     Generic,
 150
 151     /// the method's receiver (`self` argument) can't be dispatched on
 152     UndispatchableReceiver,
 153 }
 154
 155 /// Returns the object safety violations that affect
 156 /// astconv -- currently, `Self` in supertraits. This is needed
 157 /// because `object_safety_violations` can't be used during
 158 /// type collection.
 159 pub fn astconv_object_safety_violations(
 160     tcx: TyCtxt<'_>,
 161     trait_def_id: DefId,
 162 ) -> Vec<ObjectSafetyViolation> {
 163     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 164     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
 165         .filter(|&def_id| predicates_reference_self(tcx, def_id, true))
 166         .map(|_| ObjectSafetyViolation::SupertraitSelf)
 167         .collect();
 168
 169     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
 170
 171     violations
 172 }
 173
 174 pub fn object_safety_violations(
 175     tcx: TyCtxt<'_>,
 176     trait_def_id: DefId,
 177 ) -> Vec<ObjectSafetyViolation> {
 178     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 179     debug!("object_safety_violations: {:?}", trait_def_id);
 180
 181     traits::supertrait_def_ids(tcx, trait_def_id)
 182         .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id))
 183         .collect()
 184 }
 185
 186 /// We say a method is *vtable safe* if it can be invoked on a trait
 187 /// object. Note that object-safe traits can have some
 188 /// non-vtable-safe methods, so long as they require `Self: Sized` or
 189 /// otherwise ensure that they cannot be used when `Self = Trait`.
 190 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
 191     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 192     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
 193     // Any method that has a `Self: Sized` bound cannot be called.
 194     if generics_require_sized_self(tcx, method.def_id) {
 195         return false;
 196     }
 197
 198     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
 199         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
 200         Some(_) => false,
 201     }
 202 }
 203
 204 fn object_safety_violations_for_trait(
 205     tcx: TyCtxt<'_>,
 206     trait_def_id: DefId,
 207 ) -> Vec<ObjectSafetyViolation> {
 208     // Check methods for violations.
 209     let mut violations: Vec<_> = tcx
 210         .associated_items(trait_def_id)
 211         .filter(|item| item.kind == ty::AssocKind::Method)
 212         .filter_map(|item| {
 213             object_safety_violation_for_method(tcx, trait_def_id, &item)
 214                 .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span))
 215         })
 216         .filter(|violation| {
 217             if let ObjectSafetyViolation::Method(
 218                 _,
 219                 MethodViolationCode::WhereClauseReferencesSelf,
 220                 span,
 221             ) = violation
 222             {
 223                 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 224                 // It's also hard to get a use site span, so we use the method definition span.
 225                 let mut err = tcx.struct_span_lint_hir(
 226                     WHERE_CLAUSES_OBJECT_SAFETY,
 227                     hir::CRATE_HIR_ID,
 228                     *span,
 229                     &format!(
 230                         "the trait `{}` cannot be made into an object",
 231                         tcx.def_path_str(trait_def_id)
 232                     ),
 233                 );
 234                 let node = tcx.hir().get_if_local(trait_def_id);
 235                 let msg = if let Some(hir::Node::Item(item)) = node {
 236                     err.span_label(item.ident.span, "this trait cannot be made into an object...");
 237                     format!("...because {}", violation.error_msg())
 238                 } else {
 239                     format!(
 240                         "the trait cannot be made into an object because {}",
 241                         violation.error_msg()
 242                     )
 243                 };
 244                 err.span_label(*span, &msg);
 245                 match (node, violation.solution()) {
 246                     (Some(_), Some((note, None))) => {
 247                         err.help(&note);
 248                     }
 249                     (Some(_), Some((note, Some((sugg, span))))) => {
 250                         err.span_suggestion(span, &note, sugg, Applicability::MachineApplicable);
 251                     }
 252                     // Only provide the help if its a local trait, otherwise it's not actionable.
 253                     _ => {}
 254                 }
 255                 err.emit();
 256                 false
 257             } else {
 258                 true
 259             }
 260         })
 261         .collect();
 262
 263     // Check the trait itself.
 264     if trait_has_sized_self(tcx, trait_def_id) {
 265         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 266         let spans = get_sized_bounds(tcx, trait_def_id);
 267         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 268     }
 269     if predicates_reference_self(tcx, trait_def_id, false) {
 270         violations.push(ObjectSafetyViolation::SupertraitSelf);
 271     }
 272
 273     violations.extend(
 274         tcx.associated_items(trait_def_id)
 275             .filter(|item| item.kind == ty::AssocKind::Const)
 276             .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 277     );
 278
 279     debug!(
 280         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 281         trait_def_id, violations
 282     );
 283
 284     violations
 285 }
 286
 287 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 288     tcx.hir()
 289         .get_if_local(trait_def_id)
 290         .and_then(|node| match node {
 291             hir::Node::Item(hir::Item {
 292                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 293                 ..
 294             }) => Some(
 295                 generics
 296                     .where_clause
 297                     .predicates
 298                     .iter()
 299                     .filter_map(|pred| {
 300                         match pred {
 301                             hir::WherePredicate::BoundPredicate(pred)
 302                                 if pred.bounded_ty.hir_id.owner_def_id() == trait_def_id =>
 303                             {
 304                                 // Fetch spans for trait bounds that are Sized:
 305                                 // `trait T where Self: Pred`
 306                                 Some(pred.bounds.iter().filter_map(|b| match b {
 307                                     hir::GenericBound::Trait(
 308                                         trait_ref,
 309                                         hir::TraitBoundModifier::None,
 310                                     ) if trait_has_sized_self(
 311                                         tcx,
 312                                         trait_ref.trait_ref.trait_def_id(),
 313                                     ) =>
 314                                     {
 315                                         Some(trait_ref.span)
 316                                     }
 317                                     _ => None,
 318                                 }))
 319                             }
 320                             _ => None,
 321                         }
 322                     })
 323                     .flatten()
 324                     .chain(bounds.iter().filter_map(|b| match b {
 325                         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 326                             if trait_has_sized_self(tcx, trait_ref.trait_ref.trait_def_id()) =>
 327                         {
 328                             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 329                             Some(trait_ref.span)
 330                         }
 331                         _ => None,
 332                     }))
 333                     .collect::<SmallVec<[Span; 1]>>(),
 334             ),
 335             _ => None,
 336         })
 337         .unwrap_or_else(SmallVec::new)
 338 }
 339
 340 fn predicates_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId, supertraits_only: bool) -> bool {
 341     let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
 342     let predicates = if supertraits_only {
 343         tcx.super_predicates_of(trait_def_id)
 344     } else {
 345         tcx.predicates_of(trait_def_id)
 346     };
 347     let self_ty = tcx.types.self_param;
 348     let has_self_ty = |t: Ty<'_>| t.walk().any(|t| t == self_ty);
 349     predicates
 350         .predicates
 351         .iter()
 352         .map(|(predicate, _)| predicate.subst_supertrait(tcx, &trait_ref))
 353         .any(|predicate| {
 354             match predicate {
 355                 ty::Predicate::Trait(ref data, _) => {
 356                     // In the case of a trait predicate, we can skip the "self" type.
 357                     data.skip_binder().input_types().skip(1).any(has_self_ty)
 358                 }
 359                 ty::Predicate::Projection(ref data) => {
 360                     // And similarly for projections. This should be redundant with
 361                     // the previous check because any projection should have a
 362                     // matching `Trait` predicate with the same inputs, but we do
 363                     // the check to be safe.
 364                     //
 365                     // Note that we *do* allow projection *outputs* to contain
 366                     // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 367                     // we just require the user to specify *both* outputs
 368                     // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 369                     //
 370                     // This is ALT2 in issue #56288, see that for discussion of the
 371                     // possible alternatives.
 372                     data.skip_binder()
 373                         .projection_ty
 374                         .trait_ref(tcx)
 375                         .input_types()
 376                         .skip(1)
 377                         .any(has_self_ty)
 378                 }
 379                 ty::Predicate::WellFormed(..)
 380                 | ty::Predicate::ObjectSafe(..)
 381                 | ty::Predicate::TypeOutlives(..)
 382                 | ty::Predicate::RegionOutlives(..)
 383                 | ty::Predicate::ClosureKind(..)
 384                 | ty::Predicate::Subtype(..)
 385                 | ty::Predicate::ConstEvaluatable(..) => false,
 386             }
 387         })
 388 }
 389
 390 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 391     generics_require_sized_self(tcx, trait_def_id)
 392 }
 393
 394 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 395     let sized_def_id = match tcx.lang_items().sized_trait() {
 396         Some(def_id) => def_id,
 397         None => {
 398             return false; /* No Sized trait, can't require it! */
 399         }
 400     };
 401
 402     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 403     let predicates = tcx.predicates_of(def_id);
 404     let predicates = predicates.instantiate_identity(tcx).predicates;
 405     elaborate_predicates(tcx, predicates).any(|predicate| match predicate {
 406         ty::Predicate::Trait(ref trait_pred, _) => {
 407             trait_pred.def_id() == sized_def_id && trait_pred.skip_binder().self_ty().is_param(0)
 408         }
 409         ty::Predicate::Projection(..)
 410         | ty::Predicate::Subtype(..)
 411         | ty::Predicate::RegionOutlives(..)
 412         | ty::Predicate::WellFormed(..)
 413         | ty::Predicate::ObjectSafe(..)
 414         | ty::Predicate::ClosureKind(..)
 415         | ty::Predicate::TypeOutlives(..)
 416         | ty::Predicate::ConstEvaluatable(..) => false,
 417     })
 418 }
 419
 420 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 421 fn object_safety_violation_for_method(
 422     tcx: TyCtxt<'_>,
 423     trait_def_id: DefId,
 424     method: &ty::AssocItem,
 425 ) -> Option<(MethodViolationCode, Span)> {
 426     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 427     // Any method that has a `Self : Sized` requisite is otherwise
 428     // exempt from the regulations.
 429     if generics_require_sized_self(tcx, method.def_id) {
 430         return None;
 431     }
 432
 433     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 434     // Get an accurate span depending on the violation.
 435     violation.map(|v| {
 436         let node = tcx.hir().get_if_local(method.def_id);
 437         let span = match (v, node) {
 438             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 439                 .fn_decl()
 440                 .and_then(|decl| decl.inputs.get(arg + 1))
 441                 .map_or(method.ident.span, |arg| arg.span),
 442             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 443                 .fn_decl()
 444                 .and_then(|decl| decl.inputs.get(0))
 445                 .map_or(method.ident.span, |arg| arg.span),
 446             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 447                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 448             }
 449             _ => method.ident.span,
 450         };
 451         (v, span)
 452     })
 453 }
 454
 455 /// Returns `Some(_)` if this method cannot be called on a trait
 456 /// object; this does not necessarily imply that the enclosing trait
 457 /// is not object safe, because the method might have a where clause
 458 /// `Self:Sized`.
 459 fn virtual_call_violation_for_method<'tcx>(
 460     tcx: TyCtxt<'tcx>,
 461     trait_def_id: DefId,
 462     method: &ty::AssocItem,
 463 ) -> Option<MethodViolationCode> {
 464     // The method's first parameter must be named `self`
 465     if !method.method_has_self_argument {
 466         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 467         let sugg =
 468             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 469                 |generics| match generics.where_clause.predicates {
 470                     [] => (" where Self: Sized", generics.where_clause.span),
 471                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 472                 },
 473             );
 474         return Some(MethodViolationCode::StaticMethod(sugg));
 475     }
 476
 477     let sig = tcx.fn_sig(method.def_id);
 478
 479     for (i, input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 480         if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) {
 481             return Some(MethodViolationCode::ReferencesSelfInput(i));
 482         }
 483     }
 484     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output().skip_binder()) {
 485         return Some(MethodViolationCode::ReferencesSelfOutput);
 486     }
 487
 488     // We can't monomorphize things like `fn foo<A>(...)`.
 489     let own_counts = tcx.generics_of(method.def_id).own_counts();
 490     if own_counts.types + own_counts.consts != 0 {
 491         return Some(MethodViolationCode::Generic);
 492     }
 493
 494     if tcx
 495         .predicates_of(method.def_id)
 496         .predicates
 497         .iter()
 498         // A trait object can't claim to live more than the concrete type,
 499         // so outlives predicates will always hold.
 500         .cloned()
 501         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 502         .collect::<Vec<_>>()
 503         // Do a shallow visit so that `contains_illegal_self_type_reference`
 504         // may apply it's custom visiting.
 505         .visit_tys_shallow(|t| contains_illegal_self_type_reference(tcx, trait_def_id, t))
 506     {
 507         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 508     }
 509
 510     let receiver_ty =
 511         tcx.liberate_late_bound_regions(method.def_id, &sig.map_bound(|sig| sig.inputs()[0]));
 512
 513     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 514     // However, this is already considered object-safe. We allow it as a special case here.
 515     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 516     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 517     if receiver_ty != tcx.types.self_param {
 518         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 519             return Some(MethodViolationCode::UndispatchableReceiver);
 520         } else {
 521             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 522
 523             use crate::ty::layout::Abi;
 524
 525             let param_env = tcx.param_env(method.def_id);
 526
 527             let abi_of_ty = |ty: Ty<'tcx>| -> &Abi {
 528                 match tcx.layout_of(param_env.and(ty)) {
 529                     Ok(layout) => &layout.abi,
 530                     Err(err) => bug!("error: {}\n while computing layout for type {:?}", err, ty),
 531                 }
 532             };
 533
 534             // e.g., `Rc<()>`
 535             let unit_receiver_ty =
 536                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 537
 538             match abi_of_ty(unit_receiver_ty) {
 539                 &Abi::Scalar(..) => (),
 540                 abi => {
 541                     tcx.sess.delay_span_bug(
 542                         tcx.def_span(method.def_id),
 543                         &format!(
 544                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 545                             abi
 546                         ),
 547                     );
 548                 }
 549             }
 550
 551             let trait_object_ty =
 552                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 553
 554             // e.g., `Rc<dyn Trait>`
 555             let trait_object_receiver =
 556                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 557
 558             match abi_of_ty(trait_object_receiver) {
 559                 &Abi::ScalarPair(..) => (),
 560                 abi => {
 561                     tcx.sess.delay_span_bug(
 562                         tcx.def_span(method.def_id),
 563                         &format!(
 564                             "receiver when `Self = {}` should have a ScalarPair ABI; \
 565                                  found {:?}",
 566                             trait_object_ty, abi
 567                         ),
 568                     );
 569                 }
 570             }
 571         }
 572     }
 573
 574     None
 575 }
 576
 577 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 578 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 579 fn receiver_for_self_ty<'tcx>(
 580     tcx: TyCtxt<'tcx>,
 581     receiver_ty: Ty<'tcx>,
 582     self_ty: Ty<'tcx>,
 583     method_def_id: DefId,
 584 ) -> Ty<'tcx> {
 585     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 586     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 587         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 588     });
 589
 590     let result = receiver_ty.subst(tcx, substs);
 591     debug!(
 592         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 593         receiver_ty, self_ty, method_def_id, result
 594     );
 595     result
 596 }
 597
 598 /// Creates the object type for the current trait. For example,
 599 /// if the current trait is `Deref`, then this will be
 600 /// `dyn Deref<Target = Self::Target> + 'static`.
 601 fn object_ty_for_trait<'tcx>(
 602     tcx: TyCtxt<'tcx>,
 603     trait_def_id: DefId,
 604     lifetime: ty::Region<'tcx>,
 605 ) -> Ty<'tcx> {
 606     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 607
 608     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 609
 610     let trait_predicate =
 611         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 612
 613     let mut associated_types = traits::supertraits(tcx, ty::Binder::dummy(trait_ref))
 614         .flat_map(|super_trait_ref| {
 615             tcx.associated_items(super_trait_ref.def_id()).map(move |item| (super_trait_ref, item))
 616         })
 617         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 618         .collect::<Vec<_>>();
 619
 620     // existential predicates need to be in a specific order
 621     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 622
 623     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 624         // We *can* get bound lifetimes here in cases like
 625         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 626         //
 627         // binder moved to (*)...
 628         let super_trait_ref = super_trait_ref.skip_binder();
 629         ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 630             ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 631             item_def_id: item.def_id,
 632             substs: super_trait_ref.substs,
 633         })
 634     });
 635
 636     let existential_predicates =
 637         tcx.mk_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 638
 639     let object_ty = tcx.mk_dynamic(
 640         // (*) ... binder re-introduced here
 641         ty::Binder::bind(existential_predicates),
 642         lifetime,
 643     );
 644
 645     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 646
 647     object_ty
 648 }
 649
 650 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 651 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 652 /// in the following way:
 653 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 654 /// - require the following bound:
 655 ///
 656 ///   ```
 657 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 658 ///   ```
 659 ///
 660 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 661 ///   (substitution notation).
 662 ///
 663 /// Some examples of receiver types and their required obligation:
 664 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 665 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 666 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 667 ///
 668 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 669 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 670 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 671 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 672 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 673 /// contained by the trait object, because the object that needs to be coerced is behind
 674 /// a pointer.
 675 ///
 676 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 677 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 678 /// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
 679 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 680 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 681 /// Written as a chalk-style query:
 682 ///
 683 ///     forall (U: Trait + ?Sized) {
 684 ///         if (Self: Unsize<U>) {
 685 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 686 ///         }
 687 ///     }
 688 ///
 689 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 690 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 691 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 692 //
 693 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 694 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 695 // `self: Wrapper<Self>`.
 696 #[allow(dead_code)]
 697 fn receiver_is_dispatchable<'tcx>(
 698     tcx: TyCtxt<'tcx>,
 699     method: &ty::AssocItem,
 700     receiver_ty: Ty<'tcx>,
 701 ) -> bool {
 702     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 703
 704     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 705     let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
 706         (u, cu)
 707     } else {
 708         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 709         return false;
 710     };
 711
 712     // the type `U` in the query
 713     // use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
 714     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 715     // replace this with `dyn Trait`
 716     let unsized_self_ty: Ty<'tcx> =
 717         tcx.mk_ty_param(::std::u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 718
 719     // `Receiver[Self => U]`
 720     let unsized_receiver_ty =
 721         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 722
 723     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 724     // `U: ?Sized` is already implied here
 725     let param_env = {
 726         let mut param_env = tcx.param_env(method.def_id);
 727
 728         // Self: Unsize<U>
 729         let unsize_predicate = ty::TraitRef {
 730             def_id: unsize_did,
 731             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 732         }
 733         .without_const()
 734         .to_predicate();
 735
 736         // U: Trait<Arg1, ..., ArgN>
 737         let trait_predicate = {
 738             let substs =
 739                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 740                     if param.index == 0 {
 741                         unsized_self_ty.into()
 742                     } else {
 743                         tcx.mk_param_from_def(param)
 744                     }
 745                 });
 746
 747             ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate()
 748         };
 749
 750         let caller_bounds: Vec<Predicate<'tcx>> = param_env
 751             .caller_bounds
 752             .iter()
 753             .cloned()
 754             .chain(iter::once(unsize_predicate))
 755             .chain(iter::once(trait_predicate))
 756             .collect();
 757
 758         param_env.caller_bounds = tcx.intern_predicates(&caller_bounds);
 759
 760         param_env
 761     };
 762
 763     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 764     let obligation = {
 765         let predicate = ty::TraitRef {
 766             def_id: dispatch_from_dyn_did,
 767             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 768         }
 769         .without_const()
 770         .to_predicate();
 771
 772         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 773     };
 774
 775     tcx.infer_ctxt().enter(|ref infcx| {
 776         // the receiver is dispatchable iff the obligation holds
 777         infcx.predicate_must_hold_modulo_regions(&obligation)
 778     })
 779 }
 780
 781 fn contains_illegal_self_type_reference<'tcx>(
 782     tcx: TyCtxt<'tcx>,
 783     trait_def_id: DefId,
 784     ty: Ty<'tcx>,
 785 ) -> bool {
 786     // This is somewhat subtle. In general, we want to forbid
 787     // references to `Self` in the argument and return types,
 788     // since the value of `Self` is erased. However, there is one
 789     // exception: it is ok to reference `Self` in order to access
 790     // an associated type of the current trait, since we retain
 791     // the value of those associated types in the object type
 792     // itself.
 793     //
 794     // ```rust
 795     // trait SuperTrait {
 796     //     type X;
 797     // }
 798     //
 799     // trait Trait : SuperTrait {
 800     //     type Y;
 801     //     fn foo(&self, x: Self) // bad
 802     //     fn foo(&self) -> Self // bad
 803     //     fn foo(&self) -> Option<Self> // bad
 804     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 805     //     fn foo(&self) -> <Self as Trait>::Y // OK
 806     //     fn foo(&self) -> Self::X // OK, desugars to next example
 807     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 808     // }
 809     // ```
 810     //
 811     // However, it is not as simple as allowing `Self` in a projected
 812     // type, because there are illegal ways to use `Self` as well:
 813     //
 814     // ```rust
 815     // trait Trait : SuperTrait {
 816     //     ...
 817     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 818     // }
 819     // ```
 820     //
 821     // Here we will not have the type of `X` recorded in the
 822     // object type, and we cannot resolve `Self as SomeOtherTrait`
 823     // without knowing what `Self` is.
 824
 825     let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
 826     let mut error = false;
 827     let self_ty = tcx.types.self_param;
 828     ty.maybe_walk(|ty| {
 829         match ty.kind {
 830             ty::Param(_) => {
 831                 if ty == self_ty {
 832                     error = true;
 833                 }
 834
 835                 false // no contained types to walk
 836             }
 837
 838             ty::Projection(ref data) => {
 839                 // This is a projected type `<Foo as SomeTrait>::X`.
 840
 841                 // Compute supertraits of current trait lazily.
 842                 if supertraits.is_none() {
 843                     let trait_ref = ty::Binder::bind(ty::TraitRef::identity(tcx, trait_def_id));
 844                     supertraits = Some(traits::supertraits(tcx, trait_ref).collect());
 845                 }
 846
 847                 // Determine whether the trait reference `Foo as
 848                 // SomeTrait` is in fact a supertrait of the
 849                 // current trait. In that case, this type is
 850                 // legal, because the type `X` will be specified
 851                 // in the object type.  Note that we can just use
 852                 // direct equality here because all of these types
 853                 // are part of the formal parameter listing, and
 854                 // hence there should be no inference variables.
 855                 let projection_trait_ref = ty::Binder::bind(data.trait_ref(tcx));
 856                 let is_supertrait_of_current_trait =
 857                     supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 858
 859                 if is_supertrait_of_current_trait {
 860                     false // do not walk contained types, do not report error, do collect $200
 861                 } else {
 862                     true // DO walk contained types, POSSIBLY reporting an error
 863                 }
 864             }
 865
 866             _ => true, // walk contained types, if any
 867         }
 868     });
 869
 870     error
 871 }
 872
 873 pub(super) fn is_object_safe_provider(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 874     object_safety_violations(tcx, trait_def_id).is_empty()
 875 }