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, DUMMY_SP};
  23 use smallvec::{smallvec, SmallVec};
  24 use syntax::ast;
  25
  26 use std::borrow::Cow;
  27 use std::iter::{self};
  28
  29 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
  30 pub enum ObjectSafetyViolation {
  31     /// `Self: Sized` declared on the trait.
  32     SizedSelf(SmallVec<[Span; 1]>),
  33
  34     /// Supertrait reference references `Self` an in illegal location
  35     /// (e.g., `trait Foo : Bar<Self>`).
  36     SupertraitSelf(SmallVec<[Span; 1]>),
  37
  38     /// Method has something illegal.
  39     Method(ast::Name, MethodViolationCode, Span),
  40
  41     /// Associated const.
  42     AssocConst(ast::Name, Span),
  43 }
  44
  45 impl ObjectSafetyViolation {
  46     pub fn error_msg(&self) -> Cow<'static, str> {
  47         match *self {
  48             ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
  49             ObjectSafetyViolation::SupertraitSelf(ref spans) => {
  50                 if spans.iter().any(|sp| *sp != DUMMY_SP) {
  51                     "it uses `Self` as a type parameter in this".into()
  52                 } else {
  53                     "it cannot use `Self` as a type parameter in a supertrait or `where`-clause"
  54                         .into()
  55                 }
  56             }
  57             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(_), _) => {
  58                 format!("associated function `{}` has no `self` parameter", name).into()
  59             }
  60             ObjectSafetyViolation::Method(
  61                 name,
  62                 MethodViolationCode::ReferencesSelfInput(_),
  63                 DUMMY_SP,
  64             ) => format!("method `{}` references the `Self` type in its parameters", name).into(),
  65             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfInput(_), _) => {
  66                 format!("method `{}` references the `Self` type in this parameter", name).into()
  67             }
  68             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelfOutput, _) => {
  69                 format!("method `{}` references the `Self` type in its return type", name).into()
  70             }
  71             ObjectSafetyViolation::Method(
  72                 name,
  73                 MethodViolationCode::WhereClauseReferencesSelf,
  74                 _,
  75             ) => {
  76                 format!("method `{}` references the `Self` type in its `where` clause", name).into()
  77             }
  78             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
  79                 format!("method `{}` has generic type parameters", name).into()
  80             }
  81             ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) => {
  82                 format!("method `{}`'s `self` parameter cannot be dispatched on", name).into()
  83             }
  84             ObjectSafetyViolation::AssocConst(name, DUMMY_SP) => {
  85                 format!("it contains associated `const` `{}`", name).into()
  86             }
  87             ObjectSafetyViolation::AssocConst(..) => "it contains this associated `const`".into(),
  88         }
  89     }
  90
  91     pub fn solution(&self) -> Option<(String, Option<(String, Span)>)> {
  92         Some(match *self {
  93             ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf(_) => {
  94                 return None;
  95             }
  96             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod(sugg), _) => (
  97                 format!(
  98                     "consider turning `{}` into a method by giving it a `&self` argument or \
  99                      constraining it so it does not apply to trait objects",
 100                     name
 101                 ),
 102                 sugg.map(|(sugg, sp)| (sugg.to_string(), sp)),
 103             ),
 104             ObjectSafetyViolation::Method(
 105                 name,
 106                 MethodViolationCode::UndispatchableReceiver,
 107                 span,
 108             ) => (
 109                 format!("consider changing method `{}`'s `self` parameter to be `&self`", name)
 110                     .into(),
 111                 Some(("&Self".to_string(), span)),
 112             ),
 113             ObjectSafetyViolation::AssocConst(name, _)
 114             | ObjectSafetyViolation::Method(name, ..) => {
 115                 (format!("consider moving `{}` to another trait", name), None)
 116             }
 117         })
 118     }
 119
 120     pub fn spans(&self) -> SmallVec<[Span; 1]> {
 121         // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
 122         // diagnostics use a `note` instead of a `span_label`.
 123         match self {
 124             ObjectSafetyViolation::SupertraitSelf(spans)
 125             | ObjectSafetyViolation::SizedSelf(spans) => spans.clone(),
 126             ObjectSafetyViolation::AssocConst(_, span)
 127             | ObjectSafetyViolation::Method(_, _, span)
 128                 if *span != DUMMY_SP =>
 129             {
 130                 smallvec![*span]
 131             }
 132             _ => smallvec![],
 133         }
 134     }
 135 }
 136
 137 /// Reasons a method might not be object-safe.
 138 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 139 pub enum MethodViolationCode {
 140     /// e.g., `fn foo()`
 141     StaticMethod(Option<(&'static str, Span)>),
 142
 143     /// e.g., `fn foo(&self, x: Self)`
 144     ReferencesSelfInput(usize),
 145
 146     /// e.g., `fn foo(&self) -> Self`
 147     ReferencesSelfOutput,
 148
 149     /// e.g., `fn foo(&self) where Self: Clone`
 150     WhereClauseReferencesSelf,
 151
 152     /// e.g., `fn foo<A>()`
 153     Generic,
 154
 155     /// the method's receiver (`self` argument) can't be dispatched on
 156     UndispatchableReceiver,
 157 }
 158
 159 /// Returns the object safety violations that affect
 160 /// astconv -- currently, `Self` in supertraits. This is needed
 161 /// because `object_safety_violations` can't be used during
 162 /// type collection.
 163 pub fn astconv_object_safety_violations(
 164     tcx: TyCtxt<'_>,
 165     trait_def_id: DefId,
 166 ) -> Vec<ObjectSafetyViolation> {
 167     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 168     let violations = traits::supertrait_def_ids(tcx, trait_def_id)
 169         .map(|def_id| predicates_reference_self(tcx, def_id, true))
 170         .filter(|spans| !spans.is_empty())
 171         .map(|spans| ObjectSafetyViolation::SupertraitSelf(spans))
 172         .collect();
 173
 174     debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
 175
 176     violations
 177 }
 178
 179 pub fn object_safety_violations(
 180     tcx: TyCtxt<'_>,
 181     trait_def_id: DefId,
 182 ) -> Vec<ObjectSafetyViolation> {
 183     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 184     debug!("object_safety_violations: {:?}", trait_def_id);
 185
 186     traits::supertrait_def_ids(tcx, trait_def_id)
 187         .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id))
 188         .collect()
 189 }
 190
 191 /// We say a method is *vtable safe* if it can be invoked on a trait
 192 /// object. Note that object-safe traits can have some
 193 /// non-vtable-safe methods, so long as they require `Self: Sized` or
 194 /// otherwise ensure that they cannot be used when `Self = Trait`.
 195 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
 196     debug_assert!(tcx.generics_of(trait_def_id).has_self);
 197     debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
 198     // Any method that has a `Self: Sized` bound cannot be called.
 199     if generics_require_sized_self(tcx, method.def_id) {
 200         return false;
 201     }
 202
 203     match virtual_call_violation_for_method(tcx, trait_def_id, method) {
 204         None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
 205         Some(_) => false,
 206     }
 207 }
 208
 209 fn object_safety_violations_for_trait(
 210     tcx: TyCtxt<'_>,
 211     trait_def_id: DefId,
 212 ) -> Vec<ObjectSafetyViolation> {
 213     // Check methods for violations.
 214     let mut violations: Vec<_> = tcx
 215         .associated_items(trait_def_id)
 216         .iter()
 217         .filter(|item| item.kind == ty::AssocKind::Method)
 218         .filter_map(|item| {
 219             object_safety_violation_for_method(tcx, trait_def_id, &item)
 220                 .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span))
 221         })
 222         .filter(|violation| {
 223             if let ObjectSafetyViolation::Method(
 224                 _,
 225                 MethodViolationCode::WhereClauseReferencesSelf,
 226                 span,
 227             ) = violation
 228             {
 229                 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
 230                 // It's also hard to get a use site span, so we use the method definition span.
 231                 tcx.struct_span_lint_hir(
 232                     WHERE_CLAUSES_OBJECT_SAFETY,
 233                     hir::CRATE_HIR_ID,
 234                     *span,
 235                     |lint| {
 236                         let mut err = lint.build(&format!(
 237                             "the trait `{}` cannot be made into an object",
 238                             tcx.def_path_str(trait_def_id)
 239                         ));
 240                         let node = tcx.hir().get_if_local(trait_def_id);
 241                         let msg = if let Some(hir::Node::Item(item)) = node {
 242                             err.span_label(
 243                                 item.ident.span,
 244                                 "this trait cannot be made into an object...",
 245                             );
 246                             format!("...because {}", violation.error_msg())
 247                         } else {
 248                             format!(
 249                                 "the trait cannot be made into an object because {}",
 250                                 violation.error_msg()
 251                             )
 252                         };
 253                         err.span_label(*span, &msg);
 254                         match (node, violation.solution()) {
 255                             (Some(_), Some((note, None))) => {
 256                                 err.help(&note);
 257                             }
 258                             (Some(_), Some((note, Some((sugg, span))))) => {
 259                                 err.span_suggestion(
 260                                     span,
 261                                     &note,
 262                                     sugg,
 263                                     Applicability::MachineApplicable,
 264                                 );
 265                             }
 266                             // Only provide the help if its a local trait, otherwise it's not actionable.
 267                             _ => {}
 268                         }
 269                         err.emit();
 270                     },
 271                 );
 272                 false
 273             } else {
 274                 true
 275             }
 276         })
 277         .collect();
 278
 279     // Check the trait itself.
 280     if trait_has_sized_self(tcx, trait_def_id) {
 281         // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
 282         let spans = get_sized_bounds(tcx, trait_def_id);
 283         violations.push(ObjectSafetyViolation::SizedSelf(spans));
 284     }
 285     let spans = predicates_reference_self(tcx, trait_def_id, false);
 286     if !spans.is_empty() {
 287         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
 288     }
 289
 290     violations.extend(
 291         tcx.associated_items(trait_def_id)
 292             .iter()
 293             .filter(|item| item.kind == ty::AssocKind::Const)
 294             .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)),
 295     );
 296
 297     debug!(
 298         "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 299         trait_def_id, violations
 300     );
 301
 302     violations
 303 }
 304
 305 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
 306     tcx.hir()
 307         .get_if_local(trait_def_id)
 308         .and_then(|node| match node {
 309             hir::Node::Item(hir::Item {
 310                 kind: hir::ItemKind::Trait(.., generics, bounds, _),
 311                 ..
 312             }) => Some(
 313                 generics
 314                     .where_clause
 315                     .predicates
 316                     .iter()
 317                     .filter_map(|pred| {
 318                         match pred {
 319                             hir::WherePredicate::BoundPredicate(pred)
 320                                 if pred.bounded_ty.hir_id.owner_def_id() == trait_def_id =>
 321                             {
 322                                 // Fetch spans for trait bounds that are Sized:
 323                                 // `trait T where Self: Pred`
 324                                 Some(pred.bounds.iter().filter_map(|b| match b {
 325                                     hir::GenericBound::Trait(
 326                                         trait_ref,
 327                                         hir::TraitBoundModifier::None,
 328                                     ) if trait_has_sized_self(
 329                                         tcx,
 330                                         trait_ref.trait_ref.trait_def_id(),
 331                                     ) =>
 332                                     {
 333                                         Some(trait_ref.span)
 334                                     }
 335                                     _ => None,
 336                                 }))
 337                             }
 338                             _ => None,
 339                         }
 340                     })
 341                     .flatten()
 342                     .chain(bounds.iter().filter_map(|b| match b {
 343                         hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
 344                             if trait_has_sized_self(tcx, trait_ref.trait_ref.trait_def_id()) =>
 345                         {
 346                             // Fetch spans for supertraits that are `Sized`: `trait T: Super`
 347                             Some(trait_ref.span)
 348                         }
 349                         _ => None,
 350                     }))
 351                     .collect::<SmallVec<[Span; 1]>>(),
 352             ),
 353             _ => None,
 354         })
 355         .unwrap_or_else(SmallVec::new)
 356 }
 357
 358 fn predicates_reference_self(
 359     tcx: TyCtxt<'_>,
 360     trait_def_id: DefId,
 361     supertraits_only: bool,
 362 ) -> SmallVec<[Span; 1]> {
 363     let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
 364     let predicates = if supertraits_only {
 365         tcx.super_predicates_of(trait_def_id)
 366     } else {
 367         tcx.predicates_of(trait_def_id)
 368     };
 369     let self_ty = tcx.types.self_param;
 370     let has_self_ty = |t: Ty<'_>| t.walk().any(|t| t == self_ty);
 371     predicates
 372         .predicates
 373         .iter()
 374         .map(|(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
 375         .filter_map(|(predicate, &sp)| {
 376             match predicate {
 377                 ty::Predicate::Trait(ref data, _) => {
 378                     // In the case of a trait predicate, we can skip the "self" type.
 379                     if data.skip_binder().input_types().skip(1).any(has_self_ty) {
 380                         Some(sp)
 381                     } else {
 382                         None
 383                     }
 384                 }
 385                 ty::Predicate::Projection(ref data) => {
 386                     // And similarly for projections. This should be redundant with
 387                     // the previous check because any projection should have a
 388                     // matching `Trait` predicate with the same inputs, but we do
 389                     // the check to be safe.
 390                     //
 391                     // Note that we *do* allow projection *outputs* to contain
 392                     // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
 393                     // we just require the user to specify *both* outputs
 394                     // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
 395                     //
 396                     // This is ALT2 in issue #56288, see that for discussion of the
 397                     // possible alternatives.
 398                     if data
 399                         .skip_binder()
 400                         .projection_ty
 401                         .trait_ref(tcx)
 402                         .input_types()
 403                         .skip(1)
 404                         .any(has_self_ty)
 405                     {
 406                         Some(sp)
 407                     } else {
 408                         None
 409                     }
 410                 }
 411                 ty::Predicate::WellFormed(..)
 412                 | ty::Predicate::ObjectSafe(..)
 413                 | ty::Predicate::TypeOutlives(..)
 414                 | ty::Predicate::RegionOutlives(..)
 415                 | ty::Predicate::ClosureKind(..)
 416                 | ty::Predicate::Subtype(..)
 417                 | ty::Predicate::ConstEvaluatable(..) => None,
 418             }
 419         })
 420         .collect()
 421 }
 422
 423 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 424     generics_require_sized_self(tcx, trait_def_id)
 425 }
 426
 427 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
 428     let sized_def_id = match tcx.lang_items().sized_trait() {
 429         Some(def_id) => def_id,
 430         None => {
 431             return false; /* No Sized trait, can't require it! */
 432         }
 433     };
 434
 435     // Search for a predicate like `Self : Sized` amongst the trait bounds.
 436     let predicates = tcx.predicates_of(def_id);
 437     let predicates = predicates.instantiate_identity(tcx).predicates;
 438     elaborate_predicates(tcx, predicates).any(|predicate| match predicate {
 439         ty::Predicate::Trait(ref trait_pred, _) => {
 440             trait_pred.def_id() == sized_def_id && trait_pred.skip_binder().self_ty().is_param(0)
 441         }
 442         ty::Predicate::Projection(..)
 443         | ty::Predicate::Subtype(..)
 444         | ty::Predicate::RegionOutlives(..)
 445         | ty::Predicate::WellFormed(..)
 446         | ty::Predicate::ObjectSafe(..)
 447         | ty::Predicate::ClosureKind(..)
 448         | ty::Predicate::TypeOutlives(..)
 449         | ty::Predicate::ConstEvaluatable(..) => false,
 450     })
 451 }
 452
 453 /// Returns `Some(_)` if this method makes the containing trait not object safe.
 454 fn object_safety_violation_for_method(
 455     tcx: TyCtxt<'_>,
 456     trait_def_id: DefId,
 457     method: &ty::AssocItem,
 458 ) -> Option<(MethodViolationCode, Span)> {
 459     debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
 460     // Any method that has a `Self : Sized` requisite is otherwise
 461     // exempt from the regulations.
 462     if generics_require_sized_self(tcx, method.def_id) {
 463         return None;
 464     }
 465
 466     let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
 467     // Get an accurate span depending on the violation.
 468     violation.map(|v| {
 469         let node = tcx.hir().get_if_local(method.def_id);
 470         let span = match (v, node) {
 471             (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
 472                 .fn_decl()
 473                 .and_then(|decl| decl.inputs.get(arg + 1))
 474                 .map_or(method.ident.span, |arg| arg.span),
 475             (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
 476                 .fn_decl()
 477                 .and_then(|decl| decl.inputs.get(0))
 478                 .map_or(method.ident.span, |arg| arg.span),
 479             (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
 480                 node.fn_decl().map_or(method.ident.span, |decl| decl.output.span())
 481             }
 482             _ => method.ident.span,
 483         };
 484         (v, span)
 485     })
 486 }
 487
 488 /// Returns `Some(_)` if this method cannot be called on a trait
 489 /// object; this does not necessarily imply that the enclosing trait
 490 /// is not object safe, because the method might have a where clause
 491 /// `Self:Sized`.
 492 fn virtual_call_violation_for_method<'tcx>(
 493     tcx: TyCtxt<'tcx>,
 494     trait_def_id: DefId,
 495     method: &ty::AssocItem,
 496 ) -> Option<MethodViolationCode> {
 497     // The method's first parameter must be named `self`
 498     if !method.method_has_self_argument {
 499         // We'll attempt to provide a structured suggestion for `Self: Sized`.
 500         let sugg =
 501             tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
 502                 |generics| match generics.where_clause.predicates {
 503                     [] => (" where Self: Sized", generics.where_clause.span),
 504                     [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
 505                 },
 506             );
 507         return Some(MethodViolationCode::StaticMethod(sugg));
 508     }
 509
 510     let sig = tcx.fn_sig(method.def_id);
 511
 512     for (i, input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
 513         if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) {
 514             return Some(MethodViolationCode::ReferencesSelfInput(i));
 515         }
 516     }
 517     if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output().skip_binder()) {
 518         return Some(MethodViolationCode::ReferencesSelfOutput);
 519     }
 520
 521     // We can't monomorphize things like `fn foo<A>(...)`.
 522     let own_counts = tcx.generics_of(method.def_id).own_counts();
 523     if own_counts.types + own_counts.consts != 0 {
 524         return Some(MethodViolationCode::Generic);
 525     }
 526
 527     if tcx
 528         .predicates_of(method.def_id)
 529         .predicates
 530         .iter()
 531         // A trait object can't claim to live more than the concrete type,
 532         // so outlives predicates will always hold.
 533         .cloned()
 534         .filter(|(p, _)| p.to_opt_type_outlives().is_none())
 535         .collect::<Vec<_>>()
 536         // Do a shallow visit so that `contains_illegal_self_type_reference`
 537         // may apply it's custom visiting.
 538         .visit_tys_shallow(|t| contains_illegal_self_type_reference(tcx, trait_def_id, t))
 539     {
 540         return Some(MethodViolationCode::WhereClauseReferencesSelf);
 541     }
 542
 543     let receiver_ty =
 544         tcx.liberate_late_bound_regions(method.def_id, &sig.map_bound(|sig| sig.inputs()[0]));
 545
 546     // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
 547     // However, this is already considered object-safe. We allow it as a special case here.
 548     // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
 549     // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
 550     if receiver_ty != tcx.types.self_param {
 551         if !receiver_is_dispatchable(tcx, method, receiver_ty) {
 552             return Some(MethodViolationCode::UndispatchableReceiver);
 553         } else {
 554             // Do sanity check to make sure the receiver actually has the layout of a pointer.
 555
 556             use rustc::ty::layout::Abi;
 557
 558             let param_env = tcx.param_env(method.def_id);
 559
 560             let abi_of_ty = |ty: Ty<'tcx>| -> &Abi {
 561                 match tcx.layout_of(param_env.and(ty)) {
 562                     Ok(layout) => &layout.abi,
 563                     Err(err) => bug!("error: {}\n while computing layout for type {:?}", err, ty),
 564                 }
 565             };
 566
 567             // e.g., `Rc<()>`
 568             let unit_receiver_ty =
 569                 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
 570
 571             match abi_of_ty(unit_receiver_ty) {
 572                 &Abi::Scalar(..) => (),
 573                 abi => {
 574                     tcx.sess.delay_span_bug(
 575                         tcx.def_span(method.def_id),
 576                         &format!(
 577                             "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
 578                             abi
 579                         ),
 580                     );
 581                 }
 582             }
 583
 584             let trait_object_ty =
 585                 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
 586
 587             // e.g., `Rc<dyn Trait>`
 588             let trait_object_receiver =
 589                 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
 590
 591             match abi_of_ty(trait_object_receiver) {
 592                 &Abi::ScalarPair(..) => (),
 593                 abi => {
 594                     tcx.sess.delay_span_bug(
 595                         tcx.def_span(method.def_id),
 596                         &format!(
 597                             "receiver when `Self = {}` should have a ScalarPair ABI; \
 598                                  found {:?}",
 599                             trait_object_ty, abi
 600                         ),
 601                     );
 602                 }
 603             }
 604         }
 605     }
 606
 607     None
 608 }
 609
 610 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
 611 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
 612 fn receiver_for_self_ty<'tcx>(
 613     tcx: TyCtxt<'tcx>,
 614     receiver_ty: Ty<'tcx>,
 615     self_ty: Ty<'tcx>,
 616     method_def_id: DefId,
 617 ) -> Ty<'tcx> {
 618     debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
 619     let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
 620         if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
 621     });
 622
 623     let result = receiver_ty.subst(tcx, substs);
 624     debug!(
 625         "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
 626         receiver_ty, self_ty, method_def_id, result
 627     );
 628     result
 629 }
 630
 631 /// Creates the object type for the current trait. For example,
 632 /// if the current trait is `Deref`, then this will be
 633 /// `dyn Deref<Target = Self::Target> + 'static`.
 634 fn object_ty_for_trait<'tcx>(
 635     tcx: TyCtxt<'tcx>,
 636     trait_def_id: DefId,
 637     lifetime: ty::Region<'tcx>,
 638 ) -> Ty<'tcx> {
 639     debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
 640
 641     let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
 642
 643     let trait_predicate =
 644         ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 645
 646     let mut associated_types = traits::supertraits(tcx, ty::Binder::dummy(trait_ref))
 647         .flat_map(|super_trait_ref| {
 648             tcx.associated_items(super_trait_ref.def_id())
 649                 .iter()
 650                 .map(move |item| (super_trait_ref, item))
 651         })
 652         .filter(|(_, item)| item.kind == ty::AssocKind::Type)
 653         .collect::<Vec<_>>();
 654
 655     // existential predicates need to be in a specific order
 656     associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
 657
 658     let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
 659         // We *can* get bound lifetimes here in cases like
 660         // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
 661         //
 662         // binder moved to (*)...
 663         let super_trait_ref = super_trait_ref.skip_binder();
 664         ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
 665             ty: tcx.mk_projection(item.def_id, super_trait_ref.substs),
 666             item_def_id: item.def_id,
 667             substs: super_trait_ref.substs,
 668         })
 669     });
 670
 671     let existential_predicates =
 672         tcx.mk_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
 673
 674     let object_ty = tcx.mk_dynamic(
 675         // (*) ... binder re-introduced here
 676         ty::Binder::bind(existential_predicates),
 677         lifetime,
 678     );
 679
 680     debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
 681
 682     object_ty
 683 }
 684
 685 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
 686 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
 687 /// in the following way:
 688 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
 689 /// - require the following bound:
 690 ///
 691 ///   ```
 692 ///   Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
 693 ///   ```
 694 ///
 695 ///   where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
 696 ///   (substitution notation).
 697 ///
 698 /// Some examples of receiver types and their required obligation:
 699 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
 700 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
 701 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
 702 ///
 703 /// The only case where the receiver is not dispatchable, but is still a valid receiver
 704 /// type (just not object-safe), is when there is more than one level of pointer indirection.
 705 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
 706 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
 707 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
 708 /// contained by the trait object, because the object that needs to be coerced is behind
 709 /// a pointer.
 710 ///
 711 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
 712 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
 713 /// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
 714 /// Instead, we fudge a little by introducing a new type parameter `U` such that
 715 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
 716 /// Written as a chalk-style query:
 717 ///
 718 ///     forall (U: Trait + ?Sized) {
 719 ///         if (Self: Unsize<U>) {
 720 ///             Receiver: DispatchFromDyn<Receiver[Self => U]>
 721 ///         }
 722 ///     }
 723 ///
 724 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
 725 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
 726 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
 727 //
 728 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
 729 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
 730 // `self: Wrapper<Self>`.
 731 #[allow(dead_code)]
 732 fn receiver_is_dispatchable<'tcx>(
 733     tcx: TyCtxt<'tcx>,
 734     method: &ty::AssocItem,
 735     receiver_ty: Ty<'tcx>,
 736 ) -> bool {
 737     debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
 738
 739     let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
 740     let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
 741         (u, cu)
 742     } else {
 743         debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
 744         return false;
 745     };
 746
 747     // the type `U` in the query
 748     // use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
 749     // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
 750     // replace this with `dyn Trait`
 751     let unsized_self_ty: Ty<'tcx> =
 752         tcx.mk_ty_param(::std::u32::MAX, Symbol::intern("RustaceansAreAwesome"));
 753
 754     // `Receiver[Self => U]`
 755     let unsized_receiver_ty =
 756         receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
 757
 758     // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
 759     // `U: ?Sized` is already implied here
 760     let param_env = {
 761         let mut param_env = tcx.param_env(method.def_id);
 762
 763         // Self: Unsize<U>
 764         let unsize_predicate = ty::TraitRef {
 765             def_id: unsize_did,
 766             substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
 767         }
 768         .without_const()
 769         .to_predicate();
 770
 771         // U: Trait<Arg1, ..., ArgN>
 772         let trait_predicate = {
 773             let substs =
 774                 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
 775                     if param.index == 0 {
 776                         unsized_self_ty.into()
 777                     } else {
 778                         tcx.mk_param_from_def(param)
 779                     }
 780                 });
 781
 782             ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate()
 783         };
 784
 785         let caller_bounds: Vec<Predicate<'tcx>> = param_env
 786             .caller_bounds
 787             .iter()
 788             .cloned()
 789             .chain(iter::once(unsize_predicate))
 790             .chain(iter::once(trait_predicate))
 791             .collect();
 792
 793         param_env.caller_bounds = tcx.intern_predicates(&caller_bounds);
 794
 795         param_env
 796     };
 797
 798     // Receiver: DispatchFromDyn<Receiver[Self => U]>
 799     let obligation = {
 800         let predicate = ty::TraitRef {
 801             def_id: dispatch_from_dyn_did,
 802             substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
 803         }
 804         .without_const()
 805         .to_predicate();
 806
 807         Obligation::new(ObligationCause::dummy(), param_env, predicate)
 808     };
 809
 810     tcx.infer_ctxt().enter(|ref infcx| {
 811         // the receiver is dispatchable iff the obligation holds
 812         infcx.predicate_must_hold_modulo_regions(&obligation)
 813     })
 814 }
 815
 816 fn contains_illegal_self_type_reference<'tcx>(
 817     tcx: TyCtxt<'tcx>,
 818     trait_def_id: DefId,
 819     ty: Ty<'tcx>,
 820 ) -> bool {
 821     // This is somewhat subtle. In general, we want to forbid
 822     // references to `Self` in the argument and return types,
 823     // since the value of `Self` is erased. However, there is one
 824     // exception: it is ok to reference `Self` in order to access
 825     // an associated type of the current trait, since we retain
 826     // the value of those associated types in the object type
 827     // itself.
 828     //
 829     // ```rust
 830     // trait SuperTrait {
 831     //     type X;
 832     // }
 833     //
 834     // trait Trait : SuperTrait {
 835     //     type Y;
 836     //     fn foo(&self, x: Self) // bad
 837     //     fn foo(&self) -> Self // bad
 838     //     fn foo(&self) -> Option<Self> // bad
 839     //     fn foo(&self) -> Self::Y // OK, desugars to next example
 840     //     fn foo(&self) -> <Self as Trait>::Y // OK
 841     //     fn foo(&self) -> Self::X // OK, desugars to next example
 842     //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 843     // }
 844     // ```
 845     //
 846     // However, it is not as simple as allowing `Self` in a projected
 847     // type, because there are illegal ways to use `Self` as well:
 848     //
 849     // ```rust
 850     // trait Trait : SuperTrait {
 851     //     ...
 852     //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 853     // }
 854     // ```
 855     //
 856     // Here we will not have the type of `X` recorded in the
 857     // object type, and we cannot resolve `Self as SomeOtherTrait`
 858     // without knowing what `Self` is.
 859
 860     let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
 861     let mut error = false;
 862     let self_ty = tcx.types.self_param;
 863     ty.maybe_walk(|ty| {
 864         match ty.kind {
 865             ty::Param(_) => {
 866                 if ty == self_ty {
 867                     error = true;
 868                 }
 869
 870                 false // no contained types to walk
 871             }
 872
 873             ty::Projection(ref data) => {
 874                 // This is a projected type `<Foo as SomeTrait>::X`.
 875
 876                 // Compute supertraits of current trait lazily.
 877                 if supertraits.is_none() {
 878                     let trait_ref = ty::Binder::bind(ty::TraitRef::identity(tcx, trait_def_id));
 879                     supertraits = Some(traits::supertraits(tcx, trait_ref).collect());
 880                 }
 881
 882                 // Determine whether the trait reference `Foo as
 883                 // SomeTrait` is in fact a supertrait of the
 884                 // current trait. In that case, this type is
 885                 // legal, because the type `X` will be specified
 886                 // in the object type.  Note that we can just use
 887                 // direct equality here because all of these types
 888                 // are part of the formal parameter listing, and
 889                 // hence there should be no inference variables.
 890                 let projection_trait_ref = ty::Binder::bind(data.trait_ref(tcx));
 891                 let is_supertrait_of_current_trait =
 892                     supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 893
 894                 if is_supertrait_of_current_trait {
 895                     false // do not walk contained types, do not report error, do collect $200
 896                 } else {
 897                     true // DO walk contained types, POSSIBLY reporting an error
 898                 }
 899             }
 900
 901             _ => true, // walk contained types, if any
 902         }
 903     });
 904
 905     error
 906 }
 907
 908 pub(super) fn is_object_safe_provider(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
 909     object_safety_violations(tcx, trait_def_id).is_empty()
 910 }