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