src/librustc_trait_selection/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::opaque_types::required_region_bounds;
   3 use crate::traits;
   4 use rustc_hir as hir;
   5 use rustc_hir::def_id::DefId;
   6 use rustc_hir::lang_items;
   7 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
   8 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
   9 use rustc_span::Span;
  10 use std::rc::Rc;
  11
  12 /// Returns the set of obligations needed to make `ty` well-formed.
  13 /// If `ty` contains unresolved inference variables, this may include
  14 /// further WF obligations. However, if `ty` IS an unresolved
  15 /// inference variable, returns `None`, because we are not able to
  16 /// make any progress at all. This is to prevent "livelock" where we
  17 /// say "$0 is WF if $0 is WF".
  18 pub fn obligations<'a, 'tcx>(
  19     infcx: &InferCtxt<'a, 'tcx>,
  20     param_env: ty::ParamEnv<'tcx>,
  21     body_id: hir::HirId,
  22     ty: Ty<'tcx>,
  23     span: Span,
  24 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  25     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  26     if wf.compute(ty) {
  27         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  28
  29         let result = wf.normalize();
  30         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  31         Some(result)
  32     } else {
  33         None // no progress made, return None
  34     }
  35 }
  36
  37 /// Returns the obligations that make this trait reference
  38 /// well-formed.  For example, if there is a trait `Set` defined like
  39 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  40 /// if `Bar: Eq`.
  41 pub fn trait_obligations<'a, 'tcx>(
  42     infcx: &InferCtxt<'a, 'tcx>,
  43     param_env: ty::ParamEnv<'tcx>,
  44     body_id: hir::HirId,
  45     trait_ref: &ty::TraitRef<'tcx>,
  46     span: Span,
  47     item: Option<&'tcx hir::Item<'tcx>>,
  48 ) -> Vec<traits::PredicateObligation<'tcx>> {
  49     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
  50     wf.compute_trait_ref(trait_ref, Elaborate::All);
  51     wf.normalize()
  52 }
  53
  54 pub fn predicate_obligations<'a, 'tcx>(
  55     infcx: &InferCtxt<'a, 'tcx>,
  56     param_env: ty::ParamEnv<'tcx>,
  57     body_id: hir::HirId,
  58     predicate: &ty::Predicate<'tcx>,
  59     span: Span,
  60 ) -> Vec<traits::PredicateObligation<'tcx>> {
  61     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  62
  63     // (*) ok to skip binders, because wf code is prepared for it
  64     match *predicate {
  65         ty::Predicate::Trait(ref t, _) => {
  66             wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
  67         }
  68         ty::Predicate::RegionOutlives(..) => {}
  69         ty::Predicate::TypeOutlives(ref t) => {
  70             wf.compute(t.skip_binder().0);
  71         }
  72         ty::Predicate::Projection(ref t) => {
  73             let t = t.skip_binder(); // (*)
  74             wf.compute_projection(t.projection_ty);
  75             wf.compute(t.ty);
  76         }
  77         ty::Predicate::WellFormed(t) => {
  78             wf.compute(t);
  79         }
  80         ty::Predicate::ObjectSafe(_) => {}
  81         ty::Predicate::ClosureKind(..) => {}
  82         ty::Predicate::Subtype(ref data) => {
  83             wf.compute(data.skip_binder().a); // (*)
  84             wf.compute(data.skip_binder().b); // (*)
  85         }
  86         ty::Predicate::ConstEvaluatable(def_id, substs) => {
  87             let obligations = wf.nominal_obligations(def_id, substs);
  88             wf.out.extend(obligations);
  89
  90             for ty in substs.types() {
  91                 wf.compute(ty);
  92             }
  93         }
  94     }
  95
  96     wf.normalize()
  97 }
  98
  99 struct WfPredicates<'a, 'tcx> {
 100     infcx: &'a InferCtxt<'a, 'tcx>,
 101     param_env: ty::ParamEnv<'tcx>,
 102     body_id: hir::HirId,
 103     span: Span,
 104     out: Vec<traits::PredicateObligation<'tcx>>,
 105     item: Option<&'tcx hir::Item<'tcx>>,
 106 }
 107
 108 /// Controls whether we "elaborate" supertraits and so forth on the WF
 109 /// predicates. This is a kind of hack to address #43784. The
 110 /// underlying problem in that issue was a trait structure like:
 111 ///
 112 /// ```
 113 /// trait Foo: Copy { }
 114 /// trait Bar: Foo { }
 115 /// impl<T: Bar> Foo for T { }
 116 /// impl<T> Bar for T { }
 117 /// ```
 118 ///
 119 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 120 /// we decide that this is true because `T: Bar` is in the
 121 /// where-clauses (and we can elaborate that to include `T:
 122 /// Copy`). This wouldn't be a problem, except that when we check the
 123 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 124 /// impl. And so nowhere did we check that `T: Copy` holds!
 125 ///
 126 /// To resolve this, we elaborate the WF requirements that must be
 127 /// proven when checking impls. This means that (e.g.) the `impl Bar
 128 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 129 /// Copy` (which it won't be able to do, because there is no `Copy`
 130 /// impl for `T`).
 131 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 132 enum Elaborate {
 133     All,
 134     None,
 135 }
 136
 137 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 138     tcx: TyCtxt<'tcx>,
 139     trait_ref: &ty::TraitRef<'tcx>,
 140     item: Option<&hir::Item<'tcx>>,
 141     cause: &mut traits::ObligationCause<'tcx>,
 142     pred: &ty::Predicate<'_>,
 143     mut trait_assoc_items: impl Iterator<Item = ty::AssocItem>,
 144 ) {
 145     debug!(
 146         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 147         trait_ref, item, cause, pred
 148     );
 149     let items = match item {
 150         Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
 151         _ => return,
 152     };
 153     let fix_span =
 154         |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
 155             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 156             _ => impl_item_ref.span,
 157         };
 158     match pred {
 159         ty::Predicate::Projection(proj) => {
 160             // The obligation comes not from the current `impl` nor the `trait` being
 161             // implemented, but rather from a "second order" obligation, like in
 162             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs`.
 163             let trait_assoc_item = tcx.associated_item(proj.projection_def_id());
 164             if let Some(impl_item_span) =
 165                 items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 166             {
 167                 cause.span = impl_item_span;
 168             } else {
 169                 let kind = &proj.ty().skip_binder().kind;
 170                 if let ty::Projection(projection_ty) = kind {
 171                     // This happens when an associated type has a projection coming from another
 172                     // associated type. See `traits-assoc-type-in-supertrait-bad.rs`.
 173                     let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
 174                     if let Some(impl_item_span) =
 175                         items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 176                     {
 177                         cause.span = impl_item_span;
 178                     }
 179                 }
 180             }
 181         }
 182         ty::Predicate::Trait(pred, _) => {
 183             // An associated item obligation born out of the `trait` failed to be met. An example
 184             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 185             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 186             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) =
 187                 &pred.skip_binder().self_ty().kind
 188             {
 189                 if let Some(impl_item_span) = trait_assoc_items
 190                     .find(|i| i.def_id == *item_def_id)
 191                     .and_then(|trait_assoc_item| {
 192                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
 193                     })
 194                 {
 195                     cause.span = impl_item_span;
 196                 }
 197             }
 198         }
 199         _ => {}
 200     }
 201 }
 202
 203 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 204     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 205         traits::ObligationCause::new(self.span, self.body_id, code)
 206     }
 207
 208     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 209         let cause = self.cause(traits::MiscObligation);
 210         let infcx = &mut self.infcx;
 211         let param_env = self.param_env;
 212         let mut obligations = Vec::with_capacity(self.out.len());
 213         for pred in &self.out {
 214             assert!(!pred.has_escaping_bound_vars());
 215             let mut selcx = traits::SelectionContext::new(infcx);
 216             let i = obligations.len();
 217             let value =
 218                 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
 219             obligations.insert(i, value);
 220         }
 221         obligations
 222     }
 223
 224     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 225     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 226         let tcx = self.infcx.tcx;
 227         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 228
 229         debug!("compute_trait_ref obligations {:?}", obligations);
 230         let cause = self.cause(traits::MiscObligation);
 231         let param_env = self.param_env;
 232
 233         let item = self.item;
 234
 235         if let Elaborate::All = elaborate {
 236             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations.clone());
 237             let implied_obligations = implied_obligations.map(|obligation| {
 238                 debug!("compute_trait_ref implied_obligation {:?}", obligation);
 239                 debug!("compute_trait_ref implied_obligation cause {:?}", obligation.cause);
 240                 let mut cause = cause.clone();
 241                 if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
 242                     let derived_cause = traits::DerivedObligationCause {
 243                         parent_trait_ref,
 244                         parent_code: Rc::new(obligation.cause.code.clone()),
 245                     };
 246                     cause.code = traits::ObligationCauseCode::ImplDerivedObligation(derived_cause);
 247                 }
 248                 extend_cause_with_original_assoc_item_obligation(
 249                     tcx,
 250                     trait_ref,
 251                     item,
 252                     &mut cause,
 253                     &obligation.predicate,
 254                     tcx.associated_items(trait_ref.def_id).in_definition_order().copied(),
 255                 );
 256                 debug!("compute_trait_ref new cause {:?}", cause);
 257                 traits::Obligation::new(cause, param_env, obligation.predicate)
 258             });
 259             self.out.extend(implied_obligations);
 260         }
 261
 262         self.out.extend(obligations);
 263
 264         self.out.extend(trait_ref.substs.types().filter(|ty| !ty.has_escaping_bound_vars()).map(
 265             |ty| traits::Obligation::new(cause.clone(), param_env, ty::Predicate::WellFormed(ty)),
 266         ));
 267     }
 268
 269     /// Pushes the obligations required for `trait_ref::Item` to be WF
 270     /// into `self.out`.
 271     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 272         // A projection is well-formed if (a) the trait ref itself is
 273         // WF and (b) the trait-ref holds.  (It may also be
 274         // normalizable and be WF that way.)
 275         let trait_ref = data.trait_ref(self.infcx.tcx);
 276         self.compute_trait_ref(&trait_ref, Elaborate::None);
 277
 278         if !data.has_escaping_bound_vars() {
 279             let predicate = trait_ref.without_const().to_predicate();
 280             let cause = self.cause(traits::ProjectionWf(data));
 281             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 282         }
 283     }
 284
 285     /// Pushes the obligations required for an array length to be WF
 286     /// into `self.out`.
 287     fn compute_array_len(&mut self, constant: ty::Const<'tcx>) {
 288         if let ty::ConstKind::Unevaluated(def_id, substs, promoted) = constant.val {
 289             assert!(promoted.is_none());
 290
 291             let obligations = self.nominal_obligations(def_id, substs);
 292             self.out.extend(obligations);
 293
 294             let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
 295             let cause = self.cause(traits::MiscObligation);
 296             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 297         }
 298     }
 299
 300     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 301         if !subty.has_escaping_bound_vars() {
 302             let cause = self.cause(cause);
 303             let trait_ref = ty::TraitRef {
 304                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
 305                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 306             };
 307             self.out.push(traits::Obligation::new(
 308                 cause,
 309                 self.param_env,
 310                 trait_ref.without_const().to_predicate(),
 311             ));
 312         }
 313     }
 314
 315     /// Pushes new obligations into `out`. Returns `true` if it was able
 316     /// to generate all the predicates needed to validate that `ty0`
 317     /// is WF. Returns false if `ty0` is an unresolved type variable,
 318     /// in which case we are not able to simplify at all.
 319     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 320         let mut walker = ty0.walk();
 321         let param_env = self.param_env;
 322         while let Some(arg) = walker.next() {
 323             let ty = match arg.unpack() {
 324                 GenericArgKind::Type(ty) => ty,
 325
 326                 // No WF constraints for lifetimes being present, any outlives
 327                 // obligations are handled by the parent (e.g. `ty::Ref`).
 328                 GenericArgKind::Lifetime(_) => continue,
 329
 330                 // FIXME(eddyb) this is wrong and needs to be replaced
 331                 // (see https://github.com/rust-lang/rust/pull/70107).
 332                 GenericArgKind::Const(_) => continue,
 333             };
 334
 335             match ty.kind {
 336                 ty::Bool
 337                 | ty::Char
 338                 | ty::Int(..)
 339                 | ty::Uint(..)
 340                 | ty::Float(..)
 341                 | ty::Error
 342                 | ty::Str
 343                 | ty::GeneratorWitness(..)
 344                 | ty::Never
 345                 | ty::Param(_)
 346                 | ty::Bound(..)
 347                 | ty::Placeholder(..)
 348                 | ty::Foreign(..) => {
 349                     // WfScalar, WfParameter, etc
 350                 }
 351
 352                 ty::Slice(subty) => {
 353                     self.require_sized(subty, traits::SliceOrArrayElem);
 354                 }
 355
 356                 ty::Array(subty, len) => {
 357                     self.require_sized(subty, traits::SliceOrArrayElem);
 358                     // FIXME(eddyb) handle `GenericArgKind::Const` above instead.
 359                     self.compute_array_len(*len);
 360                 }
 361
 362                 ty::Tuple(ref tys) => {
 363                     if let Some((_last, rest)) = tys.split_last() {
 364                         for elem in rest {
 365                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 366                         }
 367                     }
 368                 }
 369
 370                 ty::RawPtr(_) => {
 371                     // simple cases that are WF if their type args are WF
 372                 }
 373
 374                 ty::Projection(data) => {
 375                     walker.skip_current_subtree(); // subtree handled by compute_projection
 376                     self.compute_projection(data);
 377                 }
 378
 379                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 380
 381                 ty::Adt(def, substs) => {
 382                     // WfNominalType
 383                     let obligations = self.nominal_obligations(def.did, substs);
 384                     self.out.extend(obligations);
 385                 }
 386
 387                 ty::FnDef(did, substs) => {
 388                     let obligations = self.nominal_obligations(did, substs);
 389                     self.out.extend(obligations);
 390                 }
 391
 392                 ty::Ref(r, rty, _) => {
 393                     // WfReference
 394                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 395                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 396                         self.out.push(traits::Obligation::new(
 397                             cause,
 398                             param_env,
 399                             ty::Predicate::TypeOutlives(ty::Binder::dummy(ty::OutlivesPredicate(
 400                                 rty, r,
 401                             ))),
 402                         ));
 403                     }
 404                 }
 405
 406                 ty::Generator(..) => {
 407                     // Walk ALL the types in the generator: this will
 408                     // include the upvar types as well as the yield
 409                     // type. Note that this is mildly distinct from
 410                     // the closure case, where we have to be careful
 411                     // about the signature of the closure. We don't
 412                     // have the problem of implied bounds here since
 413                     // generators don't take arguments.
 414                 }
 415
 416                 ty::Closure(_, substs) => {
 417                     // Only check the upvar types for WF, not the rest
 418                     // of the types within. This is needed because we
 419                     // capture the signature and it may not be WF
 420                     // without the implied bounds. Consider a closure
 421                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 422                     // not known to hold in the creator's context (and
 423                     // indeed the closure may not be invoked by its
 424                     // creator, but rather turned to someone who *can*
 425                     // verify that).
 426                     //
 427                     // The special treatment of closures here really
 428                     // ought not to be necessary either; the problem
 429                     // is related to #25860 -- there is no way for us
 430                     // to express a fn type complete with the implied
 431                     // bounds that it is assuming. I think in reality
 432                     // the WF rules around fn are a bit messed up, and
 433                     // that is the rot problem: `fn(&'a T)` should
 434                     // probably always be WF, because it should be
 435                     // shorthand for something like `where(T: 'a) {
 436                     // fn(&'a T) }`, as discussed in #25860.
 437                     //
 438                     // Note that we are also skipping the generic
 439                     // types. This is consistent with the `outlives`
 440                     // code, but anyway doesn't matter: within the fn
 441                     // body where they are created, the generics will
 442                     // always be WF, and outside of that fn body we
 443                     // are not directly inspecting closure types
 444                     // anyway, except via auto trait matching (which
 445                     // only inspects the upvar types).
 446                     walker.skip_current_subtree(); // subtree handled by compute_projection
 447                     for upvar_ty in substs.as_closure().upvar_tys() {
 448                         self.compute(upvar_ty);
 449                     }
 450                 }
 451
 452                 ty::FnPtr(_) => {
 453                     // let the loop iterate into the argument/return
 454                     // types appearing in the fn signature
 455                 }
 456
 457                 ty::Opaque(did, substs) => {
 458                     // all of the requirements on type parameters
 459                     // should've been checked by the instantiation
 460                     // of whatever returned this exact `impl Trait`.
 461
 462                     // for named opaque `impl Trait` types we still need to check them
 463                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 464                         let obligations = self.nominal_obligations(did, substs);
 465                         self.out.extend(obligations);
 466                     }
 467                 }
 468
 469                 ty::Dynamic(data, r) => {
 470                     // WfObject
 471                     //
 472                     // Here, we defer WF checking due to higher-ranked
 473                     // regions. This is perhaps not ideal.
 474                     self.from_object_ty(ty, data, r);
 475
 476                     // FIXME(#27579) RFC also considers adding trait
 477                     // obligations that don't refer to Self and
 478                     // checking those
 479
 480                     let defer_to_coercion = self.infcx.tcx.features().object_safe_for_dispatch;
 481
 482                     if !defer_to_coercion {
 483                         let cause = self.cause(traits::MiscObligation);
 484                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 485                         self.out.extend(component_traits.map(|did| {
 486                             traits::Obligation::new(
 487                                 cause.clone(),
 488                                 param_env,
 489                                 ty::Predicate::ObjectSafe(did),
 490                             )
 491                         }));
 492                     }
 493                 }
 494
 495                 // Inference variables are the complicated case, since we don't
 496                 // know what type they are. We do two things:
 497                 //
 498                 // 1. Check if they have been resolved, and if so proceed with
 499                 //    THAT type.
 500                 // 2. If not, check whether this is the type that we
 501                 //    started with (ty0). In that case, we've made no
 502                 //    progress at all, so return false. Otherwise,
 503                 //    we've at least simplified things (i.e., we went
 504                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 505                 //    register a pending obligation and keep
 506                 //    moving. (Goal is that an "inductive hypothesis"
 507                 //    is satisfied to ensure termination.)
 508                 ty::Infer(_) => {
 509                     let ty = self.infcx.shallow_resolve(ty);
 510                     if let ty::Infer(_) = ty.kind {
 511                         // not yet resolved...
 512                         if ty == ty0 {
 513                             // ...this is the type we started from! no progress.
 514                             return false;
 515                         }
 516
 517                         let cause = self.cause(traits::MiscObligation);
 518                         self.out.push(
 519                             // ...not the type we started from, so we made progress.
 520                             traits::Obligation::new(
 521                                 cause,
 522                                 self.param_env,
 523                                 ty::Predicate::WellFormed(ty),
 524                             ),
 525                         );
 526                     } else {
 527                         // Yes, resolved, proceed with the
 528                         // result. Should never return false because
 529                         // `ty` is not a Infer.
 530                         assert!(self.compute(ty));
 531                     }
 532                 }
 533             }
 534         }
 535
 536         // if we made it through that loop above, we made progress!
 537         true
 538     }
 539
 540     fn nominal_obligations(
 541         &mut self,
 542         def_id: DefId,
 543         substs: SubstsRef<'tcx>,
 544     ) -> Vec<traits::PredicateObligation<'tcx>> {
 545         let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
 546         predicates
 547             .predicates
 548             .into_iter()
 549             .zip(predicates.spans.into_iter())
 550             .map(|(pred, span)| {
 551                 let cause = self.cause(traits::BindingObligation(def_id, span));
 552                 traits::Obligation::new(cause, self.param_env, pred)
 553             })
 554             .filter(|pred| !pred.has_escaping_bound_vars())
 555             .collect()
 556     }
 557
 558     fn from_object_ty(
 559         &mut self,
 560         ty: Ty<'tcx>,
 561         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 562         region: ty::Region<'tcx>,
 563     ) {
 564         // Imagine a type like this:
 565         //
 566         //     trait Foo { }
 567         //     trait Bar<'c> : 'c { }
 568         //
 569         //     &'b (Foo+'c+Bar<'d>)
 570         //         ^
 571         //
 572         // In this case, the following relationships must hold:
 573         //
 574         //     'b <= 'c
 575         //     'd <= 'c
 576         //
 577         // The first conditions is due to the normal region pointer
 578         // rules, which say that a reference cannot outlive its
 579         // referent.
 580         //
 581         // The final condition may be a bit surprising. In particular,
 582         // you may expect that it would have been `'c <= 'd`, since
 583         // usually lifetimes of outer things are conservative
 584         // approximations for inner things. However, it works somewhat
 585         // differently with trait objects: here the idea is that if the
 586         // user specifies a region bound (`'c`, in this case) it is the
 587         // "master bound" that *implies* that bounds from other traits are
 588         // all met. (Remember that *all bounds* in a type like
 589         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 590         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 591         // 'y.)
 592         //
 593         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 594         // am looking forward to the future here.
 595         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 596             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 597
 598             let explicit_bound = region;
 599
 600             self.out.reserve(implicit_bounds.len());
 601             for implicit_bound in implicit_bounds {
 602                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 603                 let outlives =
 604                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 605                 self.out.push(traits::Obligation::new(
 606                     cause,
 607                     self.param_env,
 608                     outlives.to_predicate(),
 609                 ));
 610             }
 611         }
 612     }
 613 }
 614
 615 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 616 /// bounds that must hold on the elided self type. These are derived
 617 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 618 /// they declare `trait SomeTrait : 'static`, for example, then
 619 /// `'static` would appear in the list. The hard work is done by
 620 /// `infer::required_region_bounds`, see that for more information.
 621 pub fn object_region_bounds<'tcx>(
 622     tcx: TyCtxt<'tcx>,
 623     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 624 ) -> Vec<ty::Region<'tcx>> {
 625     // Since we don't actually *know* the self type for an object,
 626     // this "open(err)" serves as a kind of dummy standin -- basically
 627     // a placeholder type.
 628     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 629
 630     let predicates = existential_predicates
 631         .iter()
 632         .filter_map(|predicate| {
 633             if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 634                 None
 635             } else {
 636                 Some(predicate.with_self_ty(tcx, open_ty))
 637             }
 638         })
 639         .collect();
 640
 641     required_region_bounds(tcx, open_ty, predicates)
 642 }