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::{GenericArg, 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 `arg` well-formed.
  13 /// If `arg` contains unresolved inference variables, this may include
  14 /// further WF obligations. However, if `arg` 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     arg: GenericArg<'tcx>,
  23     span: Span,
  24 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  25     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  26     let arg = match arg.unpack() {
  27         GenericArgKind::Type(ty) => {
  28             match ty.kind {
  29                 ty::Infer(ty::TyVar(_)) => {
  30                     let resolved_ty = infcx.shallow_resolve(ty);
  31                     if resolved_ty == ty {
  32                         // No progress, bail out to prevent "livelock".
  33                         return None;
  34                     }
  35
  36                     resolved_ty
  37                 }
  38                 _ => ty,
  39             }
  40             .into()
  41         }
  42         GenericArgKind::Const(ct) => {
  43             match ct.val {
  44                 ty::ConstKind::Infer(infer) => {
  45                     let resolved = infcx.shallow_resolve(infer);
  46                     if resolved == infer {
  47                         // No progress.
  48                         return None;
  49                     }
  50
  51                     infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Infer(resolved), ty: ct.ty })
  52                 }
  53                 _ => ct,
  54             }
  55             .into()
  56         }
  57         // There is nothing we have to do for lifetimes.
  58         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  59     };
  60
  61     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  62     wf.compute(arg);
  63     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  64
  65     let result = wf.normalize();
  66     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  67     Some(result)
  68 }
  69
  70 /// Returns the obligations that make this trait reference
  71 /// well-formed.  For example, if there is a trait `Set` defined like
  72 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  73 /// if `Bar: Eq`.
  74 pub fn trait_obligations<'a, 'tcx>(
  75     infcx: &InferCtxt<'a, 'tcx>,
  76     param_env: ty::ParamEnv<'tcx>,
  77     body_id: hir::HirId,
  78     trait_ref: &ty::TraitRef<'tcx>,
  79     span: Span,
  80     item: Option<&'tcx hir::Item<'tcx>>,
  81 ) -> Vec<traits::PredicateObligation<'tcx>> {
  82     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
  83     wf.compute_trait_ref(trait_ref, Elaborate::All);
  84     wf.normalize()
  85 }
  86
  87 pub fn predicate_obligations<'a, 'tcx>(
  88     infcx: &InferCtxt<'a, 'tcx>,
  89     param_env: ty::ParamEnv<'tcx>,
  90     body_id: hir::HirId,
  91     predicate: ty::Predicate<'tcx>,
  92     span: Span,
  93 ) -> Vec<traits::PredicateObligation<'tcx>> {
  94     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  95
  96     // (*) ok to skip binders, because wf code is prepared for it
  97     match predicate.kind() {
  98         ty::PredicateKind::Trait(t, _) => {
  99             wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
 100         }
 101         ty::PredicateKind::RegionOutlives(..) => {}
 102         ty::PredicateKind::TypeOutlives(t) => {
 103             wf.compute(t.skip_binder().0.into());
 104         }
 105         ty::PredicateKind::Projection(t) => {
 106             let t = t.skip_binder(); // (*)
 107             wf.compute_projection(t.projection_ty);
 108             wf.compute(t.ty.into());
 109         }
 110         &ty::PredicateKind::WellFormed(arg) => {
 111             wf.compute(arg);
 112         }
 113         ty::PredicateKind::ObjectSafe(_) => {}
 114         ty::PredicateKind::ClosureKind(..) => {}
 115         ty::PredicateKind::Subtype(data) => {
 116             wf.compute(data.skip_binder().a.into()); // (*)
 117             wf.compute(data.skip_binder().b.into()); // (*)
 118         }
 119         &ty::PredicateKind::ConstEvaluatable(def, substs) => {
 120             let obligations = wf.nominal_obligations(def.did, substs);
 121             wf.out.extend(obligations);
 122
 123             for arg in substs.iter() {
 124                 wf.compute(arg);
 125             }
 126         }
 127         &ty::PredicateKind::ConstEquate(c1, c2) => {
 128             wf.compute(c1.into());
 129             wf.compute(c2.into());
 130         }
 131     }
 132
 133     wf.normalize()
 134 }
 135
 136 struct WfPredicates<'a, 'tcx> {
 137     infcx: &'a InferCtxt<'a, 'tcx>,
 138     param_env: ty::ParamEnv<'tcx>,
 139     body_id: hir::HirId,
 140     span: Span,
 141     out: Vec<traits::PredicateObligation<'tcx>>,
 142     item: Option<&'tcx hir::Item<'tcx>>,
 143 }
 144
 145 /// Controls whether we "elaborate" supertraits and so forth on the WF
 146 /// predicates. This is a kind of hack to address #43784. The
 147 /// underlying problem in that issue was a trait structure like:
 148 ///
 149 /// ```
 150 /// trait Foo: Copy { }
 151 /// trait Bar: Foo { }
 152 /// impl<T: Bar> Foo for T { }
 153 /// impl<T> Bar for T { }
 154 /// ```
 155 ///
 156 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 157 /// we decide that this is true because `T: Bar` is in the
 158 /// where-clauses (and we can elaborate that to include `T:
 159 /// Copy`). This wouldn't be a problem, except that when we check the
 160 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 161 /// impl. And so nowhere did we check that `T: Copy` holds!
 162 ///
 163 /// To resolve this, we elaborate the WF requirements that must be
 164 /// proven when checking impls. This means that (e.g.) the `impl Bar
 165 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 166 /// Copy` (which it won't be able to do, because there is no `Copy`
 167 /// impl for `T`).
 168 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 169 enum Elaborate {
 170     All,
 171     None,
 172 }
 173
 174 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 175     tcx: TyCtxt<'tcx>,
 176     trait_ref: &ty::TraitRef<'tcx>,
 177     item: Option<&hir::Item<'tcx>>,
 178     cause: &mut traits::ObligationCause<'tcx>,
 179     pred: &ty::Predicate<'_>,
 180     mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>,
 181 ) {
 182     debug!(
 183         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 184         trait_ref, item, cause, pred
 185     );
 186     let items = match item {
 187         Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
 188         _ => return,
 189     };
 190     let fix_span =
 191         |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
 192             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 193             _ => impl_item_ref.span,
 194         };
 195     match pred.kind() {
 196         ty::PredicateKind::Projection(proj) => {
 197             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 198             // but rather from a "second order" obligation, where an associated type has a
 199             // projection coming from another associated type. See
 200             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 201             // `traits-assoc-type-in-supertrait-bad.rs`.
 202             let kind = &proj.ty().skip_binder().kind;
 203             if let ty::Projection(projection_ty) = kind {
 204                 let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
 205                 if let Some(impl_item_span) =
 206                     items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 207                 {
 208                     cause.make_mut().span = impl_item_span;
 209                 }
 210             }
 211         }
 212         ty::PredicateKind::Trait(pred, _) => {
 213             // An associated item obligation born out of the `trait` failed to be met. An example
 214             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 215             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 216             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) =
 217                 &pred.skip_binder().self_ty().kind
 218             {
 219                 if let Some(impl_item_span) = trait_assoc_items
 220                     .find(|i| i.def_id == *item_def_id)
 221                     .and_then(|trait_assoc_item| {
 222                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
 223                     })
 224                 {
 225                     cause.make_mut().span = impl_item_span;
 226                 }
 227             }
 228         }
 229         _ => {}
 230     }
 231 }
 232
 233 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 234     fn tcx(&self) -> TyCtxt<'tcx> {
 235         self.infcx.tcx
 236     }
 237
 238     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 239         traits::ObligationCause::new(self.span, self.body_id, code)
 240     }
 241
 242     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 243         let cause = self.cause(traits::MiscObligation);
 244         let infcx = &mut self.infcx;
 245         let param_env = self.param_env;
 246         let mut obligations = Vec::with_capacity(self.out.len());
 247         for pred in &self.out {
 248             assert!(!pred.has_escaping_bound_vars());
 249             let mut selcx = traits::SelectionContext::new(infcx);
 250             let i = obligations.len();
 251             let value =
 252                 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
 253             obligations.insert(i, value);
 254         }
 255         obligations
 256     }
 257
 258     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 259     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 260         let tcx = self.infcx.tcx;
 261         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 262
 263         debug!("compute_trait_ref obligations {:?}", obligations);
 264         let cause = self.cause(traits::MiscObligation);
 265         let param_env = self.param_env;
 266
 267         let item = self.item;
 268
 269         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 270             let mut cause = cause.clone();
 271             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
 272                 let derived_cause = traits::DerivedObligationCause {
 273                     parent_trait_ref,
 274                     parent_code: Rc::new(obligation.cause.code.clone()),
 275                 };
 276                 cause.make_mut().code =
 277                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 278             }
 279             extend_cause_with_original_assoc_item_obligation(
 280                 tcx,
 281                 trait_ref,
 282                 item,
 283                 &mut cause,
 284                 &obligation.predicate,
 285                 tcx.associated_items(trait_ref.def_id).in_definition_order(),
 286             );
 287             traits::Obligation::new(cause, param_env, obligation.predicate)
 288         };
 289
 290         if let Elaborate::All = elaborate {
 291             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 292             let implied_obligations = implied_obligations.map(extend);
 293             self.out.extend(implied_obligations);
 294         } else {
 295             self.out.extend(obligations);
 296         }
 297
 298         let tcx = self.tcx();
 299         self.out.extend(
 300             trait_ref
 301                 .substs
 302                 .iter()
 303                 .filter(|arg| {
 304                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 305                 })
 306                 .filter(|arg| !arg.has_escaping_bound_vars())
 307                 .map(|arg| {
 308                     traits::Obligation::new(
 309                         cause.clone(),
 310                         param_env,
 311                         ty::PredicateKind::WellFormed(arg).to_predicate(tcx),
 312                     )
 313                 }),
 314         );
 315     }
 316
 317     /// Pushes the obligations required for `trait_ref::Item` to be WF
 318     /// into `self.out`.
 319     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 320         // A projection is well-formed if (a) the trait ref itself is
 321         // WF and (b) the trait-ref holds.  (It may also be
 322         // normalizable and be WF that way.)
 323         let trait_ref = data.trait_ref(self.infcx.tcx);
 324         self.compute_trait_ref(&trait_ref, Elaborate::None);
 325
 326         if !data.has_escaping_bound_vars() {
 327             let predicate = trait_ref.without_const().to_predicate(self.infcx.tcx);
 328             let cause = self.cause(traits::ProjectionWf(data));
 329             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 330         }
 331     }
 332
 333     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 334         if !subty.has_escaping_bound_vars() {
 335             let cause = self.cause(cause);
 336             let trait_ref = ty::TraitRef {
 337                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
 338                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 339             };
 340             self.out.push(traits::Obligation::new(
 341                 cause,
 342                 self.param_env,
 343                 trait_ref.without_const().to_predicate(self.infcx.tcx),
 344             ));
 345         }
 346     }
 347
 348     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 349     fn compute(&mut self, arg: GenericArg<'tcx>) {
 350         let mut walker = arg.walk();
 351         let param_env = self.param_env;
 352         while let Some(arg) = walker.next() {
 353             let ty = match arg.unpack() {
 354                 GenericArgKind::Type(ty) => ty,
 355
 356                 // No WF constraints for lifetimes being present, any outlives
 357                 // obligations are handled by the parent (e.g. `ty::Ref`).
 358                 GenericArgKind::Lifetime(_) => continue,
 359
 360                 GenericArgKind::Const(constant) => {
 361                     match constant.val {
 362                         ty::ConstKind::Unevaluated(def, substs, promoted) => {
 363                             assert!(promoted.is_none());
 364
 365                             let obligations = self.nominal_obligations(def.did, substs);
 366                             self.out.extend(obligations);
 367
 368                             let predicate = ty::PredicateKind::ConstEvaluatable(def, substs)
 369                                 .to_predicate(self.tcx());
 370                             let cause = self.cause(traits::MiscObligation);
 371                             self.out.push(traits::Obligation::new(
 372                                 cause,
 373                                 self.param_env,
 374                                 predicate,
 375                             ));
 376                         }
 377                         ty::ConstKind::Infer(infer) => {
 378                             let resolved = self.infcx.shallow_resolve(infer);
 379                             // the `InferConst` changed, meaning that we made progress.
 380                             if resolved != infer {
 381                                 let cause = self.cause(traits::MiscObligation);
 382
 383                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 384                                     val: ty::ConstKind::Infer(resolved),
 385                                     ..*constant
 386                                 });
 387                                 self.out.push(traits::Obligation::new(
 388                                     cause,
 389                                     self.param_env,
 390                                     ty::PredicateKind::WellFormed(resolved_constant.into())
 391                                         .to_predicate(self.tcx()),
 392                                 ));
 393                             }
 394                         }
 395                         ty::ConstKind::Error(_)
 396                         | ty::ConstKind::Param(_)
 397                         | ty::ConstKind::Bound(..)
 398                         | ty::ConstKind::Placeholder(..) => {
 399                             // These variants are trivially WF, so nothing to do here.
 400                         }
 401                         ty::ConstKind::Value(..) => {
 402                             // FIXME: Enforce that values are structurally-matchable.
 403                         }
 404                     }
 405                     continue;
 406                 }
 407             };
 408
 409             match ty.kind {
 410                 ty::Bool
 411                 | ty::Char
 412                 | ty::Int(..)
 413                 | ty::Uint(..)
 414                 | ty::Float(..)
 415                 | ty::Error(_)
 416                 | ty::Str
 417                 | ty::GeneratorWitness(..)
 418                 | ty::Never
 419                 | ty::Param(_)
 420                 | ty::Bound(..)
 421                 | ty::Placeholder(..)
 422                 | ty::Foreign(..) => {
 423                     // WfScalar, WfParameter, etc
 424                 }
 425
 426                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 427                 ty::Infer(ty::IntVar(_)) => {}
 428
 429                 // Can only infer to `ty::Float(_)`.
 430                 ty::Infer(ty::FloatVar(_)) => {}
 431
 432                 ty::Slice(subty) => {
 433                     self.require_sized(subty, traits::SliceOrArrayElem);
 434                 }
 435
 436                 ty::Array(subty, _) => {
 437                     self.require_sized(subty, traits::SliceOrArrayElem);
 438                     // Note that we handle the len is implicitly checked while walking `arg`.
 439                 }
 440
 441                 ty::Tuple(ref tys) => {
 442                     if let Some((_last, rest)) = tys.split_last() {
 443                         for elem in rest {
 444                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 445                         }
 446                     }
 447                 }
 448
 449                 ty::RawPtr(_) => {
 450                     // Simple cases that are WF if their type args are WF.
 451                 }
 452
 453                 ty::Projection(data) => {
 454                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 455                     self.compute_projection(data);
 456                 }
 457
 458                 ty::Adt(def, substs) => {
 459                     // WfNominalType
 460                     let obligations = self.nominal_obligations(def.did, substs);
 461                     self.out.extend(obligations);
 462                 }
 463
 464                 ty::FnDef(did, substs) => {
 465                     let obligations = self.nominal_obligations(did, substs);
 466                     self.out.extend(obligations);
 467                 }
 468
 469                 ty::Ref(r, rty, _) => {
 470                     // WfReference
 471                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 472                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 473                         self.out.push(traits::Obligation::new(
 474                             cause,
 475                             param_env,
 476                             ty::PredicateKind::TypeOutlives(ty::Binder::dummy(
 477                                 ty::OutlivesPredicate(rty, r),
 478                             ))
 479                             .to_predicate(self.tcx()),
 480                         ));
 481                     }
 482                 }
 483
 484                 ty::Generator(..) => {
 485                     // Walk ALL the types in the generator: this will
 486                     // include the upvar types as well as the yield
 487                     // type. Note that this is mildly distinct from
 488                     // the closure case, where we have to be careful
 489                     // about the signature of the closure. We don't
 490                     // have the problem of implied bounds here since
 491                     // generators don't take arguments.
 492                 }
 493
 494                 ty::Closure(_, substs) => {
 495                     // Only check the upvar types for WF, not the rest
 496                     // of the types within. This is needed because we
 497                     // capture the signature and it may not be WF
 498                     // without the implied bounds. Consider a closure
 499                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 500                     // not known to hold in the creator's context (and
 501                     // indeed the closure may not be invoked by its
 502                     // creator, but rather turned to someone who *can*
 503                     // verify that).
 504                     //
 505                     // The special treatment of closures here really
 506                     // ought not to be necessary either; the problem
 507                     // is related to #25860 -- there is no way for us
 508                     // to express a fn type complete with the implied
 509                     // bounds that it is assuming. I think in reality
 510                     // the WF rules around fn are a bit messed up, and
 511                     // that is the rot problem: `fn(&'a T)` should
 512                     // probably always be WF, because it should be
 513                     // shorthand for something like `where(T: 'a) {
 514                     // fn(&'a T) }`, as discussed in #25860.
 515                     //
 516                     // Note that we are also skipping the generic
 517                     // types. This is consistent with the `outlives`
 518                     // code, but anyway doesn't matter: within the fn
 519                     // body where they are created, the generics will
 520                     // always be WF, and outside of that fn body we
 521                     // are not directly inspecting closure types
 522                     // anyway, except via auto trait matching (which
 523                     // only inspects the upvar types).
 524                     walker.skip_current_subtree(); // subtree handled below
 525                     for upvar_ty in substs.as_closure().upvar_tys() {
 526                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 527                         self.compute(upvar_ty.into());
 528                     }
 529                 }
 530
 531                 ty::FnPtr(_) => {
 532                     // let the loop iterate into the argument/return
 533                     // types appearing in the fn signature
 534                 }
 535
 536                 ty::Opaque(did, substs) => {
 537                     // all of the requirements on type parameters
 538                     // should've been checked by the instantiation
 539                     // of whatever returned this exact `impl Trait`.
 540
 541                     // for named opaque `impl Trait` types we still need to check them
 542                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 543                         let obligations = self.nominal_obligations(did, substs);
 544                         self.out.extend(obligations);
 545                     }
 546                 }
 547
 548                 ty::Dynamic(data, r) => {
 549                     // WfObject
 550                     //
 551                     // Here, we defer WF checking due to higher-ranked
 552                     // regions. This is perhaps not ideal.
 553                     self.from_object_ty(ty, data, r);
 554
 555                     // FIXME(#27579) RFC also considers adding trait
 556                     // obligations that don't refer to Self and
 557                     // checking those
 558
 559                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 560
 561                     if !defer_to_coercion {
 562                         let cause = self.cause(traits::MiscObligation);
 563                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 564                         let tcx = self.tcx();
 565                         self.out.extend(component_traits.map(|did| {
 566                             traits::Obligation::new(
 567                                 cause.clone(),
 568                                 param_env,
 569                                 ty::PredicateKind::ObjectSafe(did).to_predicate(tcx),
 570                             )
 571                         }));
 572                     }
 573                 }
 574
 575                 // Inference variables are the complicated case, since we don't
 576                 // know what type they are. We do two things:
 577                 //
 578                 // 1. Check if they have been resolved, and if so proceed with
 579                 //    THAT type.
 580                 // 2. If not, we've at least simplified things (e.g., we went
 581                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 582                 //    register a pending obligation and keep
 583                 //    moving. (Goal is that an "inductive hypothesis"
 584                 //    is satisfied to ensure termination.)
 585                 // See also the comment on `fn obligations`, describing "livelock"
 586                 // prevention, which happens before this can be reached.
 587                 ty::Infer(_) => {
 588                     let ty = self.infcx.shallow_resolve(ty);
 589                     if let ty::Infer(ty::TyVar(_)) = ty.kind {
 590                         // Not yet resolved, but we've made progress.
 591                         let cause = self.cause(traits::MiscObligation);
 592                         self.out.push(traits::Obligation::new(
 593                             cause,
 594                             param_env,
 595                             ty::PredicateKind::WellFormed(ty.into()).to_predicate(self.tcx()),
 596                         ));
 597                     } else {
 598                         // Yes, resolved, proceed with the result.
 599                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 600                         self.compute(ty.into());
 601                     }
 602                 }
 603             }
 604         }
 605     }
 606
 607     fn nominal_obligations(
 608         &mut self,
 609         def_id: DefId,
 610         substs: SubstsRef<'tcx>,
 611     ) -> Vec<traits::PredicateObligation<'tcx>> {
 612         let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
 613         predicates
 614             .predicates
 615             .into_iter()
 616             .zip(predicates.spans.into_iter())
 617             .map(|(pred, span)| {
 618                 let cause = self.cause(traits::BindingObligation(def_id, span));
 619                 traits::Obligation::new(cause, self.param_env, pred)
 620             })
 621             .filter(|pred| !pred.has_escaping_bound_vars())
 622             .collect()
 623     }
 624
 625     fn from_object_ty(
 626         &mut self,
 627         ty: Ty<'tcx>,
 628         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 629         region: ty::Region<'tcx>,
 630     ) {
 631         // Imagine a type like this:
 632         //
 633         //     trait Foo { }
 634         //     trait Bar<'c> : 'c { }
 635         //
 636         //     &'b (Foo+'c+Bar<'d>)
 637         //         ^
 638         //
 639         // In this case, the following relationships must hold:
 640         //
 641         //     'b <= 'c
 642         //     'd <= 'c
 643         //
 644         // The first conditions is due to the normal region pointer
 645         // rules, which say that a reference cannot outlive its
 646         // referent.
 647         //
 648         // The final condition may be a bit surprising. In particular,
 649         // you may expect that it would have been `'c <= 'd`, since
 650         // usually lifetimes of outer things are conservative
 651         // approximations for inner things. However, it works somewhat
 652         // differently with trait objects: here the idea is that if the
 653         // user specifies a region bound (`'c`, in this case) it is the
 654         // "master bound" that *implies* that bounds from other traits are
 655         // all met. (Remember that *all bounds* in a type like
 656         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 657         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 658         // 'y.)
 659         //
 660         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 661         // am looking forward to the future here.
 662         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 663             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 664
 665             let explicit_bound = region;
 666
 667             self.out.reserve(implicit_bounds.len());
 668             for implicit_bound in implicit_bounds {
 669                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 670                 let outlives =
 671                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 672                 self.out.push(traits::Obligation::new(
 673                     cause,
 674                     self.param_env,
 675                     outlives.to_predicate(self.infcx.tcx),
 676                 ));
 677             }
 678         }
 679     }
 680 }
 681
 682 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 683 /// bounds that must hold on the elided self type. These are derived
 684 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 685 /// they declare `trait SomeTrait : 'static`, for example, then
 686 /// `'static` would appear in the list. The hard work is done by
 687 /// `infer::required_region_bounds`, see that for more information.
 688 pub fn object_region_bounds<'tcx>(
 689     tcx: TyCtxt<'tcx>,
 690     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 691 ) -> Vec<ty::Region<'tcx>> {
 692     // Since we don't actually *know* the self type for an object,
 693     // this "open(err)" serves as a kind of dummy standin -- basically
 694     // a placeholder type.
 695     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 696
 697     let predicates = existential_predicates.iter().filter_map(|predicate| {
 698         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 699             None
 700         } else {
 701             Some(predicate.with_self_ty(tcx, open_ty))
 702         }
 703     });
 704
 705     required_region_bounds(tcx, open_ty, predicates)
 706 }