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     match predicate.kind() {
  97         ty::PredicateKind::ForAll(binder) => {
  98             // It's ok to skip the binder here because wf code is prepared for it
  99             return predicate_obligations(infcx, param_env, body_id, binder.skip_binder(), span);
 100         }
 101         ty::PredicateKind::Trait(t, _) => {
 102             wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
 103         }
 104         ty::PredicateKind::RegionOutlives(..) => {}
 105         &ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 106             wf.compute(ty.into());
 107         }
 108         ty::PredicateKind::Projection(t) => {
 109             wf.compute_projection(t.projection_ty);
 110             wf.compute(t.ty.into());
 111         }
 112         &ty::PredicateKind::WellFormed(arg) => {
 113             wf.compute(arg);
 114         }
 115         ty::PredicateKind::ObjectSafe(_) => {}
 116         ty::PredicateKind::ClosureKind(..) => {}
 117         &ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 118             wf.compute(a.into());
 119             wf.compute(b.into());
 120         }
 121         &ty::PredicateKind::ConstEvaluatable(def, substs) => {
 122             let obligations = wf.nominal_obligations(def.did, substs);
 123             wf.out.extend(obligations);
 124
 125             for arg in substs.iter() {
 126                 wf.compute(arg);
 127             }
 128         }
 129         &ty::PredicateKind::ConstEquate(c1, c2) => {
 130             wf.compute(c1.into());
 131             wf.compute(c2.into());
 132         }
 133     }
 134
 135     wf.normalize()
 136 }
 137
 138 struct WfPredicates<'a, 'tcx> {
 139     infcx: &'a InferCtxt<'a, 'tcx>,
 140     param_env: ty::ParamEnv<'tcx>,
 141     body_id: hir::HirId,
 142     span: Span,
 143     out: Vec<traits::PredicateObligation<'tcx>>,
 144     item: Option<&'tcx hir::Item<'tcx>>,
 145 }
 146
 147 /// Controls whether we "elaborate" supertraits and so forth on the WF
 148 /// predicates. This is a kind of hack to address #43784. The
 149 /// underlying problem in that issue was a trait structure like:
 150 ///
 151 /// ```
 152 /// trait Foo: Copy { }
 153 /// trait Bar: Foo { }
 154 /// impl<T: Bar> Foo for T { }
 155 /// impl<T> Bar for T { }
 156 /// ```
 157 ///
 158 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 159 /// we decide that this is true because `T: Bar` is in the
 160 /// where-clauses (and we can elaborate that to include `T:
 161 /// Copy`). This wouldn't be a problem, except that when we check the
 162 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 163 /// impl. And so nowhere did we check that `T: Copy` holds!
 164 ///
 165 /// To resolve this, we elaborate the WF requirements that must be
 166 /// proven when checking impls. This means that (e.g.) the `impl Bar
 167 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 168 /// Copy` (which it won't be able to do, because there is no `Copy`
 169 /// impl for `T`).
 170 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 171 enum Elaborate {
 172     All,
 173     None,
 174 }
 175
 176 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 177     tcx: TyCtxt<'tcx>,
 178     trait_ref: &ty::TraitRef<'tcx>,
 179     item: Option<&hir::Item<'tcx>>,
 180     cause: &mut traits::ObligationCause<'tcx>,
 181     pred: &ty::Predicate<'tcx>,
 182     mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>,
 183 ) {
 184     debug!(
 185         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 186         trait_ref, item, cause, pred
 187     );
 188     let items = match item {
 189         Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
 190         _ => return,
 191     };
 192     let fix_span =
 193         |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
 194             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 195             _ => impl_item_ref.span,
 196         };
 197
 198     // It is fine to skip the binder as we don't care about regions here.
 199     match pred.ignore_quantifiers().skip_binder().kind() {
 200         ty::PredicateKind::Projection(proj) => {
 201             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 202             // but rather from a "second order" obligation, where an associated type has a
 203             // projection coming from another associated type. See
 204             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 205             // `traits-assoc-type-in-supertrait-bad.rs`.
 206             if let ty::Projection(projection_ty) = proj.ty.kind {
 207                 let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
 208                 if let Some(impl_item_span) =
 209                     items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 210                 {
 211                     cause.make_mut().span = impl_item_span;
 212                 }
 213             }
 214         }
 215         ty::PredicateKind::Trait(pred, _) => {
 216             // An associated item obligation born out of the `trait` failed to be met. An example
 217             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 218             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 219             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = pred.self_ty().kind {
 220                 if let Some(impl_item_span) = trait_assoc_items
 221                     .find(|i| i.def_id == item_def_id)
 222                     .and_then(|trait_assoc_item| {
 223                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
 224                     })
 225                 {
 226                     cause.make_mut().span = impl_item_span;
 227                 }
 228             }
 229         }
 230         _ => {}
 231     }
 232 }
 233
 234 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 235     fn tcx(&self) -> TyCtxt<'tcx> {
 236         self.infcx.tcx
 237     }
 238
 239     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 240         traits::ObligationCause::new(self.span, self.body_id, code)
 241     }
 242
 243     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 244         let cause = self.cause(traits::MiscObligation);
 245         let infcx = &mut self.infcx;
 246         let param_env = self.param_env;
 247         let mut obligations = Vec::with_capacity(self.out.len());
 248         for pred in &self.out {
 249             assert!(!pred.has_escaping_bound_vars());
 250             let mut selcx = traits::SelectionContext::new(infcx);
 251             let i = obligations.len();
 252             let value =
 253                 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
 254             obligations.insert(i, value);
 255         }
 256         obligations
 257     }
 258
 259     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 260     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 261         let tcx = self.infcx.tcx;
 262         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 263
 264         debug!("compute_trait_ref obligations {:?}", obligations);
 265         let cause = self.cause(traits::MiscObligation);
 266         let param_env = self.param_env;
 267
 268         let item = self.item;
 269
 270         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 271             let mut cause = cause.clone();
 272             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
 273                 let derived_cause = traits::DerivedObligationCause {
 274                     parent_trait_ref,
 275                     parent_code: Rc::new(obligation.cause.code.clone()),
 276                 };
 277                 cause.make_mut().code =
 278                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 279             }
 280             extend_cause_with_original_assoc_item_obligation(
 281                 tcx,
 282                 trait_ref,
 283                 item,
 284                 &mut cause,
 285                 &obligation.predicate,
 286                 tcx.associated_items(trait_ref.def_id).in_definition_order(),
 287             );
 288             traits::Obligation::new(cause, param_env, obligation.predicate)
 289         };
 290
 291         if let Elaborate::All = elaborate {
 292             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 293             let implied_obligations = implied_obligations.map(extend);
 294             self.out.extend(implied_obligations);
 295         } else {
 296             self.out.extend(obligations);
 297         }
 298
 299         let tcx = self.tcx();
 300         self.out.extend(
 301             trait_ref
 302                 .substs
 303                 .iter()
 304                 .enumerate()
 305                 .filter(|(_, arg)| {
 306                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 307                 })
 308                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 309                 .map(|(i, arg)| {
 310                     let mut new_cause = cause.clone();
 311                     // The first subst is the self ty - use the correct span for it.
 312                     if i == 0 {
 313                         if let Some(hir::ItemKind::Impl { self_ty, .. }) = item.map(|i| &i.kind) {
 314                             new_cause.make_mut().span = self_ty.span;
 315                         }
 316                     }
 317                     traits::Obligation::new(
 318                         new_cause,
 319                         param_env,
 320                         ty::PredicateKind::WellFormed(arg).to_predicate(tcx),
 321                     )
 322                 }),
 323         );
 324     }
 325
 326     /// Pushes the obligations required for `trait_ref::Item` to be WF
 327     /// into `self.out`.
 328     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 329         // A projection is well-formed if (a) the trait ref itself is
 330         // WF and (b) the trait-ref holds.  (It may also be
 331         // normalizable and be WF that way.)
 332         let trait_ref = data.trait_ref(self.infcx.tcx);
 333         self.compute_trait_ref(&trait_ref, Elaborate::None);
 334
 335         if !data.has_escaping_bound_vars() {
 336             let predicate = trait_ref.without_const().to_predicate(self.infcx.tcx);
 337             let cause = self.cause(traits::ProjectionWf(data));
 338             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 339         }
 340     }
 341
 342     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 343         if !subty.has_escaping_bound_vars() {
 344             let cause = self.cause(cause);
 345             let trait_ref = ty::TraitRef {
 346                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
 347                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 348             };
 349             self.out.push(traits::Obligation::new(
 350                 cause,
 351                 self.param_env,
 352                 trait_ref.without_const().to_predicate(self.infcx.tcx),
 353             ));
 354         }
 355     }
 356
 357     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 358     fn compute(&mut self, arg: GenericArg<'tcx>) {
 359         let mut walker = arg.walk();
 360         let param_env = self.param_env;
 361         while let Some(arg) = walker.next() {
 362             let ty = match arg.unpack() {
 363                 GenericArgKind::Type(ty) => ty,
 364
 365                 // No WF constraints for lifetimes being present, any outlives
 366                 // obligations are handled by the parent (e.g. `ty::Ref`).
 367                 GenericArgKind::Lifetime(_) => continue,
 368
 369                 GenericArgKind::Const(constant) => {
 370                     match constant.val {
 371                         ty::ConstKind::Unevaluated(def, substs, promoted) => {
 372                             assert!(promoted.is_none());
 373
 374                             let obligations = self.nominal_obligations(def.did, substs);
 375                             self.out.extend(obligations);
 376
 377                             let predicate = ty::PredicateKind::ConstEvaluatable(def, substs)
 378                                 .to_predicate(self.tcx());
 379                             let cause = self.cause(traits::MiscObligation);
 380                             self.out.push(traits::Obligation::new(
 381                                 cause,
 382                                 self.param_env,
 383                                 predicate,
 384                             ));
 385                         }
 386                         ty::ConstKind::Infer(infer) => {
 387                             let resolved = self.infcx.shallow_resolve(infer);
 388                             // the `InferConst` changed, meaning that we made progress.
 389                             if resolved != infer {
 390                                 let cause = self.cause(traits::MiscObligation);
 391
 392                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 393                                     val: ty::ConstKind::Infer(resolved),
 394                                     ..*constant
 395                                 });
 396                                 self.out.push(traits::Obligation::new(
 397                                     cause,
 398                                     self.param_env,
 399                                     ty::PredicateKind::WellFormed(resolved_constant.into())
 400                                         .to_predicate(self.tcx()),
 401                                 ));
 402                             }
 403                         }
 404                         ty::ConstKind::Error(_)
 405                         | ty::ConstKind::Param(_)
 406                         | ty::ConstKind::Bound(..)
 407                         | ty::ConstKind::Placeholder(..) => {
 408                             // These variants are trivially WF, so nothing to do here.
 409                         }
 410                         ty::ConstKind::Value(..) => {
 411                             // FIXME: Enforce that values are structurally-matchable.
 412                         }
 413                     }
 414                     continue;
 415                 }
 416             };
 417
 418             match ty.kind {
 419                 ty::Bool
 420                 | ty::Char
 421                 | ty::Int(..)
 422                 | ty::Uint(..)
 423                 | ty::Float(..)
 424                 | ty::Error(_)
 425                 | ty::Str
 426                 | ty::GeneratorWitness(..)
 427                 | ty::Never
 428                 | ty::Param(_)
 429                 | ty::Bound(..)
 430                 | ty::Placeholder(..)
 431                 | ty::Foreign(..) => {
 432                     // WfScalar, WfParameter, etc
 433                 }
 434
 435                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 436                 ty::Infer(ty::IntVar(_)) => {}
 437
 438                 // Can only infer to `ty::Float(_)`.
 439                 ty::Infer(ty::FloatVar(_)) => {}
 440
 441                 ty::Slice(subty) => {
 442                     self.require_sized(subty, traits::SliceOrArrayElem);
 443                 }
 444
 445                 ty::Array(subty, _) => {
 446                     self.require_sized(subty, traits::SliceOrArrayElem);
 447                     // Note that we handle the len is implicitly checked while walking `arg`.
 448                 }
 449
 450                 ty::Tuple(ref tys) => {
 451                     if let Some((_last, rest)) = tys.split_last() {
 452                         for elem in rest {
 453                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 454                         }
 455                     }
 456                 }
 457
 458                 ty::RawPtr(_) => {
 459                     // Simple cases that are WF if their type args are WF.
 460                 }
 461
 462                 ty::Projection(data) => {
 463                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 464                     self.compute_projection(data);
 465                 }
 466
 467                 ty::Adt(def, substs) => {
 468                     // WfNominalType
 469                     let obligations = self.nominal_obligations(def.did, substs);
 470                     self.out.extend(obligations);
 471                 }
 472
 473                 ty::FnDef(did, substs) => {
 474                     let obligations = self.nominal_obligations(did, substs);
 475                     self.out.extend(obligations);
 476                 }
 477
 478                 ty::Ref(r, rty, _) => {
 479                     // WfReference
 480                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 481                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 482                         self.out.push(traits::Obligation::new(
 483                             cause,
 484                             param_env,
 485                             ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(rty, r))
 486                                 .to_predicate(self.tcx()),
 487                         ));
 488                     }
 489                 }
 490
 491                 ty::Generator(..) => {
 492                     // Walk ALL the types in the generator: this will
 493                     // include the upvar types as well as the yield
 494                     // type. Note that this is mildly distinct from
 495                     // the closure case, where we have to be careful
 496                     // about the signature of the closure. We don't
 497                     // have the problem of implied bounds here since
 498                     // generators don't take arguments.
 499                 }
 500
 501                 ty::Closure(_, substs) => {
 502                     // Only check the upvar types for WF, not the rest
 503                     // of the types within. This is needed because we
 504                     // capture the signature and it may not be WF
 505                     // without the implied bounds. Consider a closure
 506                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 507                     // not known to hold in the creator's context (and
 508                     // indeed the closure may not be invoked by its
 509                     // creator, but rather turned to someone who *can*
 510                     // verify that).
 511                     //
 512                     // The special treatment of closures here really
 513                     // ought not to be necessary either; the problem
 514                     // is related to #25860 -- there is no way for us
 515                     // to express a fn type complete with the implied
 516                     // bounds that it is assuming. I think in reality
 517                     // the WF rules around fn are a bit messed up, and
 518                     // that is the rot problem: `fn(&'a T)` should
 519                     // probably always be WF, because it should be
 520                     // shorthand for something like `where(T: 'a) {
 521                     // fn(&'a T) }`, as discussed in #25860.
 522                     //
 523                     // Note that we are also skipping the generic
 524                     // types. This is consistent with the `outlives`
 525                     // code, but anyway doesn't matter: within the fn
 526                     // body where they are created, the generics will
 527                     // always be WF, and outside of that fn body we
 528                     // are not directly inspecting closure types
 529                     // anyway, except via auto trait matching (which
 530                     // only inspects the upvar types).
 531                     walker.skip_current_subtree(); // subtree handled below
 532                     for upvar_ty in substs.as_closure().upvar_tys() {
 533                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 534                         self.compute(upvar_ty.into());
 535                     }
 536                 }
 537
 538                 ty::FnPtr(_) => {
 539                     // let the loop iterate into the argument/return
 540                     // types appearing in the fn signature
 541                 }
 542
 543                 ty::Opaque(did, substs) => {
 544                     // all of the requirements on type parameters
 545                     // should've been checked by the instantiation
 546                     // of whatever returned this exact `impl Trait`.
 547
 548                     // for named opaque `impl Trait` types we still need to check them
 549                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 550                         let obligations = self.nominal_obligations(did, substs);
 551                         self.out.extend(obligations);
 552                     }
 553                 }
 554
 555                 ty::Dynamic(data, r) => {
 556                     // WfObject
 557                     //
 558                     // Here, we defer WF checking due to higher-ranked
 559                     // regions. This is perhaps not ideal.
 560                     self.from_object_ty(ty, data, r);
 561
 562                     // FIXME(#27579) RFC also considers adding trait
 563                     // obligations that don't refer to Self and
 564                     // checking those
 565
 566                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 567
 568                     if !defer_to_coercion {
 569                         let cause = self.cause(traits::MiscObligation);
 570                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 571                         let tcx = self.tcx();
 572                         self.out.extend(component_traits.map(|did| {
 573                             traits::Obligation::new(
 574                                 cause.clone(),
 575                                 param_env,
 576                                 ty::PredicateKind::ObjectSafe(did).to_predicate(tcx),
 577                             )
 578                         }));
 579                     }
 580                 }
 581
 582                 // Inference variables are the complicated case, since we don't
 583                 // know what type they are. We do two things:
 584                 //
 585                 // 1. Check if they have been resolved, and if so proceed with
 586                 //    THAT type.
 587                 // 2. If not, we've at least simplified things (e.g., we went
 588                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 589                 //    register a pending obligation and keep
 590                 //    moving. (Goal is that an "inductive hypothesis"
 591                 //    is satisfied to ensure termination.)
 592                 // See also the comment on `fn obligations`, describing "livelock"
 593                 // prevention, which happens before this can be reached.
 594                 ty::Infer(_) => {
 595                     let ty = self.infcx.shallow_resolve(ty);
 596                     if let ty::Infer(ty::TyVar(_)) = ty.kind {
 597                         // Not yet resolved, but we've made progress.
 598                         let cause = self.cause(traits::MiscObligation);
 599                         self.out.push(traits::Obligation::new(
 600                             cause,
 601                             param_env,
 602                             ty::PredicateKind::WellFormed(ty.into()).to_predicate(self.tcx()),
 603                         ));
 604                     } else {
 605                         // Yes, resolved, proceed with the result.
 606                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 607                         self.compute(ty.into());
 608                     }
 609                 }
 610             }
 611         }
 612     }
 613
 614     fn nominal_obligations(
 615         &mut self,
 616         def_id: DefId,
 617         substs: SubstsRef<'tcx>,
 618     ) -> Vec<traits::PredicateObligation<'tcx>> {
 619         let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
 620         predicates
 621             .predicates
 622             .into_iter()
 623             .zip(predicates.spans.into_iter())
 624             .map(|(pred, span)| {
 625                 let cause = self.cause(traits::BindingObligation(def_id, span));
 626                 traits::Obligation::new(cause, self.param_env, pred)
 627             })
 628             .filter(|pred| !pred.has_escaping_bound_vars())
 629             .collect()
 630     }
 631
 632     fn from_object_ty(
 633         &mut self,
 634         ty: Ty<'tcx>,
 635         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 636         region: ty::Region<'tcx>,
 637     ) {
 638         // Imagine a type like this:
 639         //
 640         //     trait Foo { }
 641         //     trait Bar<'c> : 'c { }
 642         //
 643         //     &'b (Foo+'c+Bar<'d>)
 644         //         ^
 645         //
 646         // In this case, the following relationships must hold:
 647         //
 648         //     'b <= 'c
 649         //     'd <= 'c
 650         //
 651         // The first conditions is due to the normal region pointer
 652         // rules, which say that a reference cannot outlive its
 653         // referent.
 654         //
 655         // The final condition may be a bit surprising. In particular,
 656         // you may expect that it would have been `'c <= 'd`, since
 657         // usually lifetimes of outer things are conservative
 658         // approximations for inner things. However, it works somewhat
 659         // differently with trait objects: here the idea is that if the
 660         // user specifies a region bound (`'c`, in this case) it is the
 661         // "master bound" that *implies* that bounds from other traits are
 662         // all met. (Remember that *all bounds* in a type like
 663         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 664         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 665         // 'y.)
 666         //
 667         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 668         // am looking forward to the future here.
 669         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 670             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 671
 672             let explicit_bound = region;
 673
 674             self.out.reserve(implicit_bounds.len());
 675             for implicit_bound in implicit_bounds {
 676                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 677                 let outlives =
 678                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 679                 self.out.push(traits::Obligation::new(
 680                     cause,
 681                     self.param_env,
 682                     outlives.to_predicate(self.infcx.tcx),
 683                 ));
 684             }
 685         }
 686     }
 687 }
 688
 689 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 690 /// bounds that must hold on the elided self type. These are derived
 691 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 692 /// they declare `trait SomeTrait : 'static`, for example, then
 693 /// `'static` would appear in the list. The hard work is done by
 694 /// `infer::required_region_bounds`, see that for more information.
 695 pub fn object_region_bounds<'tcx>(
 696     tcx: TyCtxt<'tcx>,
 697     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 698 ) -> Vec<ty::Region<'tcx>> {
 699     // Since we don't actually *know* the self type for an object,
 700     // this "open(err)" serves as a kind of dummy standin -- basically
 701     // a placeholder type.
 702     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 703
 704     let predicates = existential_predicates.iter().filter_map(|predicate| {
 705         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 706             None
 707         } else {
 708             Some(predicate.with_self_ty(tcx, open_ty))
 709         }
 710     });
 711
 712     required_region_bounds(tcx, open_ty, predicates)
 713 }