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::Atom(atom) => match atom {
 102             ty::PredicateAtom::Trait(t, _) => {
 103                 wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
 104             }
 105             ty::PredicateAtom::RegionOutlives(..) => {}
 106             ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 107                 wf.compute(ty.into());
 108             }
 109             ty::PredicateAtom::Projection(t) => {
 110                 wf.compute_projection(t.projection_ty);
 111                 wf.compute(t.ty.into());
 112             }
 113             ty::PredicateAtom::WellFormed(arg) => {
 114                 wf.compute(arg);
 115             }
 116             ty::PredicateAtom::ObjectSafe(_) => {}
 117             ty::PredicateAtom::ClosureKind(..) => {}
 118             ty::PredicateAtom::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 119                 wf.compute(a.into());
 120                 wf.compute(b.into());
 121             }
 122             ty::PredicateAtom::ConstEvaluatable(def, substs) => {
 123                 let obligations = wf.nominal_obligations(def.did, substs);
 124                 wf.out.extend(obligations);
 125
 126                 for arg in substs.iter() {
 127                     wf.compute(arg);
 128                 }
 129             }
 130             ty::PredicateAtom::ConstEquate(c1, c2) => {
 131                 wf.compute(c1.into());
 132                 wf.compute(c2.into());
 133             }
 134         },
 135     }
 136
 137     wf.normalize()
 138 }
 139
 140 struct WfPredicates<'a, 'tcx> {
 141     infcx: &'a InferCtxt<'a, 'tcx>,
 142     param_env: ty::ParamEnv<'tcx>,
 143     body_id: hir::HirId,
 144     span: Span,
 145     out: Vec<traits::PredicateObligation<'tcx>>,
 146     item: Option<&'tcx hir::Item<'tcx>>,
 147 }
 148
 149 /// Controls whether we "elaborate" supertraits and so forth on the WF
 150 /// predicates. This is a kind of hack to address #43784. The
 151 /// underlying problem in that issue was a trait structure like:
 152 ///
 153 /// ```
 154 /// trait Foo: Copy { }
 155 /// trait Bar: Foo { }
 156 /// impl<T: Bar> Foo for T { }
 157 /// impl<T> Bar for T { }
 158 /// ```
 159 ///
 160 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 161 /// we decide that this is true because `T: Bar` is in the
 162 /// where-clauses (and we can elaborate that to include `T:
 163 /// Copy`). This wouldn't be a problem, except that when we check the
 164 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 165 /// impl. And so nowhere did we check that `T: Copy` holds!
 166 ///
 167 /// To resolve this, we elaborate the WF requirements that must be
 168 /// proven when checking impls. This means that (e.g.) the `impl Bar
 169 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 170 /// Copy` (which it won't be able to do, because there is no `Copy`
 171 /// impl for `T`).
 172 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 173 enum Elaborate {
 174     All,
 175     None,
 176 }
 177
 178 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 179     tcx: TyCtxt<'tcx>,
 180     trait_ref: &ty::TraitRef<'tcx>,
 181     item: Option<&hir::Item<'tcx>>,
 182     cause: &mut traits::ObligationCause<'tcx>,
 183     pred: &ty::Predicate<'tcx>,
 184     mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>,
 185 ) {
 186     debug!(
 187         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 188         trait_ref, item, cause, pred
 189     );
 190     let items = match item {
 191         Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
 192         _ => return,
 193     };
 194     let fix_span =
 195         |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
 196             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 197             _ => impl_item_ref.span,
 198         };
 199
 200     // It is fine to skip the binder as we don't care about regions here.
 201     match pred.skip_binders() {
 202         ty::PredicateAtom::Projection(proj) => {
 203             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 204             // but rather from a "second order" obligation, where an associated type has a
 205             // projection coming from another associated type. See
 206             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 207             // `traits-assoc-type-in-supertrait-bad.rs`.
 208             if let ty::Projection(projection_ty) = proj.ty.kind {
 209                 let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
 210                 if let Some(impl_item_span) =
 211                     items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 212                 {
 213                     cause.make_mut().span = impl_item_span;
 214                 }
 215             }
 216         }
 217         ty::PredicateAtom::Trait(pred, _) => {
 218             // An associated item obligation born out of the `trait` failed to be met. An example
 219             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 220             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 221             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = pred.self_ty().kind {
 222                 if let Some(impl_item_span) = trait_assoc_items
 223                     .find(|i| i.def_id == item_def_id)
 224                     .and_then(|trait_assoc_item| {
 225                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
 226                     })
 227                 {
 228                     cause.make_mut().span = impl_item_span;
 229                 }
 230             }
 231         }
 232         _ => {}
 233     }
 234 }
 235
 236 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 237     fn tcx(&self) -> TyCtxt<'tcx> {
 238         self.infcx.tcx
 239     }
 240
 241     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 242         traits::ObligationCause::new(self.span, self.body_id, code)
 243     }
 244
 245     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 246         let cause = self.cause(traits::MiscObligation);
 247         let infcx = &mut self.infcx;
 248         let param_env = self.param_env;
 249         let mut obligations = Vec::with_capacity(self.out.len());
 250         for pred in &self.out {
 251             assert!(!pred.has_escaping_bound_vars());
 252             let mut selcx = traits::SelectionContext::new(infcx);
 253             let i = obligations.len();
 254             let value =
 255                 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
 256             obligations.insert(i, value);
 257         }
 258         obligations
 259     }
 260
 261     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 262     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 263         let tcx = self.infcx.tcx;
 264         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 265
 266         debug!("compute_trait_ref obligations {:?}", obligations);
 267         let cause = self.cause(traits::MiscObligation);
 268         let param_env = self.param_env;
 269
 270         let item = self.item;
 271
 272         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 273             let mut cause = cause.clone();
 274             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
 275                 let derived_cause = traits::DerivedObligationCause {
 276                     parent_trait_ref,
 277                     parent_code: Rc::new(obligation.cause.code.clone()),
 278                 };
 279                 cause.make_mut().code =
 280                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 281             }
 282             extend_cause_with_original_assoc_item_obligation(
 283                 tcx,
 284                 trait_ref,
 285                 item,
 286                 &mut cause,
 287                 &obligation.predicate,
 288                 tcx.associated_items(trait_ref.def_id).in_definition_order(),
 289             );
 290             traits::Obligation::new(cause, param_env, obligation.predicate)
 291         };
 292
 293         if let Elaborate::All = elaborate {
 294             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 295             let implied_obligations = implied_obligations.map(extend);
 296             self.out.extend(implied_obligations);
 297         } else {
 298             self.out.extend(obligations);
 299         }
 300
 301         let tcx = self.tcx();
 302         self.out.extend(
 303             trait_ref
 304                 .substs
 305                 .iter()
 306                 .enumerate()
 307                 .filter(|(_, arg)| {
 308                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 309                 })
 310                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 311                 .map(|(i, arg)| {
 312                     let mut new_cause = cause.clone();
 313                     // The first subst is the self ty - use the correct span for it.
 314                     if i == 0 {
 315                         if let Some(hir::ItemKind::Impl { self_ty, .. }) = item.map(|i| &i.kind) {
 316                             new_cause.make_mut().span = self_ty.span;
 317                         }
 318                     }
 319                     traits::Obligation::new(
 320                         new_cause,
 321                         param_env,
 322                         ty::PredicateAtom::WellFormed(arg).to_predicate(tcx),
 323                     )
 324                 }),
 325         );
 326     }
 327
 328     /// Pushes the obligations required for `trait_ref::Item` to be WF
 329     /// into `self.out`.
 330     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 331         // A projection is well-formed if (a) the trait ref itself is
 332         // WF and (b) the trait-ref holds.  (It may also be
 333         // normalizable and be WF that way.)
 334         let trait_ref = data.trait_ref(self.infcx.tcx);
 335         self.compute_trait_ref(&trait_ref, Elaborate::None);
 336
 337         if !data.has_escaping_bound_vars() {
 338             let predicate = trait_ref.without_const().to_predicate(self.infcx.tcx);
 339             let cause = self.cause(traits::ProjectionWf(data));
 340             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 341         }
 342     }
 343
 344     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 345         if !subty.has_escaping_bound_vars() {
 346             let cause = self.cause(cause);
 347             let trait_ref = ty::TraitRef {
 348                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
 349                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 350             };
 351             self.out.push(traits::Obligation::new(
 352                 cause,
 353                 self.param_env,
 354                 trait_ref.without_const().to_predicate(self.infcx.tcx),
 355             ));
 356         }
 357     }
 358
 359     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 360     fn compute(&mut self, arg: GenericArg<'tcx>) {
 361         let mut walker = arg.walk();
 362         let param_env = self.param_env;
 363         while let Some(arg) = walker.next() {
 364             let ty = match arg.unpack() {
 365                 GenericArgKind::Type(ty) => ty,
 366
 367                 // No WF constraints for lifetimes being present, any outlives
 368                 // obligations are handled by the parent (e.g. `ty::Ref`).
 369                 GenericArgKind::Lifetime(_) => continue,
 370
 371                 GenericArgKind::Const(constant) => {
 372                     match constant.val {
 373                         ty::ConstKind::Unevaluated(def, substs, promoted) => {
 374                             assert!(promoted.is_none());
 375
 376                             let obligations = self.nominal_obligations(def.did, substs);
 377                             self.out.extend(obligations);
 378
 379                             let predicate = ty::PredicateAtom::ConstEvaluatable(def, substs)
 380                                 .to_predicate(self.tcx());
 381                             let cause = self.cause(traits::MiscObligation);
 382                             self.out.push(traits::Obligation::new(
 383                                 cause,
 384                                 self.param_env,
 385                                 predicate,
 386                             ));
 387                         }
 388                         ty::ConstKind::Infer(infer) => {
 389                             let resolved = self.infcx.shallow_resolve(infer);
 390                             // the `InferConst` changed, meaning that we made progress.
 391                             if resolved != infer {
 392                                 let cause = self.cause(traits::MiscObligation);
 393
 394                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 395                                     val: ty::ConstKind::Infer(resolved),
 396                                     ..*constant
 397                                 });
 398                                 self.out.push(traits::Obligation::new(
 399                                     cause,
 400                                     self.param_env,
 401                                     ty::PredicateAtom::WellFormed(resolved_constant.into())
 402                                         .to_predicate(self.tcx()),
 403                                 ));
 404                             }
 405                         }
 406                         ty::ConstKind::Error(_)
 407                         | ty::ConstKind::Param(_)
 408                         | ty::ConstKind::Bound(..)
 409                         | ty::ConstKind::Placeholder(..) => {
 410                             // These variants are trivially WF, so nothing to do here.
 411                         }
 412                         ty::ConstKind::Value(..) => {
 413                             // FIXME: Enforce that values are structurally-matchable.
 414                         }
 415                     }
 416                     continue;
 417                 }
 418             };
 419
 420             match ty.kind {
 421                 ty::Bool
 422                 | ty::Char
 423                 | ty::Int(..)
 424                 | ty::Uint(..)
 425                 | ty::Float(..)
 426                 | ty::Error(_)
 427                 | ty::Str
 428                 | ty::GeneratorWitness(..)
 429                 | ty::Never
 430                 | ty::Param(_)
 431                 | ty::Bound(..)
 432                 | ty::Placeholder(..)
 433                 | ty::Foreign(..) => {
 434                     // WfScalar, WfParameter, etc
 435                 }
 436
 437                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 438                 ty::Infer(ty::IntVar(_)) => {}
 439
 440                 // Can only infer to `ty::Float(_)`.
 441                 ty::Infer(ty::FloatVar(_)) => {}
 442
 443                 ty::Slice(subty) => {
 444                     self.require_sized(subty, traits::SliceOrArrayElem);
 445                 }
 446
 447                 ty::Array(subty, _) => {
 448                     self.require_sized(subty, traits::SliceOrArrayElem);
 449                     // Note that we handle the len is implicitly checked while walking `arg`.
 450                 }
 451
 452                 ty::Tuple(ref tys) => {
 453                     if let Some((_last, rest)) = tys.split_last() {
 454                         for elem in rest {
 455                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 456                         }
 457                     }
 458                 }
 459
 460                 ty::RawPtr(_) => {
 461                     // Simple cases that are WF if their type args are WF.
 462                 }
 463
 464                 ty::Projection(data) => {
 465                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 466                     self.compute_projection(data);
 467                 }
 468
 469                 ty::Adt(def, substs) => {
 470                     // WfNominalType
 471                     let obligations = self.nominal_obligations(def.did, substs);
 472                     self.out.extend(obligations);
 473                 }
 474
 475                 ty::FnDef(did, substs) => {
 476                     let obligations = self.nominal_obligations(did, substs);
 477                     self.out.extend(obligations);
 478                 }
 479
 480                 ty::Ref(r, rty, _) => {
 481                     // WfReference
 482                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 483                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 484                         self.out.push(traits::Obligation::new(
 485                             cause,
 486                             param_env,
 487                             ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(rty, r))
 488                                 .to_predicate(self.tcx()),
 489                         ));
 490                     }
 491                 }
 492
 493                 ty::Generator(..) => {
 494                     // Walk ALL the types in the generator: this will
 495                     // include the upvar types as well as the yield
 496                     // type. Note that this is mildly distinct from
 497                     // the closure case, where we have to be careful
 498                     // about the signature of the closure. We don't
 499                     // have the problem of implied bounds here since
 500                     // generators don't take arguments.
 501                 }
 502
 503                 ty::Closure(_, substs) => {
 504                     // Only check the upvar types for WF, not the rest
 505                     // of the types within. This is needed because we
 506                     // capture the signature and it may not be WF
 507                     // without the implied bounds. Consider a closure
 508                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 509                     // not known to hold in the creator's context (and
 510                     // indeed the closure may not be invoked by its
 511                     // creator, but rather turned to someone who *can*
 512                     // verify that).
 513                     //
 514                     // The special treatment of closures here really
 515                     // ought not to be necessary either; the problem
 516                     // is related to #25860 -- there is no way for us
 517                     // to express a fn type complete with the implied
 518                     // bounds that it is assuming. I think in reality
 519                     // the WF rules around fn are a bit messed up, and
 520                     // that is the rot problem: `fn(&'a T)` should
 521                     // probably always be WF, because it should be
 522                     // shorthand for something like `where(T: 'a) {
 523                     // fn(&'a T) }`, as discussed in #25860.
 524                     //
 525                     // Note that we are also skipping the generic
 526                     // types. This is consistent with the `outlives`
 527                     // code, but anyway doesn't matter: within the fn
 528                     // body where they are created, the generics will
 529                     // always be WF, and outside of that fn body we
 530                     // are not directly inspecting closure types
 531                     // anyway, except via auto trait matching (which
 532                     // only inspects the upvar types).
 533                     walker.skip_current_subtree(); // subtree handled below
 534                     for upvar_ty in substs.as_closure().upvar_tys() {
 535                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 536                         self.compute(upvar_ty.into());
 537                     }
 538                 }
 539
 540                 ty::FnPtr(_) => {
 541                     // let the loop iterate into the argument/return
 542                     // types appearing in the fn signature
 543                 }
 544
 545                 ty::Opaque(did, substs) => {
 546                     // all of the requirements on type parameters
 547                     // should've been checked by the instantiation
 548                     // of whatever returned this exact `impl Trait`.
 549
 550                     // for named opaque `impl Trait` types we still need to check them
 551                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 552                         let obligations = self.nominal_obligations(did, substs);
 553                         self.out.extend(obligations);
 554                     }
 555                 }
 556
 557                 ty::Dynamic(data, r) => {
 558                     // WfObject
 559                     //
 560                     // Here, we defer WF checking due to higher-ranked
 561                     // regions. This is perhaps not ideal.
 562                     self.from_object_ty(ty, data, r);
 563
 564                     // FIXME(#27579) RFC also considers adding trait
 565                     // obligations that don't refer to Self and
 566                     // checking those
 567
 568                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 569
 570                     if !defer_to_coercion {
 571                         let cause = self.cause(traits::MiscObligation);
 572                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 573                         let tcx = self.tcx();
 574                         self.out.extend(component_traits.map(|did| {
 575                             traits::Obligation::new(
 576                                 cause.clone(),
 577                                 param_env,
 578                                 ty::PredicateAtom::ObjectSafe(did).to_predicate(tcx),
 579                             )
 580                         }));
 581                     }
 582                 }
 583
 584                 // Inference variables are the complicated case, since we don't
 585                 // know what type they are. We do two things:
 586                 //
 587                 // 1. Check if they have been resolved, and if so proceed with
 588                 //    THAT type.
 589                 // 2. If not, we've at least simplified things (e.g., we went
 590                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 591                 //    register a pending obligation and keep
 592                 //    moving. (Goal is that an "inductive hypothesis"
 593                 //    is satisfied to ensure termination.)
 594                 // See also the comment on `fn obligations`, describing "livelock"
 595                 // prevention, which happens before this can be reached.
 596                 ty::Infer(_) => {
 597                     let ty = self.infcx.shallow_resolve(ty);
 598                     if let ty::Infer(ty::TyVar(_)) = ty.kind {
 599                         // Not yet resolved, but we've made progress.
 600                         let cause = self.cause(traits::MiscObligation);
 601                         self.out.push(traits::Obligation::new(
 602                             cause,
 603                             param_env,
 604                             ty::PredicateAtom::WellFormed(ty.into()).to_predicate(self.tcx()),
 605                         ));
 606                     } else {
 607                         // Yes, resolved, proceed with the result.
 608                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 609                         self.compute(ty.into());
 610                     }
 611                 }
 612             }
 613         }
 614     }
 615
 616     fn nominal_obligations(
 617         &mut self,
 618         def_id: DefId,
 619         substs: SubstsRef<'tcx>,
 620     ) -> Vec<traits::PredicateObligation<'tcx>> {
 621         let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
 622         predicates
 623             .predicates
 624             .into_iter()
 625             .zip(predicates.spans.into_iter())
 626             .map(|(pred, span)| {
 627                 let cause = self.cause(traits::BindingObligation(def_id, span));
 628                 traits::Obligation::new(cause, self.param_env, pred)
 629             })
 630             .filter(|pred| !pred.has_escaping_bound_vars())
 631             .collect()
 632     }
 633
 634     fn from_object_ty(
 635         &mut self,
 636         ty: Ty<'tcx>,
 637         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 638         region: ty::Region<'tcx>,
 639     ) {
 640         // Imagine a type like this:
 641         //
 642         //     trait Foo { }
 643         //     trait Bar<'c> : 'c { }
 644         //
 645         //     &'b (Foo+'c+Bar<'d>)
 646         //         ^
 647         //
 648         // In this case, the following relationships must hold:
 649         //
 650         //     'b <= 'c
 651         //     'd <= 'c
 652         //
 653         // The first conditions is due to the normal region pointer
 654         // rules, which say that a reference cannot outlive its
 655         // referent.
 656         //
 657         // The final condition may be a bit surprising. In particular,
 658         // you may expect that it would have been `'c <= 'd`, since
 659         // usually lifetimes of outer things are conservative
 660         // approximations for inner things. However, it works somewhat
 661         // differently with trait objects: here the idea is that if the
 662         // user specifies a region bound (`'c`, in this case) it is the
 663         // "master bound" that *implies* that bounds from other traits are
 664         // all met. (Remember that *all bounds* in a type like
 665         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 666         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 667         // 'y.)
 668         //
 669         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 670         // am looking forward to the future here.
 671         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 672             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 673
 674             let explicit_bound = region;
 675
 676             self.out.reserve(implicit_bounds.len());
 677             for implicit_bound in implicit_bounds {
 678                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 679                 let outlives =
 680                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 681                 self.out.push(traits::Obligation::new(
 682                     cause,
 683                     self.param_env,
 684                     outlives.to_predicate(self.infcx.tcx),
 685                 ));
 686             }
 687         }
 688     }
 689 }
 690
 691 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 692 /// bounds that must hold on the elided self type. These are derived
 693 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 694 /// they declare `trait SomeTrait : 'static`, for example, then
 695 /// `'static` would appear in the list. The hard work is done by
 696 /// `infer::required_region_bounds`, see that for more information.
 697 pub fn object_region_bounds<'tcx>(
 698     tcx: TyCtxt<'tcx>,
 699     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 700 ) -> Vec<ty::Region<'tcx>> {
 701     // Since we don't actually *know* the self type for an object,
 702     // this "open(err)" serves as a kind of dummy standin -- basically
 703     // a placeholder type.
 704     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 705
 706     let predicates = existential_predicates.iter().filter_map(|predicate| {
 707         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 708             None
 709         } else {
 710             Some(predicate.with_self_ty(tcx, open_ty))
 711         }
 712     });
 713
 714     required_region_bounds(tcx, open_ty, predicates)
 715 }