compiler/rustc_trait_selection/src/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::LangItem;
   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
  11 use std::iter;
  12 use std::rc::Rc;
  13 /// Returns the set of obligations needed to make `arg` well-formed.
  14 /// If `arg` contains unresolved inference variables, this may include
  15 /// further WF obligations. However, if `arg` IS an unresolved
  16 /// inference variable, returns `None`, because we are not able to
  17 /// make any progress at all. This is to prevent "livelock" where we
  18 /// say "$0 is WF if $0 is WF".
  19 pub fn obligations<'a, 'tcx>(
  20     infcx: &InferCtxt<'a, 'tcx>,
  21     param_env: ty::ParamEnv<'tcx>,
  22     body_id: hir::HirId,
  23     arg: GenericArg<'tcx>,
  24     span: Span,
  25 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  26     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  27     let arg = match arg.unpack() {
  28         GenericArgKind::Type(ty) => {
  29             match ty.kind() {
  30                 ty::Infer(ty::TyVar(_)) => {
  31                     let resolved_ty = infcx.shallow_resolve(ty);
  32                     if resolved_ty == ty {
  33                         // No progress, bail out to prevent "livelock".
  34                         return None;
  35                     }
  36
  37                     resolved_ty
  38                 }
  39                 _ => ty,
  40             }
  41             .into()
  42         }
  43         GenericArgKind::Const(ct) => {
  44             match ct.val {
  45                 ty::ConstKind::Infer(infer) => {
  46                     let resolved = infcx.shallow_resolve(infer);
  47                     if resolved == infer {
  48                         // No progress.
  49                         return None;
  50                     }
  51
  52                     infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Infer(resolved), ty: ct.ty })
  53                 }
  54                 _ => ct,
  55             }
  56             .into()
  57         }
  58         // There is nothing we have to do for lifetimes.
  59         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  60     };
  61
  62     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  63     wf.compute(arg);
  64     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  65
  66     let result = wf.normalize();
  67     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  68     Some(result)
  69 }
  70
  71 /// Returns the obligations that make this trait reference
  72 /// well-formed.  For example, if there is a trait `Set` defined like
  73 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  74 /// if `Bar: Eq`.
  75 pub fn trait_obligations<'a, 'tcx>(
  76     infcx: &InferCtxt<'a, 'tcx>,
  77     param_env: ty::ParamEnv<'tcx>,
  78     body_id: hir::HirId,
  79     trait_ref: &ty::TraitRef<'tcx>,
  80     span: Span,
  81     item: Option<&'tcx hir::Item<'tcx>>,
  82 ) -> Vec<traits::PredicateObligation<'tcx>> {
  83     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
  84     wf.compute_trait_ref(trait_ref, Elaborate::All);
  85     wf.normalize()
  86 }
  87
  88 pub fn predicate_obligations<'a, 'tcx>(
  89     infcx: &InferCtxt<'a, 'tcx>,
  90     param_env: ty::ParamEnv<'tcx>,
  91     body_id: hir::HirId,
  92     predicate: ty::Predicate<'tcx>,
  93     span: Span,
  94 ) -> Vec<traits::PredicateObligation<'tcx>> {
  95     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  96
  97     // It's ok to skip the binder here because wf code is prepared for it
  98     match predicate.skip_binders() {
  99         ty::PredicateAtom::Trait(t, _) => {
 100             wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
 101         }
 102         ty::PredicateAtom::RegionOutlives(..) => {}
 103         ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 104             wf.compute(ty.into());
 105         }
 106         ty::PredicateAtom::Projection(t) => {
 107             wf.compute_projection(t.projection_ty);
 108             wf.compute(t.ty.into());
 109         }
 110         ty::PredicateAtom::WellFormed(arg) => {
 111             wf.compute(arg);
 112         }
 113         ty::PredicateAtom::ObjectSafe(_) => {}
 114         ty::PredicateAtom::ClosureKind(..) => {}
 115         ty::PredicateAtom::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 116             wf.compute(a.into());
 117             wf.compute(b.into());
 118         }
 119         ty::PredicateAtom::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::PredicateAtom::ConstEquate(c1, c2) => {
 128             wf.compute(c1.into());
 129             wf.compute(c2.into());
 130         }
 131         ty::PredicateAtom::TypeWellFormedFromEnv(..) => {
 132             bug!("TypeWellFormedFromEnv is only used for Chalk")
 133         }
 134     }
 135
 136     wf.normalize()
 137 }
 138
 139 struct WfPredicates<'a, 'tcx> {
 140     infcx: &'a InferCtxt<'a, 'tcx>,
 141     param_env: ty::ParamEnv<'tcx>,
 142     body_id: hir::HirId,
 143     span: Span,
 144     out: Vec<traits::PredicateObligation<'tcx>>,
 145     item: Option<&'tcx hir::Item<'tcx>>,
 146 }
 147
 148 /// Controls whether we "elaborate" supertraits and so forth on the WF
 149 /// predicates. This is a kind of hack to address #43784. The
 150 /// underlying problem in that issue was a trait structure like:
 151 ///
 152 /// ```
 153 /// trait Foo: Copy { }
 154 /// trait Bar: Foo { }
 155 /// impl<T: Bar> Foo for T { }
 156 /// impl<T> Bar for T { }
 157 /// ```
 158 ///
 159 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 160 /// we decide that this is true because `T: Bar` is in the
 161 /// where-clauses (and we can elaborate that to include `T:
 162 /// Copy`). This wouldn't be a problem, except that when we check the
 163 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 164 /// impl. And so nowhere did we check that `T: Copy` holds!
 165 ///
 166 /// To resolve this, we elaborate the WF requirements that must be
 167 /// proven when checking impls. This means that (e.g.) the `impl Bar
 168 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 169 /// Copy` (which it won't be able to do, because there is no `Copy`
 170 /// impl for `T`).
 171 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 172 enum Elaborate {
 173     All,
 174     None,
 175 }
 176
 177 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 178     tcx: TyCtxt<'tcx>,
 179     trait_ref: &ty::TraitRef<'tcx>,
 180     item: Option<&hir::Item<'tcx>>,
 181     cause: &mut traits::ObligationCause<'tcx>,
 182     pred: &ty::Predicate<'tcx>,
 183     mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>,
 184 ) {
 185     debug!(
 186         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 187         trait_ref, item, cause, pred
 188     );
 189     let items = match item {
 190         Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
 191         _ => return,
 192     };
 193     let fix_span =
 194         |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
 195             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 196             _ => impl_item_ref.span,
 197         };
 198
 199     // It is fine to skip the binder as we don't care about regions here.
 200     match pred.skip_binders() {
 201         ty::PredicateAtom::Projection(proj) => {
 202             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 203             // but rather from a "second order" obligation, where an associated type has a
 204             // projection coming from another associated type. See
 205             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 206             // `traits-assoc-type-in-supertrait-bad.rs`.
 207             if let ty::Projection(projection_ty) = proj.ty.kind() {
 208                 let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
 209                 if let Some(impl_item_span) =
 210                     items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
 211                 {
 212                     cause.make_mut().span = impl_item_span;
 213                 }
 214             }
 215         }
 216         ty::PredicateAtom::Trait(pred, _) => {
 217             // An associated item obligation born out of the `trait` failed to be met. An example
 218             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 219             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 220             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind() {
 221                 if let Some(impl_item_span) = trait_assoc_items
 222                     .find(|i| i.def_id == item_def_id)
 223                     .and_then(|trait_assoc_item| {
 224                         items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
 225                     })
 226                 {
 227                     cause.make_mut().span = impl_item_span;
 228                 }
 229             }
 230         }
 231         _ => {}
 232     }
 233 }
 234
 235 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 236     fn tcx(&self) -> TyCtxt<'tcx> {
 237         self.infcx.tcx
 238     }
 239
 240     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 241         traits::ObligationCause::new(self.span, self.body_id, code)
 242     }
 243
 244     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 245         let cause = self.cause(traits::MiscObligation);
 246         let infcx = &mut self.infcx;
 247         let param_env = self.param_env;
 248         let mut obligations = Vec::with_capacity(self.out.len());
 249         for pred in &self.out {
 250             assert!(!pred.has_escaping_bound_vars());
 251             let mut selcx = traits::SelectionContext::new(infcx);
 252             let i = obligations.len();
 253             let value =
 254                 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
 255             obligations.insert(i, value);
 256         }
 257         obligations
 258     }
 259
 260     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 261     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 262         let tcx = self.infcx.tcx;
 263         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 264
 265         debug!("compute_trait_ref obligations {:?}", obligations);
 266         let cause = self.cause(traits::MiscObligation);
 267         let param_env = self.param_env;
 268
 269         let item = self.item;
 270
 271         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 272             let mut cause = cause.clone();
 273             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
 274                 let derived_cause = traits::DerivedObligationCause {
 275                     parent_trait_ref,
 276                     parent_code: Rc::new(obligation.cause.code.clone()),
 277                 };
 278                 cause.make_mut().code =
 279                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 280             }
 281             extend_cause_with_original_assoc_item_obligation(
 282                 tcx,
 283                 trait_ref,
 284                 item,
 285                 &mut cause,
 286                 &obligation.predicate,
 287                 tcx.associated_items(trait_ref.def_id).in_definition_order(),
 288             );
 289             traits::Obligation::new(cause, param_env, obligation.predicate)
 290         };
 291
 292         if let Elaborate::All = elaborate {
 293             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 294             let implied_obligations = implied_obligations.map(extend);
 295             self.out.extend(implied_obligations);
 296         } else {
 297             self.out.extend(obligations);
 298         }
 299
 300         let tcx = self.tcx();
 301         self.out.extend(
 302             trait_ref
 303                 .substs
 304                 .iter()
 305                 .enumerate()
 306                 .filter(|(_, arg)| {
 307                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 308                 })
 309                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 310                 .map(|(i, arg)| {
 311                     let mut new_cause = cause.clone();
 312                     // The first subst is the self ty - use the correct span for it.
 313                     if i == 0 {
 314                         if let Some(hir::ItemKind::Impl { self_ty, .. }) = item.map(|i| &i.kind) {
 315                             new_cause.make_mut().span = self_ty.span;
 316                         }
 317                     }
 318                     traits::Obligation::new(
 319                         new_cause,
 320                         param_env,
 321                         ty::PredicateAtom::WellFormed(arg).to_predicate(tcx),
 322                     )
 323                 }),
 324         );
 325     }
 326
 327     /// Pushes the obligations required for `trait_ref::Item` to be WF
 328     /// into `self.out`.
 329     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 330         // A projection is well-formed if (a) the trait ref itself is
 331         // WF and (b) the trait-ref holds.  (It may also be
 332         // normalizable and be WF that way.)
 333         let trait_ref = data.trait_ref(self.infcx.tcx);
 334         self.compute_trait_ref(&trait_ref, Elaborate::None);
 335
 336         if !data.has_escaping_bound_vars() {
 337             let predicate = trait_ref.without_const().to_predicate(self.infcx.tcx);
 338             let cause = self.cause(traits::ProjectionWf(data));
 339             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 340         }
 341     }
 342
 343     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 344         if !subty.has_escaping_bound_vars() {
 345             let cause = self.cause(cause);
 346             let trait_ref = ty::TraitRef {
 347                 def_id: self.infcx.tcx.require_lang_item(LangItem::Sized, None),
 348                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 349             };
 350             self.out.push(traits::Obligation::new(
 351                 cause,
 352                 self.param_env,
 353                 trait_ref.without_const().to_predicate(self.infcx.tcx),
 354             ));
 355         }
 356     }
 357
 358     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 359     fn compute(&mut self, arg: GenericArg<'tcx>) {
 360         let mut walker = arg.walk();
 361         let param_env = self.param_env;
 362         while let Some(arg) = walker.next() {
 363             let ty = match arg.unpack() {
 364                 GenericArgKind::Type(ty) => ty,
 365
 366                 // No WF constraints for lifetimes being present, any outlives
 367                 // obligations are handled by the parent (e.g. `ty::Ref`).
 368                 GenericArgKind::Lifetime(_) => continue,
 369
 370                 GenericArgKind::Const(constant) => {
 371                     match constant.val {
 372                         ty::ConstKind::Unevaluated(def, substs, promoted) => {
 373                             assert!(promoted.is_none());
 374
 375                             let obligations = self.nominal_obligations(def.did, substs);
 376                             self.out.extend(obligations);
 377
 378                             let predicate = ty::PredicateAtom::ConstEvaluatable(def, substs)
 379                                 .to_predicate(self.tcx());
 380                             let cause = self.cause(traits::MiscObligation);
 381                             self.out.push(traits::Obligation::new(
 382                                 cause,
 383                                 self.param_env,
 384                                 predicate,
 385                             ));
 386                         }
 387                         ty::ConstKind::Infer(infer) => {
 388                             let resolved = self.infcx.shallow_resolve(infer);
 389                             // the `InferConst` changed, meaning that we made progress.
 390                             if resolved != infer {
 391                                 let cause = self.cause(traits::MiscObligation);
 392
 393                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 394                                     val: ty::ConstKind::Infer(resolved),
 395                                     ..*constant
 396                                 });
 397                                 self.out.push(traits::Obligation::new(
 398                                     cause,
 399                                     self.param_env,
 400                                     ty::PredicateAtom::WellFormed(resolved_constant.into())
 401                                         .to_predicate(self.tcx()),
 402                                 ));
 403                             }
 404                         }
 405                         ty::ConstKind::Error(_)
 406                         | ty::ConstKind::Param(_)
 407                         | ty::ConstKind::Bound(..)
 408                         | ty::ConstKind::Placeholder(..) => {
 409                             // These variants are trivially WF, so nothing to do here.
 410                         }
 411                         ty::ConstKind::Value(..) => {
 412                             // FIXME: Enforce that values are structurally-matchable.
 413                         }
 414                     }
 415                     continue;
 416                 }
 417             };
 418
 419             match *ty.kind() {
 420                 ty::Bool
 421                 | ty::Char
 422                 | ty::Int(..)
 423                 | ty::Uint(..)
 424                 | ty::Float(..)
 425                 | ty::Error(_)
 426                 | ty::Str
 427                 | ty::GeneratorWitness(..)
 428                 | ty::Never
 429                 | ty::Param(_)
 430                 | ty::Bound(..)
 431                 | ty::Placeholder(..)
 432                 | ty::Foreign(..) => {
 433                     // WfScalar, WfParameter, etc
 434                 }
 435
 436                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 437                 ty::Infer(ty::IntVar(_)) => {}
 438
 439                 // Can only infer to `ty::Float(_)`.
 440                 ty::Infer(ty::FloatVar(_)) => {}
 441
 442                 ty::Slice(subty) => {
 443                     self.require_sized(subty, traits::SliceOrArrayElem);
 444                 }
 445
 446                 ty::Array(subty, _) => {
 447                     self.require_sized(subty, traits::SliceOrArrayElem);
 448                     // Note that we handle the len is implicitly checked while walking `arg`.
 449                 }
 450
 451                 ty::Tuple(ref tys) => {
 452                     if let Some((_last, rest)) = tys.split_last() {
 453                         for elem in rest {
 454                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 455                         }
 456                     }
 457                 }
 458
 459                 ty::RawPtr(_) => {
 460                     // Simple cases that are WF if their type args are WF.
 461                 }
 462
 463                 ty::Projection(data) => {
 464                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 465                     self.compute_projection(data);
 466                 }
 467
 468                 ty::Adt(def, substs) => {
 469                     // WfNominalType
 470                     let obligations = self.nominal_obligations(def.did, substs);
 471                     self.out.extend(obligations);
 472                 }
 473
 474                 ty::FnDef(did, substs) => {
 475                     let obligations = self.nominal_obligations(did, substs);
 476                     self.out.extend(obligations);
 477                 }
 478
 479                 ty::Ref(r, rty, _) => {
 480                     // WfReference
 481                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 482                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 483                         self.out.push(traits::Obligation::new(
 484                             cause,
 485                             param_env,
 486                             ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(rty, r))
 487                                 .to_predicate(self.tcx()),
 488                         ));
 489                     }
 490                 }
 491
 492                 ty::Generator(..) => {
 493                     // Walk ALL the types in the generator: this will
 494                     // include the upvar types as well as the yield
 495                     // type. Note that this is mildly distinct from
 496                     // the closure case, where we have to be careful
 497                     // about the signature of the closure. We don't
 498                     // have the problem of implied bounds here since
 499                     // generators don't take arguments.
 500                 }
 501
 502                 ty::Closure(_, substs) => {
 503                     // Only check the upvar types for WF, not the rest
 504                     // of the types within. This is needed because we
 505                     // capture the signature and it may not be WF
 506                     // without the implied bounds. Consider a closure
 507                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 508                     // not known to hold in the creator's context (and
 509                     // indeed the closure may not be invoked by its
 510                     // creator, but rather turned to someone who *can*
 511                     // verify that).
 512                     //
 513                     // The special treatment of closures here really
 514                     // ought not to be necessary either; the problem
 515                     // is related to #25860 -- there is no way for us
 516                     // to express a fn type complete with the implied
 517                     // bounds that it is assuming. I think in reality
 518                     // the WF rules around fn are a bit messed up, and
 519                     // that is the rot problem: `fn(&'a T)` should
 520                     // probably always be WF, because it should be
 521                     // shorthand for something like `where(T: 'a) {
 522                     // fn(&'a T) }`, as discussed in #25860.
 523                     //
 524                     // Note that we are also skipping the generic
 525                     // types. This is consistent with the `outlives`
 526                     // code, but anyway doesn't matter: within the fn
 527                     // body where they are created, the generics will
 528                     // always be WF, and outside of that fn body we
 529                     // are not directly inspecting closure types
 530                     // anyway, except via auto trait matching (which
 531                     // only inspects the upvar types).
 532                     walker.skip_current_subtree(); // subtree handled below
 533                     for upvar_ty in substs.as_closure().upvar_tys() {
 534                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 535                         self.compute(upvar_ty.into());
 536                     }
 537                 }
 538
 539                 ty::FnPtr(_) => {
 540                     // let the loop iterate into the argument/return
 541                     // types appearing in the fn signature
 542                 }
 543
 544                 ty::Opaque(did, substs) => {
 545                     // all of the requirements on type parameters
 546                     // should've been checked by the instantiation
 547                     // of whatever returned this exact `impl Trait`.
 548
 549                     // for named opaque `impl Trait` types we still need to check them
 550                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 551                         let obligations = self.nominal_obligations(did, substs);
 552                         self.out.extend(obligations);
 553                     }
 554                 }
 555
 556                 ty::Dynamic(data, r) => {
 557                     // WfObject
 558                     //
 559                     // Here, we defer WF checking due to higher-ranked
 560                     // regions. This is perhaps not ideal.
 561                     self.from_object_ty(ty, data, r);
 562
 563                     // FIXME(#27579) RFC also considers adding trait
 564                     // obligations that don't refer to Self and
 565                     // checking those
 566
 567                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 568
 569                     if !defer_to_coercion {
 570                         let cause = self.cause(traits::MiscObligation);
 571                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 572                         let tcx = self.tcx();
 573                         self.out.extend(component_traits.map(|did| {
 574                             traits::Obligation::new(
 575                                 cause.clone(),
 576                                 param_env,
 577                                 ty::PredicateAtom::ObjectSafe(did).to_predicate(tcx),
 578                             )
 579                         }));
 580                     }
 581                 }
 582
 583                 // Inference variables are the complicated case, since we don't
 584                 // know what type they are. We do two things:
 585                 //
 586                 // 1. Check if they have been resolved, and if so proceed with
 587                 //    THAT type.
 588                 // 2. If not, we've at least simplified things (e.g., we went
 589                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 590                 //    register a pending obligation and keep
 591                 //    moving. (Goal is that an "inductive hypothesis"
 592                 //    is satisfied to ensure termination.)
 593                 // See also the comment on `fn obligations`, describing "livelock"
 594                 // prevention, which happens before this can be reached.
 595                 ty::Infer(_) => {
 596                     let ty = self.infcx.shallow_resolve(ty);
 597                     if let ty::Infer(ty::TyVar(_)) = ty.kind() {
 598                         // Not yet resolved, but we've made progress.
 599                         let cause = self.cause(traits::MiscObligation);
 600                         self.out.push(traits::Obligation::new(
 601                             cause,
 602                             param_env,
 603                             ty::PredicateAtom::WellFormed(ty.into()).to_predicate(self.tcx()),
 604                         ));
 605                     } else {
 606                         // Yes, resolved, proceed with the result.
 607                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 608                         self.compute(ty.into());
 609                     }
 610                 }
 611             }
 612         }
 613     }
 614
 615     fn nominal_obligations(
 616         &mut self,
 617         def_id: DefId,
 618         substs: SubstsRef<'tcx>,
 619     ) -> Vec<traits::PredicateObligation<'tcx>> {
 620         let predicates = self.infcx.tcx.predicates_of(def_id);
 621         let mut origins = vec![def_id; predicates.predicates.len()];
 622         let mut head = predicates;
 623         while let Some(parent) = head.parent {
 624             head = self.infcx.tcx.predicates_of(parent);
 625             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 626         }
 627
 628         let predicates = predicates.instantiate(self.infcx.tcx, substs);
 629         debug_assert_eq!(predicates.predicates.len(), origins.len());
 630
 631         predicates
 632             .predicates
 633             .into_iter()
 634             .zip(predicates.spans.into_iter())
 635             .zip(origins.into_iter().rev())
 636             .map(|((pred, span), origin_def_id)| {
 637                 let cause = self.cause(traits::BindingObligation(origin_def_id, span));
 638                 traits::Obligation::new(cause, self.param_env, pred)
 639             })
 640             .filter(|pred| !pred.has_escaping_bound_vars())
 641             .collect()
 642     }
 643
 644     fn from_object_ty(
 645         &mut self,
 646         ty: Ty<'tcx>,
 647         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 648         region: ty::Region<'tcx>,
 649     ) {
 650         // Imagine a type like this:
 651         //
 652         //     trait Foo { }
 653         //     trait Bar<'c> : 'c { }
 654         //
 655         //     &'b (Foo+'c+Bar<'d>)
 656         //         ^
 657         //
 658         // In this case, the following relationships must hold:
 659         //
 660         //     'b <= 'c
 661         //     'd <= 'c
 662         //
 663         // The first conditions is due to the normal region pointer
 664         // rules, which say that a reference cannot outlive its
 665         // referent.
 666         //
 667         // The final condition may be a bit surprising. In particular,
 668         // you may expect that it would have been `'c <= 'd`, since
 669         // usually lifetimes of outer things are conservative
 670         // approximations for inner things. However, it works somewhat
 671         // differently with trait objects: here the idea is that if the
 672         // user specifies a region bound (`'c`, in this case) it is the
 673         // "master bound" that *implies* that bounds from other traits are
 674         // all met. (Remember that *all bounds* in a type like
 675         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 676         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 677         // 'y.)
 678         //
 679         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 680         // am looking forward to the future here.
 681         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 682             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 683
 684             let explicit_bound = region;
 685
 686             self.out.reserve(implicit_bounds.len());
 687             for implicit_bound in implicit_bounds {
 688                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 689                 let outlives =
 690                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 691                 self.out.push(traits::Obligation::new(
 692                     cause,
 693                     self.param_env,
 694                     outlives.to_predicate(self.infcx.tcx),
 695                 ));
 696             }
 697         }
 698     }
 699 }
 700
 701 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 702 /// bounds that must hold on the elided self type. These are derived
 703 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 704 /// they declare `trait SomeTrait : 'static`, for example, then
 705 /// `'static` would appear in the list. The hard work is done by
 706 /// `infer::required_region_bounds`, see that for more information.
 707 pub fn object_region_bounds<'tcx>(
 708     tcx: TyCtxt<'tcx>,
 709     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 710 ) -> Vec<ty::Region<'tcx>> {
 711     // Since we don't actually *know* the self type for an object,
 712     // this "open(err)" serves as a kind of dummy standin -- basically
 713     // a placeholder type.
 714     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 715
 716     let predicates = existential_predicates.iter().filter_map(|predicate| {
 717         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 718             None
 719         } else {
 720             Some(predicate.with_self_ty(tcx, open_ty))
 721         }
 722     });
 723
 724     required_region_bounds(tcx, open_ty, predicates)
 725 }