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