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, TypeVisitable};
   9 use rustc_span::Span;
  10
  11 use std::iter;
  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     recursion_depth: usize,
  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.kind() {
  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
  53                         .tcx
  54                         .mk_const(ty::ConstS { kind: ty::ConstKind::Infer(resolved), ty: ct.ty() })
  55                 }
  56                 _ => ct,
  57             }
  58             .into()
  59         }
  60         // There is nothing we have to do for lifetimes.
  61         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  62     };
  63
  64     let mut wf =
  65         WfPredicates { infcx, param_env, body_id, span, out: vec![], recursion_depth, item: None };
  66     wf.compute(arg);
  67     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  68
  69     let result = wf.normalize();
  70     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  71     Some(result)
  72 }
  73
  74 /// Returns the obligations that make this trait reference
  75 /// well-formed.  For example, if there is a trait `Set` defined like
  76 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  77 /// if `Bar: Eq`.
  78 pub fn trait_obligations<'a, 'tcx>(
  79     infcx: &InferCtxt<'a, 'tcx>,
  80     param_env: ty::ParamEnv<'tcx>,
  81     body_id: hir::HirId,
  82     trait_ref: &ty::TraitRef<'tcx>,
  83     span: Span,
  84     item: &'tcx hir::Item<'tcx>,
  85 ) -> Vec<traits::PredicateObligation<'tcx>> {
  86     let mut wf = WfPredicates {
  87         infcx,
  88         param_env,
  89         body_id,
  90         span,
  91         out: vec![],
  92         recursion_depth: 0,
  93         item: Some(item),
  94     };
  95     wf.compute_trait_ref(trait_ref, Elaborate::All);
  96     debug!(obligations = ?wf.out);
  97     wf.normalize()
  98 }
  99
 100 pub fn predicate_obligations<'a, 'tcx>(
 101     infcx: &InferCtxt<'a, 'tcx>,
 102     param_env: ty::ParamEnv<'tcx>,
 103     body_id: hir::HirId,
 104     predicate: ty::Predicate<'tcx>,
 105     span: Span,
 106 ) -> Vec<traits::PredicateObligation<'tcx>> {
 107     let mut wf = WfPredicates {
 108         infcx,
 109         param_env,
 110         body_id,
 111         span,
 112         out: vec![],
 113         recursion_depth: 0,
 114         item: None,
 115     };
 116
 117     // It's ok to skip the binder here because wf code is prepared for it
 118     match predicate.kind().skip_binder() {
 119         ty::PredicateKind::Trait(t) => {
 120             wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
 121         }
 122         ty::PredicateKind::RegionOutlives(..) => {}
 123         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 124             wf.compute(ty.into());
 125         }
 126         ty::PredicateKind::Projection(t) => {
 127             wf.compute_projection(t.projection_ty);
 128             wf.compute(match t.term {
 129                 ty::Term::Ty(ty) => ty.into(),
 130                 ty::Term::Const(c) => c.into(),
 131             })
 132         }
 133         ty::PredicateKind::WellFormed(arg) => {
 134             wf.compute(arg);
 135         }
 136         ty::PredicateKind::ObjectSafe(_) => {}
 137         ty::PredicateKind::ClosureKind(..) => {}
 138         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 139             wf.compute(a.into());
 140             wf.compute(b.into());
 141         }
 142         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 143             wf.compute(a.into());
 144             wf.compute(b.into());
 145         }
 146         ty::PredicateKind::ConstEvaluatable(uv) => {
 147             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 148             wf.out.extend(obligations);
 149
 150             for arg in uv.substs.iter() {
 151                 wf.compute(arg);
 152             }
 153         }
 154         ty::PredicateKind::ConstEquate(c1, c2) => {
 155             wf.compute(c1.into());
 156             wf.compute(c2.into());
 157         }
 158         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 159             bug!("TypeWellFormedFromEnv is only used for Chalk")
 160         }
 161     }
 162
 163     wf.normalize()
 164 }
 165
 166 struct WfPredicates<'a, 'tcx> {
 167     infcx: &'a InferCtxt<'a, 'tcx>,
 168     param_env: ty::ParamEnv<'tcx>,
 169     body_id: hir::HirId,
 170     span: Span,
 171     out: Vec<traits::PredicateObligation<'tcx>>,
 172     recursion_depth: usize,
 173     item: Option<&'tcx hir::Item<'tcx>>,
 174 }
 175
 176 /// Controls whether we "elaborate" supertraits and so forth on the WF
 177 /// predicates. This is a kind of hack to address #43784. The
 178 /// underlying problem in that issue was a trait structure like:
 179 ///
 180 /// ```ignore (illustrative)
 181 /// trait Foo: Copy { }
 182 /// trait Bar: Foo { }
 183 /// impl<T: Bar> Foo for T { }
 184 /// impl<T> Bar for T { }
 185 /// ```
 186 ///
 187 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 188 /// we decide that this is true because `T: Bar` is in the
 189 /// where-clauses (and we can elaborate that to include `T:
 190 /// Copy`). This wouldn't be a problem, except that when we check the
 191 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 192 /// impl. And so nowhere did we check that `T: Copy` holds!
 193 ///
 194 /// To resolve this, we elaborate the WF requirements that must be
 195 /// proven when checking impls. This means that (e.g.) the `impl Bar
 196 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 197 /// Copy` (which it won't be able to do, because there is no `Copy`
 198 /// impl for `T`).
 199 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 200 enum Elaborate {
 201     All,
 202     None,
 203 }
 204
 205 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 206     tcx: TyCtxt<'tcx>,
 207     trait_ref: &ty::TraitRef<'tcx>,
 208     item: Option<&hir::Item<'tcx>>,
 209     cause: &mut traits::ObligationCause<'tcx>,
 210     pred: ty::Predicate<'tcx>,
 211 ) {
 212     debug!(
 213         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 214         trait_ref, item, cause, pred
 215     );
 216     let (items, impl_def_id) = match item {
 217         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 218         _ => return,
 219     };
 220     let fix_span =
 221         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 222             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 223             _ => impl_item_ref.span,
 224         };
 225
 226     // It is fine to skip the binder as we don't care about regions here.
 227     match pred.kind().skip_binder() {
 228         ty::PredicateKind::Projection(proj) => {
 229             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 230             // but rather from a "second order" obligation, where an associated type has a
 231             // projection coming from another associated type. See
 232             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 233             // `traits-assoc-type-in-supertrait-bad.rs`.
 234             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 235                 && let Some(&impl_item_id) =
 236                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 237                 && let Some(impl_item_span) = items
 238                     .iter()
 239                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 240                     .map(fix_span)
 241             {
 242                 cause.span = impl_item_span;
 243             }
 244         }
 245         ty::PredicateKind::Trait(pred) => {
 246             // An associated item obligation born out of the `trait` failed to be met. An example
 247             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 248             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 249             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 250                 && let Some(&impl_item_id) =
 251                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 252                 && let Some(impl_item_span) = items
 253                     .iter()
 254                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 255                     .map(fix_span)
 256             {
 257                 cause.span = impl_item_span;
 258             }
 259         }
 260         _ => {}
 261     }
 262 }
 263
 264 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 265     fn tcx(&self) -> TyCtxt<'tcx> {
 266         self.infcx.tcx
 267     }
 268
 269     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 270         traits::ObligationCause::new(self.span, self.body_id, code)
 271     }
 272
 273     fn normalize(mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 274         let cause = self.cause(traits::MiscObligation);
 275         let infcx = &mut self.infcx;
 276         let param_env = self.param_env;
 277         let mut obligations = Vec::with_capacity(self.out.len());
 278         for mut obligation in self.out {
 279             assert!(!obligation.has_escaping_bound_vars());
 280             let mut selcx = traits::SelectionContext::new(infcx);
 281             // Don't normalize the whole obligation, the param env is either
 282             // already normalized, or we're currently normalizing the
 283             // param_env. Either way we should only normalize the predicate.
 284             let normalized_predicate = traits::project::normalize_with_depth_to(
 285                 &mut selcx,
 286                 param_env,
 287                 cause.clone(),
 288                 self.recursion_depth,
 289                 obligation.predicate,
 290                 &mut obligations,
 291             );
 292             obligation.predicate = normalized_predicate;
 293             obligations.push(obligation);
 294         }
 295         obligations
 296     }
 297
 298     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 299     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 300         let tcx = self.infcx.tcx;
 301         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 302
 303         debug!("compute_trait_ref obligations {:?}", obligations);
 304         let param_env = self.param_env;
 305         let depth = self.recursion_depth;
 306
 307         let item = self.item;
 308
 309         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 310             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 311                 cause = cause.derived_cause(
 312                     parent_trait_pred,
 313                     traits::ObligationCauseCode::DerivedObligation,
 314                 );
 315             }
 316             extend_cause_with_original_assoc_item_obligation(
 317                 tcx, trait_ref, item, &mut cause, predicate,
 318             );
 319             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 320         };
 321
 322         if let Elaborate::All = elaborate {
 323             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 324             let implied_obligations = implied_obligations.map(extend);
 325             self.out.extend(implied_obligations);
 326         } else {
 327             self.out.extend(obligations);
 328         }
 329
 330         let tcx = self.tcx();
 331         self.out.extend(
 332             trait_ref
 333                 .substs
 334                 .iter()
 335                 .enumerate()
 336                 .filter(|(_, arg)| {
 337                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 338                 })
 339                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 340                 .map(|(i, arg)| {
 341                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 342                     // The first subst is the self ty - use the correct span for it.
 343                     if i == 0 {
 344                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 345                             item.map(|i| &i.kind)
 346                         {
 347                             cause.span = self_ty.span;
 348                         }
 349                     }
 350                     traits::Obligation::with_depth(
 351                         cause,
 352                         depth,
 353                         param_env,
 354                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 355                     )
 356                 }),
 357         );
 358     }
 359
 360     /// Pushes the obligations required for `trait_ref::Item` to be WF
 361     /// into `self.out`.
 362     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 363         // A projection is well-formed if
 364         //
 365         // (a) its predicates hold (*)
 366         // (b) its substs are wf
 367         //
 368         // (*) The predicates of an associated type include the predicates of
 369         //     the trait that it's contained in. For example, given
 370         //
 371         // trait A<T>: Clone {
 372         //     type X where T: Copy;
 373         // }
 374         //
 375         // The predicates of `<() as A<i32>>::X` are:
 376         // [
 377         //     `(): Sized`
 378         //     `(): Clone`
 379         //     `(): A<i32>`
 380         //     `i32: Sized`
 381         //     `i32: Clone`
 382         //     `i32: Copy`
 383         // ]
 384         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 385         self.out.extend(obligations);
 386
 387         let tcx = self.tcx();
 388         let cause = self.cause(traits::MiscObligation);
 389         let param_env = self.param_env;
 390         let depth = self.recursion_depth;
 391
 392         self.out.extend(
 393             data.substs
 394                 .iter()
 395                 .filter(|arg| {
 396                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 397                 })
 398                 .filter(|arg| !arg.has_escaping_bound_vars())
 399                 .map(|arg| {
 400                     traits::Obligation::with_depth(
 401                         cause.clone(),
 402                         depth,
 403                         param_env,
 404                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 405                     )
 406                 }),
 407         );
 408     }
 409
 410     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 411         if !subty.has_escaping_bound_vars() {
 412             let cause = self.cause(cause);
 413             let trait_ref = ty::TraitRef {
 414                 def_id: self.infcx.tcx.require_lang_item(LangItem::Sized, None),
 415                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 416             };
 417             self.out.push(traits::Obligation::with_depth(
 418                 cause,
 419                 self.recursion_depth,
 420                 self.param_env,
 421                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.infcx.tcx),
 422             ));
 423         }
 424     }
 425
 426     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 427     fn compute(&mut self, arg: GenericArg<'tcx>) {
 428         let mut walker = arg.walk();
 429         let param_env = self.param_env;
 430         let depth = self.recursion_depth;
 431         while let Some(arg) = walker.next() {
 432             let ty = match arg.unpack() {
 433                 GenericArgKind::Type(ty) => ty,
 434
 435                 // No WF constraints for lifetimes being present, any outlives
 436                 // obligations are handled by the parent (e.g. `ty::Ref`).
 437                 GenericArgKind::Lifetime(_) => continue,
 438
 439                 GenericArgKind::Const(constant) => {
 440                     match constant.kind() {
 441                         ty::ConstKind::Unevaluated(uv) => {
 442                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 443                             self.out.extend(obligations);
 444
 445                             let predicate =
 446                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 447                                     .to_predicate(self.tcx());
 448                             let cause = self.cause(traits::MiscObligation);
 449                             self.out.push(traits::Obligation::with_depth(
 450                                 cause,
 451                                 self.recursion_depth,
 452                                 self.param_env,
 453                                 predicate,
 454                             ));
 455                         }
 456                         ty::ConstKind::Infer(infer) => {
 457                             let resolved = self.infcx.shallow_resolve(infer);
 458                             // the `InferConst` changed, meaning that we made progress.
 459                             if resolved != infer {
 460                                 let cause = self.cause(traits::MiscObligation);
 461
 462                                 let resolved_constant = self.infcx.tcx.mk_const(ty::ConstS {
 463                                     kind: ty::ConstKind::Infer(resolved),
 464                                     ty: constant.ty(),
 465                                 });
 466                                 self.out.push(traits::Obligation::with_depth(
 467                                     cause,
 468                                     self.recursion_depth,
 469                                     self.param_env,
 470                                     ty::Binder::dummy(ty::PredicateKind::WellFormed(
 471                                         resolved_constant.into(),
 472                                     ))
 473                                     .to_predicate(self.tcx()),
 474                                 ));
 475                             }
 476                         }
 477                         ty::ConstKind::Error(_)
 478                         | ty::ConstKind::Param(_)
 479                         | ty::ConstKind::Bound(..)
 480                         | ty::ConstKind::Placeholder(..) => {
 481                             // These variants are trivially WF, so nothing to do here.
 482                         }
 483                         ty::ConstKind::Value(..) => {
 484                             // FIXME: Enforce that values are structurally-matchable.
 485                         }
 486                     }
 487                     continue;
 488                 }
 489             };
 490
 491             match *ty.kind() {
 492                 ty::Bool
 493                 | ty::Char
 494                 | ty::Int(..)
 495                 | ty::Uint(..)
 496                 | ty::Float(..)
 497                 | ty::Error(_)
 498                 | ty::Str
 499                 | ty::GeneratorWitness(..)
 500                 | ty::Never
 501                 | ty::Param(_)
 502                 | ty::Bound(..)
 503                 | ty::Placeholder(..)
 504                 | ty::Foreign(..) => {
 505                     // WfScalar, WfParameter, etc
 506                 }
 507
 508                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 509                 ty::Infer(ty::IntVar(_)) => {}
 510
 511                 // Can only infer to `ty::Float(_)`.
 512                 ty::Infer(ty::FloatVar(_)) => {}
 513
 514                 ty::Slice(subty) => {
 515                     self.require_sized(subty, traits::SliceOrArrayElem);
 516                 }
 517
 518                 ty::Array(subty, _) => {
 519                     self.require_sized(subty, traits::SliceOrArrayElem);
 520                     // Note that we handle the len is implicitly checked while walking `arg`.
 521                 }
 522
 523                 ty::Tuple(ref tys) => {
 524                     if let Some((_last, rest)) = tys.split_last() {
 525                         for &elem in rest {
 526                             self.require_sized(elem, traits::TupleElem);
 527                         }
 528                     }
 529                 }
 530
 531                 ty::RawPtr(_) => {
 532                     // Simple cases that are WF if their type args are WF.
 533                 }
 534
 535                 ty::Projection(data) => {
 536                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 537                     self.compute_projection(data);
 538                 }
 539
 540                 ty::Adt(def, substs) => {
 541                     // WfNominalType
 542                     let obligations = self.nominal_obligations(def.did(), substs);
 543                     self.out.extend(obligations);
 544                 }
 545
 546                 ty::FnDef(did, substs) => {
 547                     let obligations = self.nominal_obligations(did, substs);
 548                     self.out.extend(obligations);
 549                 }
 550
 551                 ty::Ref(r, rty, _) => {
 552                     // WfReference
 553                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 554                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 555                         self.out.push(traits::Obligation::with_depth(
 556                             cause,
 557                             depth,
 558                             param_env,
 559                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 560                                 ty::OutlivesPredicate(rty, r),
 561                             ))
 562                             .to_predicate(self.tcx()),
 563                         ));
 564                     }
 565                 }
 566
 567                 ty::Generator(..) => {
 568                     // Walk ALL the types in the generator: this will
 569                     // include the upvar types as well as the yield
 570                     // type. Note that this is mildly distinct from
 571                     // the closure case, where we have to be careful
 572                     // about the signature of the closure. We don't
 573                     // have the problem of implied bounds here since
 574                     // generators don't take arguments.
 575                 }
 576
 577                 ty::Closure(did, substs) => {
 578                     // Only check the upvar types for WF, not the rest
 579                     // of the types within. This is needed because we
 580                     // capture the signature and it may not be WF
 581                     // without the implied bounds. Consider a closure
 582                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 583                     // not known to hold in the creator's context (and
 584                     // indeed the closure may not be invoked by its
 585                     // creator, but rather turned to someone who *can*
 586                     // verify that).
 587                     //
 588                     // The special treatment of closures here really
 589                     // ought not to be necessary either; the problem
 590                     // is related to #25860 -- there is no way for us
 591                     // to express a fn type complete with the implied
 592                     // bounds that it is assuming. I think in reality
 593                     // the WF rules around fn are a bit messed up, and
 594                     // that is the rot problem: `fn(&'a T)` should
 595                     // probably always be WF, because it should be
 596                     // shorthand for something like `where(T: 'a) {
 597                     // fn(&'a T) }`, as discussed in #25860.
 598                     walker.skip_current_subtree(); // subtree handled below
 599                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 600                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 601                     // Note that we cannot skip the generic types
 602                     // types. Normally, within the fn
 603                     // body where they are created, the generics will
 604                     // always be WF, and outside of that fn body we
 605                     // are not directly inspecting closure types
 606                     // anyway, except via auto trait matching (which
 607                     // only inspects the upvar types).
 608                     // But when a closure is part of a type-alias-impl-trait
 609                     // then the function that created the defining site may
 610                     // have had more bounds available than the type alias
 611                     // specifies. This may cause us to have a closure in the
 612                     // hidden type that is not actually well formed and
 613                     // can cause compiler crashes when the user abuses unsafe
 614                     // code to procure such a closure.
 615                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 616                     let obligations = self.nominal_obligations(did, substs);
 617                     self.out.extend(obligations);
 618                 }
 619
 620                 ty::FnPtr(_) => {
 621                     // let the loop iterate into the argument/return
 622                     // types appearing in the fn signature
 623                 }
 624
 625                 ty::Opaque(did, substs) => {
 626                     // all of the requirements on type parameters
 627                     // should've been checked by the instantiation
 628                     // of whatever returned this exact `impl Trait`.
 629
 630                     // for named opaque `impl Trait` types we still need to check them
 631                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 632                         let obligations = self.nominal_obligations(did, substs);
 633                         self.out.extend(obligations);
 634                     }
 635                 }
 636
 637                 ty::Dynamic(data, r) => {
 638                     // WfObject
 639                     //
 640                     // Here, we defer WF checking due to higher-ranked
 641                     // regions. This is perhaps not ideal.
 642                     self.from_object_ty(ty, data, r);
 643
 644                     // FIXME(#27579) RFC also considers adding trait
 645                     // obligations that don't refer to Self and
 646                     // checking those
 647
 648                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 649
 650                     if !defer_to_coercion {
 651                         let cause = self.cause(traits::MiscObligation);
 652                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 653                         let tcx = self.tcx();
 654                         self.out.extend(component_traits.map(|did| {
 655                             traits::Obligation::with_depth(
 656                                 cause.clone(),
 657                                 depth,
 658                                 param_env,
 659                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 660                                     .to_predicate(tcx),
 661                             )
 662                         }));
 663                     }
 664                 }
 665
 666                 // Inference variables are the complicated case, since we don't
 667                 // know what type they are. We do two things:
 668                 //
 669                 // 1. Check if they have been resolved, and if so proceed with
 670                 //    THAT type.
 671                 // 2. If not, we've at least simplified things (e.g., we went
 672                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 673                 //    register a pending obligation and keep
 674                 //    moving. (Goal is that an "inductive hypothesis"
 675                 //    is satisfied to ensure termination.)
 676                 // See also the comment on `fn obligations`, describing "livelock"
 677                 // prevention, which happens before this can be reached.
 678                 ty::Infer(_) => {
 679                     let ty = self.infcx.shallow_resolve(ty);
 680                     if let ty::Infer(ty::TyVar(_)) = ty.kind() {
 681                         // Not yet resolved, but we've made progress.
 682                         let cause = self.cause(traits::MiscObligation);
 683                         self.out.push(traits::Obligation::with_depth(
 684                             cause,
 685                             self.recursion_depth,
 686                             param_env,
 687                             ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 688                                 .to_predicate(self.tcx()),
 689                         ));
 690                     } else {
 691                         // Yes, resolved, proceed with the result.
 692                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 693                         self.compute(ty.into());
 694                     }
 695                 }
 696             }
 697         }
 698     }
 699
 700     fn nominal_obligations(
 701         &mut self,
 702         def_id: DefId,
 703         substs: SubstsRef<'tcx>,
 704     ) -> Vec<traits::PredicateObligation<'tcx>> {
 705         let predicates = self.infcx.tcx.predicates_of(def_id);
 706         let mut origins = vec![def_id; predicates.predicates.len()];
 707         let mut head = predicates;
 708         while let Some(parent) = head.parent {
 709             head = self.infcx.tcx.predicates_of(parent);
 710             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 711         }
 712
 713         let predicates = predicates.instantiate(self.infcx.tcx, substs);
 714         debug_assert_eq!(predicates.predicates.len(), origins.len());
 715
 716         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 717             .map(|((pred, span), origin_def_id)| {
 718                 let code = if span.is_dummy() {
 719                     traits::MiscObligation
 720                 } else {
 721                     traits::BindingObligation(origin_def_id, span)
 722                 };
 723                 let cause = self.cause(code);
 724                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 725             })
 726             .filter(|pred| !pred.has_escaping_bound_vars())
 727             .collect()
 728     }
 729
 730     fn from_object_ty(
 731         &mut self,
 732         ty: Ty<'tcx>,
 733         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 734         region: ty::Region<'tcx>,
 735     ) {
 736         // Imagine a type like this:
 737         //
 738         //     trait Foo { }
 739         //     trait Bar<'c> : 'c { }
 740         //
 741         //     &'b (Foo+'c+Bar<'d>)
 742         //         ^
 743         //
 744         // In this case, the following relationships must hold:
 745         //
 746         //     'b <= 'c
 747         //     'd <= 'c
 748         //
 749         // The first conditions is due to the normal region pointer
 750         // rules, which say that a reference cannot outlive its
 751         // referent.
 752         //
 753         // The final condition may be a bit surprising. In particular,
 754         // you may expect that it would have been `'c <= 'd`, since
 755         // usually lifetimes of outer things are conservative
 756         // approximations for inner things. However, it works somewhat
 757         // differently with trait objects: here the idea is that if the
 758         // user specifies a region bound (`'c`, in this case) it is the
 759         // "master bound" that *implies* that bounds from other traits are
 760         // all met. (Remember that *all bounds* in a type like
 761         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 762         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 763         // 'y.)
 764         //
 765         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 766         // am looking forward to the future here.
 767         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 768             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 769
 770             let explicit_bound = region;
 771
 772             self.out.reserve(implicit_bounds.len());
 773             for implicit_bound in implicit_bounds {
 774                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 775                 let outlives =
 776                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 777                 self.out.push(traits::Obligation::with_depth(
 778                     cause,
 779                     self.recursion_depth,
 780                     self.param_env,
 781                     outlives.to_predicate(self.infcx.tcx),
 782                 ));
 783             }
 784         }
 785     }
 786 }
 787
 788 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 789 /// bounds that must hold on the elided self type. These are derived
 790 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 791 /// they declare `trait SomeTrait : 'static`, for example, then
 792 /// `'static` would appear in the list. The hard work is done by
 793 /// `infer::required_region_bounds`, see that for more information.
 794 pub fn object_region_bounds<'tcx>(
 795     tcx: TyCtxt<'tcx>,
 796     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 797 ) -> Vec<ty::Region<'tcx>> {
 798     // Since we don't actually *know* the self type for an object,
 799     // this "open(err)" serves as a kind of dummy standin -- basically
 800     // a placeholder type.
 801     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 802
 803     let predicates = existential_predicates.iter().filter_map(|predicate| {
 804         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 805             None
 806         } else {
 807             Some(predicate.with_self_ty(tcx, open_ty))
 808         }
 809     });
 810
 811     required_region_bounds(tcx, open_ty, predicates)
 812 }