compiler/rustc_trait_selection/src/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::traits;
   3 use rustc_hir as hir;
   4 use rustc_hir::def_id::DefId;
   5 use rustc_hir::lang_items::LangItem;
   6 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
   7 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeVisitable};
   8 use rustc_span::Span;
   9
  10 use std::iter;
  11 /// Returns the set of obligations needed to make `arg` well-formed.
  12 /// If `arg` contains unresolved inference variables, this may include
  13 /// further WF obligations. However, if `arg` IS an unresolved
  14 /// inference variable, returns `None`, because we are not able to
  15 /// make any progress at all. This is to prevent "livelock" where we
  16 /// say "$0 is WF if $0 is WF".
  17 pub fn obligations<'tcx>(
  18     infcx: &InferCtxt<'tcx>,
  19     param_env: ty::ParamEnv<'tcx>,
  20     body_id: hir::HirId,
  21     recursion_depth: usize,
  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                     } else {
  35                         resolved_ty
  36                     }
  37                 }
  38                 _ => ty,
  39             }
  40             .into()
  41         }
  42         GenericArgKind::Const(ct) => {
  43             match ct.kind() {
  44                 ty::ConstKind::Infer(_) => {
  45                     let resolved = infcx.shallow_resolve(ct);
  46                     if resolved == ct {
  47                         // No progress.
  48                         return None;
  49                     } else {
  50                         resolved
  51                     }
  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 {
  62         tcx: infcx.tcx,
  63         param_env,
  64         body_id,
  65         span,
  66         out: vec![],
  67         recursion_depth,
  68         item: None,
  69     };
  70     wf.compute(arg);
  71     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  72
  73     let result = wf.normalize(infcx);
  74     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  75     Some(result)
  76 }
  77
  78 /// Returns the obligations that make this trait reference
  79 /// well-formed.  For example, if there is a trait `Set` defined like
  80 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  81 /// if `Bar: Eq`.
  82 pub fn trait_obligations<'tcx>(
  83     infcx: &InferCtxt<'tcx>,
  84     param_env: ty::ParamEnv<'tcx>,
  85     body_id: hir::HirId,
  86     trait_pred: &ty::TraitPredicate<'tcx>,
  87     span: Span,
  88     item: &'tcx hir::Item<'tcx>,
  89 ) -> Vec<traits::PredicateObligation<'tcx>> {
  90     let mut wf = WfPredicates {
  91         tcx: infcx.tcx,
  92         param_env,
  93         body_id,
  94         span,
  95         out: vec![],
  96         recursion_depth: 0,
  97         item: Some(item),
  98     };
  99     wf.compute_trait_pred(trait_pred, Elaborate::All);
 100     debug!(obligations = ?wf.out);
 101     wf.normalize(infcx)
 102 }
 103
 104 #[instrument(skip(infcx), ret)]
 105 pub fn predicate_obligations<'tcx>(
 106     infcx: &InferCtxt<'tcx>,
 107     param_env: ty::ParamEnv<'tcx>,
 108     body_id: hir::HirId,
 109     predicate: ty::Predicate<'tcx>,
 110     span: Span,
 111 ) -> Vec<traits::PredicateObligation<'tcx>> {
 112     let mut wf = WfPredicates {
 113         tcx: infcx.tcx,
 114         param_env,
 115         body_id,
 116         span,
 117         out: vec![],
 118         recursion_depth: 0,
 119         item: None,
 120     };
 121
 122     // It's ok to skip the binder here because wf code is prepared for it
 123     match predicate.kind().skip_binder() {
 124         ty::PredicateKind::Trait(t) => {
 125             wf.compute_trait_pred(&t, Elaborate::None);
 126         }
 127         ty::PredicateKind::RegionOutlives(..) => {}
 128         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 129             wf.compute(ty.into());
 130         }
 131         ty::PredicateKind::Projection(t) => {
 132             wf.compute_projection(t.projection_ty);
 133             wf.compute(match t.term.unpack() {
 134                 ty::TermKind::Ty(ty) => ty.into(),
 135                 ty::TermKind::Const(c) => c.into(),
 136             })
 137         }
 138         ty::PredicateKind::WellFormed(arg) => {
 139             wf.compute(arg);
 140         }
 141         ty::PredicateKind::ObjectSafe(_) => {}
 142         ty::PredicateKind::ClosureKind(..) => {}
 143         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 144             wf.compute(a.into());
 145             wf.compute(b.into());
 146         }
 147         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 148             wf.compute(a.into());
 149             wf.compute(b.into());
 150         }
 151         ty::PredicateKind::ConstEvaluatable(ct) => {
 152             wf.compute(ct.into());
 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(infcx)
 164 }
 165
 166 struct WfPredicates<'tcx> {
 167     tcx: TyCtxt<'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_), owner_id, .. }) => {
 218             (impl_.items, *owner_id)
 219         }
 220         _ => return,
 221     };
 222     let fix_span =
 223         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 224             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
 225             _ => impl_item_ref.span,
 226         };
 227
 228     // It is fine to skip the binder as we don't care about regions here.
 229     match pred.kind().skip_binder() {
 230         ty::PredicateKind::Projection(proj) => {
 231             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 232             // but rather from a "second order" obligation, where an associated type has a
 233             // projection coming from another associated type. See
 234             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 235             // `traits-assoc-type-in-supertrait-bad.rs`.
 236             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 237                 && let Some(&impl_item_id) =
 238                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 239                 && let Some(impl_item_span) = items
 240                     .iter()
 241                     .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
 242                     .map(fix_span)
 243             {
 244                 cause.span = impl_item_span;
 245             }
 246         }
 247         ty::PredicateKind::Trait(pred) => {
 248             // An associated item obligation born out of the `trait` failed to be met. An example
 249             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 250             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 251             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 252                 && let Some(&impl_item_id) =
 253                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 254                 && let Some(impl_item_span) = items
 255                     .iter()
 256                     .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
 257                     .map(fix_span)
 258             {
 259                 cause.span = impl_item_span;
 260             }
 261         }
 262         _ => {}
 263     }
 264 }
 265
 266 impl<'tcx> WfPredicates<'tcx> {
 267     fn tcx(&self) -> TyCtxt<'tcx> {
 268         self.tcx
 269     }
 270
 271     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 272         traits::ObligationCause::new(self.span, self.body_id, code)
 273     }
 274
 275     fn normalize(self, infcx: &InferCtxt<'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 276         let cause = self.cause(traits::WellFormed(None));
 277         let param_env = self.param_env;
 278         let mut obligations = Vec::with_capacity(self.out.len());
 279         for mut obligation in self.out {
 280             assert!(!obligation.has_escaping_bound_vars());
 281             let mut selcx = traits::SelectionContext::new(infcx);
 282             // Don't normalize the whole obligation, the param env is either
 283             // already normalized, or we're currently normalizing the
 284             // param_env. Either way we should only normalize the predicate.
 285             let normalized_predicate = traits::project::normalize_with_depth_to(
 286                 &mut selcx,
 287                 param_env,
 288                 cause.clone(),
 289                 self.recursion_depth,
 290                 obligation.predicate,
 291                 &mut obligations,
 292             );
 293             obligation.predicate = normalized_predicate;
 294             obligations.push(obligation);
 295         }
 296         obligations
 297     }
 298
 299     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 300     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 301         let tcx = self.tcx;
 302         let trait_ref = &trait_pred.trait_ref;
 303
 304         // if the trait predicate is not const, the wf obligations should not be const as well.
 305         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 306             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 307         } else {
 308             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 309         };
 310
 311         debug!("compute_trait_pred obligations {:?}", obligations);
 312         let param_env = self.param_env;
 313         let depth = self.recursion_depth;
 314
 315         let item = self.item;
 316
 317         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 318             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 319                 cause = cause.derived_cause(
 320                     parent_trait_pred,
 321                     traits::ObligationCauseCode::DerivedObligation,
 322                 );
 323             }
 324             extend_cause_with_original_assoc_item_obligation(
 325                 tcx, trait_ref, item, &mut cause, predicate,
 326             );
 327             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 328         };
 329
 330         if let Elaborate::All = elaborate {
 331             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 332             let implied_obligations = implied_obligations.map(extend);
 333             self.out.extend(implied_obligations);
 334         } else {
 335             self.out.extend(obligations);
 336         }
 337
 338         let tcx = self.tcx();
 339         self.out.extend(
 340             trait_ref
 341                 .substs
 342                 .iter()
 343                 .enumerate()
 344                 .filter(|(_, arg)| {
 345                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 346                 })
 347                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 348                 .map(|(i, arg)| {
 349                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 350                     // The first subst is the self ty - use the correct span for it.
 351                     if i == 0 {
 352                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 353                             item.map(|i| &i.kind)
 354                         {
 355                             cause.span = self_ty.span;
 356                         }
 357                     }
 358                     traits::Obligation::with_depth(
 359                         cause,
 360                         depth,
 361                         param_env,
 362                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 363                     )
 364                 }),
 365         );
 366     }
 367
 368     /// Pushes the obligations required for `trait_ref::Item` to be WF
 369     /// into `self.out`.
 370     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 371         // A projection is well-formed if
 372         //
 373         // (a) its predicates hold (*)
 374         // (b) its substs are wf
 375         //
 376         // (*) The predicates of an associated type include the predicates of
 377         //     the trait that it's contained in. For example, given
 378         //
 379         // trait A<T>: Clone {
 380         //     type X where T: Copy;
 381         // }
 382         //
 383         // The predicates of `<() as A<i32>>::X` are:
 384         // [
 385         //     `(): Sized`
 386         //     `(): Clone`
 387         //     `(): A<i32>`
 388         //     `i32: Sized`
 389         //     `i32: Clone`
 390         //     `i32: Copy`
 391         // ]
 392         // Projection types do not require const predicates.
 393         let obligations = self.nominal_obligations_without_const(data.item_def_id, data.substs);
 394         self.out.extend(obligations);
 395
 396         let tcx = self.tcx();
 397         let cause = self.cause(traits::WellFormed(None));
 398         let param_env = self.param_env;
 399         let depth = self.recursion_depth;
 400
 401         self.out.extend(
 402             data.substs
 403                 .iter()
 404                 .filter(|arg| {
 405                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 406                 })
 407                 .filter(|arg| !arg.has_escaping_bound_vars())
 408                 .map(|arg| {
 409                     traits::Obligation::with_depth(
 410                         cause.clone(),
 411                         depth,
 412                         param_env,
 413                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 414                     )
 415                 }),
 416         );
 417     }
 418
 419     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 420         if !subty.has_escaping_bound_vars() {
 421             let cause = self.cause(cause);
 422             let trait_ref = ty::TraitRef {
 423                 def_id: self.tcx.require_lang_item(LangItem::Sized, None),
 424                 substs: self.tcx.mk_substs_trait(subty, &[]),
 425             };
 426             self.out.push(traits::Obligation::with_depth(
 427                 cause,
 428                 self.recursion_depth,
 429                 self.param_env,
 430                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx),
 431             ));
 432         }
 433     }
 434
 435     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 436     #[instrument(level = "debug", skip(self))]
 437     fn compute(&mut self, arg: GenericArg<'tcx>) {
 438         let mut walker = arg.walk();
 439         let param_env = self.param_env;
 440         let depth = self.recursion_depth;
 441         while let Some(arg) = walker.next() {
 442             debug!(?arg, ?self.out);
 443             let ty = match arg.unpack() {
 444                 GenericArgKind::Type(ty) => ty,
 445
 446                 // No WF constraints for lifetimes being present, any outlives
 447                 // obligations are handled by the parent (e.g. `ty::Ref`).
 448                 GenericArgKind::Lifetime(_) => continue,
 449
 450                 GenericArgKind::Const(ct) => {
 451                     match ct.kind() {
 452                         ty::ConstKind::Unevaluated(uv) => {
 453                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 454                             self.out.extend(obligations);
 455
 456                             let predicate =
 457                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(ct))
 458                                     .to_predicate(self.tcx());
 459                             let cause = self.cause(traits::WellFormed(None));
 460                             self.out.push(traits::Obligation::with_depth(
 461                                 cause,
 462                                 self.recursion_depth,
 463                                 self.param_env,
 464                                 predicate,
 465                             ));
 466                         }
 467                         ty::ConstKind::Infer(_) => {
 468                             let cause = self.cause(traits::WellFormed(None));
 469
 470                             self.out.push(traits::Obligation::with_depth(
 471                                 cause,
 472                                 self.recursion_depth,
 473                                 self.param_env,
 474                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(ct.into()))
 475                                     .to_predicate(self.tcx()),
 476                             ));
 477                         }
 478                         ty::ConstKind::Error(_)
 479                         | ty::ConstKind::Param(_)
 480                         | ty::ConstKind::Bound(..)
 481                         | ty::ConstKind::Placeholder(..) => {
 482                             // These variants are trivially WF, so nothing to do here.
 483                         }
 484                         ty::ConstKind::Value(..) => {
 485                             // FIXME: Enforce that values are structurally-matchable.
 486                         }
 487                     }
 488                     continue;
 489                 }
 490             };
 491
 492             debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
 493
 494             match *ty.kind() {
 495                 ty::Bool
 496                 | ty::Char
 497                 | ty::Int(..)
 498                 | ty::Uint(..)
 499                 | ty::Float(..)
 500                 | ty::Error(_)
 501                 | ty::Str
 502                 | ty::GeneratorWitness(..)
 503                 | ty::Never
 504                 | ty::Param(_)
 505                 | ty::Bound(..)
 506                 | ty::Placeholder(..)
 507                 | ty::Foreign(..) => {
 508                     // WfScalar, WfParameter, etc
 509                 }
 510
 511                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 512                 ty::Infer(ty::IntVar(_)) => {}
 513
 514                 // Can only infer to `ty::Float(_)`.
 515                 ty::Infer(ty::FloatVar(_)) => {}
 516
 517                 ty::Slice(subty) => {
 518                     self.require_sized(subty, traits::SliceOrArrayElem);
 519                 }
 520
 521                 ty::Array(subty, _) => {
 522                     self.require_sized(subty, traits::SliceOrArrayElem);
 523                     // Note that we handle the len is implicitly checked while walking `arg`.
 524                 }
 525
 526                 ty::Tuple(ref tys) => {
 527                     if let Some((_last, rest)) = tys.split_last() {
 528                         for &elem in rest {
 529                             self.require_sized(elem, traits::TupleElem);
 530                         }
 531                     }
 532                 }
 533
 534                 ty::RawPtr(_) => {
 535                     // Simple cases that are WF if their type args are WF.
 536                 }
 537
 538                 ty::Projection(data) => {
 539                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 540                     self.compute_projection(data);
 541                 }
 542
 543                 ty::Adt(def, substs) => {
 544                     // WfNominalType
 545                     let obligations = self.nominal_obligations(def.did(), substs);
 546                     self.out.extend(obligations);
 547                 }
 548
 549                 ty::FnDef(did, substs) => {
 550                     let obligations = self.nominal_obligations_without_const(did, substs);
 551                     self.out.extend(obligations);
 552                 }
 553
 554                 ty::Ref(r, rty, _) => {
 555                     // WfReference
 556                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 557                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 558                         self.out.push(traits::Obligation::with_depth(
 559                             cause,
 560                             depth,
 561                             param_env,
 562                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 563                                 ty::OutlivesPredicate(rty, r),
 564                             ))
 565                             .to_predicate(self.tcx()),
 566                         ));
 567                     }
 568                 }
 569
 570                 ty::Generator(did, substs, ..) => {
 571                     // Walk ALL the types in the generator: this will
 572                     // include the upvar types as well as the yield
 573                     // type. Note that this is mildly distinct from
 574                     // the closure case, where we have to be careful
 575                     // about the signature of the closure. We don't
 576                     // have the problem of implied bounds here since
 577                     // generators don't take arguments.
 578                     let obligations = self.nominal_obligations(did, substs);
 579                     self.out.extend(obligations);
 580                 }
 581
 582                 ty::Closure(did, substs) => {
 583                     // Only check the upvar types for WF, not the rest
 584                     // of the types within. This is needed because we
 585                     // capture the signature and it may not be WF
 586                     // without the implied bounds. Consider a closure
 587                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 588                     // not known to hold in the creator's context (and
 589                     // indeed the closure may not be invoked by its
 590                     // creator, but rather turned to someone who *can*
 591                     // verify that).
 592                     //
 593                     // The special treatment of closures here really
 594                     // ought not to be necessary either; the problem
 595                     // is related to #25860 -- there is no way for us
 596                     // to express a fn type complete with the implied
 597                     // bounds that it is assuming. I think in reality
 598                     // the WF rules around fn are a bit messed up, and
 599                     // that is the rot problem: `fn(&'a T)` should
 600                     // probably always be WF, because it should be
 601                     // shorthand for something like `where(T: 'a) {
 602                     // fn(&'a T) }`, as discussed in #25860.
 603                     walker.skip_current_subtree(); // subtree handled below
 604                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 605                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 606                     // Note that we cannot skip the generic types
 607                     // types. Normally, within the fn
 608                     // body where they are created, the generics will
 609                     // always be WF, and outside of that fn body we
 610                     // are not directly inspecting closure types
 611                     // anyway, except via auto trait matching (which
 612                     // only inspects the upvar types).
 613                     // But when a closure is part of a type-alias-impl-trait
 614                     // then the function that created the defining site may
 615                     // have had more bounds available than the type alias
 616                     // specifies. This may cause us to have a closure in the
 617                     // hidden type that is not actually well formed and
 618                     // can cause compiler crashes when the user abuses unsafe
 619                     // code to procure such a closure.
 620                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 621                     let obligations = self.nominal_obligations(did, substs);
 622                     self.out.extend(obligations);
 623                 }
 624
 625                 ty::FnPtr(_) => {
 626                     // let the loop iterate into the argument/return
 627                     // types appearing in the fn signature
 628                 }
 629
 630                 ty::Opaque(did, substs) => {
 631                     // All of the requirements on type parameters
 632                     // have already been checked for `impl Trait` in
 633                     // return position. We do need to check type-alias-impl-trait though.
 634                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 635                         let obligations = self.nominal_obligations(did, substs);
 636                         self.out.extend(obligations);
 637                     }
 638                 }
 639
 640                 ty::Dynamic(data, r, _) => {
 641                     // WfObject
 642                     //
 643                     // Here, we defer WF checking due to higher-ranked
 644                     // regions. This is perhaps not ideal.
 645                     self.from_object_ty(ty, data, r);
 646
 647                     // FIXME(#27579) RFC also considers adding trait
 648                     // obligations that don't refer to Self and
 649                     // checking those
 650
 651                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 652
 653                     if !defer_to_coercion {
 654                         let cause = self.cause(traits::WellFormed(None));
 655                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 656                         let tcx = self.tcx();
 657                         self.out.extend(component_traits.map(|did| {
 658                             traits::Obligation::with_depth(
 659                                 cause.clone(),
 660                                 depth,
 661                                 param_env,
 662                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 663                                     .to_predicate(tcx),
 664                             )
 665                         }));
 666                     }
 667                 }
 668
 669                 // Inference variables are the complicated case, since we don't
 670                 // know what type they are. We do two things:
 671                 //
 672                 // 1. Check if they have been resolved, and if so proceed with
 673                 //    THAT type.
 674                 // 2. If not, we've at least simplified things (e.g., we went
 675                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 676                 //    register a pending obligation and keep
 677                 //    moving. (Goal is that an "inductive hypothesis"
 678                 //    is satisfied to ensure termination.)
 679                 // See also the comment on `fn obligations`, describing "livelock"
 680                 // prevention, which happens before this can be reached.
 681                 ty::Infer(_) => {
 682                     let cause = self.cause(traits::WellFormed(None));
 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                 }
 691             }
 692
 693             debug!(?self.out);
 694         }
 695     }
 696
 697     #[instrument(level = "debug", skip(self))]
 698     fn nominal_obligations_inner(
 699         &mut self,
 700         def_id: DefId,
 701         substs: SubstsRef<'tcx>,
 702         remap_constness: bool,
 703     ) -> Vec<traits::PredicateObligation<'tcx>> {
 704         let predicates = self.tcx.predicates_of(def_id);
 705         let mut origins = vec![def_id; predicates.predicates.len()];
 706         let mut head = predicates;
 707         while let Some(parent) = head.parent {
 708             head = self.tcx.predicates_of(parent);
 709             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 710         }
 711
 712         let predicates = predicates.instantiate(self.tcx, substs);
 713         trace!("{:#?}", predicates);
 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(|((mut pred, span), origin_def_id)| {
 718                 let code = if span.is_dummy() {
 719                     traits::ItemObligation(origin_def_id)
 720                 } else {
 721                     traits::BindingObligation(origin_def_id, span)
 722                 };
 723                 let cause = self.cause(code);
 724                 if remap_constness {
 725                     pred = pred.without_const(self.tcx);
 726                 }
 727                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 728             })
 729             .filter(|pred| !pred.has_escaping_bound_vars())
 730             .collect()
 731     }
 732
 733     fn nominal_obligations(
 734         &mut self,
 735         def_id: DefId,
 736         substs: SubstsRef<'tcx>,
 737     ) -> Vec<traits::PredicateObligation<'tcx>> {
 738         self.nominal_obligations_inner(def_id, substs, false)
 739     }
 740
 741     fn nominal_obligations_without_const(
 742         &mut self,
 743         def_id: DefId,
 744         substs: SubstsRef<'tcx>,
 745     ) -> Vec<traits::PredicateObligation<'tcx>> {
 746         self.nominal_obligations_inner(def_id, substs, true)
 747     }
 748
 749     fn from_object_ty(
 750         &mut self,
 751         ty: Ty<'tcx>,
 752         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 753         region: ty::Region<'tcx>,
 754     ) {
 755         // Imagine a type like this:
 756         //
 757         //     trait Foo { }
 758         //     trait Bar<'c> : 'c { }
 759         //
 760         //     &'b (Foo+'c+Bar<'d>)
 761         //         ^
 762         //
 763         // In this case, the following relationships must hold:
 764         //
 765         //     'b <= 'c
 766         //     'd <= 'c
 767         //
 768         // The first conditions is due to the normal region pointer
 769         // rules, which say that a reference cannot outlive its
 770         // referent.
 771         //
 772         // The final condition may be a bit surprising. In particular,
 773         // you may expect that it would have been `'c <= 'd`, since
 774         // usually lifetimes of outer things are conservative
 775         // approximations for inner things. However, it works somewhat
 776         // differently with trait objects: here the idea is that if the
 777         // user specifies a region bound (`'c`, in this case) it is the
 778         // "master bound" that *implies* that bounds from other traits are
 779         // all met. (Remember that *all bounds* in a type like
 780         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 781         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 782         // 'y.)
 783         //
 784         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 785         // am looking forward to the future here.
 786         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 787             let implicit_bounds = object_region_bounds(self.tcx, data);
 788
 789             let explicit_bound = region;
 790
 791             self.out.reserve(implicit_bounds.len());
 792             for implicit_bound in implicit_bounds {
 793                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 794                 let outlives =
 795                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 796                 self.out.push(traits::Obligation::with_depth(
 797                     cause,
 798                     self.recursion_depth,
 799                     self.param_env,
 800                     outlives.to_predicate(self.tcx),
 801                 ));
 802             }
 803         }
 804     }
 805 }
 806
 807 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 808 /// bounds that must hold on the elided self type. These are derived
 809 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 810 /// they declare `trait SomeTrait : 'static`, for example, then
 811 /// `'static` would appear in the list. The hard work is done by
 812 /// `infer::required_region_bounds`, see that for more information.
 813 pub fn object_region_bounds<'tcx>(
 814     tcx: TyCtxt<'tcx>,
 815     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 816 ) -> Vec<ty::Region<'tcx>> {
 817     // Since we don't actually *know* the self type for an object,
 818     // this "open(err)" serves as a kind of dummy standin -- basically
 819     // a placeholder type.
 820     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 821
 822     let predicates = existential_predicates.iter().filter_map(|predicate| {
 823         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 824             None
 825         } else {
 826             Some(predicate.with_self_ty(tcx, open_ty))
 827         }
 828     });
 829
 830     required_region_bounds(tcx, open_ty, predicates)
 831 }
 832
 833 /// Given a set of predicates that apply to an object type, returns
 834 /// the region bounds that the (erased) `Self` type must
 835 /// outlive. Precisely *because* the `Self` type is erased, the
 836 /// parameter `erased_self_ty` must be supplied to indicate what type
 837 /// has been used to represent `Self` in the predicates
 838 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 839 /// popular choice.
 840 ///
 841 /// N.B., in some cases, particularly around higher-ranked bounds,
 842 /// this function returns a kind of conservative approximation.
 843 /// That is, all regions returned by this function are definitely
 844 /// required, but there may be other region bounds that are not
 845 /// returned, as well as requirements like `for<'a> T: 'a`.
 846 ///
 847 /// Requires that trait definitions have been processed so that we can
 848 /// elaborate predicates and walk supertraits.
 849 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 850 pub(crate) fn required_region_bounds<'tcx>(
 851     tcx: TyCtxt<'tcx>,
 852     erased_self_ty: Ty<'tcx>,
 853     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 854 ) -> Vec<ty::Region<'tcx>> {
 855     assert!(!erased_self_ty.has_escaping_bound_vars());
 856
 857     traits::elaborate_predicates(tcx, predicates)
 858         .filter_map(|obligation| {
 859             debug!(?obligation);
 860             match obligation.predicate.kind().skip_binder() {
 861                 ty::PredicateKind::Projection(..)
 862                 | ty::PredicateKind::Trait(..)
 863                 | ty::PredicateKind::Subtype(..)
 864                 | ty::PredicateKind::Coerce(..)
 865                 | ty::PredicateKind::WellFormed(..)
 866                 | ty::PredicateKind::ObjectSafe(..)
 867                 | ty::PredicateKind::ClosureKind(..)
 868                 | ty::PredicateKind::RegionOutlives(..)
 869                 | ty::PredicateKind::ConstEvaluatable(..)
 870                 | ty::PredicateKind::ConstEquate(..)
 871                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 872                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
 873                     // Search for a bound of the form `erased_self_ty
 874                     // : 'a`, but be wary of something like `for<'a>
 875                     // erased_self_ty : 'a` (we interpret a
 876                     // higher-ranked bound like that as 'static,
 877                     // though at present the code in `fulfill.rs`
 878                     // considers such bounds to be unsatisfiable, so
 879                     // it's kind of a moot point since you could never
 880                     // construct such an object, but this seems
 881                     // correct even if that code changes).
 882                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 883                         Some(*r)
 884                     } else {
 885                         None
 886                     }
 887                 }
 888             }
 889         })
 890         .collect()
 891 }