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<'a, 'tcx>(
  18     infcx: &InferCtxt<'a, '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                     }
  35
  36                     resolved_ty
  37                 }
  38                 _ => ty,
  39             }
  40             .into()
  41         }
  42         GenericArgKind::Const(ct) => {
  43             match ct.kind() {
  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
  52                         .tcx
  53                         .mk_const(ty::ConstS { kind: ty::ConstKind::Infer(resolved), ty: ct.ty() })
  54                 }
  55                 _ => ct,
  56             }
  57             .into()
  58         }
  59         // There is nothing we have to do for lifetimes.
  60         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  61     };
  62
  63     let mut wf = WfPredicates {
  64         tcx: infcx.tcx,
  65         param_env,
  66         body_id,
  67         span,
  68         out: vec![],
  69         recursion_depth,
  70         item: None,
  71     };
  72     wf.compute(arg);
  73     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  74
  75     let result = wf.normalize(infcx);
  76     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  77     Some(result)
  78 }
  79
  80 /// Returns the obligations that make this trait reference
  81 /// well-formed.  For example, if there is a trait `Set` defined like
  82 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  83 /// if `Bar: Eq`.
  84 pub fn trait_obligations<'a, 'tcx>(
  85     infcx: &InferCtxt<'a, 'tcx>,
  86     param_env: ty::ParamEnv<'tcx>,
  87     body_id: hir::HirId,
  88     trait_pred: &ty::TraitPredicate<'tcx>,
  89     span: Span,
  90     item: &'tcx hir::Item<'tcx>,
  91 ) -> Vec<traits::PredicateObligation<'tcx>> {
  92     let mut wf = WfPredicates {
  93         tcx: infcx.tcx,
  94         param_env,
  95         body_id,
  96         span,
  97         out: vec![],
  98         recursion_depth: 0,
  99         item: Some(item),
 100     };
 101     wf.compute_trait_pred(trait_pred, Elaborate::All);
 102     debug!(obligations = ?wf.out);
 103     wf.normalize(infcx)
 104 }
 105
 106 pub fn predicate_obligations<'a, 'tcx>(
 107     infcx: &InferCtxt<'a, 'tcx>,
 108     param_env: ty::ParamEnv<'tcx>,
 109     body_id: hir::HirId,
 110     predicate: ty::Predicate<'tcx>,
 111     span: Span,
 112 ) -> Vec<traits::PredicateObligation<'tcx>> {
 113     let mut wf = WfPredicates {
 114         tcx: infcx.tcx,
 115         param_env,
 116         body_id,
 117         span,
 118         out: vec![],
 119         recursion_depth: 0,
 120         item: None,
 121     };
 122
 123     // It's ok to skip the binder here because wf code is prepared for it
 124     match predicate.kind().skip_binder() {
 125         ty::PredicateKind::Trait(t) => {
 126             wf.compute_trait_pred(&t, Elaborate::None);
 127         }
 128         ty::PredicateKind::RegionOutlives(..) => {}
 129         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 130             wf.compute(ty.into());
 131         }
 132         ty::PredicateKind::Projection(t) => {
 133             wf.compute_projection(t.projection_ty);
 134             wf.compute(match t.term {
 135                 ty::Term::Ty(ty) => ty.into(),
 136                 ty::Term::Const(c) => c.into(),
 137             })
 138         }
 139         ty::PredicateKind::WellFormed(arg) => {
 140             wf.compute(arg);
 141         }
 142         ty::PredicateKind::ObjectSafe(_) => {}
 143         ty::PredicateKind::ClosureKind(..) => {}
 144         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 145             wf.compute(a.into());
 146             wf.compute(b.into());
 147         }
 148         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 149             wf.compute(a.into());
 150             wf.compute(b.into());
 151         }
 152         ty::PredicateKind::ConstEvaluatable(uv) => {
 153             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 154             wf.out.extend(obligations);
 155
 156             for arg in uv.substs.iter() {
 157                 wf.compute(arg);
 158             }
 159         }
 160         ty::PredicateKind::ConstEquate(c1, c2) => {
 161             wf.compute(c1.into());
 162             wf.compute(c2.into());
 163         }
 164         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 165             bug!("TypeWellFormedFromEnv is only used for Chalk")
 166         }
 167     }
 168
 169     wf.normalize(infcx)
 170 }
 171
 172 struct WfPredicates<'tcx> {
 173     tcx: TyCtxt<'tcx>,
 174     param_env: ty::ParamEnv<'tcx>,
 175     body_id: hir::HirId,
 176     span: Span,
 177     out: Vec<traits::PredicateObligation<'tcx>>,
 178     recursion_depth: usize,
 179     item: Option<&'tcx hir::Item<'tcx>>,
 180 }
 181
 182 /// Controls whether we "elaborate" supertraits and so forth on the WF
 183 /// predicates. This is a kind of hack to address #43784. The
 184 /// underlying problem in that issue was a trait structure like:
 185 ///
 186 /// ```ignore (illustrative)
 187 /// trait Foo: Copy { }
 188 /// trait Bar: Foo { }
 189 /// impl<T: Bar> Foo for T { }
 190 /// impl<T> Bar for T { }
 191 /// ```
 192 ///
 193 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 194 /// we decide that this is true because `T: Bar` is in the
 195 /// where-clauses (and we can elaborate that to include `T:
 196 /// Copy`). This wouldn't be a problem, except that when we check the
 197 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 198 /// impl. And so nowhere did we check that `T: Copy` holds!
 199 ///
 200 /// To resolve this, we elaborate the WF requirements that must be
 201 /// proven when checking impls. This means that (e.g.) the `impl Bar
 202 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 203 /// Copy` (which it won't be able to do, because there is no `Copy`
 204 /// impl for `T`).
 205 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 206 enum Elaborate {
 207     All,
 208     None,
 209 }
 210
 211 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 212     tcx: TyCtxt<'tcx>,
 213     trait_ref: &ty::TraitRef<'tcx>,
 214     item: Option<&hir::Item<'tcx>>,
 215     cause: &mut traits::ObligationCause<'tcx>,
 216     pred: ty::Predicate<'tcx>,
 217 ) {
 218     debug!(
 219         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 220         trait_ref, item, cause, pred
 221     );
 222     let (items, impl_def_id) = match item {
 223         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 224         _ => return,
 225     };
 226     let fix_span =
 227         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 228             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 229             _ => impl_item_ref.span,
 230         };
 231
 232     // It is fine to skip the binder as we don't care about regions here.
 233     match pred.kind().skip_binder() {
 234         ty::PredicateKind::Projection(proj) => {
 235             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 236             // but rather from a "second order" obligation, where an associated type has a
 237             // projection coming from another associated type. See
 238             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 239             // `traits-assoc-type-in-supertrait-bad.rs`.
 240             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 241                 && let Some(&impl_item_id) =
 242                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 243                 && let Some(impl_item_span) = items
 244                     .iter()
 245                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 246                     .map(fix_span)
 247             {
 248                 cause.span = impl_item_span;
 249             }
 250         }
 251         ty::PredicateKind::Trait(pred) => {
 252             // An associated item obligation born out of the `trait` failed to be met. An example
 253             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 254             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 255             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 256                 && let Some(&impl_item_id) =
 257                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 258                 && let Some(impl_item_span) = items
 259                     .iter()
 260                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 261                     .map(fix_span)
 262             {
 263                 cause.span = impl_item_span;
 264             }
 265         }
 266         _ => {}
 267     }
 268 }
 269
 270 impl<'tcx> WfPredicates<'tcx> {
 271     fn tcx(&self) -> TyCtxt<'tcx> {
 272         self.tcx
 273     }
 274
 275     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 276         traits::ObligationCause::new(self.span, self.body_id, code)
 277     }
 278
 279     fn normalize(self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 280         let cause = self.cause(traits::WellFormed(None));
 281         let param_env = self.param_env;
 282         let mut obligations = Vec::with_capacity(self.out.len());
 283         for mut obligation in self.out {
 284             assert!(!obligation.has_escaping_bound_vars());
 285             let mut selcx = traits::SelectionContext::new(infcx);
 286             // Don't normalize the whole obligation, the param env is either
 287             // already normalized, or we're currently normalizing the
 288             // param_env. Either way we should only normalize the predicate.
 289             let normalized_predicate = traits::project::normalize_with_depth_to(
 290                 &mut selcx,
 291                 param_env,
 292                 cause.clone(),
 293                 self.recursion_depth,
 294                 obligation.predicate,
 295                 &mut obligations,
 296             );
 297             obligation.predicate = normalized_predicate;
 298             obligations.push(obligation);
 299         }
 300         obligations
 301     }
 302
 303     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 304     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 305         let tcx = self.tcx;
 306         let trait_ref = &trait_pred.trait_ref;
 307
 308         // if the trait predicate is not const, the wf obligations should not be const as well.
 309         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 310             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 311         } else {
 312             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 313         };
 314
 315         debug!("compute_trait_pred obligations {:?}", obligations);
 316         let param_env = self.param_env;
 317         let depth = self.recursion_depth;
 318
 319         let item = self.item;
 320
 321         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 322             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 323                 cause = cause.derived_cause(
 324                     parent_trait_pred,
 325                     traits::ObligationCauseCode::DerivedObligation,
 326                 );
 327             }
 328             extend_cause_with_original_assoc_item_obligation(
 329                 tcx, trait_ref, item, &mut cause, predicate,
 330             );
 331             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 332         };
 333
 334         if let Elaborate::All = elaborate {
 335             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 336             let implied_obligations = implied_obligations.map(extend);
 337             self.out.extend(implied_obligations);
 338         } else {
 339             self.out.extend(obligations);
 340         }
 341
 342         let tcx = self.tcx();
 343         self.out.extend(
 344             trait_ref
 345                 .substs
 346                 .iter()
 347                 .enumerate()
 348                 .filter(|(_, arg)| {
 349                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 350                 })
 351                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 352                 .map(|(i, arg)| {
 353                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 354                     // The first subst is the self ty - use the correct span for it.
 355                     if i == 0 {
 356                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 357                             item.map(|i| &i.kind)
 358                         {
 359                             cause.span = self_ty.span;
 360                         }
 361                     }
 362                     traits::Obligation::with_depth(
 363                         cause,
 364                         depth,
 365                         param_env,
 366                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 367                     )
 368                 }),
 369         );
 370     }
 371
 372     /// Pushes the obligations required for `trait_ref::Item` to be WF
 373     /// into `self.out`.
 374     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 375         // A projection is well-formed if
 376         //
 377         // (a) its predicates hold (*)
 378         // (b) its substs are wf
 379         //
 380         // (*) The predicates of an associated type include the predicates of
 381         //     the trait that it's contained in. For example, given
 382         //
 383         // trait A<T>: Clone {
 384         //     type X where T: Copy;
 385         // }
 386         //
 387         // The predicates of `<() as A<i32>>::X` are:
 388         // [
 389         //     `(): Sized`
 390         //     `(): Clone`
 391         //     `(): A<i32>`
 392         //     `i32: Sized`
 393         //     `i32: Clone`
 394         //     `i32: Copy`
 395         // ]
 396         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 397         self.out.extend(obligations);
 398
 399         let tcx = self.tcx();
 400         let cause = self.cause(traits::WellFormed(None));
 401         let param_env = self.param_env;
 402         let depth = self.recursion_depth;
 403
 404         self.out.extend(
 405             data.substs
 406                 .iter()
 407                 .filter(|arg| {
 408                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 409                 })
 410                 .filter(|arg| !arg.has_escaping_bound_vars())
 411                 .map(|arg| {
 412                     traits::Obligation::with_depth(
 413                         cause.clone(),
 414                         depth,
 415                         param_env,
 416                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 417                     )
 418                 }),
 419         );
 420     }
 421
 422     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 423         if !subty.has_escaping_bound_vars() {
 424             let cause = self.cause(cause);
 425             let trait_ref = ty::TraitRef {
 426                 def_id: self.tcx.require_lang_item(LangItem::Sized, None),
 427                 substs: self.tcx.mk_substs_trait(subty, &[]),
 428             };
 429             self.out.push(traits::Obligation::with_depth(
 430                 cause,
 431                 self.recursion_depth,
 432                 self.param_env,
 433                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx),
 434             ));
 435         }
 436     }
 437
 438     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 439     fn compute(&mut self, arg: GenericArg<'tcx>) {
 440         let mut walker = arg.walk();
 441         let param_env = self.param_env;
 442         let depth = self.recursion_depth;
 443         while let Some(arg) = walker.next() {
 444             let ty = match arg.unpack() {
 445                 GenericArgKind::Type(ty) => ty,
 446
 447                 // No WF constraints for lifetimes being present, any outlives
 448                 // obligations are handled by the parent (e.g. `ty::Ref`).
 449                 GenericArgKind::Lifetime(_) => continue,
 450
 451                 GenericArgKind::Const(constant) => {
 452                     match constant.kind() {
 453                         ty::ConstKind::Unevaluated(uv) => {
 454                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 455                             self.out.extend(obligations);
 456
 457                             let predicate =
 458                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 459                                     .to_predicate(self.tcx());
 460                             let cause = self.cause(traits::WellFormed(None));
 461                             self.out.push(traits::Obligation::with_depth(
 462                                 cause,
 463                                 self.recursion_depth,
 464                                 self.param_env,
 465                                 predicate,
 466                             ));
 467                         }
 468                         ty::ConstKind::Infer(_) => {
 469                             let cause = self.cause(traits::WellFormed(None));
 470
 471                             self.out.push(traits::Obligation::with_depth(
 472                                 cause,
 473                                 self.recursion_depth,
 474                                 self.param_env,
 475                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(constant.into()))
 476                                     .to_predicate(self.tcx()),
 477                             ));
 478                         }
 479                         ty::ConstKind::Error(_)
 480                         | ty::ConstKind::Param(_)
 481                         | ty::ConstKind::Bound(..)
 482                         | ty::ConstKind::Placeholder(..) => {
 483                             // These variants are trivially WF, so nothing to do here.
 484                         }
 485                         ty::ConstKind::Value(..) => {
 486                             // FIXME: Enforce that values are structurally-matchable.
 487                         }
 488                     }
 489                     continue;
 490                 }
 491             };
 492
 493             match *ty.kind() {
 494                 ty::Bool
 495                 | ty::Char
 496                 | ty::Int(..)
 497                 | ty::Uint(..)
 498                 | ty::Float(..)
 499                 | ty::Error(_)
 500                 | ty::Str
 501                 | ty::GeneratorWitness(..)
 502                 | ty::Never
 503                 | ty::Param(_)
 504                 | ty::Bound(..)
 505                 | ty::Placeholder(..)
 506                 | ty::Foreign(..) => {
 507                     // WfScalar, WfParameter, etc
 508                 }
 509
 510                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 511                 ty::Infer(ty::IntVar(_)) => {}
 512
 513                 // Can only infer to `ty::Float(_)`.
 514                 ty::Infer(ty::FloatVar(_)) => {}
 515
 516                 ty::Slice(subty) => {
 517                     self.require_sized(subty, traits::SliceOrArrayElem);
 518                 }
 519
 520                 ty::Array(subty, _) => {
 521                     self.require_sized(subty, traits::SliceOrArrayElem);
 522                     // Note that we handle the len is implicitly checked while walking `arg`.
 523                 }
 524
 525                 ty::Tuple(ref tys) => {
 526                     if let Some((_last, rest)) = tys.split_last() {
 527                         for &elem in rest {
 528                             self.require_sized(elem, traits::TupleElem);
 529                         }
 530                     }
 531                 }
 532
 533                 ty::RawPtr(_) => {
 534                     // Simple cases that are WF if their type args are WF.
 535                 }
 536
 537                 ty::Projection(data) => {
 538                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 539                     self.compute_projection(data);
 540                 }
 541
 542                 ty::Adt(def, substs) => {
 543                     // WfNominalType
 544                     let obligations = self.nominal_obligations(def.did(), substs);
 545                     self.out.extend(obligations);
 546                 }
 547
 548                 ty::FnDef(did, substs) => {
 549                     let obligations = self.nominal_obligations(did, substs);
 550                     self.out.extend(obligations);
 551                 }
 552
 553                 ty::Ref(r, rty, _) => {
 554                     // WfReference
 555                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 556                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 557                         self.out.push(traits::Obligation::with_depth(
 558                             cause,
 559                             depth,
 560                             param_env,
 561                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 562                                 ty::OutlivesPredicate(rty, r),
 563                             ))
 564                             .to_predicate(self.tcx()),
 565                         ));
 566                     }
 567                 }
 568
 569                 ty::Generator(did, substs, ..) => {
 570                     // Walk ALL the types in the generator: this will
 571                     // include the upvar types as well as the yield
 572                     // type. Note that this is mildly distinct from
 573                     // the closure case, where we have to be careful
 574                     // about the signature of the closure. We don't
 575                     // have the problem of implied bounds here since
 576                     // generators don't take arguments.
 577                     let obligations = self.nominal_obligations(did, substs);
 578                     self.out.extend(obligations);
 579                 }
 580
 581                 ty::Closure(did, substs) => {
 582                     // Only check the upvar types for WF, not the rest
 583                     // of the types within. This is needed because we
 584                     // capture the signature and it may not be WF
 585                     // without the implied bounds. Consider a closure
 586                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 587                     // not known to hold in the creator's context (and
 588                     // indeed the closure may not be invoked by its
 589                     // creator, but rather turned to someone who *can*
 590                     // verify that).
 591                     //
 592                     // The special treatment of closures here really
 593                     // ought not to be necessary either; the problem
 594                     // is related to #25860 -- there is no way for us
 595                     // to express a fn type complete with the implied
 596                     // bounds that it is assuming. I think in reality
 597                     // the WF rules around fn are a bit messed up, and
 598                     // that is the rot problem: `fn(&'a T)` should
 599                     // probably always be WF, because it should be
 600                     // shorthand for something like `where(T: 'a) {
 601                     // fn(&'a T) }`, as discussed in #25860.
 602                     walker.skip_current_subtree(); // subtree handled below
 603                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 604                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 605                     // Note that we cannot skip the generic types
 606                     // types. Normally, within the fn
 607                     // body where they are created, the generics will
 608                     // always be WF, and outside of that fn body we
 609                     // are not directly inspecting closure types
 610                     // anyway, except via auto trait matching (which
 611                     // only inspects the upvar types).
 612                     // But when a closure is part of a type-alias-impl-trait
 613                     // then the function that created the defining site may
 614                     // have had more bounds available than the type alias
 615                     // specifies. This may cause us to have a closure in the
 616                     // hidden type that is not actually well formed and
 617                     // can cause compiler crashes when the user abuses unsafe
 618                     // code to procure such a closure.
 619                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 620                     let obligations = self.nominal_obligations(did, substs);
 621                     self.out.extend(obligations);
 622                 }
 623
 624                 ty::FnPtr(_) => {
 625                     // let the loop iterate into the argument/return
 626                     // types appearing in the fn signature
 627                 }
 628
 629                 ty::Opaque(did, substs) => {
 630                     // All of the requirements on type parameters
 631                     // have already been checked for `impl Trait` in
 632                     // return position. We do need to check type-alias-impl-trait though.
 633                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 634                         let obligations = self.nominal_obligations(did, substs);
 635                         self.out.extend(obligations);
 636                     }
 637                 }
 638
 639                 ty::Dynamic(data, r) => {
 640                     // WfObject
 641                     //
 642                     // Here, we defer WF checking due to higher-ranked
 643                     // regions. This is perhaps not ideal.
 644                     self.from_object_ty(ty, data, r);
 645
 646                     // FIXME(#27579) RFC also considers adding trait
 647                     // obligations that don't refer to Self and
 648                     // checking those
 649
 650                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 651
 652                     if !defer_to_coercion {
 653                         let cause = self.cause(traits::WellFormed(None));
 654                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 655                         let tcx = self.tcx();
 656                         self.out.extend(component_traits.map(|did| {
 657                             traits::Obligation::with_depth(
 658                                 cause.clone(),
 659                                 depth,
 660                                 param_env,
 661                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 662                                     .to_predicate(tcx),
 663                             )
 664                         }));
 665                     }
 666                 }
 667
 668                 // Inference variables are the complicated case, since we don't
 669                 // know what type they are. We do two things:
 670                 //
 671                 // 1. Check if they have been resolved, and if so proceed with
 672                 //    THAT type.
 673                 // 2. If not, we've at least simplified things (e.g., we went
 674                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 675                 //    register a pending obligation and keep
 676                 //    moving. (Goal is that an "inductive hypothesis"
 677                 //    is satisfied to ensure termination.)
 678                 // See also the comment on `fn obligations`, describing "livelock"
 679                 // prevention, which happens before this can be reached.
 680                 ty::Infer(_) => {
 681                     let cause = self.cause(traits::WellFormed(None));
 682                     self.out.push(traits::Obligation::with_depth(
 683                         cause,
 684                         self.recursion_depth,
 685                         param_env,
 686                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 687                             .to_predicate(self.tcx()),
 688                     ));
 689                 }
 690             }
 691         }
 692     }
 693
 694     #[instrument(level = "debug", skip(self))]
 695     fn nominal_obligations_inner(
 696         &mut self,
 697         def_id: DefId,
 698         substs: SubstsRef<'tcx>,
 699         remap_constness: bool,
 700     ) -> Vec<traits::PredicateObligation<'tcx>> {
 701         let predicates = self.tcx.predicates_of(def_id);
 702         let mut origins = vec![def_id; predicates.predicates.len()];
 703         let mut head = predicates;
 704         while let Some(parent) = head.parent {
 705             head = self.tcx.predicates_of(parent);
 706             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 707         }
 708
 709         let predicates = predicates.instantiate(self.tcx, substs);
 710         trace!("{:#?}", predicates);
 711         debug_assert_eq!(predicates.predicates.len(), origins.len());
 712
 713         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 714             .map(|((mut pred, span), origin_def_id)| {
 715                 let code = if span.is_dummy() {
 716                     traits::MiscObligation
 717                 } else {
 718                     traits::BindingObligation(origin_def_id, span)
 719                 };
 720                 let cause = self.cause(code);
 721                 if remap_constness {
 722                     pred = pred.without_const(self.tcx);
 723                 }
 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 nominal_obligations(
 731         &mut self,
 732         def_id: DefId,
 733         substs: SubstsRef<'tcx>,
 734     ) -> Vec<traits::PredicateObligation<'tcx>> {
 735         self.nominal_obligations_inner(def_id, substs, false)
 736     }
 737
 738     fn nominal_obligations_without_const(
 739         &mut self,
 740         def_id: DefId,
 741         substs: SubstsRef<'tcx>,
 742     ) -> Vec<traits::PredicateObligation<'tcx>> {
 743         self.nominal_obligations_inner(def_id, substs, true)
 744     }
 745
 746     fn from_object_ty(
 747         &mut self,
 748         ty: Ty<'tcx>,
 749         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 750         region: ty::Region<'tcx>,
 751     ) {
 752         // Imagine a type like this:
 753         //
 754         //     trait Foo { }
 755         //     trait Bar<'c> : 'c { }
 756         //
 757         //     &'b (Foo+'c+Bar<'d>)
 758         //         ^
 759         //
 760         // In this case, the following relationships must hold:
 761         //
 762         //     'b <= 'c
 763         //     'd <= 'c
 764         //
 765         // The first conditions is due to the normal region pointer
 766         // rules, which say that a reference cannot outlive its
 767         // referent.
 768         //
 769         // The final condition may be a bit surprising. In particular,
 770         // you may expect that it would have been `'c <= 'd`, since
 771         // usually lifetimes of outer things are conservative
 772         // approximations for inner things. However, it works somewhat
 773         // differently with trait objects: here the idea is that if the
 774         // user specifies a region bound (`'c`, in this case) it is the
 775         // "master bound" that *implies* that bounds from other traits are
 776         // all met. (Remember that *all bounds* in a type like
 777         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 778         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 779         // 'y.)
 780         //
 781         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 782         // am looking forward to the future here.
 783         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 784             let implicit_bounds = object_region_bounds(self.tcx, data);
 785
 786             let explicit_bound = region;
 787
 788             self.out.reserve(implicit_bounds.len());
 789             for implicit_bound in implicit_bounds {
 790                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 791                 let outlives =
 792                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 793                 self.out.push(traits::Obligation::with_depth(
 794                     cause,
 795                     self.recursion_depth,
 796                     self.param_env,
 797                     outlives.to_predicate(self.tcx),
 798                 ));
 799             }
 800         }
 801     }
 802 }
 803
 804 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 805 /// bounds that must hold on the elided self type. These are derived
 806 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 807 /// they declare `trait SomeTrait : 'static`, for example, then
 808 /// `'static` would appear in the list. The hard work is done by
 809 /// `infer::required_region_bounds`, see that for more information.
 810 pub fn object_region_bounds<'tcx>(
 811     tcx: TyCtxt<'tcx>,
 812     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 813 ) -> Vec<ty::Region<'tcx>> {
 814     // Since we don't actually *know* the self type for an object,
 815     // this "open(err)" serves as a kind of dummy standin -- basically
 816     // a placeholder type.
 817     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 818
 819     let predicates = existential_predicates.iter().filter_map(|predicate| {
 820         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 821             None
 822         } else {
 823             Some(predicate.with_self_ty(tcx, open_ty))
 824         }
 825     });
 826
 827     required_region_bounds(tcx, open_ty, predicates)
 828 }
 829
 830 /// Given a set of predicates that apply to an object type, returns
 831 /// the region bounds that the (erased) `Self` type must
 832 /// outlive. Precisely *because* the `Self` type is erased, the
 833 /// parameter `erased_self_ty` must be supplied to indicate what type
 834 /// has been used to represent `Self` in the predicates
 835 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 836 /// popular choice.
 837 ///
 838 /// N.B., in some cases, particularly around higher-ranked bounds,
 839 /// this function returns a kind of conservative approximation.
 840 /// That is, all regions returned by this function are definitely
 841 /// required, but there may be other region bounds that are not
 842 /// returned, as well as requirements like `for<'a> T: 'a`.
 843 ///
 844 /// Requires that trait definitions have been processed so that we can
 845 /// elaborate predicates and walk supertraits.
 846 #[instrument(skip(tcx, predicates), level = "debug")]
 847 pub(crate) fn required_region_bounds<'tcx>(
 848     tcx: TyCtxt<'tcx>,
 849     erased_self_ty: Ty<'tcx>,
 850     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 851 ) -> Vec<ty::Region<'tcx>> {
 852     assert!(!erased_self_ty.has_escaping_bound_vars());
 853
 854     traits::elaborate_predicates(tcx, predicates)
 855         .filter_map(|obligation| {
 856             debug!(?obligation);
 857             match obligation.predicate.kind().skip_binder() {
 858                 ty::PredicateKind::Projection(..)
 859                 | ty::PredicateKind::Trait(..)
 860                 | ty::PredicateKind::Subtype(..)
 861                 | ty::PredicateKind::Coerce(..)
 862                 | ty::PredicateKind::WellFormed(..)
 863                 | ty::PredicateKind::ObjectSafe(..)
 864                 | ty::PredicateKind::ClosureKind(..)
 865                 | ty::PredicateKind::RegionOutlives(..)
 866                 | ty::PredicateKind::ConstEvaluatable(..)
 867                 | ty::PredicateKind::ConstEquate(..)
 868                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 869                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
 870                     // Search for a bound of the form `erased_self_ty
 871                     // : 'a`, but be wary of something like `for<'a>
 872                     // erased_self_ty : 'a` (we interpret a
 873                     // higher-ranked bound like that as 'static,
 874                     // though at present the code in `fulfill.rs`
 875                     // considers such bounds to be unsatisfiable, so
 876                     // it's kind of a moot point since you could never
 877                     // construct such an object, but this seems
 878                     // correct even if that code changes).
 879                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 880                         Some(*r)
 881                     } else {
 882                         None
 883                     }
 884                 }
 885             }
 886         })
 887         .collect()
 888 }