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                     } 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<'a, 'tcx>(
  83     infcx: &InferCtxt<'a, '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<'a, 'tcx>(
 106     infcx: &InferCtxt<'a, '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(uv) => {
 152             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 153             wf.out.extend(obligations);
 154
 155             for arg in uv.substs.iter() {
 156                 wf.compute(arg);
 157             }
 158         }
 159         ty::PredicateKind::ConstEquate(c1, c2) => {
 160             wf.compute(c1.into());
 161             wf.compute(c2.into());
 162         }
 163         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 164             bug!("TypeWellFormedFromEnv is only used for Chalk")
 165         }
 166     }
 167
 168     wf.normalize(infcx)
 169 }
 170
 171 struct WfPredicates<'tcx> {
 172     tcx: TyCtxt<'tcx>,
 173     param_env: ty::ParamEnv<'tcx>,
 174     body_id: hir::HirId,
 175     span: Span,
 176     out: Vec<traits::PredicateObligation<'tcx>>,
 177     recursion_depth: usize,
 178     item: Option<&'tcx hir::Item<'tcx>>,
 179 }
 180
 181 /// Controls whether we "elaborate" supertraits and so forth on the WF
 182 /// predicates. This is a kind of hack to address #43784. The
 183 /// underlying problem in that issue was a trait structure like:
 184 ///
 185 /// ```ignore (illustrative)
 186 /// trait Foo: Copy { }
 187 /// trait Bar: Foo { }
 188 /// impl<T: Bar> Foo for T { }
 189 /// impl<T> Bar for T { }
 190 /// ```
 191 ///
 192 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 193 /// we decide that this is true because `T: Bar` is in the
 194 /// where-clauses (and we can elaborate that to include `T:
 195 /// Copy`). This wouldn't be a problem, except that when we check the
 196 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 197 /// impl. And so nowhere did we check that `T: Copy` holds!
 198 ///
 199 /// To resolve this, we elaborate the WF requirements that must be
 200 /// proven when checking impls. This means that (e.g.) the `impl Bar
 201 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 202 /// Copy` (which it won't be able to do, because there is no `Copy`
 203 /// impl for `T`).
 204 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 205 enum Elaborate {
 206     All,
 207     None,
 208 }
 209
 210 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 211     tcx: TyCtxt<'tcx>,
 212     trait_ref: &ty::TraitRef<'tcx>,
 213     item: Option<&hir::Item<'tcx>>,
 214     cause: &mut traits::ObligationCause<'tcx>,
 215     pred: ty::Predicate<'tcx>,
 216 ) {
 217     debug!(
 218         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 219         trait_ref, item, cause, pred
 220     );
 221     let (items, impl_def_id) = match item {
 222         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 223         _ => return,
 224     };
 225     let fix_span =
 226         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 227             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 228             _ => impl_item_ref.span,
 229         };
 230
 231     // It is fine to skip the binder as we don't care about regions here.
 232     match pred.kind().skip_binder() {
 233         ty::PredicateKind::Projection(proj) => {
 234             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 235             // but rather from a "second order" obligation, where an associated type has a
 236             // projection coming from another associated type. See
 237             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 238             // `traits-assoc-type-in-supertrait-bad.rs`.
 239             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 240                 && let Some(&impl_item_id) =
 241                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 242                 && let Some(impl_item_span) = items
 243                     .iter()
 244                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 245                     .map(fix_span)
 246             {
 247                 cause.span = impl_item_span;
 248             }
 249         }
 250         ty::PredicateKind::Trait(pred) => {
 251             // An associated item obligation born out of the `trait` failed to be met. An example
 252             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 253             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 254             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 255                 && let Some(&impl_item_id) =
 256                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 257                 && let Some(impl_item_span) = items
 258                     .iter()
 259                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 260                     .map(fix_span)
 261             {
 262                 cause.span = impl_item_span;
 263             }
 264         }
 265         _ => {}
 266     }
 267 }
 268
 269 impl<'tcx> WfPredicates<'tcx> {
 270     fn tcx(&self) -> TyCtxt<'tcx> {
 271         self.tcx
 272     }
 273
 274     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 275         traits::ObligationCause::new(self.span, self.body_id, code)
 276     }
 277
 278     fn normalize(self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 279         let cause = self.cause(traits::WellFormed(None));
 280         let param_env = self.param_env;
 281         let mut obligations = Vec::with_capacity(self.out.len());
 282         for mut obligation in self.out {
 283             assert!(!obligation.has_escaping_bound_vars());
 284             let mut selcx = traits::SelectionContext::new(infcx);
 285             // Don't normalize the whole obligation, the param env is either
 286             // already normalized, or we're currently normalizing the
 287             // param_env. Either way we should only normalize the predicate.
 288             let normalized_predicate = traits::project::normalize_with_depth_to(
 289                 &mut selcx,
 290                 param_env,
 291                 cause.clone(),
 292                 self.recursion_depth,
 293                 obligation.predicate,
 294                 &mut obligations,
 295             );
 296             obligation.predicate = normalized_predicate;
 297             obligations.push(obligation);
 298         }
 299         obligations
 300     }
 301
 302     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 303     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 304         let tcx = self.tcx;
 305         let trait_ref = &trait_pred.trait_ref;
 306
 307         // if the trait predicate is not const, the wf obligations should not be const as well.
 308         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 309             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 310         } else {
 311             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 312         };
 313
 314         debug!("compute_trait_pred obligations {:?}", obligations);
 315         let param_env = self.param_env;
 316         let depth = self.recursion_depth;
 317
 318         let item = self.item;
 319
 320         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 321             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 322                 cause = cause.derived_cause(
 323                     parent_trait_pred,
 324                     traits::ObligationCauseCode::DerivedObligation,
 325                 );
 326             }
 327             extend_cause_with_original_assoc_item_obligation(
 328                 tcx, trait_ref, item, &mut cause, predicate,
 329             );
 330             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 331         };
 332
 333         if let Elaborate::All = elaborate {
 334             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 335             let implied_obligations = implied_obligations.map(extend);
 336             self.out.extend(implied_obligations);
 337         } else {
 338             self.out.extend(obligations);
 339         }
 340
 341         let tcx = self.tcx();
 342         self.out.extend(
 343             trait_ref
 344                 .substs
 345                 .iter()
 346                 .enumerate()
 347                 .filter(|(_, arg)| {
 348                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 349                 })
 350                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 351                 .map(|(i, arg)| {
 352                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 353                     // The first subst is the self ty - use the correct span for it.
 354                     if i == 0 {
 355                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 356                             item.map(|i| &i.kind)
 357                         {
 358                             cause.span = self_ty.span;
 359                         }
 360                     }
 361                     traits::Obligation::with_depth(
 362                         cause,
 363                         depth,
 364                         param_env,
 365                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 366                     )
 367                 }),
 368         );
 369     }
 370
 371     /// Pushes the obligations required for `trait_ref::Item` to be WF
 372     /// into `self.out`.
 373     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 374         // A projection is well-formed if
 375         //
 376         // (a) its predicates hold (*)
 377         // (b) its substs are wf
 378         //
 379         // (*) The predicates of an associated type include the predicates of
 380         //     the trait that it's contained in. For example, given
 381         //
 382         // trait A<T>: Clone {
 383         //     type X where T: Copy;
 384         // }
 385         //
 386         // The predicates of `<() as A<i32>>::X` are:
 387         // [
 388         //     `(): Sized`
 389         //     `(): Clone`
 390         //     `(): A<i32>`
 391         //     `i32: Sized`
 392         //     `i32: Clone`
 393         //     `i32: Copy`
 394         // ]
 395         // Projection types do not require const predicates.
 396         let obligations = self.nominal_obligations_without_const(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     #[instrument(level = "debug", skip(self))]
 440     fn compute(&mut self, arg: GenericArg<'tcx>) {
 441         let mut walker = arg.walk();
 442         let param_env = self.param_env;
 443         let depth = self.recursion_depth;
 444         while let Some(arg) = walker.next() {
 445             debug!(?arg, ?self.out);
 446             let ty = match arg.unpack() {
 447                 GenericArgKind::Type(ty) => ty,
 448
 449                 // No WF constraints for lifetimes being present, any outlives
 450                 // obligations are handled by the parent (e.g. `ty::Ref`).
 451                 GenericArgKind::Lifetime(_) => continue,
 452
 453                 GenericArgKind::Const(constant) => {
 454                     match constant.kind() {
 455                         ty::ConstKind::Unevaluated(uv) => {
 456                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 457                             self.out.extend(obligations);
 458
 459                             let predicate =
 460                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 461                                     .to_predicate(self.tcx());
 462                             let cause = self.cause(traits::WellFormed(None));
 463                             self.out.push(traits::Obligation::with_depth(
 464                                 cause,
 465                                 self.recursion_depth,
 466                                 self.param_env,
 467                                 predicate,
 468                             ));
 469                         }
 470                         ty::ConstKind::Infer(_) => {
 471                             let cause = self.cause(traits::WellFormed(None));
 472
 473                             self.out.push(traits::Obligation::with_depth(
 474                                 cause,
 475                                 self.recursion_depth,
 476                                 self.param_env,
 477                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(constant.into()))
 478                                     .to_predicate(self.tcx()),
 479                             ));
 480                         }
 481                         ty::ConstKind::Error(_)
 482                         | ty::ConstKind::Param(_)
 483                         | ty::ConstKind::Bound(..)
 484                         | ty::ConstKind::Placeholder(..) => {
 485                             // These variants are trivially WF, so nothing to do here.
 486                         }
 487                         ty::ConstKind::Value(..) => {
 488                             // FIXME: Enforce that values are structurally-matchable.
 489                         }
 490                     }
 491                     continue;
 492                 }
 493             };
 494
 495             debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
 496
 497             match *ty.kind() {
 498                 ty::Bool
 499                 | ty::Char
 500                 | ty::Int(..)
 501                 | ty::Uint(..)
 502                 | ty::Float(..)
 503                 | ty::Error(_)
 504                 | ty::Str
 505                 | ty::GeneratorWitness(..)
 506                 | ty::Never
 507                 | ty::Param(_)
 508                 | ty::Bound(..)
 509                 | ty::Placeholder(..)
 510                 | ty::Foreign(..) => {
 511                     // WfScalar, WfParameter, etc
 512                 }
 513
 514                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 515                 ty::Infer(ty::IntVar(_)) => {}
 516
 517                 // Can only infer to `ty::Float(_)`.
 518                 ty::Infer(ty::FloatVar(_)) => {}
 519
 520                 ty::Slice(subty) => {
 521                     self.require_sized(subty, traits::SliceOrArrayElem);
 522                 }
 523
 524                 ty::Array(subty, _) => {
 525                     self.require_sized(subty, traits::SliceOrArrayElem);
 526                     // Note that we handle the len is implicitly checked while walking `arg`.
 527                 }
 528
 529                 ty::Tuple(ref tys) => {
 530                     if let Some((_last, rest)) = tys.split_last() {
 531                         for &elem in rest {
 532                             self.require_sized(elem, traits::TupleElem);
 533                         }
 534                     }
 535                 }
 536
 537                 ty::RawPtr(_) => {
 538                     // Simple cases that are WF if their type args are WF.
 539                 }
 540
 541                 ty::Projection(data) => {
 542                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 543                     self.compute_projection(data);
 544                 }
 545
 546                 ty::Adt(def, substs) => {
 547                     // WfNominalType
 548                     let obligations = self.nominal_obligations(def.did(), substs);
 549                     self.out.extend(obligations);
 550                 }
 551
 552                 ty::FnDef(did, substs) => {
 553                     let obligations = self.nominal_obligations(did, substs);
 554                     self.out.extend(obligations);
 555                 }
 556
 557                 ty::Ref(r, rty, _) => {
 558                     // WfReference
 559                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 560                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 561                         self.out.push(traits::Obligation::with_depth(
 562                             cause,
 563                             depth,
 564                             param_env,
 565                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 566                                 ty::OutlivesPredicate(rty, r),
 567                             ))
 568                             .to_predicate(self.tcx()),
 569                         ));
 570                     }
 571                 }
 572
 573                 ty::Generator(did, substs, ..) => {
 574                     // Walk ALL the types in the generator: this will
 575                     // include the upvar types as well as the yield
 576                     // type. Note that this is mildly distinct from
 577                     // the closure case, where we have to be careful
 578                     // about the signature of the closure. We don't
 579                     // have the problem of implied bounds here since
 580                     // generators don't take arguments.
 581                     let obligations = self.nominal_obligations(did, substs);
 582                     self.out.extend(obligations);
 583                 }
 584
 585                 ty::Closure(did, substs) => {
 586                     // Only check the upvar types for WF, not the rest
 587                     // of the types within. This is needed because we
 588                     // capture the signature and it may not be WF
 589                     // without the implied bounds. Consider a closure
 590                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 591                     // not known to hold in the creator's context (and
 592                     // indeed the closure may not be invoked by its
 593                     // creator, but rather turned to someone who *can*
 594                     // verify that).
 595                     //
 596                     // The special treatment of closures here really
 597                     // ought not to be necessary either; the problem
 598                     // is related to #25860 -- there is no way for us
 599                     // to express a fn type complete with the implied
 600                     // bounds that it is assuming. I think in reality
 601                     // the WF rules around fn are a bit messed up, and
 602                     // that is the rot problem: `fn(&'a T)` should
 603                     // probably always be WF, because it should be
 604                     // shorthand for something like `where(T: 'a) {
 605                     // fn(&'a T) }`, as discussed in #25860.
 606                     walker.skip_current_subtree(); // subtree handled below
 607                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 608                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 609                     // Note that we cannot skip the generic types
 610                     // types. Normally, within the fn
 611                     // body where they are created, the generics will
 612                     // always be WF, and outside of that fn body we
 613                     // are not directly inspecting closure types
 614                     // anyway, except via auto trait matching (which
 615                     // only inspects the upvar types).
 616                     // But when a closure is part of a type-alias-impl-trait
 617                     // then the function that created the defining site may
 618                     // have had more bounds available than the type alias
 619                     // specifies. This may cause us to have a closure in the
 620                     // hidden type that is not actually well formed and
 621                     // can cause compiler crashes when the user abuses unsafe
 622                     // code to procure such a closure.
 623                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 624                     let obligations = self.nominal_obligations(did, substs);
 625                     self.out.extend(obligations);
 626                 }
 627
 628                 ty::FnPtr(_) => {
 629                     // let the loop iterate into the argument/return
 630                     // types appearing in the fn signature
 631                 }
 632
 633                 ty::Opaque(did, substs) => {
 634                     // All of the requirements on type parameters
 635                     // have already been checked for `impl Trait` in
 636                     // return position. We do need to check type-alias-impl-trait though.
 637                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 638                         let obligations = self.nominal_obligations(did, substs);
 639                         self.out.extend(obligations);
 640                     }
 641                 }
 642
 643                 ty::Dynamic(data, r, _) => {
 644                     // WfObject
 645                     //
 646                     // Here, we defer WF checking due to higher-ranked
 647                     // regions. This is perhaps not ideal.
 648                     self.from_object_ty(ty, data, r);
 649
 650                     // FIXME(#27579) RFC also considers adding trait
 651                     // obligations that don't refer to Self and
 652                     // checking those
 653
 654                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 655
 656                     if !defer_to_coercion {
 657                         let cause = self.cause(traits::WellFormed(None));
 658                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 659                         let tcx = self.tcx();
 660                         self.out.extend(component_traits.map(|did| {
 661                             traits::Obligation::with_depth(
 662                                 cause.clone(),
 663                                 depth,
 664                                 param_env,
 665                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 666                                     .to_predicate(tcx),
 667                             )
 668                         }));
 669                     }
 670                 }
 671
 672                 // Inference variables are the complicated case, since we don't
 673                 // know what type they are. We do two things:
 674                 //
 675                 // 1. Check if they have been resolved, and if so proceed with
 676                 //    THAT type.
 677                 // 2. If not, we've at least simplified things (e.g., we went
 678                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 679                 //    register a pending obligation and keep
 680                 //    moving. (Goal is that an "inductive hypothesis"
 681                 //    is satisfied to ensure termination.)
 682                 // See also the comment on `fn obligations`, describing "livelock"
 683                 // prevention, which happens before this can be reached.
 684                 ty::Infer(_) => {
 685                     let cause = self.cause(traits::WellFormed(None));
 686                     self.out.push(traits::Obligation::with_depth(
 687                         cause,
 688                         self.recursion_depth,
 689                         param_env,
 690                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 691                             .to_predicate(self.tcx()),
 692                     ));
 693                 }
 694             }
 695
 696             debug!(?self.out);
 697         }
 698     }
 699
 700     #[instrument(level = "debug", skip(self))]
 701     fn nominal_obligations_inner(
 702         &mut self,
 703         def_id: DefId,
 704         substs: SubstsRef<'tcx>,
 705         remap_constness: bool,
 706     ) -> Vec<traits::PredicateObligation<'tcx>> {
 707         let predicates = self.tcx.predicates_of(def_id);
 708         let mut origins = vec![def_id; predicates.predicates.len()];
 709         let mut head = predicates;
 710         while let Some(parent) = head.parent {
 711             head = self.tcx.predicates_of(parent);
 712             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 713         }
 714
 715         let predicates = predicates.instantiate(self.tcx, substs);
 716         trace!("{:#?}", predicates);
 717         debug_assert_eq!(predicates.predicates.len(), origins.len());
 718
 719         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 720             .map(|((mut pred, span), origin_def_id)| {
 721                 let code = if span.is_dummy() {
 722                     traits::ItemObligation(origin_def_id)
 723                 } else {
 724                     traits::BindingObligation(origin_def_id, span)
 725                 };
 726                 let cause = self.cause(code);
 727                 if remap_constness {
 728                     pred = pred.without_const(self.tcx);
 729                 }
 730                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 731             })
 732             .filter(|pred| !pred.has_escaping_bound_vars())
 733             .collect()
 734     }
 735
 736     fn nominal_obligations(
 737         &mut self,
 738         def_id: DefId,
 739         substs: SubstsRef<'tcx>,
 740     ) -> Vec<traits::PredicateObligation<'tcx>> {
 741         self.nominal_obligations_inner(def_id, substs, false)
 742     }
 743
 744     fn nominal_obligations_without_const(
 745         &mut self,
 746         def_id: DefId,
 747         substs: SubstsRef<'tcx>,
 748     ) -> Vec<traits::PredicateObligation<'tcx>> {
 749         self.nominal_obligations_inner(def_id, substs, true)
 750     }
 751
 752     fn from_object_ty(
 753         &mut self,
 754         ty: Ty<'tcx>,
 755         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 756         region: ty::Region<'tcx>,
 757     ) {
 758         // Imagine a type like this:
 759         //
 760         //     trait Foo { }
 761         //     trait Bar<'c> : 'c { }
 762         //
 763         //     &'b (Foo+'c+Bar<'d>)
 764         //         ^
 765         //
 766         // In this case, the following relationships must hold:
 767         //
 768         //     'b <= 'c
 769         //     'd <= 'c
 770         //
 771         // The first conditions is due to the normal region pointer
 772         // rules, which say that a reference cannot outlive its
 773         // referent.
 774         //
 775         // The final condition may be a bit surprising. In particular,
 776         // you may expect that it would have been `'c <= 'd`, since
 777         // usually lifetimes of outer things are conservative
 778         // approximations for inner things. However, it works somewhat
 779         // differently with trait objects: here the idea is that if the
 780         // user specifies a region bound (`'c`, in this case) it is the
 781         // "master bound" that *implies* that bounds from other traits are
 782         // all met. (Remember that *all bounds* in a type like
 783         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 784         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 785         // 'y.)
 786         //
 787         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 788         // am looking forward to the future here.
 789         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 790             let implicit_bounds = object_region_bounds(self.tcx, data);
 791
 792             let explicit_bound = region;
 793
 794             self.out.reserve(implicit_bounds.len());
 795             for implicit_bound in implicit_bounds {
 796                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 797                 let outlives =
 798                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 799                 self.out.push(traits::Obligation::with_depth(
 800                     cause,
 801                     self.recursion_depth,
 802                     self.param_env,
 803                     outlives.to_predicate(self.tcx),
 804                 ));
 805             }
 806         }
 807     }
 808 }
 809
 810 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 811 /// bounds that must hold on the elided self type. These are derived
 812 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 813 /// they declare `trait SomeTrait : 'static`, for example, then
 814 /// `'static` would appear in the list. The hard work is done by
 815 /// `infer::required_region_bounds`, see that for more information.
 816 pub fn object_region_bounds<'tcx>(
 817     tcx: TyCtxt<'tcx>,
 818     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 819 ) -> Vec<ty::Region<'tcx>> {
 820     // Since we don't actually *know* the self type for an object,
 821     // this "open(err)" serves as a kind of dummy standin -- basically
 822     // a placeholder type.
 823     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 824
 825     let predicates = existential_predicates.iter().filter_map(|predicate| {
 826         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 827             None
 828         } else {
 829             Some(predicate.with_self_ty(tcx, open_ty))
 830         }
 831     });
 832
 833     required_region_bounds(tcx, open_ty, predicates)
 834 }
 835
 836 /// Given a set of predicates that apply to an object type, returns
 837 /// the region bounds that the (erased) `Self` type must
 838 /// outlive. Precisely *because* the `Self` type is erased, the
 839 /// parameter `erased_self_ty` must be supplied to indicate what type
 840 /// has been used to represent `Self` in the predicates
 841 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 842 /// popular choice.
 843 ///
 844 /// N.B., in some cases, particularly around higher-ranked bounds,
 845 /// this function returns a kind of conservative approximation.
 846 /// That is, all regions returned by this function are definitely
 847 /// required, but there may be other region bounds that are not
 848 /// returned, as well as requirements like `for<'a> T: 'a`.
 849 ///
 850 /// Requires that trait definitions have been processed so that we can
 851 /// elaborate predicates and walk supertraits.
 852 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 853 pub(crate) fn required_region_bounds<'tcx>(
 854     tcx: TyCtxt<'tcx>,
 855     erased_self_ty: Ty<'tcx>,
 856     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 857 ) -> Vec<ty::Region<'tcx>> {
 858     assert!(!erased_self_ty.has_escaping_bound_vars());
 859
 860     traits::elaborate_predicates(tcx, predicates)
 861         .filter_map(|obligation| {
 862             debug!(?obligation);
 863             match obligation.predicate.kind().skip_binder() {
 864                 ty::PredicateKind::Projection(..)
 865                 | ty::PredicateKind::Trait(..)
 866                 | ty::PredicateKind::Subtype(..)
 867                 | ty::PredicateKind::Coerce(..)
 868                 | ty::PredicateKind::WellFormed(..)
 869                 | ty::PredicateKind::ObjectSafe(..)
 870                 | ty::PredicateKind::ClosureKind(..)
 871                 | ty::PredicateKind::RegionOutlives(..)
 872                 | ty::PredicateKind::ConstEvaluatable(..)
 873                 | ty::PredicateKind::ConstEquate(..)
 874                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 875                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
 876                     // Search for a bound of the form `erased_self_ty
 877                     // : 'a`, but be wary of something like `for<'a>
 878                     // erased_self_ty : 'a` (we interpret a
 879                     // higher-ranked bound like that as 'static,
 880                     // though at present the code in `fulfill.rs`
 881                     // considers such bounds to be unsatisfiable, so
 882                     // it's kind of a moot point since you could never
 883                     // construct such an object, but this seems
 884                     // correct even if that code changes).
 885                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 886                         Some(*r)
 887                     } else {
 888                         None
 889                     }
 890                 }
 891             }
 892         })
 893         .collect()
 894 }