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, 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::Clause(ty::Clause::Trait(t)) => {
 125             wf.compute_trait_pred(&t, Elaborate::None);
 126         }
 127         ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..)) => {}
 128         ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(ty, _reg))) => {
 129             wf.compute(ty.into());
 130         }
 131         ty::PredicateKind::Clause(ty::Clause::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::Ambiguous => {}
 159         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 160             bug!("TypeWellFormedFromEnv is only used for Chalk")
 161         }
 162     }
 163
 164     wf.normalize(infcx)
 165 }
 166
 167 struct WfPredicates<'tcx> {
 168     tcx: TyCtxt<'tcx>,
 169     param_env: ty::ParamEnv<'tcx>,
 170     body_id: hir::HirId,
 171     span: Span,
 172     out: Vec<traits::PredicateObligation<'tcx>>,
 173     recursion_depth: usize,
 174     item: Option<&'tcx hir::Item<'tcx>>,
 175 }
 176
 177 /// Controls whether we "elaborate" supertraits and so forth on the WF
 178 /// predicates. This is a kind of hack to address #43784. The
 179 /// underlying problem in that issue was a trait structure like:
 180 ///
 181 /// ```ignore (illustrative)
 182 /// trait Foo: Copy { }
 183 /// trait Bar: Foo { }
 184 /// impl<T: Bar> Foo for T { }
 185 /// impl<T> Bar for T { }
 186 /// ```
 187 ///
 188 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 189 /// we decide that this is true because `T: Bar` is in the
 190 /// where-clauses (and we can elaborate that to include `T:
 191 /// Copy`). This wouldn't be a problem, except that when we check the
 192 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 193 /// impl. And so nowhere did we check that `T: Copy` holds!
 194 ///
 195 /// To resolve this, we elaborate the WF requirements that must be
 196 /// proven when checking impls. This means that (e.g.) the `impl Bar
 197 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 198 /// Copy` (which it won't be able to do, because there is no `Copy`
 199 /// impl for `T`).
 200 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 201 enum Elaborate {
 202     All,
 203     None,
 204 }
 205
 206 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 207     tcx: TyCtxt<'tcx>,
 208     trait_ref: &ty::TraitRef<'tcx>,
 209     item: Option<&hir::Item<'tcx>>,
 210     cause: &mut traits::ObligationCause<'tcx>,
 211     pred: ty::Predicate<'tcx>,
 212 ) {
 213     debug!(
 214         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 215         trait_ref, item, cause, pred
 216     );
 217     let (items, impl_def_id) = match item {
 218         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), owner_id, .. }) => {
 219             (impl_.items, *owner_id)
 220         }
 221         _ => return,
 222     };
 223     let fix_span =
 224         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 225             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
 226             _ => impl_item_ref.span,
 227         };
 228
 229     // It is fine to skip the binder as we don't care about regions here.
 230     match pred.kind().skip_binder() {
 231         ty::PredicateKind::Clause(ty::Clause::Projection(proj)) => {
 232             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 233             // but rather from a "second order" obligation, where an associated type has a
 234             // projection coming from another associated type. See
 235             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 236             // `traits-assoc-type-in-supertrait-bad.rs`.
 237             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 238                 && let Some(&impl_item_id) =
 239                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 240                 && let Some(impl_item_span) = items
 241                     .iter()
 242                     .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
 243                     .map(fix_span)
 244             {
 245                 cause.span = impl_item_span;
 246             }
 247         }
 248         ty::PredicateKind::Clause(ty::Clause::Trait(pred)) => {
 249             // An associated item obligation born out of the `trait` failed to be met. An example
 250             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 251             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 252             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 253                 && let Some(&impl_item_id) =
 254                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 255                 && let Some(impl_item_span) = items
 256                     .iter()
 257                     .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
 258                     .map(fix_span)
 259             {
 260                 cause.span = impl_item_span;
 261             }
 262         }
 263         _ => {}
 264     }
 265 }
 266
 267 impl<'tcx> WfPredicates<'tcx> {
 268     fn tcx(&self) -> TyCtxt<'tcx> {
 269         self.tcx
 270     }
 271
 272     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 273         traits::ObligationCause::new(self.span, self.body_id, code)
 274     }
 275
 276     fn normalize(self, infcx: &InferCtxt<'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 277         let cause = self.cause(traits::WellFormed(None));
 278         let param_env = self.param_env;
 279         let mut obligations = Vec::with_capacity(self.out.len());
 280         for mut obligation in self.out {
 281             assert!(!obligation.has_escaping_bound_vars());
 282             let mut selcx = traits::SelectionContext::new(infcx);
 283             // Don't normalize the whole obligation, the param env is either
 284             // already normalized, or we're currently normalizing the
 285             // param_env. Either way we should only normalize the predicate.
 286             let normalized_predicate = traits::project::normalize_with_depth_to(
 287                 &mut selcx,
 288                 param_env,
 289                 cause.clone(),
 290                 self.recursion_depth,
 291                 obligation.predicate,
 292                 &mut obligations,
 293             );
 294             obligation.predicate = normalized_predicate;
 295             obligations.push(obligation);
 296         }
 297         obligations
 298     }
 299
 300     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 301     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 302         let tcx = self.tcx;
 303         let trait_ref = &trait_pred.trait_ref;
 304
 305         // if the trait predicate is not const, the wf obligations should not be const as well.
 306         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 307             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 308         } else {
 309             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 310         };
 311
 312         debug!("compute_trait_pred obligations {:?}", obligations);
 313         let param_env = self.param_env;
 314         let depth = self.recursion_depth;
 315
 316         let item = self.item;
 317
 318         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 319             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 320                 cause = cause.derived_cause(
 321                     parent_trait_pred,
 322                     traits::ObligationCauseCode::DerivedObligation,
 323                 );
 324             }
 325             extend_cause_with_original_assoc_item_obligation(
 326                 tcx, trait_ref, item, &mut cause, predicate,
 327             );
 328             traits::Obligation::with_depth(tcx, cause, depth, param_env, predicate)
 329         };
 330
 331         if let Elaborate::All = elaborate {
 332             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 333             let implied_obligations = implied_obligations.map(extend);
 334             self.out.extend(implied_obligations);
 335         } else {
 336             self.out.extend(obligations);
 337         }
 338
 339         let tcx = self.tcx();
 340         self.out.extend(
 341             trait_ref
 342                 .substs
 343                 .iter()
 344                 .enumerate()
 345                 .filter(|(_, arg)| {
 346                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 347                 })
 348                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 349                 .map(|(i, arg)| {
 350                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 351                     // The first subst is the self ty - use the correct span for it.
 352                     if i == 0 {
 353                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 354                             item.map(|i| &i.kind)
 355                         {
 356                             cause.span = self_ty.span;
 357                         }
 358                     }
 359                     traits::Obligation::with_depth(
 360                         tcx,
 361                         cause,
 362                         depth,
 363                         param_env,
 364                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)),
 365                     )
 366                 }),
 367         );
 368     }
 369
 370     /// Pushes the obligations required for `trait_ref::Item` to be WF
 371     /// into `self.out`.
 372     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 373         // A projection is well-formed if
 374         //
 375         // (a) its predicates hold (*)
 376         // (b) its substs are wf
 377         //
 378         // (*) The predicates of an associated type include the predicates of
 379         //     the trait that it's contained in. For example, given
 380         //
 381         // trait A<T>: Clone {
 382         //     type X where T: Copy;
 383         // }
 384         //
 385         // The predicates of `<() as A<i32>>::X` are:
 386         // [
 387         //     `(): Sized`
 388         //     `(): Clone`
 389         //     `(): A<i32>`
 390         //     `i32: Sized`
 391         //     `i32: Clone`
 392         //     `i32: Copy`
 393         // ]
 394         // Projection types do not require const predicates.
 395         let obligations = self.nominal_obligations_without_const(data.item_def_id, data.substs);
 396         self.out.extend(obligations);
 397
 398         let tcx = self.tcx();
 399         let cause = self.cause(traits::WellFormed(None));
 400         let param_env = self.param_env;
 401         let depth = self.recursion_depth;
 402
 403         self.out.extend(
 404             data.substs
 405                 .iter()
 406                 .filter(|arg| {
 407                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 408                 })
 409                 .filter(|arg| !arg.has_escaping_bound_vars())
 410                 .map(|arg| {
 411                     traits::Obligation::with_depth(
 412                         tcx,
 413                         cause.clone(),
 414                         depth,
 415                         param_env,
 416                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)),
 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 = self.tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [subty]);
 426             self.out.push(traits::Obligation::with_depth(
 427                 self.tcx,
 428                 cause,
 429                 self.recursion_depth,
 430                 self.param_env,
 431                 ty::Binder::dummy(trait_ref).without_const(),
 432             ));
 433         }
 434     }
 435
 436     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 437     #[instrument(level = "debug", skip(self))]
 438     fn compute(&mut self, arg: GenericArg<'tcx>) {
 439         let mut walker = arg.walk();
 440         let param_env = self.param_env;
 441         let depth = self.recursion_depth;
 442         while let Some(arg) = walker.next() {
 443             debug!(?arg, ?self.out);
 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(ct) => {
 452                     match ct.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(ct));
 459                             let cause = self.cause(traits::WellFormed(None));
 460                             self.out.push(traits::Obligation::with_depth(
 461                                 self.tcx(),
 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                                 self.tcx(),
 473                                 cause,
 474                                 self.recursion_depth,
 475                                 self.param_env,
 476                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(ct.into())),
 477                             ));
 478                         }
 479                         // FIXME(generic_const_exprs): This seems wrong but I could not find a way to get this to trigger
 480                         ty::ConstKind::Expr(_) => {
 481                             bug!("checking wfness of `ConstKind::Expr` is unsupported")
 482                         }
 483
 484                         ty::ConstKind::Error(_)
 485                         | ty::ConstKind::Param(_)
 486                         | ty::ConstKind::Bound(..)
 487                         | ty::ConstKind::Placeholder(..) => {
 488                             // These variants are trivially WF, so nothing to do here.
 489                         }
 490                         ty::ConstKind::Value(..) => {
 491                             // FIXME: Enforce that values are structurally-matchable.
 492                         }
 493                     }
 494                     continue;
 495                 }
 496             };
 497
 498             debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
 499
 500             match *ty.kind() {
 501                 ty::Bool
 502                 | ty::Char
 503                 | ty::Int(..)
 504                 | ty::Uint(..)
 505                 | ty::Float(..)
 506                 | ty::Error(_)
 507                 | ty::Str
 508                 | ty::GeneratorWitness(..)
 509                 | ty::Never
 510                 | ty::Param(_)
 511                 | ty::Bound(..)
 512                 | ty::Placeholder(..)
 513                 | ty::Foreign(..) => {
 514                     // WfScalar, WfParameter, etc
 515                 }
 516
 517                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 518                 ty::Infer(ty::IntVar(_)) => {}
 519
 520                 // Can only infer to `ty::Float(_)`.
 521                 ty::Infer(ty::FloatVar(_)) => {}
 522
 523                 ty::Slice(subty) => {
 524                     self.require_sized(subty, traits::SliceOrArrayElem);
 525                 }
 526
 527                 ty::Array(subty, _) => {
 528                     self.require_sized(subty, traits::SliceOrArrayElem);
 529                     // Note that we handle the len is implicitly checked while walking `arg`.
 530                 }
 531
 532                 ty::Tuple(ref tys) => {
 533                     if let Some((_last, rest)) = tys.split_last() {
 534                         for &elem in rest {
 535                             self.require_sized(elem, traits::TupleElem);
 536                         }
 537                     }
 538                 }
 539
 540                 ty::RawPtr(_) => {
 541                     // Simple cases that are WF if their type args are WF.
 542                 }
 543
 544                 ty::Projection(data) => {
 545                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 546                     self.compute_projection(data);
 547                 }
 548
 549                 ty::Adt(def, substs) => {
 550                     // WfNominalType
 551                     let obligations = self.nominal_obligations(def.did(), substs);
 552                     self.out.extend(obligations);
 553                 }
 554
 555                 ty::FnDef(did, substs) => {
 556                     let obligations = self.nominal_obligations_without_const(did, substs);
 557                     self.out.extend(obligations);
 558                 }
 559
 560                 ty::Ref(r, rty, _) => {
 561                     // WfReference
 562                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 563                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 564                         self.out.push(traits::Obligation::with_depth(
 565                             self.tcx(),
 566                             cause,
 567                             depth,
 568                             param_env,
 569                             ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::TypeOutlives(
 570                                 ty::OutlivesPredicate(rty, r),
 571                             ))),
 572                         ));
 573                     }
 574                 }
 575
 576                 ty::Generator(did, substs, ..) => {
 577                     // Walk ALL the types in the generator: this will
 578                     // include the upvar types as well as the yield
 579                     // type. Note that this is mildly distinct from
 580                     // the closure case, where we have to be careful
 581                     // about the signature of the closure. We don't
 582                     // have the problem of implied bounds here since
 583                     // generators don't take arguments.
 584                     let obligations = self.nominal_obligations(did, substs);
 585                     self.out.extend(obligations);
 586                 }
 587
 588                 ty::Closure(did, substs) => {
 589                     // Only check the upvar types for WF, not the rest
 590                     // of the types within. This is needed because we
 591                     // capture the signature and it may not be WF
 592                     // without the implied bounds. Consider a closure
 593                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 594                     // not known to hold in the creator's context (and
 595                     // indeed the closure may not be invoked by its
 596                     // creator, but rather turned to someone who *can*
 597                     // verify that).
 598                     //
 599                     // The special treatment of closures here really
 600                     // ought not to be necessary either; the problem
 601                     // is related to #25860 -- there is no way for us
 602                     // to express a fn type complete with the implied
 603                     // bounds that it is assuming. I think in reality
 604                     // the WF rules around fn are a bit messed up, and
 605                     // that is the rot problem: `fn(&'a T)` should
 606                     // probably always be WF, because it should be
 607                     // shorthand for something like `where(T: 'a) {
 608                     // fn(&'a T) }`, as discussed in #25860.
 609                     walker.skip_current_subtree(); // subtree handled below
 610                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 611                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 612                     // Note that we cannot skip the generic types
 613                     // types. Normally, within the fn
 614                     // body where they are created, the generics will
 615                     // always be WF, and outside of that fn body we
 616                     // are not directly inspecting closure types
 617                     // anyway, except via auto trait matching (which
 618                     // only inspects the upvar types).
 619                     // But when a closure is part of a type-alias-impl-trait
 620                     // then the function that created the defining site may
 621                     // have had more bounds available than the type alias
 622                     // specifies. This may cause us to have a closure in the
 623                     // hidden type that is not actually well formed and
 624                     // can cause compiler crashes when the user abuses unsafe
 625                     // code to procure such a closure.
 626                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 627                     let obligations = self.nominal_obligations(did, substs);
 628                     self.out.extend(obligations);
 629                 }
 630
 631                 ty::FnPtr(_) => {
 632                     // let the loop iterate into the argument/return
 633                     // types appearing in the fn signature
 634                 }
 635
 636                 ty::Opaque(did, substs) => {
 637                     // All of the requirements on type parameters
 638                     // have already been checked for `impl Trait` in
 639                     // return position. We do need to check type-alias-impl-trait though.
 640                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 641                         let obligations = self.nominal_obligations(did, substs);
 642                         self.out.extend(obligations);
 643                     }
 644                 }
 645
 646                 ty::Dynamic(data, r, _) => {
 647                     // WfObject
 648                     //
 649                     // Here, we defer WF checking due to higher-ranked
 650                     // regions. This is perhaps not ideal.
 651                     self.from_object_ty(ty, data, r);
 652
 653                     // FIXME(#27579) RFC also considers adding trait
 654                     // obligations that don't refer to Self and
 655                     // checking those
 656
 657                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 658
 659                     if !defer_to_coercion {
 660                         let cause = self.cause(traits::WellFormed(None));
 661                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 662                         let tcx = self.tcx();
 663                         self.out.extend(component_traits.map(|did| {
 664                             traits::Obligation::with_depth(
 665                                 tcx,
 666                                 cause.clone(),
 667                                 depth,
 668                                 param_env,
 669                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did)),
 670                             )
 671                         }));
 672                     }
 673                 }
 674
 675                 // Inference variables are the complicated case, since we don't
 676                 // know what type they are. We do two things:
 677                 //
 678                 // 1. Check if they have been resolved, and if so proceed with
 679                 //    THAT type.
 680                 // 2. If not, we've at least simplified things (e.g., we went
 681                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 682                 //    register a pending obligation and keep
 683                 //    moving. (Goal is that an "inductive hypothesis"
 684                 //    is satisfied to ensure termination.)
 685                 // See also the comment on `fn obligations`, describing "livelock"
 686                 // prevention, which happens before this can be reached.
 687                 ty::Infer(_) => {
 688                     let cause = self.cause(traits::WellFormed(None));
 689                     self.out.push(traits::Obligation::with_depth(
 690                         self.tcx(),
 691                         cause,
 692                         self.recursion_depth,
 693                         param_env,
 694                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into())),
 695                     ));
 696                 }
 697             }
 698
 699             debug!(?self.out);
 700         }
 701     }
 702
 703     #[instrument(level = "debug", skip(self))]
 704     fn nominal_obligations_inner(
 705         &mut self,
 706         def_id: DefId,
 707         substs: SubstsRef<'tcx>,
 708         remap_constness: bool,
 709     ) -> Vec<traits::PredicateObligation<'tcx>> {
 710         let predicates = self.tcx.predicates_of(def_id);
 711         let mut origins = vec![def_id; predicates.predicates.len()];
 712         let mut head = predicates;
 713         while let Some(parent) = head.parent {
 714             head = self.tcx.predicates_of(parent);
 715             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 716         }
 717
 718         let predicates = predicates.instantiate(self.tcx, substs);
 719         trace!("{:#?}", predicates);
 720         debug_assert_eq!(predicates.predicates.len(), origins.len());
 721
 722         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 723             .map(|((mut pred, span), origin_def_id)| {
 724                 let code = if span.is_dummy() {
 725                     traits::ItemObligation(origin_def_id)
 726                 } else {
 727                     traits::BindingObligation(origin_def_id, span)
 728                 };
 729                 let cause = self.cause(code);
 730                 if remap_constness {
 731                     pred = pred.without_const(self.tcx);
 732                 }
 733                 traits::Obligation::with_depth(
 734                     self.tcx,
 735                     cause,
 736                     self.recursion_depth,
 737                     self.param_env,
 738                     pred,
 739                 )
 740             })
 741             .filter(|pred| !pred.has_escaping_bound_vars())
 742             .collect()
 743     }
 744
 745     fn nominal_obligations(
 746         &mut self,
 747         def_id: DefId,
 748         substs: SubstsRef<'tcx>,
 749     ) -> Vec<traits::PredicateObligation<'tcx>> {
 750         self.nominal_obligations_inner(def_id, substs, false)
 751     }
 752
 753     fn nominal_obligations_without_const(
 754         &mut self,
 755         def_id: DefId,
 756         substs: SubstsRef<'tcx>,
 757     ) -> Vec<traits::PredicateObligation<'tcx>> {
 758         self.nominal_obligations_inner(def_id, substs, true)
 759     }
 760
 761     fn from_object_ty(
 762         &mut self,
 763         ty: Ty<'tcx>,
 764         data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
 765         region: ty::Region<'tcx>,
 766     ) {
 767         // Imagine a type like this:
 768         //
 769         //     trait Foo { }
 770         //     trait Bar<'c> : 'c { }
 771         //
 772         //     &'b (Foo+'c+Bar<'d>)
 773         //         ^
 774         //
 775         // In this case, the following relationships must hold:
 776         //
 777         //     'b <= 'c
 778         //     'd <= 'c
 779         //
 780         // The first conditions is due to the normal region pointer
 781         // rules, which say that a reference cannot outlive its
 782         // referent.
 783         //
 784         // The final condition may be a bit surprising. In particular,
 785         // you may expect that it would have been `'c <= 'd`, since
 786         // usually lifetimes of outer things are conservative
 787         // approximations for inner things. However, it works somewhat
 788         // differently with trait objects: here the idea is that if the
 789         // user specifies a region bound (`'c`, in this case) it is the
 790         // "master bound" that *implies* that bounds from other traits are
 791         // all met. (Remember that *all bounds* in a type like
 792         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 793         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 794         // 'y.)
 795         //
 796         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 797         // am looking forward to the future here.
 798         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 799             let implicit_bounds = object_region_bounds(self.tcx, data);
 800
 801             let explicit_bound = region;
 802
 803             self.out.reserve(implicit_bounds.len());
 804             for implicit_bound in implicit_bounds {
 805                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 806                 let outlives =
 807                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 808                 self.out.push(traits::Obligation::with_depth(
 809                     self.tcx,
 810                     cause,
 811                     self.recursion_depth,
 812                     self.param_env,
 813                     outlives,
 814                 ));
 815             }
 816         }
 817     }
 818 }
 819
 820 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 821 /// bounds that must hold on the elided self type. These are derived
 822 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 823 /// they declare `trait SomeTrait : 'static`, for example, then
 824 /// `'static` would appear in the list. The hard work is done by
 825 /// `infer::required_region_bounds`, see that for more information.
 826 pub fn object_region_bounds<'tcx>(
 827     tcx: TyCtxt<'tcx>,
 828     existential_predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
 829 ) -> Vec<ty::Region<'tcx>> {
 830     // Since we don't actually *know* the self type for an object,
 831     // this "open(err)" serves as a kind of dummy standin -- basically
 832     // a placeholder type.
 833     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 834
 835     let predicates = existential_predicates.iter().filter_map(|predicate| {
 836         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 837             None
 838         } else {
 839             Some(predicate.with_self_ty(tcx, open_ty))
 840         }
 841     });
 842
 843     required_region_bounds(tcx, open_ty, predicates)
 844 }
 845
 846 /// Given a set of predicates that apply to an object type, returns
 847 /// the region bounds that the (erased) `Self` type must
 848 /// outlive. Precisely *because* the `Self` type is erased, the
 849 /// parameter `erased_self_ty` must be supplied to indicate what type
 850 /// has been used to represent `Self` in the predicates
 851 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 852 /// popular choice.
 853 ///
 854 /// N.B., in some cases, particularly around higher-ranked bounds,
 855 /// this function returns a kind of conservative approximation.
 856 /// That is, all regions returned by this function are definitely
 857 /// required, but there may be other region bounds that are not
 858 /// returned, as well as requirements like `for<'a> T: 'a`.
 859 ///
 860 /// Requires that trait definitions have been processed so that we can
 861 /// elaborate predicates and walk supertraits.
 862 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 863 pub(crate) fn required_region_bounds<'tcx>(
 864     tcx: TyCtxt<'tcx>,
 865     erased_self_ty: Ty<'tcx>,
 866     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 867 ) -> Vec<ty::Region<'tcx>> {
 868     assert!(!erased_self_ty.has_escaping_bound_vars());
 869
 870     traits::elaborate_predicates(tcx, predicates)
 871         .filter_map(|obligation| {
 872             debug!(?obligation);
 873             match obligation.predicate.kind().skip_binder() {
 874                 ty::PredicateKind::Clause(ty::Clause::Projection(..))
 875                 | ty::PredicateKind::Clause(ty::Clause::Trait(..))
 876                 | ty::PredicateKind::Subtype(..)
 877                 | ty::PredicateKind::Coerce(..)
 878                 | ty::PredicateKind::WellFormed(..)
 879                 | ty::PredicateKind::ObjectSafe(..)
 880                 | ty::PredicateKind::ClosureKind(..)
 881                 | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
 882                 | ty::PredicateKind::ConstEvaluatable(..)
 883                 | ty::PredicateKind::ConstEquate(..)
 884                 | ty::PredicateKind::Ambiguous
 885                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 886                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 887                     ref t,
 888                     ref r,
 889                 ))) => {
 890                     // Search for a bound of the form `erased_self_ty
 891                     // : 'a`, but be wary of something like `for<'a>
 892                     // erased_self_ty : 'a` (we interpret a
 893                     // higher-ranked bound like that as 'static,
 894                     // though at present the code in `fulfill.rs`
 895                     // considers such bounds to be unsatisfiable, so
 896                     // it's kind of a moot point since you could never
 897                     // construct such an object, but this seems
 898                     // correct even if that code changes).
 899                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 900                         Some(*r)
 901                     } else {
 902                         None
 903                     }
 904                 }
 905             }
 906         })
 907         .collect()
 908 }