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 pub fn predicate_obligations<'a, 'tcx>(
 105     infcx: &InferCtxt<'a, 'tcx>,
 106     param_env: ty::ParamEnv<'tcx>,
 107     body_id: hir::HirId,
 108     predicate: ty::Predicate<'tcx>,
 109     span: Span,
 110 ) -> Vec<traits::PredicateObligation<'tcx>> {
 111     let mut wf = WfPredicates {
 112         tcx: infcx.tcx,
 113         param_env,
 114         body_id,
 115         span,
 116         out: vec![],
 117         recursion_depth: 0,
 118         item: None,
 119     };
 120
 121     // It's ok to skip the binder here because wf code is prepared for it
 122     match predicate.kind().skip_binder() {
 123         ty::PredicateKind::Trait(t) => {
 124             wf.compute_trait_pred(&t, Elaborate::None);
 125         }
 126         ty::PredicateKind::RegionOutlives(..) => {}
 127         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 128             wf.compute(ty.into());
 129         }
 130         ty::PredicateKind::Projection(t) => {
 131             wf.compute_projection(t.projection_ty);
 132             wf.compute(match t.term {
 133                 ty::Term::Ty(ty) => ty.into(),
 134                 ty::Term::Const(c) => c.into(),
 135             })
 136         }
 137         ty::PredicateKind::WellFormed(arg) => {
 138             wf.compute(arg);
 139         }
 140         ty::PredicateKind::ObjectSafe(_) => {}
 141         ty::PredicateKind::ClosureKind(..) => {}
 142         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 143             wf.compute(a.into());
 144             wf.compute(b.into());
 145         }
 146         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 147             wf.compute(a.into());
 148             wf.compute(b.into());
 149         }
 150         ty::PredicateKind::ConstEvaluatable(uv) => {
 151             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 152             wf.out.extend(obligations);
 153
 154             for arg in uv.substs.iter() {
 155                 wf.compute(arg);
 156             }
 157         }
 158         ty::PredicateKind::ConstEquate(c1, c2) => {
 159             wf.compute(c1.into());
 160             wf.compute(c2.into());
 161         }
 162         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 163             bug!("TypeWellFormedFromEnv is only used for Chalk")
 164         }
 165     }
 166
 167     wf.normalize(infcx)
 168 }
 169
 170 struct WfPredicates<'tcx> {
 171     tcx: TyCtxt<'tcx>,
 172     param_env: ty::ParamEnv<'tcx>,
 173     body_id: hir::HirId,
 174     span: Span,
 175     out: Vec<traits::PredicateObligation<'tcx>>,
 176     recursion_depth: usize,
 177     item: Option<&'tcx hir::Item<'tcx>>,
 178 }
 179
 180 /// Controls whether we "elaborate" supertraits and so forth on the WF
 181 /// predicates. This is a kind of hack to address #43784. The
 182 /// underlying problem in that issue was a trait structure like:
 183 ///
 184 /// ```ignore (illustrative)
 185 /// trait Foo: Copy { }
 186 /// trait Bar: Foo { }
 187 /// impl<T: Bar> Foo for T { }
 188 /// impl<T> Bar for T { }
 189 /// ```
 190 ///
 191 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 192 /// we decide that this is true because `T: Bar` is in the
 193 /// where-clauses (and we can elaborate that to include `T:
 194 /// Copy`). This wouldn't be a problem, except that when we check the
 195 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 196 /// impl. And so nowhere did we check that `T: Copy` holds!
 197 ///
 198 /// To resolve this, we elaborate the WF requirements that must be
 199 /// proven when checking impls. This means that (e.g.) the `impl Bar
 200 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 201 /// Copy` (which it won't be able to do, because there is no `Copy`
 202 /// impl for `T`).
 203 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 204 enum Elaborate {
 205     All,
 206     None,
 207 }
 208
 209 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 210     tcx: TyCtxt<'tcx>,
 211     trait_ref: &ty::TraitRef<'tcx>,
 212     item: Option<&hir::Item<'tcx>>,
 213     cause: &mut traits::ObligationCause<'tcx>,
 214     pred: ty::Predicate<'tcx>,
 215 ) {
 216     debug!(
 217         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 218         trait_ref, item, cause, pred
 219     );
 220     let (items, impl_def_id) = match item {
 221         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 222         _ => return,
 223     };
 224     let fix_span =
 225         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 226             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 227             _ => impl_item_ref.span,
 228         };
 229
 230     // It is fine to skip the binder as we don't care about regions here.
 231     match pred.kind().skip_binder() {
 232         ty::PredicateKind::Projection(proj) => {
 233             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 234             // but rather from a "second order" obligation, where an associated type has a
 235             // projection coming from another associated type. See
 236             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 237             // `traits-assoc-type-in-supertrait-bad.rs`.
 238             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 239                 && let Some(&impl_item_id) =
 240                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 241                 && let Some(impl_item_span) = items
 242                     .iter()
 243                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 244                     .map(fix_span)
 245             {
 246                 cause.span = impl_item_span;
 247             }
 248         }
 249         ty::PredicateKind::Trait(pred) => {
 250             // An associated item obligation born out of the `trait` failed to be met. An example
 251             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 252             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 253             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 254                 && let Some(&impl_item_id) =
 255                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 256                 && let Some(impl_item_span) = items
 257                     .iter()
 258                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 259                     .map(fix_span)
 260             {
 261                 cause.span = impl_item_span;
 262             }
 263         }
 264         _ => {}
 265     }
 266 }
 267
 268 impl<'tcx> WfPredicates<'tcx> {
 269     fn tcx(&self) -> TyCtxt<'tcx> {
 270         self.tcx
 271     }
 272
 273     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 274         traits::ObligationCause::new(self.span, self.body_id, code)
 275     }
 276
 277     fn normalize(self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 278         let cause = self.cause(traits::WellFormed(None));
 279         let param_env = self.param_env;
 280         let mut obligations = Vec::with_capacity(self.out.len());
 281         for mut obligation in self.out {
 282             assert!(!obligation.has_escaping_bound_vars());
 283             let mut selcx = traits::SelectionContext::new(infcx);
 284             // Don't normalize the whole obligation, the param env is either
 285             // already normalized, or we're currently normalizing the
 286             // param_env. Either way we should only normalize the predicate.
 287             let normalized_predicate = traits::project::normalize_with_depth_to(
 288                 &mut selcx,
 289                 param_env,
 290                 cause.clone(),
 291                 self.recursion_depth,
 292                 obligation.predicate,
 293                 &mut obligations,
 294             );
 295             obligation.predicate = normalized_predicate;
 296             obligations.push(obligation);
 297         }
 298         obligations
 299     }
 300
 301     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 302     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 303         let tcx = self.tcx;
 304         let trait_ref = &trait_pred.trait_ref;
 305
 306         // if the trait predicate is not const, the wf obligations should not be const as well.
 307         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 308             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 309         } else {
 310             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 311         };
 312
 313         debug!("compute_trait_pred obligations {:?}", obligations);
 314         let param_env = self.param_env;
 315         let depth = self.recursion_depth;
 316
 317         let item = self.item;
 318
 319         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 320             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 321                 cause = cause.derived_cause(
 322                     parent_trait_pred,
 323                     traits::ObligationCauseCode::DerivedObligation,
 324                 );
 325             }
 326             extend_cause_with_original_assoc_item_obligation(
 327                 tcx, trait_ref, item, &mut cause, predicate,
 328             );
 329             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 330         };
 331
 332         if let Elaborate::All = elaborate {
 333             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 334             let implied_obligations = implied_obligations.map(extend);
 335             self.out.extend(implied_obligations);
 336         } else {
 337             self.out.extend(obligations);
 338         }
 339
 340         let tcx = self.tcx();
 341         self.out.extend(
 342             trait_ref
 343                 .substs
 344                 .iter()
 345                 .enumerate()
 346                 .filter(|(_, arg)| {
 347                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 348                 })
 349                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 350                 .map(|(i, arg)| {
 351                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 352                     // The first subst is the self ty - use the correct span for it.
 353                     if i == 0 {
 354                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 355                             item.map(|i| &i.kind)
 356                         {
 357                             cause.span = self_ty.span;
 358                         }
 359                     }
 360                     traits::Obligation::with_depth(
 361                         cause,
 362                         depth,
 363                         param_env,
 364                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 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         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 395         self.out.extend(obligations);
 396
 397         let tcx = self.tcx();
 398         let cause = self.cause(traits::WellFormed(None));
 399         let param_env = self.param_env;
 400         let depth = self.recursion_depth;
 401
 402         self.out.extend(
 403             data.substs
 404                 .iter()
 405                 .filter(|arg| {
 406                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 407                 })
 408                 .filter(|arg| !arg.has_escaping_bound_vars())
 409                 .map(|arg| {
 410                     traits::Obligation::with_depth(
 411                         cause.clone(),
 412                         depth,
 413                         param_env,
 414                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 415                     )
 416                 }),
 417         );
 418     }
 419
 420     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 421         if !subty.has_escaping_bound_vars() {
 422             let cause = self.cause(cause);
 423             let trait_ref = ty::TraitRef {
 424                 def_id: self.tcx.require_lang_item(LangItem::Sized, None),
 425                 substs: self.tcx.mk_substs_trait(subty, &[]),
 426             };
 427             self.out.push(traits::Obligation::with_depth(
 428                 cause,
 429                 self.recursion_depth,
 430                 self.param_env,
 431                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx),
 432             ));
 433         }
 434     }
 435
 436     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 437     fn compute(&mut self, arg: GenericArg<'tcx>) {
 438         let mut walker = arg.walk();
 439         let param_env = self.param_env;
 440         let depth = self.recursion_depth;
 441         while let Some(arg) = walker.next() {
 442             let ty = match arg.unpack() {
 443                 GenericArgKind::Type(ty) => ty,
 444
 445                 // No WF constraints for lifetimes being present, any outlives
 446                 // obligations are handled by the parent (e.g. `ty::Ref`).
 447                 GenericArgKind::Lifetime(_) => continue,
 448
 449                 GenericArgKind::Const(constant) => {
 450                     match constant.kind() {
 451                         ty::ConstKind::Unevaluated(uv) => {
 452                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 453                             self.out.extend(obligations);
 454
 455                             let predicate =
 456                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 457                                     .to_predicate(self.tcx());
 458                             let cause = self.cause(traits::WellFormed(None));
 459                             self.out.push(traits::Obligation::with_depth(
 460                                 cause,
 461                                 self.recursion_depth,
 462                                 self.param_env,
 463                                 predicate,
 464                             ));
 465                         }
 466                         ty::ConstKind::Infer(_) => {
 467                             let cause = self.cause(traits::WellFormed(None));
 468
 469                             self.out.push(traits::Obligation::with_depth(
 470                                 cause,
 471                                 self.recursion_depth,
 472                                 self.param_env,
 473                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(constant.into()))
 474                                     .to_predicate(self.tcx()),
 475                             ));
 476                         }
 477                         ty::ConstKind::Error(_)
 478                         | ty::ConstKind::Param(_)
 479                         | ty::ConstKind::Bound(..)
 480                         | ty::ConstKind::Placeholder(..) => {
 481                             // These variants are trivially WF, so nothing to do here.
 482                         }
 483                         ty::ConstKind::Value(..) => {
 484                             // FIXME: Enforce that values are structurally-matchable.
 485                         }
 486                     }
 487                     continue;
 488                 }
 489             };
 490
 491             match *ty.kind() {
 492                 ty::Bool
 493                 | ty::Char
 494                 | ty::Int(..)
 495                 | ty::Uint(..)
 496                 | ty::Float(..)
 497                 | ty::Error(_)
 498                 | ty::Str
 499                 | ty::GeneratorWitness(..)
 500                 | ty::Never
 501                 | ty::Param(_)
 502                 | ty::Bound(..)
 503                 | ty::Placeholder(..)
 504                 | ty::Foreign(..) => {
 505                     // WfScalar, WfParameter, etc
 506                 }
 507
 508                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 509                 ty::Infer(ty::IntVar(_)) => {}
 510
 511                 // Can only infer to `ty::Float(_)`.
 512                 ty::Infer(ty::FloatVar(_)) => {}
 513
 514                 ty::Slice(subty) => {
 515                     self.require_sized(subty, traits::SliceOrArrayElem);
 516                 }
 517
 518                 ty::Array(subty, _) => {
 519                     self.require_sized(subty, traits::SliceOrArrayElem);
 520                     // Note that we handle the len is implicitly checked while walking `arg`.
 521                 }
 522
 523                 ty::Tuple(ref tys) => {
 524                     if let Some((_last, rest)) = tys.split_last() {
 525                         for &elem in rest {
 526                             self.require_sized(elem, traits::TupleElem);
 527                         }
 528                     }
 529                 }
 530
 531                 ty::RawPtr(_) => {
 532                     // Simple cases that are WF if their type args are WF.
 533                 }
 534
 535                 ty::Projection(data) => {
 536                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 537                     self.compute_projection(data);
 538                 }
 539
 540                 ty::Adt(def, substs) => {
 541                     // WfNominalType
 542                     let obligations = self.nominal_obligations(def.did(), substs);
 543                     self.out.extend(obligations);
 544                 }
 545
 546                 ty::FnDef(did, substs) => {
 547                     let obligations = self.nominal_obligations(did, substs);
 548                     self.out.extend(obligations);
 549                 }
 550
 551                 ty::Ref(r, rty, _) => {
 552                     // WfReference
 553                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 554                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 555                         self.out.push(traits::Obligation::with_depth(
 556                             cause,
 557                             depth,
 558                             param_env,
 559                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 560                                 ty::OutlivesPredicate(rty, r),
 561                             ))
 562                             .to_predicate(self.tcx()),
 563                         ));
 564                     }
 565                 }
 566
 567                 ty::Generator(did, substs, ..) => {
 568                     // Walk ALL the types in the generator: this will
 569                     // include the upvar types as well as the yield
 570                     // type. Note that this is mildly distinct from
 571                     // the closure case, where we have to be careful
 572                     // about the signature of the closure. We don't
 573                     // have the problem of implied bounds here since
 574                     // generators don't take arguments.
 575                     let obligations = self.nominal_obligations(did, substs);
 576                     self.out.extend(obligations);
 577                 }
 578
 579                 ty::Closure(did, substs) => {
 580                     // Only check the upvar types for WF, not the rest
 581                     // of the types within. This is needed because we
 582                     // capture the signature and it may not be WF
 583                     // without the implied bounds. Consider a closure
 584                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 585                     // not known to hold in the creator's context (and
 586                     // indeed the closure may not be invoked by its
 587                     // creator, but rather turned to someone who *can*
 588                     // verify that).
 589                     //
 590                     // The special treatment of closures here really
 591                     // ought not to be necessary either; the problem
 592                     // is related to #25860 -- there is no way for us
 593                     // to express a fn type complete with the implied
 594                     // bounds that it is assuming. I think in reality
 595                     // the WF rules around fn are a bit messed up, and
 596                     // that is the rot problem: `fn(&'a T)` should
 597                     // probably always be WF, because it should be
 598                     // shorthand for something like `where(T: 'a) {
 599                     // fn(&'a T) }`, as discussed in #25860.
 600                     walker.skip_current_subtree(); // subtree handled below
 601                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 602                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 603                     // Note that we cannot skip the generic types
 604                     // types. Normally, within the fn
 605                     // body where they are created, the generics will
 606                     // always be WF, and outside of that fn body we
 607                     // are not directly inspecting closure types
 608                     // anyway, except via auto trait matching (which
 609                     // only inspects the upvar types).
 610                     // But when a closure is part of a type-alias-impl-trait
 611                     // then the function that created the defining site may
 612                     // have had more bounds available than the type alias
 613                     // specifies. This may cause us to have a closure in the
 614                     // hidden type that is not actually well formed and
 615                     // can cause compiler crashes when the user abuses unsafe
 616                     // code to procure such a closure.
 617                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 618                     let obligations = self.nominal_obligations(did, substs);
 619                     self.out.extend(obligations);
 620                 }
 621
 622                 ty::FnPtr(_) => {
 623                     // let the loop iterate into the argument/return
 624                     // types appearing in the fn signature
 625                 }
 626
 627                 ty::Opaque(did, substs) => {
 628                     // All of the requirements on type parameters
 629                     // have already been checked for `impl Trait` in
 630                     // return position. We do need to check type-alias-impl-trait though.
 631                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 632                         let obligations = self.nominal_obligations(did, substs);
 633                         self.out.extend(obligations);
 634                     }
 635                 }
 636
 637                 ty::Dynamic(data, r) => {
 638                     // WfObject
 639                     //
 640                     // Here, we defer WF checking due to higher-ranked
 641                     // regions. This is perhaps not ideal.
 642                     self.from_object_ty(ty, data, r);
 643
 644                     // FIXME(#27579) RFC also considers adding trait
 645                     // obligations that don't refer to Self and
 646                     // checking those
 647
 648                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 649
 650                     if !defer_to_coercion {
 651                         let cause = self.cause(traits::WellFormed(None));
 652                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 653                         let tcx = self.tcx();
 654                         self.out.extend(component_traits.map(|did| {
 655                             traits::Obligation::with_depth(
 656                                 cause.clone(),
 657                                 depth,
 658                                 param_env,
 659                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 660                                     .to_predicate(tcx),
 661                             )
 662                         }));
 663                     }
 664                 }
 665
 666                 // Inference variables are the complicated case, since we don't
 667                 // know what type they are. We do two things:
 668                 //
 669                 // 1. Check if they have been resolved, and if so proceed with
 670                 //    THAT type.
 671                 // 2. If not, we've at least simplified things (e.g., we went
 672                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 673                 //    register a pending obligation and keep
 674                 //    moving. (Goal is that an "inductive hypothesis"
 675                 //    is satisfied to ensure termination.)
 676                 // See also the comment on `fn obligations`, describing "livelock"
 677                 // prevention, which happens before this can be reached.
 678                 ty::Infer(_) => {
 679                     let cause = self.cause(traits::WellFormed(None));
 680                     self.out.push(traits::Obligation::with_depth(
 681                         cause,
 682                         self.recursion_depth,
 683                         param_env,
 684                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 685                             .to_predicate(self.tcx()),
 686                     ));
 687                 }
 688             }
 689         }
 690     }
 691
 692     #[instrument(level = "debug", skip(self))]
 693     fn nominal_obligations_inner(
 694         &mut self,
 695         def_id: DefId,
 696         substs: SubstsRef<'tcx>,
 697         remap_constness: bool,
 698     ) -> Vec<traits::PredicateObligation<'tcx>> {
 699         let predicates = self.tcx.predicates_of(def_id);
 700         let mut origins = vec![def_id; predicates.predicates.len()];
 701         let mut head = predicates;
 702         while let Some(parent) = head.parent {
 703             head = self.tcx.predicates_of(parent);
 704             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 705         }
 706
 707         let predicates = predicates.instantiate(self.tcx, substs);
 708         trace!("{:#?}", predicates);
 709         debug_assert_eq!(predicates.predicates.len(), origins.len());
 710
 711         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 712             .map(|((mut pred, span), origin_def_id)| {
 713                 let code = if span.is_dummy() {
 714                     traits::ItemObligation(origin_def_id)
 715                 } else {
 716                     traits::BindingObligation(origin_def_id, span)
 717                 };
 718                 let cause = self.cause(code);
 719                 if remap_constness {
 720                     pred = pred.without_const(self.tcx);
 721                 }
 722                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 723             })
 724             .filter(|pred| !pred.has_escaping_bound_vars())
 725             .collect()
 726     }
 727
 728     fn nominal_obligations(
 729         &mut self,
 730         def_id: DefId,
 731         substs: SubstsRef<'tcx>,
 732     ) -> Vec<traits::PredicateObligation<'tcx>> {
 733         self.nominal_obligations_inner(def_id, substs, false)
 734     }
 735
 736     fn nominal_obligations_without_const(
 737         &mut self,
 738         def_id: DefId,
 739         substs: SubstsRef<'tcx>,
 740     ) -> Vec<traits::PredicateObligation<'tcx>> {
 741         self.nominal_obligations_inner(def_id, substs, true)
 742     }
 743
 744     fn from_object_ty(
 745         &mut self,
 746         ty: Ty<'tcx>,
 747         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 748         region: ty::Region<'tcx>,
 749     ) {
 750         // Imagine a type like this:
 751         //
 752         //     trait Foo { }
 753         //     trait Bar<'c> : 'c { }
 754         //
 755         //     &'b (Foo+'c+Bar<'d>)
 756         //         ^
 757         //
 758         // In this case, the following relationships must hold:
 759         //
 760         //     'b <= 'c
 761         //     'd <= 'c
 762         //
 763         // The first conditions is due to the normal region pointer
 764         // rules, which say that a reference cannot outlive its
 765         // referent.
 766         //
 767         // The final condition may be a bit surprising. In particular,
 768         // you may expect that it would have been `'c <= 'd`, since
 769         // usually lifetimes of outer things are conservative
 770         // approximations for inner things. However, it works somewhat
 771         // differently with trait objects: here the idea is that if the
 772         // user specifies a region bound (`'c`, in this case) it is the
 773         // "master bound" that *implies* that bounds from other traits are
 774         // all met. (Remember that *all bounds* in a type like
 775         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 776         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 777         // 'y.)
 778         //
 779         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 780         // am looking forward to the future here.
 781         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 782             let implicit_bounds = object_region_bounds(self.tcx, data);
 783
 784             let explicit_bound = region;
 785
 786             self.out.reserve(implicit_bounds.len());
 787             for implicit_bound in implicit_bounds {
 788                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 789                 let outlives =
 790                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 791                 self.out.push(traits::Obligation::with_depth(
 792                     cause,
 793                     self.recursion_depth,
 794                     self.param_env,
 795                     outlives.to_predicate(self.tcx),
 796                 ));
 797             }
 798         }
 799     }
 800 }
 801
 802 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 803 /// bounds that must hold on the elided self type. These are derived
 804 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 805 /// they declare `trait SomeTrait : 'static`, for example, then
 806 /// `'static` would appear in the list. The hard work is done by
 807 /// `infer::required_region_bounds`, see that for more information.
 808 pub fn object_region_bounds<'tcx>(
 809     tcx: TyCtxt<'tcx>,
 810     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 811 ) -> Vec<ty::Region<'tcx>> {
 812     // Since we don't actually *know* the self type for an object,
 813     // this "open(err)" serves as a kind of dummy standin -- basically
 814     // a placeholder type.
 815     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 816
 817     let predicates = existential_predicates.iter().filter_map(|predicate| {
 818         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 819             None
 820         } else {
 821             Some(predicate.with_self_ty(tcx, open_ty))
 822         }
 823     });
 824
 825     required_region_bounds(tcx, open_ty, predicates)
 826 }
 827
 828 /// Given a set of predicates that apply to an object type, returns
 829 /// the region bounds that the (erased) `Self` type must
 830 /// outlive. Precisely *because* the `Self` type is erased, the
 831 /// parameter `erased_self_ty` must be supplied to indicate what type
 832 /// has been used to represent `Self` in the predicates
 833 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 834 /// popular choice.
 835 ///
 836 /// N.B., in some cases, particularly around higher-ranked bounds,
 837 /// this function returns a kind of conservative approximation.
 838 /// That is, all regions returned by this function are definitely
 839 /// required, but there may be other region bounds that are not
 840 /// returned, as well as requirements like `for<'a> T: 'a`.
 841 ///
 842 /// Requires that trait definitions have been processed so that we can
 843 /// elaborate predicates and walk supertraits.
 844 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 845 pub(crate) fn required_region_bounds<'tcx>(
 846     tcx: TyCtxt<'tcx>,
 847     erased_self_ty: Ty<'tcx>,
 848     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 849 ) -> Vec<ty::Region<'tcx>> {
 850     assert!(!erased_self_ty.has_escaping_bound_vars());
 851
 852     traits::elaborate_predicates(tcx, predicates)
 853         .filter_map(|obligation| {
 854             debug!(?obligation);
 855             match obligation.predicate.kind().skip_binder() {
 856                 ty::PredicateKind::Projection(..)
 857                 | ty::PredicateKind::Trait(..)
 858                 | ty::PredicateKind::Subtype(..)
 859                 | ty::PredicateKind::Coerce(..)
 860                 | ty::PredicateKind::WellFormed(..)
 861                 | ty::PredicateKind::ObjectSafe(..)
 862                 | ty::PredicateKind::ClosureKind(..)
 863                 | ty::PredicateKind::RegionOutlives(..)
 864                 | ty::PredicateKind::ConstEvaluatable(..)
 865                 | ty::PredicateKind::ConstEquate(..)
 866                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 867                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
 868                     // Search for a bound of the form `erased_self_ty
 869                     // : 'a`, but be wary of something like `for<'a>
 870                     // erased_self_ty : 'a` (we interpret a
 871                     // higher-ranked bound like that as 'static,
 872                     // though at present the code in `fulfill.rs`
 873                     // considers such bounds to be unsatisfiable, so
 874                     // it's kind of a moot point since you could never
 875                     // construct such an object, but this seems
 876                     // correct even if that code changes).
 877                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 878                         Some(*r)
 879                     } else {
 880                         None
 881                     }
 882                 }
 883             }
 884         })
 885         .collect()
 886 }