compiler/rustc_trait_selection/src/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::traits;
   3 use rustc_hir as hir;
   4 use rustc_hir::def_id::DefId;
   5 use rustc_hir::lang_items::LangItem;
   6 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
   7 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeVisitable};
   8 use rustc_span::Span;
   9
  10 use std::iter;
  11 /// Returns the set of obligations needed to make `arg` well-formed.
  12 /// If `arg` contains unresolved inference variables, this may include
  13 /// further WF obligations. However, if `arg` IS an unresolved
  14 /// inference variable, returns `None`, because we are not able to
  15 /// make any progress at all. This is to prevent "livelock" where we
  16 /// say "$0 is WF if $0 is WF".
  17 pub fn obligations<'tcx>(
  18     infcx: &InferCtxt<'tcx>,
  19     param_env: ty::ParamEnv<'tcx>,
  20     body_id: hir::HirId,
  21     recursion_depth: usize,
  22     arg: GenericArg<'tcx>,
  23     span: Span,
  24 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  25     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  26     let arg = match arg.unpack() {
  27         GenericArgKind::Type(ty) => {
  28             match ty.kind() {
  29                 ty::Infer(ty::TyVar(_)) => {
  30                     let resolved_ty = infcx.shallow_resolve(ty);
  31                     if resolved_ty == ty {
  32                         // No progress, bail out to prevent "livelock".
  33                         return None;
  34                     } else {
  35                         resolved_ty
  36                     }
  37                 }
  38                 _ => ty,
  39             }
  40             .into()
  41         }
  42         GenericArgKind::Const(ct) => {
  43             match ct.kind() {
  44                 ty::ConstKind::Infer(_) => {
  45                     let resolved = infcx.shallow_resolve(ct);
  46                     if resolved == ct {
  47                         // No progress.
  48                         return None;
  49                     } else {
  50                         resolved
  51                     }
  52                 }
  53                 _ => ct,
  54             }
  55             .into()
  56         }
  57         // There is nothing we have to do for lifetimes.
  58         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  59     };
  60
  61     let mut wf = WfPredicates {
  62         tcx: infcx.tcx,
  63         param_env,
  64         body_id,
  65         span,
  66         out: vec![],
  67         recursion_depth,
  68         item: None,
  69     };
  70     wf.compute(arg);
  71     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  72
  73     let result = wf.normalize(infcx);
  74     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  75     Some(result)
  76 }
  77
  78 /// Returns the obligations that make this trait reference
  79 /// well-formed.  For example, if there is a trait `Set` defined like
  80 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  81 /// if `Bar: Eq`.
  82 pub fn trait_obligations<'tcx>(
  83     infcx: &InferCtxt<'tcx>,
  84     param_env: ty::ParamEnv<'tcx>,
  85     body_id: hir::HirId,
  86     trait_pred: &ty::TraitPredicate<'tcx>,
  87     span: Span,
  88     item: &'tcx hir::Item<'tcx>,
  89 ) -> Vec<traits::PredicateObligation<'tcx>> {
  90     let mut wf = WfPredicates {
  91         tcx: infcx.tcx,
  92         param_env,
  93         body_id,
  94         span,
  95         out: vec![],
  96         recursion_depth: 0,
  97         item: Some(item),
  98     };
  99     wf.compute_trait_pred(trait_pred, Elaborate::All);
 100     debug!(obligations = ?wf.out);
 101     wf.normalize(infcx)
 102 }
 103
 104 #[instrument(skip(infcx), ret)]
 105 pub fn predicate_obligations<'tcx>(
 106     infcx: &InferCtxt<'tcx>,
 107     param_env: ty::ParamEnv<'tcx>,
 108     body_id: hir::HirId,
 109     predicate: ty::Predicate<'tcx>,
 110     span: Span,
 111 ) -> Vec<traits::PredicateObligation<'tcx>> {
 112     let mut wf = WfPredicates {
 113         tcx: infcx.tcx,
 114         param_env,
 115         body_id,
 116         span,
 117         out: vec![],
 118         recursion_depth: 0,
 119         item: None,
 120     };
 121
 122     // It's ok to skip the binder here because wf code is prepared for it
 123     match predicate.kind().skip_binder() {
 124         ty::PredicateKind::Trait(t) => {
 125             wf.compute_trait_pred(&t, Elaborate::None);
 126         }
 127         ty::PredicateKind::RegionOutlives(..) => {}
 128         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 129             wf.compute(ty.into());
 130         }
 131         ty::PredicateKind::Projection(t) => {
 132             wf.compute_projection(t.projection_ty);
 133             wf.compute(match t.term.unpack() {
 134                 ty::TermKind::Ty(ty) => ty.into(),
 135                 ty::TermKind::Const(c) => c.into(),
 136             })
 137         }
 138         ty::PredicateKind::WellFormed(arg) => {
 139             wf.compute(arg);
 140         }
 141         ty::PredicateKind::ObjectSafe(_) => {}
 142         ty::PredicateKind::ClosureKind(..) => {}
 143         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 144             wf.compute(a.into());
 145             wf.compute(b.into());
 146         }
 147         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 148             wf.compute(a.into());
 149             wf.compute(b.into());
 150         }
 151         ty::PredicateKind::ConstEvaluatable(ct) => {
 152             wf.compute(ct.into());
 153         }
 154         ty::PredicateKind::ConstEquate(c1, c2) => {
 155             wf.compute(c1.into());
 156             wf.compute(c2.into());
 157         }
 158         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 159             bug!("TypeWellFormedFromEnv is only used for Chalk")
 160         }
 161     }
 162
 163     wf.normalize(infcx)
 164 }
 165
 166 struct WfPredicates<'tcx> {
 167     tcx: TyCtxt<'tcx>,
 168     param_env: ty::ParamEnv<'tcx>,
 169     body_id: hir::HirId,
 170     span: Span,
 171     out: Vec<traits::PredicateObligation<'tcx>>,
 172     recursion_depth: usize,
 173     item: Option<&'tcx hir::Item<'tcx>>,
 174 }
 175
 176 /// Controls whether we "elaborate" supertraits and so forth on the WF
 177 /// predicates. This is a kind of hack to address #43784. The
 178 /// underlying problem in that issue was a trait structure like:
 179 ///
 180 /// ```ignore (illustrative)
 181 /// trait Foo: Copy { }
 182 /// trait Bar: Foo { }
 183 /// impl<T: Bar> Foo for T { }
 184 /// impl<T> Bar for T { }
 185 /// ```
 186 ///
 187 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 188 /// we decide that this is true because `T: Bar` is in the
 189 /// where-clauses (and we can elaborate that to include `T:
 190 /// Copy`). This wouldn't be a problem, except that when we check the
 191 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 192 /// impl. And so nowhere did we check that `T: Copy` holds!
 193 ///
 194 /// To resolve this, we elaborate the WF requirements that must be
 195 /// proven when checking impls. This means that (e.g.) the `impl Bar
 196 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 197 /// Copy` (which it won't be able to do, because there is no `Copy`
 198 /// impl for `T`).
 199 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 200 enum Elaborate {
 201     All,
 202     None,
 203 }
 204
 205 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 206     tcx: TyCtxt<'tcx>,
 207     trait_ref: &ty::TraitRef<'tcx>,
 208     item: Option<&hir::Item<'tcx>>,
 209     cause: &mut traits::ObligationCause<'tcx>,
 210     pred: ty::Predicate<'tcx>,
 211 ) {
 212     debug!(
 213         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 214         trait_ref, item, cause, pred
 215     );
 216     let (items, impl_def_id) = match item {
 217         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 218         _ => return,
 219     };
 220     let fix_span =
 221         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 222             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
 223             _ => impl_item_ref.span,
 224         };
 225
 226     // It is fine to skip the binder as we don't care about regions here.
 227     match pred.kind().skip_binder() {
 228         ty::PredicateKind::Projection(proj) => {
 229             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 230             // but rather from a "second order" obligation, where an associated type has a
 231             // projection coming from another associated type. See
 232             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 233             // `traits-assoc-type-in-supertrait-bad.rs`.
 234             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 235                 && let Some(&impl_item_id) =
 236                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 237                 && let Some(impl_item_span) = items
 238                     .iter()
 239                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 240                     .map(fix_span)
 241             {
 242                 cause.span = impl_item_span;
 243             }
 244         }
 245         ty::PredicateKind::Trait(pred) => {
 246             // An associated item obligation born out of the `trait` failed to be met. An example
 247             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 248             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 249             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 250                 && let Some(&impl_item_id) =
 251                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 252                 && let Some(impl_item_span) = items
 253                     .iter()
 254                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 255                     .map(fix_span)
 256             {
 257                 cause.span = impl_item_span;
 258             }
 259         }
 260         _ => {}
 261     }
 262 }
 263
 264 impl<'tcx> WfPredicates<'tcx> {
 265     fn tcx(&self) -> TyCtxt<'tcx> {
 266         self.tcx
 267     }
 268
 269     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 270         traits::ObligationCause::new(self.span, self.body_id, code)
 271     }
 272
 273     fn normalize(self, infcx: &InferCtxt<'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 274         let cause = self.cause(traits::WellFormed(None));
 275         let param_env = self.param_env;
 276         let mut obligations = Vec::with_capacity(self.out.len());
 277         for mut obligation in self.out {
 278             assert!(!obligation.has_escaping_bound_vars());
 279             let mut selcx = traits::SelectionContext::new(infcx);
 280             // Don't normalize the whole obligation, the param env is either
 281             // already normalized, or we're currently normalizing the
 282             // param_env. Either way we should only normalize the predicate.
 283             let normalized_predicate = traits::project::normalize_with_depth_to(
 284                 &mut selcx,
 285                 param_env,
 286                 cause.clone(),
 287                 self.recursion_depth,
 288                 obligation.predicate,
 289                 &mut obligations,
 290             );
 291             obligation.predicate = normalized_predicate;
 292             obligations.push(obligation);
 293         }
 294         obligations
 295     }
 296
 297     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 298     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 299         let tcx = self.tcx;
 300         let trait_ref = &trait_pred.trait_ref;
 301
 302         // if the trait predicate is not const, the wf obligations should not be const as well.
 303         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 304             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 305         } else {
 306             if !tcx.has_attr(trait_ref.def_id, rustc_span::sym::const_trait) {
 307                 if let Some(item) = self.item &&
 308                    let hir::ItemKind::Impl(impl_) = item.kind &&
 309                    let Some(trait_) = &impl_.of_trait &&
 310                    let Some(def_id) = trait_.trait_def_id() &&
 311                    def_id == trait_ref.def_id
 312                 {
 313                     let trait_name = tcx.item_name(def_id);
 314                     let mut err = tcx.sess.struct_span_err(
 315                         self.span,
 316                         &format!("const `impl` for trait `{trait_name}` which is not marked with `#[const_trait]`"),
 317                     );
 318                     if def_id.is_local() {
 319                         let sp = tcx.def_span(def_id).shrink_to_lo();
 320                         err.span_suggestion(sp, &format!("mark `{trait_name}` as const"), "#[const_trait]", rustc_errors::Applicability::MachineApplicable);
 321                     }
 322                     err.note("marking a trait with `#[const_trait]` ensures all default method bodies are `const`");
 323                     err.note("adding a non-const method body in the future would be a breaking change");
 324                     err.emit();
 325                 } else {
 326                     tcx.sess.span_err(
 327                         self.span,
 328                         "~const can only be applied to `#[const_trait]` traits",
 329                     );
 330                 }
 331             }
 332             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 333         };
 334
 335         debug!("compute_trait_pred obligations {:?}", obligations);
 336         let param_env = self.param_env;
 337         let depth = self.recursion_depth;
 338
 339         let item = self.item;
 340
 341         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 342             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 343                 cause = cause.derived_cause(
 344                     parent_trait_pred,
 345                     traits::ObligationCauseCode::DerivedObligation,
 346                 );
 347             }
 348             extend_cause_with_original_assoc_item_obligation(
 349                 tcx, trait_ref, item, &mut cause, predicate,
 350             );
 351             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 352         };
 353
 354         if let Elaborate::All = elaborate {
 355             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 356             let implied_obligations = implied_obligations.map(extend);
 357             self.out.extend(implied_obligations);
 358         } else {
 359             self.out.extend(obligations);
 360         }
 361
 362         let tcx = self.tcx();
 363         self.out.extend(
 364             trait_ref
 365                 .substs
 366                 .iter()
 367                 .enumerate()
 368                 .filter(|(_, arg)| {
 369                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 370                 })
 371                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 372                 .map(|(i, arg)| {
 373                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 374                     // The first subst is the self ty - use the correct span for it.
 375                     if i == 0 {
 376                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 377                             item.map(|i| &i.kind)
 378                         {
 379                             cause.span = self_ty.span;
 380                         }
 381                     }
 382                     traits::Obligation::with_depth(
 383                         cause,
 384                         depth,
 385                         param_env,
 386                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 387                     )
 388                 }),
 389         );
 390     }
 391
 392     /// Pushes the obligations required for `trait_ref::Item` to be WF
 393     /// into `self.out`.
 394     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 395         // A projection is well-formed if
 396         //
 397         // (a) its predicates hold (*)
 398         // (b) its substs are wf
 399         //
 400         // (*) The predicates of an associated type include the predicates of
 401         //     the trait that it's contained in. For example, given
 402         //
 403         // trait A<T>: Clone {
 404         //     type X where T: Copy;
 405         // }
 406         //
 407         // The predicates of `<() as A<i32>>::X` are:
 408         // [
 409         //     `(): Sized`
 410         //     `(): Clone`
 411         //     `(): A<i32>`
 412         //     `i32: Sized`
 413         //     `i32: Clone`
 414         //     `i32: Copy`
 415         // ]
 416         // Projection types do not require const predicates.
 417         let obligations = self.nominal_obligations_without_const(data.item_def_id, data.substs);
 418         self.out.extend(obligations);
 419
 420         let tcx = self.tcx();
 421         let cause = self.cause(traits::WellFormed(None));
 422         let param_env = self.param_env;
 423         let depth = self.recursion_depth;
 424
 425         self.out.extend(
 426             data.substs
 427                 .iter()
 428                 .filter(|arg| {
 429                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 430                 })
 431                 .filter(|arg| !arg.has_escaping_bound_vars())
 432                 .map(|arg| {
 433                     traits::Obligation::with_depth(
 434                         cause.clone(),
 435                         depth,
 436                         param_env,
 437                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 438                     )
 439                 }),
 440         );
 441     }
 442
 443     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 444         if !subty.has_escaping_bound_vars() {
 445             let cause = self.cause(cause);
 446             let trait_ref = ty::TraitRef {
 447                 def_id: self.tcx.require_lang_item(LangItem::Sized, None),
 448                 substs: self.tcx.mk_substs_trait(subty, &[]),
 449             };
 450             self.out.push(traits::Obligation::with_depth(
 451                 cause,
 452                 self.recursion_depth,
 453                 self.param_env,
 454                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx),
 455             ));
 456         }
 457     }
 458
 459     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 460     #[instrument(level = "debug", skip(self))]
 461     fn compute(&mut self, arg: GenericArg<'tcx>) {
 462         let mut walker = arg.walk();
 463         let param_env = self.param_env;
 464         let depth = self.recursion_depth;
 465         while let Some(arg) = walker.next() {
 466             debug!(?arg, ?self.out);
 467             let ty = match arg.unpack() {
 468                 GenericArgKind::Type(ty) => ty,
 469
 470                 // No WF constraints for lifetimes being present, any outlives
 471                 // obligations are handled by the parent (e.g. `ty::Ref`).
 472                 GenericArgKind::Lifetime(_) => continue,
 473
 474                 GenericArgKind::Const(ct) => {
 475                     match ct.kind() {
 476                         ty::ConstKind::Unevaluated(uv) => {
 477                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 478                             self.out.extend(obligations);
 479
 480                             let predicate =
 481                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(ct))
 482                                     .to_predicate(self.tcx());
 483                             let cause = self.cause(traits::WellFormed(None));
 484                             self.out.push(traits::Obligation::with_depth(
 485                                 cause,
 486                                 self.recursion_depth,
 487                                 self.param_env,
 488                                 predicate,
 489                             ));
 490                         }
 491                         ty::ConstKind::Infer(_) => {
 492                             let cause = self.cause(traits::WellFormed(None));
 493
 494                             self.out.push(traits::Obligation::with_depth(
 495                                 cause,
 496                                 self.recursion_depth,
 497                                 self.param_env,
 498                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(ct.into()))
 499                                     .to_predicate(self.tcx()),
 500                             ));
 501                         }
 502                         ty::ConstKind::Error(_)
 503                         | ty::ConstKind::Param(_)
 504                         | ty::ConstKind::Bound(..)
 505                         | ty::ConstKind::Placeholder(..) => {
 506                             // These variants are trivially WF, so nothing to do here.
 507                         }
 508                         ty::ConstKind::Value(..) => {
 509                             // FIXME: Enforce that values are structurally-matchable.
 510                         }
 511                     }
 512                     continue;
 513                 }
 514             };
 515
 516             debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
 517
 518             match *ty.kind() {
 519                 ty::Bool
 520                 | ty::Char
 521                 | ty::Int(..)
 522                 | ty::Uint(..)
 523                 | ty::Float(..)
 524                 | ty::Error(_)
 525                 | ty::Str
 526                 | ty::GeneratorWitness(..)
 527                 | ty::Never
 528                 | ty::Param(_)
 529                 | ty::Bound(..)
 530                 | ty::Placeholder(..)
 531                 | ty::Foreign(..) => {
 532                     // WfScalar, WfParameter, etc
 533                 }
 534
 535                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 536                 ty::Infer(ty::IntVar(_)) => {}
 537
 538                 // Can only infer to `ty::Float(_)`.
 539                 ty::Infer(ty::FloatVar(_)) => {}
 540
 541                 ty::Slice(subty) => {
 542                     self.require_sized(subty, traits::SliceOrArrayElem);
 543                 }
 544
 545                 ty::Array(subty, _) => {
 546                     self.require_sized(subty, traits::SliceOrArrayElem);
 547                     // Note that we handle the len is implicitly checked while walking `arg`.
 548                 }
 549
 550                 ty::Tuple(ref tys) => {
 551                     if let Some((_last, rest)) = tys.split_last() {
 552                         for &elem in rest {
 553                             self.require_sized(elem, traits::TupleElem);
 554                         }
 555                     }
 556                 }
 557
 558                 ty::RawPtr(_) => {
 559                     // Simple cases that are WF if their type args are WF.
 560                 }
 561
 562                 ty::Projection(data) => {
 563                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 564                     self.compute_projection(data);
 565                 }
 566
 567                 ty::Adt(def, substs) => {
 568                     // WfNominalType
 569                     let obligations = self.nominal_obligations(def.did(), substs);
 570                     self.out.extend(obligations);
 571                 }
 572
 573                 ty::FnDef(did, substs) => {
 574                     let obligations = self.nominal_obligations(did, substs);
 575                     self.out.extend(obligations);
 576                 }
 577
 578                 ty::Ref(r, rty, _) => {
 579                     // WfReference
 580                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 581                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 582                         self.out.push(traits::Obligation::with_depth(
 583                             cause,
 584                             depth,
 585                             param_env,
 586                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 587                                 ty::OutlivesPredicate(rty, r),
 588                             ))
 589                             .to_predicate(self.tcx()),
 590                         ));
 591                     }
 592                 }
 593
 594                 ty::Generator(did, substs, ..) => {
 595                     // Walk ALL the types in the generator: this will
 596                     // include the upvar types as well as the yield
 597                     // type. Note that this is mildly distinct from
 598                     // the closure case, where we have to be careful
 599                     // about the signature of the closure. We don't
 600                     // have the problem of implied bounds here since
 601                     // generators don't take arguments.
 602                     let obligations = self.nominal_obligations(did, substs);
 603                     self.out.extend(obligations);
 604                 }
 605
 606                 ty::Closure(did, substs) => {
 607                     // Only check the upvar types for WF, not the rest
 608                     // of the types within. This is needed because we
 609                     // capture the signature and it may not be WF
 610                     // without the implied bounds. Consider a closure
 611                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 612                     // not known to hold in the creator's context (and
 613                     // indeed the closure may not be invoked by its
 614                     // creator, but rather turned to someone who *can*
 615                     // verify that).
 616                     //
 617                     // The special treatment of closures here really
 618                     // ought not to be necessary either; the problem
 619                     // is related to #25860 -- there is no way for us
 620                     // to express a fn type complete with the implied
 621                     // bounds that it is assuming. I think in reality
 622                     // the WF rules around fn are a bit messed up, and
 623                     // that is the rot problem: `fn(&'a T)` should
 624                     // probably always be WF, because it should be
 625                     // shorthand for something like `where(T: 'a) {
 626                     // fn(&'a T) }`, as discussed in #25860.
 627                     walker.skip_current_subtree(); // subtree handled below
 628                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 629                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 630                     // Note that we cannot skip the generic types
 631                     // types. Normally, within the fn
 632                     // body where they are created, the generics will
 633                     // always be WF, and outside of that fn body we
 634                     // are not directly inspecting closure types
 635                     // anyway, except via auto trait matching (which
 636                     // only inspects the upvar types).
 637                     // But when a closure is part of a type-alias-impl-trait
 638                     // then the function that created the defining site may
 639                     // have had more bounds available than the type alias
 640                     // specifies. This may cause us to have a closure in the
 641                     // hidden type that is not actually well formed and
 642                     // can cause compiler crashes when the user abuses unsafe
 643                     // code to procure such a closure.
 644                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 645                     let obligations = self.nominal_obligations(did, substs);
 646                     self.out.extend(obligations);
 647                 }
 648
 649                 ty::FnPtr(_) => {
 650                     // let the loop iterate into the argument/return
 651                     // types appearing in the fn signature
 652                 }
 653
 654                 ty::Opaque(did, substs) => {
 655                     // All of the requirements on type parameters
 656                     // have already been checked for `impl Trait` in
 657                     // return position. We do need to check type-alias-impl-trait though.
 658                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 659                         let obligations = self.nominal_obligations(did, substs);
 660                         self.out.extend(obligations);
 661                     }
 662                 }
 663
 664                 ty::Dynamic(data, r, _) => {
 665                     // WfObject
 666                     //
 667                     // Here, we defer WF checking due to higher-ranked
 668                     // regions. This is perhaps not ideal.
 669                     self.from_object_ty(ty, data, r);
 670
 671                     // FIXME(#27579) RFC also considers adding trait
 672                     // obligations that don't refer to Self and
 673                     // checking those
 674
 675                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 676
 677                     if !defer_to_coercion {
 678                         let cause = self.cause(traits::WellFormed(None));
 679                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 680                         let tcx = self.tcx();
 681                         self.out.extend(component_traits.map(|did| {
 682                             traits::Obligation::with_depth(
 683                                 cause.clone(),
 684                                 depth,
 685                                 param_env,
 686                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 687                                     .to_predicate(tcx),
 688                             )
 689                         }));
 690                     }
 691                 }
 692
 693                 // Inference variables are the complicated case, since we don't
 694                 // know what type they are. We do two things:
 695                 //
 696                 // 1. Check if they have been resolved, and if so proceed with
 697                 //    THAT type.
 698                 // 2. If not, we've at least simplified things (e.g., we went
 699                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 700                 //    register a pending obligation and keep
 701                 //    moving. (Goal is that an "inductive hypothesis"
 702                 //    is satisfied to ensure termination.)
 703                 // See also the comment on `fn obligations`, describing "livelock"
 704                 // prevention, which happens before this can be reached.
 705                 ty::Infer(_) => {
 706                     let cause = self.cause(traits::WellFormed(None));
 707                     self.out.push(traits::Obligation::with_depth(
 708                         cause,
 709                         self.recursion_depth,
 710                         param_env,
 711                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 712                             .to_predicate(self.tcx()),
 713                     ));
 714                 }
 715             }
 716
 717             debug!(?self.out);
 718         }
 719     }
 720
 721     #[instrument(level = "debug", skip(self))]
 722     fn nominal_obligations_inner(
 723         &mut self,
 724         def_id: DefId,
 725         substs: SubstsRef<'tcx>,
 726         remap_constness: bool,
 727     ) -> Vec<traits::PredicateObligation<'tcx>> {
 728         let predicates = self.tcx.predicates_of(def_id);
 729         let mut origins = vec![def_id; predicates.predicates.len()];
 730         let mut head = predicates;
 731         while let Some(parent) = head.parent {
 732             head = self.tcx.predicates_of(parent);
 733             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 734         }
 735
 736         let predicates = predicates.instantiate(self.tcx, substs);
 737         trace!("{:#?}", predicates);
 738         debug_assert_eq!(predicates.predicates.len(), origins.len());
 739
 740         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 741             .map(|((mut pred, span), origin_def_id)| {
 742                 let code = if span.is_dummy() {
 743                     traits::ItemObligation(origin_def_id)
 744                 } else {
 745                     traits::BindingObligation(origin_def_id, span)
 746                 };
 747                 let cause = self.cause(code);
 748                 if remap_constness {
 749                     pred = pred.without_const(self.tcx);
 750                 }
 751                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 752             })
 753             .filter(|pred| !pred.has_escaping_bound_vars())
 754             .collect()
 755     }
 756
 757     fn nominal_obligations(
 758         &mut self,
 759         def_id: DefId,
 760         substs: SubstsRef<'tcx>,
 761     ) -> Vec<traits::PredicateObligation<'tcx>> {
 762         self.nominal_obligations_inner(def_id, substs, false)
 763     }
 764
 765     fn nominal_obligations_without_const(
 766         &mut self,
 767         def_id: DefId,
 768         substs: SubstsRef<'tcx>,
 769     ) -> Vec<traits::PredicateObligation<'tcx>> {
 770         self.nominal_obligations_inner(def_id, substs, true)
 771     }
 772
 773     fn from_object_ty(
 774         &mut self,
 775         ty: Ty<'tcx>,
 776         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 777         region: ty::Region<'tcx>,
 778     ) {
 779         // Imagine a type like this:
 780         //
 781         //     trait Foo { }
 782         //     trait Bar<'c> : 'c { }
 783         //
 784         //     &'b (Foo+'c+Bar<'d>)
 785         //         ^
 786         //
 787         // In this case, the following relationships must hold:
 788         //
 789         //     'b <= 'c
 790         //     'd <= 'c
 791         //
 792         // The first conditions is due to the normal region pointer
 793         // rules, which say that a reference cannot outlive its
 794         // referent.
 795         //
 796         // The final condition may be a bit surprising. In particular,
 797         // you may expect that it would have been `'c <= 'd`, since
 798         // usually lifetimes of outer things are conservative
 799         // approximations for inner things. However, it works somewhat
 800         // differently with trait objects: here the idea is that if the
 801         // user specifies a region bound (`'c`, in this case) it is the
 802         // "master bound" that *implies* that bounds from other traits are
 803         // all met. (Remember that *all bounds* in a type like
 804         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 805         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 806         // 'y.)
 807         //
 808         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 809         // am looking forward to the future here.
 810         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 811             let implicit_bounds = object_region_bounds(self.tcx, data);
 812
 813             let explicit_bound = region;
 814
 815             self.out.reserve(implicit_bounds.len());
 816             for implicit_bound in implicit_bounds {
 817                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 818                 let outlives =
 819                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 820                 self.out.push(traits::Obligation::with_depth(
 821                     cause,
 822                     self.recursion_depth,
 823                     self.param_env,
 824                     outlives.to_predicate(self.tcx),
 825                 ));
 826             }
 827         }
 828     }
 829 }
 830
 831 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 832 /// bounds that must hold on the elided self type. These are derived
 833 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 834 /// they declare `trait SomeTrait : 'static`, for example, then
 835 /// `'static` would appear in the list. The hard work is done by
 836 /// `infer::required_region_bounds`, see that for more information.
 837 pub fn object_region_bounds<'tcx>(
 838     tcx: TyCtxt<'tcx>,
 839     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 840 ) -> Vec<ty::Region<'tcx>> {
 841     // Since we don't actually *know* the self type for an object,
 842     // this "open(err)" serves as a kind of dummy standin -- basically
 843     // a placeholder type.
 844     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 845
 846     let predicates = existential_predicates.iter().filter_map(|predicate| {
 847         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 848             None
 849         } else {
 850             Some(predicate.with_self_ty(tcx, open_ty))
 851         }
 852     });
 853
 854     required_region_bounds(tcx, open_ty, predicates)
 855 }
 856
 857 /// Given a set of predicates that apply to an object type, returns
 858 /// the region bounds that the (erased) `Self` type must
 859 /// outlive. Precisely *because* the `Self` type is erased, the
 860 /// parameter `erased_self_ty` must be supplied to indicate what type
 861 /// has been used to represent `Self` in the predicates
 862 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 863 /// popular choice.
 864 ///
 865 /// N.B., in some cases, particularly around higher-ranked bounds,
 866 /// this function returns a kind of conservative approximation.
 867 /// That is, all regions returned by this function are definitely
 868 /// required, but there may be other region bounds that are not
 869 /// returned, as well as requirements like `for<'a> T: 'a`.
 870 ///
 871 /// Requires that trait definitions have been processed so that we can
 872 /// elaborate predicates and walk supertraits.
 873 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 874 pub(crate) fn required_region_bounds<'tcx>(
 875     tcx: TyCtxt<'tcx>,
 876     erased_self_ty: Ty<'tcx>,
 877     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 878 ) -> Vec<ty::Region<'tcx>> {
 879     assert!(!erased_self_ty.has_escaping_bound_vars());
 880
 881     traits::elaborate_predicates(tcx, predicates)
 882         .filter_map(|obligation| {
 883             debug!(?obligation);
 884             match obligation.predicate.kind().skip_binder() {
 885                 ty::PredicateKind::Projection(..)
 886                 | ty::PredicateKind::Trait(..)
 887                 | ty::PredicateKind::Subtype(..)
 888                 | ty::PredicateKind::Coerce(..)
 889                 | ty::PredicateKind::WellFormed(..)
 890                 | ty::PredicateKind::ObjectSafe(..)
 891                 | ty::PredicateKind::ClosureKind(..)
 892                 | ty::PredicateKind::RegionOutlives(..)
 893                 | ty::PredicateKind::ConstEvaluatable(..)
 894                 | ty::PredicateKind::ConstEquate(..)
 895                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 896                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
 897                     // Search for a bound of the form `erased_self_ty
 898                     // : 'a`, but be wary of something like `for<'a>
 899                     // erased_self_ty : 'a` (we interpret a
 900                     // higher-ranked bound like that as 'static,
 901                     // though at present the code in `fulfill.rs`
 902                     // considers such bounds to be unsatisfiable, so
 903                     // it's kind of a moot point since you could never
 904                     // construct such an object, but this seems
 905                     // correct even if that code changes).
 906                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 907                         Some(*r)
 908                     } else {
 909                         None
 910                     }
 911                 }
 912             }
 913         })
 914         .collect()
 915 }