compiler/rustc_trait_selection/src/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::traits;
   3 use rustc_hir as hir;
   4 use rustc_hir::def_id::DefId;
   5 use rustc_hir::lang_items::LangItem;
   6 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
   7 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeVisitable};
   8 use rustc_span::Span;
   9
  10 use std::iter;
  11 /// Returns the set of obligations needed to make `arg` well-formed.
  12 /// If `arg` contains unresolved inference variables, this may include
  13 /// further WF obligations. However, if `arg` IS an unresolved
  14 /// inference variable, returns `None`, because we are not able to
  15 /// make any progress at all. This is to prevent "livelock" where we
  16 /// say "$0 is WF if $0 is WF".
  17 pub fn obligations<'a, 'tcx>(
  18     infcx: &InferCtxt<'a, 'tcx>,
  19     param_env: ty::ParamEnv<'tcx>,
  20     body_id: hir::HirId,
  21     recursion_depth: usize,
  22     arg: GenericArg<'tcx>,
  23     span: Span,
  24 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  25     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  26     let arg = match arg.unpack() {
  27         GenericArgKind::Type(ty) => {
  28             match ty.kind() {
  29                 ty::Infer(ty::TyVar(_)) => {
  30                     let resolved_ty = infcx.shallow_resolve(ty);
  31                     if resolved_ty == ty {
  32                         // No progress, bail out to prevent "livelock".
  33                         return None;
  34                     } else {
  35                         resolved_ty
  36                     }
  37                 }
  38                 _ => ty,
  39             }
  40             .into()
  41         }
  42         GenericArgKind::Const(ct) => {
  43             match ct.kind() {
  44                 ty::ConstKind::Infer(_) => {
  45                     let resolved = infcx.shallow_resolve(ct);
  46                     if resolved == ct {
  47                         // No progress.
  48                         return None;
  49                     } else {
  50                         resolved
  51                     }
  52                 }
  53                 _ => ct,
  54             }
  55             .into()
  56         }
  57         // There is nothing we have to do for lifetimes.
  58         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  59     };
  60
  61     let mut wf = WfPredicates {
  62         tcx: infcx.tcx,
  63         param_env,
  64         body_id,
  65         span,
  66         out: vec![],
  67         recursion_depth,
  68         item: None,
  69     };
  70     wf.compute(arg);
  71     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  72
  73     let result = wf.normalize(infcx);
  74     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  75     Some(result)
  76 }
  77
  78 /// Returns the obligations that make this trait reference
  79 /// well-formed.  For example, if there is a trait `Set` defined like
  80 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  81 /// if `Bar: Eq`.
  82 pub fn trait_obligations<'a, 'tcx>(
  83     infcx: &InferCtxt<'a, 'tcx>,
  84     param_env: ty::ParamEnv<'tcx>,
  85     body_id: hir::HirId,
  86     trait_pred: &ty::TraitPredicate<'tcx>,
  87     span: Span,
  88     item: &'tcx hir::Item<'tcx>,
  89 ) -> Vec<traits::PredicateObligation<'tcx>> {
  90     let mut wf = WfPredicates {
  91         tcx: infcx.tcx,
  92         param_env,
  93         body_id,
  94         span,
  95         out: vec![],
  96         recursion_depth: 0,
  97         item: Some(item),
  98     };
  99     wf.compute_trait_pred(trait_pred, Elaborate::All);
 100     debug!(obligations = ?wf.out);
 101     wf.normalize(infcx)
 102 }
 103
 104 #[instrument(skip(infcx), ret)]
 105 pub fn predicate_obligations<'a, 'tcx>(
 106     infcx: &InferCtxt<'a, 'tcx>,
 107     param_env: ty::ParamEnv<'tcx>,
 108     body_id: hir::HirId,
 109     predicate: ty::Predicate<'tcx>,
 110     span: Span,
 111 ) -> Vec<traits::PredicateObligation<'tcx>> {
 112     let mut wf = WfPredicates {
 113         tcx: infcx.tcx,
 114         param_env,
 115         body_id,
 116         span,
 117         out: vec![],
 118         recursion_depth: 0,
 119         item: None,
 120     };
 121
 122     // It's ok to skip the binder here because wf code is prepared for it
 123     match predicate.kind().skip_binder() {
 124         ty::PredicateKind::Trait(t) => {
 125             wf.compute_trait_pred(&t, Elaborate::None);
 126         }
 127         ty::PredicateKind::RegionOutlives(..) => {}
 128         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 129             wf.compute(ty.into());
 130         }
 131         ty::PredicateKind::Projection(t) => {
 132             wf.compute_projection(t.projection_ty);
 133             wf.compute(match t.term.unpack() {
 134                 ty::TermKind::Ty(ty) => ty.into(),
 135                 ty::TermKind::Const(c) => c.into(),
 136             })
 137         }
 138         ty::PredicateKind::WellFormed(arg) => {
 139             wf.compute(arg);
 140         }
 141         ty::PredicateKind::ObjectSafe(_) => {}
 142         ty::PredicateKind::ClosureKind(..) => {}
 143         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 144             wf.compute(a.into());
 145             wf.compute(b.into());
 146         }
 147         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 148             wf.compute(a.into());
 149             wf.compute(b.into());
 150         }
 151         ty::PredicateKind::ConstEvaluatable(uv) => {
 152             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 153             wf.out.extend(obligations);
 154
 155             for arg in uv.substs.iter() {
 156                 wf.compute(arg);
 157             }
 158         }
 159         ty::PredicateKind::ConstEquate(c1, c2) => {
 160             wf.compute(c1.into());
 161             wf.compute(c2.into());
 162         }
 163         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 164             bug!("TypeWellFormedFromEnv is only used for Chalk")
 165         }
 166     }
 167
 168     wf.normalize(infcx)
 169 }
 170
 171 struct WfPredicates<'tcx> {
 172     tcx: TyCtxt<'tcx>,
 173     param_env: ty::ParamEnv<'tcx>,
 174     body_id: hir::HirId,
 175     span: Span,
 176     out: Vec<traits::PredicateObligation<'tcx>>,
 177     recursion_depth: usize,
 178     item: Option<&'tcx hir::Item<'tcx>>,
 179 }
 180
 181 /// Controls whether we "elaborate" supertraits and so forth on the WF
 182 /// predicates. This is a kind of hack to address #43784. The
 183 /// underlying problem in that issue was a trait structure like:
 184 ///
 185 /// ```ignore (illustrative)
 186 /// trait Foo: Copy { }
 187 /// trait Bar: Foo { }
 188 /// impl<T: Bar> Foo for T { }
 189 /// impl<T> Bar for T { }
 190 /// ```
 191 ///
 192 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 193 /// we decide that this is true because `T: Bar` is in the
 194 /// where-clauses (and we can elaborate that to include `T:
 195 /// Copy`). This wouldn't be a problem, except that when we check the
 196 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 197 /// impl. And so nowhere did we check that `T: Copy` holds!
 198 ///
 199 /// To resolve this, we elaborate the WF requirements that must be
 200 /// proven when checking impls. This means that (e.g.) the `impl Bar
 201 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 202 /// Copy` (which it won't be able to do, because there is no `Copy`
 203 /// impl for `T`).
 204 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 205 enum Elaborate {
 206     All,
 207     None,
 208 }
 209
 210 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 211     tcx: TyCtxt<'tcx>,
 212     trait_ref: &ty::TraitRef<'tcx>,
 213     item: Option<&hir::Item<'tcx>>,
 214     cause: &mut traits::ObligationCause<'tcx>,
 215     pred: ty::Predicate<'tcx>,
 216 ) {
 217     debug!(
 218         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 219         trait_ref, item, cause, pred
 220     );
 221     let (items, impl_def_id) = match item {
 222         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 223         _ => return,
 224     };
 225     let fix_span =
 226         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 227             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 228             _ => impl_item_ref.span,
 229         };
 230
 231     // It is fine to skip the binder as we don't care about regions here.
 232     match pred.kind().skip_binder() {
 233         ty::PredicateKind::Projection(proj) => {
 234             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 235             // but rather from a "second order" obligation, where an associated type has a
 236             // projection coming from another associated type. See
 237             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 238             // `traits-assoc-type-in-supertrait-bad.rs`.
 239             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 240                 && let Some(&impl_item_id) =
 241                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 242                 && let Some(impl_item_span) = items
 243                     .iter()
 244                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 245                     .map(fix_span)
 246             {
 247                 cause.span = impl_item_span;
 248             }
 249         }
 250         ty::PredicateKind::Trait(pred) => {
 251             // An associated item obligation born out of the `trait` failed to be met. An example
 252             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 253             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 254             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind()
 255                 && let Some(&impl_item_id) =
 256                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 257                 && let Some(impl_item_span) = items
 258                     .iter()
 259                     .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 260                     .map(fix_span)
 261             {
 262                 cause.span = impl_item_span;
 263             }
 264         }
 265         _ => {}
 266     }
 267 }
 268
 269 impl<'tcx> WfPredicates<'tcx> {
 270     fn tcx(&self) -> TyCtxt<'tcx> {
 271         self.tcx
 272     }
 273
 274     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 275         traits::ObligationCause::new(self.span, self.body_id, code)
 276     }
 277
 278     fn normalize(self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 279         let cause = self.cause(traits::WellFormed(None));
 280         let param_env = self.param_env;
 281         let mut obligations = Vec::with_capacity(self.out.len());
 282         for mut obligation in self.out {
 283             assert!(!obligation.has_escaping_bound_vars());
 284             let mut selcx = traits::SelectionContext::new(infcx);
 285             // Don't normalize the whole obligation, the param env is either
 286             // already normalized, or we're currently normalizing the
 287             // param_env. Either way we should only normalize the predicate.
 288             let normalized_predicate = traits::project::normalize_with_depth_to(
 289                 &mut selcx,
 290                 param_env,
 291                 cause.clone(),
 292                 self.recursion_depth,
 293                 obligation.predicate,
 294                 &mut obligations,
 295             );
 296             obligation.predicate = normalized_predicate;
 297             obligations.push(obligation);
 298         }
 299         obligations
 300     }
 301
 302     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 303     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 304         let tcx = self.tcx;
 305         let trait_ref = &trait_pred.trait_ref;
 306
 307         // if the trait predicate is not const, the wf obligations should not be const as well.
 308         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 309             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 310         } else {
 311             if !tcx.has_attr(trait_ref.def_id, rustc_span::sym::const_trait) {
 312                 if let Some(item) = self.item &&
 313                    let hir::ItemKind::Impl(impl_) = item.kind &&
 314                    let Some(trait_) = &impl_.of_trait &&
 315                    let Some(def_id) = trait_.trait_def_id() &&
 316                    def_id == trait_ref.def_id
 317                 {
 318                     let trait_name = tcx.item_name(def_id);
 319                     let mut err = tcx.sess.struct_span_err(
 320                         self.span,
 321                         &format!("const `impl` for trait `{trait_name}` which is not marked with `#[const_trait]`"),
 322                     );
 323                     if def_id.is_local() {
 324                         let sp = tcx.def_span(def_id).shrink_to_lo();
 325                         err.span_suggestion(sp, &format!("mark `{trait_name}` as const"), "#[const_trait]", rustc_errors::Applicability::MachineApplicable);
 326                     }
 327                     err.note("marking a trait with `#[const_trait]` ensures all default method bodies are `const`");
 328                     err.note("adding a non-const method body in the future would be a breaking change");
 329                     err.emit();
 330                 } else {
 331                     tcx.sess.span_err(
 332                         self.span,
 333                         "~const can only be applied to `#[const_trait]` traits",
 334                     );
 335                 }
 336             }
 337             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 338         };
 339
 340         debug!("compute_trait_pred obligations {:?}", obligations);
 341         let param_env = self.param_env;
 342         let depth = self.recursion_depth;
 343
 344         let item = self.item;
 345
 346         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 347             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 348                 cause = cause.derived_cause(
 349                     parent_trait_pred,
 350                     traits::ObligationCauseCode::DerivedObligation,
 351                 );
 352             }
 353             extend_cause_with_original_assoc_item_obligation(
 354                 tcx, trait_ref, item, &mut cause, predicate,
 355             );
 356             traits::Obligation::with_depth(cause, depth, param_env, predicate)
 357         };
 358
 359         if let Elaborate::All = elaborate {
 360             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 361             let implied_obligations = implied_obligations.map(extend);
 362             self.out.extend(implied_obligations);
 363         } else {
 364             self.out.extend(obligations);
 365         }
 366
 367         let tcx = self.tcx();
 368         self.out.extend(
 369             trait_ref
 370                 .substs
 371                 .iter()
 372                 .enumerate()
 373                 .filter(|(_, arg)| {
 374                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 375                 })
 376                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 377                 .map(|(i, arg)| {
 378                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 379                     // The first subst is the self ty - use the correct span for it.
 380                     if i == 0 {
 381                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 382                             item.map(|i| &i.kind)
 383                         {
 384                             cause.span = self_ty.span;
 385                         }
 386                     }
 387                     traits::Obligation::with_depth(
 388                         cause,
 389                         depth,
 390                         param_env,
 391                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 392                     )
 393                 }),
 394         );
 395     }
 396
 397     /// Pushes the obligations required for `trait_ref::Item` to be WF
 398     /// into `self.out`.
 399     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 400         // A projection is well-formed if
 401         //
 402         // (a) its predicates hold (*)
 403         // (b) its substs are wf
 404         //
 405         // (*) The predicates of an associated type include the predicates of
 406         //     the trait that it's contained in. For example, given
 407         //
 408         // trait A<T>: Clone {
 409         //     type X where T: Copy;
 410         // }
 411         //
 412         // The predicates of `<() as A<i32>>::X` are:
 413         // [
 414         //     `(): Sized`
 415         //     `(): Clone`
 416         //     `(): A<i32>`
 417         //     `i32: Sized`
 418         //     `i32: Clone`
 419         //     `i32: Copy`
 420         // ]
 421         // Projection types do not require const predicates.
 422         let obligations = self.nominal_obligations_without_const(data.item_def_id, data.substs);
 423         self.out.extend(obligations);
 424
 425         let tcx = self.tcx();
 426         let cause = self.cause(traits::WellFormed(None));
 427         let param_env = self.param_env;
 428         let depth = self.recursion_depth;
 429
 430         self.out.extend(
 431             data.substs
 432                 .iter()
 433                 .filter(|arg| {
 434                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 435                 })
 436                 .filter(|arg| !arg.has_escaping_bound_vars())
 437                 .map(|arg| {
 438                     traits::Obligation::with_depth(
 439                         cause.clone(),
 440                         depth,
 441                         param_env,
 442                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 443                     )
 444                 }),
 445         );
 446     }
 447
 448     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 449         if !subty.has_escaping_bound_vars() {
 450             let cause = self.cause(cause);
 451             let trait_ref = ty::TraitRef {
 452                 def_id: self.tcx.require_lang_item(LangItem::Sized, None),
 453                 substs: self.tcx.mk_substs_trait(subty, &[]),
 454             };
 455             self.out.push(traits::Obligation::with_depth(
 456                 cause,
 457                 self.recursion_depth,
 458                 self.param_env,
 459                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx),
 460             ));
 461         }
 462     }
 463
 464     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 465     #[instrument(level = "debug", skip(self))]
 466     fn compute(&mut self, arg: GenericArg<'tcx>) {
 467         let mut walker = arg.walk();
 468         let param_env = self.param_env;
 469         let depth = self.recursion_depth;
 470         while let Some(arg) = walker.next() {
 471             debug!(?arg, ?self.out);
 472             let ty = match arg.unpack() {
 473                 GenericArgKind::Type(ty) => ty,
 474
 475                 // No WF constraints for lifetimes being present, any outlives
 476                 // obligations are handled by the parent (e.g. `ty::Ref`).
 477                 GenericArgKind::Lifetime(_) => continue,
 478
 479                 GenericArgKind::Const(constant) => {
 480                     match constant.kind() {
 481                         ty::ConstKind::Unevaluated(uv) => {
 482                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 483                             self.out.extend(obligations);
 484
 485                             let predicate =
 486                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv))
 487                                     .to_predicate(self.tcx());
 488                             let cause = self.cause(traits::WellFormed(None));
 489                             self.out.push(traits::Obligation::with_depth(
 490                                 cause,
 491                                 self.recursion_depth,
 492                                 self.param_env,
 493                                 predicate,
 494                             ));
 495                         }
 496                         ty::ConstKind::Infer(_) => {
 497                             let cause = self.cause(traits::WellFormed(None));
 498
 499                             self.out.push(traits::Obligation::with_depth(
 500                                 cause,
 501                                 self.recursion_depth,
 502                                 self.param_env,
 503                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(constant.into()))
 504                                     .to_predicate(self.tcx()),
 505                             ));
 506                         }
 507                         ty::ConstKind::Error(_)
 508                         | ty::ConstKind::Param(_)
 509                         | ty::ConstKind::Bound(..)
 510                         | ty::ConstKind::Placeholder(..) => {
 511                             // These variants are trivially WF, so nothing to do here.
 512                         }
 513                         ty::ConstKind::Value(..) => {
 514                             // FIXME: Enforce that values are structurally-matchable.
 515                         }
 516                     }
 517                     continue;
 518                 }
 519             };
 520
 521             debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
 522
 523             match *ty.kind() {
 524                 ty::Bool
 525                 | ty::Char
 526                 | ty::Int(..)
 527                 | ty::Uint(..)
 528                 | ty::Float(..)
 529                 | ty::Error(_)
 530                 | ty::Str
 531                 | ty::GeneratorWitness(..)
 532                 | ty::Never
 533                 | ty::Param(_)
 534                 | ty::Bound(..)
 535                 | ty::Placeholder(..)
 536                 | ty::Foreign(..) => {
 537                     // WfScalar, WfParameter, etc
 538                 }
 539
 540                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 541                 ty::Infer(ty::IntVar(_)) => {}
 542
 543                 // Can only infer to `ty::Float(_)`.
 544                 ty::Infer(ty::FloatVar(_)) => {}
 545
 546                 ty::Slice(subty) => {
 547                     self.require_sized(subty, traits::SliceOrArrayElem);
 548                 }
 549
 550                 ty::Array(subty, _) => {
 551                     self.require_sized(subty, traits::SliceOrArrayElem);
 552                     // Note that we handle the len is implicitly checked while walking `arg`.
 553                 }
 554
 555                 ty::Tuple(ref tys) => {
 556                     if let Some((_last, rest)) = tys.split_last() {
 557                         for &elem in rest {
 558                             self.require_sized(elem, traits::TupleElem);
 559                         }
 560                     }
 561                 }
 562
 563                 ty::RawPtr(_) => {
 564                     // Simple cases that are WF if their type args are WF.
 565                 }
 566
 567                 ty::Projection(data) => {
 568                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 569                     self.compute_projection(data);
 570                 }
 571
 572                 ty::Adt(def, substs) => {
 573                     // WfNominalType
 574                     let obligations = self.nominal_obligations(def.did(), substs);
 575                     self.out.extend(obligations);
 576                 }
 577
 578                 ty::FnDef(did, substs) => {
 579                     let obligations = self.nominal_obligations(did, substs);
 580                     self.out.extend(obligations);
 581                 }
 582
 583                 ty::Ref(r, rty, _) => {
 584                     // WfReference
 585                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 586                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 587                         self.out.push(traits::Obligation::with_depth(
 588                             cause,
 589                             depth,
 590                             param_env,
 591                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 592                                 ty::OutlivesPredicate(rty, r),
 593                             ))
 594                             .to_predicate(self.tcx()),
 595                         ));
 596                     }
 597                 }
 598
 599                 ty::Generator(did, substs, ..) => {
 600                     // Walk ALL the types in the generator: this will
 601                     // include the upvar types as well as the yield
 602                     // type. Note that this is mildly distinct from
 603                     // the closure case, where we have to be careful
 604                     // about the signature of the closure. We don't
 605                     // have the problem of implied bounds here since
 606                     // generators don't take arguments.
 607                     let obligations = self.nominal_obligations(did, substs);
 608                     self.out.extend(obligations);
 609                 }
 610
 611                 ty::Closure(did, substs) => {
 612                     // Only check the upvar types for WF, not the rest
 613                     // of the types within. This is needed because we
 614                     // capture the signature and it may not be WF
 615                     // without the implied bounds. Consider a closure
 616                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 617                     // not known to hold in the creator's context (and
 618                     // indeed the closure may not be invoked by its
 619                     // creator, but rather turned to someone who *can*
 620                     // verify that).
 621                     //
 622                     // The special treatment of closures here really
 623                     // ought not to be necessary either; the problem
 624                     // is related to #25860 -- there is no way for us
 625                     // to express a fn type complete with the implied
 626                     // bounds that it is assuming. I think in reality
 627                     // the WF rules around fn are a bit messed up, and
 628                     // that is the rot problem: `fn(&'a T)` should
 629                     // probably always be WF, because it should be
 630                     // shorthand for something like `where(T: 'a) {
 631                     // fn(&'a T) }`, as discussed in #25860.
 632                     walker.skip_current_subtree(); // subtree handled below
 633                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 634                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 635                     // Note that we cannot skip the generic types
 636                     // types. Normally, within the fn
 637                     // body where they are created, the generics will
 638                     // always be WF, and outside of that fn body we
 639                     // are not directly inspecting closure types
 640                     // anyway, except via auto trait matching (which
 641                     // only inspects the upvar types).
 642                     // But when a closure is part of a type-alias-impl-trait
 643                     // then the function that created the defining site may
 644                     // have had more bounds available than the type alias
 645                     // specifies. This may cause us to have a closure in the
 646                     // hidden type that is not actually well formed and
 647                     // can cause compiler crashes when the user abuses unsafe
 648                     // code to procure such a closure.
 649                     // See src/test/ui/type-alias-impl-trait/wf_check_closures.rs
 650                     let obligations = self.nominal_obligations(did, substs);
 651                     self.out.extend(obligations);
 652                 }
 653
 654                 ty::FnPtr(_) => {
 655                     // let the loop iterate into the argument/return
 656                     // types appearing in the fn signature
 657                 }
 658
 659                 ty::Opaque(did, substs) => {
 660                     // All of the requirements on type parameters
 661                     // have already been checked for `impl Trait` in
 662                     // return position. We do need to check type-alias-impl-trait though.
 663                     if ty::is_impl_trait_defn(self.tcx, did).is_none() {
 664                         let obligations = self.nominal_obligations(did, substs);
 665                         self.out.extend(obligations);
 666                     }
 667                 }
 668
 669                 ty::Dynamic(data, r, _) => {
 670                     // WfObject
 671                     //
 672                     // Here, we defer WF checking due to higher-ranked
 673                     // regions. This is perhaps not ideal.
 674                     self.from_object_ty(ty, data, r);
 675
 676                     // FIXME(#27579) RFC also considers adding trait
 677                     // obligations that don't refer to Self and
 678                     // checking those
 679
 680                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 681
 682                     if !defer_to_coercion {
 683                         let cause = self.cause(traits::WellFormed(None));
 684                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 685                         let tcx = self.tcx();
 686                         self.out.extend(component_traits.map(|did| {
 687                             traits::Obligation::with_depth(
 688                                 cause.clone(),
 689                                 depth,
 690                                 param_env,
 691                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 692                                     .to_predicate(tcx),
 693                             )
 694                         }));
 695                     }
 696                 }
 697
 698                 // Inference variables are the complicated case, since we don't
 699                 // know what type they are. We do two things:
 700                 //
 701                 // 1. Check if they have been resolved, and if so proceed with
 702                 //    THAT type.
 703                 // 2. If not, we've at least simplified things (e.g., we went
 704                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 705                 //    register a pending obligation and keep
 706                 //    moving. (Goal is that an "inductive hypothesis"
 707                 //    is satisfied to ensure termination.)
 708                 // See also the comment on `fn obligations`, describing "livelock"
 709                 // prevention, which happens before this can be reached.
 710                 ty::Infer(_) => {
 711                     let cause = self.cause(traits::WellFormed(None));
 712                     self.out.push(traits::Obligation::with_depth(
 713                         cause,
 714                         self.recursion_depth,
 715                         param_env,
 716                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 717                             .to_predicate(self.tcx()),
 718                     ));
 719                 }
 720             }
 721
 722             debug!(?self.out);
 723         }
 724     }
 725
 726     #[instrument(level = "debug", skip(self))]
 727     fn nominal_obligations_inner(
 728         &mut self,
 729         def_id: DefId,
 730         substs: SubstsRef<'tcx>,
 731         remap_constness: bool,
 732     ) -> Vec<traits::PredicateObligation<'tcx>> {
 733         let predicates = self.tcx.predicates_of(def_id);
 734         let mut origins = vec![def_id; predicates.predicates.len()];
 735         let mut head = predicates;
 736         while let Some(parent) = head.parent {
 737             head = self.tcx.predicates_of(parent);
 738             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 739         }
 740
 741         let predicates = predicates.instantiate(self.tcx, substs);
 742         trace!("{:#?}", predicates);
 743         debug_assert_eq!(predicates.predicates.len(), origins.len());
 744
 745         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 746             .map(|((mut pred, span), origin_def_id)| {
 747                 let code = if span.is_dummy() {
 748                     traits::ItemObligation(origin_def_id)
 749                 } else {
 750                     traits::BindingObligation(origin_def_id, span)
 751                 };
 752                 let cause = self.cause(code);
 753                 if remap_constness {
 754                     pred = pred.without_const(self.tcx);
 755                 }
 756                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 757             })
 758             .filter(|pred| !pred.has_escaping_bound_vars())
 759             .collect()
 760     }
 761
 762     fn nominal_obligations(
 763         &mut self,
 764         def_id: DefId,
 765         substs: SubstsRef<'tcx>,
 766     ) -> Vec<traits::PredicateObligation<'tcx>> {
 767         self.nominal_obligations_inner(def_id, substs, false)
 768     }
 769
 770     fn nominal_obligations_without_const(
 771         &mut self,
 772         def_id: DefId,
 773         substs: SubstsRef<'tcx>,
 774     ) -> Vec<traits::PredicateObligation<'tcx>> {
 775         self.nominal_obligations_inner(def_id, substs, true)
 776     }
 777
 778     fn from_object_ty(
 779         &mut self,
 780         ty: Ty<'tcx>,
 781         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 782         region: ty::Region<'tcx>,
 783     ) {
 784         // Imagine a type like this:
 785         //
 786         //     trait Foo { }
 787         //     trait Bar<'c> : 'c { }
 788         //
 789         //     &'b (Foo+'c+Bar<'d>)
 790         //         ^
 791         //
 792         // In this case, the following relationships must hold:
 793         //
 794         //     'b <= 'c
 795         //     'd <= 'c
 796         //
 797         // The first conditions is due to the normal region pointer
 798         // rules, which say that a reference cannot outlive its
 799         // referent.
 800         //
 801         // The final condition may be a bit surprising. In particular,
 802         // you may expect that it would have been `'c <= 'd`, since
 803         // usually lifetimes of outer things are conservative
 804         // approximations for inner things. However, it works somewhat
 805         // differently with trait objects: here the idea is that if the
 806         // user specifies a region bound (`'c`, in this case) it is the
 807         // "master bound" that *implies* that bounds from other traits are
 808         // all met. (Remember that *all bounds* in a type like
 809         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 810         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 811         // 'y.)
 812         //
 813         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 814         // am looking forward to the future here.
 815         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 816             let implicit_bounds = object_region_bounds(self.tcx, data);
 817
 818             let explicit_bound = region;
 819
 820             self.out.reserve(implicit_bounds.len());
 821             for implicit_bound in implicit_bounds {
 822                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 823                 let outlives =
 824                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 825                 self.out.push(traits::Obligation::with_depth(
 826                     cause,
 827                     self.recursion_depth,
 828                     self.param_env,
 829                     outlives.to_predicate(self.tcx),
 830                 ));
 831             }
 832         }
 833     }
 834 }
 835
 836 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 837 /// bounds that must hold on the elided self type. These are derived
 838 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 839 /// they declare `trait SomeTrait : 'static`, for example, then
 840 /// `'static` would appear in the list. The hard work is done by
 841 /// `infer::required_region_bounds`, see that for more information.
 842 pub fn object_region_bounds<'tcx>(
 843     tcx: TyCtxt<'tcx>,
 844     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 845 ) -> Vec<ty::Region<'tcx>> {
 846     // Since we don't actually *know* the self type for an object,
 847     // this "open(err)" serves as a kind of dummy standin -- basically
 848     // a placeholder type.
 849     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 850
 851     let predicates = existential_predicates.iter().filter_map(|predicate| {
 852         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 853             None
 854         } else {
 855             Some(predicate.with_self_ty(tcx, open_ty))
 856         }
 857     });
 858
 859     required_region_bounds(tcx, open_ty, predicates)
 860 }
 861
 862 /// Given a set of predicates that apply to an object type, returns
 863 /// the region bounds that the (erased) `Self` type must
 864 /// outlive. Precisely *because* the `Self` type is erased, the
 865 /// parameter `erased_self_ty` must be supplied to indicate what type
 866 /// has been used to represent `Self` in the predicates
 867 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 868 /// popular choice.
 869 ///
 870 /// N.B., in some cases, particularly around higher-ranked bounds,
 871 /// this function returns a kind of conservative approximation.
 872 /// That is, all regions returned by this function are definitely
 873 /// required, but there may be other region bounds that are not
 874 /// returned, as well as requirements like `for<'a> T: 'a`.
 875 ///
 876 /// Requires that trait definitions have been processed so that we can
 877 /// elaborate predicates and walk supertraits.
 878 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 879 pub(crate) fn required_region_bounds<'tcx>(
 880     tcx: TyCtxt<'tcx>,
 881     erased_self_ty: Ty<'tcx>,
 882     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 883 ) -> Vec<ty::Region<'tcx>> {
 884     assert!(!erased_self_ty.has_escaping_bound_vars());
 885
 886     traits::elaborate_predicates(tcx, predicates)
 887         .filter_map(|obligation| {
 888             debug!(?obligation);
 889             match obligation.predicate.kind().skip_binder() {
 890                 ty::PredicateKind::Projection(..)
 891                 | ty::PredicateKind::Trait(..)
 892                 | ty::PredicateKind::Subtype(..)
 893                 | ty::PredicateKind::Coerce(..)
 894                 | ty::PredicateKind::WellFormed(..)
 895                 | ty::PredicateKind::ObjectSafe(..)
 896                 | ty::PredicateKind::ClosureKind(..)
 897                 | ty::PredicateKind::RegionOutlives(..)
 898                 | ty::PredicateKind::ConstEvaluatable(..)
 899                 | ty::PredicateKind::ConstEquate(..)
 900                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 901                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
 902                     // Search for a bound of the form `erased_self_ty
 903                     // : 'a`, but be wary of something like `for<'a>
 904                     // erased_self_ty : 'a` (we interpret a
 905                     // higher-ranked bound like that as 'static,
 906                     // though at present the code in `fulfill.rs`
 907                     // considers such bounds to be unsatisfiable, so
 908                     // it's kind of a moot point since you could never
 909                     // construct such an object, but this seems
 910                     // correct even if that code changes).
 911                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 912                         Some(*r)
 913                     } else {
 914                         None
 915                     }
 916                 }
 917             }
 918         })
 919         .collect()
 920 }