compiler/rustc_trait_selection/src/traits/wf.rs

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