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(t.ty.into());
 120         }
 121         ty::PredicateKind::WellFormed(arg) => {
 122             wf.compute(arg);
 123         }
 124         ty::PredicateKind::ObjectSafe(_) => {}
 125         ty::PredicateKind::ClosureKind(..) => {}
 126         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 127             wf.compute(a.into());
 128             wf.compute(b.into());
 129         }
 130         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 131             wf.compute(a.into());
 132             wf.compute(b.into());
 133         }
 134         ty::PredicateKind::ConstEvaluatable(uv) => {
 135             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 136             wf.out.extend(obligations);
 137
 138             for arg in uv.substs.iter() {
 139                 wf.compute(arg);
 140             }
 141         }
 142         ty::PredicateKind::ConstEquate(c1, c2) => {
 143             wf.compute(c1.into());
 144             wf.compute(c2.into());
 145         }
 146         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 147             bug!("TypeWellFormedFromEnv is only used for Chalk")
 148         }
 149     }
 150
 151     wf.normalize()
 152 }
 153
 154 struct WfPredicates<'a, 'tcx> {
 155     infcx: &'a InferCtxt<'a, 'tcx>,
 156     param_env: ty::ParamEnv<'tcx>,
 157     body_id: hir::HirId,
 158     span: Span,
 159     out: Vec<traits::PredicateObligation<'tcx>>,
 160     recursion_depth: usize,
 161     item: Option<&'tcx hir::Item<'tcx>>,
 162 }
 163
 164 /// Controls whether we "elaborate" supertraits and so forth on the WF
 165 /// predicates. This is a kind of hack to address #43784. The
 166 /// underlying problem in that issue was a trait structure like:
 167 ///
 168 /// ```
 169 /// trait Foo: Copy { }
 170 /// trait Bar: Foo { }
 171 /// impl<T: Bar> Foo for T { }
 172 /// impl<T> Bar for T { }
 173 /// ```
 174 ///
 175 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 176 /// we decide that this is true because `T: Bar` is in the
 177 /// where-clauses (and we can elaborate that to include `T:
 178 /// Copy`). This wouldn't be a problem, except that when we check the
 179 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 180 /// impl. And so nowhere did we check that `T: Copy` holds!
 181 ///
 182 /// To resolve this, we elaborate the WF requirements that must be
 183 /// proven when checking impls. This means that (e.g.) the `impl Bar
 184 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 185 /// Copy` (which it won't be able to do, because there is no `Copy`
 186 /// impl for `T`).
 187 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 188 enum Elaborate {
 189     All,
 190     None,
 191 }
 192
 193 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 194     tcx: TyCtxt<'tcx>,
 195     trait_ref: &ty::TraitRef<'tcx>,
 196     item: Option<&hir::Item<'tcx>>,
 197     cause: &mut traits::ObligationCause<'tcx>,
 198     pred: &ty::Predicate<'tcx>,
 199 ) {
 200     debug!(
 201         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 202         trait_ref, item, cause, pred
 203     );
 204     let (items, impl_def_id) = match item {
 205         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 206         _ => return,
 207     };
 208     let fix_span =
 209         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 210             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 211             _ => impl_item_ref.span,
 212         };
 213
 214     // It is fine to skip the binder as we don't care about regions here.
 215     match pred.kind().skip_binder() {
 216         ty::PredicateKind::Projection(proj) => {
 217             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 218             // but rather from a "second order" obligation, where an associated type has a
 219             // projection coming from another associated type. See
 220             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 221             // `traits-assoc-type-in-supertrait-bad.rs`.
 222             if let ty::Projection(projection_ty) = proj.ty.kind() {
 223                 if let Some(&impl_item_id) =
 224                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 225                 {
 226                     if let Some(impl_item_span) = items
 227                         .iter()
 228                         .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 229                         .map(fix_span)
 230                     {
 231                         cause.span = impl_item_span;
 232                     }
 233                 }
 234             }
 235         }
 236         ty::PredicateKind::Trait(pred) => {
 237             // An associated item obligation born out of the `trait` failed to be met. An example
 238             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 239             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 240             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind() {
 241                 if let Some(&impl_item_id) =
 242                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 243                 {
 244                     if let Some(impl_item_span) = items
 245                         .iter()
 246                         .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 247                         .map(fix_span)
 248                     {
 249                         cause.span = impl_item_span;
 250                     }
 251                 }
 252             }
 253         }
 254         _ => {}
 255     }
 256 }
 257
 258 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 259     fn tcx(&self) -> TyCtxt<'tcx> {
 260         self.infcx.tcx
 261     }
 262
 263     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 264         traits::ObligationCause::new(self.span, self.body_id, code)
 265     }
 266
 267     fn normalize(mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 268         let cause = self.cause(traits::MiscObligation);
 269         let infcx = &mut self.infcx;
 270         let param_env = self.param_env;
 271         let mut obligations = Vec::with_capacity(self.out.len());
 272         for mut obligation in self.out {
 273             assert!(!obligation.has_escaping_bound_vars());
 274             let mut selcx = traits::SelectionContext::new(infcx);
 275             // Don't normalize the whole obligation, the param env is either
 276             // already normalized, or we're currently normalizing the
 277             // param_env. Either way we should only normalize the predicate.
 278             let normalized_predicate = traits::project::normalize_with_depth_to(
 279                 &mut selcx,
 280                 param_env,
 281                 cause.clone(),
 282                 self.recursion_depth,
 283                 obligation.predicate,
 284                 &mut obligations,
 285             );
 286             obligation.predicate = normalized_predicate;
 287             obligations.push(obligation);
 288         }
 289         obligations
 290     }
 291
 292     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 293     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 294         let tcx = self.infcx.tcx;
 295         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 296
 297         debug!("compute_trait_ref obligations {:?}", obligations);
 298         let cause = self.cause(traits::MiscObligation);
 299         let param_env = self.param_env;
 300         let depth = self.recursion_depth;
 301
 302         let item = self.item;
 303
 304         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 305             let mut cause = cause.clone();
 306             if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_pred() {
 307                 let derived_cause = traits::DerivedObligationCause {
 308                     // FIXME(fee1-dead): when improving error messages, change this to PolyTraitPredicate
 309                     parent_trait_ref: parent_trait_ref.map_bound(|t| t.trait_ref),
 310                     parent_code: obligation.cause.clone_code(),
 311                 };
 312                 *cause.make_mut_code() =
 313                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 314             }
 315             extend_cause_with_original_assoc_item_obligation(
 316                 tcx,
 317                 trait_ref,
 318                 item,
 319                 &mut cause,
 320                 &obligation.predicate,
 321             );
 322             traits::Obligation::with_depth(cause, depth, param_env, obligation.predicate)
 323         };
 324
 325         if let Elaborate::All = elaborate {
 326             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 327             let implied_obligations = implied_obligations.map(extend);
 328             self.out.extend(implied_obligations);
 329         } else {
 330             self.out.extend(obligations);
 331         }
 332
 333         let tcx = self.tcx();
 334         self.out.extend(
 335             trait_ref
 336                 .substs
 337                 .iter()
 338                 .enumerate()
 339                 .filter(|(_, arg)| {
 340                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 341                 })
 342                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 343                 .map(|(i, arg)| {
 344                     let mut new_cause = cause.clone();
 345                     // The first subst is the self ty - use the correct span for it.
 346                     if i == 0 {
 347                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 348                             item.map(|i| &i.kind)
 349                         {
 350                             new_cause.span = self_ty.span;
 351                         }
 352                     }
 353                     traits::Obligation::with_depth(
 354                         new_cause,
 355                         depth,
 356                         param_env,
 357                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 358                     )
 359                 }),
 360         );
 361     }
 362
 363     /// Pushes the obligations required for `trait_ref::Item` to be WF
 364     /// into `self.out`.
 365     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 366         // A projection is well-formed if
 367         //
 368         // (a) its predicates hold (*)
 369         // (b) its substs are wf
 370         //
 371         // (*) The predicates of an associated type include the predicates of
 372         //     the trait that it's contained in. For example, given
 373         //
 374         // trait A<T>: Clone {
 375         //     type X where T: Copy;
 376         // }
 377         //
 378         // The predicates of `<() as A<i32>>::X` are:
 379         // [
 380         //     `(): Sized`
 381         //     `(): Clone`
 382         //     `(): A<i32>`
 383         //     `i32: Sized`
 384         //     `i32: Clone`
 385         //     `i32: Copy`
 386         // ]
 387         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 388         self.out.extend(obligations);
 389
 390         let tcx = self.tcx();
 391         let cause = self.cause(traits::MiscObligation);
 392         let param_env = self.param_env;
 393         let depth = self.recursion_depth;
 394
 395         self.out.extend(
 396             data.substs
 397                 .iter()
 398                 .filter(|arg| {
 399                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 400                 })
 401                 .filter(|arg| !arg.has_escaping_bound_vars())
 402                 .map(|arg| {
 403                     traits::Obligation::with_depth(
 404                         cause.clone(),
 405                         depth,
 406                         param_env,
 407                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 408                     )
 409                 }),
 410         );
 411     }
 412
 413     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 414         if !subty.has_escaping_bound_vars() {
 415             let cause = self.cause(cause);
 416             let trait_ref = ty::TraitRef {
 417                 def_id: self.infcx.tcx.require_lang_item(LangItem::Sized, None),
 418                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 419             };
 420             self.out.push(traits::Obligation::with_depth(
 421                 cause,
 422                 self.recursion_depth,
 423                 self.param_env,
 424                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.infcx.tcx),
 425             ));
 426         }
 427     }
 428
 429     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 430     fn compute(&mut self, arg: GenericArg<'tcx>) {
 431         let mut walker = arg.walk();
 432         let param_env = self.param_env;
 433         let depth = self.recursion_depth;
 434         while let Some(arg) = walker.next() {
 435             let ty = match arg.unpack() {
 436                 GenericArgKind::Type(ty) => ty,
 437
 438                 // No WF constraints for lifetimes being present, any outlives
 439                 // obligations are handled by the parent (e.g. `ty::Ref`).
 440                 GenericArgKind::Lifetime(_) => continue,
 441
 442                 GenericArgKind::Const(constant) => {
 443                     match constant.val {
 444                         ty::ConstKind::Unevaluated(uv) => {
 445                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 446                             self.out.extend(obligations);
 447
 448                             let predicate =
 449                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 450                                     .to_predicate(self.tcx());
 451                             let cause = self.cause(traits::MiscObligation);
 452                             self.out.push(traits::Obligation::with_depth(
 453                                 cause,
 454                                 self.recursion_depth,
 455                                 self.param_env,
 456                                 predicate,
 457                             ));
 458                         }
 459                         ty::ConstKind::Infer(infer) => {
 460                             let resolved = self.infcx.shallow_resolve(infer);
 461                             // the `InferConst` changed, meaning that we made progress.
 462                             if resolved != infer {
 463                                 let cause = self.cause(traits::MiscObligation);
 464
 465                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 466                                     val: ty::ConstKind::Infer(resolved),
 467                                     ..*constant
 468                                 });
 469                                 self.out.push(traits::Obligation::with_depth(
 470                                     cause,
 471                                     self.recursion_depth,
 472                                     self.param_env,
 473                                     ty::Binder::dummy(ty::PredicateKind::WellFormed(
 474                                         resolved_constant.into(),
 475                                     ))
 476                                     .to_predicate(self.tcx()),
 477                                 ));
 478                             }
 479                         }
 480                         ty::ConstKind::Error(_)
 481                         | ty::ConstKind::Param(_)
 482                         | ty::ConstKind::Bound(..)
 483                         | ty::ConstKind::Placeholder(..) => {
 484                             // These variants are trivially WF, so nothing to do here.
 485                         }
 486                         ty::ConstKind::Value(..) => {
 487                             // FIXME: Enforce that values are structurally-matchable.
 488                         }
 489                     }
 490                     continue;
 491                 }
 492             };
 493
 494             match *ty.kind() {
 495                 ty::Bool
 496                 | ty::Char
 497                 | ty::Int(..)
 498                 | ty::Uint(..)
 499                 | ty::Float(..)
 500                 | ty::Error(_)
 501                 | ty::Str
 502                 | ty::GeneratorWitness(..)
 503                 | ty::Never
 504                 | ty::Param(_)
 505                 | ty::Bound(..)
 506                 | ty::Placeholder(..)
 507                 | ty::Foreign(..) => {
 508                     // WfScalar, WfParameter, etc
 509                 }
 510
 511                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 512                 ty::Infer(ty::IntVar(_)) => {}
 513
 514                 // Can only infer to `ty::Float(_)`.
 515                 ty::Infer(ty::FloatVar(_)) => {}
 516
 517                 ty::Slice(subty) => {
 518                     self.require_sized(subty, traits::SliceOrArrayElem);
 519                 }
 520
 521                 ty::Array(subty, _) => {
 522                     self.require_sized(subty, traits::SliceOrArrayElem);
 523                     // Note that we handle the len is implicitly checked while walking `arg`.
 524                 }
 525
 526                 ty::Tuple(ref tys) => {
 527                     if let Some((_last, rest)) = tys.split_last() {
 528                         for elem in rest {
 529                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 530                         }
 531                     }
 532                 }
 533
 534                 ty::RawPtr(_) => {
 535                     // Simple cases that are WF if their type args are WF.
 536                 }
 537
 538                 ty::Projection(data) => {
 539                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 540                     self.compute_projection(data);
 541                 }
 542
 543                 ty::Adt(def, substs) => {
 544                     // WfNominalType
 545                     let obligations = self.nominal_obligations(def.did, substs);
 546                     self.out.extend(obligations);
 547                 }
 548
 549                 ty::FnDef(did, substs) => {
 550                     let obligations = self.nominal_obligations(did, substs);
 551                     self.out.extend(obligations);
 552                 }
 553
 554                 ty::Ref(r, rty, _) => {
 555                     // WfReference
 556                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 557                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 558                         self.out.push(traits::Obligation::with_depth(
 559                             cause,
 560                             depth,
 561                             param_env,
 562                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 563                                 ty::OutlivesPredicate(rty, r),
 564                             ))
 565                             .to_predicate(self.tcx()),
 566                         ));
 567                     }
 568                 }
 569
 570                 ty::Generator(..) => {
 571                     // Walk ALL the types in the generator: this will
 572                     // include the upvar types as well as the yield
 573                     // type. Note that this is mildly distinct from
 574                     // the closure case, where we have to be careful
 575                     // about the signature of the closure. We don't
 576                     // have the problem of implied bounds here since
 577                     // generators don't take arguments.
 578                 }
 579
 580                 ty::Closure(_, substs) => {
 581                     // Only check the upvar types for WF, not the rest
 582                     // of the types within. This is needed because we
 583                     // capture the signature and it may not be WF
 584                     // without the implied bounds. Consider a closure
 585                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 586                     // not known to hold in the creator's context (and
 587                     // indeed the closure may not be invoked by its
 588                     // creator, but rather turned to someone who *can*
 589                     // verify that).
 590                     //
 591                     // The special treatment of closures here really
 592                     // ought not to be necessary either; the problem
 593                     // is related to #25860 -- there is no way for us
 594                     // to express a fn type complete with the implied
 595                     // bounds that it is assuming. I think in reality
 596                     // the WF rules around fn are a bit messed up, and
 597                     // that is the rot problem: `fn(&'a T)` should
 598                     // probably always be WF, because it should be
 599                     // shorthand for something like `where(T: 'a) {
 600                     // fn(&'a T) }`, as discussed in #25860.
 601                     //
 602                     // Note that we are also skipping the generic
 603                     // types. This is consistent with the `outlives`
 604                     // code, but anyway doesn't matter: within the fn
 605                     // body where they are created, the generics will
 606                     // always be WF, and outside of that fn body we
 607                     // are not directly inspecting closure types
 608                     // anyway, except via auto trait matching (which
 609                     // only inspects the upvar types).
 610                     walker.skip_current_subtree(); // subtree handled below
 611                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 612                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 613                 }
 614
 615                 ty::FnPtr(_) => {
 616                     // let the loop iterate into the argument/return
 617                     // types appearing in the fn signature
 618                 }
 619
 620                 ty::Opaque(did, substs) => {
 621                     // all of the requirements on type parameters
 622                     // should've been checked by the instantiation
 623                     // of whatever returned this exact `impl Trait`.
 624
 625                     // for named opaque `impl Trait` types we still need to check them
 626                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 627                         let obligations = self.nominal_obligations(did, substs);
 628                         self.out.extend(obligations);
 629                     }
 630                 }
 631
 632                 ty::Dynamic(data, r) => {
 633                     // WfObject
 634                     //
 635                     // Here, we defer WF checking due to higher-ranked
 636                     // regions. This is perhaps not ideal.
 637                     self.from_object_ty(ty, data, r);
 638
 639                     // FIXME(#27579) RFC also considers adding trait
 640                     // obligations that don't refer to Self and
 641                     // checking those
 642
 643                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 644
 645                     if !defer_to_coercion {
 646                         let cause = self.cause(traits::MiscObligation);
 647                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 648                         let tcx = self.tcx();
 649                         self.out.extend(component_traits.map(|did| {
 650                             traits::Obligation::with_depth(
 651                                 cause.clone(),
 652                                 depth,
 653                                 param_env,
 654                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 655                                     .to_predicate(tcx),
 656                             )
 657                         }));
 658                     }
 659                 }
 660
 661                 // Inference variables are the complicated case, since we don't
 662                 // know what type they are. We do two things:
 663                 //
 664                 // 1. Check if they have been resolved, and if so proceed with
 665                 //    THAT type.
 666                 // 2. If not, we've at least simplified things (e.g., we went
 667                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 668                 //    register a pending obligation and keep
 669                 //    moving. (Goal is that an "inductive hypothesis"
 670                 //    is satisfied to ensure termination.)
 671                 // See also the comment on `fn obligations`, describing "livelock"
 672                 // prevention, which happens before this can be reached.
 673                 ty::Infer(_) => {
 674                     let ty = self.infcx.shallow_resolve(ty);
 675                     if let ty::Infer(ty::TyVar(_)) = ty.kind() {
 676                         // Not yet resolved, but we've made progress.
 677                         let cause = self.cause(traits::MiscObligation);
 678                         self.out.push(traits::Obligation::with_depth(
 679                             cause,
 680                             self.recursion_depth,
 681                             param_env,
 682                             ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 683                                 .to_predicate(self.tcx()),
 684                         ));
 685                     } else {
 686                         // Yes, resolved, proceed with the result.
 687                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 688                         self.compute(ty.into());
 689                     }
 690                 }
 691             }
 692         }
 693     }
 694
 695     fn nominal_obligations(
 696         &mut self,
 697         def_id: DefId,
 698         substs: SubstsRef<'tcx>,
 699     ) -> Vec<traits::PredicateObligation<'tcx>> {
 700         let predicates = self.infcx.tcx.predicates_of(def_id);
 701         let mut origins = vec![def_id; predicates.predicates.len()];
 702         let mut head = predicates;
 703         while let Some(parent) = head.parent {
 704             head = self.infcx.tcx.predicates_of(parent);
 705             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 706         }
 707
 708         let predicates = predicates.instantiate(self.infcx.tcx, substs);
 709         debug_assert_eq!(predicates.predicates.len(), origins.len());
 710
 711         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 712             .map(|((pred, span), origin_def_id)| {
 713                 let code = if span.is_dummy() {
 714                     traits::MiscObligation
 715                 } else {
 716                     traits::BindingObligation(origin_def_id, span)
 717                 };
 718                 let cause = self.cause(code);
 719                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 720             })
 721             .filter(|pred| !pred.has_escaping_bound_vars())
 722             .collect()
 723     }
 724
 725     fn from_object_ty(
 726         &mut self,
 727         ty: Ty<'tcx>,
 728         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 729         region: ty::Region<'tcx>,
 730     ) {
 731         // Imagine a type like this:
 732         //
 733         //     trait Foo { }
 734         //     trait Bar<'c> : 'c { }
 735         //
 736         //     &'b (Foo+'c+Bar<'d>)
 737         //         ^
 738         //
 739         // In this case, the following relationships must hold:
 740         //
 741         //     'b <= 'c
 742         //     'd <= 'c
 743         //
 744         // The first conditions is due to the normal region pointer
 745         // rules, which say that a reference cannot outlive its
 746         // referent.
 747         //
 748         // The final condition may be a bit surprising. In particular,
 749         // you may expect that it would have been `'c <= 'd`, since
 750         // usually lifetimes of outer things are conservative
 751         // approximations for inner things. However, it works somewhat
 752         // differently with trait objects: here the idea is that if the
 753         // user specifies a region bound (`'c`, in this case) it is the
 754         // "master bound" that *implies* that bounds from other traits are
 755         // all met. (Remember that *all bounds* in a type like
 756         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 757         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 758         // 'y.)
 759         //
 760         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 761         // am looking forward to the future here.
 762         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 763             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 764
 765             let explicit_bound = region;
 766
 767             self.out.reserve(implicit_bounds.len());
 768             for implicit_bound in implicit_bounds {
 769                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 770                 let outlives =
 771                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 772                 self.out.push(traits::Obligation::with_depth(
 773                     cause,
 774                     self.recursion_depth,
 775                     self.param_env,
 776                     outlives.to_predicate(self.infcx.tcx),
 777                 ));
 778             }
 779         }
 780     }
 781 }
 782
 783 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 784 /// bounds that must hold on the elided self type. These are derived
 785 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 786 /// they declare `trait SomeTrait : 'static`, for example, then
 787 /// `'static` would appear in the list. The hard work is done by
 788 /// `infer::required_region_bounds`, see that for more information.
 789 pub fn object_region_bounds<'tcx>(
 790     tcx: TyCtxt<'tcx>,
 791     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 792 ) -> Vec<ty::Region<'tcx>> {
 793     // Since we don't actually *know* the self type for an object,
 794     // this "open(err)" serves as a kind of dummy standin -- basically
 795     // a placeholder type.
 796     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 797
 798     let predicates = existential_predicates.iter().filter_map(|predicate| {
 799         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 800             None
 801         } else {
 802             Some(predicate.with_self_ty(tcx, open_ty))
 803         }
 804     });
 805
 806     required_region_bounds(tcx, open_ty, predicates)
 807 }