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