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 /// ```
 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                 if let Some(&impl_item_id) =
 229                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 230                 {
 231                     if let Some(impl_item_span) = items
 232                         .iter()
 233                         .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 234                         .map(fix_span)
 235                     {
 236                         cause.span = impl_item_span;
 237                     }
 238                 }
 239             }
 240         }
 241         ty::PredicateKind::Trait(pred) => {
 242             // An associated item obligation born out of the `trait` failed to be met. An example
 243             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 244             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 245             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind() {
 246                 if let Some(&impl_item_id) =
 247                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 248                 {
 249                     if let Some(impl_item_span) = items
 250                         .iter()
 251                         .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 252                         .map(fix_span)
 253                     {
 254                         cause.span = impl_item_span;
 255                     }
 256                 }
 257             }
 258         }
 259         _ => {}
 260     }
 261 }
 262
 263 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 264     fn tcx(&self) -> TyCtxt<'tcx> {
 265         self.infcx.tcx
 266     }
 267
 268     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 269         traits::ObligationCause::new(self.span, self.body_id, code)
 270     }
 271
 272     fn normalize(mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 273         let cause = self.cause(traits::MiscObligation);
 274         let infcx = &mut self.infcx;
 275         let param_env = self.param_env;
 276         let mut obligations = Vec::with_capacity(self.out.len());
 277         for mut obligation in self.out {
 278             assert!(!obligation.has_escaping_bound_vars());
 279             let mut selcx = traits::SelectionContext::new(infcx);
 280             // Don't normalize the whole obligation, the param env is either
 281             // already normalized, or we're currently normalizing the
 282             // param_env. Either way we should only normalize the predicate.
 283             let normalized_predicate = traits::project::normalize_with_depth_to(
 284                 &mut selcx,
 285                 param_env,
 286                 cause.clone(),
 287                 self.recursion_depth,
 288                 obligation.predicate,
 289                 &mut obligations,
 290             );
 291             obligation.predicate = normalized_predicate;
 292             obligations.push(obligation);
 293         }
 294         obligations
 295     }
 296
 297     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 298     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 299         let tcx = self.infcx.tcx;
 300         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 301
 302         debug!("compute_trait_ref obligations {:?}", obligations);
 303         let cause = self.cause(traits::MiscObligation);
 304         let param_env = self.param_env;
 305         let depth = self.recursion_depth;
 306
 307         let item = self.item;
 308
 309         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 310             let mut cause = cause.clone();
 311             if let Some(parent_trait_pred) = obligation.predicate.to_opt_poly_trait_pred() {
 312                 let derived_cause = traits::DerivedObligationCause {
 313                     parent_trait_pred,
 314                     parent_code: obligation.cause.clone_code(),
 315                 };
 316                 *cause.make_mut_code() =
 317                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 318             }
 319             extend_cause_with_original_assoc_item_obligation(
 320                 tcx,
 321                 trait_ref,
 322                 item,
 323                 &mut cause,
 324                 obligation.predicate,
 325             );
 326             traits::Obligation::with_depth(cause, depth, param_env, obligation.predicate)
 327         };
 328
 329         if let Elaborate::All = elaborate {
 330             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 331             let implied_obligations = implied_obligations.map(extend);
 332             self.out.extend(implied_obligations);
 333         } else {
 334             self.out.extend(obligations);
 335         }
 336
 337         let tcx = self.tcx();
 338         self.out.extend(
 339             trait_ref
 340                 .substs
 341                 .iter()
 342                 .enumerate()
 343                 .filter(|(_, arg)| {
 344                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 345                 })
 346                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 347                 .map(|(i, arg)| {
 348                     let mut new_cause = cause.clone();
 349                     // The first subst is the self ty - use the correct span for it.
 350                     if i == 0 {
 351                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 352                             item.map(|i| &i.kind)
 353                         {
 354                             new_cause.span = self_ty.span;
 355                         }
 356                     }
 357                     traits::Obligation::with_depth(
 358                         new_cause,
 359                         depth,
 360                         param_env,
 361                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 362                     )
 363                 }),
 364         );
 365     }
 366
 367     /// Pushes the obligations required for `trait_ref::Item` to be WF
 368     /// into `self.out`.
 369     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 370         // A projection is well-formed if
 371         //
 372         // (a) its predicates hold (*)
 373         // (b) its substs are wf
 374         //
 375         // (*) The predicates of an associated type include the predicates of
 376         //     the trait that it's contained in. For example, given
 377         //
 378         // trait A<T>: Clone {
 379         //     type X where T: Copy;
 380         // }
 381         //
 382         // The predicates of `<() as A<i32>>::X` are:
 383         // [
 384         //     `(): Sized`
 385         //     `(): Clone`
 386         //     `(): A<i32>`
 387         //     `i32: Sized`
 388         //     `i32: Clone`
 389         //     `i32: Copy`
 390         // ]
 391         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 392         self.out.extend(obligations);
 393
 394         let tcx = self.tcx();
 395         let cause = self.cause(traits::MiscObligation);
 396         let param_env = self.param_env;
 397         let depth = self.recursion_depth;
 398
 399         self.out.extend(
 400             data.substs
 401                 .iter()
 402                 .filter(|arg| {
 403                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 404                 })
 405                 .filter(|arg| !arg.has_escaping_bound_vars())
 406                 .map(|arg| {
 407                     traits::Obligation::with_depth(
 408                         cause.clone(),
 409                         depth,
 410                         param_env,
 411                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 412                     )
 413                 }),
 414         );
 415     }
 416
 417     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 418         if !subty.has_escaping_bound_vars() {
 419             let cause = self.cause(cause);
 420             let trait_ref = ty::TraitRef {
 421                 def_id: self.infcx.tcx.require_lang_item(LangItem::Sized, None),
 422                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 423             };
 424             self.out.push(traits::Obligation::with_depth(
 425                 cause,
 426                 self.recursion_depth,
 427                 self.param_env,
 428                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.infcx.tcx),
 429             ));
 430         }
 431     }
 432
 433     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 434     fn compute(&mut self, arg: GenericArg<'tcx>) {
 435         let mut walker = arg.walk();
 436         let param_env = self.param_env;
 437         let depth = self.recursion_depth;
 438         while let Some(arg) = walker.next() {
 439             let ty = match arg.unpack() {
 440                 GenericArgKind::Type(ty) => ty,
 441
 442                 // No WF constraints for lifetimes being present, any outlives
 443                 // obligations are handled by the parent (e.g. `ty::Ref`).
 444                 GenericArgKind::Lifetime(_) => continue,
 445
 446                 GenericArgKind::Const(constant) => {
 447                     match constant.val() {
 448                         ty::ConstKind::Unevaluated(uv) => {
 449                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 450                             self.out.extend(obligations);
 451
 452                             let predicate =
 453                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 454                                     .to_predicate(self.tcx());
 455                             let cause = self.cause(traits::MiscObligation);
 456                             self.out.push(traits::Obligation::with_depth(
 457                                 cause,
 458                                 self.recursion_depth,
 459                                 self.param_env,
 460                                 predicate,
 461                             ));
 462                         }
 463                         ty::ConstKind::Infer(infer) => {
 464                             let resolved = self.infcx.shallow_resolve(infer);
 465                             // the `InferConst` changed, meaning that we made progress.
 466                             if resolved != infer {
 467                                 let cause = self.cause(traits::MiscObligation);
 468
 469                                 let resolved_constant = self.infcx.tcx.mk_const(ty::ConstS {
 470                                     val: ty::ConstKind::Infer(resolved),
 471                                     ty: constant.ty(),
 472                                 });
 473                                 self.out.push(traits::Obligation::with_depth(
 474                                     cause,
 475                                     self.recursion_depth,
 476                                     self.param_env,
 477                                     ty::Binder::dummy(ty::PredicateKind::WellFormed(
 478                                         resolved_constant.into(),
 479                                     ))
 480                                     .to_predicate(self.tcx()),
 481                                 ));
 482                             }
 483                         }
 484                         ty::ConstKind::Error(_)
 485                         | ty::ConstKind::Param(_)
 486                         | ty::ConstKind::Bound(..)
 487                         | ty::ConstKind::Placeholder(..) => {
 488                             // These variants are trivially WF, so nothing to do here.
 489                         }
 490                         ty::ConstKind::Value(..) => {
 491                             // FIXME: Enforce that values are structurally-matchable.
 492                         }
 493                     }
 494                     continue;
 495                 }
 496             };
 497
 498             match *ty.kind() {
 499                 ty::Bool
 500                 | ty::Char
 501                 | ty::Int(..)
 502                 | ty::Uint(..)
 503                 | ty::Float(..)
 504                 | ty::Error(_)
 505                 | ty::Str
 506                 | ty::GeneratorWitness(..)
 507                 | ty::Never
 508                 | ty::Param(_)
 509                 | ty::Bound(..)
 510                 | ty::Placeholder(..)
 511                 | ty::Foreign(..) => {
 512                     // WfScalar, WfParameter, etc
 513                 }
 514
 515                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 516                 ty::Infer(ty::IntVar(_)) => {}
 517
 518                 // Can only infer to `ty::Float(_)`.
 519                 ty::Infer(ty::FloatVar(_)) => {}
 520
 521                 ty::Slice(subty) => {
 522                     self.require_sized(subty, traits::SliceOrArrayElem);
 523                 }
 524
 525                 ty::Array(subty, _) => {
 526                     self.require_sized(subty, traits::SliceOrArrayElem);
 527                     // Note that we handle the len is implicitly checked while walking `arg`.
 528                 }
 529
 530                 ty::Tuple(ref tys) => {
 531                     if let Some((_last, rest)) = tys.split_last() {
 532                         for elem in rest {
 533                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 534                         }
 535                     }
 536                 }
 537
 538                 ty::RawPtr(_) => {
 539                     // Simple cases that are WF if their type args are WF.
 540                 }
 541
 542                 ty::Projection(data) => {
 543                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 544                     self.compute_projection(data);
 545                 }
 546
 547                 ty::Adt(def, substs) => {
 548                     // WfNominalType
 549                     let obligations = self.nominal_obligations(def.did, substs);
 550                     self.out.extend(obligations);
 551                 }
 552
 553                 ty::FnDef(did, substs) => {
 554                     let obligations = self.nominal_obligations(did, substs);
 555                     self.out.extend(obligations);
 556                 }
 557
 558                 ty::Ref(r, rty, _) => {
 559                     // WfReference
 560                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 561                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 562                         self.out.push(traits::Obligation::with_depth(
 563                             cause,
 564                             depth,
 565                             param_env,
 566                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 567                                 ty::OutlivesPredicate(rty, r),
 568                             ))
 569                             .to_predicate(self.tcx()),
 570                         ));
 571                     }
 572                 }
 573
 574                 ty::Generator(..) => {
 575                     // Walk ALL the types in the generator: this will
 576                     // include the upvar types as well as the yield
 577                     // type. Note that this is mildly distinct from
 578                     // the closure case, where we have to be careful
 579                     // about the signature of the closure. We don't
 580                     // have the problem of implied bounds here since
 581                     // generators don't take arguments.
 582                 }
 583
 584                 ty::Closure(_, substs) => {
 585                     // Only check the upvar types for WF, not the rest
 586                     // of the types within. This is needed because we
 587                     // capture the signature and it may not be WF
 588                     // without the implied bounds. Consider a closure
 589                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 590                     // not known to hold in the creator's context (and
 591                     // indeed the closure may not be invoked by its
 592                     // creator, but rather turned to someone who *can*
 593                     // verify that).
 594                     //
 595                     // The special treatment of closures here really
 596                     // ought not to be necessary either; the problem
 597                     // is related to #25860 -- there is no way for us
 598                     // to express a fn type complete with the implied
 599                     // bounds that it is assuming. I think in reality
 600                     // the WF rules around fn are a bit messed up, and
 601                     // that is the rot problem: `fn(&'a T)` should
 602                     // probably always be WF, because it should be
 603                     // shorthand for something like `where(T: 'a) {
 604                     // fn(&'a T) }`, as discussed in #25860.
 605                     //
 606                     // Note that we are also skipping the generic
 607                     // types. This is consistent with the `outlives`
 608                     // code, but anyway doesn't matter: within the fn
 609                     // body where they are created, the generics will
 610                     // always be WF, and outside of that fn body we
 611                     // are not directly inspecting closure types
 612                     // anyway, except via auto trait matching (which
 613                     // only inspects the upvar types).
 614                     walker.skip_current_subtree(); // subtree handled below
 615                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 616                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 617                 }
 618
 619                 ty::FnPtr(_) => {
 620                     // let the loop iterate into the argument/return
 621                     // types appearing in the fn signature
 622                 }
 623
 624                 ty::Opaque(did, substs) => {
 625                     // all of the requirements on type parameters
 626                     // should've been checked by the instantiation
 627                     // of whatever returned this exact `impl Trait`.
 628
 629                     // for named opaque `impl Trait` types we still need to check them
 630                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 631                         let obligations = self.nominal_obligations(did, substs);
 632                         self.out.extend(obligations);
 633                     }
 634                 }
 635
 636                 ty::Dynamic(data, r) => {
 637                     // WfObject
 638                     //
 639                     // Here, we defer WF checking due to higher-ranked
 640                     // regions. This is perhaps not ideal.
 641                     self.from_object_ty(ty, data, r);
 642
 643                     // FIXME(#27579) RFC also considers adding trait
 644                     // obligations that don't refer to Self and
 645                     // checking those
 646
 647                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 648
 649                     if !defer_to_coercion {
 650                         let cause = self.cause(traits::MiscObligation);
 651                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 652                         let tcx = self.tcx();
 653                         self.out.extend(component_traits.map(|did| {
 654                             traits::Obligation::with_depth(
 655                                 cause.clone(),
 656                                 depth,
 657                                 param_env,
 658                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 659                                     .to_predicate(tcx),
 660                             )
 661                         }));
 662                     }
 663                 }
 664
 665                 // Inference variables are the complicated case, since we don't
 666                 // know what type they are. We do two things:
 667                 //
 668                 // 1. Check if they have been resolved, and if so proceed with
 669                 //    THAT type.
 670                 // 2. If not, we've at least simplified things (e.g., we went
 671                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 672                 //    register a pending obligation and keep
 673                 //    moving. (Goal is that an "inductive hypothesis"
 674                 //    is satisfied to ensure termination.)
 675                 // See also the comment on `fn obligations`, describing "livelock"
 676                 // prevention, which happens before this can be reached.
 677                 ty::Infer(_) => {
 678                     let ty = self.infcx.shallow_resolve(ty);
 679                     if let ty::Infer(ty::TyVar(_)) = ty.kind() {
 680                         // Not yet resolved, but we've made progress.
 681                         let cause = self.cause(traits::MiscObligation);
 682                         self.out.push(traits::Obligation::with_depth(
 683                             cause,
 684                             self.recursion_depth,
 685                             param_env,
 686                             ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 687                                 .to_predicate(self.tcx()),
 688                         ));
 689                     } else {
 690                         // Yes, resolved, proceed with the result.
 691                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 692                         self.compute(ty.into());
 693                     }
 694                 }
 695             }
 696         }
 697     }
 698
 699     fn nominal_obligations(
 700         &mut self,
 701         def_id: DefId,
 702         substs: SubstsRef<'tcx>,
 703     ) -> Vec<traits::PredicateObligation<'tcx>> {
 704         let predicates = self.infcx.tcx.predicates_of(def_id);
 705         let mut origins = vec![def_id; predicates.predicates.len()];
 706         let mut head = predicates;
 707         while let Some(parent) = head.parent {
 708             head = self.infcx.tcx.predicates_of(parent);
 709             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 710         }
 711
 712         let predicates = predicates.instantiate(self.infcx.tcx, substs);
 713         debug_assert_eq!(predicates.predicates.len(), origins.len());
 714
 715         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 716             .map(|((pred, span), origin_def_id)| {
 717                 let code = if span.is_dummy() {
 718                     traits::MiscObligation
 719                 } else {
 720                     traits::BindingObligation(origin_def_id, span)
 721                 };
 722                 let cause = self.cause(code);
 723                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 724             })
 725             .filter(|pred| !pred.has_escaping_bound_vars())
 726             .collect()
 727     }
 728
 729     fn from_object_ty(
 730         &mut self,
 731         ty: Ty<'tcx>,
 732         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 733         region: ty::Region<'tcx>,
 734     ) {
 735         // Imagine a type like this:
 736         //
 737         //     trait Foo { }
 738         //     trait Bar<'c> : 'c { }
 739         //
 740         //     &'b (Foo+'c+Bar<'d>)
 741         //         ^
 742         //
 743         // In this case, the following relationships must hold:
 744         //
 745         //     'b <= 'c
 746         //     'd <= 'c
 747         //
 748         // The first conditions is due to the normal region pointer
 749         // rules, which say that a reference cannot outlive its
 750         // referent.
 751         //
 752         // The final condition may be a bit surprising. In particular,
 753         // you may expect that it would have been `'c <= 'd`, since
 754         // usually lifetimes of outer things are conservative
 755         // approximations for inner things. However, it works somewhat
 756         // differently with trait objects: here the idea is that if the
 757         // user specifies a region bound (`'c`, in this case) it is the
 758         // "master bound" that *implies* that bounds from other traits are
 759         // all met. (Remember that *all bounds* in a type like
 760         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 761         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 762         // 'y.)
 763         //
 764         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 765         // am looking forward to the future here.
 766         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 767             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 768
 769             let explicit_bound = region;
 770
 771             self.out.reserve(implicit_bounds.len());
 772             for implicit_bound in implicit_bounds {
 773                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 774                 let outlives =
 775                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 776                 self.out.push(traits::Obligation::with_depth(
 777                     cause,
 778                     self.recursion_depth,
 779                     self.param_env,
 780                     outlives.to_predicate(self.infcx.tcx),
 781                 ));
 782             }
 783         }
 784     }
 785 }
 786
 787 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 788 /// bounds that must hold on the elided self type. These are derived
 789 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 790 /// they declare `trait SomeTrait : 'static`, for example, then
 791 /// `'static` would appear in the list. The hard work is done by
 792 /// `infer::required_region_bounds`, see that for more information.
 793 pub fn object_region_bounds<'tcx>(
 794     tcx: TyCtxt<'tcx>,
 795     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 796 ) -> Vec<ty::Region<'tcx>> {
 797     // Since we don't actually *know* the self type for an object,
 798     // this "open(err)" serves as a kind of dummy standin -- basically
 799     // a placeholder type.
 800     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 801
 802     let predicates = existential_predicates.iter().filter_map(|predicate| {
 803         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 804             None
 805         } else {
 806             Some(predicate.with_self_ty(tcx, open_ty))
 807         }
 808     });
 809
 810     required_region_bounds(tcx, open_ty, predicates)
 811 }