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