compiler/rustc_trait_selection/src/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::opaque_types::required_region_bounds;
   3 use crate::traits;
   4 use rustc_hir as hir;
   5 use rustc_hir::def_id::DefId;
   6 use rustc_hir::lang_items::LangItem;
   7 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
   8 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
   9 use rustc_span::Span;
  10
  11 use std::iter;
  12 /// Returns the set of obligations needed to make `arg` well-formed.
  13 /// If `arg` contains unresolved inference variables, this may include
  14 /// further WF obligations. However, if `arg` IS an unresolved
  15 /// inference variable, returns `None`, because we are not able to
  16 /// make any progress at all. This is to prevent "livelock" where we
  17 /// say "$0 is WF if $0 is WF".
  18 pub fn obligations<'a, 'tcx>(
  19     infcx: &InferCtxt<'a, 'tcx>,
  20     param_env: ty::ParamEnv<'tcx>,
  21     body_id: hir::HirId,
  22     recursion_depth: usize,
  23     arg: GenericArg<'tcx>,
  24     span: Span,
  25 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  26     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  27     let arg = match arg.unpack() {
  28         GenericArgKind::Type(ty) => {
  29             match ty.kind() {
  30                 ty::Infer(ty::TyVar(_)) => {
  31                     let resolved_ty = infcx.shallow_resolve(ty);
  32                     if resolved_ty == ty {
  33                         // No progress, bail out to prevent "livelock".
  34                         return None;
  35                     }
  36
  37                     resolved_ty
  38                 }
  39                 _ => ty,
  40             }
  41             .into()
  42         }
  43         GenericArgKind::Const(ct) => {
  44             match ct.val {
  45                 ty::ConstKind::Infer(infer) => {
  46                     let resolved = infcx.shallow_resolve(infer);
  47                     if resolved == infer {
  48                         // No progress.
  49                         return None;
  50                     }
  51
  52                     infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Infer(resolved), ty: ct.ty })
  53                 }
  54                 _ => ct,
  55             }
  56             .into()
  57         }
  58         // There is nothing we have to do for lifetimes.
  59         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  60     };
  61
  62     let mut wf =
  63         WfPredicates { infcx, param_env, body_id, span, out: vec![], recursion_depth, item: None };
  64     wf.compute(arg);
  65     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  66
  67     let result = wf.normalize();
  68     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  69     Some(result)
  70 }
  71
  72 /// Returns the obligations that make this trait reference
  73 /// well-formed.  For example, if there is a trait `Set` defined like
  74 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  75 /// if `Bar: Eq`.
  76 pub fn trait_obligations<'a, 'tcx>(
  77     infcx: &InferCtxt<'a, 'tcx>,
  78     param_env: ty::ParamEnv<'tcx>,
  79     body_id: hir::HirId,
  80     trait_ref: &ty::TraitRef<'tcx>,
  81     span: Span,
  82     item: Option<&'tcx hir::Item<'tcx>>,
  83 ) -> Vec<traits::PredicateObligation<'tcx>> {
  84     let mut wf =
  85         WfPredicates { infcx, param_env, body_id, span, out: vec![], recursion_depth: 0, item };
  86     wf.compute_trait_ref(trait_ref, Elaborate::All);
  87     debug!(obligations = ?wf.out);
  88     wf.normalize()
  89 }
  90
  91 pub fn predicate_obligations<'a, 'tcx>(
  92     infcx: &InferCtxt<'a, 'tcx>,
  93     param_env: ty::ParamEnv<'tcx>,
  94     body_id: hir::HirId,
  95     predicate: ty::Predicate<'tcx>,
  96     span: Span,
  97 ) -> Vec<traits::PredicateObligation<'tcx>> {
  98     let mut wf = WfPredicates {
  99         infcx,
 100         param_env,
 101         body_id,
 102         span,
 103         out: vec![],
 104         recursion_depth: 0,
 105         item: None,
 106     };
 107
 108     // It's ok to skip the binder here because wf code is prepared for it
 109     match predicate.kind().skip_binder() {
 110         ty::PredicateKind::Trait(t) => {
 111             wf.compute_trait_ref(&t.trait_ref, Elaborate::None);
 112         }
 113         ty::PredicateKind::RegionOutlives(..) => {}
 114         ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty, _reg)) => {
 115             wf.compute(ty.into());
 116         }
 117         ty::PredicateKind::Projection(t) => {
 118             wf.compute_projection(t.projection_ty);
 119             wf.compute(match t.term {
 120                 ty::Term::Ty(ty) => ty.into(),
 121                 ty::Term::Const(c) => c.into(),
 122             })
 123         }
 124         ty::PredicateKind::WellFormed(arg) => {
 125             wf.compute(arg);
 126         }
 127         ty::PredicateKind::ObjectSafe(_) => {}
 128         ty::PredicateKind::ClosureKind(..) => {}
 129         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 130             wf.compute(a.into());
 131             wf.compute(b.into());
 132         }
 133         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 134             wf.compute(a.into());
 135             wf.compute(b.into());
 136         }
 137         ty::PredicateKind::ConstEvaluatable(uv) => {
 138             let obligations = wf.nominal_obligations(uv.def.did, uv.substs);
 139             wf.out.extend(obligations);
 140
 141             for arg in uv.substs.iter() {
 142                 wf.compute(arg);
 143             }
 144         }
 145         ty::PredicateKind::ConstEquate(c1, c2) => {
 146             wf.compute(c1.into());
 147             wf.compute(c2.into());
 148         }
 149         ty::PredicateKind::OpaqueType(opaque, ty) => {
 150             wf.compute(opaque.into());
 151             wf.compute(ty.into());
 152         }
 153         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 154             bug!("TypeWellFormedFromEnv is only used for Chalk")
 155         }
 156     }
 157
 158     wf.normalize()
 159 }
 160
 161 struct WfPredicates<'a, 'tcx> {
 162     infcx: &'a InferCtxt<'a, 'tcx>,
 163     param_env: ty::ParamEnv<'tcx>,
 164     body_id: hir::HirId,
 165     span: Span,
 166     out: Vec<traits::PredicateObligation<'tcx>>,
 167     recursion_depth: usize,
 168     item: Option<&'tcx hir::Item<'tcx>>,
 169 }
 170
 171 /// Controls whether we "elaborate" supertraits and so forth on the WF
 172 /// predicates. This is a kind of hack to address #43784. The
 173 /// underlying problem in that issue was a trait structure like:
 174 ///
 175 /// ```
 176 /// trait Foo: Copy { }
 177 /// trait Bar: Foo { }
 178 /// impl<T: Bar> Foo for T { }
 179 /// impl<T> Bar for T { }
 180 /// ```
 181 ///
 182 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 183 /// we decide that this is true because `T: Bar` is in the
 184 /// where-clauses (and we can elaborate that to include `T:
 185 /// Copy`). This wouldn't be a problem, except that when we check the
 186 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 187 /// impl. And so nowhere did we check that `T: Copy` holds!
 188 ///
 189 /// To resolve this, we elaborate the WF requirements that must be
 190 /// proven when checking impls. This means that (e.g.) the `impl Bar
 191 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 192 /// Copy` (which it won't be able to do, because there is no `Copy`
 193 /// impl for `T`).
 194 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 195 enum Elaborate {
 196     All,
 197     None,
 198 }
 199
 200 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 201     tcx: TyCtxt<'tcx>,
 202     trait_ref: &ty::TraitRef<'tcx>,
 203     item: Option<&hir::Item<'tcx>>,
 204     cause: &mut traits::ObligationCause<'tcx>,
 205     pred: &ty::Predicate<'tcx>,
 206 ) {
 207     debug!(
 208         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 209         trait_ref, item, cause, pred
 210     );
 211     let (items, impl_def_id) = match item {
 212         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), def_id, .. }) => (impl_.items, *def_id),
 213         _ => return,
 214     };
 215     let fix_span =
 216         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 217             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
 218             _ => impl_item_ref.span,
 219         };
 220
 221     // It is fine to skip the binder as we don't care about regions here.
 222     match pred.kind().skip_binder() {
 223         ty::PredicateKind::Projection(proj) => {
 224             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 225             // but rather from a "second order" obligation, where an associated type has a
 226             // projection coming from another associated type. See
 227             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 228             // `traits-assoc-type-in-supertrait-bad.rs`.
 229             if let Some(ty::Projection(projection_ty)) = proj.term.ty().map(|ty| ty.kind()) {
 230                 if let Some(&impl_item_id) =
 231                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.item_def_id)
 232                 {
 233                     if let Some(impl_item_span) = items
 234                         .iter()
 235                         .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 236                         .map(fix_span)
 237                     {
 238                         cause.span = impl_item_span;
 239                     }
 240                 }
 241             }
 242         }
 243         ty::PredicateKind::Trait(pred) => {
 244             // An associated item obligation born out of the `trait` failed to be met. An example
 245             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 246             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 247             if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = *pred.self_ty().kind() {
 248                 if let Some(&impl_item_id) =
 249                     tcx.impl_item_implementor_ids(impl_def_id).get(&item_def_id)
 250                 {
 251                     if let Some(impl_item_span) = items
 252                         .iter()
 253                         .find(|item| item.id.def_id.to_def_id() == impl_item_id)
 254                         .map(fix_span)
 255                     {
 256                         cause.span = impl_item_span;
 257                     }
 258                 }
 259             }
 260         }
 261         _ => {}
 262     }
 263 }
 264
 265 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 266     fn tcx(&self) -> TyCtxt<'tcx> {
 267         self.infcx.tcx
 268     }
 269
 270     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 271         traits::ObligationCause::new(self.span, self.body_id, code)
 272     }
 273
 274     fn normalize(mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 275         let cause = self.cause(traits::MiscObligation);
 276         let infcx = &mut self.infcx;
 277         let param_env = self.param_env;
 278         let mut obligations = Vec::with_capacity(self.out.len());
 279         for mut obligation in self.out {
 280             assert!(!obligation.has_escaping_bound_vars());
 281             let mut selcx = traits::SelectionContext::new(infcx);
 282             // Don't normalize the whole obligation, the param env is either
 283             // already normalized, or we're currently normalizing the
 284             // param_env. Either way we should only normalize the predicate.
 285             let normalized_predicate = traits::project::normalize_with_depth_to(
 286                 &mut selcx,
 287                 param_env,
 288                 cause.clone(),
 289                 self.recursion_depth,
 290                 obligation.predicate,
 291                 &mut obligations,
 292             );
 293             obligation.predicate = normalized_predicate;
 294             obligations.push(obligation);
 295         }
 296         obligations
 297     }
 298
 299     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 300     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 301         let tcx = self.infcx.tcx;
 302         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 303
 304         debug!("compute_trait_ref obligations {:?}", obligations);
 305         let cause = self.cause(traits::MiscObligation);
 306         let param_env = self.param_env;
 307         let depth = self.recursion_depth;
 308
 309         let item = self.item;
 310
 311         let extend = |obligation: traits::PredicateObligation<'tcx>| {
 312             let mut cause = cause.clone();
 313             if let Some(parent_trait_pred) = obligation.predicate.to_opt_poly_trait_pred() {
 314                 let derived_cause = traits::DerivedObligationCause {
 315                     parent_trait_pred,
 316                     parent_code: obligation.cause.clone_code(),
 317                 };
 318                 *cause.make_mut_code() =
 319                     traits::ObligationCauseCode::DerivedObligation(derived_cause);
 320             }
 321             extend_cause_with_original_assoc_item_obligation(
 322                 tcx,
 323                 trait_ref,
 324                 item,
 325                 &mut cause,
 326                 &obligation.predicate,
 327             );
 328             traits::Obligation::with_depth(cause, depth, param_env, obligation.predicate)
 329         };
 330
 331         if let Elaborate::All = elaborate {
 332             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 333             let implied_obligations = implied_obligations.map(extend);
 334             self.out.extend(implied_obligations);
 335         } else {
 336             self.out.extend(obligations);
 337         }
 338
 339         let tcx = self.tcx();
 340         self.out.extend(
 341             trait_ref
 342                 .substs
 343                 .iter()
 344                 .enumerate()
 345                 .filter(|(_, arg)| {
 346                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 347                 })
 348                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 349                 .map(|(i, arg)| {
 350                     let mut new_cause = cause.clone();
 351                     // The first subst is the self ty - use the correct span for it.
 352                     if i == 0 {
 353                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 354                             item.map(|i| &i.kind)
 355                         {
 356                             new_cause.span = self_ty.span;
 357                         }
 358                     }
 359                     traits::Obligation::with_depth(
 360                         new_cause,
 361                         depth,
 362                         param_env,
 363                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 364                     )
 365                 }),
 366         );
 367     }
 368
 369     /// Pushes the obligations required for `trait_ref::Item` to be WF
 370     /// into `self.out`.
 371     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 372         // A projection is well-formed if
 373         //
 374         // (a) its predicates hold (*)
 375         // (b) its substs are wf
 376         //
 377         // (*) The predicates of an associated type include the predicates of
 378         //     the trait that it's contained in. For example, given
 379         //
 380         // trait A<T>: Clone {
 381         //     type X where T: Copy;
 382         // }
 383         //
 384         // The predicates of `<() as A<i32>>::X` are:
 385         // [
 386         //     `(): Sized`
 387         //     `(): Clone`
 388         //     `(): A<i32>`
 389         //     `i32: Sized`
 390         //     `i32: Clone`
 391         //     `i32: Copy`
 392         // ]
 393         let obligations = self.nominal_obligations(data.item_def_id, data.substs);
 394         self.out.extend(obligations);
 395
 396         let tcx = self.tcx();
 397         let cause = self.cause(traits::MiscObligation);
 398         let param_env = self.param_env;
 399         let depth = self.recursion_depth;
 400
 401         self.out.extend(
 402             data.substs
 403                 .iter()
 404                 .filter(|arg| {
 405                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 406                 })
 407                 .filter(|arg| !arg.has_escaping_bound_vars())
 408                 .map(|arg| {
 409                     traits::Obligation::with_depth(
 410                         cause.clone(),
 411                         depth,
 412                         param_env,
 413                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(tcx),
 414                     )
 415                 }),
 416         );
 417     }
 418
 419     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 420         if !subty.has_escaping_bound_vars() {
 421             let cause = self.cause(cause);
 422             let trait_ref = ty::TraitRef {
 423                 def_id: self.infcx.tcx.require_lang_item(LangItem::Sized, None),
 424                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 425             };
 426             self.out.push(traits::Obligation::with_depth(
 427                 cause,
 428                 self.recursion_depth,
 429                 self.param_env,
 430                 ty::Binder::dummy(trait_ref).without_const().to_predicate(self.infcx.tcx),
 431             ));
 432         }
 433     }
 434
 435     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 436     fn compute(&mut self, arg: GenericArg<'tcx>) {
 437         let mut walker = arg.walk();
 438         let param_env = self.param_env;
 439         let depth = self.recursion_depth;
 440         while let Some(arg) = walker.next() {
 441             let ty = match arg.unpack() {
 442                 GenericArgKind::Type(ty) => ty,
 443
 444                 // No WF constraints for lifetimes being present, any outlives
 445                 // obligations are handled by the parent (e.g. `ty::Ref`).
 446                 GenericArgKind::Lifetime(_) => continue,
 447
 448                 GenericArgKind::Const(constant) => {
 449                     match constant.val {
 450                         ty::ConstKind::Unevaluated(uv) => {
 451                             let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 452                             self.out.extend(obligations);
 453
 454                             let predicate =
 455                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(uv.shrink()))
 456                                     .to_predicate(self.tcx());
 457                             let cause = self.cause(traits::MiscObligation);
 458                             self.out.push(traits::Obligation::with_depth(
 459                                 cause,
 460                                 self.recursion_depth,
 461                                 self.param_env,
 462                                 predicate,
 463                             ));
 464                         }
 465                         ty::ConstKind::Infer(infer) => {
 466                             let resolved = self.infcx.shallow_resolve(infer);
 467                             // the `InferConst` changed, meaning that we made progress.
 468                             if resolved != infer {
 469                                 let cause = self.cause(traits::MiscObligation);
 470
 471                                 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
 472                                     val: ty::ConstKind::Infer(resolved),
 473                                     ..*constant
 474                                 });
 475                                 self.out.push(traits::Obligation::with_depth(
 476                                     cause,
 477                                     self.recursion_depth,
 478                                     self.param_env,
 479                                     ty::Binder::dummy(ty::PredicateKind::WellFormed(
 480                                         resolved_constant.into(),
 481                                     ))
 482                                     .to_predicate(self.tcx()),
 483                                 ));
 484                             }
 485                         }
 486                         ty::ConstKind::Error(_)
 487                         | ty::ConstKind::Param(_)
 488                         | ty::ConstKind::Bound(..)
 489                         | ty::ConstKind::Placeholder(..) => {
 490                             // These variants are trivially WF, so nothing to do here.
 491                         }
 492                         ty::ConstKind::Value(..) => {
 493                             // FIXME: Enforce that values are structurally-matchable.
 494                         }
 495                     }
 496                     continue;
 497                 }
 498             };
 499
 500             match *ty.kind() {
 501                 ty::Bool
 502                 | ty::Char
 503                 | ty::Int(..)
 504                 | ty::Uint(..)
 505                 | ty::Float(..)
 506                 | ty::Error(_)
 507                 | ty::Str
 508                 | ty::GeneratorWitness(..)
 509                 | ty::Never
 510                 | ty::Param(_)
 511                 | ty::Bound(..)
 512                 | ty::Placeholder(..)
 513                 | ty::Foreign(..) => {
 514                     // WfScalar, WfParameter, etc
 515                 }
 516
 517                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 518                 ty::Infer(ty::IntVar(_)) => {}
 519
 520                 // Can only infer to `ty::Float(_)`.
 521                 ty::Infer(ty::FloatVar(_)) => {}
 522
 523                 ty::Slice(subty) => {
 524                     self.require_sized(subty, traits::SliceOrArrayElem);
 525                 }
 526
 527                 ty::Array(subty, _) => {
 528                     self.require_sized(subty, traits::SliceOrArrayElem);
 529                     // Note that we handle the len is implicitly checked while walking `arg`.
 530                 }
 531
 532                 ty::Tuple(ref tys) => {
 533                     if let Some((_last, rest)) = tys.split_last() {
 534                         for elem in rest {
 535                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 536                         }
 537                     }
 538                 }
 539
 540                 ty::RawPtr(_) => {
 541                     // Simple cases that are WF if their type args are WF.
 542                 }
 543
 544                 ty::Projection(data) => {
 545                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 546                     self.compute_projection(data);
 547                 }
 548
 549                 ty::Adt(def, substs) => {
 550                     // WfNominalType
 551                     let obligations = self.nominal_obligations(def.did, substs);
 552                     self.out.extend(obligations);
 553                 }
 554
 555                 ty::FnDef(did, substs) => {
 556                     let obligations = self.nominal_obligations(did, substs);
 557                     self.out.extend(obligations);
 558                 }
 559
 560                 ty::Ref(r, rty, _) => {
 561                     // WfReference
 562                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 563                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 564                         self.out.push(traits::Obligation::with_depth(
 565                             cause,
 566                             depth,
 567                             param_env,
 568                             ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
 569                                 ty::OutlivesPredicate(rty, r),
 570                             ))
 571                             .to_predicate(self.tcx()),
 572                         ));
 573                     }
 574                 }
 575
 576                 ty::Generator(..) => {
 577                     // Walk ALL the types in the generator: this will
 578                     // include the upvar types as well as the yield
 579                     // type. Note that this is mildly distinct from
 580                     // the closure case, where we have to be careful
 581                     // about the signature of the closure. We don't
 582                     // have the problem of implied bounds here since
 583                     // generators don't take arguments.
 584                 }
 585
 586                 ty::Closure(_, substs) => {
 587                     // Only check the upvar types for WF, not the rest
 588                     // of the types within. This is needed because we
 589                     // capture the signature and it may not be WF
 590                     // without the implied bounds. Consider a closure
 591                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 592                     // not known to hold in the creator's context (and
 593                     // indeed the closure may not be invoked by its
 594                     // creator, but rather turned to someone who *can*
 595                     // verify that).
 596                     //
 597                     // The special treatment of closures here really
 598                     // ought not to be necessary either; the problem
 599                     // is related to #25860 -- there is no way for us
 600                     // to express a fn type complete with the implied
 601                     // bounds that it is assuming. I think in reality
 602                     // the WF rules around fn are a bit messed up, and
 603                     // that is the rot problem: `fn(&'a T)` should
 604                     // probably always be WF, because it should be
 605                     // shorthand for something like `where(T: 'a) {
 606                     // fn(&'a T) }`, as discussed in #25860.
 607                     //
 608                     // Note that we are also skipping the generic
 609                     // types. This is consistent with the `outlives`
 610                     // code, but anyway doesn't matter: within the fn
 611                     // body where they are created, the generics will
 612                     // always be WF, and outside of that fn body we
 613                     // are not directly inspecting closure types
 614                     // anyway, except via auto trait matching (which
 615                     // only inspects the upvar types).
 616                     walker.skip_current_subtree(); // subtree handled below
 617                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 618                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 619                 }
 620
 621                 ty::FnPtr(_) => {
 622                     // let the loop iterate into the argument/return
 623                     // types appearing in the fn signature
 624                 }
 625
 626                 ty::Opaque(did, substs) => {
 627                     // all of the requirements on type parameters
 628                     // should've been checked by the instantiation
 629                     // of whatever returned this exact `impl Trait`.
 630
 631                     // for named opaque `impl Trait` types we still need to check them
 632                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 633                         let obligations = self.nominal_obligations(did, substs);
 634                         self.out.extend(obligations);
 635                     }
 636                 }
 637
 638                 ty::Dynamic(data, r) => {
 639                     // WfObject
 640                     //
 641                     // Here, we defer WF checking due to higher-ranked
 642                     // regions. This is perhaps not ideal.
 643                     self.from_object_ty(ty, data, r);
 644
 645                     // FIXME(#27579) RFC also considers adding trait
 646                     // obligations that don't refer to Self and
 647                     // checking those
 648
 649                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 650
 651                     if !defer_to_coercion {
 652                         let cause = self.cause(traits::MiscObligation);
 653                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 654                         let tcx = self.tcx();
 655                         self.out.extend(component_traits.map(|did| {
 656                             traits::Obligation::with_depth(
 657                                 cause.clone(),
 658                                 depth,
 659                                 param_env,
 660                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did))
 661                                     .to_predicate(tcx),
 662                             )
 663                         }));
 664                     }
 665                 }
 666
 667                 // Inference variables are the complicated case, since we don't
 668                 // know what type they are. We do two things:
 669                 //
 670                 // 1. Check if they have been resolved, and if so proceed with
 671                 //    THAT type.
 672                 // 2. If not, we've at least simplified things (e.g., we went
 673                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 674                 //    register a pending obligation and keep
 675                 //    moving. (Goal is that an "inductive hypothesis"
 676                 //    is satisfied to ensure termination.)
 677                 // See also the comment on `fn obligations`, describing "livelock"
 678                 // prevention, which happens before this can be reached.
 679                 ty::Infer(_) => {
 680                     let ty = self.infcx.shallow_resolve(ty);
 681                     if let ty::Infer(ty::TyVar(_)) = ty.kind() {
 682                         // Not yet resolved, but we've made progress.
 683                         let cause = self.cause(traits::MiscObligation);
 684                         self.out.push(traits::Obligation::with_depth(
 685                             cause,
 686                             self.recursion_depth,
 687                             param_env,
 688                             ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into()))
 689                                 .to_predicate(self.tcx()),
 690                         ));
 691                     } else {
 692                         // Yes, resolved, proceed with the result.
 693                         // FIXME(eddyb) add the type to `walker` instead of recursing.
 694                         self.compute(ty.into());
 695                     }
 696                 }
 697             }
 698         }
 699     }
 700
 701     fn nominal_obligations(
 702         &mut self,
 703         def_id: DefId,
 704         substs: SubstsRef<'tcx>,
 705     ) -> Vec<traits::PredicateObligation<'tcx>> {
 706         let predicates = self.infcx.tcx.predicates_of(def_id);
 707         let mut origins = vec![def_id; predicates.predicates.len()];
 708         let mut head = predicates;
 709         while let Some(parent) = head.parent {
 710             head = self.infcx.tcx.predicates_of(parent);
 711             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 712         }
 713
 714         let predicates = predicates.instantiate(self.infcx.tcx, substs);
 715         debug_assert_eq!(predicates.predicates.len(), origins.len());
 716
 717         iter::zip(iter::zip(predicates.predicates, predicates.spans), origins.into_iter().rev())
 718             .map(|((pred, span), origin_def_id)| {
 719                 let code = if span.is_dummy() {
 720                     traits::MiscObligation
 721                 } else {
 722                     traits::BindingObligation(origin_def_id, span)
 723                 };
 724                 let cause = self.cause(code);
 725                 traits::Obligation::with_depth(cause, self.recursion_depth, self.param_env, pred)
 726             })
 727             .filter(|pred| !pred.has_escaping_bound_vars())
 728             .collect()
 729     }
 730
 731     fn from_object_ty(
 732         &mut self,
 733         ty: Ty<'tcx>,
 734         data: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 735         region: ty::Region<'tcx>,
 736     ) {
 737         // Imagine a type like this:
 738         //
 739         //     trait Foo { }
 740         //     trait Bar<'c> : 'c { }
 741         //
 742         //     &'b (Foo+'c+Bar<'d>)
 743         //         ^
 744         //
 745         // In this case, the following relationships must hold:
 746         //
 747         //     'b <= 'c
 748         //     'd <= 'c
 749         //
 750         // The first conditions is due to the normal region pointer
 751         // rules, which say that a reference cannot outlive its
 752         // referent.
 753         //
 754         // The final condition may be a bit surprising. In particular,
 755         // you may expect that it would have been `'c <= 'd`, since
 756         // usually lifetimes of outer things are conservative
 757         // approximations for inner things. However, it works somewhat
 758         // differently with trait objects: here the idea is that if the
 759         // user specifies a region bound (`'c`, in this case) it is the
 760         // "master bound" that *implies* that bounds from other traits are
 761         // all met. (Remember that *all bounds* in a type like
 762         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 763         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 764         // 'y.)
 765         //
 766         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 767         // am looking forward to the future here.
 768         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 769             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 770
 771             let explicit_bound = region;
 772
 773             self.out.reserve(implicit_bounds.len());
 774             for implicit_bound in implicit_bounds {
 775                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 776                 let outlives =
 777                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 778                 self.out.push(traits::Obligation::with_depth(
 779                     cause,
 780                     self.recursion_depth,
 781                     self.param_env,
 782                     outlives.to_predicate(self.infcx.tcx),
 783                 ));
 784             }
 785         }
 786     }
 787 }
 788
 789 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 790 /// bounds that must hold on the elided self type. These are derived
 791 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 792 /// they declare `trait SomeTrait : 'static`, for example, then
 793 /// `'static` would appear in the list. The hard work is done by
 794 /// `infer::required_region_bounds`, see that for more information.
 795 pub fn object_region_bounds<'tcx>(
 796     tcx: TyCtxt<'tcx>,
 797     existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
 798 ) -> Vec<ty::Region<'tcx>> {
 799     // Since we don't actually *know* the self type for an object,
 800     // this "open(err)" serves as a kind of dummy standin -- basically
 801     // a placeholder type.
 802     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 803
 804     let predicates = existential_predicates.iter().filter_map(|predicate| {
 805         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 806             None
 807         } else {
 808             Some(predicate.with_self_ty(tcx, open_ty))
 809         }
 810     });
 811
 812     required_region_bounds(tcx, open_ty, predicates)
 813 }