compiler/rustc_trait_selection/src/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::traits;
   3 use rustc_hir as hir;
   4 use rustc_hir::lang_items::LangItem;
   5 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
   6 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
   7 use rustc_span::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
   8 use rustc_span::{Span, DUMMY_SP};
   9
  10 use std::iter;
  11 /// Returns the set of obligations needed to make `arg` well-formed.
  12 /// If `arg` contains unresolved inference variables, this may include
  13 /// further WF obligations. However, if `arg` IS an unresolved
  14 /// inference variable, returns `None`, because we are not able to
  15 /// make any progress at all. This is to prevent "livelock" where we
  16 /// say "$0 is WF if $0 is WF".
  17 pub fn obligations<'tcx>(
  18     infcx: &InferCtxt<'tcx>,
  19     param_env: ty::ParamEnv<'tcx>,
  20     body_id: LocalDefId,
  21     recursion_depth: usize,
  22     arg: GenericArg<'tcx>,
  23     span: Span,
  24 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  25     // Handle the "livelock" case (see comment above) by bailing out if necessary.
  26     let arg = match arg.unpack() {
  27         GenericArgKind::Type(ty) => {
  28             match ty.kind() {
  29                 ty::Infer(ty::TyVar(_)) => {
  30                     let resolved_ty = infcx.shallow_resolve(ty);
  31                     if resolved_ty == ty {
  32                         // No progress, bail out to prevent "livelock".
  33                         return None;
  34                     } else {
  35                         resolved_ty
  36                     }
  37                 }
  38                 _ => ty,
  39             }
  40             .into()
  41         }
  42         GenericArgKind::Const(ct) => {
  43             match ct.kind() {
  44                 ty::ConstKind::Infer(_) => {
  45                     let resolved = infcx.shallow_resolve(ct);
  46                     if resolved == ct {
  47                         // No progress.
  48                         return None;
  49                     } else {
  50                         resolved
  51                     }
  52                 }
  53                 _ => ct,
  54             }
  55             .into()
  56         }
  57         // There is nothing we have to do for lifetimes.
  58         GenericArgKind::Lifetime(..) => return Some(Vec::new()),
  59     };
  60
  61     let mut wf = WfPredicates {
  62         tcx: infcx.tcx,
  63         param_env,
  64         body_id,
  65         span,
  66         out: vec![],
  67         recursion_depth,
  68         item: None,
  69     };
  70     wf.compute(arg);
  71     debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
  72
  73     let result = wf.normalize(infcx);
  74     debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
  75     Some(result)
  76 }
  77
  78 /// Compute the predicates that are required for a type to be well-formed.
  79 ///
  80 /// This is only intended to be used in the new solver, since it does not
  81 /// take into account recursion depth or proper error-reporting spans.
  82 pub fn unnormalized_obligations<'tcx>(
  83     infcx: &InferCtxt<'tcx>,
  84     param_env: ty::ParamEnv<'tcx>,
  85     arg: GenericArg<'tcx>,
  86 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  87     if let ty::GenericArgKind::Lifetime(..) = arg.unpack() {
  88         return Some(vec![]);
  89     }
  90
  91     debug_assert_eq!(arg, infcx.resolve_vars_if_possible(arg));
  92
  93     let mut wf = WfPredicates {
  94         tcx: infcx.tcx,
  95         param_env,
  96         body_id: CRATE_DEF_ID,
  97         span: DUMMY_SP,
  98         out: vec![],
  99         recursion_depth: 0,
 100         item: None,
 101     };
 102     wf.compute(arg);
 103     Some(wf.out)
 104 }
 105
 106 /// Returns the obligations that make this trait reference
 107 /// well-formed. For example, if there is a trait `Set` defined like
 108 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
 109 /// if `Bar: Eq`.
 110 pub fn trait_obligations<'tcx>(
 111     infcx: &InferCtxt<'tcx>,
 112     param_env: ty::ParamEnv<'tcx>,
 113     body_id: LocalDefId,
 114     trait_pred: &ty::TraitPredicate<'tcx>,
 115     span: Span,
 116     item: &'tcx hir::Item<'tcx>,
 117 ) -> Vec<traits::PredicateObligation<'tcx>> {
 118     let mut wf = WfPredicates {
 119         tcx: infcx.tcx,
 120         param_env,
 121         body_id,
 122         span,
 123         out: vec![],
 124         recursion_depth: 0,
 125         item: Some(item),
 126     };
 127     wf.compute_trait_pred(trait_pred, Elaborate::All);
 128     debug!(obligations = ?wf.out);
 129     wf.normalize(infcx)
 130 }
 131
 132 #[instrument(skip(infcx), ret)]
 133 pub fn predicate_obligations<'tcx>(
 134     infcx: &InferCtxt<'tcx>,
 135     param_env: ty::ParamEnv<'tcx>,
 136     body_id: LocalDefId,
 137     predicate: ty::Predicate<'tcx>,
 138     span: Span,
 139 ) -> Vec<traits::PredicateObligation<'tcx>> {
 140     let mut wf = WfPredicates {
 141         tcx: infcx.tcx,
 142         param_env,
 143         body_id,
 144         span,
 145         out: vec![],
 146         recursion_depth: 0,
 147         item: None,
 148     };
 149
 150     // It's ok to skip the binder here because wf code is prepared for it
 151     match predicate.kind().skip_binder() {
 152         ty::PredicateKind::Clause(ty::Clause::Trait(t)) => {
 153             wf.compute_trait_pred(&t, Elaborate::None);
 154         }
 155         ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..)) => {}
 156         ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(ty, _reg))) => {
 157             wf.compute(ty.into());
 158         }
 159         ty::PredicateKind::Clause(ty::Clause::Projection(t)) => {
 160             wf.compute_projection(t.projection_ty);
 161             wf.compute(match t.term.unpack() {
 162                 ty::TermKind::Ty(ty) => ty.into(),
 163                 ty::TermKind::Const(c) => c.into(),
 164             })
 165         }
 166         ty::PredicateKind::WellFormed(arg) => {
 167             wf.compute(arg);
 168         }
 169         ty::PredicateKind::ObjectSafe(_) => {}
 170         ty::PredicateKind::ClosureKind(..) => {}
 171         ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, a_is_expected: _ }) => {
 172             wf.compute(a.into());
 173             wf.compute(b.into());
 174         }
 175         ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
 176             wf.compute(a.into());
 177             wf.compute(b.into());
 178         }
 179         ty::PredicateKind::ConstEvaluatable(ct) => {
 180             wf.compute(ct.into());
 181         }
 182         ty::PredicateKind::ConstEquate(c1, c2) => {
 183             wf.compute(c1.into());
 184             wf.compute(c2.into());
 185         }
 186         ty::PredicateKind::Ambiguous => {}
 187         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 188             bug!("TypeWellFormedFromEnv is only used for Chalk")
 189         }
 190     }
 191
 192     wf.normalize(infcx)
 193 }
 194
 195 struct WfPredicates<'tcx> {
 196     tcx: TyCtxt<'tcx>,
 197     param_env: ty::ParamEnv<'tcx>,
 198     body_id: LocalDefId,
 199     span: Span,
 200     out: Vec<traits::PredicateObligation<'tcx>>,
 201     recursion_depth: usize,
 202     item: Option<&'tcx hir::Item<'tcx>>,
 203 }
 204
 205 /// Controls whether we "elaborate" supertraits and so forth on the WF
 206 /// predicates. This is a kind of hack to address #43784. The
 207 /// underlying problem in that issue was a trait structure like:
 208 ///
 209 /// ```ignore (illustrative)
 210 /// trait Foo: Copy { }
 211 /// trait Bar: Foo { }
 212 /// impl<T: Bar> Foo for T { }
 213 /// impl<T> Bar for T { }
 214 /// ```
 215 ///
 216 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 217 /// we decide that this is true because `T: Bar` is in the
 218 /// where-clauses (and we can elaborate that to include `T:
 219 /// Copy`). This wouldn't be a problem, except that when we check the
 220 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 221 /// impl. And so nowhere did we check that `T: Copy` holds!
 222 ///
 223 /// To resolve this, we elaborate the WF requirements that must be
 224 /// proven when checking impls. This means that (e.g.) the `impl Bar
 225 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 226 /// Copy` (which it won't be able to do, because there is no `Copy`
 227 /// impl for `T`).
 228 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 229 enum Elaborate {
 230     All,
 231     None,
 232 }
 233
 234 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 235     tcx: TyCtxt<'tcx>,
 236     trait_ref: &ty::TraitRef<'tcx>,
 237     item: Option<&hir::Item<'tcx>>,
 238     cause: &mut traits::ObligationCause<'tcx>,
 239     pred: ty::Predicate<'tcx>,
 240 ) {
 241     debug!(
 242         "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
 243         trait_ref, item, cause, pred
 244     );
 245     let (items, impl_def_id) = match item {
 246         Some(hir::Item { kind: hir::ItemKind::Impl(impl_), owner_id, .. }) => {
 247             (impl_.items, *owner_id)
 248         }
 249         _ => return,
 250     };
 251     let fix_span =
 252         |impl_item_ref: &hir::ImplItemRef| match tcx.hir().impl_item(impl_item_ref.id).kind {
 253             hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::Type(ty) => ty.span,
 254             _ => impl_item_ref.span,
 255         };
 256
 257     // It is fine to skip the binder as we don't care about regions here.
 258     match pred.kind().skip_binder() {
 259         ty::PredicateKind::Clause(ty::Clause::Projection(proj)) => {
 260             // The obligation comes not from the current `impl` nor the `trait` being implemented,
 261             // but rather from a "second order" obligation, where an associated type has a
 262             // projection coming from another associated type. See
 263             // `tests/ui/associated-types/point-at-type-on-obligation-failure.rs` and
 264             // `traits-assoc-type-in-supertrait-bad.rs`.
 265             if let Some(ty::Alias(ty::Projection, projection_ty)) = proj.term.ty().map(|ty| ty.kind())
 266                 && let Some(&impl_item_id) =
 267                     tcx.impl_item_implementor_ids(impl_def_id).get(&projection_ty.def_id)
 268                 && let Some(impl_item_span) = items
 269                     .iter()
 270                     .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
 271                     .map(fix_span)
 272             {
 273                 cause.span = impl_item_span;
 274             }
 275         }
 276         ty::PredicateKind::Clause(ty::Clause::Trait(pred)) => {
 277             // An associated item obligation born out of the `trait` failed to be met. An example
 278             // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
 279             debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
 280             if let ty::Alias(ty::Projection, ty::AliasTy { def_id, .. }) = *pred.self_ty().kind()
 281                 && let Some(&impl_item_id) =
 282                     tcx.impl_item_implementor_ids(impl_def_id).get(&def_id)
 283                 && let Some(impl_item_span) = items
 284                     .iter()
 285                     .find(|item| item.id.owner_id.to_def_id() == impl_item_id)
 286                     .map(fix_span)
 287             {
 288                 cause.span = impl_item_span;
 289             }
 290         }
 291         _ => {}
 292     }
 293 }
 294
 295 impl<'tcx> WfPredicates<'tcx> {
 296     fn tcx(&self) -> TyCtxt<'tcx> {
 297         self.tcx
 298     }
 299
 300     fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 301         traits::ObligationCause::new(self.span, self.body_id, code)
 302     }
 303
 304     fn normalize(self, infcx: &InferCtxt<'tcx>) -> Vec<traits::PredicateObligation<'tcx>> {
 305         let cause = self.cause(traits::WellFormed(None));
 306         let param_env = self.param_env;
 307         let mut obligations = Vec::with_capacity(self.out.len());
 308         for mut obligation in self.out {
 309             assert!(!obligation.has_escaping_bound_vars());
 310             let mut selcx = traits::SelectionContext::new(infcx);
 311             // Don't normalize the whole obligation, the param env is either
 312             // already normalized, or we're currently normalizing the
 313             // param_env. Either way we should only normalize the predicate.
 314             let normalized_predicate = traits::project::normalize_with_depth_to(
 315                 &mut selcx,
 316                 param_env,
 317                 cause.clone(),
 318                 self.recursion_depth,
 319                 obligation.predicate,
 320                 &mut obligations,
 321             );
 322             obligation.predicate = normalized_predicate;
 323             obligations.push(obligation);
 324         }
 325         obligations
 326     }
 327
 328     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 329     fn compute_trait_pred(&mut self, trait_pred: &ty::TraitPredicate<'tcx>, elaborate: Elaborate) {
 330         let tcx = self.tcx;
 331         let trait_ref = &trait_pred.trait_ref;
 332
 333         // if the trait predicate is not const, the wf obligations should not be const as well.
 334         let obligations = if trait_pred.constness == ty::BoundConstness::NotConst {
 335             self.nominal_obligations_without_const(trait_ref.def_id, trait_ref.substs)
 336         } else {
 337             self.nominal_obligations(trait_ref.def_id, trait_ref.substs)
 338         };
 339
 340         debug!("compute_trait_pred obligations {:?}", obligations);
 341         let param_env = self.param_env;
 342         let depth = self.recursion_depth;
 343
 344         let item = self.item;
 345
 346         let extend = |traits::PredicateObligation { predicate, mut cause, .. }| {
 347             if let Some(parent_trait_pred) = predicate.to_opt_poly_trait_pred() {
 348                 cause = cause.derived_cause(
 349                     parent_trait_pred,
 350                     traits::ObligationCauseCode::DerivedObligation,
 351                 );
 352             }
 353             extend_cause_with_original_assoc_item_obligation(
 354                 tcx, trait_ref, item, &mut cause, predicate,
 355             );
 356             traits::Obligation::with_depth(tcx, cause, depth, param_env, predicate)
 357         };
 358
 359         if let Elaborate::All = elaborate {
 360             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
 361             let implied_obligations = implied_obligations.map(extend);
 362             self.out.extend(implied_obligations);
 363         } else {
 364             self.out.extend(obligations);
 365         }
 366
 367         let tcx = self.tcx();
 368         self.out.extend(
 369             trait_ref
 370                 .substs
 371                 .iter()
 372                 .enumerate()
 373                 .filter(|(_, arg)| {
 374                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 375                 })
 376                 .filter(|(_, arg)| !arg.has_escaping_bound_vars())
 377                 .map(|(i, arg)| {
 378                     let mut cause = traits::ObligationCause::misc(self.span, self.body_id);
 379                     // The first subst is the self ty - use the correct span for it.
 380                     if i == 0 {
 381                         if let Some(hir::ItemKind::Impl(hir::Impl { self_ty, .. })) =
 382                             item.map(|i| &i.kind)
 383                         {
 384                             cause.span = self_ty.span;
 385                         }
 386                     }
 387                     traits::Obligation::with_depth(
 388                         tcx,
 389                         cause,
 390                         depth,
 391                         param_env,
 392                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)),
 393                     )
 394                 }),
 395         );
 396     }
 397
 398     /// Pushes the obligations required for `trait_ref::Item` to be WF
 399     /// into `self.out`.
 400     fn compute_projection(&mut self, data: ty::AliasTy<'tcx>) {
 401         // A projection is well-formed if
 402         //
 403         // (a) its predicates hold (*)
 404         // (b) its substs are wf
 405         //
 406         // (*) The predicates of an associated type include the predicates of
 407         //     the trait that it's contained in. For example, given
 408         //
 409         // trait A<T>: Clone {
 410         //     type X where T: Copy;
 411         // }
 412         //
 413         // The predicates of `<() as A<i32>>::X` are:
 414         // [
 415         //     `(): Sized`
 416         //     `(): Clone`
 417         //     `(): A<i32>`
 418         //     `i32: Sized`
 419         //     `i32: Clone`
 420         //     `i32: Copy`
 421         // ]
 422         // Projection types do not require const predicates.
 423         let obligations = self.nominal_obligations_without_const(data.def_id, data.substs);
 424         self.out.extend(obligations);
 425
 426         let tcx = self.tcx();
 427         let cause = self.cause(traits::WellFormed(None));
 428         let param_env = self.param_env;
 429         let depth = self.recursion_depth;
 430
 431         self.out.extend(
 432             data.substs
 433                 .iter()
 434                 .filter(|arg| {
 435                     matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
 436                 })
 437                 .filter(|arg| !arg.has_escaping_bound_vars())
 438                 .map(|arg| {
 439                     traits::Obligation::with_depth(
 440                         tcx,
 441                         cause.clone(),
 442                         depth,
 443                         param_env,
 444                         ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)),
 445                     )
 446                 }),
 447         );
 448     }
 449
 450     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 451         if !subty.has_escaping_bound_vars() {
 452             let cause = self.cause(cause);
 453             let trait_ref = self.tcx.at(cause.span).mk_trait_ref(LangItem::Sized, [subty]);
 454             self.out.push(traits::Obligation::with_depth(
 455                 self.tcx,
 456                 cause,
 457                 self.recursion_depth,
 458                 self.param_env,
 459                 ty::Binder::dummy(trait_ref).without_const(),
 460             ));
 461         }
 462     }
 463
 464     /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
 465     #[instrument(level = "debug", skip(self))]
 466     fn compute(&mut self, arg: GenericArg<'tcx>) {
 467         let mut walker = arg.walk();
 468         let param_env = self.param_env;
 469         let depth = self.recursion_depth;
 470         while let Some(arg) = walker.next() {
 471             debug!(?arg, ?self.out);
 472             let ty = match arg.unpack() {
 473                 GenericArgKind::Type(ty) => ty,
 474
 475                 // No WF constraints for lifetimes being present, any outlives
 476                 // obligations are handled by the parent (e.g. `ty::Ref`).
 477                 GenericArgKind::Lifetime(_) => continue,
 478
 479                 GenericArgKind::Const(ct) => {
 480                     match ct.kind() {
 481                         ty::ConstKind::Unevaluated(uv) => {
 482                             if !ct.has_escaping_bound_vars() {
 483                                 let obligations = self.nominal_obligations(uv.def.did, uv.substs);
 484                                 self.out.extend(obligations);
 485
 486                                 let predicate =
 487                                     ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(ct));
 488                                 let cause = self.cause(traits::WellFormed(None));
 489                                 self.out.push(traits::Obligation::with_depth(
 490                                     self.tcx(),
 491                                     cause,
 492                                     self.recursion_depth,
 493                                     self.param_env,
 494                                     predicate,
 495                                 ));
 496                             }
 497                         }
 498                         ty::ConstKind::Infer(_) => {
 499                             let cause = self.cause(traits::WellFormed(None));
 500
 501                             self.out.push(traits::Obligation::with_depth(
 502                                 self.tcx(),
 503                                 cause,
 504                                 self.recursion_depth,
 505                                 self.param_env,
 506                                 ty::Binder::dummy(ty::PredicateKind::WellFormed(ct.into())),
 507                             ));
 508                         }
 509                         ty::ConstKind::Expr(_) => {
 510                             // FIXME(generic_const_exprs): this doesnt verify that given `Expr(N + 1)` the
 511                             // trait bound `typeof(N): Add<typeof(1)>` holds. This is currently unnecessary
 512                             // as `ConstKind::Expr` is only produced via normalization of `ConstKind::Unevaluated`
 513                             // which means that the `DefId` would have been typeck'd elsewhere. However in
 514                             // the future we may allow directly lowering to `ConstKind::Expr` in which case
 515                             // we would not be proving bounds we should.
 516
 517                             let predicate =
 518                                 ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(ct));
 519                             let cause = self.cause(traits::WellFormed(None));
 520                             self.out.push(traits::Obligation::with_depth(
 521                                 self.tcx(),
 522                                 cause,
 523                                 self.recursion_depth,
 524                                 self.param_env,
 525                                 predicate,
 526                             ));
 527                         }
 528
 529                         ty::ConstKind::Error(_)
 530                         | ty::ConstKind::Param(_)
 531                         | ty::ConstKind::Bound(..)
 532                         | ty::ConstKind::Placeholder(..) => {
 533                             // These variants are trivially WF, so nothing to do here.
 534                         }
 535                         ty::ConstKind::Value(..) => {
 536                             // FIXME: Enforce that values are structurally-matchable.
 537                         }
 538                     }
 539                     continue;
 540                 }
 541             };
 542
 543             debug!("wf bounds for ty={:?} ty.kind={:#?}", ty, ty.kind());
 544
 545             match *ty.kind() {
 546                 ty::Bool
 547                 | ty::Char
 548                 | ty::Int(..)
 549                 | ty::Uint(..)
 550                 | ty::Float(..)
 551                 | ty::Error(_)
 552                 | ty::Str
 553                 | ty::GeneratorWitness(..)
 554                 | ty::Never
 555                 | ty::Param(_)
 556                 | ty::Bound(..)
 557                 | ty::Placeholder(..)
 558                 | ty::Foreign(..) => {
 559                     // WfScalar, WfParameter, etc
 560                 }
 561
 562                 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
 563                 ty::Infer(ty::IntVar(_)) => {}
 564
 565                 // Can only infer to `ty::Float(_)`.
 566                 ty::Infer(ty::FloatVar(_)) => {}
 567
 568                 ty::Slice(subty) => {
 569                     self.require_sized(subty, traits::SliceOrArrayElem);
 570                 }
 571
 572                 ty::Array(subty, _) => {
 573                     self.require_sized(subty, traits::SliceOrArrayElem);
 574                     // Note that we handle the len is implicitly checked while walking `arg`.
 575                 }
 576
 577                 ty::Tuple(ref tys) => {
 578                     if let Some((_last, rest)) = tys.split_last() {
 579                         for &elem in rest {
 580                             self.require_sized(elem, traits::TupleElem);
 581                         }
 582                     }
 583                 }
 584
 585                 ty::RawPtr(_) => {
 586                     // Simple cases that are WF if their type args are WF.
 587                 }
 588
 589                 ty::Alias(ty::Projection, data) => {
 590                     walker.skip_current_subtree(); // Subtree handled by compute_projection.
 591                     self.compute_projection(data);
 592                 }
 593
 594                 ty::Adt(def, substs) => {
 595                     // WfNominalType
 596                     let obligations = self.nominal_obligations(def.did(), substs);
 597                     self.out.extend(obligations);
 598                 }
 599
 600                 ty::FnDef(did, substs) => {
 601                     let obligations = self.nominal_obligations_without_const(did, substs);
 602                     self.out.extend(obligations);
 603                 }
 604
 605                 ty::Ref(r, rty, _) => {
 606                     // WfReference
 607                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 608                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 609                         self.out.push(traits::Obligation::with_depth(
 610                             self.tcx(),
 611                             cause,
 612                             depth,
 613                             param_env,
 614                             ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::TypeOutlives(
 615                                 ty::OutlivesPredicate(rty, r),
 616                             ))),
 617                         ));
 618                     }
 619                 }
 620
 621                 ty::Generator(did, substs, ..) => {
 622                     // Walk ALL the types in the generator: this will
 623                     // include the upvar types as well as the yield
 624                     // type. Note that this is mildly distinct from
 625                     // the closure case, where we have to be careful
 626                     // about the signature of the closure. We don't
 627                     // have the problem of implied bounds here since
 628                     // generators don't take arguments.
 629                     let obligations = self.nominal_obligations(did, substs);
 630                     self.out.extend(obligations);
 631                 }
 632
 633                 ty::Closure(did, substs) => {
 634                     // Only check the upvar types for WF, not the rest
 635                     // of the types within. This is needed because we
 636                     // capture the signature and it may not be WF
 637                     // without the implied bounds. Consider a closure
 638                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 639                     // not known to hold in the creator's context (and
 640                     // indeed the closure may not be invoked by its
 641                     // creator, but rather turned to someone who *can*
 642                     // verify that).
 643                     //
 644                     // The special treatment of closures here really
 645                     // ought not to be necessary either; the problem
 646                     // is related to #25860 -- there is no way for us
 647                     // to express a fn type complete with the implied
 648                     // bounds that it is assuming. I think in reality
 649                     // the WF rules around fn are a bit messed up, and
 650                     // that is the rot problem: `fn(&'a T)` should
 651                     // probably always be WF, because it should be
 652                     // shorthand for something like `where(T: 'a) {
 653                     // fn(&'a T) }`, as discussed in #25860.
 654                     walker.skip_current_subtree(); // subtree handled below
 655                     // FIXME(eddyb) add the type to `walker` instead of recursing.
 656                     self.compute(substs.as_closure().tupled_upvars_ty().into());
 657                     // Note that we cannot skip the generic types
 658                     // types. Normally, within the fn
 659                     // body where they are created, the generics will
 660                     // always be WF, and outside of that fn body we
 661                     // are not directly inspecting closure types
 662                     // anyway, except via auto trait matching (which
 663                     // only inspects the upvar types).
 664                     // But when a closure is part of a type-alias-impl-trait
 665                     // then the function that created the defining site may
 666                     // have had more bounds available than the type alias
 667                     // specifies. This may cause us to have a closure in the
 668                     // hidden type that is not actually well formed and
 669                     // can cause compiler crashes when the user abuses unsafe
 670                     // code to procure such a closure.
 671                     // See tests/ui/type-alias-impl-trait/wf_check_closures.rs
 672                     let obligations = self.nominal_obligations(did, substs);
 673                     self.out.extend(obligations);
 674                 }
 675
 676                 ty::FnPtr(_) => {
 677                     // let the loop iterate into the argument/return
 678                     // types appearing in the fn signature
 679                 }
 680
 681                 ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
 682                     // All of the requirements on type parameters
 683                     // have already been checked for `impl Trait` in
 684                     // return position. We do need to check type-alias-impl-trait though.
 685                     if self.tcx.is_type_alias_impl_trait(def_id) {
 686                         let obligations = self.nominal_obligations(def_id, substs);
 687                         self.out.extend(obligations);
 688                     }
 689                 }
 690
 691                 ty::Dynamic(data, r, _) => {
 692                     // WfObject
 693                     //
 694                     // Here, we defer WF checking due to higher-ranked
 695                     // regions. This is perhaps not ideal.
 696                     self.from_object_ty(ty, data, r);
 697
 698                     // FIXME(#27579) RFC also considers adding trait
 699                     // obligations that don't refer to Self and
 700                     // checking those
 701
 702                     let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
 703
 704                     if !defer_to_coercion {
 705                         let cause = self.cause(traits::WellFormed(None));
 706                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 707                         let tcx = self.tcx();
 708                         self.out.extend(component_traits.map(|did| {
 709                             traits::Obligation::with_depth(
 710                                 tcx,
 711                                 cause.clone(),
 712                                 depth,
 713                                 param_env,
 714                                 ty::Binder::dummy(ty::PredicateKind::ObjectSafe(did)),
 715                             )
 716                         }));
 717                     }
 718                 }
 719
 720                 // Inference variables are the complicated case, since we don't
 721                 // know what type they are. We do two things:
 722                 //
 723                 // 1. Check if they have been resolved, and if so proceed with
 724                 //    THAT type.
 725                 // 2. If not, we've at least simplified things (e.g., we went
 726                 //    from `Vec<$0>: WF` to `$0: WF`), so we can
 727                 //    register a pending obligation and keep
 728                 //    moving. (Goal is that an "inductive hypothesis"
 729                 //    is satisfied to ensure termination.)
 730                 // See also the comment on `fn obligations`, describing "livelock"
 731                 // prevention, which happens before this can be reached.
 732                 ty::Infer(_) => {
 733                     let cause = self.cause(traits::WellFormed(None));
 734                     self.out.push(traits::Obligation::with_depth(
 735                         self.tcx(),
 736                         cause,
 737                         self.recursion_depth,
 738                         param_env,
 739                         ty::Binder::dummy(ty::PredicateKind::WellFormed(ty.into())),
 740                     ));
 741                 }
 742             }
 743
 744             debug!(?self.out);
 745         }
 746     }
 747
 748     #[instrument(level = "debug", skip(self))]
 749     fn nominal_obligations_inner(
 750         &mut self,
 751         def_id: DefId,
 752         substs: SubstsRef<'tcx>,
 753         remap_constness: bool,
 754     ) -> Vec<traits::PredicateObligation<'tcx>> {
 755         let predicates = self.tcx.predicates_of(def_id);
 756         let mut origins = vec![def_id; predicates.predicates.len()];
 757         let mut head = predicates;
 758         while let Some(parent) = head.parent {
 759             head = self.tcx.predicates_of(parent);
 760             origins.extend(iter::repeat(parent).take(head.predicates.len()));
 761         }
 762
 763         let predicates = predicates.instantiate(self.tcx, substs);
 764         trace!("{:#?}", predicates);
 765         debug_assert_eq!(predicates.predicates.len(), origins.len());
 766
 767         iter::zip(predicates, origins.into_iter().rev())
 768             .map(|((mut pred, span), origin_def_id)| {
 769                 let code = if span.is_dummy() {
 770                     traits::ItemObligation(origin_def_id)
 771                 } else {
 772                     traits::BindingObligation(origin_def_id, span)
 773                 };
 774                 let cause = self.cause(code);
 775                 if remap_constness {
 776                     pred = pred.without_const(self.tcx);
 777                 }
 778                 traits::Obligation::with_depth(
 779                     self.tcx,
 780                     cause,
 781                     self.recursion_depth,
 782                     self.param_env,
 783                     pred,
 784                 )
 785             })
 786             .filter(|pred| !pred.has_escaping_bound_vars())
 787             .collect()
 788     }
 789
 790     fn nominal_obligations(
 791         &mut self,
 792         def_id: DefId,
 793         substs: SubstsRef<'tcx>,
 794     ) -> Vec<traits::PredicateObligation<'tcx>> {
 795         self.nominal_obligations_inner(def_id, substs, false)
 796     }
 797
 798     fn nominal_obligations_without_const(
 799         &mut self,
 800         def_id: DefId,
 801         substs: SubstsRef<'tcx>,
 802     ) -> Vec<traits::PredicateObligation<'tcx>> {
 803         self.nominal_obligations_inner(def_id, substs, true)
 804     }
 805
 806     fn from_object_ty(
 807         &mut self,
 808         ty: Ty<'tcx>,
 809         data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
 810         region: ty::Region<'tcx>,
 811     ) {
 812         // Imagine a type like this:
 813         //
 814         //     trait Foo { }
 815         //     trait Bar<'c> : 'c { }
 816         //
 817         //     &'b (Foo+'c+Bar<'d>)
 818         //         ^
 819         //
 820         // In this case, the following relationships must hold:
 821         //
 822         //     'b <= 'c
 823         //     'd <= 'c
 824         //
 825         // The first conditions is due to the normal region pointer
 826         // rules, which say that a reference cannot outlive its
 827         // referent.
 828         //
 829         // The final condition may be a bit surprising. In particular,
 830         // you may expect that it would have been `'c <= 'd`, since
 831         // usually lifetimes of outer things are conservative
 832         // approximations for inner things. However, it works somewhat
 833         // differently with trait objects: here the idea is that if the
 834         // user specifies a region bound (`'c`, in this case) it is the
 835         // "master bound" that *implies* that bounds from other traits are
 836         // all met. (Remember that *all bounds* in a type like
 837         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 838         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 839         // 'y.)
 840         //
 841         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 842         // am looking forward to the future here.
 843         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 844             let implicit_bounds = object_region_bounds(self.tcx, data);
 845
 846             let explicit_bound = region;
 847
 848             self.out.reserve(implicit_bounds.len());
 849             for implicit_bound in implicit_bounds {
 850                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 851                 let outlives =
 852                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 853                 self.out.push(traits::Obligation::with_depth(
 854                     self.tcx,
 855                     cause,
 856                     self.recursion_depth,
 857                     self.param_env,
 858                     outlives,
 859                 ));
 860             }
 861         }
 862     }
 863 }
 864
 865 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 866 /// bounds that must hold on the elided self type. These are derived
 867 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 868 /// they declare `trait SomeTrait : 'static`, for example, then
 869 /// `'static` would appear in the list. The hard work is done by
 870 /// `infer::required_region_bounds`, see that for more information.
 871 pub fn object_region_bounds<'tcx>(
 872     tcx: TyCtxt<'tcx>,
 873     existential_predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
 874 ) -> Vec<ty::Region<'tcx>> {
 875     // Since we don't actually *know* the self type for an object,
 876     // this "open(err)" serves as a kind of dummy standin -- basically
 877     // a placeholder type.
 878     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 879
 880     let predicates = existential_predicates.iter().filter_map(|predicate| {
 881         if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
 882             None
 883         } else {
 884             Some(predicate.with_self_ty(tcx, open_ty))
 885         }
 886     });
 887
 888     required_region_bounds(tcx, open_ty, predicates)
 889 }
 890
 891 /// Given a set of predicates that apply to an object type, returns
 892 /// the region bounds that the (erased) `Self` type must
 893 /// outlive. Precisely *because* the `Self` type is erased, the
 894 /// parameter `erased_self_ty` must be supplied to indicate what type
 895 /// has been used to represent `Self` in the predicates
 896 /// themselves. This should really be a unique type; `FreshTy(0)` is a
 897 /// popular choice.
 898 ///
 899 /// N.B., in some cases, particularly around higher-ranked bounds,
 900 /// this function returns a kind of conservative approximation.
 901 /// That is, all regions returned by this function are definitely
 902 /// required, but there may be other region bounds that are not
 903 /// returned, as well as requirements like `for<'a> T: 'a`.
 904 ///
 905 /// Requires that trait definitions have been processed so that we can
 906 /// elaborate predicates and walk supertraits.
 907 #[instrument(skip(tcx, predicates), level = "debug", ret)]
 908 pub(crate) fn required_region_bounds<'tcx>(
 909     tcx: TyCtxt<'tcx>,
 910     erased_self_ty: Ty<'tcx>,
 911     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 912 ) -> Vec<ty::Region<'tcx>> {
 913     assert!(!erased_self_ty.has_escaping_bound_vars());
 914
 915     traits::elaborate_predicates(tcx, predicates)
 916         .filter_map(|obligation| {
 917             debug!(?obligation);
 918             match obligation.predicate.kind().skip_binder() {
 919                 ty::PredicateKind::Clause(ty::Clause::Projection(..))
 920                 | ty::PredicateKind::Clause(ty::Clause::Trait(..))
 921                 | ty::PredicateKind::Subtype(..)
 922                 | ty::PredicateKind::Coerce(..)
 923                 | ty::PredicateKind::WellFormed(..)
 924                 | ty::PredicateKind::ObjectSafe(..)
 925                 | ty::PredicateKind::ClosureKind(..)
 926                 | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
 927                 | ty::PredicateKind::ConstEvaluatable(..)
 928                 | ty::PredicateKind::ConstEquate(..)
 929                 | ty::PredicateKind::Ambiguous
 930                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 931                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 932                     ref t,
 933                     ref r,
 934                 ))) => {
 935                     // Search for a bound of the form `erased_self_ty
 936                     // : 'a`, but be wary of something like `for<'a>
 937                     // erased_self_ty : 'a` (we interpret a
 938                     // higher-ranked bound like that as 'static,
 939                     // though at present the code in `fulfill.rs`
 940                     // considers such bounds to be unsatisfiable, so
 941                     // it's kind of a moot point since you could never
 942                     // construct such an object, but this seems
 943                     // correct even if that code changes).
 944                     if t == &erased_self_ty && !r.has_escaping_bound_vars() {
 945                         Some(*r)
 946                     } else {
 947                         None
 948                     }
 949                 }
 950             }
 951         })
 952         .collect()
 953 }