src/librustc_trait_selection/traits/wf.rs

   1 use crate::infer::InferCtxt;
   2 use crate::opaque_types::required_region_bounds;
   3 use crate::traits::{self, AssocTypeBoundData};
   4 use rustc_hir as hir;
   5 use rustc_hir::def_id::DefId;
   6 use rustc_hir::lang_items;
   7 use rustc_middle::ty::subst::{GenericArgKind, SubstsRef};
   8 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
   9 use rustc_span::symbol::{kw, Ident};
  10 use rustc_span::Span;
  11 use std::rc::Rc;
  12
  13 /// Returns the set of obligations needed to make `ty` well-formed.
  14 /// If `ty` contains unresolved inference variables, this may include
  15 /// further WF obligations. However, if `ty` 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     ty: Ty<'tcx>,
  24     span: Span,
  25 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
  26     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  27     if wf.compute(ty) {
  28         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  29
  30         let result = wf.normalize();
  31         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  32         Some(result)
  33     } else {
  34         None // no progress made, return None
  35     }
  36 }
  37
  38 /// Returns the obligations that make this trait reference
  39 /// well-formed.  For example, if there is a trait `Set` defined like
  40 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  41 /// if `Bar: Eq`.
  42 pub fn trait_obligations<'a, 'tcx>(
  43     infcx: &InferCtxt<'a, 'tcx>,
  44     param_env: ty::ParamEnv<'tcx>,
  45     body_id: hir::HirId,
  46     trait_ref: &ty::TraitRef<'tcx>,
  47     span: Span,
  48     item: Option<&'tcx hir::Item<'tcx>>,
  49 ) -> Vec<traits::PredicateObligation<'tcx>> {
  50     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
  51     wf.compute_trait_ref(trait_ref, Elaborate::All);
  52     wf.normalize()
  53 }
  54
  55 pub fn predicate_obligations<'a, 'tcx>(
  56     infcx: &InferCtxt<'a, 'tcx>,
  57     param_env: ty::ParamEnv<'tcx>,
  58     body_id: hir::HirId,
  59     predicate: &ty::Predicate<'tcx>,
  60     span: Span,
  61 ) -> Vec<traits::PredicateObligation<'tcx>> {
  62     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
  63
  64     // (*) ok to skip binders, because wf code is prepared for it
  65     match *predicate {
  66         ty::Predicate::Trait(ref t, _) => {
  67             wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
  68         }
  69         ty::Predicate::RegionOutlives(..) => {}
  70         ty::Predicate::TypeOutlives(ref t) => {
  71             wf.compute(t.skip_binder().0);
  72         }
  73         ty::Predicate::Projection(ref t) => {
  74             let t = t.skip_binder(); // (*)
  75             wf.compute_projection(t.projection_ty);
  76             wf.compute(t.ty);
  77         }
  78         ty::Predicate::WellFormed(t) => {
  79             wf.compute(t);
  80         }
  81         ty::Predicate::ObjectSafe(_) => {}
  82         ty::Predicate::ClosureKind(..) => {}
  83         ty::Predicate::Subtype(ref data) => {
  84             wf.compute(data.skip_binder().a); // (*)
  85             wf.compute(data.skip_binder().b); // (*)
  86         }
  87         ty::Predicate::ConstEvaluatable(def_id, substs) => {
  88             let obligations = wf.nominal_obligations(def_id, substs);
  89             wf.out.extend(obligations);
  90
  91             for ty in substs.types() {
  92                 wf.compute(ty);
  93             }
  94         }
  95     }
  96
  97     wf.normalize()
  98 }
  99
 100 struct WfPredicates<'a, 'tcx> {
 101     infcx: &'a InferCtxt<'a, 'tcx>,
 102     param_env: ty::ParamEnv<'tcx>,
 103     body_id: hir::HirId,
 104     span: Span,
 105     out: Vec<traits::PredicateObligation<'tcx>>,
 106     item: Option<&'tcx hir::Item<'tcx>>,
 107 }
 108
 109 /// Controls whether we "elaborate" supertraits and so forth on the WF
 110 /// predicates. This is a kind of hack to address #43784. The
 111 /// underlying problem in that issue was a trait structure like:
 112 ///
 113 /// ```
 114 /// trait Foo: Copy { }
 115 /// trait Bar: Foo { }
 116 /// impl<T: Bar> Foo for T { }
 117 /// impl<T> Bar for T { }
 118 /// ```
 119 ///
 120 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 121 /// we decide that this is true because `T: Bar` is in the
 122 /// where-clauses (and we can elaborate that to include `T:
 123 /// Copy`). This wouldn't be a problem, except that when we check the
 124 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 125 /// impl. And so nowhere did we check that `T: Copy` holds!
 126 ///
 127 /// To resolve this, we elaborate the WF requirements that must be
 128 /// proven when checking impls. This means that (e.g.) the `impl Bar
 129 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 130 /// Copy` (which it won't be able to do, because there is no `Copy`
 131 /// impl for `T`).
 132 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 133 enum Elaborate {
 134     All,
 135     None,
 136 }
 137
 138 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
 139     tcx: TyCtxt<'tcx>,
 140     trait_ref: &ty::TraitRef<'tcx>,
 141     item: Option<&hir::Item<'tcx>>,
 142     cause: &mut traits::ObligationCause<'tcx>,
 143     pred: &ty::Predicate<'_>,
 144     mut trait_assoc_items: impl Iterator<Item = ty::AssocItem>,
 145 ) {
 146     let trait_item =
 147         tcx.hir().as_local_hir_id(trait_ref.def_id).and_then(|trait_id| tcx.hir().find(trait_id));
 148     let (trait_name, trait_generics) = match trait_item {
 149         Some(hir::Node::Item(hir::Item {
 150             ident,
 151             kind: hir::ItemKind::Trait(.., generics, _, _),
 152             ..
 153         }))
 154         | Some(hir::Node::Item(hir::Item {
 155             ident,
 156             kind: hir::ItemKind::TraitAlias(generics, _),
 157             ..
 158         })) => (Some(ident), Some(generics)),
 159         _ => (None, None),
 160     };
 161
 162     let item_span = item.map(|i| tcx.sess.source_map().guess_head_span(i.span));
 163     match pred {
 164         ty::Predicate::Projection(proj) => {
 165             // The obligation comes not from the current `impl` nor the `trait` being
 166             // implemented, but rather from a "second order" obligation, like in
 167             // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs`:
 168             //
 169             //   error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
 170             //     --> $DIR/point-at-type-on-obligation-failure.rs:13:5
 171             //      |
 172             //   LL |     type Ok;
 173             //      |          -- associated type defined here
 174             //   ...
 175             //   LL | impl Bar for Foo {
 176             //      | ---------------- in this `impl` item
 177             //   LL |     type Ok = ();
 178             //      |     ^^^^^^^^^^^^^ expected `u32`, found `()`
 179             //      |
 180             //      = note: expected type `u32`
 181             //                 found type `()`
 182             //
 183             // FIXME: we would want to point a span to all places that contributed to this
 184             // obligation. In the case above, it should be closer to:
 185             //
 186             //   error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
 187             //     --> $DIR/point-at-type-on-obligation-failure.rs:13:5
 188             //      |
 189             //   LL |     type Ok;
 190             //      |          -- associated type defined here
 191             //   LL |     type Sibling: Bar2<Ok=Self::Ok>;
 192             //      |     -------------------------------- obligation set here
 193             //   ...
 194             //   LL | impl Bar for Foo {
 195             //      | ---------------- in this `impl` item
 196             //   LL |     type Ok = ();
 197             //      |     ^^^^^^^^^^^^^ expected `u32`, found `()`
 198             //   ...
 199             //   LL | impl Bar2 for Foo2 {
 200             //      | ---------------- in this `impl` item
 201             //   LL |     type Ok = u32;
 202             //      |     -------------- obligation set here
 203             //      |
 204             //      = note: expected type `u32`
 205             //                 found type `()`
 206             if let Some(hir::ItemKind::Impl { items, .. }) = item.map(|i| &i.kind) {
 207                 let trait_assoc_item = tcx.associated_item(proj.projection_def_id());
 208                 if let Some(impl_item) =
 209                     items.iter().find(|item| item.ident == trait_assoc_item.ident)
 210                 {
 211                     cause.span = impl_item.span;
 212                     cause.code = traits::AssocTypeBound(Box::new(AssocTypeBoundData {
 213                         impl_span: item_span,
 214                         original: trait_assoc_item.ident.span,
 215                         bounds: vec![],
 216                     }));
 217                 }
 218             }
 219         }
 220         ty::Predicate::Trait(proj, _) => {
 221             // An associated item obligation born out of the `trait` failed to be met.
 222             // Point at the `impl` that failed the obligation, the associated item that
 223             // needed to meet the obligation, and the definition of that associated item,
 224             // which should hold the obligation in most cases. An example can be seen in
 225             // `src/test/ui/associated-types/point-at-type-on-obligation-failure-2.rs`:
 226             //
 227             //   error[E0277]: the trait bound `bool: Bar` is not satisfied
 228             //     --> $DIR/point-at-type-on-obligation-failure-2.rs:8:5
 229             //      |
 230             //   LL |     type Assoc: Bar;
 231             //      |          ----- associated type defined here
 232             //   ...
 233             //   LL | impl Foo for () {
 234             //      | --------------- in this `impl` item
 235             //   LL |     type Assoc = bool;
 236             //      |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
 237             //
 238             // If the obligation comes from the where clause in the `trait`, we point at it:
 239             //
 240             //   error[E0277]: the trait bound `bool: Bar` is not satisfied
 241             //     --> $DIR/point-at-type-on-obligation-failure-2.rs:8:5
 242             //      |
 243             //      | trait Foo where <Self as Foo>>::Assoc: Bar {
 244             //      |                 -------------------------- restricted in this bound
 245             //   LL |     type Assoc;
 246             //      |          ----- associated type defined here
 247             //   ...
 248             //   LL | impl Foo for () {
 249             //      | --------------- in this `impl` item
 250             //   LL |     type Assoc = bool;
 251             //      |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
 252             if let (
 253                 ty::Projection(ty::ProjectionTy { item_def_id, .. }),
 254                 Some(hir::ItemKind::Impl { items, .. }),
 255             ) = (&proj.skip_binder().self_ty().kind, item.map(|i| &i.kind))
 256             {
 257                 if let Some((impl_item, trait_assoc_item)) = trait_assoc_items
 258                     .find(|i| i.def_id == *item_def_id)
 259                     .and_then(|trait_assoc_item| {
 260                         items
 261                             .iter()
 262                             .find(|i| i.ident == trait_assoc_item.ident)
 263                             .map(|impl_item| (impl_item, trait_assoc_item))
 264                     })
 265                 {
 266                     let bounds = trait_generics
 267                         .map(|generics| {
 268                             get_generic_bound_spans(&generics, trait_name, trait_assoc_item.ident)
 269                         })
 270                         .unwrap_or_else(Vec::new);
 271                     cause.span = impl_item.span;
 272                     cause.code = traits::AssocTypeBound(Box::new(AssocTypeBoundData {
 273                         impl_span: item_span,
 274                         original: trait_assoc_item.ident.span,
 275                         bounds,
 276                     }));
 277                 }
 278             }
 279         }
 280         _ => {}
 281     }
 282 }
 283
 284 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
 285     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 286         traits::ObligationCause::new(self.span, self.body_id, code)
 287     }
 288
 289     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 290         let cause = self.cause(traits::MiscObligation);
 291         let infcx = &mut self.infcx;
 292         let param_env = self.param_env;
 293         let mut obligations = Vec::with_capacity(self.out.len());
 294         for pred in &self.out {
 295             assert!(!pred.has_escaping_bound_vars());
 296             let mut selcx = traits::SelectionContext::new(infcx);
 297             let i = obligations.len();
 298             let value =
 299                 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
 300             obligations.insert(i, value);
 301         }
 302         obligations
 303     }
 304
 305     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 306     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 307         let tcx = self.infcx.tcx;
 308         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 309
 310         let cause = self.cause(traits::MiscObligation);
 311         let param_env = self.param_env;
 312
 313         let item = self.item;
 314
 315         if let Elaborate::All = elaborate {
 316             let implied_obligations = traits::util::elaborate_obligations(tcx, obligations.clone());
 317             let implied_obligations = implied_obligations.map(|obligation| {
 318                 let mut cause = cause.clone();
 319                 let parent_trait_ref = obligation
 320                     .predicate
 321                     .to_opt_poly_trait_ref()
 322                     .unwrap_or_else(|| ty::Binder::dummy(*trait_ref));
 323                 let derived_cause = traits::DerivedObligationCause {
 324                     parent_trait_ref,
 325                     parent_code: Rc::new(obligation.cause.code.clone()),
 326                 };
 327                 cause.code = traits::ObligationCauseCode::ImplDerivedObligation(derived_cause);
 328                 extend_cause_with_original_assoc_item_obligation(
 329                     tcx,
 330                     trait_ref,
 331                     item,
 332                     &mut cause,
 333                     &obligation.predicate,
 334                     tcx.associated_items(trait_ref.def_id).in_definition_order().copied(),
 335                 );
 336                 traits::Obligation::new(cause, param_env, obligation.predicate)
 337             });
 338             self.out.extend(implied_obligations);
 339         }
 340
 341         self.out.extend(obligations);
 342
 343         self.out.extend(trait_ref.substs.types().filter(|ty| !ty.has_escaping_bound_vars()).map(
 344             |ty| traits::Obligation::new(cause.clone(), param_env, ty::Predicate::WellFormed(ty)),
 345         ));
 346     }
 347
 348     /// Pushes the obligations required for `trait_ref::Item` to be WF
 349     /// into `self.out`.
 350     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 351         // A projection is well-formed if (a) the trait ref itself is
 352         // WF and (b) the trait-ref holds.  (It may also be
 353         // normalizable and be WF that way.)
 354         let trait_ref = data.trait_ref(self.infcx.tcx);
 355         self.compute_trait_ref(&trait_ref, Elaborate::None);
 356
 357         if !data.has_escaping_bound_vars() {
 358             let predicate = trait_ref.without_const().to_predicate();
 359             let cause = self.cause(traits::ProjectionWf(data));
 360             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 361         }
 362     }
 363
 364     /// Pushes the obligations required for an array length to be WF
 365     /// into `self.out`.
 366     fn compute_array_len(&mut self, constant: ty::Const<'tcx>) {
 367         if let ty::ConstKind::Unevaluated(def_id, substs, promoted) = constant.val {
 368             assert!(promoted.is_none());
 369
 370             let obligations = self.nominal_obligations(def_id, substs);
 371             self.out.extend(obligations);
 372
 373             let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
 374             let cause = self.cause(traits::MiscObligation);
 375             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 376         }
 377     }
 378
 379     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 380         if !subty.has_escaping_bound_vars() {
 381             let cause = self.cause(cause);
 382             let trait_ref = ty::TraitRef {
 383                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
 384                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 385             };
 386             self.out.push(traits::Obligation::new(
 387                 cause,
 388                 self.param_env,
 389                 trait_ref.without_const().to_predicate(),
 390             ));
 391         }
 392     }
 393
 394     /// Pushes new obligations into `out`. Returns `true` if it was able
 395     /// to generate all the predicates needed to validate that `ty0`
 396     /// is WF. Returns false if `ty0` is an unresolved type variable,
 397     /// in which case we are not able to simplify at all.
 398     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 399         let mut walker = ty0.walk();
 400         let param_env = self.param_env;
 401         while let Some(arg) = walker.next() {
 402             let ty = match arg.unpack() {
 403                 GenericArgKind::Type(ty) => ty,
 404
 405                 // No WF constraints for lifetimes being present, any outlives
 406                 // obligations are handled by the parent (e.g. `ty::Ref`).
 407                 GenericArgKind::Lifetime(_) => continue,
 408
 409                 // FIXME(eddyb) this is wrong and needs to be replaced
 410                 // (see https://github.com/rust-lang/rust/pull/70107).
 411                 GenericArgKind::Const(_) => continue,
 412             };
 413
 414             match ty.kind {
 415                 ty::Bool
 416                 | ty::Char
 417                 | ty::Int(..)
 418                 | ty::Uint(..)
 419                 | ty::Float(..)
 420                 | ty::Error
 421                 | ty::Str
 422                 | ty::GeneratorWitness(..)
 423                 | ty::Never
 424                 | ty::Param(_)
 425                 | ty::Bound(..)
 426                 | ty::Placeholder(..)
 427                 | ty::Foreign(..) => {
 428                     // WfScalar, WfParameter, etc
 429                 }
 430
 431                 ty::Slice(subty) => {
 432                     self.require_sized(subty, traits::SliceOrArrayElem);
 433                 }
 434
 435                 ty::Array(subty, len) => {
 436                     self.require_sized(subty, traits::SliceOrArrayElem);
 437                     // FIXME(eddyb) handle `GenericArgKind::Const` above instead.
 438                     self.compute_array_len(*len);
 439                 }
 440
 441                 ty::Tuple(ref tys) => {
 442                     if let Some((_last, rest)) = tys.split_last() {
 443                         for elem in rest {
 444                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 445                         }
 446                     }
 447                 }
 448
 449                 ty::RawPtr(_) => {
 450                     // simple cases that are WF if their type args are WF
 451                 }
 452
 453                 ty::Projection(data) => {
 454                     walker.skip_current_subtree(); // subtree handled by compute_projection
 455                     self.compute_projection(data);
 456                 }
 457
 458                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 459
 460                 ty::Adt(def, substs) => {
 461                     // WfNominalType
 462                     let obligations = self.nominal_obligations(def.did, substs);
 463                     self.out.extend(obligations);
 464                 }
 465
 466                 ty::FnDef(did, substs) => {
 467                     let obligations = self.nominal_obligations(did, substs);
 468                     self.out.extend(obligations);
 469                 }
 470
 471                 ty::Ref(r, rty, _) => {
 472                     // WfReference
 473                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 474                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 475                         self.out.push(traits::Obligation::new(
 476                             cause,
 477                             param_env,
 478                             ty::Predicate::TypeOutlives(ty::Binder::dummy(ty::OutlivesPredicate(
 479                                 rty, r,
 480                             ))),
 481                         ));
 482                     }
 483                 }
 484
 485                 ty::Generator(..) => {
 486                     // Walk ALL the types in the generator: this will
 487                     // include the upvar types as well as the yield
 488                     // type. Note that this is mildly distinct from
 489                     // the closure case, where we have to be careful
 490                     // about the signature of the closure. We don't
 491                     // have the problem of implied bounds here since
 492                     // generators don't take arguments.
 493                 }
 494
 495                 ty::Closure(_, substs) => {
 496                     // Only check the upvar types for WF, not the rest
 497                     // of the types within. This is needed because we
 498                     // capture the signature and it may not be WF
 499                     // without the implied bounds. Consider a closure
 500                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 501                     // not known to hold in the creator's context (and
 502                     // indeed the closure may not be invoked by its
 503                     // creator, but rather turned to someone who *can*
 504                     // verify that).
 505                     //
 506                     // The special treatment of closures here really
 507                     // ought not to be necessary either; the problem
 508                     // is related to #25860 -- there is no way for us
 509                     // to express a fn type complete with the implied
 510                     // bounds that it is assuming. I think in reality
 511                     // the WF rules around fn are a bit messed up, and
 512                     // that is the rot problem: `fn(&'a T)` should
 513                     // probably always be WF, because it should be
 514                     // shorthand for something like `where(T: 'a) {
 515                     // fn(&'a T) }`, as discussed in #25860.
 516                     //
 517                     // Note that we are also skipping the generic
 518                     // types. This is consistent with the `outlives`
 519                     // code, but anyway doesn't matter: within the fn
 520                     // body where they are created, the generics will
 521                     // always be WF, and outside of that fn body we
 522                     // are not directly inspecting closure types
 523                     // anyway, except via auto trait matching (which
 524                     // only inspects the upvar types).
 525                     walker.skip_current_subtree(); // subtree handled by compute_projection
 526                     for upvar_ty in substs.as_closure().upvar_tys() {
 527                         self.compute(upvar_ty);
 528                     }
 529                 }
 530
 531                 ty::FnPtr(_) => {
 532                     // let the loop iterate into the argument/return
 533                     // types appearing in the fn signature
 534                 }
 535
 536                 ty::Opaque(did, substs) => {
 537                     // all of the requirements on type parameters
 538                     // should've been checked by the instantiation
 539                     // of whatever returned this exact `impl Trait`.
 540
 541                     // for named opaque `impl Trait` types we still need to check them
 542                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 543                         let obligations = self.nominal_obligations(did, substs);
 544                         self.out.extend(obligations);
 545                     }
 546                 }
 547
 548                 ty::Dynamic(data, r) => {
 549                     // WfObject
 550                     //
 551                     // Here, we defer WF checking due to higher-ranked
 552                     // regions. This is perhaps not ideal.
 553                     self.from_object_ty(ty, data, r);
 554
 555                     // FIXME(#27579) RFC also considers adding trait
 556                     // obligations that don't refer to Self and
 557                     // checking those
 558
 559                     let defer_to_coercion = self.infcx.tcx.features().object_safe_for_dispatch;
 560
 561                     if !defer_to_coercion {
 562                         let cause = self.cause(traits::MiscObligation);
 563                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 564                         self.out.extend(component_traits.map(|did| {
 565                             traits::Obligation::new(
 566                                 cause.clone(),
 567                                 param_env,
 568                                 ty::Predicate::ObjectSafe(did),
 569                             )
 570                         }));
 571                     }
 572                 }
 573
 574                 // Inference variables are the complicated case, since we don't
 575                 // know what type they are. We do two things:
 576                 //
 577                 // 1. Check if they have been resolved, and if so proceed with
 578                 //    THAT type.
 579                 // 2. If not, check whether this is the type that we
 580                 //    started with (ty0). In that case, we've made no
 581                 //    progress at all, so return false. Otherwise,
 582                 //    we've at least simplified things (i.e., we went
 583                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 584                 //    register a pending obligation and keep
 585                 //    moving. (Goal is that an "inductive hypothesis"
 586                 //    is satisfied to ensure termination.)
 587                 ty::Infer(_) => {
 588                     let ty = self.infcx.shallow_resolve(ty);
 589                     if let ty::Infer(_) = ty.kind {
 590                         // not yet resolved...
 591                         if ty == ty0 {
 592                             // ...this is the type we started from! no progress.
 593                             return false;
 594                         }
 595
 596                         let cause = self.cause(traits::MiscObligation);
 597                         self.out.push(
 598                             // ...not the type we started from, so we made progress.
 599                             traits::Obligation::new(
 600                                 cause,
 601                                 self.param_env,
 602                                 ty::Predicate::WellFormed(ty),
 603                             ),
 604                         );
 605                     } else {
 606                         // Yes, resolved, proceed with the
 607                         // result. Should never return false because
 608                         // `ty` is not a Infer.
 609                         assert!(self.compute(ty));
 610                     }
 611                 }
 612             }
 613         }
 614
 615         // if we made it through that loop above, we made progress!
 616         true
 617     }
 618
 619     fn nominal_obligations(
 620         &mut self,
 621         def_id: DefId,
 622         substs: SubstsRef<'tcx>,
 623     ) -> Vec<traits::PredicateObligation<'tcx>> {
 624         let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
 625         predicates
 626             .predicates
 627             .into_iter()
 628             .zip(predicates.spans.into_iter())
 629             .map(|(pred, span)| {
 630                 let cause = self.cause(traits::BindingObligation(def_id, span));
 631                 traits::Obligation::new(cause, self.param_env, pred)
 632             })
 633             .filter(|pred| !pred.has_escaping_bound_vars())
 634             .collect()
 635     }
 636
 637     fn from_object_ty(
 638         &mut self,
 639         ty: Ty<'tcx>,
 640         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 641         region: ty::Region<'tcx>,
 642     ) {
 643         // Imagine a type like this:
 644         //
 645         //     trait Foo { }
 646         //     trait Bar<'c> : 'c { }
 647         //
 648         //     &'b (Foo+'c+Bar<'d>)
 649         //         ^
 650         //
 651         // In this case, the following relationships must hold:
 652         //
 653         //     'b <= 'c
 654         //     'd <= 'c
 655         //
 656         // The first conditions is due to the normal region pointer
 657         // rules, which say that a reference cannot outlive its
 658         // referent.
 659         //
 660         // The final condition may be a bit surprising. In particular,
 661         // you may expect that it would have been `'c <= 'd`, since
 662         // usually lifetimes of outer things are conservative
 663         // approximations for inner things. However, it works somewhat
 664         // differently with trait objects: here the idea is that if the
 665         // user specifies a region bound (`'c`, in this case) it is the
 666         // "master bound" that *implies* that bounds from other traits are
 667         // all met. (Remember that *all bounds* in a type like
 668         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 669         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 670         // 'y.)
 671         //
 672         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 673         // am looking forward to the future here.
 674         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 675             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 676
 677             let explicit_bound = region;
 678
 679             self.out.reserve(implicit_bounds.len());
 680             for implicit_bound in implicit_bounds {
 681                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 682                 let outlives =
 683                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 684                 self.out.push(traits::Obligation::new(
 685                     cause,
 686                     self.param_env,
 687                     outlives.to_predicate(),
 688                 ));
 689             }
 690         }
 691     }
 692 }
 693
 694 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 695 /// bounds that must hold on the elided self type. These are derived
 696 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 697 /// they declare `trait SomeTrait : 'static`, for example, then
 698 /// `'static` would appear in the list. The hard work is done by
 699 /// `infer::required_region_bounds`, see that for more information.
 700 pub fn object_region_bounds<'tcx>(
 701     tcx: TyCtxt<'tcx>,
 702     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 703 ) -> Vec<ty::Region<'tcx>> {
 704     // Since we don't actually *know* the self type for an object,
 705     // this "open(err)" serves as a kind of dummy standin -- basically
 706     // a placeholder type.
 707     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 708
 709     let predicates = existential_predicates
 710         .iter()
 711         .filter_map(|predicate| {
 712             if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 713                 None
 714             } else {
 715                 Some(predicate.with_self_ty(tcx, open_ty))
 716             }
 717         })
 718         .collect();
 719
 720     required_region_bounds(tcx, open_ty, predicates)
 721 }
 722
 723 /// Find the span of a generic bound affecting an associated type.
 724 fn get_generic_bound_spans(
 725     generics: &hir::Generics<'_>,
 726     trait_name: Option<&Ident>,
 727     assoc_item_name: Ident,
 728 ) -> Vec<Span> {
 729     let mut bounds = vec![];
 730     for clause in generics.where_clause.predicates.iter() {
 731         if let hir::WherePredicate::BoundPredicate(pred) = clause {
 732             match &pred.bounded_ty.kind {
 733                 hir::TyKind::Path(hir::QPath::Resolved(Some(ty), path)) => {
 734                     let mut s = path.segments.iter();
 735                     if let (a, Some(b), None) = (s.next(), s.next(), s.next()) {
 736                         if a.map(|s| &s.ident) == trait_name
 737                             && b.ident == assoc_item_name
 738                             && is_self_path(&ty.kind)
 739                         {
 740                             // `<Self as Foo>::Bar`
 741                             bounds.push(pred.span);
 742                         }
 743                     }
 744                 }
 745                 hir::TyKind::Path(hir::QPath::TypeRelative(ty, segment)) => {
 746                     if segment.ident == assoc_item_name {
 747                         if is_self_path(&ty.kind) {
 748                             // `Self::Bar`
 749                             bounds.push(pred.span);
 750                         }
 751                     }
 752                 }
 753                 _ => {}
 754             }
 755         }
 756     }
 757     bounds
 758 }
 759
 760 fn is_self_path(kind: &hir::TyKind<'_>) -> bool {
 761     if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = kind {
 762         let mut s = path.segments.iter();
 763         if let (Some(segment), None) = (s.next(), s.next()) {
 764             if segment.ident.name == kw::SelfUpper {
 765                 // `type(Self)`
 766                 return true;
 767             }
 768         }
 769     }
 770     false
 771 }