src/librustc/ty/wf.rs

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