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