src/librustc/traits/wf.rs

   1 use crate::infer::opaque_types::required_region_bounds;
   2 use crate::infer::InferCtxt;
   3 use crate::middle::lang_items;
   4 use crate::traits::{self, AssocTypeBoundData};
   5 use crate::ty::subst::SubstsRef;
   6 use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
   7 use rustc_hir as hir;
   8 use rustc_hir::def_id::DefId;
   9 use rustc_span::symbol::{kw, Ident};
  10 use rustc_span::Span;
  11 use std::iter::once;
  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         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         self.out
 147             .iter()
 148             .inspect(|pred| assert!(!pred.has_escaping_bound_vars()))
 149             .flat_map(|pred| {
 150                 let mut selcx = traits::SelectionContext::new(infcx);
 151                 let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
 152                 once(pred.value).chain(pred.obligations)
 153             })
 154             .collect()
 155     }
 156
 157     /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
 158     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 159         let tcx = self.infcx.tcx;
 160         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 161
 162         let cause = self.cause(traits::MiscObligation);
 163         let param_env = self.param_env;
 164
 165         let item = &self.item;
 166         let extend_cause_with_original_assoc_item_obligation =
 167             |cause: &mut traits::ObligationCause<'_>,
 168              pred: &ty::Predicate<'_>,
 169              trait_assoc_items: ty::AssocItemsIterator<'_>| {
 170                 let trait_item = tcx
 171                     .hir()
 172                     .as_local_hir_id(trait_ref.def_id)
 173                     .and_then(|trait_id| tcx.hir().find(trait_id));
 174                 let (trait_name, trait_generics) = match trait_item {
 175                     Some(hir::Node::Item(hir::Item {
 176                         ident,
 177                         kind: hir::ItemKind::Trait(.., generics, _, _),
 178                         ..
 179                     }))
 180                     | Some(hir::Node::Item(hir::Item {
 181                         ident,
 182                         kind: hir::ItemKind::TraitAlias(generics, _),
 183                         ..
 184                     })) => (Some(ident), Some(generics)),
 185                     _ => (None, None),
 186                 };
 187
 188                 let item_span = item.map(|i| tcx.sess.source_map().def_span(i.span));
 189                 match pred {
 190                     ty::Predicate::Projection(proj) => {
 191                         // The obligation comes not from the current `impl` nor the `trait` being
 192                         // implemented, but rather from a "second order" obligation, like in
 193                         // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs`:
 194                         //
 195                         //   error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
 196                         //     --> $DIR/point-at-type-on-obligation-failure.rs:13:5
 197                         //      |
 198                         //   LL |     type Ok;
 199                         //      |          -- associated type defined here
 200                         //   ...
 201                         //   LL | impl Bar for Foo {
 202                         //      | ---------------- in this `impl` item
 203                         //   LL |     type Ok = ();
 204                         //      |     ^^^^^^^^^^^^^ expected `u32`, found `()`
 205                         //      |
 206                         //      = note: expected type `u32`
 207                         //                 found type `()`
 208                         //
 209                         // FIXME: we would want to point a span to all places that contributed to this
 210                         // obligation. In the case above, it should be closer to:
 211                         //
 212                         //   error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
 213                         //     --> $DIR/point-at-type-on-obligation-failure.rs:13:5
 214                         //      |
 215                         //   LL |     type Ok;
 216                         //      |          -- associated type defined here
 217                         //   LL |     type Sibling: Bar2<Ok=Self::Ok>;
 218                         //      |     -------------------------------- obligation set here
 219                         //   ...
 220                         //   LL | impl Bar for Foo {
 221                         //      | ---------------- in this `impl` item
 222                         //   LL |     type Ok = ();
 223                         //      |     ^^^^^^^^^^^^^ expected `u32`, found `()`
 224                         //   ...
 225                         //   LL | impl Bar2 for Foo2 {
 226                         //      | ---------------- in this `impl` item
 227                         //   LL |     type Ok = u32;
 228                         //      |     -------------- obligation set here
 229                         //      |
 230                         //      = note: expected type `u32`
 231                         //                 found type `()`
 232                         if let Some(hir::ItemKind::Impl(.., impl_items)) = item.map(|i| &i.kind) {
 233                             let trait_assoc_item = tcx.associated_item(proj.projection_def_id());
 234                             if let Some(impl_item) = impl_items
 235                                 .iter()
 236                                 .filter(|item| item.ident == trait_assoc_item.ident)
 237                                 .next()
 238                             {
 239                                 cause.span = impl_item.span;
 240                                 cause.code = traits::AssocTypeBound(Box::new(AssocTypeBoundData {
 241                                     impl_span: item_span,
 242                                     original: trait_assoc_item.ident.span,
 243                                     bounds: vec![],
 244                                 }));
 245                             }
 246                         }
 247                     }
 248                     ty::Predicate::Trait(proj) => {
 249                         // An associated item obligation born out of the `trait` failed to be met.
 250                         // Point at the `impl` that failed the obligation, the associated item that
 251                         // needed to meet the obligation, and the definition of that associated item,
 252                         // which should hold the obligation in most cases. An example can be seen in
 253                         // `src/test/ui/associated-types/point-at-type-on-obligation-failure-2.rs`:
 254                         //
 255                         //   error[E0277]: the trait bound `bool: Bar` is not satisfied
 256                         //     --> $DIR/point-at-type-on-obligation-failure-2.rs:8:5
 257                         //      |
 258                         //   LL |     type Assoc: Bar;
 259                         //      |          ----- associated type defined here
 260                         //   ...
 261                         //   LL | impl Foo for () {
 262                         //      | --------------- in this `impl` item
 263                         //   LL |     type Assoc = bool;
 264                         //      |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
 265                         //
 266                         // If the obligation comes from the where clause in the `trait`, we point at it:
 267                         //
 268                         //   error[E0277]: the trait bound `bool: Bar` is not satisfied
 269                         //     --> $DIR/point-at-type-on-obligation-failure-2.rs:8:5
 270                         //      |
 271                         //      | trait Foo where <Self as Foo>>::Assoc: Bar {
 272                         //      |                 -------------------------- restricted in this bound
 273                         //   LL |     type Assoc;
 274                         //      |          ----- associated type defined here
 275                         //   ...
 276                         //   LL | impl Foo for () {
 277                         //      | --------------- in this `impl` item
 278                         //   LL |     type Assoc = bool;
 279                         //      |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
 280                         if let (
 281                             ty::Projection(ty::ProjectionTy { item_def_id, .. }),
 282                             Some(hir::ItemKind::Impl(.., impl_items)),
 283                         ) = (&proj.skip_binder().self_ty().kind, item.map(|i| &i.kind))
 284                         {
 285                             if let Some((impl_item, trait_assoc_item)) = trait_assoc_items
 286                                 .filter(|i| i.def_id == *item_def_id)
 287                                 .next()
 288                                 .and_then(|trait_assoc_item| {
 289                                     impl_items
 290                                         .iter()
 291                                         .filter(|i| i.ident == trait_assoc_item.ident)
 292                                         .next()
 293                                         .map(|impl_item| (impl_item, trait_assoc_item))
 294                                 })
 295                             {
 296                                 let bounds = trait_generics
 297                                     .map(|generics| {
 298                                         get_generic_bound_spans(
 299                                             &generics,
 300                                             trait_name,
 301                                             trait_assoc_item.ident,
 302                                         )
 303                                     })
 304                                     .unwrap_or_else(Vec::new);
 305                                 cause.span = impl_item.span;
 306                                 cause.code = traits::AssocTypeBound(Box::new(AssocTypeBoundData {
 307                                     impl_span: item_span,
 308                                     original: trait_assoc_item.ident.span,
 309                                     bounds,
 310                                 }));
 311                             }
 312                         }
 313                     }
 314                     _ => {}
 315                 }
 316             };
 317
 318         if let Elaborate::All = elaborate {
 319             let trait_assoc_items = tcx.associated_items(trait_ref.def_id);
 320
 321             let predicates =
 322                 obligations.iter().map(|obligation| obligation.predicate.clone()).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.clone(),
 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.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) = constant.val {
 363             let obligations = self.nominal_obligations(def_id, substs);
 364             self.out.extend(obligations);
 365
 366             let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
 367             let cause = self.cause(traits::MiscObligation);
 368             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 369         }
 370     }
 371
 372     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 373         if !subty.has_escaping_bound_vars() {
 374             let cause = self.cause(cause);
 375             let trait_ref = ty::TraitRef {
 376                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
 377                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 378             };
 379             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
 380         }
 381     }
 382
 383     /// Pushes new obligations into `out`. Returns `true` if it was able
 384     /// to generate all the predicates needed to validate that `ty0`
 385     /// is WF. Returns false if `ty0` is an unresolved type variable,
 386     /// in which case we are not able to simplify at all.
 387     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 388         let mut subtys = ty0.walk();
 389         let param_env = self.param_env;
 390         while let Some(ty) = subtys.next() {
 391             match ty.kind {
 392                 ty::Bool
 393                 | ty::Char
 394                 | ty::Int(..)
 395                 | ty::Uint(..)
 396                 | ty::Float(..)
 397                 | ty::Error
 398                 | ty::Str
 399                 | ty::GeneratorWitness(..)
 400                 | ty::Never
 401                 | ty::Param(_)
 402                 | ty::Bound(..)
 403                 | ty::Placeholder(..)
 404                 | ty::Foreign(..) => {
 405                     // WfScalar, WfParameter, etc
 406                 }
 407
 408                 ty::Slice(subty) => {
 409                     self.require_sized(subty, traits::SliceOrArrayElem);
 410                 }
 411
 412                 ty::Array(subty, len) => {
 413                     self.require_sized(subty, traits::SliceOrArrayElem);
 414                     self.compute_array_len(*len);
 415                 }
 416
 417                 ty::Tuple(ref tys) => {
 418                     if let Some((_last, rest)) = tys.split_last() {
 419                         for elem in rest {
 420                             self.require_sized(elem.expect_ty(), traits::TupleElem);
 421                         }
 422                     }
 423                 }
 424
 425                 ty::RawPtr(_) => {
 426                     // simple cases that are WF if their type args are WF
 427                 }
 428
 429                 ty::Projection(data) => {
 430                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 431                     self.compute_projection(data);
 432                 }
 433
 434                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 435
 436                 ty::Adt(def, substs) => {
 437                     // WfNominalType
 438                     let obligations = self.nominal_obligations(def.did, substs);
 439                     self.out.extend(obligations);
 440                 }
 441
 442                 ty::FnDef(did, substs) => {
 443                     let obligations = self.nominal_obligations(did, substs);
 444                     self.out.extend(obligations);
 445                 }
 446
 447                 ty::Ref(r, rty, _) => {
 448                     // WfReference
 449                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 450                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 451                         self.out.push(traits::Obligation::new(
 452                             cause,
 453                             param_env,
 454                             ty::Predicate::TypeOutlives(ty::Binder::dummy(ty::OutlivesPredicate(
 455                                 rty, r,
 456                             ))),
 457                         ));
 458                     }
 459                 }
 460
 461                 ty::Generator(..) => {
 462                     // Walk ALL the types in the generator: this will
 463                     // include the upvar types as well as the yield
 464                     // type. Note that this is mildly distinct from
 465                     // the closure case, where we have to be careful
 466                     // about the signature of the closure. We don't
 467                     // have the problem of implied bounds here since
 468                     // generators don't take arguments.
 469                 }
 470
 471                 ty::Closure(def_id, substs) => {
 472                     // Only check the upvar types for WF, not the rest
 473                     // of the types within. This is needed because we
 474                     // capture the signature and it may not be WF
 475                     // without the implied bounds. Consider a closure
 476                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 477                     // not known to hold in the creator's context (and
 478                     // indeed the closure may not be invoked by its
 479                     // creator, but rather turned to someone who *can*
 480                     // verify that).
 481                     //
 482                     // The special treatment of closures here really
 483                     // ought not to be necessary either; the problem
 484                     // is related to #25860 -- there is no way for us
 485                     // to express a fn type complete with the implied
 486                     // bounds that it is assuming. I think in reality
 487                     // the WF rules around fn are a bit messed up, and
 488                     // that is the rot problem: `fn(&'a T)` should
 489                     // probably always be WF, because it should be
 490                     // shorthand for something like `where(T: 'a) {
 491                     // fn(&'a T) }`, as discussed in #25860.
 492                     //
 493                     // Note that we are also skipping the generic
 494                     // types. This is consistent with the `outlives`
 495                     // code, but anyway doesn't matter: within the fn
 496                     // body where they are created, the generics will
 497                     // always be WF, and outside of that fn body we
 498                     // are not directly inspecting closure types
 499                     // anyway, except via auto trait matching (which
 500                     // only inspects the upvar types).
 501                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 502                     for upvar_ty in substs.as_closure().upvar_tys(def_id, self.infcx.tcx) {
 503                         self.compute(upvar_ty);
 504                     }
 505                 }
 506
 507                 ty::FnPtr(_) => {
 508                     // let the loop iterate into the argument/return
 509                     // types appearing in the fn signature
 510                 }
 511
 512                 ty::Opaque(did, substs) => {
 513                     // all of the requirements on type parameters
 514                     // should've been checked by the instantiation
 515                     // of whatever returned this exact `impl Trait`.
 516
 517                     // for named opaque `impl Trait` types we still need to check them
 518                     if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 519                         let obligations = self.nominal_obligations(did, substs);
 520                         self.out.extend(obligations);
 521                     }
 522                 }
 523
 524                 ty::Dynamic(data, r) => {
 525                     // WfObject
 526                     //
 527                     // Here, we defer WF checking due to higher-ranked
 528                     // regions. This is perhaps not ideal.
 529                     self.from_object_ty(ty, data, r);
 530
 531                     // FIXME(#27579) RFC also considers adding trait
 532                     // obligations that don't refer to Self and
 533                     // checking those
 534
 535                     let defer_to_coercion = self.infcx.tcx.features().object_safe_for_dispatch;
 536
 537                     if !defer_to_coercion {
 538                         let cause = self.cause(traits::MiscObligation);
 539                         let component_traits = data.auto_traits().chain(data.principal_def_id());
 540                         self.out.extend(component_traits.map(|did| {
 541                             traits::Obligation::new(
 542                                 cause.clone(),
 543                                 param_env,
 544                                 ty::Predicate::ObjectSafe(did),
 545                             )
 546                         }));
 547                     }
 548                 }
 549
 550                 // Inference variables are the complicated case, since we don't
 551                 // know what type they are. We do two things:
 552                 //
 553                 // 1. Check if they have been resolved, and if so proceed with
 554                 //    THAT type.
 555                 // 2. If not, check whether this is the type that we
 556                 //    started with (ty0). In that case, we've made no
 557                 //    progress at all, so return false. Otherwise,
 558                 //    we've at least simplified things (i.e., we went
 559                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 560                 //    register a pending obligation and keep
 561                 //    moving. (Goal is that an "inductive hypothesis"
 562                 //    is satisfied to ensure termination.)
 563                 ty::Infer(_) => {
 564                     let ty = self.infcx.shallow_resolve(ty);
 565                     if let ty::Infer(_) = ty.kind {
 566                         // not yet resolved...
 567                         if ty == ty0 {
 568                             // ...this is the type we started from! no progress.
 569                             return false;
 570                         }
 571
 572                         let cause = self.cause(traits::MiscObligation);
 573                         self.out.push(
 574                             // ...not the type we started from, so we made progress.
 575                             traits::Obligation::new(
 576                                 cause,
 577                                 self.param_env,
 578                                 ty::Predicate::WellFormed(ty),
 579                             ),
 580                         );
 581                     } else {
 582                         // Yes, resolved, proceed with the
 583                         // result. Should never return false because
 584                         // `ty` is not a Infer.
 585                         assert!(self.compute(ty));
 586                     }
 587                 }
 588             }
 589         }
 590
 591         // if we made it through that loop above, we made progress!
 592         return true;
 593     }
 594
 595     fn nominal_obligations(
 596         &mut self,
 597         def_id: DefId,
 598         substs: SubstsRef<'tcx>,
 599     ) -> Vec<traits::PredicateObligation<'tcx>> {
 600         let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
 601         let cause = self.cause(traits::ItemObligation(def_id));
 602         predicates
 603             .predicates
 604             .into_iter()
 605             .map(|pred| traits::Obligation::new(cause.clone(), self.param_env, pred))
 606             .filter(|pred| !pred.has_escaping_bound_vars())
 607             .collect()
 608     }
 609
 610     fn from_object_ty(
 611         &mut self,
 612         ty: Ty<'tcx>,
 613         data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 614         region: ty::Region<'tcx>,
 615     ) {
 616         // Imagine a type like this:
 617         //
 618         //     trait Foo { }
 619         //     trait Bar<'c> : 'c { }
 620         //
 621         //     &'b (Foo+'c+Bar<'d>)
 622         //         ^
 623         //
 624         // In this case, the following relationships must hold:
 625         //
 626         //     'b <= 'c
 627         //     'd <= 'c
 628         //
 629         // The first conditions is due to the normal region pointer
 630         // rules, which say that a reference cannot outlive its
 631         // referent.
 632         //
 633         // The final condition may be a bit surprising. In particular,
 634         // you may expect that it would have been `'c <= 'd`, since
 635         // usually lifetimes of outer things are conservative
 636         // approximations for inner things. However, it works somewhat
 637         // differently with trait objects: here the idea is that if the
 638         // user specifies a region bound (`'c`, in this case) it is the
 639         // "master bound" that *implies* that bounds from other traits are
 640         // all met. (Remember that *all bounds* in a type like
 641         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 642         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 643         // 'y.)
 644         //
 645         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 646         // am looking forward to the future here.
 647         if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
 648             let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
 649
 650             let explicit_bound = region;
 651
 652             self.out.reserve(implicit_bounds.len());
 653             for implicit_bound in implicit_bounds {
 654                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 655                 let outlives =
 656                     ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 657                 self.out.push(traits::Obligation::new(
 658                     cause,
 659                     self.param_env,
 660                     outlives.to_predicate(),
 661                 ));
 662             }
 663         }
 664     }
 665 }
 666
 667 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 668 /// bounds that must hold on the elided self type. These are derived
 669 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 670 /// they declare `trait SomeTrait : 'static`, for example, then
 671 /// `'static` would appear in the list. The hard work is done by
 672 /// `infer::required_region_bounds`, see that for more information.
 673 pub fn object_region_bounds<'tcx>(
 674     tcx: TyCtxt<'tcx>,
 675     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 676 ) -> Vec<ty::Region<'tcx>> {
 677     // Since we don't actually *know* the self type for an object,
 678     // this "open(err)" serves as a kind of dummy standin -- basically
 679     // a placeholder type.
 680     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 681
 682     let predicates = existential_predicates
 683         .iter()
 684         .filter_map(|predicate| {
 685             if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 686                 None
 687             } else {
 688                 Some(predicate.with_self_ty(tcx, open_ty))
 689             }
 690         })
 691         .collect();
 692
 693     required_region_bounds(tcx, open_ty, predicates)
 694 }
 695
 696 /// Find the span of a generic bound affecting an associated type.
 697 fn get_generic_bound_spans(
 698     generics: &hir::Generics<'_>,
 699     trait_name: Option<&Ident>,
 700     assoc_item_name: Ident,
 701 ) -> Vec<Span> {
 702     let mut bounds = vec![];
 703     for clause in generics.where_clause.predicates.iter() {
 704         if let hir::WherePredicate::BoundPredicate(pred) = clause {
 705             match &pred.bounded_ty.kind {
 706                 hir::TyKind::Path(hir::QPath::Resolved(Some(ty), path)) => {
 707                     let mut s = path.segments.iter();
 708                     if let (a, Some(b), None) = (s.next(), s.next(), s.next()) {
 709                         if a.map(|s| &s.ident) == trait_name
 710                             && b.ident == assoc_item_name
 711                             && is_self_path(&ty.kind)
 712                         {
 713                             // `<Self as Foo>::Bar`
 714                             bounds.push(pred.span);
 715                         }
 716                     }
 717                 }
 718                 hir::TyKind::Path(hir::QPath::TypeRelative(ty, segment)) => {
 719                     if segment.ident == assoc_item_name {
 720                         if is_self_path(&ty.kind) {
 721                             // `Self::Bar`
 722                             bounds.push(pred.span);
 723                         }
 724                     }
 725                 }
 726                 _ => {}
 727             }
 728         }
 729     }
 730     bounds
 731 }
 732
 733 fn is_self_path(kind: &hir::TyKind<'_>) -> bool {
 734     match kind {
 735         hir::TyKind::Path(hir::QPath::Resolved(None, path)) => {
 736             let mut s = path.segments.iter();
 737             if let (Some(segment), None) = (s.next(), s.next()) {
 738                 if segment.ident.name == kw::SelfUpper {
 739                     // `type(Self)`
 740                     return true;
 741                 }
 742             }
 743         }
 744         _ => {}
 745     }
 746     false
 747 }