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