src/librustc/ty/wf.rs

   1 use crate::hir;
   2 use crate::hir::def_id::DefId;
   3 use crate::infer::InferCtxt;
   4 use crate::ty::subst::SubstsRef;
   5 use crate::traits;
   6 use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
   7 use std::iter::once;
   8 use syntax_pos::Span;
   9 use crate::middle::lang_items;
  10
  11 /// Returns the set of obligations needed to make `ty` well-formed.
  12 /// If `ty` contains unresolved inference variables, this may include
  13 /// further WF obligations. However, if `ty` IS an unresolved
  14 /// inference variable, returns `None`, because we are not able to
  15 /// make any progress at all. This is to prevent "livelock" where we
  16 /// say "$0 is WF if $0 is WF".
  17 pub fn obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  18                                    param_env: ty::ParamEnv<'tcx>,
  19                                    body_id: hir::HirId,
  20                                    ty: Ty<'tcx>,
  21                                    span: Span)
  22                                    -> Option<Vec<traits::PredicateObligation<'tcx>>>
  23 {
  24     let mut wf = WfPredicates { infcx,
  25                                 param_env,
  26                                 body_id,
  27                                 span,
  28                                 out: vec![] };
  29     if wf.compute(ty) {
  30         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  31         let result = wf.normalize();
  32         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  33         Some(result)
  34     } else {
  35         None // no progress made, return None
  36     }
  37 }
  38
  39 /// Returns the obligations that make this trait reference
  40 /// well-formed.  For example, if there is a trait `Set` defined like
  41 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  42 /// if `Bar: Eq`.
  43 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  44                                          param_env: ty::ParamEnv<'tcx>,
  45                                          body_id: hir::HirId,
  46                                          trait_ref: &ty::TraitRef<'tcx>,
  47                                          span: Span)
  48                                          -> Vec<traits::PredicateObligation<'tcx>>
  49 {
  50     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
  51     wf.compute_trait_ref(trait_ref, Elaborate::All);
  52     wf.normalize()
  53 }
  54
  55 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, '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 {
  62     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
  63
  64     // (*) ok to skip binders, because wf code is prepared for it
  65     match *predicate {
  66         ty::Predicate::Trait(ref t) => {
  67             wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
  68         }
  69         ty::Predicate::RegionOutlives(..) => {
  70         }
  71         ty::Predicate::TypeOutlives(ref t) => {
  72             wf.compute(t.skip_binder().0);
  73         }
  74         ty::Predicate::Projection(ref t) => {
  75             let t = t.skip_binder(); // (*)
  76             wf.compute_projection(t.projection_ty);
  77             wf.compute(t.ty);
  78         }
  79         ty::Predicate::WellFormed(t) => {
  80             wf.compute(t);
  81         }
  82         ty::Predicate::ObjectSafe(_) => {
  83         }
  84         ty::Predicate::ClosureKind(..) => {
  85         }
  86         ty::Predicate::Subtype(ref data) => {
  87             wf.compute(data.skip_binder().a); // (*)
  88             wf.compute(data.skip_binder().b); // (*)
  89         }
  90         ty::Predicate::ConstEvaluatable(def_id, substs) => {
  91             let obligations = wf.nominal_obligations(def_id, substs);
  92             wf.out.extend(obligations);
  93
  94             for ty in substs.types() {
  95                 wf.compute(ty);
  96             }
  97         }
  98     }
  99
 100     wf.normalize()
 101 }
 102
 103 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 104     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 105     param_env: ty::ParamEnv<'tcx>,
 106     body_id: hir::HirId,
 107     span: Span,
 108     out: Vec<traits::PredicateObligation<'tcx>>,
 109 }
 110
 111 /// Controls whether we "elaborate" supertraits and so forth on the WF
 112 /// predicates. This is a kind of hack to address #43784. The
 113 /// underlying problem in that issue was a trait structure like:
 114 ///
 115 /// ```
 116 /// trait Foo: Copy { }
 117 /// trait Bar: Foo { }
 118 /// impl<T: Bar> Foo for T { }
 119 /// impl<T> Bar for T { }
 120 /// ```
 121 ///
 122 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 123 /// we decide that this is true because `T: Bar` is in the
 124 /// where-clauses (and we can elaborate that to include `T:
 125 /// Copy`). This wouldn't be a problem, except that when we check the
 126 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 127 /// impl. And so nowhere did we check that `T: Copy` holds!
 128 ///
 129 /// To resolve this, we elaborate the WF requirements that must be
 130 /// proven when checking impls. This means that (e.g.) the `impl Bar
 131 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 132 /// Copy` (which it won't be able to do, because there is no `Copy`
 133 /// impl for `T`).
 134 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 135 enum Elaborate {
 136     All,
 137     None,
 138 }
 139
 140 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 141     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 142         traits::ObligationCause::new(self.span, self.body_id, code)
 143     }
 144
 145     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 146         let cause = self.cause(traits::MiscObligation);
 147         let infcx = &mut self.infcx;
 148         let param_env = self.param_env;
 149         self.out.iter()
 150                 .inspect(|pred| assert!(!pred.has_escaping_bound_vars()))
 151                 .flat_map(|pred| {
 152                     let mut selcx = traits::SelectionContext::new(infcx);
 153                     let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
 154                     once(pred.value).chain(pred.obligations)
 155                 })
 156                 .collect()
 157     }
 158
 159     /// Pushes the obligations required for `trait_ref` to be WF into
 160     /// `self.out`.
 161     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 162         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 163
 164         let cause = self.cause(traits::MiscObligation);
 165         let param_env = self.param_env;
 166
 167         if let Elaborate::All = elaborate {
 168             let predicates = obligations.iter()
 169                                         .map(|obligation| obligation.predicate.clone())
 170                                         .collect();
 171             let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates);
 172             let implied_obligations = implied_obligations.map(|pred| {
 173                 traits::Obligation::new(cause.clone(), param_env, pred)
 174             });
 175             self.out.extend(implied_obligations);
 176         }
 177
 178         self.out.extend(obligations);
 179
 180         self.out.extend(
 181             trait_ref.substs.types()
 182                             .filter(|ty| !ty.has_escaping_bound_vars())
 183                             .map(|ty| traits::Obligation::new(cause.clone(),
 184                                                               param_env,
 185                                                               ty::Predicate::WellFormed(ty))));
 186     }
 187
 188     /// Pushes the obligations required for `trait_ref::Item` to be WF
 189     /// into `self.out`.
 190     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 191         // A projection is well-formed if (a) the trait ref itself is
 192         // WF and (b) the trait-ref holds.  (It may also be
 193         // normalizable and be WF that way.)
 194         let trait_ref = data.trait_ref(self.infcx.tcx);
 195         self.compute_trait_ref(&trait_ref, Elaborate::None);
 196
 197         if !data.has_escaping_bound_vars() {
 198             let predicate = trait_ref.to_predicate();
 199             let cause = self.cause(traits::ProjectionWf(data));
 200             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 201         }
 202     }
 203
 204     /// Pushes the obligations required for an array length to be WF
 205     /// into `self.out`.
 206     fn compute_array_len(&mut self, constant: ty::LazyConst<'tcx>) {
 207         if let ty::LazyConst::Unevaluated(def_id, substs) = constant {
 208             let obligations = self.nominal_obligations(def_id, substs);
 209             self.out.extend(obligations);
 210
 211             let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
 212             let cause = self.cause(traits::MiscObligation);
 213             self.out.push(traits::Obligation::new(cause,
 214                                                   self.param_env,
 215                                                   predicate));
 216         }
 217     }
 218
 219     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 220         if !subty.has_escaping_bound_vars() {
 221             let cause = self.cause(cause);
 222             let trait_ref = ty::TraitRef {
 223                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 224                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 225             };
 226             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
 227         }
 228     }
 229
 230     /// Pushes new obligations into `out`. Returns `true` if it was able
 231     /// to generate all the predicates needed to validate that `ty0`
 232     /// is WF. Returns false if `ty0` is an unresolved type variable,
 233     /// in which case we are not able to simplify at all.
 234     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 235         let mut subtys = ty0.walk();
 236         let param_env = self.param_env;
 237         while let Some(ty) = subtys.next() {
 238             match ty.sty {
 239                 ty::Bool |
 240                 ty::Char |
 241                 ty::Int(..) |
 242                 ty::Uint(..) |
 243                 ty::Float(..) |
 244                 ty::Error |
 245                 ty::Str |
 246                 ty::GeneratorWitness(..) |
 247                 ty::Never |
 248                 ty::Param(_) |
 249                 ty::Bound(..) |
 250                 ty::Placeholder(..) |
 251                 ty::Foreign(..) => {
 252                     // WfScalar, WfParameter, etc
 253                 }
 254
 255                 ty::Slice(subty) => {
 256                     self.require_sized(subty, traits::SliceOrArrayElem);
 257                 }
 258
 259                 ty::Array(subty, len) => {
 260                     self.require_sized(subty, traits::SliceOrArrayElem);
 261                     self.compute_array_len(*len);
 262                 }
 263
 264                 ty::Tuple(ref tys) => {
 265                     if let Some((_last, rest)) = tys.split_last() {
 266                         for elem in rest {
 267                             self.require_sized(elem, traits::TupleElem);
 268                         }
 269                     }
 270                 }
 271
 272                 ty::RawPtr(_) => {
 273                     // simple cases that are WF if their type args are WF
 274                 }
 275
 276                 ty::Projection(data) => {
 277                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 278                     self.compute_projection(data);
 279                 }
 280
 281                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 282
 283                 ty::Adt(def, substs) => {
 284                     // WfNominalType
 285                     let obligations = self.nominal_obligations(def.did, substs);
 286                     self.out.extend(obligations);
 287                 }
 288
 289                 ty::FnDef(did, substs) => {
 290                     let obligations = self.nominal_obligations(did, substs);
 291                     self.out.extend(obligations);
 292                 }
 293
 294                 ty::Ref(r, rty, _) => {
 295                     // WfReference
 296                     if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
 297                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 298                         self.out.push(
 299                             traits::Obligation::new(
 300                                 cause,
 301                                 param_env,
 302                                 ty::Predicate::TypeOutlives(
 303                                     ty::Binder::dummy(
 304                                         ty::OutlivesPredicate(rty, r)))));
 305                     }
 306                 }
 307
 308                 ty::Generator(..) => {
 309                     // Walk ALL the types in the generator: this will
 310                     // include the upvar types as well as the yield
 311                     // type. Note that this is mildly distinct from
 312                     // the closure case, where we have to be careful
 313                     // about the signature of the closure. We don't
 314                     // have the problem of implied bounds here since
 315                     // generators don't take arguments.
 316                 }
 317
 318                 ty::Closure(def_id, substs) => {
 319                     // Only check the upvar types for WF, not the rest
 320                     // of the types within. This is needed because we
 321                     // capture the signature and it may not be WF
 322                     // without the implied bounds. Consider a closure
 323                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 324                     // not known to hold in the creator's context (and
 325                     // indeed the closure may not be invoked by its
 326                     // creator, but rather turned to someone who *can*
 327                     // verify that).
 328                     //
 329                     // The special treatment of closures here really
 330                     // ought not to be necessary either; the problem
 331                     // is related to #25860 -- there is no way for us
 332                     // to express a fn type complete with the implied
 333                     // bounds that it is assuming. I think in reality
 334                     // the WF rules around fn are a bit messed up, and
 335                     // that is the rot problem: `fn(&'a T)` should
 336                     // probably always be WF, because it should be
 337                     // shorthand for something like `where(T: 'a) {
 338                     // fn(&'a T) }`, as discussed in #25860.
 339                     //
 340                     // Note that we are also skipping the generic
 341                     // types. This is consistent with the `outlives`
 342                     // code, but anyway doesn't matter: within the fn
 343                     // body where they are created, the generics will
 344                     // always be WF, and outside of that fn body we
 345                     // are not directly inspecting closure types
 346                     // anyway, except via auto trait matching (which
 347                     // only inspects the upvar types).
 348                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 349                     for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) {
 350                         self.compute(upvar_ty);
 351                     }
 352                 }
 353
 354                 ty::FnPtr(_) => {
 355                     // let the loop iterate into the argument/return
 356                     // types appearing in the fn signature
 357                 }
 358
 359                 ty::Opaque(did, substs) => {
 360                     // all of the requirements on type parameters
 361                     // should've been checked by the instantiation
 362                     // of whatever returned this exact `impl Trait`.
 363
 364                     // for named existential types we still need to check them
 365                     if super::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
 366                         let obligations = self.nominal_obligations(did, substs);
 367                         self.out.extend(obligations);
 368                     }
 369                 }
 370
 371                 ty::Dynamic(data, r) => {
 372                     // WfObject
 373                     //
 374                     // Here, we defer WF checking due to higher-ranked
 375                     // regions. This is perhaps not ideal.
 376                     self.from_object_ty(ty, data, r);
 377
 378                     // FIXME(#27579) RFC also considers adding trait
 379                     // obligations that don't refer to Self and
 380                     // checking those
 381
 382                     let cause = self.cause(traits::MiscObligation);
 383                     let component_traits =
 384                         data.auto_traits().chain(data.principal_def_id());
 385                     self.out.extend(
 386                         component_traits.map(|did| traits::Obligation::new(
 387                             cause.clone(),
 388                             param_env,
 389                             ty::Predicate::ObjectSafe(did)
 390                         ))
 391                     );
 392                 }
 393
 394                 // Inference variables are the complicated case, since we don't
 395                 // know what type they are. We do two things:
 396                 //
 397                 // 1. Check if they have been resolved, and if so proceed with
 398                 //    THAT type.
 399                 // 2. If not, check whether this is the type that we
 400                 //    started with (ty0). In that case, we've made no
 401                 //    progress at all, so return false. Otherwise,
 402                 //    we've at least simplified things (i.e., we went
 403                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 404                 //    register a pending obligation and keep
 405                 //    moving. (Goal is that an "inductive hypothesis"
 406                 //    is satisfied to ensure termination.)
 407                 ty::Infer(_) => {
 408                     let ty = self.infcx.shallow_resolve(ty);
 409                     if let ty::Infer(_) = ty.sty { // not yet resolved...
 410                         if ty == ty0 { // ...this is the type we started from! no progress.
 411                             return false;
 412                         }
 413
 414                         let cause = self.cause(traits::MiscObligation);
 415                         self.out.push( // ...not the type we started from, so we made progress.
 416                             traits::Obligation::new(cause,
 417                                                     self.param_env,
 418                                                     ty::Predicate::WellFormed(ty)));
 419                     } else {
 420                         // Yes, resolved, proceed with the
 421                         // result. Should never return false because
 422                         // `ty` is not a Infer.
 423                         assert!(self.compute(ty));
 424                     }
 425                 }
 426             }
 427         }
 428
 429         // if we made it through that loop above, we made progress!
 430         return true;
 431     }
 432
 433     fn nominal_obligations(&mut self,
 434                            def_id: DefId,
 435                            substs: SubstsRef<'tcx>)
 436                            -> Vec<traits::PredicateObligation<'tcx>>
 437     {
 438         let predicates =
 439             self.infcx.tcx.predicates_of(def_id)
 440                           .instantiate(self.infcx.tcx, substs);
 441         let cause = self.cause(traits::ItemObligation(def_id));
 442         predicates.predicates
 443                   .into_iter()
 444                   .map(|pred| traits::Obligation::new(cause.clone(),
 445                                                       self.param_env,
 446                                                       pred))
 447                   .filter(|pred| !pred.has_escaping_bound_vars())
 448                   .collect()
 449     }
 450
 451     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 452                       data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
 453                       region: ty::Region<'tcx>) {
 454         // Imagine a type like this:
 455         //
 456         //     trait Foo { }
 457         //     trait Bar<'c> : 'c { }
 458         //
 459         //     &'b (Foo+'c+Bar<'d>)
 460         //         ^
 461         //
 462         // In this case, the following relationships must hold:
 463         //
 464         //     'b <= 'c
 465         //     'd <= 'c
 466         //
 467         // The first conditions is due to the normal region pointer
 468         // rules, which say that a reference cannot outlive its
 469         // referent.
 470         //
 471         // The final condition may be a bit surprising. In particular,
 472         // you may expect that it would have been `'c <= 'd`, since
 473         // usually lifetimes of outer things are conservative
 474         // approximations for inner things. However, it works somewhat
 475         // differently with trait objects: here the idea is that if the
 476         // user specifies a region bound (`'c`, in this case) it is the
 477         // "master bound" that *implies* that bounds from other traits are
 478         // all met. (Remember that *all bounds* in a type like
 479         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 480         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 481         // 'y.)
 482         //
 483         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 484         // am looking forward to the future here.
 485
 486         if !data.has_escaping_bound_vars() {
 487             let implicit_bounds =
 488                 object_region_bounds(self.infcx.tcx, data);
 489
 490             let explicit_bound = region;
 491
 492             self.out.reserve(implicit_bounds.len());
 493             for implicit_bound in implicit_bounds {
 494                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 495                 let outlives = ty::Binder::dummy(
 496                     ty::OutlivesPredicate(explicit_bound, implicit_bound));
 497                 self.out.push(traits::Obligation::new(cause,
 498                                                       self.param_env,
 499                                                       outlives.to_predicate()));
 500             }
 501         }
 502     }
 503 }
 504
 505 /// Given an object type like `SomeTrait + Send`, computes the lifetime
 506 /// bounds that must hold on the elided self type. These are derived
 507 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 508 /// they declare `trait SomeTrait : 'static`, for example, then
 509 /// `'static` would appear in the list. The hard work is done by
 510 /// `ty::required_region_bounds`, see that for more information.
 511 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 512     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 513     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>)
 514     -> Vec<ty::Region<'tcx>>
 515 {
 516     // Since we don't actually *know* the self type for an object,
 517     // this "open(err)" serves as a kind of dummy standin -- basically
 518     // a placeholder type.
 519     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 520
 521     let predicates = existential_predicates.iter().filter_map(|predicate| {
 522         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 523             None
 524         } else {
 525             Some(predicate.with_self_ty(tcx, open_ty))
 526         }
 527     }).collect();
 528
 529     tcx.required_region_bounds(open_ty, predicates)
 530 }