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