src/librustc/ty/wf.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use hir::def_id::DefId;
  12 use infer::InferCtxt;
  13 use ty::outlives::Component;
  14 use ty::subst::Substs;
  15 use traits;
  16 use ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
  17 use std::iter::once;
  18 use syntax::ast;
  19 use syntax_pos::Span;
  20 use middle::lang_items;
  21
  22 /// Returns the set of obligations needed to make `ty` well-formed.
  23 /// If `ty` contains unresolved inference variables, this may include
  24 /// further WF obligations. However, if `ty` IS an unresolved
  25 /// inference variable, returns `None`, because we are not able to
  26 /// make any progress at all. This is to prevent "livelock" where we
  27 /// say "$0 is WF if $0 is WF".
  28 pub fn obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  29                                    body_id: ast::NodeId,
  30                                    ty: Ty<'tcx>,
  31                                    span: Span)
  32                                    -> Option<Vec<traits::PredicateObligation<'tcx>>>
  33 {
  34     let mut wf = WfPredicates { infcx: infcx,
  35                                 body_id: body_id,
  36                                 span: span,
  37                                 out: vec![] };
  38     if wf.compute(ty) {
  39         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  40         let result = wf.normalize();
  41         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  42         Some(result)
  43     } else {
  44         None // no progress made, return None
  45     }
  46 }
  47
  48 /// Returns the obligations that make this trait reference
  49 /// well-formed.  For example, if there is a trait `Set` defined like
  50 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  51 /// if `Bar: Eq`.
  52 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  53                                          body_id: ast::NodeId,
  54                                          trait_ref: &ty::TraitRef<'tcx>,
  55                                          span: Span)
  56                                          -> Vec<traits::PredicateObligation<'tcx>>
  57 {
  58     let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span, out: vec![] };
  59     wf.compute_trait_ref(trait_ref);
  60     wf.normalize()
  61 }
  62
  63 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  64                                              body_id: ast::NodeId,
  65                                              predicate: &ty::Predicate<'tcx>,
  66                                              span: Span)
  67                                              -> Vec<traits::PredicateObligation<'tcx>>
  68 {
  69     let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span, out: vec![] };
  70
  71     // (*) ok to skip binders, because wf code is prepared for it
  72     match *predicate {
  73         ty::Predicate::Trait(ref t) => {
  74             wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
  75         }
  76         ty::Predicate::Equate(ref t) => {
  77             wf.compute(t.skip_binder().0);
  78             wf.compute(t.skip_binder().1);
  79         }
  80         ty::Predicate::RegionOutlives(..) => {
  81         }
  82         ty::Predicate::TypeOutlives(ref t) => {
  83             wf.compute(t.skip_binder().0);
  84         }
  85         ty::Predicate::Projection(ref t) => {
  86             let t = t.skip_binder(); // (*)
  87             wf.compute_projection(t.projection_ty);
  88             wf.compute(t.ty);
  89         }
  90         ty::Predicate::WellFormed(t) => {
  91             wf.compute(t);
  92         }
  93         ty::Predicate::ObjectSafe(_) => {
  94         }
  95         ty::Predicate::ClosureKind(..) => {
  96         }
  97         ty::Predicate::Subtype(ref data) => {
  98             wf.compute(data.skip_binder().a); // (*)
  99             wf.compute(data.skip_binder().b); // (*)
 100         }
 101     }
 102
 103     wf.normalize()
 104 }
 105
 106 /// Implied bounds are region relationships that we deduce
 107 /// automatically.  The idea is that (e.g.) a caller must check that a
 108 /// function's argument types are well-formed immediately before
 109 /// calling that fn, and hence the *callee* can assume that its
 110 /// argument types are well-formed. This may imply certain relationships
 111 /// between generic parameters. For example:
 112 ///
 113 ///     fn foo<'a,T>(x: &'a T)
 114 ///
 115 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
 116 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
 117 #[derive(Debug)]
 118 pub enum ImpliedBound<'tcx> {
 119     RegionSubRegion(&'tcx ty::Region, &'tcx ty::Region),
 120     RegionSubParam(&'tcx ty::Region, ty::ParamTy),
 121     RegionSubProjection(&'tcx ty::Region, ty::ProjectionTy<'tcx>),
 122 }
 123
 124 /// Compute the implied bounds that a callee/impl can assume based on
 125 /// the fact that caller/projector has ensured that `ty` is WF.  See
 126 /// the `ImpliedBound` type for more details.
 127 pub fn implied_bounds<'a, 'gcx, 'tcx>(
 128     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 129     body_id: ast::NodeId,
 130     ty: Ty<'tcx>,
 131     span: Span)
 132     -> Vec<ImpliedBound<'tcx>>
 133 {
 134     // Sometimes when we ask what it takes for T: WF, we get back that
 135     // U: WF is required; in that case, we push U onto this stack and
 136     // process it next. Currently (at least) these resulting
 137     // predicates are always guaranteed to be a subset of the original
 138     // type, so we need not fear non-termination.
 139     let mut wf_types = vec![ty];
 140
 141     let mut implied_bounds = vec![];
 142
 143     while let Some(ty) = wf_types.pop() {
 144         // Compute the obligations for `ty` to be well-formed. If `ty` is
 145         // an unresolved inference variable, just substituted an empty set
 146         // -- because the return type here is going to be things we *add*
 147         // to the environment, it's always ok for this set to be smaller
 148         // than the ultimate set. (Note: normally there won't be
 149         // unresolved inference variables here anyway, but there might be
 150         // during typeck under some circumstances.)
 151         let obligations = obligations(infcx, body_id, ty, span).unwrap_or(vec![]);
 152
 153         // From the full set of obligations, just filter down to the
 154         // region relationships.
 155         implied_bounds.extend(
 156             obligations
 157             .into_iter()
 158             .flat_map(|obligation| {
 159                 assert!(!obligation.has_escaping_regions());
 160                 match obligation.predicate {
 161                     ty::Predicate::Trait(..) |
 162                     ty::Predicate::Equate(..) |
 163                     ty::Predicate::Subtype(..) |
 164                     ty::Predicate::Projection(..) |
 165                     ty::Predicate::ClosureKind(..) |
 166                     ty::Predicate::ObjectSafe(..) =>
 167                         vec![],
 168
 169                     ty::Predicate::WellFormed(subty) => {
 170                         wf_types.push(subty);
 171                         vec![]
 172                     }
 173
 174                     ty::Predicate::RegionOutlives(ref data) =>
 175                         match infcx.tcx.no_late_bound_regions(data) {
 176                             None =>
 177                                 vec![],
 178                             Some(ty::OutlivesPredicate(r_a, r_b)) =>
 179                                 vec![ImpliedBound::RegionSubRegion(r_b, r_a)],
 180                         },
 181
 182                     ty::Predicate::TypeOutlives(ref data) =>
 183                         match infcx.tcx.no_late_bound_regions(data) {
 184                             None => vec![],
 185                             Some(ty::OutlivesPredicate(ty_a, r_b)) => {
 186                                 let ty_a = infcx.resolve_type_vars_if_possible(&ty_a);
 187                                 let components = infcx.tcx.outlives_components(ty_a);
 188                                 implied_bounds_from_components(r_b, components)
 189                             }
 190                         },
 191                 }}));
 192     }
 193
 194     implied_bounds
 195 }
 196
 197 /// When we have an implied bound that `T: 'a`, we can further break
 198 /// this down to determine what relationships would have to hold for
 199 /// `T: 'a` to hold. We get to assume that the caller has validated
 200 /// those relationships.
 201 fn implied_bounds_from_components<'tcx>(sub_region: &'tcx ty::Region,
 202                                         sup_components: Vec<Component<'tcx>>)
 203                                         -> Vec<ImpliedBound<'tcx>>
 204 {
 205     sup_components
 206         .into_iter()
 207         .flat_map(|component| {
 208             match component {
 209                 Component::Region(r) =>
 210                     vec![ImpliedBound::RegionSubRegion(sub_region, r)],
 211                 Component::Param(p) =>
 212                     vec![ImpliedBound::RegionSubParam(sub_region, p)],
 213                 Component::Projection(p) =>
 214                     vec![ImpliedBound::RegionSubProjection(sub_region, p)],
 215                 Component::EscapingProjection(_) =>
 216                     // If the projection has escaping regions, don't
 217                     // try to infer any implied bounds even for its
 218                     // free components. This is conservative, because
 219                     // the caller will still have to prove that those
 220                     // free components outlive `sub_region`. But the
 221                     // idea is that the WAY that the caller proves
 222                     // that may change in the future and we want to
 223                     // give ourselves room to get smarter here.
 224                     vec![],
 225                 Component::UnresolvedInferenceVariable(..) =>
 226                     vec![],
 227             }
 228         })
 229         .collect()
 230 }
 231
 232 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 233     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 234     body_id: ast::NodeId,
 235     span: Span,
 236     out: Vec<traits::PredicateObligation<'tcx>>,
 237 }
 238
 239 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 240     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 241         traits::ObligationCause::new(self.span, self.body_id, code)
 242     }
 243
 244     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 245         let cause = self.cause(traits::MiscObligation);
 246         let infcx = &mut self.infcx;
 247         self.out.iter()
 248                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 249                 .flat_map(|pred| {
 250                     let mut selcx = traits::SelectionContext::new(infcx);
 251                     let pred = traits::normalize(&mut selcx, cause.clone(), pred);
 252                     once(pred.value).chain(pred.obligations)
 253                 })
 254                 .collect()
 255     }
 256
 257     /// Pushes the obligations required for `trait_ref` to be WF into
 258     /// `self.out`.
 259     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
 260         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 261         self.out.extend(obligations);
 262
 263         let cause = self.cause(traits::MiscObligation);
 264         self.out.extend(
 265             trait_ref.substs.types()
 266                             .filter(|ty| !ty.has_escaping_regions())
 267                             .map(|ty| traits::Obligation::new(cause.clone(),
 268                                                               ty::Predicate::WellFormed(ty))));
 269     }
 270
 271     /// Pushes the obligations required for `trait_ref::Item` to be WF
 272     /// into `self.out`.
 273     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 274         // A projection is well-formed if (a) the trait ref itself is
 275         // WF and (b) the trait-ref holds.  (It may also be
 276         // normalizable and be WF that way.)
 277
 278         self.compute_trait_ref(&data.trait_ref);
 279
 280         if !data.has_escaping_regions() {
 281             let predicate = data.trait_ref.to_predicate();
 282             let cause = self.cause(traits::ProjectionWf(data));
 283             self.out.push(traits::Obligation::new(cause, predicate));
 284         }
 285     }
 286
 287     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 288         if !subty.has_escaping_regions() {
 289             let cause = self.cause(cause);
 290             let trait_ref = ty::TraitRef {
 291                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 292                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 293             };
 294             self.out.push(traits::Obligation::new(cause, trait_ref.to_predicate()));
 295         }
 296     }
 297
 298     /// Push new obligations into `out`. Returns true if it was able
 299     /// to generate all the predicates needed to validate that `ty0`
 300     /// is WF. Returns false if `ty0` is an unresolved type variable,
 301     /// in which case we are not able to simplify at all.
 302     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 303         let mut subtys = ty0.walk();
 304         while let Some(ty) = subtys.next() {
 305             match ty.sty {
 306                 ty::TyBool |
 307                 ty::TyChar |
 308                 ty::TyInt(..) |
 309                 ty::TyUint(..) |
 310                 ty::TyFloat(..) |
 311                 ty::TyError |
 312                 ty::TyStr |
 313                 ty::TyNever |
 314                 ty::TyParam(_) => {
 315                     // WfScalar, WfParameter, etc
 316                 }
 317
 318                 ty::TySlice(subty) |
 319                 ty::TyArray(subty, _) => {
 320                     self.require_sized(subty, traits::SliceOrArrayElem);
 321                 }
 322
 323                 ty::TyTuple(ref tys, _) => {
 324                     if let Some((_last, rest)) = tys.split_last() {
 325                         for elem in rest {
 326                             self.require_sized(elem, traits::TupleElem);
 327                         }
 328                     }
 329                 }
 330
 331                 ty::TyRawPtr(_) => {
 332                     // simple cases that are WF if their type args are WF
 333                 }
 334
 335                 ty::TyProjection(data) => {
 336                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 337                     self.compute_projection(data);
 338                 }
 339
 340                 ty::TyAdt(def, substs) => {
 341                     // WfNominalType
 342                     let obligations = self.nominal_obligations(def.did, substs);
 343                     self.out.extend(obligations);
 344                 }
 345
 346                 ty::TyRef(r, mt) => {
 347                     // WfReference
 348                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 349                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 350                         self.out.push(
 351                             traits::Obligation::new(
 352                                 cause,
 353                                 ty::Predicate::TypeOutlives(
 354                                     ty::Binder(
 355                                         ty::OutlivesPredicate(mt.ty, r)))));
 356                     }
 357                 }
 358
 359                 ty::TyClosure(..) => {
 360                     // the types in a closure are always the types of
 361                     // local variables (or possibly references to local
 362                     // variables), we'll walk those.
 363                     //
 364                     // (Though, local variables are probably not
 365                     // needed, as they are separately checked w/r/t
 366                     // WFedness.)
 367                 }
 368
 369                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 370                     // let the loop iterate into the argument/return
 371                     // types appearing in the fn signature
 372                 }
 373
 374                 ty::TyAnon(..) => {
 375                     // all of the requirements on type parameters
 376                     // should've been checked by the instantiation
 377                     // of whatever returned this exact `impl Trait`.
 378                 }
 379
 380                 ty::TyDynamic(data, r) => {
 381                     // WfObject
 382                     //
 383                     // Here, we defer WF checking due to higher-ranked
 384                     // regions. This is perhaps not ideal.
 385                     self.from_object_ty(ty, data, r);
 386
 387                     // FIXME(#27579) RFC also considers adding trait
 388                     // obligations that don't refer to Self and
 389                     // checking those
 390
 391                     let cause = self.cause(traits::MiscObligation);
 392
 393                     let component_traits =
 394                         data.auto_traits().chain(data.principal().map(|p| p.def_id()));
 395                     self.out.extend(
 396                         component_traits.map(|did| traits::Obligation::new(
 397                             cause.clone(),
 398                             ty::Predicate::ObjectSafe(did)
 399                         ))
 400                     );
 401                 }
 402
 403                 // Inference variables are the complicated case, since we don't
 404                 // know what type they are. We do two things:
 405                 //
 406                 // 1. Check if they have been resolved, and if so proceed with
 407                 //    THAT type.
 408                 // 2. If not, check whether this is the type that we
 409                 //    started with (ty0). In that case, we've made no
 410                 //    progress at all, so return false. Otherwise,
 411                 //    we've at least simplified things (i.e., we went
 412                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 413                 //    register a pending obligation and keep
 414                 //    moving. (Goal is that an "inductive hypothesis"
 415                 //    is satisfied to ensure termination.)
 416                 ty::TyInfer(_) => {
 417                     let ty = self.infcx.shallow_resolve(ty);
 418                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 419                         if ty == ty0 { // ...this is the type we started from! no progress.
 420                             return false;
 421                         }
 422
 423                         let cause = self.cause(traits::MiscObligation);
 424                         self.out.push( // ...not the type we started from, so we made progress.
 425                             traits::Obligation::new(cause, ty::Predicate::WellFormed(ty)));
 426                     } else {
 427                         // Yes, resolved, proceed with the
 428                         // result. Should never return false because
 429                         // `ty` is not a TyInfer.
 430                         assert!(self.compute(ty));
 431                     }
 432                 }
 433             }
 434         }
 435
 436         // if we made it through that loop above, we made progress!
 437         return true;
 438     }
 439
 440     fn nominal_obligations(&mut self,
 441                            def_id: DefId,
 442                            substs: &Substs<'tcx>)
 443                            -> Vec<traits::PredicateObligation<'tcx>>
 444     {
 445         let predicates =
 446             self.infcx.tcx.item_predicates(def_id)
 447                           .instantiate(self.infcx.tcx, substs);
 448         let cause = self.cause(traits::ItemObligation(def_id));
 449         predicates.predicates
 450                   .into_iter()
 451                   .map(|pred| traits::Obligation::new(cause.clone(), pred))
 452                   .filter(|pred| !pred.has_escaping_regions())
 453                   .collect()
 454     }
 455
 456     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 457                       data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
 458                       region: &'tcx ty::Region) {
 459         // Imagine a type like this:
 460         //
 461         //     trait Foo { }
 462         //     trait Bar<'c> : 'c { }
 463         //
 464         //     &'b (Foo+'c+Bar<'d>)
 465         //         ^
 466         //
 467         // In this case, the following relationships must hold:
 468         //
 469         //     'b <= 'c
 470         //     'd <= 'c
 471         //
 472         // The first conditions is due to the normal region pointer
 473         // rules, which say that a reference cannot outlive its
 474         // referent.
 475         //
 476         // The final condition may be a bit surprising. In particular,
 477         // you may expect that it would have been `'c <= 'd`, since
 478         // usually lifetimes of outer things are conservative
 479         // approximations for inner things. However, it works somewhat
 480         // differently with trait objects: here the idea is that if the
 481         // user specifies a region bound (`'c`, in this case) it is the
 482         // "master bound" that *implies* that bounds from other traits are
 483         // all met. (Remember that *all bounds* in a type like
 484         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 485         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 486         // 'y.)
 487         //
 488         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 489         // am looking forward to the future here.
 490
 491         if !data.has_escaping_regions() {
 492             let implicit_bounds =
 493                 object_region_bounds(self.infcx.tcx, data);
 494
 495             let explicit_bound = region;
 496
 497             for implicit_bound in implicit_bounds {
 498                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 499                 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 500                 self.out.push(traits::Obligation::new(cause, outlives.to_predicate()));
 501             }
 502         }
 503     }
 504 }
 505
 506 /// Given an object type like `SomeTrait+Send`, computes the lifetime
 507 /// bounds that must hold on the elided self type. These are derived
 508 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 509 /// they declare `trait SomeTrait : 'static`, for example, then
 510 /// `'static` would appear in the list. The hard work is done by
 511 /// `ty::required_region_bounds`, see that for more information.
 512 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 513     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 514     existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
 515     -> Vec<&'tcx ty::Region>
 516 {
 517     // Since we don't actually *know* the self type for an object,
 518     // this "open(err)" serves as a kind of dummy standin -- basically
 519     // a skolemized type.
 520     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 521
 522     let predicates = existential_predicates.iter().filter_map(|predicate| {
 523         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 524             None
 525         } else {
 526             Some(predicate.with_self_ty(tcx, open_ty))
 527         }
 528     }).collect();
 529
 530     tcx.required_region_bounds(open_ty, predicates)
 531 }