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     }
  98
  99     wf.normalize()
 100 }
 101
 102 /// Implied bounds are region relationships that we deduce
 103 /// automatically.  The idea is that (e.g.) a caller must check that a
 104 /// function's argument types are well-formed immediately before
 105 /// calling that fn, and hence the *callee* can assume that its
 106 /// argument types are well-formed. This may imply certain relationships
 107 /// between generic parameters. For example:
 108 ///
 109 ///     fn foo<'a,T>(x: &'a T)
 110 ///
 111 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
 112 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
 113 #[derive(Debug)]
 114 pub enum ImpliedBound<'tcx> {
 115     RegionSubRegion(&'tcx ty::Region, &'tcx ty::Region),
 116     RegionSubParam(&'tcx ty::Region, ty::ParamTy),
 117     RegionSubProjection(&'tcx ty::Region, ty::ProjectionTy<'tcx>),
 118 }
 119
 120 /// Compute the implied bounds that a callee/impl can assume based on
 121 /// the fact that caller/projector has ensured that `ty` is WF.  See
 122 /// the `ImpliedBound` type for more details.
 123 pub fn implied_bounds<'a, 'gcx, 'tcx>(
 124     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 125     body_id: ast::NodeId,
 126     ty: Ty<'tcx>,
 127     span: Span)
 128     -> Vec<ImpliedBound<'tcx>>
 129 {
 130     // Sometimes when we ask what it takes for T: WF, we get back that
 131     // U: WF is required; in that case, we push U onto this stack and
 132     // process it next. Currently (at least) these resulting
 133     // predicates are always guaranteed to be a subset of the original
 134     // type, so we need not fear non-termination.
 135     let mut wf_types = vec![ty];
 136
 137     let mut implied_bounds = vec![];
 138
 139     while let Some(ty) = wf_types.pop() {
 140         // Compute the obligations for `ty` to be well-formed. If `ty` is
 141         // an unresolved inference variable, just substituted an empty set
 142         // -- because the return type here is going to be things we *add*
 143         // to the environment, it's always ok for this set to be smaller
 144         // than the ultimate set. (Note: normally there won't be
 145         // unresolved inference variables here anyway, but there might be
 146         // during typeck under some circumstances.)
 147         let obligations = obligations(infcx, body_id, ty, span).unwrap_or(vec![]);
 148
 149         // From the full set of obligations, just filter down to the
 150         // region relationships.
 151         implied_bounds.extend(
 152             obligations
 153             .into_iter()
 154             .flat_map(|obligation| {
 155                 assert!(!obligation.has_escaping_regions());
 156                 match obligation.predicate {
 157                     ty::Predicate::Trait(..) |
 158                     ty::Predicate::Equate(..) |
 159                     ty::Predicate::Projection(..) |
 160                     ty::Predicate::ClosureKind(..) |
 161                     ty::Predicate::ObjectSafe(..) =>
 162                         vec![],
 163
 164                     ty::Predicate::WellFormed(subty) => {
 165                         wf_types.push(subty);
 166                         vec![]
 167                     }
 168
 169                     ty::Predicate::RegionOutlives(ref data) =>
 170                         match infcx.tcx.no_late_bound_regions(data) {
 171                             None =>
 172                                 vec![],
 173                             Some(ty::OutlivesPredicate(r_a, r_b)) =>
 174                                 vec![ImpliedBound::RegionSubRegion(r_b, r_a)],
 175                         },
 176
 177                     ty::Predicate::TypeOutlives(ref data) =>
 178                         match infcx.tcx.no_late_bound_regions(data) {
 179                             None => vec![],
 180                             Some(ty::OutlivesPredicate(ty_a, r_b)) => {
 181                                 let ty_a = infcx.resolve_type_vars_if_possible(&ty_a);
 182                                 let components = infcx.tcx.outlives_components(ty_a);
 183                                 implied_bounds_from_components(r_b, components)
 184                             }
 185                         },
 186                 }}));
 187     }
 188
 189     implied_bounds
 190 }
 191
 192 /// When we have an implied bound that `T: 'a`, we can further break
 193 /// this down to determine what relationships would have to hold for
 194 /// `T: 'a` to hold. We get to assume that the caller has validated
 195 /// those relationships.
 196 fn implied_bounds_from_components<'tcx>(sub_region: &'tcx ty::Region,
 197                                         sup_components: Vec<Component<'tcx>>)
 198                                         -> Vec<ImpliedBound<'tcx>>
 199 {
 200     sup_components
 201         .into_iter()
 202         .flat_map(|component| {
 203             match component {
 204                 Component::Region(r) =>
 205                     vec![ImpliedBound::RegionSubRegion(sub_region, r)],
 206                 Component::Param(p) =>
 207                     vec![ImpliedBound::RegionSubParam(sub_region, p)],
 208                 Component::Projection(p) =>
 209                     vec![ImpliedBound::RegionSubProjection(sub_region, p)],
 210                 Component::EscapingProjection(_) =>
 211                     // If the projection has escaping regions, don't
 212                     // try to infer any implied bounds even for its
 213                     // free components. This is conservative, because
 214                     // the caller will still have to prove that those
 215                     // free components outlive `sub_region`. But the
 216                     // idea is that the WAY that the caller proves
 217                     // that may change in the future and we want to
 218                     // give ourselves room to get smarter here.
 219                     vec![],
 220                 Component::UnresolvedInferenceVariable(..) =>
 221                     vec![],
 222             }
 223         })
 224         .collect()
 225 }
 226
 227 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 228     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 229     body_id: ast::NodeId,
 230     span: Span,
 231     out: Vec<traits::PredicateObligation<'tcx>>,
 232 }
 233
 234 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 235     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 236         traits::ObligationCause::new(self.span, self.body_id, code)
 237     }
 238
 239     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 240         let cause = self.cause(traits::MiscObligation);
 241         let infcx = &mut self.infcx;
 242         self.out.iter()
 243                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 244                 .flat_map(|pred| {
 245                     let mut selcx = traits::SelectionContext::new(infcx);
 246                     let pred = traits::normalize(&mut selcx, cause.clone(), pred);
 247                     once(pred.value).chain(pred.obligations)
 248                 })
 249                 .collect()
 250     }
 251
 252     /// Pushes the obligations required for `trait_ref` to be WF into
 253     /// `self.out`.
 254     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
 255         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 256         self.out.extend(obligations);
 257
 258         let cause = self.cause(traits::MiscObligation);
 259         self.out.extend(
 260             trait_ref.substs.types()
 261                             .filter(|ty| !ty.has_escaping_regions())
 262                             .map(|ty| traits::Obligation::new(cause.clone(),
 263                                                               ty::Predicate::WellFormed(ty))));
 264     }
 265
 266     /// Pushes the obligations required for `trait_ref::Item` to be WF
 267     /// into `self.out`.
 268     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 269         // A projection is well-formed if (a) the trait ref itself is
 270         // WF and (b) the trait-ref holds.  (It may also be
 271         // normalizable and be WF that way.)
 272
 273         self.compute_trait_ref(&data.trait_ref);
 274
 275         if !data.has_escaping_regions() {
 276             let predicate = data.trait_ref.to_predicate();
 277             let cause = self.cause(traits::ProjectionWf(data));
 278             self.out.push(traits::Obligation::new(cause, predicate));
 279         }
 280     }
 281
 282     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 283         if !subty.has_escaping_regions() {
 284             let cause = self.cause(cause);
 285             let trait_ref = ty::TraitRef {
 286                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 287                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 288             };
 289             self.out.push(traits::Obligation::new(cause, trait_ref.to_predicate()));
 290         }
 291     }
 292
 293     /// Push new obligations into `out`. Returns true if it was able
 294     /// to generate all the predicates needed to validate that `ty0`
 295     /// is WF. Returns false if `ty0` is an unresolved type variable,
 296     /// in which case we are not able to simplify at all.
 297     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 298         let mut subtys = ty0.walk();
 299         while let Some(ty) = subtys.next() {
 300             match ty.sty {
 301                 ty::TyBool |
 302                 ty::TyChar |
 303                 ty::TyInt(..) |
 304                 ty::TyUint(..) |
 305                 ty::TyFloat(..) |
 306                 ty::TyError |
 307                 ty::TyStr |
 308                 ty::TyNever |
 309                 ty::TyParam(_) => {
 310                     // WfScalar, WfParameter, etc
 311                 }
 312
 313                 ty::TySlice(subty) |
 314                 ty::TyArray(subty, _) => {
 315                     self.require_sized(subty, traits::SliceOrArrayElem);
 316                 }
 317
 318                 ty::TyTuple(ref tys, _) => {
 319                     if let Some((_last, rest)) = tys.split_last() {
 320                         for elem in rest {
 321                             self.require_sized(elem, traits::TupleElem);
 322                         }
 323                     }
 324                 }
 325
 326                 ty::TyRawPtr(_) => {
 327                     // simple cases that are WF if their type args are WF
 328                 }
 329
 330                 ty::TyProjection(data) => {
 331                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 332                     self.compute_projection(data);
 333                 }
 334
 335                 ty::TyAdt(def, substs) => {
 336                     // WfNominalType
 337                     let obligations = self.nominal_obligations(def.did, substs);
 338                     self.out.extend(obligations);
 339                 }
 340
 341                 ty::TyRef(r, mt) => {
 342                     // WfReference
 343                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 344                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 345                         self.out.push(
 346                             traits::Obligation::new(
 347                                 cause,
 348                                 ty::Predicate::TypeOutlives(
 349                                     ty::Binder(
 350                                         ty::OutlivesPredicate(mt.ty, r)))));
 351                     }
 352                 }
 353
 354                 ty::TyClosure(..) => {
 355                     // the types in a closure are always the types of
 356                     // local variables (or possibly references to local
 357                     // variables), we'll walk those.
 358                     //
 359                     // (Though, local variables are probably not
 360                     // needed, as they are separately checked w/r/t
 361                     // WFedness.)
 362                 }
 363
 364                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 365                     // let the loop iterate into the argument/return
 366                     // types appearing in the fn signature
 367                 }
 368
 369                 ty::TyAnon(..) => {
 370                     // all of the requirements on type parameters
 371                     // should've been checked by the instantiation
 372                     // of whatever returned this exact `impl Trait`.
 373                 }
 374
 375                 ty::TyDynamic(data, r) => {
 376                     // WfObject
 377                     //
 378                     // Here, we defer WF checking due to higher-ranked
 379                     // regions. This is perhaps not ideal.
 380                     self.from_object_ty(ty, data, r);
 381
 382                     // FIXME(#27579) RFC also considers adding trait
 383                     // obligations that don't refer to Self and
 384                     // checking those
 385
 386                     let cause = self.cause(traits::MiscObligation);
 387
 388                     let component_traits =
 389                         data.auto_traits().chain(data.principal().map(|p| p.def_id()));
 390                     self.out.extend(
 391                         component_traits.map(|did| traits::Obligation::new(
 392                             cause.clone(),
 393                             ty::Predicate::ObjectSafe(did)
 394                         ))
 395                     );
 396                 }
 397
 398                 // Inference variables are the complicated case, since we don't
 399                 // know what type they are. We do two things:
 400                 //
 401                 // 1. Check if they have been resolved, and if so proceed with
 402                 //    THAT type.
 403                 // 2. If not, check whether this is the type that we
 404                 //    started with (ty0). In that case, we've made no
 405                 //    progress at all, so return false. Otherwise,
 406                 //    we've at least simplified things (i.e., we went
 407                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 408                 //    register a pending obligation and keep
 409                 //    moving. (Goal is that an "inductive hypothesis"
 410                 //    is satisfied to ensure termination.)
 411                 ty::TyInfer(_) => {
 412                     let ty = self.infcx.shallow_resolve(ty);
 413                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 414                         if ty == ty0 { // ...this is the type we started from! no progress.
 415                             return false;
 416                         }
 417
 418                         let cause = self.cause(traits::MiscObligation);
 419                         self.out.push( // ...not the type we started from, so we made progress.
 420                             traits::Obligation::new(cause, ty::Predicate::WellFormed(ty)));
 421                     } else {
 422                         // Yes, resolved, proceed with the
 423                         // result. Should never return false because
 424                         // `ty` is not a TyInfer.
 425                         assert!(self.compute(ty));
 426                     }
 427                 }
 428             }
 429         }
 430
 431         // if we made it through that loop above, we made progress!
 432         return true;
 433     }
 434
 435     fn nominal_obligations(&mut self,
 436                            def_id: DefId,
 437                            substs: &Substs<'tcx>)
 438                            -> Vec<traits::PredicateObligation<'tcx>>
 439     {
 440         let predicates =
 441             self.infcx.tcx.item_predicates(def_id)
 442                           .instantiate(self.infcx.tcx, substs);
 443         let cause = self.cause(traits::ItemObligation(def_id));
 444         predicates.predicates
 445                   .into_iter()
 446                   .map(|pred| traits::Obligation::new(cause.clone(), pred))
 447                   .filter(|pred| !pred.has_escaping_regions())
 448                   .collect()
 449     }
 450
 451     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 452                       data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
 453                       region: &'tcx ty::Region) {
 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_regions() {
 487             let implicit_bounds =
 488                 object_region_bounds(self.infcx.tcx, data);
 489
 490             let explicit_bound = region;
 491
 492             for implicit_bound in implicit_bounds {
 493                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 494                 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 495                 self.out.push(traits::Obligation::new(cause, outlives.to_predicate()));
 496             }
 497         }
 498     }
 499 }
 500
 501 /// Given an object type like `SomeTrait+Send`, computes the lifetime
 502 /// bounds that must hold on the elided self type. These are derived
 503 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 504 /// they declare `trait SomeTrait : 'static`, for example, then
 505 /// `'static` would appear in the list. The hard work is done by
 506 /// `ty::required_region_bounds`, see that for more information.
 507 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 508     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 509     existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
 510     -> Vec<&'tcx ty::Region>
 511 {
 512     // Since we don't actually *know* the self type for an object,
 513     // this "open(err)" serves as a kind of dummy standin -- basically
 514     // a skolemized type.
 515     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 516
 517     let predicates = existential_predicates.iter().filter_map(|predicate| {
 518         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 519             None
 520         } else {
 521             Some(predicate.with_self_ty(tcx, open_ty))
 522         }
 523     }).collect();
 524
 525     tcx.required_region_bounds(open_ty, predicates)
 526 }