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                                    param_env: ty::ParamEnv<'tcx>,
  30                                    body_id: ast::NodeId,
  31                                    ty: Ty<'tcx>,
  32                                    span: Span)
  33                                    -> Option<Vec<traits::PredicateObligation<'tcx>>>
  34 {
  35     let mut wf = WfPredicates { infcx: infcx,
  36                                 param_env: param_env,
  37                                 body_id: body_id,
  38                                 span: span,
  39                                 out: vec![] };
  40     if wf.compute(ty) {
  41         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  42         let result = wf.normalize();
  43         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  44         Some(result)
  45     } else {
  46         None // no progress made, return None
  47     }
  48 }
  49
  50 /// Returns the obligations that make this trait reference
  51 /// well-formed.  For example, if there is a trait `Set` defined like
  52 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  53 /// if `Bar: Eq`.
  54 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  55                                          param_env: ty::ParamEnv<'tcx>,
  56                                          body_id: ast::NodeId,
  57                                          trait_ref: &ty::TraitRef<'tcx>,
  58                                          span: Span)
  59                                          -> Vec<traits::PredicateObligation<'tcx>>
  60 {
  61     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
  62     wf.compute_trait_ref(trait_ref);
  63     wf.normalize()
  64 }
  65
  66 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  67                                              param_env: ty::ParamEnv<'tcx>,
  68                                              body_id: ast::NodeId,
  69                                              predicate: &ty::Predicate<'tcx>,
  70                                              span: Span)
  71                                              -> Vec<traits::PredicateObligation<'tcx>>
  72 {
  73     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
  74
  75     // (*) ok to skip binders, because wf code is prepared for it
  76     match *predicate {
  77         ty::Predicate::Trait(ref t) => {
  78             wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
  79         }
  80         ty::Predicate::Equate(ref t) => {
  81             wf.compute(t.skip_binder().0);
  82             wf.compute(t.skip_binder().1);
  83         }
  84         ty::Predicate::RegionOutlives(..) => {
  85         }
  86         ty::Predicate::TypeOutlives(ref t) => {
  87             wf.compute(t.skip_binder().0);
  88         }
  89         ty::Predicate::Projection(ref t) => {
  90             let t = t.skip_binder(); // (*)
  91             wf.compute_projection(t.projection_ty);
  92             wf.compute(t.ty);
  93         }
  94         ty::Predicate::WellFormed(t) => {
  95             wf.compute(t);
  96         }
  97         ty::Predicate::ObjectSafe(_) => {
  98         }
  99         ty::Predicate::ClosureKind(..) => {
 100         }
 101         ty::Predicate::Subtype(ref data) => {
 102             wf.compute(data.skip_binder().a); // (*)
 103             wf.compute(data.skip_binder().b); // (*)
 104         }
 105     }
 106
 107     wf.normalize()
 108 }
 109
 110 /// Implied bounds are region relationships that we deduce
 111 /// automatically.  The idea is that (e.g.) a caller must check that a
 112 /// function's argument types are well-formed immediately before
 113 /// calling that fn, and hence the *callee* can assume that its
 114 /// argument types are well-formed. This may imply certain relationships
 115 /// between generic parameters. For example:
 116 ///
 117 ///     fn foo<'a,T>(x: &'a T)
 118 ///
 119 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
 120 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
 121 #[derive(Debug)]
 122 pub enum ImpliedBound<'tcx> {
 123     RegionSubRegion(ty::Region<'tcx>, ty::Region<'tcx>),
 124     RegionSubParam(ty::Region<'tcx>, ty::ParamTy),
 125     RegionSubProjection(ty::Region<'tcx>, ty::ProjectionTy<'tcx>),
 126 }
 127
 128 /// Compute the implied bounds that a callee/impl can assume based on
 129 /// the fact that caller/projector has ensured that `ty` is WF.  See
 130 /// the `ImpliedBound` type for more details.
 131 pub fn implied_bounds<'a, 'gcx, 'tcx>(
 132     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 133     param_env: ty::ParamEnv<'tcx>,
 134     body_id: ast::NodeId,
 135     ty: Ty<'tcx>,
 136     span: Span)
 137     -> Vec<ImpliedBound<'tcx>>
 138 {
 139     // Sometimes when we ask what it takes for T: WF, we get back that
 140     // U: WF is required; in that case, we push U onto this stack and
 141     // process it next. Currently (at least) these resulting
 142     // predicates are always guaranteed to be a subset of the original
 143     // type, so we need not fear non-termination.
 144     let mut wf_types = vec![ty];
 145
 146     let mut implied_bounds = vec![];
 147
 148     while let Some(ty) = wf_types.pop() {
 149         // Compute the obligations for `ty` to be well-formed. If `ty` is
 150         // an unresolved inference variable, just substituted an empty set
 151         // -- because the return type here is going to be things we *add*
 152         // to the environment, it's always ok for this set to be smaller
 153         // than the ultimate set. (Note: normally there won't be
 154         // unresolved inference variables here anyway, but there might be
 155         // during typeck under some circumstances.)
 156         let obligations = obligations(infcx, param_env, body_id, ty, span).unwrap_or(vec![]);
 157
 158         // From the full set of obligations, just filter down to the
 159         // region relationships.
 160         implied_bounds.extend(
 161             obligations
 162             .into_iter()
 163             .flat_map(|obligation| {
 164                 assert!(!obligation.has_escaping_regions());
 165                 match obligation.predicate {
 166                     ty::Predicate::Trait(..) |
 167                     ty::Predicate::Equate(..) |
 168                     ty::Predicate::Subtype(..) |
 169                     ty::Predicate::Projection(..) |
 170                     ty::Predicate::ClosureKind(..) |
 171                     ty::Predicate::ObjectSafe(..) =>
 172                         vec![],
 173
 174                     ty::Predicate::WellFormed(subty) => {
 175                         wf_types.push(subty);
 176                         vec![]
 177                     }
 178
 179                     ty::Predicate::RegionOutlives(ref data) =>
 180                         match infcx.tcx.no_late_bound_regions(data) {
 181                             None =>
 182                                 vec![],
 183                             Some(ty::OutlivesPredicate(r_a, r_b)) =>
 184                                 vec![ImpliedBound::RegionSubRegion(r_b, r_a)],
 185                         },
 186
 187                     ty::Predicate::TypeOutlives(ref data) =>
 188                         match infcx.tcx.no_late_bound_regions(data) {
 189                             None => vec![],
 190                             Some(ty::OutlivesPredicate(ty_a, r_b)) => {
 191                                 let ty_a = infcx.resolve_type_vars_if_possible(&ty_a);
 192                                 let components = infcx.tcx.outlives_components(ty_a);
 193                                 implied_bounds_from_components(r_b, components)
 194                             }
 195                         },
 196                 }}));
 197     }
 198
 199     implied_bounds
 200 }
 201
 202 /// When we have an implied bound that `T: 'a`, we can further break
 203 /// this down to determine what relationships would have to hold for
 204 /// `T: 'a` to hold. We get to assume that the caller has validated
 205 /// those relationships.
 206 fn implied_bounds_from_components<'tcx>(sub_region: ty::Region<'tcx>,
 207                                         sup_components: Vec<Component<'tcx>>)
 208                                         -> Vec<ImpliedBound<'tcx>>
 209 {
 210     sup_components
 211         .into_iter()
 212         .flat_map(|component| {
 213             match component {
 214                 Component::Region(r) =>
 215                     vec![ImpliedBound::RegionSubRegion(sub_region, r)],
 216                 Component::Param(p) =>
 217                     vec![ImpliedBound::RegionSubParam(sub_region, p)],
 218                 Component::Projection(p) =>
 219                     vec![ImpliedBound::RegionSubProjection(sub_region, p)],
 220                 Component::EscapingProjection(_) =>
 221                     // If the projection has escaping regions, don't
 222                     // try to infer any implied bounds even for its
 223                     // free components. This is conservative, because
 224                     // the caller will still have to prove that those
 225                     // free components outlive `sub_region`. But the
 226                     // idea is that the WAY that the caller proves
 227                     // that may change in the future and we want to
 228                     // give ourselves room to get smarter here.
 229                     vec![],
 230                 Component::UnresolvedInferenceVariable(..) =>
 231                     vec![],
 232             }
 233         })
 234         .collect()
 235 }
 236
 237 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 238     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 239     param_env: ty::ParamEnv<'tcx>,
 240     body_id: ast::NodeId,
 241     span: Span,
 242     out: Vec<traits::PredicateObligation<'tcx>>,
 243 }
 244
 245 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 246     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 247         traits::ObligationCause::new(self.span, self.body_id, code)
 248     }
 249
 250     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 251         let cause = self.cause(traits::MiscObligation);
 252         let infcx = &mut self.infcx;
 253         let param_env = self.param_env;
 254         self.out.iter()
 255                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 256                 .flat_map(|pred| {
 257                     let mut selcx = traits::SelectionContext::new(infcx);
 258                     let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
 259                     once(pred.value).chain(pred.obligations)
 260                 })
 261                 .collect()
 262     }
 263
 264     /// Pushes the obligations required for `trait_ref` to be WF into
 265     /// `self.out`.
 266     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
 267         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 268         self.out.extend(obligations);
 269
 270         let cause = self.cause(traits::MiscObligation);
 271         let param_env = self.param_env;
 272         self.out.extend(
 273             trait_ref.substs.types()
 274                             .filter(|ty| !ty.has_escaping_regions())
 275                             .map(|ty| traits::Obligation::new(cause.clone(),
 276                                                               param_env,
 277                                                               ty::Predicate::WellFormed(ty))));
 278     }
 279
 280     /// Pushes the obligations required for `trait_ref::Item` to be WF
 281     /// into `self.out`.
 282     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 283         // A projection is well-formed if (a) the trait ref itself is
 284         // WF and (b) the trait-ref holds.  (It may also be
 285         // normalizable and be WF that way.)
 286
 287         self.compute_trait_ref(&data.trait_ref);
 288
 289         if !data.has_escaping_regions() {
 290             let predicate = data.trait_ref.to_predicate();
 291             let cause = self.cause(traits::ProjectionWf(data));
 292             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 293         }
 294     }
 295
 296     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 297         if !subty.has_escaping_regions() {
 298             let cause = self.cause(cause);
 299             let trait_ref = ty::TraitRef {
 300                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 301                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 302             };
 303             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
 304         }
 305     }
 306
 307     /// Push new obligations into `out`. Returns true if it was able
 308     /// to generate all the predicates needed to validate that `ty0`
 309     /// is WF. Returns false if `ty0` is an unresolved type variable,
 310     /// in which case we are not able to simplify at all.
 311     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 312         let mut subtys = ty0.walk();
 313         let param_env = self.param_env;
 314         while let Some(ty) = subtys.next() {
 315             match ty.sty {
 316                 ty::TyBool |
 317                 ty::TyChar |
 318                 ty::TyInt(..) |
 319                 ty::TyUint(..) |
 320                 ty::TyFloat(..) |
 321                 ty::TyError |
 322                 ty::TyStr |
 323                 ty::TyNever |
 324                 ty::TyParam(_) => {
 325                     // WfScalar, WfParameter, etc
 326                 }
 327
 328                 ty::TySlice(subty) |
 329                 ty::TyArray(subty, _) => {
 330                     self.require_sized(subty, traits::SliceOrArrayElem);
 331                 }
 332
 333                 ty::TyTuple(ref tys, _) => {
 334                     if let Some((_last, rest)) = tys.split_last() {
 335                         for elem in rest {
 336                             self.require_sized(elem, traits::TupleElem);
 337                         }
 338                     }
 339                 }
 340
 341                 ty::TyRawPtr(_) => {
 342                     // simple cases that are WF if their type args are WF
 343                 }
 344
 345                 ty::TyProjection(data) => {
 346                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 347                     self.compute_projection(data);
 348                 }
 349
 350                 ty::TyAdt(def, substs) => {
 351                     // WfNominalType
 352                     let obligations = self.nominal_obligations(def.did, substs);
 353                     self.out.extend(obligations);
 354                 }
 355
 356                 ty::TyRef(r, mt) => {
 357                     // WfReference
 358                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 359                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 360                         self.out.push(
 361                             traits::Obligation::new(
 362                                 cause,
 363                                 param_env,
 364                                 ty::Predicate::TypeOutlives(
 365                                     ty::Binder(
 366                                         ty::OutlivesPredicate(mt.ty, r)))));
 367                     }
 368                 }
 369
 370                 ty::TyClosure(..) => {
 371                     // the types in a closure are always the types of
 372                     // local variables (or possibly references to local
 373                     // variables), we'll walk those.
 374                     //
 375                     // (Though, local variables are probably not
 376                     // needed, as they are separately checked w/r/t
 377                     // WFedness.)
 378                 }
 379
 380                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 381                     // let the loop iterate into the argument/return
 382                     // types appearing in the fn signature
 383                 }
 384
 385                 ty::TyAnon(..) => {
 386                     // all of the requirements on type parameters
 387                     // should've been checked by the instantiation
 388                     // of whatever returned this exact `impl Trait`.
 389                 }
 390
 391                 ty::TyDynamic(data, r) => {
 392                     // WfObject
 393                     //
 394                     // Here, we defer WF checking due to higher-ranked
 395                     // regions. This is perhaps not ideal.
 396                     self.from_object_ty(ty, data, r);
 397
 398                     // FIXME(#27579) RFC also considers adding trait
 399                     // obligations that don't refer to Self and
 400                     // checking those
 401
 402                     let cause = self.cause(traits::MiscObligation);
 403                     let component_traits =
 404                         data.auto_traits().chain(data.principal().map(|p| p.def_id()));
 405                     self.out.extend(
 406                         component_traits.map(|did| traits::Obligation::new(
 407                             cause.clone(),
 408                             param_env,
 409                             ty::Predicate::ObjectSafe(did)
 410                         ))
 411                     );
 412                 }
 413
 414                 // Inference variables are the complicated case, since we don't
 415                 // know what type they are. We do two things:
 416                 //
 417                 // 1. Check if they have been resolved, and if so proceed with
 418                 //    THAT type.
 419                 // 2. If not, check whether this is the type that we
 420                 //    started with (ty0). In that case, we've made no
 421                 //    progress at all, so return false. Otherwise,
 422                 //    we've at least simplified things (i.e., we went
 423                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 424                 //    register a pending obligation and keep
 425                 //    moving. (Goal is that an "inductive hypothesis"
 426                 //    is satisfied to ensure termination.)
 427                 ty::TyInfer(_) => {
 428                     let ty = self.infcx.shallow_resolve(ty);
 429                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 430                         if ty == ty0 { // ...this is the type we started from! no progress.
 431                             return false;
 432                         }
 433
 434                         let cause = self.cause(traits::MiscObligation);
 435                         self.out.push( // ...not the type we started from, so we made progress.
 436                             traits::Obligation::new(cause,
 437                                                     self.param_env,
 438                                                     ty::Predicate::WellFormed(ty)));
 439                     } else {
 440                         // Yes, resolved, proceed with the
 441                         // result. Should never return false because
 442                         // `ty` is not a TyInfer.
 443                         assert!(self.compute(ty));
 444                     }
 445                 }
 446             }
 447         }
 448
 449         // if we made it through that loop above, we made progress!
 450         return true;
 451     }
 452
 453     fn nominal_obligations(&mut self,
 454                            def_id: DefId,
 455                            substs: &Substs<'tcx>)
 456                            -> Vec<traits::PredicateObligation<'tcx>>
 457     {
 458         let predicates =
 459             self.infcx.tcx.predicates_of(def_id)
 460                           .instantiate(self.infcx.tcx, substs);
 461         let cause = self.cause(traits::ItemObligation(def_id));
 462         predicates.predicates
 463                   .into_iter()
 464                   .map(|pred| traits::Obligation::new(cause.clone(),
 465                                                       self.param_env,
 466                                                       pred))
 467                   .filter(|pred| !pred.has_escaping_regions())
 468                   .collect()
 469     }
 470
 471     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 472                       data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
 473                       region: ty::Region<'tcx>) {
 474         // Imagine a type like this:
 475         //
 476         //     trait Foo { }
 477         //     trait Bar<'c> : 'c { }
 478         //
 479         //     &'b (Foo+'c+Bar<'d>)
 480         //         ^
 481         //
 482         // In this case, the following relationships must hold:
 483         //
 484         //     'b <= 'c
 485         //     'd <= 'c
 486         //
 487         // The first conditions is due to the normal region pointer
 488         // rules, which say that a reference cannot outlive its
 489         // referent.
 490         //
 491         // The final condition may be a bit surprising. In particular,
 492         // you may expect that it would have been `'c <= 'd`, since
 493         // usually lifetimes of outer things are conservative
 494         // approximations for inner things. However, it works somewhat
 495         // differently with trait objects: here the idea is that if the
 496         // user specifies a region bound (`'c`, in this case) it is the
 497         // "master bound" that *implies* that bounds from other traits are
 498         // all met. (Remember that *all bounds* in a type like
 499         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 500         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 501         // 'y.)
 502         //
 503         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 504         // am looking forward to the future here.
 505
 506         if !data.has_escaping_regions() {
 507             let implicit_bounds =
 508                 object_region_bounds(self.infcx.tcx, data);
 509
 510             let explicit_bound = region;
 511
 512             for implicit_bound in implicit_bounds {
 513                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 514                 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 515                 self.out.push(traits::Obligation::new(cause,
 516                                                       self.param_env,
 517                                                       outlives.to_predicate()));
 518             }
 519         }
 520     }
 521 }
 522
 523 /// Given an object type like `SomeTrait+Send`, computes the lifetime
 524 /// bounds that must hold on the elided self type. These are derived
 525 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 526 /// they declare `trait SomeTrait : 'static`, for example, then
 527 /// `'static` would appear in the list. The hard work is done by
 528 /// `ty::required_region_bounds`, see that for more information.
 529 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 530     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 531     existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
 532     -> Vec<ty::Region<'tcx>>
 533 {
 534     // Since we don't actually *know* the self type for an object,
 535     // this "open(err)" serves as a kind of dummy standin -- basically
 536     // a skolemized type.
 537     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 538
 539     let predicates = existential_predicates.iter().filter_map(|predicate| {
 540         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 541             None
 542         } else {
 543             Some(predicate.with_self_ty(tcx, open_ty))
 544         }
 545     }).collect();
 546
 547     tcx.required_region_bounds(open_ty, predicates)
 548 }