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 middle::const_val::{ConstVal, ConstAggregate};
  13 use infer::InferCtxt;
  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,
  36                                 param_env,
  37                                 body_id,
  38                                 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, Elaborate::All);
  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, Elaborate::None); // (*)
  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         ty::Predicate::ConstEvaluatable(def_id, substs) => {
 106             let obligations = wf.nominal_obligations(def_id, substs);
 107             wf.out.extend(obligations);
 108
 109             for ty in substs.types() {
 110                 wf.compute(ty);
 111             }
 112         }
 113     }
 114
 115     wf.normalize()
 116 }
 117
 118 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 119     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 120     param_env: ty::ParamEnv<'tcx>,
 121     body_id: ast::NodeId,
 122     span: Span,
 123     out: Vec<traits::PredicateObligation<'tcx>>,
 124 }
 125
 126 /// Controls whether we "elaborate" supertraits and so forth on the WF
 127 /// predicates. This is a kind of hack to address #43784. The
 128 /// underlying problem in that issue was a trait structure like:
 129 ///
 130 /// ```
 131 /// trait Foo: Copy { }
 132 /// trait Bar: Foo { }
 133 /// impl<T: Bar> Foo for T { }
 134 /// impl<T> Bar for T { }
 135 /// ```
 136 ///
 137 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 138 /// we decide that this is true because `T: Bar` is in the
 139 /// where-clauses (and we can elaborate that to include `T:
 140 /// Copy`). This wouldn't be a problem, except that when we check the
 141 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 142 /// impl. And so nowhere did we check that `T: Copy` holds!
 143 ///
 144 /// To resolve this, we elaborate the WF requirements that must be
 145 /// proven when checking impls. This means that (e.g.) the `impl Bar
 146 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 147 /// Copy` (which it won't be able to do, because there is no `Copy`
 148 /// impl for `T`).
 149 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 150 enum Elaborate {
 151     All,
 152     None,
 153 }
 154
 155 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 156     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 157         traits::ObligationCause::new(self.span, self.body_id, code)
 158     }
 159
 160     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 161         let cause = self.cause(traits::MiscObligation);
 162         let infcx = &mut self.infcx;
 163         let param_env = self.param_env;
 164         self.out.iter()
 165                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 166                 .flat_map(|pred| {
 167                     let mut selcx = traits::SelectionContext::new(infcx);
 168                     let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
 169                     once(pred.value).chain(pred.obligations)
 170                 })
 171                 .collect()
 172     }
 173
 174     /// Pushes the obligations required for `trait_ref` to be WF into
 175     /// `self.out`.
 176     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 177         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 178
 179         let cause = self.cause(traits::MiscObligation);
 180         let param_env = self.param_env;
 181
 182         if let Elaborate::All = elaborate {
 183             let predicates = obligations.iter()
 184                                         .map(|obligation| obligation.predicate.clone())
 185                                         .collect();
 186             let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates);
 187             let implied_obligations = implied_obligations.map(|pred| {
 188                 traits::Obligation::new(cause.clone(), param_env, pred)
 189             });
 190             self.out.extend(implied_obligations);
 191         }
 192
 193         self.out.extend(obligations);
 194
 195         self.out.extend(
 196             trait_ref.substs.types()
 197                             .filter(|ty| !ty.has_escaping_regions())
 198                             .map(|ty| traits::Obligation::new(cause.clone(),
 199                                                               param_env,
 200                                                               ty::Predicate::WellFormed(ty))));
 201     }
 202
 203     /// Pushes the obligations required for `trait_ref::Item` to be WF
 204     /// into `self.out`.
 205     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 206         // A projection is well-formed if (a) the trait ref itself is
 207         // WF and (b) the trait-ref holds.  (It may also be
 208         // normalizable and be WF that way.)
 209         let trait_ref = data.trait_ref(self.infcx.tcx);
 210         self.compute_trait_ref(&trait_ref, Elaborate::None);
 211
 212         if !data.has_escaping_regions() {
 213             let predicate = trait_ref.to_predicate();
 214             let cause = self.cause(traits::ProjectionWf(data));
 215             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 216         }
 217     }
 218
 219     /// Pushes the obligations required for a constant value to be WF
 220     /// into `self.out`.
 221     fn compute_const(&mut self, constant: &'tcx ty::Const<'tcx>) {
 222         self.require_sized(constant.ty, traits::ConstSized);
 223         match constant.val {
 224             ConstVal::Integral(_) |
 225             ConstVal::Float(_) |
 226             ConstVal::Str(_) |
 227             ConstVal::ByteStr(_) |
 228             ConstVal::Bool(_) |
 229             ConstVal::Char(_) |
 230             ConstVal::Variant(_) |
 231             ConstVal::Function(..) => {}
 232             ConstVal::Aggregate(ConstAggregate::Struct(fields)) => {
 233                 for &(_, v) in fields {
 234                     self.compute_const(v);
 235                 }
 236             }
 237             ConstVal::Aggregate(ConstAggregate::Tuple(fields)) |
 238             ConstVal::Aggregate(ConstAggregate::Array(fields)) => {
 239                 for v in fields {
 240                     self.compute_const(v);
 241                 }
 242             }
 243             ConstVal::Aggregate(ConstAggregate::Repeat(v, _)) => {
 244                 self.compute_const(v);
 245             }
 246             ConstVal::Unevaluated(def_id, substs) => {
 247                 let obligations = self.nominal_obligations(def_id, substs);
 248                 self.out.extend(obligations);
 249
 250                 let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
 251                 let cause = self.cause(traits::MiscObligation);
 252                 self.out.push(traits::Obligation::new(cause,
 253                                                       self.param_env,
 254                                                       predicate));
 255             }
 256         }
 257     }
 258
 259     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 260         if !subty.has_escaping_regions() {
 261             let cause = self.cause(cause);
 262             let trait_ref = ty::TraitRef {
 263                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 264                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 265             };
 266             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
 267         }
 268     }
 269
 270     /// Push new obligations into `out`. Returns true if it was able
 271     /// to generate all the predicates needed to validate that `ty0`
 272     /// is WF. Returns false if `ty0` is an unresolved type variable,
 273     /// in which case we are not able to simplify at all.
 274     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 275         let mut subtys = ty0.walk();
 276         let param_env = self.param_env;
 277         while let Some(ty) = subtys.next() {
 278             match ty.sty {
 279                 ty::TyBool |
 280                 ty::TyChar |
 281                 ty::TyInt(..) |
 282                 ty::TyUint(..) |
 283                 ty::TyFloat(..) |
 284                 ty::TyError |
 285                 ty::TyStr |
 286                 ty::TyNever |
 287                 ty::TyParam(_) |
 288                 ty::TyForeign(..) => {
 289                     // WfScalar, WfParameter, etc
 290                 }
 291
 292                 ty::TySlice(subty) => {
 293                     self.require_sized(subty, traits::SliceOrArrayElem);
 294                 }
 295
 296                 ty::TyArray(subty, len) => {
 297                     self.require_sized(subty, traits::SliceOrArrayElem);
 298                     assert_eq!(len.ty, self.infcx.tcx.types.usize);
 299                     self.compute_const(len);
 300                 }
 301
 302                 ty::TyTuple(ref tys, _) => {
 303                     if let Some((_last, rest)) = tys.split_last() {
 304                         for elem in rest {
 305                             self.require_sized(elem, traits::TupleElem);
 306                         }
 307                     }
 308                 }
 309
 310                 ty::TyRawPtr(_) => {
 311                     // simple cases that are WF if their type args are WF
 312                 }
 313
 314                 ty::TyProjection(data) => {
 315                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 316                     self.compute_projection(data);
 317                 }
 318
 319                 ty::TyAdt(def, substs) => {
 320                     // WfNominalType
 321                     let obligations = self.nominal_obligations(def.did, substs);
 322                     self.out.extend(obligations);
 323                 }
 324
 325                 ty::TyRef(r, mt) => {
 326                     // WfReference
 327                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 328                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 329                         self.out.push(
 330                             traits::Obligation::new(
 331                                 cause,
 332                                 param_env,
 333                                 ty::Predicate::TypeOutlives(
 334                                     ty::Binder(
 335                                         ty::OutlivesPredicate(mt.ty, r)))));
 336                     }
 337                 }
 338
 339                 ty::TyGenerator(..) => {
 340                     // Walk ALL the types in the generator: this will
 341                     // include the upvar types as well as the yield
 342                     // type. Note that this is mildly distinct from
 343                     // the closure case, where we have to be careful
 344                     // about the signature of the closure. We don't
 345                     // have the problem of implied bounds here since
 346                     // generators don't take arguments.
 347                 }
 348
 349                 ty::TyClosure(def_id, substs) => {
 350                     // Only check the upvar types for WF, not the rest
 351                     // of the types within. This is needed because we
 352                     // capture the signature and it may not be WF
 353                     // without the implied bounds. Consider a closure
 354                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 355                     // not known to hold in the creator's context (and
 356                     // indeed the closure may not be invoked by its
 357                     // creator, but rather turned to someone who *can*
 358                     // verify that).
 359                     //
 360                     // The special treatment of closures here really
 361                     // ought not to be necessary either; the problem
 362                     // is related to #25860 -- there is no way for us
 363                     // to express a fn type complete with the implied
 364                     // bounds that it is assuming. I think in reality
 365                     // the WF rules around fn are a bit messed up, and
 366                     // that is the rot problem: `fn(&'a T)` should
 367                     // probably always be WF, because it should be
 368                     // shorthand for something like `where(T: 'a) {
 369                     // fn(&'a T) }`, as discussed in #25860.
 370                     //
 371                     // Note that we are also skipping the generic
 372                     // types. This is consistent with the `outlives`
 373                     // code, but anyway doesn't matter: within the fn
 374                     // body where they are created, the generics will
 375                     // always be WF, and outside of that fn body we
 376                     // are not directly inspecting closure types
 377                     // anyway, except via auto trait matching (which
 378                     // only inspects the upvar types).
 379                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 380                     for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) {
 381                         self.compute(upvar_ty);
 382                     }
 383                 }
 384
 385                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 386                     // let the loop iterate into the argument/return
 387                     // types appearing in the fn signature
 388                 }
 389
 390                 ty::TyAnon(..) => {
 391                     // all of the requirements on type parameters
 392                     // should've been checked by the instantiation
 393                     // of whatever returned this exact `impl Trait`.
 394                 }
 395
 396                 ty::TyDynamic(data, r) => {
 397                     // WfObject
 398                     //
 399                     // Here, we defer WF checking due to higher-ranked
 400                     // regions. This is perhaps not ideal.
 401                     self.from_object_ty(ty, data, r);
 402
 403                     // FIXME(#27579) RFC also considers adding trait
 404                     // obligations that don't refer to Self and
 405                     // checking those
 406
 407                     let cause = self.cause(traits::MiscObligation);
 408                     let component_traits =
 409                         data.auto_traits().chain(data.principal().map(|p| p.def_id()));
 410                     self.out.extend(
 411                         component_traits.map(|did| traits::Obligation::new(
 412                             cause.clone(),
 413                             param_env,
 414                             ty::Predicate::ObjectSafe(did)
 415                         ))
 416                     );
 417                 }
 418
 419                 // Inference variables are the complicated case, since we don't
 420                 // know what type they are. We do two things:
 421                 //
 422                 // 1. Check if they have been resolved, and if so proceed with
 423                 //    THAT type.
 424                 // 2. If not, check whether this is the type that we
 425                 //    started with (ty0). In that case, we've made no
 426                 //    progress at all, so return false. Otherwise,
 427                 //    we've at least simplified things (i.e., we went
 428                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 429                 //    register a pending obligation and keep
 430                 //    moving. (Goal is that an "inductive hypothesis"
 431                 //    is satisfied to ensure termination.)
 432                 ty::TyInfer(_) => {
 433                     let ty = self.infcx.shallow_resolve(ty);
 434                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 435                         if ty == ty0 { // ...this is the type we started from! no progress.
 436                             return false;
 437                         }
 438
 439                         let cause = self.cause(traits::MiscObligation);
 440                         self.out.push( // ...not the type we started from, so we made progress.
 441                             traits::Obligation::new(cause,
 442                                                     self.param_env,
 443                                                     ty::Predicate::WellFormed(ty)));
 444                     } else {
 445                         // Yes, resolved, proceed with the
 446                         // result. Should never return false because
 447                         // `ty` is not a TyInfer.
 448                         assert!(self.compute(ty));
 449                     }
 450                 }
 451             }
 452         }
 453
 454         // if we made it through that loop above, we made progress!
 455         return true;
 456     }
 457
 458     fn nominal_obligations(&mut self,
 459                            def_id: DefId,
 460                            substs: &Substs<'tcx>)
 461                            -> Vec<traits::PredicateObligation<'tcx>>
 462     {
 463         let predicates =
 464             self.infcx.tcx.predicates_of(def_id)
 465                           .instantiate(self.infcx.tcx, substs);
 466         let cause = self.cause(traits::ItemObligation(def_id));
 467         predicates.predicates
 468                   .into_iter()
 469                   .map(|pred| traits::Obligation::new(cause.clone(),
 470                                                       self.param_env,
 471                                                       pred))
 472                   .filter(|pred| !pred.has_escaping_regions())
 473                   .collect()
 474     }
 475
 476     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 477                       data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
 478                       region: ty::Region<'tcx>) {
 479         // Imagine a type like this:
 480         //
 481         //     trait Foo { }
 482         //     trait Bar<'c> : 'c { }
 483         //
 484         //     &'b (Foo+'c+Bar<'d>)
 485         //         ^
 486         //
 487         // In this case, the following relationships must hold:
 488         //
 489         //     'b <= 'c
 490         //     'd <= 'c
 491         //
 492         // The first conditions is due to the normal region pointer
 493         // rules, which say that a reference cannot outlive its
 494         // referent.
 495         //
 496         // The final condition may be a bit surprising. In particular,
 497         // you may expect that it would have been `'c <= 'd`, since
 498         // usually lifetimes of outer things are conservative
 499         // approximations for inner things. However, it works somewhat
 500         // differently with trait objects: here the idea is that if the
 501         // user specifies a region bound (`'c`, in this case) it is the
 502         // "master bound" that *implies* that bounds from other traits are
 503         // all met. (Remember that *all bounds* in a type like
 504         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 505         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 506         // 'y.)
 507         //
 508         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 509         // am looking forward to the future here.
 510
 511         if !data.has_escaping_regions() {
 512             let implicit_bounds =
 513                 object_region_bounds(self.infcx.tcx, data);
 514
 515             let explicit_bound = region;
 516
 517             for implicit_bound in implicit_bounds {
 518                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 519                 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 520                 self.out.push(traits::Obligation::new(cause,
 521                                                       self.param_env,
 522                                                       outlives.to_predicate()));
 523             }
 524         }
 525     }
 526 }
 527
 528 /// Given an object type like `SomeTrait+Send`, computes the lifetime
 529 /// bounds that must hold on the elided self type. These are derived
 530 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 531 /// they declare `trait SomeTrait : 'static`, for example, then
 532 /// `'static` would appear in the list. The hard work is done by
 533 /// `ty::required_region_bounds`, see that for more information.
 534 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 535     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 536     existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
 537     -> Vec<ty::Region<'tcx>>
 538 {
 539     // Since we don't actually *know* the self type for an object,
 540     // this "open(err)" serves as a kind of dummy standin -- basically
 541     // a skolemized type.
 542     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 543
 544     let predicates = existential_predicates.iter().filter_map(|predicate| {
 545         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 546             None
 547         } else {
 548             Some(predicate.with_self_ty(tcx, open_ty))
 549         }
 550     }).collect();
 551
 552     tcx.required_region_bounds(open_ty, predicates)
 553 }