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;
  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::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         ty::Predicate::ConstEvaluatable(def_id, substs) => {
 102             let obligations = wf.nominal_obligations(def_id, substs);
 103             wf.out.extend(obligations);
 104
 105             for ty in substs.types() {
 106                 wf.compute(ty);
 107             }
 108         }
 109     }
 110
 111     wf.normalize()
 112 }
 113
 114 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 115     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 116     param_env: ty::ParamEnv<'tcx>,
 117     body_id: ast::NodeId,
 118     span: Span,
 119     out: Vec<traits::PredicateObligation<'tcx>>,
 120 }
 121
 122 /// Controls whether we "elaborate" supertraits and so forth on the WF
 123 /// predicates. This is a kind of hack to address #43784. The
 124 /// underlying problem in that issue was a trait structure like:
 125 ///
 126 /// ```
 127 /// trait Foo: Copy { }
 128 /// trait Bar: Foo { }
 129 /// impl<T: Bar> Foo for T { }
 130 /// impl<T> Bar for T { }
 131 /// ```
 132 ///
 133 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
 134 /// we decide that this is true because `T: Bar` is in the
 135 /// where-clauses (and we can elaborate that to include `T:
 136 /// Copy`). This wouldn't be a problem, except that when we check the
 137 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
 138 /// impl. And so nowhere did we check that `T: Copy` holds!
 139 ///
 140 /// To resolve this, we elaborate the WF requirements that must be
 141 /// proven when checking impls. This means that (e.g.) the `impl Bar
 142 /// for T` will be forced to prove not only that `T: Foo` but also `T:
 143 /// Copy` (which it won't be able to do, because there is no `Copy`
 144 /// impl for `T`).
 145 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 146 enum Elaborate {
 147     All,
 148     None,
 149 }
 150
 151 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 152     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 153         traits::ObligationCause::new(self.span, self.body_id, code)
 154     }
 155
 156     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 157         let cause = self.cause(traits::MiscObligation);
 158         let infcx = &mut self.infcx;
 159         let param_env = self.param_env;
 160         self.out.iter()
 161                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 162                 .flat_map(|pred| {
 163                     let mut selcx = traits::SelectionContext::new(infcx);
 164                     let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
 165                     once(pred.value).chain(pred.obligations)
 166                 })
 167                 .collect()
 168     }
 169
 170     /// Pushes the obligations required for `trait_ref` to be WF into
 171     /// `self.out`.
 172     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
 173         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 174
 175         let cause = self.cause(traits::MiscObligation);
 176         let param_env = self.param_env;
 177
 178         if let Elaborate::All = elaborate {
 179             let predicates = obligations.iter()
 180                                         .map(|obligation| obligation.predicate.clone())
 181                                         .collect();
 182             let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates);
 183             let implied_obligations = implied_obligations.map(|pred| {
 184                 traits::Obligation::new(cause.clone(), param_env, pred)
 185             });
 186             self.out.extend(implied_obligations);
 187         }
 188
 189         self.out.extend(obligations);
 190
 191         self.out.extend(
 192             trait_ref.substs.types()
 193                             .filter(|ty| !ty.has_escaping_regions())
 194                             .map(|ty| traits::Obligation::new(cause.clone(),
 195                                                               param_env,
 196                                                               ty::Predicate::WellFormed(ty))));
 197     }
 198
 199     /// Pushes the obligations required for `trait_ref::Item` to be WF
 200     /// into `self.out`.
 201     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 202         // A projection is well-formed if (a) the trait ref itself is
 203         // WF and (b) the trait-ref holds.  (It may also be
 204         // normalizable and be WF that way.)
 205         let trait_ref = data.trait_ref(self.infcx.tcx);
 206         self.compute_trait_ref(&trait_ref, Elaborate::None);
 207
 208         if !data.has_escaping_regions() {
 209             let predicate = trait_ref.to_predicate();
 210             let cause = self.cause(traits::ProjectionWf(data));
 211             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 212         }
 213     }
 214
 215     /// Pushes the obligations required for a constant value to be WF
 216     /// into `self.out`.
 217     fn compute_const(&mut self, constant: &'tcx ty::Const<'tcx>) {
 218         self.require_sized(constant.ty, traits::ConstSized);
 219         match constant.val {
 220             ConstVal::Value(_) => {}
 221             ConstVal::Unevaluated(def_id, substs) => {
 222                 let obligations = self.nominal_obligations(def_id, substs);
 223                 self.out.extend(obligations);
 224
 225                 let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
 226                 let cause = self.cause(traits::MiscObligation);
 227                 self.out.push(traits::Obligation::new(cause,
 228                                                       self.param_env,
 229                                                       predicate));
 230             }
 231         }
 232     }
 233
 234     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 235         if !subty.has_escaping_regions() {
 236             let cause = self.cause(cause);
 237             let trait_ref = ty::TraitRef {
 238                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 239                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 240             };
 241             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
 242         }
 243     }
 244
 245     /// Push new obligations into `out`. Returns true if it was able
 246     /// to generate all the predicates needed to validate that `ty0`
 247     /// is WF. Returns false if `ty0` is an unresolved type variable,
 248     /// in which case we are not able to simplify at all.
 249     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 250         let mut subtys = ty0.walk();
 251         let param_env = self.param_env;
 252         while let Some(ty) = subtys.next() {
 253             match ty.sty {
 254                 ty::TyBool |
 255                 ty::TyChar |
 256                 ty::TyInt(..) |
 257                 ty::TyUint(..) |
 258                 ty::TyFloat(..) |
 259                 ty::TyError |
 260                 ty::TyStr |
 261                 ty::TyGeneratorWitness(..) |
 262                 ty::TyNever |
 263                 ty::TyParam(_) |
 264                 ty::TyForeign(..) => {
 265                     // WfScalar, WfParameter, etc
 266                 }
 267
 268                 ty::TySlice(subty) => {
 269                     self.require_sized(subty, traits::SliceOrArrayElem);
 270                 }
 271
 272                 ty::TyArray(subty, len) => {
 273                     self.require_sized(subty, traits::SliceOrArrayElem);
 274                     assert_eq!(len.ty, self.infcx.tcx.types.usize);
 275                     self.compute_const(len);
 276                 }
 277
 278                 ty::TyTuple(ref tys) => {
 279                     if let Some((_last, rest)) = tys.split_last() {
 280                         for elem in rest {
 281                             self.require_sized(elem, traits::TupleElem);
 282                         }
 283                     }
 284                 }
 285
 286                 ty::TyRawPtr(_) => {
 287                     // simple cases that are WF if their type args are WF
 288                 }
 289
 290                 ty::TyProjection(data) => {
 291                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 292                     self.compute_projection(data);
 293                 }
 294
 295                 ty::TyAdt(def, substs) => {
 296                     // WfNominalType
 297                     let obligations = self.nominal_obligations(def.did, substs);
 298                     self.out.extend(obligations);
 299                 }
 300
 301                 ty::TyRef(r, mt) => {
 302                     // WfReference
 303                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 304                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 305                         self.out.push(
 306                             traits::Obligation::new(
 307                                 cause,
 308                                 param_env,
 309                                 ty::Predicate::TypeOutlives(
 310                                     ty::Binder::dummy(
 311                                         ty::OutlivesPredicate(mt.ty, r)))));
 312                     }
 313                 }
 314
 315                 ty::TyGenerator(..) => {
 316                     // Walk ALL the types in the generator: this will
 317                     // include the upvar types as well as the yield
 318                     // type. Note that this is mildly distinct from
 319                     // the closure case, where we have to be careful
 320                     // about the signature of the closure. We don't
 321                     // have the problem of implied bounds here since
 322                     // generators don't take arguments.
 323                 }
 324
 325                 ty::TyClosure(def_id, substs) => {
 326                     // Only check the upvar types for WF, not the rest
 327                     // of the types within. This is needed because we
 328                     // capture the signature and it may not be WF
 329                     // without the implied bounds. Consider a closure
 330                     // like `|x: &'a T|` -- it may be that `T: 'a` is
 331                     // not known to hold in the creator's context (and
 332                     // indeed the closure may not be invoked by its
 333                     // creator, but rather turned to someone who *can*
 334                     // verify that).
 335                     //
 336                     // The special treatment of closures here really
 337                     // ought not to be necessary either; the problem
 338                     // is related to #25860 -- there is no way for us
 339                     // to express a fn type complete with the implied
 340                     // bounds that it is assuming. I think in reality
 341                     // the WF rules around fn are a bit messed up, and
 342                     // that is the rot problem: `fn(&'a T)` should
 343                     // probably always be WF, because it should be
 344                     // shorthand for something like `where(T: 'a) {
 345                     // fn(&'a T) }`, as discussed in #25860.
 346                     //
 347                     // Note that we are also skipping the generic
 348                     // types. This is consistent with the `outlives`
 349                     // code, but anyway doesn't matter: within the fn
 350                     // body where they are created, the generics will
 351                     // always be WF, and outside of that fn body we
 352                     // are not directly inspecting closure types
 353                     // anyway, except via auto trait matching (which
 354                     // only inspects the upvar types).
 355                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 356                     for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) {
 357                         self.compute(upvar_ty);
 358                     }
 359                 }
 360
 361                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 362                     // let the loop iterate into the argument/return
 363                     // types appearing in the fn signature
 364                 }
 365
 366                 ty::TyAnon(..) => {
 367                     // all of the requirements on type parameters
 368                     // should've been checked by the instantiation
 369                     // of whatever returned this exact `impl Trait`.
 370                 }
 371
 372                 ty::TyDynamic(data, r) => {
 373                     // WfObject
 374                     //
 375                     // Here, we defer WF checking due to higher-ranked
 376                     // regions. This is perhaps not ideal.
 377                     self.from_object_ty(ty, data, r);
 378
 379                     // FIXME(#27579) RFC also considers adding trait
 380                     // obligations that don't refer to Self and
 381                     // checking those
 382
 383                     let cause = self.cause(traits::MiscObligation);
 384                     let component_traits =
 385                         data.auto_traits().chain(data.principal().map(|p| p.def_id()));
 386                     self.out.extend(
 387                         component_traits.map(|did| traits::Obligation::new(
 388                             cause.clone(),
 389                             param_env,
 390                             ty::Predicate::ObjectSafe(did)
 391                         ))
 392                     );
 393                 }
 394
 395                 // Inference variables are the complicated case, since we don't
 396                 // know what type they are. We do two things:
 397                 //
 398                 // 1. Check if they have been resolved, and if so proceed with
 399                 //    THAT type.
 400                 // 2. If not, check whether this is the type that we
 401                 //    started with (ty0). In that case, we've made no
 402                 //    progress at all, so return false. Otherwise,
 403                 //    we've at least simplified things (i.e., we went
 404                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 405                 //    register a pending obligation and keep
 406                 //    moving. (Goal is that an "inductive hypothesis"
 407                 //    is satisfied to ensure termination.)
 408                 ty::TyInfer(_) => {
 409                     let ty = self.infcx.shallow_resolve(ty);
 410                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 411                         if ty == ty0 { // ...this is the type we started from! no progress.
 412                             return false;
 413                         }
 414
 415                         let cause = self.cause(traits::MiscObligation);
 416                         self.out.push( // ...not the type we started from, so we made progress.
 417                             traits::Obligation::new(cause,
 418                                                     self.param_env,
 419                                                     ty::Predicate::WellFormed(ty)));
 420                     } else {
 421                         // Yes, resolved, proceed with the
 422                         // result. Should never return false because
 423                         // `ty` is not a TyInfer.
 424                         assert!(self.compute(ty));
 425                     }
 426                 }
 427             }
 428         }
 429
 430         // if we made it through that loop above, we made progress!
 431         return true;
 432     }
 433
 434     fn nominal_obligations(&mut self,
 435                            def_id: DefId,
 436                            substs: &Substs<'tcx>)
 437                            -> Vec<traits::PredicateObligation<'tcx>>
 438     {
 439         let predicates =
 440             self.infcx.tcx.predicates_of(def_id)
 441                           .instantiate(self.infcx.tcx, substs);
 442         let cause = self.cause(traits::ItemObligation(def_id));
 443         predicates.predicates
 444                   .into_iter()
 445                   .map(|pred| traits::Obligation::new(cause.clone(),
 446                                                       self.param_env,
 447                                                       pred))
 448                   .filter(|pred| !pred.has_escaping_regions())
 449                   .collect()
 450     }
 451
 452     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 453                       data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
 454                       region: ty::Region<'tcx>) {
 455         // Imagine a type like this:
 456         //
 457         //     trait Foo { }
 458         //     trait Bar<'c> : 'c { }
 459         //
 460         //     &'b (Foo+'c+Bar<'d>)
 461         //         ^
 462         //
 463         // In this case, the following relationships must hold:
 464         //
 465         //     'b <= 'c
 466         //     'd <= 'c
 467         //
 468         // The first conditions is due to the normal region pointer
 469         // rules, which say that a reference cannot outlive its
 470         // referent.
 471         //
 472         // The final condition may be a bit surprising. In particular,
 473         // you may expect that it would have been `'c <= 'd`, since
 474         // usually lifetimes of outer things are conservative
 475         // approximations for inner things. However, it works somewhat
 476         // differently with trait objects: here the idea is that if the
 477         // user specifies a region bound (`'c`, in this case) it is the
 478         // "master bound" that *implies* that bounds from other traits are
 479         // all met. (Remember that *all bounds* in a type like
 480         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 481         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 482         // 'y.)
 483         //
 484         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 485         // am looking forward to the future here.
 486
 487         if !data.has_escaping_regions() {
 488             let implicit_bounds =
 489                 object_region_bounds(self.infcx.tcx, data);
 490
 491             let explicit_bound = region;
 492
 493             for implicit_bound in implicit_bounds {
 494                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 495                 let outlives = ty::Binder::dummy(
 496                     ty::OutlivesPredicate(explicit_bound, implicit_bound));
 497                 self.out.push(traits::Obligation::new(cause,
 498                                                       self.param_env,
 499                                                       outlives.to_predicate()));
 500             }
 501         }
 502     }
 503 }
 504
 505 /// Given an object type like `SomeTrait+Send`, computes the lifetime
 506 /// bounds that must hold on the elided self type. These are derived
 507 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 508 /// they declare `trait SomeTrait : 'static`, for example, then
 509 /// `'static` would appear in the list. The hard work is done by
 510 /// `ty::required_region_bounds`, see that for more information.
 511 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 512     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 513     existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
 514     -> Vec<ty::Region<'tcx>>
 515 {
 516     // Since we don't actually *know* the self type for an object,
 517     // this "open(err)" serves as a kind of dummy standin -- basically
 518     // a skolemized type.
 519     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 520
 521     let predicates = existential_predicates.iter().filter_map(|predicate| {
 522         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 523             None
 524         } else {
 525             Some(predicate.with_self_ty(tcx, open_ty))
 526         }
 527     }).collect();
 528
 529     tcx.required_region_bounds(open_ty, predicates)
 530 }