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 util::common::ErrorReported;
  21
  22 /// Returns the set of obligations needed to make `ty` well-formed.
  23 /// If `ty` contains unresolved inference variables, this may include
  24 /// further WF obligations. However, if `ty` IS an unresolved
  25 /// inference variable, returns `None`, because we are not able to
  26 /// make any progress at all. This is to prevent "livelock" where we
  27 /// say "$0 is WF if $0 is WF".
  28 pub fn obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  29                                    body_id: ast::NodeId,
  30                                    ty: Ty<'tcx>,
  31                                    span: Span)
  32                                    -> Option<Vec<traits::PredicateObligation<'tcx>>>
  33 {
  34     let mut wf = WfPredicates { infcx: infcx,
  35                                 body_id: body_id,
  36                                 span: span,
  37                                 out: vec![] };
  38     if wf.compute(ty) {
  39         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  40         let result = wf.normalize();
  41         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  42         Some(result)
  43     } else {
  44         None // no progress made, return None
  45     }
  46 }
  47
  48 /// Returns the obligations that make this trait reference
  49 /// well-formed.  For example, if there is a trait `Set` defined like
  50 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  51 /// if `Bar: Eq`.
  52 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  53                                          body_id: ast::NodeId,
  54                                          trait_ref: &ty::TraitRef<'tcx>,
  55                                          span: Span)
  56                                          -> Vec<traits::PredicateObligation<'tcx>>
  57 {
  58     let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span, out: vec![] };
  59     wf.compute_trait_ref(trait_ref);
  60     wf.normalize()
  61 }
  62
  63 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  64                                              body_id: ast::NodeId,
  65                                              predicate: &ty::Predicate<'tcx>,
  66                                              span: Span)
  67                                              -> Vec<traits::PredicateObligation<'tcx>>
  68 {
  69     let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span, out: vec![] };
  70
  71     // (*) ok to skip binders, because wf code is prepared for it
  72     match *predicate {
  73         ty::Predicate::Trait(ref t) => {
  74             wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
  75         }
  76         ty::Predicate::Equate(ref t) => {
  77             wf.compute(t.skip_binder().0);
  78             wf.compute(t.skip_binder().1);
  79         }
  80         ty::Predicate::RegionOutlives(..) => {
  81         }
  82         ty::Predicate::TypeOutlives(ref t) => {
  83             wf.compute(t.skip_binder().0);
  84         }
  85         ty::Predicate::Projection(ref t) => {
  86             let t = t.skip_binder(); // (*)
  87             wf.compute_projection(t.projection_ty);
  88             wf.compute(t.ty);
  89         }
  90         ty::Predicate::WellFormed(t) => {
  91             wf.compute(t);
  92         }
  93         ty::Predicate::ObjectSafe(_) => {
  94         }
  95         ty::Predicate::ClosureKind(..) => {
  96         }
  97         ty::Predicate::Rfc1592(ref data) => {
  98             bug!("RFC1592 predicate `{:?}` in predicate_obligations", data);
  99         }
 100     }
 101
 102     wf.normalize()
 103 }
 104
 105 /// Implied bounds are region relationships that we deduce
 106 /// automatically.  The idea is that (e.g.) a caller must check that a
 107 /// function's argument types are well-formed immediately before
 108 /// calling that fn, and hence the *callee* can assume that its
 109 /// argument types are well-formed. This may imply certain relationships
 110 /// between generic parameters. For example:
 111 ///
 112 ///     fn foo<'a,T>(x: &'a T)
 113 ///
 114 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
 115 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
 116 #[derive(Debug)]
 117 pub enum ImpliedBound<'tcx> {
 118     RegionSubRegion(ty::Region, ty::Region),
 119     RegionSubParam(ty::Region, ty::ParamTy),
 120     RegionSubProjection(ty::Region, ty::ProjectionTy<'tcx>),
 121 }
 122
 123 /// Compute the implied bounds that a callee/impl can assume based on
 124 /// the fact that caller/projector has ensured that `ty` is WF.  See
 125 /// the `ImpliedBound` type for more details.
 126 pub fn implied_bounds<'a, 'gcx, 'tcx>(
 127     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 128     body_id: ast::NodeId,
 129     ty: Ty<'tcx>,
 130     span: Span)
 131     -> Vec<ImpliedBound<'tcx>>
 132 {
 133     // Sometimes when we ask what it takes for T: WF, we get back that
 134     // U: WF is required; in that case, we push U onto this stack and
 135     // process it next. Currently (at least) these resulting
 136     // predicates are always guaranteed to be a subset of the original
 137     // type, so we need not fear non-termination.
 138     let mut wf_types = vec![ty];
 139
 140     let mut implied_bounds = vec![];
 141
 142     while let Some(ty) = wf_types.pop() {
 143         // Compute the obligations for `ty` to be well-formed. If `ty` is
 144         // an unresolved inference variable, just substituted an empty set
 145         // -- because the return type here is going to be things we *add*
 146         // to the environment, it's always ok for this set to be smaller
 147         // than the ultimate set. (Note: normally there won't be
 148         // unresolved inference variables here anyway, but there might be
 149         // during typeck under some circumstances.)
 150         let obligations = obligations(infcx, body_id, ty, span).unwrap_or(vec![]);
 151
 152         // From the full set of obligations, just filter down to the
 153         // region relationships.
 154         implied_bounds.extend(
 155             obligations
 156             .into_iter()
 157             .flat_map(|obligation| {
 158                 assert!(!obligation.has_escaping_regions());
 159                 match obligation.predicate {
 160                     ty::Predicate::Trait(..) |
 161                     ty::Predicate::Rfc1592(..) |
 162                     ty::Predicate::Equate(..) |
 163                     ty::Predicate::Projection(..) |
 164                     ty::Predicate::ClosureKind(..) |
 165                     ty::Predicate::ObjectSafe(..) =>
 166                         vec![],
 167
 168                     ty::Predicate::WellFormed(subty) => {
 169                         wf_types.push(subty);
 170                         vec![]
 171                     }
 172
 173                     ty::Predicate::RegionOutlives(ref data) =>
 174                         match infcx.tcx.no_late_bound_regions(data) {
 175                             None =>
 176                                 vec![],
 177                             Some(ty::OutlivesPredicate(r_a, r_b)) =>
 178                                 vec![ImpliedBound::RegionSubRegion(r_b, r_a)],
 179                         },
 180
 181                     ty::Predicate::TypeOutlives(ref data) =>
 182                         match infcx.tcx.no_late_bound_regions(data) {
 183                             None => vec![],
 184                             Some(ty::OutlivesPredicate(ty_a, r_b)) => {
 185                                 let components = infcx.outlives_components(ty_a);
 186                                 implied_bounds_from_components(r_b, components)
 187                             }
 188                         },
 189                 }}));
 190     }
 191
 192     implied_bounds
 193 }
 194
 195 /// When we have an implied bound that `T: 'a`, we can further break
 196 /// this down to determine what relationships would have to hold for
 197 /// `T: 'a` to hold. We get to assume that the caller has validated
 198 /// those relationships.
 199 fn implied_bounds_from_components<'tcx>(sub_region: ty::Region,
 200                                         sup_components: Vec<Component<'tcx>>)
 201                                         -> Vec<ImpliedBound<'tcx>>
 202 {
 203     sup_components
 204         .into_iter()
 205         .flat_map(|component| {
 206             match component {
 207                 Component::Region(r) =>
 208                     vec!(ImpliedBound::RegionSubRegion(sub_region, r)),
 209                 Component::Param(p) =>
 210                     vec!(ImpliedBound::RegionSubParam(sub_region, p)),
 211                 Component::Projection(p) =>
 212                     vec!(ImpliedBound::RegionSubProjection(sub_region, p)),
 213                 Component::EscapingProjection(_) =>
 214                     // If the projection has escaping regions, don't
 215                     // try to infer any implied bounds even for its
 216                     // free components. This is conservative, because
 217                     // the caller will still have to prove that those
 218                     // free components outlive `sub_region`. But the
 219                     // idea is that the WAY that the caller proves
 220                     // that may change in the future and we want to
 221                     // give ourselves room to get smarter here.
 222                     vec!(),
 223                 Component::UnresolvedInferenceVariable(..) =>
 224                     vec!(),
 225             }
 226         })
 227         .collect()
 228 }
 229
 230 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 231     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 232     body_id: ast::NodeId,
 233     span: Span,
 234     out: Vec<traits::PredicateObligation<'tcx>>,
 235 }
 236
 237 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 238     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 239         traits::ObligationCause::new(self.span, self.body_id, code)
 240     }
 241
 242     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 243         let cause = self.cause(traits::MiscObligation);
 244         let infcx = &mut self.infcx;
 245         self.out.iter()
 246                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 247                 .flat_map(|pred| {
 248                     let mut selcx = traits::SelectionContext::new(infcx);
 249                     let pred = traits::normalize(&mut selcx, cause.clone(), pred);
 250                     once(pred.value).chain(pred.obligations)
 251                 })
 252                 .collect()
 253     }
 254
 255     /// Pushes the obligations required for `trait_ref` to be WF into
 256     /// `self.out`.
 257     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
 258         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 259         self.out.extend(obligations);
 260
 261         let cause = self.cause(traits::MiscObligation);
 262         self.out.extend(
 263             trait_ref.substs.types
 264                             .as_slice()
 265                             .iter()
 266                             .filter(|ty| !ty.has_escaping_regions())
 267                             .map(|ty| traits::Obligation::new(cause.clone(),
 268                                                               ty::Predicate::WellFormed(ty))));
 269     }
 270
 271     /// Pushes the obligations required for `trait_ref::Item` to be WF
 272     /// into `self.out`.
 273     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 274         // A projection is well-formed if (a) the trait ref itself is
 275         // WF and (b) the trait-ref holds.  (It may also be
 276         // normalizable and be WF that way.)
 277
 278         self.compute_trait_ref(&data.trait_ref);
 279
 280         if !data.has_escaping_regions() {
 281             let predicate = data.trait_ref.to_predicate();
 282             let cause = self.cause(traits::ProjectionWf(data));
 283             self.out.push(traits::Obligation::new(cause, predicate));
 284         }
 285     }
 286
 287     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>,
 288                      rfc1592: bool) {
 289         if !subty.has_escaping_regions() {
 290             let cause = self.cause(cause);
 291             match self.infcx.tcx.trait_ref_for_builtin_bound(ty::BoundSized, subty) {
 292                 Ok(trait_ref) => {
 293                     let predicate = trait_ref.to_predicate();
 294                     let predicate = if rfc1592 {
 295                         ty::Predicate::Rfc1592(box predicate)
 296                     } else {
 297                         predicate
 298                     };
 299                     self.out.push(
 300                         traits::Obligation::new(cause,
 301                                                 predicate));
 302                 }
 303                 Err(ErrorReported) => { }
 304             }
 305         }
 306     }
 307
 308     /// Push new obligations into `out`. Returns true if it was able
 309     /// to generate all the predicates needed to validate that `ty0`
 310     /// is WF. Returns false if `ty0` is an unresolved type variable,
 311     /// in which case we are not able to simplify at all.
 312     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 313         let tcx = self.infcx.tcx;
 314         let mut subtys = ty0.walk();
 315         while let Some(ty) = subtys.next() {
 316             match ty.sty {
 317                 ty::TyBool |
 318                 ty::TyChar |
 319                 ty::TyInt(..) |
 320                 ty::TyUint(..) |
 321                 ty::TyFloat(..) |
 322                 ty::TyError |
 323                 ty::TyStr |
 324                 ty::TyParam(_) => {
 325                     // WfScalar, WfParameter, etc
 326                 }
 327
 328                 ty::TySlice(subty) |
 329                 ty::TyArray(subty, _) => {
 330                     self.require_sized(subty, traits::SliceOrArrayElem, false);
 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, true);
 337                         }
 338                     }
 339                 }
 340
 341                 ty::TyBox(_) |
 342                 ty::TyRawPtr(_) => {
 343                     // simple cases that are WF if their type args are WF
 344                 }
 345
 346                 ty::TyProjection(data) => {
 347                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 348                     self.compute_projection(data);
 349                 }
 350
 351                 ty::TyEnum(def, substs) |
 352                 ty::TyStruct(def, substs) => {
 353                     // WfNominalType
 354                     let obligations = self.nominal_obligations(def.did, substs);
 355                     self.out.extend(obligations);
 356                 }
 357
 358                 ty::TyRef(r, mt) => {
 359                     // WfReference
 360                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 361                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 362                         self.out.push(
 363                             traits::Obligation::new(
 364                                 cause,
 365                                 ty::Predicate::TypeOutlives(
 366                                     ty::Binder(
 367                                         ty::OutlivesPredicate(mt.ty, *r)))));
 368                     }
 369                 }
 370
 371                 ty::TyClosure(..) => {
 372                     // the types in a closure are always the types of
 373                     // local variables (or possibly references to local
 374                     // variables), we'll walk those.
 375                     //
 376                     // (Though, local variables are probably not
 377                     // needed, as they are separately checked w/r/t
 378                     // WFedness.)
 379                 }
 380
 381                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 382                     // let the loop iterate into the argument/return
 383                     // types appearing in the fn signature
 384                 }
 385
 386                 ty::TyAnon(..) => {
 387                     // all of the requirements on type parameters
 388                     // should've been checked by the instantiation
 389                     // of whatever returned this exact `impl Trait`.
 390                 }
 391
 392                 ty::TyTrait(ref data) => {
 393                     // WfObject
 394                     //
 395                     // Here, we defer WF checking due to higher-ranked
 396                     // regions. This is perhaps not ideal.
 397                     self.from_object_ty(ty, data);
 398
 399                     // FIXME(#27579) RFC also considers adding trait
 400                     // obligations that don't refer to Self and
 401                     // checking those
 402
 403                     let cause = self.cause(traits::MiscObligation);
 404
 405                     // FIXME(#33243): remove RFC1592
 406                     self.out.push(traits::Obligation::new(
 407                         cause.clone(),
 408                         ty::Predicate::ObjectSafe(data.principal_def_id())
 409                     ));
 410                     let component_traits =
 411                         data.bounds.builtin_bounds.iter().flat_map(|bound| {
 412                             tcx.lang_items.from_builtin_kind(bound).ok()
 413                         });
 414 //                        .chain(Some(data.principal_def_id()));
 415                     self.out.extend(
 416                         component_traits.map(|did| { traits::Obligation::new(
 417                             cause.clone(),
 418                             ty::Predicate::Rfc1592(
 419                                 box ty::Predicate::ObjectSafe(did)
 420                             )
 421                         )})
 422                     );
 423                 }
 424
 425                 // Inference variables are the complicated case, since we don't
 426                 // know what type they are. We do two things:
 427                 //
 428                 // 1. Check if they have been resolved, and if so proceed with
 429                 //    THAT type.
 430                 // 2. If not, check whether this is the type that we
 431                 //    started with (ty0). In that case, we've made no
 432                 //    progress at all, so return false. Otherwise,
 433                 //    we've at least simplified things (i.e., we went
 434                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 435                 //    register a pending obligation and keep
 436                 //    moving. (Goal is that an "inductive hypothesis"
 437                 //    is satisfied to ensure termination.)
 438                 ty::TyInfer(_) => {
 439                     let ty = self.infcx.shallow_resolve(ty);
 440                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 441                         if ty == ty0 { // ...this is the type we started from! no progress.
 442                             return false;
 443                         }
 444
 445                         let cause = self.cause(traits::MiscObligation);
 446                         self.out.push( // ...not the type we started from, so we made progress.
 447                             traits::Obligation::new(cause, ty::Predicate::WellFormed(ty)));
 448                     } else {
 449                         // Yes, resolved, proceed with the
 450                         // result. Should never return false because
 451                         // `ty` is not a TyInfer.
 452                         assert!(self.compute(ty));
 453                     }
 454                 }
 455             }
 456         }
 457
 458         // if we made it through that loop above, we made progress!
 459         return true;
 460     }
 461
 462     fn nominal_obligations(&mut self,
 463                            def_id: DefId,
 464                            substs: &Substs<'tcx>)
 465                            -> Vec<traits::PredicateObligation<'tcx>>
 466     {
 467         let predicates =
 468             self.infcx.tcx.lookup_predicates(def_id)
 469                           .instantiate(self.infcx.tcx, substs);
 470         let cause = self.cause(traits::ItemObligation(def_id));
 471         predicates.predicates
 472                   .into_iter()
 473                   .map(|pred| traits::Obligation::new(cause.clone(), pred))
 474                   .filter(|pred| !pred.has_escaping_regions())
 475                   .collect()
 476     }
 477
 478     fn from_object_ty(&mut self, ty: Ty<'tcx>, data: &ty::TraitTy<'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,
 514                                      &data.principal,
 515                                      data.bounds.builtin_bounds);
 516
 517             let explicit_bound = data.bounds.region_bound;
 518
 519             for implicit_bound in implicit_bounds {
 520                 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 521                 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 522                 self.out.push(traits::Obligation::new(cause, 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     principal: &ty::PolyTraitRef<'tcx>,
 537     others: ty::BuiltinBounds)
 538     -> Vec<ty::Region>
 539 {
 540     // Since we don't actually *know* the self type for an object,
 541     // this "open(err)" serves as a kind of dummy standin -- basically
 542     // a skolemized type.
 543     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 544
 545     // Note that we preserve the overall binding levels here.
 546     assert!(!open_ty.has_escaping_regions());
 547     let substs = tcx.mk_substs(principal.0.substs.with_self_ty(open_ty));
 548     let trait_refs = vec!(ty::Binder(ty::TraitRef::new(principal.0.def_id, substs)));
 549
 550     let mut predicates = others.to_predicates(tcx, open_ty);
 551     predicates.extend(trait_refs.iter().map(|t| t.to_predicate()));
 552
 553     tcx.required_region_bounds(open_ty, predicates)
 554 }