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