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::subst::Substs;
  14 use traits;
  15 use ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
  16 use std::iter::once;
  17 use syntax::ast;
  18 use syntax_pos::Span;
  19 use middle::lang_items;
  20
  21 /// Returns the set of obligations needed to make `ty` well-formed.
  22 /// If `ty` contains unresolved inference variables, this may include
  23 /// further WF obligations. However, if `ty` IS an unresolved
  24 /// inference variable, returns `None`, because we are not able to
  25 /// make any progress at all. This is to prevent "livelock" where we
  26 /// say "$0 is WF if $0 is WF".
  27 pub fn obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  28                                    param_env: ty::ParamEnv<'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,
  35                                 param_env,
  36                                 body_id,
  37                                 span,
  38                                 out: vec![] };
  39     if wf.compute(ty) {
  40         debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
  41         let result = wf.normalize();
  42         debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
  43         Some(result)
  44     } else {
  45         None // no progress made, return None
  46     }
  47 }
  48
  49 /// Returns the obligations that make this trait reference
  50 /// well-formed.  For example, if there is a trait `Set` defined like
  51 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
  52 /// if `Bar: Eq`.
  53 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  54                                          param_env: ty::ParamEnv<'tcx>,
  55                                          body_id: ast::NodeId,
  56                                          trait_ref: &ty::TraitRef<'tcx>,
  57                                          span: Span)
  58                                          -> Vec<traits::PredicateObligation<'tcx>>
  59 {
  60     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
  61     wf.compute_trait_ref(trait_ref);
  62     wf.normalize()
  63 }
  64
  65 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  66                                              param_env: ty::ParamEnv<'tcx>,
  67                                              body_id: ast::NodeId,
  68                                              predicate: &ty::Predicate<'tcx>,
  69                                              span: Span)
  70                                              -> Vec<traits::PredicateObligation<'tcx>>
  71 {
  72     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
  73
  74     // (*) ok to skip binders, because wf code is prepared for it
  75     match *predicate {
  76         ty::Predicate::Trait(ref t) => {
  77             wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
  78         }
  79         ty::Predicate::Equate(ref t) => {
  80             wf.compute(t.skip_binder().0);
  81             wf.compute(t.skip_binder().1);
  82         }
  83         ty::Predicate::RegionOutlives(..) => {
  84         }
  85         ty::Predicate::TypeOutlives(ref t) => {
  86             wf.compute(t.skip_binder().0);
  87         }
  88         ty::Predicate::Projection(ref t) => {
  89             let t = t.skip_binder(); // (*)
  90             wf.compute_projection(t.projection_ty);
  91             wf.compute(t.ty);
  92         }
  93         ty::Predicate::WellFormed(t) => {
  94             wf.compute(t);
  95         }
  96         ty::Predicate::ObjectSafe(_) => {
  97         }
  98         ty::Predicate::ClosureKind(..) => {
  99         }
 100         ty::Predicate::Subtype(ref data) => {
 101             wf.compute(data.skip_binder().a); // (*)
 102             wf.compute(data.skip_binder().b); // (*)
 103         }
 104     }
 105
 106     wf.normalize()
 107 }
 108
 109 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 110     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 111     param_env: ty::ParamEnv<'tcx>,
 112     body_id: ast::NodeId,
 113     span: Span,
 114     out: Vec<traits::PredicateObligation<'tcx>>,
 115 }
 116
 117 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
 118     fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
 119         traits::ObligationCause::new(self.span, self.body_id, code)
 120     }
 121
 122     fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
 123         let cause = self.cause(traits::MiscObligation);
 124         let infcx = &mut self.infcx;
 125         let param_env = self.param_env;
 126         self.out.iter()
 127                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
 128                 .flat_map(|pred| {
 129                     let mut selcx = traits::SelectionContext::new(infcx);
 130                     let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
 131                     once(pred.value).chain(pred.obligations)
 132                 })
 133                 .collect()
 134     }
 135
 136     /// Pushes the obligations required for `trait_ref` to be WF into
 137     /// `self.out`.
 138     fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
 139         let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
 140         self.out.extend(obligations);
 141
 142         let cause = self.cause(traits::MiscObligation);
 143         let param_env = self.param_env;
 144         self.out.extend(
 145             trait_ref.substs.types()
 146                             .filter(|ty| !ty.has_escaping_regions())
 147                             .map(|ty| traits::Obligation::new(cause.clone(),
 148                                                               param_env,
 149                                                               ty::Predicate::WellFormed(ty))));
 150     }
 151
 152     /// Pushes the obligations required for `trait_ref::Item` to be WF
 153     /// into `self.out`.
 154     fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
 155         // A projection is well-formed if (a) the trait ref itself is
 156         // WF and (b) the trait-ref holds.  (It may also be
 157         // normalizable and be WF that way.)
 158         let trait_ref = data.trait_ref(self.infcx.tcx);
 159         self.compute_trait_ref(&trait_ref);
 160
 161         if !data.has_escaping_regions() {
 162             let predicate = trait_ref.to_predicate();
 163             let cause = self.cause(traits::ProjectionWf(data));
 164             self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
 165         }
 166     }
 167
 168     fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
 169         if !subty.has_escaping_regions() {
 170             let cause = self.cause(cause);
 171             let trait_ref = ty::TraitRef {
 172                 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 173                 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
 174             };
 175             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
 176         }
 177     }
 178
 179     /// Push new obligations into `out`. Returns true if it was able
 180     /// to generate all the predicates needed to validate that `ty0`
 181     /// is WF. Returns false if `ty0` is an unresolved type variable,
 182     /// in which case we are not able to simplify at all.
 183     fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
 184         let mut subtys = ty0.walk();
 185         let param_env = self.param_env;
 186         while let Some(ty) = subtys.next() {
 187             match ty.sty {
 188                 ty::TyBool |
 189                 ty::TyChar |
 190                 ty::TyInt(..) |
 191                 ty::TyUint(..) |
 192                 ty::TyFloat(..) |
 193                 ty::TyError |
 194                 ty::TyStr |
 195                 ty::TyNever |
 196                 ty::TyParam(_) => {
 197                     // WfScalar, WfParameter, etc
 198                 }
 199
 200                 ty::TySlice(subty) |
 201                 ty::TyArray(subty, _) => {
 202                     self.require_sized(subty, traits::SliceOrArrayElem);
 203                 }
 204
 205                 ty::TyTuple(ref tys, _) => {
 206                     if let Some((_last, rest)) = tys.split_last() {
 207                         for elem in rest {
 208                             self.require_sized(elem, traits::TupleElem);
 209                         }
 210                     }
 211                 }
 212
 213                 ty::TyRawPtr(_) => {
 214                     // simple cases that are WF if their type args are WF
 215                 }
 216
 217                 ty::TyProjection(data) => {
 218                     subtys.skip_current_subtree(); // subtree handled by compute_projection
 219                     self.compute_projection(data);
 220                 }
 221
 222                 ty::TyAdt(def, substs) => {
 223                     // WfNominalType
 224                     let obligations = self.nominal_obligations(def.did, substs);
 225                     self.out.extend(obligations);
 226                 }
 227
 228                 ty::TyRef(r, mt) => {
 229                     // WfReference
 230                     if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
 231                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
 232                         self.out.push(
 233                             traits::Obligation::new(
 234                                 cause,
 235                                 param_env,
 236                                 ty::Predicate::TypeOutlives(
 237                                     ty::Binder(
 238                                         ty::OutlivesPredicate(mt.ty, r)))));
 239                     }
 240                 }
 241
 242                 ty::TyClosure(..) => {
 243                     // the types in a closure are always the types of
 244                     // local variables (or possibly references to local
 245                     // variables), we'll walk those.
 246                     //
 247                     // (Though, local variables are probably not
 248                     // needed, as they are separately checked w/r/t
 249                     // WFedness.)
 250                 }
 251
 252                 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 253                     // let the loop iterate into the argument/return
 254                     // types appearing in the fn signature
 255                 }
 256
 257                 ty::TyAnon(..) => {
 258                     // all of the requirements on type parameters
 259                     // should've been checked by the instantiation
 260                     // of whatever returned this exact `impl Trait`.
 261                 }
 262
 263                 ty::TyDynamic(data, r) => {
 264                     // WfObject
 265                     //
 266                     // Here, we defer WF checking due to higher-ranked
 267                     // regions. This is perhaps not ideal.
 268                     self.from_object_ty(ty, data, r);
 269
 270                     // FIXME(#27579) RFC also considers adding trait
 271                     // obligations that don't refer to Self and
 272                     // checking those
 273
 274                     let cause = self.cause(traits::MiscObligation);
 275                     let component_traits =
 276                         data.auto_traits().chain(data.principal().map(|p| p.def_id()));
 277                     self.out.extend(
 278                         component_traits.map(|did| traits::Obligation::new(
 279                             cause.clone(),
 280                             param_env,
 281                             ty::Predicate::ObjectSafe(did)
 282                         ))
 283                     );
 284                 }
 285
 286                 // Inference variables are the complicated case, since we don't
 287                 // know what type they are. We do two things:
 288                 //
 289                 // 1. Check if they have been resolved, and if so proceed with
 290                 //    THAT type.
 291                 // 2. If not, check whether this is the type that we
 292                 //    started with (ty0). In that case, we've made no
 293                 //    progress at all, so return false. Otherwise,
 294                 //    we've at least simplified things (i.e., we went
 295                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
 296                 //    register a pending obligation and keep
 297                 //    moving. (Goal is that an "inductive hypothesis"
 298                 //    is satisfied to ensure termination.)
 299                 ty::TyInfer(_) => {
 300                     let ty = self.infcx.shallow_resolve(ty);
 301                     if let ty::TyInfer(_) = ty.sty { // not yet resolved...
 302                         if ty == ty0 { // ...this is the type we started from! no progress.
 303                             return false;
 304                         }
 305
 306                         let cause = self.cause(traits::MiscObligation);
 307                         self.out.push( // ...not the type we started from, so we made progress.
 308                             traits::Obligation::new(cause,
 309                                                     self.param_env,
 310                                                     ty::Predicate::WellFormed(ty)));
 311                     } else {
 312                         // Yes, resolved, proceed with the
 313                         // result. Should never return false because
 314                         // `ty` is not a TyInfer.
 315                         assert!(self.compute(ty));
 316                     }
 317                 }
 318             }
 319         }
 320
 321         // if we made it through that loop above, we made progress!
 322         return true;
 323     }
 324
 325     fn nominal_obligations(&mut self,
 326                            def_id: DefId,
 327                            substs: &Substs<'tcx>)
 328                            -> Vec<traits::PredicateObligation<'tcx>>
 329     {
 330         let predicates =
 331             self.infcx.tcx.predicates_of(def_id)
 332                           .instantiate(self.infcx.tcx, substs);
 333         let cause = self.cause(traits::ItemObligation(def_id));
 334         predicates.predicates
 335                   .into_iter()
 336                   .map(|pred| traits::Obligation::new(cause.clone(),
 337                                                       self.param_env,
 338                                                       pred))
 339                   .filter(|pred| !pred.has_escaping_regions())
 340                   .collect()
 341     }
 342
 343     fn from_object_ty(&mut self, ty: Ty<'tcx>,
 344                       data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
 345                       region: ty::Region<'tcx>) {
 346         // Imagine a type like this:
 347         //
 348         //     trait Foo { }
 349         //     trait Bar<'c> : 'c { }
 350         //
 351         //     &'b (Foo+'c+Bar<'d>)
 352         //         ^
 353         //
 354         // In this case, the following relationships must hold:
 355         //
 356         //     'b <= 'c
 357         //     'd <= 'c
 358         //
 359         // The first conditions is due to the normal region pointer
 360         // rules, which say that a reference cannot outlive its
 361         // referent.
 362         //
 363         // The final condition may be a bit surprising. In particular,
 364         // you may expect that it would have been `'c <= 'd`, since
 365         // usually lifetimes of outer things are conservative
 366         // approximations for inner things. However, it works somewhat
 367         // differently with trait objects: here the idea is that if the
 368         // user specifies a region bound (`'c`, in this case) it is the
 369         // "master bound" that *implies* that bounds from other traits are
 370         // all met. (Remember that *all bounds* in a type like
 371         // `Foo+Bar+Zed` must be met, not just one, hence if we write
 372         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
 373         // 'y.)
 374         //
 375         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
 376         // am looking forward to the future here.
 377
 378         if !data.has_escaping_regions() {
 379             let implicit_bounds =
 380                 object_region_bounds(self.infcx.tcx, data);
 381
 382             let explicit_bound = region;
 383
 384             for implicit_bound in implicit_bounds {
 385                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
 386                 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
 387                 self.out.push(traits::Obligation::new(cause,
 388                                                       self.param_env,
 389                                                       outlives.to_predicate()));
 390             }
 391         }
 392     }
 393 }
 394
 395 /// Given an object type like `SomeTrait+Send`, computes the lifetime
 396 /// bounds that must hold on the elided self type. These are derived
 397 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
 398 /// they declare `trait SomeTrait : 'static`, for example, then
 399 /// `'static` would appear in the list. The hard work is done by
 400 /// `ty::required_region_bounds`, see that for more information.
 401 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
 402     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 403     existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
 404     -> Vec<ty::Region<'tcx>>
 405 {
 406     // Since we don't actually *know* the self type for an object,
 407     // this "open(err)" serves as a kind of dummy standin -- basically
 408     // a skolemized type.
 409     let open_ty = tcx.mk_infer(ty::FreshTy(0));
 410
 411     let predicates = existential_predicates.iter().filter_map(|predicate| {
 412         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
 413             None
 414         } else {
 415             Some(predicate.with_self_ty(tcx, open_ty))
 416         }
 417     }).collect();
 418
 419     tcx.required_region_bounds(open_ty, predicates)
 420 }