src/librustc/traits/util.rs

   1 // Copyright 2014 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 ty::subst::{Kind, Subst, Substs};
  13 use ty::{self, Ty, TyCtxt, ToPredicate, ToPolyTraitRef};
  14 use ty::outlives::Component;
  15 use util::nodemap::FxHashSet;
  16 use hir::{self};
  17 use traits::specialize::specialization_graph::NodeItem;
  18
  19 use super::{Obligation, ObligationCause, PredicateObligation, SelectionContext, Normalized};
  20
  21 fn anonymize_predicate<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
  22                                        pred: &ty::Predicate<'tcx>)
  23                                        -> ty::Predicate<'tcx> {
  24     match *pred {
  25         ty::Predicate::Trait(ref data) =>
  26             ty::Predicate::Trait(tcx.anonymize_late_bound_regions(data)),
  27
  28         ty::Predicate::RegionOutlives(ref data) =>
  29             ty::Predicate::RegionOutlives(tcx.anonymize_late_bound_regions(data)),
  30
  31         ty::Predicate::TypeOutlives(ref data) =>
  32             ty::Predicate::TypeOutlives(tcx.anonymize_late_bound_regions(data)),
  33
  34         ty::Predicate::Projection(ref data) =>
  35             ty::Predicate::Projection(tcx.anonymize_late_bound_regions(data)),
  36
  37         ty::Predicate::WellFormed(data) =>
  38             ty::Predicate::WellFormed(data),
  39
  40         ty::Predicate::ObjectSafe(data) =>
  41             ty::Predicate::ObjectSafe(data),
  42
  43         ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) =>
  44             ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind),
  45
  46         ty::Predicate::Subtype(ref data) =>
  47             ty::Predicate::Subtype(tcx.anonymize_late_bound_regions(data)),
  48
  49         ty::Predicate::ConstEvaluatable(def_id, substs) =>
  50             ty::Predicate::ConstEvaluatable(def_id, substs),
  51     }
  52 }
  53
  54
  55 struct PredicateSet<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
  56     tcx: TyCtxt<'a, 'gcx, 'tcx>,
  57     set: FxHashSet<ty::Predicate<'tcx>>,
  58 }
  59
  60 impl<'a, 'gcx, 'tcx> PredicateSet<'a, 'gcx, 'tcx> {
  61     fn new(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> PredicateSet<'a, 'gcx, 'tcx> {
  62         PredicateSet { tcx: tcx, set: Default::default() }
  63     }
  64
  65     fn insert(&mut self, pred: &ty::Predicate<'tcx>) -> bool {
  66         // We have to be careful here because we want
  67         //
  68         //    for<'a> Foo<&'a int>
  69         //
  70         // and
  71         //
  72         //    for<'b> Foo<&'b int>
  73         //
  74         // to be considered equivalent. So normalize all late-bound
  75         // regions before we throw things into the underlying set.
  76         self.set.insert(anonymize_predicate(self.tcx, pred))
  77     }
  78 }
  79
  80 ///////////////////////////////////////////////////////////////////////////
  81 // `Elaboration` iterator
  82 ///////////////////////////////////////////////////////////////////////////
  83
  84 /// "Elaboration" is the process of identifying all the predicates that
  85 /// are implied by a source predicate. Currently this basically means
  86 /// walking the "supertraits" and other similar assumptions. For
  87 /// example, if we know that `T : Ord`, the elaborator would deduce
  88 /// that `T : PartialOrd` holds as well. Similarly, if we have `trait
  89 /// Foo : 'static`, and we know that `T : Foo`, then we know that `T :
  90 /// 'static`.
  91 pub struct Elaborator<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
  92     stack: Vec<ty::Predicate<'tcx>>,
  93     visited: PredicateSet<'a, 'gcx, 'tcx>,
  94 }
  95
  96 pub fn elaborate_trait_ref<'cx, 'gcx, 'tcx>(
  97     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
  98     trait_ref: ty::PolyTraitRef<'tcx>)
  99     -> Elaborator<'cx, 'gcx, 'tcx>
 100 {
 101     elaborate_predicates(tcx, vec![trait_ref.to_predicate()])
 102 }
 103
 104 pub fn elaborate_trait_refs<'cx, 'gcx, 'tcx>(
 105     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 106     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>)
 107     -> Elaborator<'cx, 'gcx, 'tcx>
 108 {
 109     let predicates = trait_refs.map(|trait_ref| trait_ref.to_predicate())
 110                                .collect();
 111     elaborate_predicates(tcx, predicates)
 112 }
 113
 114 pub fn elaborate_predicates<'cx, 'gcx, 'tcx>(
 115     tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 116     mut predicates: Vec<ty::Predicate<'tcx>>)
 117     -> Elaborator<'cx, 'gcx, 'tcx>
 118 {
 119     let mut visited = PredicateSet::new(tcx);
 120     predicates.retain(|pred| visited.insert(pred));
 121     Elaborator { stack: predicates, visited: visited }
 122 }
 123
 124 impl<'cx, 'gcx, 'tcx> Elaborator<'cx, 'gcx, 'tcx> {
 125     pub fn filter_to_traits(self) -> FilterToTraits<Self> {
 126         FilterToTraits::new(self)
 127     }
 128
 129     fn push(&mut self, predicate: &ty::Predicate<'tcx>) {
 130         let tcx = self.visited.tcx;
 131         match *predicate {
 132             ty::Predicate::Trait(ref data) => {
 133                 // Predicates declared on the trait.
 134                 let predicates = tcx.super_predicates_of(data.def_id());
 135
 136                 let mut predicates: Vec<_> =
 137                     predicates.predicates
 138                               .iter()
 139                               .map(|(p, _)| p.subst_supertrait(tcx, &data.to_poly_trait_ref()))
 140                               .collect();
 141
 142                 debug!("super_predicates: data={:?} predicates={:?}",
 143                        data, predicates);
 144
 145                 // Only keep those bounds that we haven't already
 146                 // seen.  This is necessary to prevent infinite
 147                 // recursion in some cases.  One common case is when
 148                 // people define `trait Sized: Sized { }` rather than `trait
 149                 // Sized { }`.
 150                 predicates.retain(|r| self.visited.insert(r));
 151
 152                 self.stack.extend(predicates);
 153             }
 154             ty::Predicate::WellFormed(..) => {
 155                 // Currently, we do not elaborate WF predicates,
 156                 // although we easily could.
 157             }
 158             ty::Predicate::ObjectSafe(..) => {
 159                 // Currently, we do not elaborate object-safe
 160                 // predicates.
 161             }
 162             ty::Predicate::Subtype(..) => {
 163                 // Currently, we do not "elaborate" predicates like `X
 164                 // <: Y`, though conceivably we might.
 165             }
 166             ty::Predicate::Projection(..) => {
 167                 // Nothing to elaborate in a projection predicate.
 168             }
 169             ty::Predicate::ClosureKind(..) => {
 170                 // Nothing to elaborate when waiting for a closure's kind to be inferred.
 171             }
 172             ty::Predicate::ConstEvaluatable(..) => {
 173                 // Currently, we do not elaborate const-evaluatable
 174                 // predicates.
 175             }
 176
 177             ty::Predicate::RegionOutlives(..) => {
 178                 // Nothing to elaborate from `'a: 'b`.
 179             }
 180
 181             ty::Predicate::TypeOutlives(ref data) => {
 182                 // We know that `T: 'a` for some type `T`. We can
 183                 // often elaborate this. For example, if we know that
 184                 // `[U]: 'a`, that implies that `U: 'a`. Similarly, if
 185                 // we know `&'a U: 'b`, then we know that `'a: 'b` and
 186                 // `U: 'b`.
 187                 //
 188                 // We can basically ignore bound regions here. So for
 189                 // example `for<'c> Foo<'a,'c>: 'b` can be elaborated to
 190                 // `'a: 'b`.
 191
 192                 // Ignore `for<'a> T: 'a` -- we might in the future
 193                 // consider this as evidence that `T: 'static`, but
 194                 // I'm a bit wary of such constructions and so for now
 195                 // I want to be conservative. --nmatsakis
 196                 let ty_max = data.skip_binder().0;
 197                 let r_min = data.skip_binder().1;
 198                 if r_min.is_late_bound() {
 199                     return;
 200                 }
 201
 202                 let visited = &mut self.visited;
 203                 self.stack.extend(
 204                     tcx.outlives_components(ty_max)
 205                        .into_iter()
 206                        .filter_map(|component| match component {
 207                            Component::Region(r) => if r.is_late_bound() {
 208                                None
 209                            } else {
 210                                Some(ty::Predicate::RegionOutlives(
 211                                    ty::Binder::dummy(ty::OutlivesPredicate(r, r_min))))
 212                            },
 213
 214                            Component::Param(p) => {
 215                                let ty = tcx.mk_ty_param(p.idx, p.name);
 216                                Some(ty::Predicate::TypeOutlives(
 217                                    ty::Binder::dummy(ty::OutlivesPredicate(ty, r_min))))
 218                            },
 219
 220                            Component::UnresolvedInferenceVariable(_) => {
 221                                None
 222                            },
 223
 224                            Component::Projection(_) |
 225                            Component::EscapingProjection(_) => {
 226                                // We can probably do more here. This
 227                                // corresponds to a case like `<T as
 228                                // Foo<'a>>::U: 'b`.
 229                                None
 230                            },
 231                        })
 232                        .filter(|p| visited.insert(p)));
 233             }
 234         }
 235     }
 236 }
 237
 238 impl<'cx, 'gcx, 'tcx> Iterator for Elaborator<'cx, 'gcx, 'tcx> {
 239     type Item = ty::Predicate<'tcx>;
 240
 241     fn size_hint(&self) -> (usize, Option<usize>) {
 242         (self.stack.len(), None)
 243     }
 244
 245     fn next(&mut self) -> Option<ty::Predicate<'tcx>> {
 246         // Extract next item from top-most stack frame, if any.
 247         let next_predicate = match self.stack.pop() {
 248             Some(predicate) => predicate,
 249             None => {
 250                 // No more stack frames. Done.
 251                 return None;
 252             }
 253         };
 254         self.push(&next_predicate);
 255         return Some(next_predicate);
 256     }
 257 }
 258
 259 ///////////////////////////////////////////////////////////////////////////
 260 // Supertrait iterator
 261 ///////////////////////////////////////////////////////////////////////////
 262
 263 pub type Supertraits<'cx, 'gcx, 'tcx> = FilterToTraits<Elaborator<'cx, 'gcx, 'tcx>>;
 264
 265 pub fn supertraits<'cx, 'gcx, 'tcx>(tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 266                                     trait_ref: ty::PolyTraitRef<'tcx>)
 267                                     -> Supertraits<'cx, 'gcx, 'tcx>
 268 {
 269     elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
 270 }
 271
 272 pub fn transitive_bounds<'cx, 'gcx, 'tcx>(tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 273                                           bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>)
 274                                           -> Supertraits<'cx, 'gcx, 'tcx>
 275 {
 276     elaborate_trait_refs(tcx, bounds).filter_to_traits()
 277 }
 278
 279 ///////////////////////////////////////////////////////////////////////////
 280 // Iterator over def-ids of supertraits
 281
 282 pub struct SupertraitDefIds<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 283     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 284     stack: Vec<DefId>,
 285     visited: FxHashSet<DefId>,
 286 }
 287
 288 pub fn supertrait_def_ids<'cx, 'gcx, 'tcx>(tcx: TyCtxt<'cx, 'gcx, 'tcx>,
 289                                            trait_def_id: DefId)
 290                                            -> SupertraitDefIds<'cx, 'gcx, 'tcx>
 291 {
 292     SupertraitDefIds {
 293         tcx,
 294         stack: vec![trait_def_id],
 295         visited: Some(trait_def_id).into_iter().collect(),
 296     }
 297 }
 298
 299 impl<'cx, 'gcx, 'tcx> Iterator for SupertraitDefIds<'cx, 'gcx, 'tcx> {
 300     type Item = DefId;
 301
 302     fn next(&mut self) -> Option<DefId> {
 303         let def_id = match self.stack.pop() {
 304             Some(def_id) => def_id,
 305             None => { return None; }
 306         };
 307
 308         let predicates = self.tcx.super_predicates_of(def_id);
 309         let visited = &mut self.visited;
 310         self.stack.extend(
 311             predicates.predicates
 312                       .iter()
 313                       .filter_map(|(p, _)| p.to_opt_poly_trait_ref())
 314                       .map(|t| t.def_id())
 315                       .filter(|&super_def_id| visited.insert(super_def_id)));
 316         Some(def_id)
 317     }
 318 }
 319
 320 ///////////////////////////////////////////////////////////////////////////
 321 // Other
 322 ///////////////////////////////////////////////////////////////////////////
 323
 324 /// A filter around an iterator of predicates that makes it yield up
 325 /// just trait references.
 326 pub struct FilterToTraits<I> {
 327     base_iterator: I
 328 }
 329
 330 impl<I> FilterToTraits<I> {
 331     fn new(base: I) -> FilterToTraits<I> {
 332         FilterToTraits { base_iterator: base }
 333     }
 334 }
 335
 336 impl<'tcx,I:Iterator<Item = ty::Predicate<'tcx>>> Iterator for FilterToTraits<I> {
 337     type Item = ty::PolyTraitRef<'tcx>;
 338
 339     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
 340         loop {
 341             match self.base_iterator.next() {
 342                 None => {
 343                     return None;
 344                 }
 345                 Some(ty::Predicate::Trait(data)) => {
 346                     return Some(data.to_poly_trait_ref());
 347                 }
 348                 Some(_) => {}
 349             }
 350         }
 351     }
 352
 353     fn size_hint(&self) -> (usize, Option<usize>) {
 354         let (_, upper) = self.base_iterator.size_hint();
 355         (0, upper)
 356     }
 357 }
 358
 359 ///////////////////////////////////////////////////////////////////////////
 360 // Other
 361 ///////////////////////////////////////////////////////////////////////////
 362
 363 /// Instantiate all bound parameters of the impl with the given substs,
 364 /// returning the resulting trait ref and all obligations that arise.
 365 /// The obligations are closed under normalization.
 366 pub fn impl_trait_ref_and_oblig<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 367                                                 param_env: ty::ParamEnv<'tcx>,
 368                                                 impl_def_id: DefId,
 369                                                 impl_substs: &Substs<'tcx>)
 370                                                 -> (ty::TraitRef<'tcx>,
 371                                                     Vec<PredicateObligation<'tcx>>)
 372 {
 373     let impl_trait_ref =
 374         selcx.tcx().impl_trait_ref(impl_def_id).unwrap();
 375     let impl_trait_ref =
 376         impl_trait_ref.subst(selcx.tcx(), impl_substs);
 377     let Normalized { value: impl_trait_ref, obligations: normalization_obligations1 } =
 378         super::normalize(selcx, param_env, ObligationCause::dummy(), &impl_trait_ref);
 379
 380     let predicates = selcx.tcx().predicates_of(impl_def_id);
 381     let predicates = predicates.instantiate(selcx.tcx(), impl_substs);
 382     let Normalized { value: predicates, obligations: normalization_obligations2 } =
 383         super::normalize(selcx, param_env, ObligationCause::dummy(), &predicates);
 384     let impl_obligations =
 385         predicates_for_generics(ObligationCause::dummy(), 0, param_env, &predicates);
 386
 387     let impl_obligations: Vec<_> =
 388         impl_obligations.into_iter()
 389         .chain(normalization_obligations1)
 390         .chain(normalization_obligations2)
 391         .collect();
 392
 393     (impl_trait_ref, impl_obligations)
 394 }
 395
 396 /// See `super::obligations_for_generics`
 397 pub fn predicates_for_generics<'tcx>(cause: ObligationCause<'tcx>,
 398                                      recursion_depth: usize,
 399                                      param_env: ty::ParamEnv<'tcx>,
 400                                      generic_bounds: &ty::InstantiatedPredicates<'tcx>)
 401                                      -> Vec<PredicateObligation<'tcx>>
 402 {
 403     debug!("predicates_for_generics(generic_bounds={:?})",
 404            generic_bounds);
 405
 406     generic_bounds.predicates.iter().map(|predicate| {
 407         Obligation { cause: cause.clone(),
 408                      recursion_depth,
 409                      param_env,
 410                      predicate: predicate.clone() }
 411     }).collect()
 412 }
 413
 414 pub fn predicate_for_trait_ref<'tcx>(
 415     cause: ObligationCause<'tcx>,
 416     param_env: ty::ParamEnv<'tcx>,
 417     trait_ref: ty::TraitRef<'tcx>,
 418     recursion_depth: usize)
 419     -> PredicateObligation<'tcx>
 420 {
 421     Obligation {
 422         cause,
 423         param_env,
 424         recursion_depth,
 425         predicate: trait_ref.to_predicate(),
 426     }
 427 }
 428
 429 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 430     pub fn predicate_for_trait_def(self,
 431                                    param_env: ty::ParamEnv<'tcx>,
 432                                    cause: ObligationCause<'tcx>,
 433                                    trait_def_id: DefId,
 434                                    recursion_depth: usize,
 435                                    self_ty: Ty<'tcx>,
 436                                    params: &[Kind<'tcx>])
 437         -> PredicateObligation<'tcx>
 438     {
 439         let trait_ref = ty::TraitRef {
 440             def_id: trait_def_id,
 441             substs: self.mk_substs_trait(self_ty, params)
 442         };
 443         predicate_for_trait_ref(cause, param_env, trait_ref, recursion_depth)
 444     }
 445
 446     /// Cast a trait reference into a reference to one of its super
 447     /// traits; returns `None` if `target_trait_def_id` is not a
 448     /// supertrait.
 449     pub fn upcast_choices(self,
 450                           source_trait_ref: ty::PolyTraitRef<'tcx>,
 451                           target_trait_def_id: DefId)
 452                           -> Vec<ty::PolyTraitRef<'tcx>>
 453     {
 454         if source_trait_ref.def_id() == target_trait_def_id {
 455             return vec![source_trait_ref]; // shorcut the most common case
 456         }
 457
 458         supertraits(self, source_trait_ref)
 459             .filter(|r| r.def_id() == target_trait_def_id)
 460             .collect()
 461     }
 462
 463     /// Given a trait `trait_ref`, returns the number of vtable entries
 464     /// that come from `trait_ref`, excluding its supertraits. Used in
 465     /// computing the vtable base for an upcast trait of a trait object.
 466     pub fn count_own_vtable_entries(self, trait_ref: ty::PolyTraitRef<'tcx>) -> usize {
 467         let mut entries = 0;
 468         // Count number of methods and add them to the total offset.
 469         // Skip over associated types and constants.
 470         for trait_item in self.associated_items(trait_ref.def_id()) {
 471             if trait_item.kind == ty::AssociatedKind::Method {
 472                 entries += 1;
 473             }
 474         }
 475         entries
 476     }
 477
 478     /// Given an upcast trait object described by `object`, returns the
 479     /// index of the method `method_def_id` (which should be part of
 480     /// `object.upcast_trait_ref`) within the vtable for `object`.
 481     pub fn get_vtable_index_of_object_method<N>(self,
 482                                                 object: &super::VtableObjectData<'tcx, N>,
 483                                                 method_def_id: DefId) -> usize {
 484         // Count number of methods preceding the one we are selecting and
 485         // add them to the total offset.
 486         // Skip over associated types and constants.
 487         let mut entries = object.vtable_base;
 488         for trait_item in self.associated_items(object.upcast_trait_ref.def_id()) {
 489             if trait_item.def_id == method_def_id {
 490                 // The item with the ID we were given really ought to be a method.
 491                 assert_eq!(trait_item.kind, ty::AssociatedKind::Method);
 492                 return entries;
 493             }
 494             if trait_item.kind == ty::AssociatedKind::Method {
 495                 entries += 1;
 496             }
 497         }
 498
 499         bug!("get_vtable_index_of_object_method: {:?} was not found",
 500              method_def_id);
 501     }
 502
 503     pub fn closure_trait_ref_and_return_type(self,
 504         fn_trait_def_id: DefId,
 505         self_ty: Ty<'tcx>,
 506         sig: ty::PolyFnSig<'tcx>,
 507         tuple_arguments: TupleArgumentsFlag)
 508         -> ty::Binder<(ty::TraitRef<'tcx>, Ty<'tcx>)>
 509     {
 510         let arguments_tuple = match tuple_arguments {
 511             TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
 512             TupleArgumentsFlag::Yes =>
 513                 self.intern_tup(sig.skip_binder().inputs()),
 514         };
 515         let trait_ref = ty::TraitRef {
 516             def_id: fn_trait_def_id,
 517             substs: self.mk_substs_trait(self_ty, &[arguments_tuple.into()]),
 518         };
 519         ty::Binder::bind((trait_ref, sig.skip_binder().output()))
 520     }
 521
 522     pub fn generator_trait_ref_and_outputs(self,
 523         fn_trait_def_id: DefId,
 524         self_ty: Ty<'tcx>,
 525         sig: ty::PolyGenSig<'tcx>)
 526         -> ty::Binder<(ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>)>
 527     {
 528         let trait_ref = ty::TraitRef {
 529             def_id: fn_trait_def_id,
 530             substs: self.mk_substs_trait(self_ty, &[]),
 531         };
 532         ty::Binder::bind((trait_ref, sig.skip_binder().yield_ty, sig.skip_binder().return_ty))
 533     }
 534
 535     pub fn impl_is_default(self, node_item_def_id: DefId) -> bool {
 536         match self.hir.as_local_node_id(node_item_def_id) {
 537             Some(node_id) => {
 538                 let item = self.hir.expect_item(node_id);
 539                 if let hir::ItemKind::Impl(_, _, defaultness, ..) = item.node {
 540                     defaultness.is_default()
 541                 } else {
 542                     false
 543                 }
 544             }
 545             None => {
 546                 self.global_tcx()
 547                     .impl_defaultness(node_item_def_id)
 548                     .is_default()
 549             }
 550         }
 551     }
 552
 553     pub fn impl_item_is_final(self, node_item: &NodeItem<hir::Defaultness>) -> bool {
 554         node_item.item.is_final() && !self.impl_is_default(node_item.node.def_id())
 555     }
 556 }
 557
 558 pub enum TupleArgumentsFlag { Yes, No }