src/librustc_infer/traits/util.rs

   1 use smallvec::smallvec;
   2
   3 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
   4 use rustc_data_structures::fx::FxHashSet;
   5 use rustc_middle::ty::outlives::Component;
   6 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, TyCtxt, WithConstness};
   7 use rustc_span::Span;
   8
   9 pub fn anonymize_predicate<'tcx>(
  10     tcx: TyCtxt<'tcx>,
  11     pred: &ty::Predicate<'tcx>,
  12 ) -> ty::Predicate<'tcx> {
  13     match *pred {
  14         ty::Predicate::Trait(ref data, constness) => {
  15             ty::Predicate::Trait(tcx.anonymize_late_bound_regions(data), constness)
  16         }
  17
  18         ty::Predicate::RegionOutlives(ref data) => {
  19             ty::Predicate::RegionOutlives(tcx.anonymize_late_bound_regions(data))
  20         }
  21
  22         ty::Predicate::TypeOutlives(ref data) => {
  23             ty::Predicate::TypeOutlives(tcx.anonymize_late_bound_regions(data))
  24         }
  25
  26         ty::Predicate::Projection(ref data) => {
  27             ty::Predicate::Projection(tcx.anonymize_late_bound_regions(data))
  28         }
  29
  30         ty::Predicate::WellFormed(data) => ty::Predicate::WellFormed(data),
  31
  32         ty::Predicate::ObjectSafe(data) => ty::Predicate::ObjectSafe(data),
  33
  34         ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
  35             ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind)
  36         }
  37
  38         ty::Predicate::Subtype(ref data) => {
  39             ty::Predicate::Subtype(tcx.anonymize_late_bound_regions(data))
  40         }
  41
  42         ty::Predicate::ConstEvaluatable(def_id, substs) => {
  43             ty::Predicate::ConstEvaluatable(def_id, substs)
  44         }
  45     }
  46 }
  47
  48 struct PredicateSet<'tcx> {
  49     tcx: TyCtxt<'tcx>,
  50     set: FxHashSet<ty::Predicate<'tcx>>,
  51 }
  52
  53 impl PredicateSet<'tcx> {
  54     fn new(tcx: TyCtxt<'tcx>) -> Self {
  55         Self { tcx, set: Default::default() }
  56     }
  57
  58     fn insert(&mut self, pred: &ty::Predicate<'tcx>) -> bool {
  59         // We have to be careful here because we want
  60         //
  61         //    for<'a> Foo<&'a int>
  62         //
  63         // and
  64         //
  65         //    for<'b> Foo<&'b int>
  66         //
  67         // to be considered equivalent. So normalize all late-bound
  68         // regions before we throw things into the underlying set.
  69         self.set.insert(anonymize_predicate(self.tcx, pred))
  70     }
  71 }
  72
  73 impl<T: AsRef<ty::Predicate<'tcx>>> Extend<T> for PredicateSet<'tcx> {
  74     fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
  75         for pred in iter {
  76             self.insert(pred.as_ref());
  77         }
  78     }
  79 }
  80
  81 ///////////////////////////////////////////////////////////////////////////
  82 // `Elaboration` iterator
  83 ///////////////////////////////////////////////////////////////////////////
  84
  85 /// "Elaboration" is the process of identifying all the predicates that
  86 /// are implied by a source predicate. Currently, this basically means
  87 /// walking the "supertraits" and other similar assumptions. For example,
  88 /// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd`
  89 /// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that
  90 /// `T: Foo`, then we know that `T: 'static`.
  91 pub struct Elaborator<'tcx> {
  92     stack: Vec<PredicateObligation<'tcx>>,
  93     visited: PredicateSet<'tcx>,
  94 }
  95
  96 pub fn elaborate_trait_ref<'tcx>(
  97     tcx: TyCtxt<'tcx>,
  98     trait_ref: ty::PolyTraitRef<'tcx>,
  99 ) -> Elaborator<'tcx> {
 100     elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate()))
 101 }
 102
 103 pub fn elaborate_trait_refs<'tcx>(
 104     tcx: TyCtxt<'tcx>,
 105     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 106 ) -> Elaborator<'tcx> {
 107     let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate());
 108     elaborate_predicates(tcx, predicates)
 109 }
 110
 111 pub fn elaborate_predicates<'tcx>(
 112     tcx: TyCtxt<'tcx>,
 113     predicates: impl IntoIterator<Item = ty::Predicate<'tcx>>,
 114 ) -> Elaborator<'tcx> {
 115     let obligations =
 116         predicates.into_iter().map(|predicate| predicate_obligation(predicate, None)).collect();
 117     elaborate_obligations(tcx, obligations)
 118 }
 119
 120 pub fn elaborate_obligations<'tcx>(
 121     tcx: TyCtxt<'tcx>,
 122     mut obligations: Vec<PredicateObligation<'tcx>>,
 123 ) -> Elaborator<'tcx> {
 124     let mut visited = PredicateSet::new(tcx);
 125     obligations.retain(|obligation| visited.insert(&obligation.predicate));
 126     Elaborator { stack: obligations, visited }
 127 }
 128
 129 fn predicate_obligation<'tcx>(
 130     predicate: ty::Predicate<'tcx>,
 131     span: Option<Span>,
 132 ) -> PredicateObligation<'tcx> {
 133     let mut cause = ObligationCause::dummy();
 134     if let Some(span) = span {
 135         cause.span = span;
 136     }
 137     Obligation { cause, param_env: ty::ParamEnv::empty(), recursion_depth: 0, predicate }
 138 }
 139
 140 impl Elaborator<'tcx> {
 141     pub fn filter_to_traits(self) -> FilterToTraits<Self> {
 142         FilterToTraits::new(self)
 143     }
 144
 145     fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) {
 146         let tcx = self.visited.tcx;
 147         match obligation.predicate {
 148             ty::Predicate::Trait(ref data, _) => {
 149                 // Get predicates declared on the trait.
 150                 let predicates = tcx.super_predicates_of(data.def_id());
 151
 152                 let obligations = predicates.predicates.into_iter().map(|(pred, span)| {
 153                     predicate_obligation(
 154                         pred.subst_supertrait(tcx, &data.to_poly_trait_ref()),
 155                         Some(*span),
 156                     )
 157                 });
 158                 debug!("super_predicates: data={:?}", data);
 159
 160                 // Only keep those bounds that we haven't already seen.
 161                 // This is necessary to prevent infinite recursion in some
 162                 // cases. One common case is when people define
 163                 // `trait Sized: Sized { }` rather than `trait Sized { }`.
 164                 let visited = &mut self.visited;
 165                 let obligations = obligations.filter(|o| visited.insert(&o.predicate));
 166
 167                 self.stack.extend(obligations);
 168             }
 169             ty::Predicate::WellFormed(..) => {
 170                 // Currently, we do not elaborate WF predicates,
 171                 // although we easily could.
 172             }
 173             ty::Predicate::ObjectSafe(..) => {
 174                 // Currently, we do not elaborate object-safe
 175                 // predicates.
 176             }
 177             ty::Predicate::Subtype(..) => {
 178                 // Currently, we do not "elaborate" predicates like `X <: Y`,
 179                 // though conceivably we might.
 180             }
 181             ty::Predicate::Projection(..) => {
 182                 // Nothing to elaborate in a projection predicate.
 183             }
 184             ty::Predicate::ClosureKind(..) => {
 185                 // Nothing to elaborate when waiting for a closure's kind to be inferred.
 186             }
 187             ty::Predicate::ConstEvaluatable(..) => {
 188                 // Currently, we do not elaborate const-evaluatable
 189                 // predicates.
 190             }
 191             ty::Predicate::RegionOutlives(..) => {
 192                 // Nothing to elaborate from `'a: 'b`.
 193             }
 194             ty::Predicate::TypeOutlives(ref data) => {
 195                 // We know that `T: 'a` for some type `T`. We can
 196                 // often elaborate this. For example, if we know that
 197                 // `[U]: 'a`, that implies that `U: 'a`. Similarly, if
 198                 // we know `&'a U: 'b`, then we know that `'a: 'b` and
 199                 // `U: 'b`.
 200                 //
 201                 // We can basically ignore bound regions here. So for
 202                 // example `for<'c> Foo<'a,'c>: 'b` can be elaborated to
 203                 // `'a: 'b`.
 204
 205                 // Ignore `for<'a> T: 'a` -- we might in the future
 206                 // consider this as evidence that `T: 'static`, but
 207                 // I'm a bit wary of such constructions and so for now
 208                 // I want to be conservative. --nmatsakis
 209                 let ty_max = data.skip_binder().0;
 210                 let r_min = data.skip_binder().1;
 211                 if r_min.is_late_bound() {
 212                     return;
 213                 }
 214
 215                 let visited = &mut self.visited;
 216                 let mut components = smallvec![];
 217                 tcx.push_outlives_components(ty_max, &mut components);
 218                 self.stack.extend(
 219                     components
 220                         .into_iter()
 221                         .filter_map(|component| match component {
 222                             Component::Region(r) => {
 223                                 if r.is_late_bound() {
 224                                     None
 225                                 } else {
 226                                     Some(ty::Predicate::RegionOutlives(ty::Binder::dummy(
 227                                         ty::OutlivesPredicate(r, r_min),
 228                                     )))
 229                                 }
 230                             }
 231
 232                             Component::Param(p) => {
 233                                 let ty = tcx.mk_ty_param(p.index, p.name);
 234                                 Some(ty::Predicate::TypeOutlives(ty::Binder::dummy(
 235                                     ty::OutlivesPredicate(ty, r_min),
 236                                 )))
 237                             }
 238
 239                             Component::UnresolvedInferenceVariable(_) => None,
 240
 241                             Component::Projection(_) | Component::EscapingProjection(_) => {
 242                                 // We can probably do more here. This
 243                                 // corresponds to a case like `<T as
 244                                 // Foo<'a>>::U: 'b`.
 245                                 None
 246                             }
 247                         })
 248                         .filter(|p| visited.insert(p))
 249                         .map(|p| predicate_obligation(p, None)),
 250                 );
 251             }
 252         }
 253     }
 254 }
 255
 256 impl Iterator for Elaborator<'tcx> {
 257     type Item = PredicateObligation<'tcx>;
 258
 259     fn size_hint(&self) -> (usize, Option<usize>) {
 260         (self.stack.len(), None)
 261     }
 262
 263     fn next(&mut self) -> Option<Self::Item> {
 264         // Extract next item from top-most stack frame, if any.
 265         if let Some(obligation) = self.stack.pop() {
 266             self.elaborate(&obligation);
 267             Some(obligation)
 268         } else {
 269             None
 270         }
 271     }
 272 }
 273
 274 ///////////////////////////////////////////////////////////////////////////
 275 // Supertrait iterator
 276 ///////////////////////////////////////////////////////////////////////////
 277
 278 pub type Supertraits<'tcx> = FilterToTraits<Elaborator<'tcx>>;
 279
 280 pub fn supertraits<'tcx>(
 281     tcx: TyCtxt<'tcx>,
 282     trait_ref: ty::PolyTraitRef<'tcx>,
 283 ) -> Supertraits<'tcx> {
 284     elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
 285 }
 286
 287 pub fn transitive_bounds<'tcx>(
 288     tcx: TyCtxt<'tcx>,
 289     bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 290 ) -> Supertraits<'tcx> {
 291     elaborate_trait_refs(tcx, bounds).filter_to_traits()
 292 }
 293
 294 ///////////////////////////////////////////////////////////////////////////
 295 // Other
 296 ///////////////////////////////////////////////////////////////////////////
 297
 298 /// A filter around an iterator of predicates that makes it yield up
 299 /// just trait references.
 300 pub struct FilterToTraits<I> {
 301     base_iterator: I,
 302 }
 303
 304 impl<I> FilterToTraits<I> {
 305     fn new(base: I) -> FilterToTraits<I> {
 306         FilterToTraits { base_iterator: base }
 307     }
 308 }
 309
 310 impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToTraits<I> {
 311     type Item = ty::PolyTraitRef<'tcx>;
 312
 313     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
 314         while let Some(obligation) = self.base_iterator.next() {
 315             if let ty::Predicate::Trait(data, _) = obligation.predicate {
 316                 return Some(data.to_poly_trait_ref());
 317             }
 318         }
 319         None
 320     }
 321
 322     fn size_hint(&self) -> (usize, Option<usize>) {
 323         let (_, upper) = self.base_iterator.size_hint();
 324         (0, upper)
 325     }
 326 }