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     let kind = pred.kint(tcx);
  14     let new = match kind {
  15         ty::PredicateKint::ForAll(binder) => {
  16             ty::PredicateKint::ForAll(tcx.anonymize_late_bound_regions(binder))
  17         }
  18         &ty::PredicateKint::Trait(data, constness) => ty::PredicateKint::Trait(data, constness),
  19
  20         &ty::PredicateKint::RegionOutlives(data) => ty::PredicateKint::RegionOutlives(data),
  21
  22         &ty::PredicateKint::TypeOutlives(data) => ty::PredicateKint::TypeOutlives(data),
  23
  24         &ty::PredicateKint::Projection(data) => ty::PredicateKint::Projection(data),
  25
  26         &ty::PredicateKint::WellFormed(data) => ty::PredicateKint::WellFormed(data),
  27
  28         &ty::PredicateKint::ObjectSafe(data) => ty::PredicateKint::ObjectSafe(data),
  29
  30         &ty::PredicateKint::ClosureKind(closure_def_id, closure_substs, kind) => {
  31             ty::PredicateKint::ClosureKind(closure_def_id, closure_substs, kind)
  32         }
  33
  34         &ty::PredicateKint::Subtype(data) => ty::PredicateKint::Subtype(data),
  35
  36         &ty::PredicateKint::ConstEvaluatable(def_id, substs) => {
  37             ty::PredicateKint::ConstEvaluatable(def_id, substs)
  38         }
  39
  40         &ty::PredicateKint::ConstEquate(c1, c2) => ty::PredicateKint::ConstEquate(c1, c2),
  41     };
  42
  43     if new != *kind { new.to_predicate(tcx) } else { pred }
  44 }
  45
  46 struct PredicateSet<'tcx> {
  47     tcx: TyCtxt<'tcx>,
  48     set: FxHashSet<ty::Predicate<'tcx>>,
  49 }
  50
  51 impl PredicateSet<'tcx> {
  52     fn new(tcx: TyCtxt<'tcx>) -> Self {
  53         Self { tcx, set: Default::default() }
  54     }
  55
  56     fn insert(&mut self, pred: ty::Predicate<'tcx>) -> bool {
  57         // We have to be careful here because we want
  58         //
  59         //    for<'a> Foo<&'a i32>
  60         //
  61         // and
  62         //
  63         //    for<'b> Foo<&'b i32>
  64         //
  65         // to be considered equivalent. So normalize all late-bound
  66         // regions before we throw things into the underlying set.
  67         self.set.insert(anonymize_predicate(self.tcx, pred))
  68     }
  69 }
  70
  71 impl Extend<ty::Predicate<'tcx>> for PredicateSet<'tcx> {
  72     fn extend<I: IntoIterator<Item = ty::Predicate<'tcx>>>(&mut self, iter: I) {
  73         for pred in iter {
  74             self.insert(pred);
  75         }
  76     }
  77
  78     fn extend_one(&mut self, pred: ty::Predicate<'tcx>) {
  79         self.insert(pred);
  80     }
  81
  82     fn extend_reserve(&mut self, additional: usize) {
  83         Extend::<ty::Predicate<'tcx>>::extend_reserve(&mut self.set, additional);
  84     }
  85 }
  86
  87 ///////////////////////////////////////////////////////////////////////////
  88 // `Elaboration` iterator
  89 ///////////////////////////////////////////////////////////////////////////
  90
  91 /// "Elaboration" is the process of identifying all the predicates that
  92 /// are implied by a source predicate. Currently, this basically means
  93 /// walking the "supertraits" and other similar assumptions. For example,
  94 /// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd`
  95 /// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that
  96 /// `T: Foo`, then we know that `T: 'static`.
  97 pub struct Elaborator<'tcx> {
  98     stack: Vec<PredicateObligation<'tcx>>,
  99     visited: PredicateSet<'tcx>,
 100 }
 101
 102 pub fn elaborate_trait_ref<'tcx>(
 103     tcx: TyCtxt<'tcx>,
 104     trait_ref: ty::PolyTraitRef<'tcx>,
 105 ) -> Elaborator<'tcx> {
 106     elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate(tcx)))
 107 }
 108
 109 pub fn elaborate_trait_refs<'tcx>(
 110     tcx: TyCtxt<'tcx>,
 111     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 112 ) -> Elaborator<'tcx> {
 113     let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate(tcx));
 114     elaborate_predicates(tcx, predicates)
 115 }
 116
 117 pub fn elaborate_predicates<'tcx>(
 118     tcx: TyCtxt<'tcx>,
 119     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 120 ) -> Elaborator<'tcx> {
 121     let obligations = predicates.map(|predicate| predicate_obligation(predicate, None)).collect();
 122     elaborate_obligations(tcx, obligations)
 123 }
 124
 125 pub fn elaborate_obligations<'tcx>(
 126     tcx: TyCtxt<'tcx>,
 127     mut obligations: Vec<PredicateObligation<'tcx>>,
 128 ) -> Elaborator<'tcx> {
 129     let mut visited = PredicateSet::new(tcx);
 130     obligations.retain(|obligation| visited.insert(obligation.predicate));
 131     Elaborator { stack: obligations, visited }
 132 }
 133
 134 fn predicate_obligation<'tcx>(
 135     predicate: ty::Predicate<'tcx>,
 136     span: Option<Span>,
 137 ) -> PredicateObligation<'tcx> {
 138     let cause = if let Some(span) = span {
 139         ObligationCause::dummy_with_span(span)
 140     } else {
 141         ObligationCause::dummy()
 142     };
 143
 144     Obligation { cause, param_env: ty::ParamEnv::empty(), recursion_depth: 0, predicate }
 145 }
 146
 147 impl Elaborator<'tcx> {
 148     pub fn filter_to_traits(self) -> FilterToTraits<Self> {
 149         FilterToTraits::new(self)
 150     }
 151
 152     fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) {
 153         let tcx = self.visited.tcx;
 154         match obligation.predicate.kind() {
 155             ty::PredicateKind::Trait(ref data, _) => {
 156                 // Get predicates declared on the trait.
 157                 let predicates = tcx.super_predicates_of(data.def_id());
 158
 159                 let obligations = predicates.predicates.iter().map(|(pred, span)| {
 160                     predicate_obligation(
 161                         pred.subst_supertrait(tcx, &data.to_poly_trait_ref()),
 162                         Some(*span),
 163                     )
 164                 });
 165                 debug!("super_predicates: data={:?}", data);
 166
 167                 // Only keep those bounds that we haven't already seen.
 168                 // This is necessary to prevent infinite recursion in some
 169                 // cases. One common case is when people define
 170                 // `trait Sized: Sized { }` rather than `trait Sized { }`.
 171                 let visited = &mut self.visited;
 172                 let obligations = obligations.filter(|o| visited.insert(o.predicate));
 173
 174                 self.stack.extend(obligations);
 175             }
 176             ty::PredicateKind::WellFormed(..) => {
 177                 // Currently, we do not elaborate WF predicates,
 178                 // although we easily could.
 179             }
 180             ty::PredicateKind::ObjectSafe(..) => {
 181                 // Currently, we do not elaborate object-safe
 182                 // predicates.
 183             }
 184             ty::PredicateKind::Subtype(..) => {
 185                 // Currently, we do not "elaborate" predicates like `X <: Y`,
 186                 // though conceivably we might.
 187             }
 188             ty::PredicateKind::Projection(..) => {
 189                 // Nothing to elaborate in a projection predicate.
 190             }
 191             ty::PredicateKind::ClosureKind(..) => {
 192                 // Nothing to elaborate when waiting for a closure's kind to be inferred.
 193             }
 194             ty::PredicateKind::ConstEvaluatable(..) => {
 195                 // Currently, we do not elaborate const-evaluatable
 196                 // predicates.
 197             }
 198             ty::PredicateKind::ConstEquate(..) => {
 199                 // Currently, we do not elaborate const-equate
 200                 // predicates.
 201             }
 202             ty::PredicateKind::RegionOutlives(..) => {
 203                 // Nothing to elaborate from `'a: 'b`.
 204             }
 205             ty::PredicateKind::TypeOutlives(ref data) => {
 206                 // We know that `T: 'a` for some type `T`. We can
 207                 // often elaborate this. For example, if we know that
 208                 // `[U]: 'a`, that implies that `U: 'a`. Similarly, if
 209                 // we know `&'a U: 'b`, then we know that `'a: 'b` and
 210                 // `U: 'b`.
 211                 //
 212                 // We can basically ignore bound regions here. So for
 213                 // example `for<'c> Foo<'a,'c>: 'b` can be elaborated to
 214                 // `'a: 'b`.
 215
 216                 // Ignore `for<'a> T: 'a` -- we might in the future
 217                 // consider this as evidence that `T: 'static`, but
 218                 // I'm a bit wary of such constructions and so for now
 219                 // I want to be conservative. --nmatsakis
 220                 let ty_max = data.skip_binder().0;
 221                 let r_min = data.skip_binder().1;
 222                 if r_min.is_late_bound() {
 223                     return;
 224                 }
 225
 226                 let visited = &mut self.visited;
 227                 let mut components = smallvec![];
 228                 tcx.push_outlives_components(ty_max, &mut components);
 229                 self.stack.extend(
 230                     components
 231                         .into_iter()
 232                         .filter_map(|component| match component {
 233                             Component::Region(r) => {
 234                                 if r.is_late_bound() {
 235                                     None
 236                                 } else {
 237                                     Some(ty::PredicateKint::RegionOutlives(ty::OutlivesPredicate(
 238                                         r, r_min,
 239                                     )))
 240                                 }
 241                             }
 242
 243                             Component::Param(p) => {
 244                                 let ty = tcx.mk_ty_param(p.index, p.name);
 245                                 Some(ty::PredicateKint::TypeOutlives(ty::OutlivesPredicate(
 246                                     ty, r_min,
 247                                 )))
 248                             }
 249
 250                             Component::UnresolvedInferenceVariable(_) => None,
 251
 252                             Component::Projection(_) | Component::EscapingProjection(_) => {
 253                                 // We can probably do more here. This
 254                                 // corresponds to a case like `<T as
 255                                 // Foo<'a>>::U: 'b`.
 256                                 None
 257                             }
 258                         })
 259                         .map(|predicate_kind| predicate_kind.to_predicate(tcx))
 260                         .filter(|&predicate| visited.insert(predicate))
 261                         .map(|predicate| predicate_obligation(predicate, None)),
 262                 );
 263             }
 264         }
 265     }
 266 }
 267
 268 impl Iterator for Elaborator<'tcx> {
 269     type Item = PredicateObligation<'tcx>;
 270
 271     fn size_hint(&self) -> (usize, Option<usize>) {
 272         (self.stack.len(), None)
 273     }
 274
 275     fn next(&mut self) -> Option<Self::Item> {
 276         // Extract next item from top-most stack frame, if any.
 277         if let Some(obligation) = self.stack.pop() {
 278             self.elaborate(&obligation);
 279             Some(obligation)
 280         } else {
 281             None
 282         }
 283     }
 284 }
 285
 286 ///////////////////////////////////////////////////////////////////////////
 287 // Supertrait iterator
 288 ///////////////////////////////////////////////////////////////////////////
 289
 290 pub type Supertraits<'tcx> = FilterToTraits<Elaborator<'tcx>>;
 291
 292 pub fn supertraits<'tcx>(
 293     tcx: TyCtxt<'tcx>,
 294     trait_ref: ty::PolyTraitRef<'tcx>,
 295 ) -> Supertraits<'tcx> {
 296     elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
 297 }
 298
 299 pub fn transitive_bounds<'tcx>(
 300     tcx: TyCtxt<'tcx>,
 301     bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 302 ) -> Supertraits<'tcx> {
 303     elaborate_trait_refs(tcx, bounds).filter_to_traits()
 304 }
 305
 306 ///////////////////////////////////////////////////////////////////////////
 307 // Other
 308 ///////////////////////////////////////////////////////////////////////////
 309
 310 /// A filter around an iterator of predicates that makes it yield up
 311 /// just trait references.
 312 pub struct FilterToTraits<I> {
 313     base_iterator: I,
 314 }
 315
 316 impl<I> FilterToTraits<I> {
 317     fn new(base: I) -> FilterToTraits<I> {
 318         FilterToTraits { base_iterator: base }
 319     }
 320 }
 321
 322 impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToTraits<I> {
 323     type Item = ty::PolyTraitRef<'tcx>;
 324
 325     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
 326         while let Some(obligation) = self.base_iterator.next() {
 327             if let ty::PredicateKind::Trait(data, _) = obligation.predicate.kind() {
 328                 return Some(data.to_poly_trait_ref());
 329             }
 330         }
 331         None
 332     }
 333
 334     fn size_hint(&self) -> (usize, Option<usize>) {
 335         let (_, upper) = self.base_iterator.size_hint();
 336         (0, upper)
 337     }
 338 }