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, 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.kind();
  14     match kind {
  15         ty::PredicateKind::ForAll(binder) => {
  16             let new = ty::PredicateKind::ForAll(tcx.anonymize_late_bound_regions(binder));
  17             if new != *kind { new.to_predicate(tcx) } else { pred }
  18         }
  19         ty::PredicateKind::Trait(_, _)
  20         | ty::PredicateKind::RegionOutlives(_)
  21         | ty::PredicateKind::TypeOutlives(_)
  22         | ty::PredicateKind::Projection(_)
  23         | ty::PredicateKind::WellFormed(_)
  24         | ty::PredicateKind::ObjectSafe(_)
  25         | ty::PredicateKind::ClosureKind(_, _, _)
  26         | ty::PredicateKind::Subtype(_)
  27         | ty::PredicateKind::ConstEvaluatable(_, _)
  28         | ty::PredicateKind::ConstEquate(_, _) => pred,
  29     }
  30 }
  31
  32 struct PredicateSet<'tcx> {
  33     tcx: TyCtxt<'tcx>,
  34     set: FxHashSet<ty::Predicate<'tcx>>,
  35 }
  36
  37 impl PredicateSet<'tcx> {
  38     fn new(tcx: TyCtxt<'tcx>) -> Self {
  39         Self { tcx, set: Default::default() }
  40     }
  41
  42     fn insert(&mut self, pred: ty::Predicate<'tcx>) -> bool {
  43         // We have to be careful here because we want
  44         //
  45         //    for<'a> Foo<&'a i32>
  46         //
  47         // and
  48         //
  49         //    for<'b> Foo<&'b i32>
  50         //
  51         // to be considered equivalent. So normalize all late-bound
  52         // regions before we throw things into the underlying set.
  53         self.set.insert(anonymize_predicate(self.tcx, pred))
  54     }
  55 }
  56
  57 impl Extend<ty::Predicate<'tcx>> for PredicateSet<'tcx> {
  58     fn extend<I: IntoIterator<Item = ty::Predicate<'tcx>>>(&mut self, iter: I) {
  59         for pred in iter {
  60             self.insert(pred);
  61         }
  62     }
  63
  64     fn extend_one(&mut self, pred: ty::Predicate<'tcx>) {
  65         self.insert(pred);
  66     }
  67
  68     fn extend_reserve(&mut self, additional: usize) {
  69         Extend::<ty::Predicate<'tcx>>::extend_reserve(&mut self.set, additional);
  70     }
  71 }
  72
  73 ///////////////////////////////////////////////////////////////////////////
  74 // `Elaboration` iterator
  75 ///////////////////////////////////////////////////////////////////////////
  76
  77 /// "Elaboration" is the process of identifying all the predicates that
  78 /// are implied by a source predicate. Currently, this basically means
  79 /// walking the "supertraits" and other similar assumptions. For example,
  80 /// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd`
  81 /// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that
  82 /// `T: Foo`, then we know that `T: 'static`.
  83 pub struct Elaborator<'tcx> {
  84     stack: Vec<PredicateObligation<'tcx>>,
  85     visited: PredicateSet<'tcx>,
  86 }
  87
  88 pub fn elaborate_trait_ref<'tcx>(
  89     tcx: TyCtxt<'tcx>,
  90     trait_ref: ty::PolyTraitRef<'tcx>,
  91 ) -> Elaborator<'tcx> {
  92     elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate(tcx)))
  93 }
  94
  95 pub fn elaborate_trait_refs<'tcx>(
  96     tcx: TyCtxt<'tcx>,
  97     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
  98 ) -> Elaborator<'tcx> {
  99     let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate(tcx));
 100     elaborate_predicates(tcx, predicates)
 101 }
 102
 103 pub fn elaborate_predicates<'tcx>(
 104     tcx: TyCtxt<'tcx>,
 105     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 106 ) -> Elaborator<'tcx> {
 107     let obligations = predicates.map(|predicate| predicate_obligation(predicate, None)).collect();
 108     elaborate_obligations(tcx, obligations)
 109 }
 110
 111 pub fn elaborate_obligations<'tcx>(
 112     tcx: TyCtxt<'tcx>,
 113     mut obligations: Vec<PredicateObligation<'tcx>>,
 114 ) -> Elaborator<'tcx> {
 115     let mut visited = PredicateSet::new(tcx);
 116     obligations.retain(|obligation| visited.insert(obligation.predicate));
 117     Elaborator { stack: obligations, visited }
 118 }
 119
 120 fn predicate_obligation<'tcx>(
 121     predicate: ty::Predicate<'tcx>,
 122     span: Option<Span>,
 123 ) -> PredicateObligation<'tcx> {
 124     let cause = if let Some(span) = span {
 125         ObligationCause::dummy_with_span(span)
 126     } else {
 127         ObligationCause::dummy()
 128     };
 129
 130     Obligation { cause, param_env: ty::ParamEnv::empty(), recursion_depth: 0, predicate }
 131 }
 132
 133 impl Elaborator<'tcx> {
 134     pub fn filter_to_traits(self) -> FilterToTraits<Self> {
 135         FilterToTraits::new(self)
 136     }
 137
 138     fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) {
 139         let tcx = self.visited.tcx;
 140
 141         match obligation.predicate.ignore_quantifiers().skip_binder().kind() {
 142             ty::PredicateKind::ForAll(_) => {
 143                 bug!("unexpected predicate: {:?}", obligation.predicate)
 144             }
 145             ty::PredicateKind::Trait(data, _) => {
 146                 // Get predicates declared on the trait.
 147                 let predicates = tcx.super_predicates_of(data.def_id());
 148
 149                 let obligations = predicates.predicates.iter().map(|&(pred, span)| {
 150                     predicate_obligation(
 151                         pred.subst_supertrait(tcx, &ty::Binder::bind(data.trait_ref)),
 152                         Some(span),
 153                     )
 154                 });
 155                 debug!("super_predicates: data={:?}", data);
 156
 157                 // Only keep those bounds that we haven't already seen.
 158                 // This is necessary to prevent infinite recursion in some
 159                 // cases. One common case is when people define
 160                 // `trait Sized: Sized { }` rather than `trait Sized { }`.
 161                 let visited = &mut self.visited;
 162                 let obligations = obligations.filter(|o| visited.insert(o.predicate));
 163
 164                 self.stack.extend(obligations);
 165             }
 166             ty::PredicateKind::WellFormed(..) => {
 167                 // Currently, we do not elaborate WF predicates,
 168                 // although we easily could.
 169             }
 170             ty::PredicateKind::ObjectSafe(..) => {
 171                 // Currently, we do not elaborate object-safe
 172                 // predicates.
 173             }
 174             ty::PredicateKind::Subtype(..) => {
 175                 // Currently, we do not "elaborate" predicates like `X <: Y`,
 176                 // though conceivably we might.
 177             }
 178             ty::PredicateKind::Projection(..) => {
 179                 // Nothing to elaborate in a projection predicate.
 180             }
 181             ty::PredicateKind::ClosureKind(..) => {
 182                 // Nothing to elaborate when waiting for a closure's kind to be inferred.
 183             }
 184             ty::PredicateKind::ConstEvaluatable(..) => {
 185                 // Currently, we do not elaborate const-evaluatable
 186                 // predicates.
 187             }
 188             ty::PredicateKind::ConstEquate(..) => {
 189                 // Currently, we do not elaborate const-equate
 190                 // predicates.
 191             }
 192             ty::PredicateKind::RegionOutlives(..) => {
 193                 // Nothing to elaborate from `'a: 'b`.
 194             }
 195             ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_max, r_min)) => {
 196                 // We know that `T: 'a` for some type `T`. We can
 197                 // often elaborate this. For example, if we know that
 198                 // `[U]: 'a`, that implies that `U: 'a`. Similarly, if
 199                 // we know `&'a U: 'b`, then we know that `'a: 'b` and
 200                 // `U: 'b`.
 201                 //
 202                 // We can basically ignore bound regions here. So for
 203                 // example `for<'c> Foo<'a,'c>: 'b` can be elaborated to
 204                 // `'a: 'b`.
 205
 206                 // Ignore `for<'a> T: 'a` -- we might in the future
 207                 // consider this as evidence that `T: 'static`, but
 208                 // I'm a bit wary of such constructions and so for now
 209                 // I want to be conservative. --nmatsakis
 210                 if r_min.is_late_bound() {
 211                     return;
 212                 }
 213
 214                 let visited = &mut self.visited;
 215                 let mut components = smallvec![];
 216                 tcx.push_outlives_components(ty_max, &mut components);
 217                 self.stack.extend(
 218                     components
 219                         .into_iter()
 220                         .filter_map(|component| match component {
 221                             Component::Region(r) => {
 222                                 if r.is_late_bound() {
 223                                     None
 224                                 } else {
 225                                     Some(ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(
 226                                         r, r_min,
 227                                     )))
 228                                 }
 229                             }
 230
 231                             Component::Param(p) => {
 232                                 let ty = tcx.mk_ty_param(p.index, p.name);
 233                                 Some(ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(
 234                                     ty, r_min,
 235                                 )))
 236                             }
 237
 238                             Component::UnresolvedInferenceVariable(_) => None,
 239
 240                             Component::Projection(_) | Component::EscapingProjection(_) => {
 241                                 // We can probably do more here. This
 242                                 // corresponds to a case like `<T as
 243                                 // Foo<'a>>::U: 'b`.
 244                                 None
 245                             }
 246                         })
 247                         .map(|predicate_kind| predicate_kind.to_predicate(tcx))
 248                         .filter(|&predicate| visited.insert(predicate))
 249                         .map(|predicate| predicate_obligation(predicate, 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 Some(data) = obligation.predicate.to_opt_poly_trait_ref() {
 316                 return Some(data);
 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 }