compiler/rustc_infer/src/traits/util.rs

   1 use smallvec::smallvec;
   2
   3 use crate::infer::outlives::components::{push_outlives_components, Component};
   4 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
   5 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
   6 use rustc_middle::ty::{self, ToPredicate, TyCtxt};
   7 use rustc_span::symbol::Ident;
   8 use rustc_span::Span;
   9
  10 pub fn anonymize_predicate<'tcx>(
  11     tcx: TyCtxt<'tcx>,
  12     pred: ty::Predicate<'tcx>,
  13 ) -> ty::Predicate<'tcx> {
  14     let new = tcx.anonymize_late_bound_regions(pred.kind());
  15     tcx.reuse_or_mk_predicate(pred, new)
  16 }
  17
  18 pub struct PredicateSet<'tcx> {
  19     tcx: TyCtxt<'tcx>,
  20     set: FxHashSet<ty::Predicate<'tcx>>,
  21 }
  22
  23 impl<'tcx> PredicateSet<'tcx> {
  24     pub fn new(tcx: TyCtxt<'tcx>) -> Self {
  25         Self { tcx, set: Default::default() }
  26     }
  27
  28     pub fn insert(&mut self, pred: ty::Predicate<'tcx>) -> bool {
  29         // We have to be careful here because we want
  30         //
  31         //    for<'a> Foo<&'a i32>
  32         //
  33         // and
  34         //
  35         //    for<'b> Foo<&'b i32>
  36         //
  37         // to be considered equivalent. So normalize all late-bound
  38         // regions before we throw things into the underlying set.
  39         self.set.insert(anonymize_predicate(self.tcx, pred))
  40     }
  41 }
  42
  43 impl<'tcx> Extend<ty::Predicate<'tcx>> for PredicateSet<'tcx> {
  44     fn extend<I: IntoIterator<Item = ty::Predicate<'tcx>>>(&mut self, iter: I) {
  45         for pred in iter {
  46             self.insert(pred);
  47         }
  48     }
  49
  50     fn extend_one(&mut self, pred: ty::Predicate<'tcx>) {
  51         self.insert(pred);
  52     }
  53
  54     fn extend_reserve(&mut self, additional: usize) {
  55         Extend::<ty::Predicate<'tcx>>::extend_reserve(&mut self.set, additional);
  56     }
  57 }
  58
  59 ///////////////////////////////////////////////////////////////////////////
  60 // `Elaboration` iterator
  61 ///////////////////////////////////////////////////////////////////////////
  62
  63 /// "Elaboration" is the process of identifying all the predicates that
  64 /// are implied by a source predicate. Currently, this basically means
  65 /// walking the "supertraits" and other similar assumptions. For example,
  66 /// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd`
  67 /// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that
  68 /// `T: Foo`, then we know that `T: 'static`.
  69 pub struct Elaborator<'tcx> {
  70     stack: Vec<PredicateObligation<'tcx>>,
  71     visited: PredicateSet<'tcx>,
  72 }
  73
  74 pub fn elaborate_trait_ref<'tcx>(
  75     tcx: TyCtxt<'tcx>,
  76     trait_ref: ty::PolyTraitRef<'tcx>,
  77 ) -> Elaborator<'tcx> {
  78     elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate(tcx)))
  79 }
  80
  81 pub fn elaborate_trait_refs<'tcx>(
  82     tcx: TyCtxt<'tcx>,
  83     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
  84 ) -> Elaborator<'tcx> {
  85     let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate(tcx));
  86     elaborate_predicates(tcx, predicates)
  87 }
  88
  89 pub fn elaborate_predicates<'tcx>(
  90     tcx: TyCtxt<'tcx>,
  91     predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
  92 ) -> Elaborator<'tcx> {
  93     let obligations = predicates
  94         .map(|predicate| {
  95             predicate_obligation(predicate, ty::ParamEnv::empty(), ObligationCause::dummy())
  96         })
  97         .collect();
  98     elaborate_obligations(tcx, obligations)
  99 }
 100
 101 pub fn elaborate_predicates_with_span<'tcx>(
 102     tcx: TyCtxt<'tcx>,
 103     predicates: impl Iterator<Item = (ty::Predicate<'tcx>, Span)>,
 104 ) -> Elaborator<'tcx> {
 105     let obligations = predicates
 106         .map(|(predicate, span)| {
 107             predicate_obligation(
 108                 predicate,
 109                 ty::ParamEnv::empty(),
 110                 ObligationCause::dummy_with_span(span),
 111             )
 112         })
 113         .collect();
 114     elaborate_obligations(tcx, obligations)
 115 }
 116
 117 pub fn elaborate_obligations<'tcx>(
 118     tcx: TyCtxt<'tcx>,
 119     mut obligations: Vec<PredicateObligation<'tcx>>,
 120 ) -> Elaborator<'tcx> {
 121     let mut visited = PredicateSet::new(tcx);
 122     obligations.retain(|obligation| visited.insert(obligation.predicate));
 123     Elaborator { stack: obligations, visited }
 124 }
 125
 126 fn predicate_obligation<'tcx>(
 127     predicate: ty::Predicate<'tcx>,
 128     param_env: ty::ParamEnv<'tcx>,
 129     cause: ObligationCause<'tcx>,
 130 ) -> PredicateObligation<'tcx> {
 131     Obligation { cause, param_env, recursion_depth: 0, predicate }
 132 }
 133
 134 impl<'tcx> Elaborator<'tcx> {
 135     pub fn filter_to_traits(self) -> FilterToTraits<Self> {
 136         FilterToTraits::new(self)
 137     }
 138
 139     fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) {
 140         let tcx = self.visited.tcx;
 141
 142         let bound_predicate = obligation.predicate.kind();
 143         match bound_predicate.skip_binder() {
 144             ty::PredicateKind::Trait(data) => {
 145                 // Get predicates declared on the trait.
 146                 let predicates = tcx.super_predicates_of(data.def_id());
 147
 148                 let obligations = predicates.predicates.iter().map(|&(pred, _)| {
 149                     predicate_obligation(
 150                         pred.subst_supertrait(tcx, &bound_predicate.rebind(data.trait_ref)),
 151                         obligation.param_env,
 152                         obligation.cause.clone(),
 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::Coerce(..) => {
 179                 // Currently, we do not "elaborate" predicates like `X -> Y`,
 180                 // though conceivably we might.
 181             }
 182             ty::PredicateKind::Projection(..) => {
 183                 // Nothing to elaborate in a projection predicate.
 184             }
 185             ty::PredicateKind::ClosureKind(..) => {
 186                 // Nothing to elaborate when waiting for a closure's kind to be inferred.
 187             }
 188             ty::PredicateKind::ConstEvaluatable(..) => {
 189                 // Currently, we do not elaborate const-evaluatable
 190                 // predicates.
 191             }
 192             ty::PredicateKind::ConstEquate(..) => {
 193                 // Currently, we do not elaborate const-equate
 194                 // predicates.
 195             }
 196             ty::PredicateKind::RegionOutlives(..) => {
 197                 // Nothing to elaborate from `'a: 'b`.
 198             }
 199             ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_max, r_min)) => {
 200                 // We know that `T: 'a` for some type `T`. We can
 201                 // often elaborate this. For example, if we know that
 202                 // `[U]: 'a`, that implies that `U: 'a`. Similarly, if
 203                 // we know `&'a U: 'b`, then we know that `'a: 'b` and
 204                 // `U: 'b`.
 205                 //
 206                 // We can basically ignore bound regions here. So for
 207                 // example `for<'c> Foo<'a,'c>: 'b` can be elaborated to
 208                 // `'a: 'b`.
 209
 210                 // Ignore `for<'a> T: 'a` -- we might in the future
 211                 // consider this as evidence that `T: 'static`, but
 212                 // I'm a bit wary of such constructions and so for now
 213                 // I want to be conservative. --nmatsakis
 214                 if r_min.is_late_bound() {
 215                     return;
 216                 }
 217
 218                 let visited = &mut self.visited;
 219                 let mut components = smallvec![];
 220                 push_outlives_components(tcx, ty_max, &mut components);
 221                 self.stack.extend(
 222                     components
 223                         .into_iter()
 224                         .filter_map(|component| match component {
 225                             Component::Region(r) => {
 226                                 if r.is_late_bound() {
 227                                     None
 228                                 } else {
 229                                     Some(ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(
 230                                         r, r_min,
 231                                     )))
 232                                 }
 233                             }
 234
 235                             Component::Param(p) => {
 236                                 let ty = tcx.mk_ty_param(p.index, p.name);
 237                                 Some(ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(
 238                                     ty, r_min,
 239                                 )))
 240                             }
 241
 242                             Component::UnresolvedInferenceVariable(_) => None,
 243
 244                             Component::Projection(_) | Component::EscapingProjection(_) => {
 245                                 // We can probably do more here. This
 246                                 // corresponds to a case like `<T as
 247                                 // Foo<'a>>::U: 'b`.
 248                                 None
 249                             }
 250                         })
 251                         .map(ty::Binder::dummy)
 252                         .map(|predicate_kind| predicate_kind.to_predicate(tcx))
 253                         .filter(|&predicate| visited.insert(predicate))
 254                         .map(|predicate| {
 255                             predicate_obligation(
 256                                 predicate,
 257                                 obligation.param_env,
 258                                 obligation.cause.clone(),
 259                             )
 260                         }),
 261                 );
 262             }
 263             ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 264                 // Nothing to elaborate
 265             }
 266         }
 267     }
 268 }
 269
 270 impl<'tcx> Iterator for Elaborator<'tcx> {
 271     type Item = PredicateObligation<'tcx>;
 272
 273     fn size_hint(&self) -> (usize, Option<usize>) {
 274         (self.stack.len(), None)
 275     }
 276
 277     fn next(&mut self) -> Option<Self::Item> {
 278         // Extract next item from top-most stack frame, if any.
 279         if let Some(obligation) = self.stack.pop() {
 280             self.elaborate(&obligation);
 281             Some(obligation)
 282         } else {
 283             None
 284         }
 285     }
 286 }
 287
 288 ///////////////////////////////////////////////////////////////////////////
 289 // Supertrait iterator
 290 ///////////////////////////////////////////////////////////////////////////
 291
 292 pub type Supertraits<'tcx> = FilterToTraits<Elaborator<'tcx>>;
 293
 294 pub fn supertraits<'tcx>(
 295     tcx: TyCtxt<'tcx>,
 296     trait_ref: ty::PolyTraitRef<'tcx>,
 297 ) -> Supertraits<'tcx> {
 298     elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
 299 }
 300
 301 pub fn transitive_bounds<'tcx>(
 302     tcx: TyCtxt<'tcx>,
 303     bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 304 ) -> Supertraits<'tcx> {
 305     elaborate_trait_refs(tcx, bounds).filter_to_traits()
 306 }
 307
 308 /// A specialized variant of `elaborate_trait_refs` that only elaborates trait references that may
 309 /// define the given associated type `assoc_name`. It uses the
 310 /// `super_predicates_that_define_assoc_type` query to avoid enumerating super-predicates that
 311 /// aren't related to `assoc_item`.  This is used when resolving types like `Self::Item` or
 312 /// `T::Item` and helps to avoid cycle errors (see e.g. #35237).
 313 pub fn transitive_bounds_that_define_assoc_type<'tcx>(
 314     tcx: TyCtxt<'tcx>,
 315     bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 316     assoc_name: Ident,
 317 ) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
 318     let mut stack: Vec<_> = bounds.collect();
 319     let mut visited = FxIndexSet::default();
 320
 321     std::iter::from_fn(move || {
 322         while let Some(trait_ref) = stack.pop() {
 323             let anon_trait_ref = tcx.anonymize_late_bound_regions(trait_ref);
 324             if visited.insert(anon_trait_ref) {
 325                 let super_predicates = tcx.super_predicates_that_define_assoc_type((
 326                     trait_ref.def_id(),
 327                     Some(assoc_name),
 328                 ));
 329                 for (super_predicate, _) in super_predicates.predicates {
 330                     let subst_predicate = super_predicate.subst_supertrait(tcx, &trait_ref);
 331                     if let Some(binder) = subst_predicate.to_opt_poly_trait_pred() {
 332                         stack.push(binder.map_bound(|t| t.trait_ref));
 333                     }
 334                 }
 335
 336                 return Some(trait_ref);
 337             }
 338         }
 339
 340         return None;
 341     })
 342 }
 343
 344 ///////////////////////////////////////////////////////////////////////////
 345 // Other
 346 ///////////////////////////////////////////////////////////////////////////
 347
 348 /// A filter around an iterator of predicates that makes it yield up
 349 /// just trait references.
 350 pub struct FilterToTraits<I> {
 351     base_iterator: I,
 352 }
 353
 354 impl<I> FilterToTraits<I> {
 355     fn new(base: I) -> FilterToTraits<I> {
 356         FilterToTraits { base_iterator: base }
 357     }
 358 }
 359
 360 impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToTraits<I> {
 361     type Item = ty::PolyTraitRef<'tcx>;
 362
 363     fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> {
 364         while let Some(obligation) = self.base_iterator.next() {
 365             if let Some(data) = obligation.predicate.to_opt_poly_trait_pred() {
 366                 return Some(data.map_bound(|t| t.trait_ref));
 367             }
 368         }
 369         None
 370     }
 371
 372     fn size_hint(&self) -> (usize, Option<usize>) {
 373         let (_, upper) = self.base_iterator.size_hint();
 374         (0, upper)
 375     }
 376 }