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Diffstat (limited to '')
| -rw-r--r-- | compiler/rustc_infer/src/traits/util.rs | 390 |
1 files changed, 390 insertions, 0 deletions
diff --git a/compiler/rustc_infer/src/traits/util.rs b/compiler/rustc_infer/src/traits/util.rs new file mode 100644 index 000000000..f5a1edf6d --- /dev/null +++ b/compiler/rustc_infer/src/traits/util.rs @@ -0,0 +1,390 @@ +use smallvec::smallvec; + +use crate::infer::outlives::components::{push_outlives_components, Component}; +use crate::traits::{Obligation, ObligationCause, PredicateObligation}; +use rustc_data_structures::fx::{FxHashSet, FxIndexSet}; +use rustc_middle::ty::{self, ToPredicate, TyCtxt}; +use rustc_span::symbol::Ident; +use rustc_span::Span; + +pub fn anonymize_predicate<'tcx>( + tcx: TyCtxt<'tcx>, + pred: ty::Predicate<'tcx>, +) -> ty::Predicate<'tcx> { + let new = tcx.anonymize_bound_vars(pred.kind()); + tcx.reuse_or_mk_predicate(pred, new) +} + +pub struct PredicateSet<'tcx> { + tcx: TyCtxt<'tcx>, + set: FxHashSet<ty::Predicate<'tcx>>, +} + +impl<'tcx> PredicateSet<'tcx> { + pub fn new(tcx: TyCtxt<'tcx>) -> Self { + Self { tcx, set: Default::default() } + } + + pub fn insert(&mut self, pred: ty::Predicate<'tcx>) -> bool { + // We have to be careful here because we want + // + // for<'a> Foo<&'a i32> + // + // and + // + // for<'b> Foo<&'b i32> + // + // to be considered equivalent. So normalize all late-bound + // regions before we throw things into the underlying set. + self.set.insert(anonymize_predicate(self.tcx, pred)) + } +} + +impl<'tcx> Extend<ty::Predicate<'tcx>> for PredicateSet<'tcx> { + fn extend<I: IntoIterator<Item = ty::Predicate<'tcx>>>(&mut self, iter: I) { + for pred in iter { + self.insert(pred); + } + } + + fn extend_one(&mut self, pred: ty::Predicate<'tcx>) { + self.insert(pred); + } + + fn extend_reserve(&mut self, additional: usize) { + Extend::<ty::Predicate<'tcx>>::extend_reserve(&mut self.set, additional); + } +} + +/////////////////////////////////////////////////////////////////////////// +// `Elaboration` iterator +/////////////////////////////////////////////////////////////////////////// + +/// "Elaboration" is the process of identifying all the predicates that +/// are implied by a source predicate. Currently, this basically means +/// walking the "supertraits" and other similar assumptions. For example, +/// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd` +/// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that +/// `T: Foo`, then we know that `T: 'static`. +pub struct Elaborator<'tcx> { + stack: Vec<PredicateObligation<'tcx>>, + visited: PredicateSet<'tcx>, +} + +pub fn elaborate_trait_ref<'tcx>( + tcx: TyCtxt<'tcx>, + trait_ref: ty::PolyTraitRef<'tcx>, +) -> Elaborator<'tcx> { + elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate(tcx))) +} + +pub fn elaborate_trait_refs<'tcx>( + tcx: TyCtxt<'tcx>, + trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>, +) -> Elaborator<'tcx> { + let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate(tcx)); + elaborate_predicates(tcx, predicates) +} + +pub fn elaborate_predicates<'tcx>( + tcx: TyCtxt<'tcx>, + predicates: impl Iterator<Item = ty::Predicate<'tcx>>, +) -> Elaborator<'tcx> { + let obligations = predicates + .map(|predicate| { + predicate_obligation(predicate, ty::ParamEnv::empty(), ObligationCause::dummy()) + }) + .collect(); + elaborate_obligations(tcx, obligations) +} + +pub fn elaborate_predicates_with_span<'tcx>( + tcx: TyCtxt<'tcx>, + predicates: impl Iterator<Item = (ty::Predicate<'tcx>, Span)>, +) -> Elaborator<'tcx> { + let obligations = predicates + .map(|(predicate, span)| { + predicate_obligation( + predicate, + ty::ParamEnv::empty(), + ObligationCause::dummy_with_span(span), + ) + }) + .collect(); + elaborate_obligations(tcx, obligations) +} + +pub fn elaborate_obligations<'tcx>( + tcx: TyCtxt<'tcx>, + mut obligations: Vec<PredicateObligation<'tcx>>, +) -> Elaborator<'tcx> { + let mut visited = PredicateSet::new(tcx); + obligations.retain(|obligation| visited.insert(obligation.predicate)); + Elaborator { stack: obligations, visited } +} + +fn predicate_obligation<'tcx>( + predicate: ty::Predicate<'tcx>, + param_env: ty::ParamEnv<'tcx>, + cause: ObligationCause<'tcx>, +) -> PredicateObligation<'tcx> { + Obligation { cause, param_env, recursion_depth: 0, predicate } +} + +impl<'tcx> Elaborator<'tcx> { + pub fn filter_to_traits(self) -> FilterToTraits<Self> { + FilterToTraits::new(self) + } + + fn elaborate(&mut self, obligation: &PredicateObligation<'tcx>) { + let tcx = self.visited.tcx; + + let bound_predicate = obligation.predicate.kind(); + match bound_predicate.skip_binder() { + ty::PredicateKind::Trait(data) => { + // Get predicates declared on the trait. + let predicates = tcx.super_predicates_of(data.def_id()); + + let obligations = predicates.predicates.iter().map(|&(mut pred, _)| { + // when parent predicate is non-const, elaborate it to non-const predicates. + if data.constness == ty::BoundConstness::NotConst { + pred = pred.without_const(tcx); + } + + predicate_obligation( + pred.subst_supertrait(tcx, &bound_predicate.rebind(data.trait_ref)), + obligation.param_env, + obligation.cause.clone(), + ) + }); + debug!(?data, ?obligations, "super_predicates"); + + // Only keep those bounds that we haven't already seen. + // This is necessary to prevent infinite recursion in some + // cases. One common case is when people define + // `trait Sized: Sized { }` rather than `trait Sized { }`. + let visited = &mut self.visited; + let obligations = obligations.filter(|o| visited.insert(o.predicate)); + + self.stack.extend(obligations); + } + ty::PredicateKind::WellFormed(..) => { + // Currently, we do not elaborate WF predicates, + // although we easily could. + } + ty::PredicateKind::ObjectSafe(..) => { + // Currently, we do not elaborate object-safe + // predicates. + } + ty::PredicateKind::Subtype(..) => { + // Currently, we do not "elaborate" predicates like `X <: Y`, + // though conceivably we might. + } + ty::PredicateKind::Coerce(..) => { + // Currently, we do not "elaborate" predicates like `X -> Y`, + // though conceivably we might. + } + ty::PredicateKind::Projection(..) => { + // Nothing to elaborate in a projection predicate. + } + ty::PredicateKind::ClosureKind(..) => { + // Nothing to elaborate when waiting for a closure's kind to be inferred. + } + ty::PredicateKind::ConstEvaluatable(..) => { + // Currently, we do not elaborate const-evaluatable + // predicates. + } + ty::PredicateKind::ConstEquate(..) => { + // Currently, we do not elaborate const-equate + // predicates. + } + ty::PredicateKind::RegionOutlives(..) => { + // Nothing to elaborate from `'a: 'b`. + } + ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_max, r_min)) => { + // We know that `T: 'a` for some type `T`. We can + // often elaborate this. For example, if we know that + // `[U]: 'a`, that implies that `U: 'a`. Similarly, if + // we know `&'a U: 'b`, then we know that `'a: 'b` and + // `U: 'b`. + // + // We can basically ignore bound regions here. So for + // example `for<'c> Foo<'a,'c>: 'b` can be elaborated to + // `'a: 'b`. + + // Ignore `for<'a> T: 'a` -- we might in the future + // consider this as evidence that `T: 'static`, but + // I'm a bit wary of such constructions and so for now + // I want to be conservative. --nmatsakis + if r_min.is_late_bound() { + return; + } + + let visited = &mut self.visited; + let mut components = smallvec![]; + push_outlives_components(tcx, ty_max, &mut components); + self.stack.extend( + components + .into_iter() + .filter_map(|component| match component { + Component::Region(r) => { + if r.is_late_bound() { + None + } else { + Some(ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate( + r, r_min, + ))) + } + } + + Component::Param(p) => { + let ty = tcx.mk_ty_param(p.index, p.name); + Some(ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate( + ty, r_min, + ))) + } + + Component::UnresolvedInferenceVariable(_) => None, + + Component::Projection(projection) => { + // We might end up here if we have `Foo<<Bar as Baz>::Assoc>: 'a`. + // With this, we can deduce that `<Bar as Baz>::Assoc: 'a`. + let ty = + tcx.mk_projection(projection.item_def_id, projection.substs); + Some(ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate( + ty, r_min, + ))) + } + + Component::EscapingProjection(_) => { + // We might be able to do more here, but we don't + // want to deal with escaping vars right now. + None + } + }) + .map(ty::Binder::dummy) + .map(|predicate_kind| predicate_kind.to_predicate(tcx)) + .filter(|&predicate| visited.insert(predicate)) + .map(|predicate| { + predicate_obligation( + predicate, + obligation.param_env, + obligation.cause.clone(), + ) + }), + ); + } + ty::PredicateKind::TypeWellFormedFromEnv(..) => { + // Nothing to elaborate + } + } + } +} + +impl<'tcx> Iterator for Elaborator<'tcx> { + type Item = PredicateObligation<'tcx>; + + fn size_hint(&self) -> (usize, Option<usize>) { + (self.stack.len(), None) + } + + fn next(&mut self) -> Option<Self::Item> { + // Extract next item from top-most stack frame, if any. + if let Some(obligation) = self.stack.pop() { + self.elaborate(&obligation); + Some(obligation) + } else { + None + } + } +} + +/////////////////////////////////////////////////////////////////////////// +// Supertrait iterator +/////////////////////////////////////////////////////////////////////////// + +pub type Supertraits<'tcx> = FilterToTraits<Elaborator<'tcx>>; + +pub fn supertraits<'tcx>( + tcx: TyCtxt<'tcx>, + trait_ref: ty::PolyTraitRef<'tcx>, +) -> Supertraits<'tcx> { + elaborate_trait_ref(tcx, trait_ref).filter_to_traits() +} + +pub fn transitive_bounds<'tcx>( + tcx: TyCtxt<'tcx>, + bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>, +) -> Supertraits<'tcx> { + elaborate_trait_refs(tcx, bounds).filter_to_traits() +} + +/// A specialized variant of `elaborate_trait_refs` that only elaborates trait references that may +/// define the given associated type `assoc_name`. It uses the +/// `super_predicates_that_define_assoc_type` query to avoid enumerating super-predicates that +/// aren't related to `assoc_item`. This is used when resolving types like `Self::Item` or +/// `T::Item` and helps to avoid cycle errors (see e.g. #35237). +pub fn transitive_bounds_that_define_assoc_type<'tcx>( + tcx: TyCtxt<'tcx>, + bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>, + assoc_name: Ident, +) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> { + let mut stack: Vec<_> = bounds.collect(); + let mut visited = FxIndexSet::default(); + + std::iter::from_fn(move || { + while let Some(trait_ref) = stack.pop() { + let anon_trait_ref = tcx.anonymize_bound_vars(trait_ref); + if visited.insert(anon_trait_ref) { + let super_predicates = tcx.super_predicates_that_define_assoc_type(( + trait_ref.def_id(), + Some(assoc_name), + )); + for (super_predicate, _) in super_predicates.predicates { + let subst_predicate = super_predicate.subst_supertrait(tcx, &trait_ref); + if let Some(binder) = subst_predicate.to_opt_poly_trait_pred() { + stack.push(binder.map_bound(|t| t.trait_ref)); + } + } + + return Some(trait_ref); + } + } + + return None; + }) +} + +/////////////////////////////////////////////////////////////////////////// +// Other +/////////////////////////////////////////////////////////////////////////// + +/// A filter around an iterator of predicates that makes it yield up +/// just trait references. +pub struct FilterToTraits<I> { + base_iterator: I, +} + +impl<I> FilterToTraits<I> { + fn new(base: I) -> FilterToTraits<I> { + FilterToTraits { base_iterator: base } + } +} + +impl<'tcx, I: Iterator<Item = PredicateObligation<'tcx>>> Iterator for FilterToTraits<I> { + type Item = ty::PolyTraitRef<'tcx>; + + fn next(&mut self) -> Option<ty::PolyTraitRef<'tcx>> { + while let Some(obligation) = self.base_iterator.next() { + if let Some(data) = obligation.predicate.to_opt_poly_trait_pred() { + return Some(data.map_bound(|t| t.trait_ref)); + } + } + None + } + + fn size_hint(&self) -> (usize, Option<usize>) { + let (_, upper) = self.base_iterator.size_hint(); + (0, upper) + } +} |