rust/compiler/rustc_infer/src/traits/util.rs

use smallvec::smallvec;

use crate::traits::{Obligation, ObligationCause, PredicateObligation};
use rustc_data_structures::fx::FxHashSet;
use rustc_middle::ty::outlives::Component;
use rustc_middle::ty::{self, ToPredicate, TyCtxt, WithConstness};

pub fn anonymize_predicate<'tcx>(
    tcx: TyCtxt<'tcx>,
    pred: ty::Predicate<'tcx>,
) -> ty::Predicate<'tcx> {
    match pred.kind() {
        ty::PredicateKind::ForAll(binder) => {
            let new = ty::PredicateKind::ForAll(tcx.anonymize_late_bound_regions(binder));
            tcx.reuse_or_mk_predicate(pred, new)
        }
        ty::PredicateKind::Atom(_) => pred,
    }
}

struct PredicateSet<'tcx> {
    tcx: TyCtxt<'tcx>,
    set: FxHashSet<ty::Predicate<'tcx>>,
}

impl PredicateSet<'tcx> {
    fn new(tcx: TyCtxt<'tcx>) -> Self {
        Self { tcx, set: Default::default() }
    }

    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 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_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 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.bound_atom(tcx);
        match bound_predicate.skip_binder() {
            ty::PredicateAtom::Trait(data, _) => {
                // Get predicates declared on the trait.
                let predicates = tcx.super_predicates_of(data.def_id());

                let obligations = predicates.predicates.iter().map(|&(pred, _)| {
                    predicate_obligation(
                        pred.subst_supertrait(tcx, &bound_predicate.map_bound(|_| data.trait_ref)),
                        obligation.param_env,
                        obligation.cause.clone(),
                    )
                });
                debug!("super_predicates: data={:?}", data);

                // 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::PredicateAtom::WellFormed(..) => {
                // Currently, we do not elaborate WF predicates,
                // although we easily could.
            }
            ty::PredicateAtom::ObjectSafe(..) => {
                // Currently, we do not elaborate object-safe
                // predicates.
            }
            ty::PredicateAtom::Subtype(..) => {
                // Currently, we do not "elaborate" predicates like `X <: Y`,
                // though conceivably we might.
            }
            ty::PredicateAtom::Projection(..) => {
                // Nothing to elaborate in a projection predicate.
            }
            ty::PredicateAtom::ClosureKind(..) => {
                // Nothing to elaborate when waiting for a closure's kind to be inferred.
            }
            ty::PredicateAtom::ConstEvaluatable(..) => {
                // Currently, we do not elaborate const-evaluatable
                // predicates.
            }
            ty::PredicateAtom::ConstEquate(..) => {
                // Currently, we do not elaborate const-equate
                // predicates.
            }
            ty::PredicateAtom::RegionOutlives(..) => {
                // Nothing to elaborate from `'a: 'b`.
            }
            ty::PredicateAtom::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![];
                tcx.push_outlives_components(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::PredicateAtom::RegionOutlives(ty::OutlivesPredicate(
                                        r, r_min,
                                    )))
                                }
                            }

                            Component::Param(p) => {
                                let ty = tcx.mk_ty_param(p.index, p.name);
                                Some(ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(
                                    ty, r_min,
                                )))
                            }

                            Component::UnresolvedInferenceVariable(_) => None,

                            Component::Projection(_) | Component::EscapingProjection(_) => {
                                // We can probably do more here. This
                                // corresponds to a case like `<T as
                                // Foo<'a>>::U: 'b`.
                                None
                            }
                        })
                        .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::PredicateAtom::TypeWellFormedFromEnv(..) => {
                // Nothing to elaborate
            }
        }
    }
}

impl 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()
}

///////////////////////////////////////////////////////////////////////////
// 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_ref() {
                return Some(data);
            }
        }
        None
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.base_iterator.size_hint();
        (0, upper)
    }
}