rustc_trait_selection/traits/

util.rs

use std::collections::VecDeque;

use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
use rustc_hir::LangItem;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::InferCtxt;
use rustc_infer::traits::PolyTraitObligation;
pub use rustc_infer::traits::util::*;
use rustc_middle::bug;
use rustc_middle::ty::fast_reject::DeepRejectCtxt;
use rustc_middle::ty::{
    self, PolyTraitPredicate, PredicatePolarity, SizedTraitKind, TraitPredicate, TraitRef, Ty,
    TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt, Unnormalized,
};
pub use rustc_next_trait_solver::placeholder::BoundVarReplacer;
use rustc_span::Span;
use smallvec::{SmallVec, smallvec};
use tracing::debug;

/// Return the trait and projection predicates that come from eagerly expanding the
/// trait aliases in the list of clauses. For each trait predicate, record a stack
/// of spans that trace from the user-written trait alias bound. For projection predicates,
/// just record the span of the projection itself.
///
/// For trait aliases, we don't deduplicte the predicates, since we currently do not
/// consider duplicated traits as a single trait for the purposes of our "one trait principal"
/// restriction; however, for projections we do deduplicate them.
///
/// ```rust,ignore (fails)
/// trait Bar {}
/// trait Foo = Bar + Bar;
///
/// let dyn_incompatible: dyn Foo; // bad, two `Bar` principals.
/// ```
pub fn expand_trait_aliases<'tcx>(
    tcx: TyCtxt<'tcx>,
    clauses: impl IntoIterator<Item = (ty::Clause<'tcx>, Span)>,
) -> (
    Vec<(ty::PolyTraitPredicate<'tcx>, SmallVec<[Span; 1]>)>,
    Vec<(ty::PolyProjectionPredicate<'tcx>, Span)>,
) {
    let mut trait_preds = vec![];
    let mut projection_preds = vec![];
    let mut seen_projection_preds = FxHashSet::default();

    let mut queue: VecDeque<_> = clauses.into_iter().map(|(p, s)| (p, smallvec![s])).collect();

    while let Some((clause, spans)) = queue.pop_front() {
        match clause.kind().skip_binder() {
            ty::ClauseKind::Trait(trait_pred) => {
                if tcx.is_trait_alias(trait_pred.def_id()) {
                    queue.extend(
                        tcx.explicit_super_predicates_of(trait_pred.def_id())
                            .iter_identity_copied()
                            .map(Unnormalized::skip_norm_wip)
                            .map(|(super_clause, span)| {
                                let mut spans = spans.clone();
                                spans.push(span);
                                (
                                    super_clause.instantiate_supertrait(
                                        tcx,
                                        clause.kind().rebind(trait_pred.trait_ref),
                                    ),
                                    spans,
                                )
                            }),
                    );
                } else {
                    trait_preds.push((clause.kind().rebind(trait_pred), spans));
                }
            }
            ty::ClauseKind::Projection(projection_pred) => {
                let projection_pred = clause.kind().rebind(projection_pred);
                if !seen_projection_preds.insert(tcx.anonymize_bound_vars(projection_pred)) {
                    continue;
                }
                projection_preds.push((projection_pred, *spans.last().unwrap()));
            }
            ty::ClauseKind::RegionOutlives(..)
            | ty::ClauseKind::TypeOutlives(..)
            | ty::ClauseKind::ConstArgHasType(_, _)
            | ty::ClauseKind::WellFormed(_)
            | ty::ClauseKind::ConstEvaluatable(_)
            | ty::ClauseKind::UnstableFeature(_)
            | ty::ClauseKind::HostEffect(..) => {}
        }
    }

    (trait_preds, projection_preds)
}

///////////////////////////////////////////////////////////////////////////
// Other
///////////////////////////////////////////////////////////////////////////

/// Casts a trait reference into a reference to one of its super
/// traits; returns `None` if `target_trait_def_id` is not a
/// supertrait.
pub fn upcast_choices<'tcx>(
    tcx: TyCtxt<'tcx>,
    source_trait_ref: ty::PolyTraitRef<'tcx>,
    target_trait_def_id: DefId,
) -> Vec<ty::PolyTraitRef<'tcx>> {
    if source_trait_ref.def_id() == target_trait_def_id {
        return vec![source_trait_ref]; // Shortcut the most common case.
    }

    supertraits(tcx, source_trait_ref).filter(|r| r.def_id() == target_trait_def_id).collect()
}

pub(crate) fn closure_trait_ref_and_return_type<'tcx>(
    tcx: TyCtxt<'tcx>,
    fn_trait_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::PolyFnSig<'tcx>,
    tuple_arguments: TupleArgumentsFlag,
) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>)> {
    assert!(!self_ty.has_escaping_bound_vars());
    let arguments_tuple = match tuple_arguments {
        TupleArgumentsFlag::No => sig.skip_binder().inputs()[0],
        TupleArgumentsFlag::Yes => Ty::new_tup(tcx, sig.skip_binder().inputs()),
    };
    let trait_ref = ty::TraitRef::new(tcx, fn_trait_def_id, [self_ty, arguments_tuple]);
    sig.map_bound(|sig| (trait_ref, sig.output()))
}

pub(crate) fn coroutine_trait_ref_and_outputs<'tcx>(
    tcx: TyCtxt<'tcx>,
    fn_trait_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::GenSig<TyCtxt<'tcx>>,
) -> (ty::TraitRef<'tcx>, Ty<'tcx>, Ty<'tcx>) {
    assert!(!self_ty.has_escaping_bound_vars());
    let trait_ref = ty::TraitRef::new(tcx, fn_trait_def_id, [self_ty, sig.resume_ty]);
    (trait_ref, sig.yield_ty, sig.return_ty)
}

pub(crate) fn future_trait_ref_and_outputs<'tcx>(
    tcx: TyCtxt<'tcx>,
    fn_trait_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::GenSig<TyCtxt<'tcx>>,
) -> (ty::TraitRef<'tcx>, Ty<'tcx>) {
    assert!(!self_ty.has_escaping_bound_vars());
    let trait_ref = ty::TraitRef::new(tcx, fn_trait_def_id, [self_ty]);
    (trait_ref, sig.return_ty)
}

pub(crate) fn iterator_trait_ref_and_outputs<'tcx>(
    tcx: TyCtxt<'tcx>,
    iterator_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::GenSig<TyCtxt<'tcx>>,
) -> (ty::TraitRef<'tcx>, Ty<'tcx>) {
    assert!(!self_ty.has_escaping_bound_vars());
    let trait_ref = ty::TraitRef::new(tcx, iterator_def_id, [self_ty]);
    (trait_ref, sig.yield_ty)
}

pub(crate) fn async_iterator_trait_ref_and_outputs<'tcx>(
    tcx: TyCtxt<'tcx>,
    async_iterator_def_id: DefId,
    self_ty: Ty<'tcx>,
    sig: ty::GenSig<TyCtxt<'tcx>>,
) -> (ty::TraitRef<'tcx>, Ty<'tcx>) {
    assert!(!self_ty.has_escaping_bound_vars());
    let trait_ref = ty::TraitRef::new(tcx, async_iterator_def_id, [self_ty]);
    (trait_ref, sig.yield_ty)
}

pub fn impl_item_is_final(tcx: TyCtxt<'_>, assoc_item: &ty::AssocItem) -> bool {
    assoc_item.defaultness(tcx).is_final()
        && tcx.defaultness(assoc_item.container_id(tcx)).is_final()
}

pub(crate) enum TupleArgumentsFlag {
    Yes,
    No,
}

/// Executes `f` on `value` after replacing all escaping bound variables with placeholders
/// and then replaces these placeholders with the original bound variables in the result.
///
/// In most places, bound variables should be replaced right when entering a binder, making
/// this function unnecessary. However, normalization currently does not do that, so we have
/// to do this lazily.
///
/// You should not add any additional uses of this function, at least not without first
/// discussing it with t-types.
///
/// FIXME(@lcnr): We may even consider experimenting with eagerly replacing bound vars during
/// normalization as well, at which point this function will be unnecessary and can be removed.
pub fn with_replaced_escaping_bound_vars<
    'a,
    'tcx,
    T: TypeFoldable<TyCtxt<'tcx>>,
    R: TypeFoldable<TyCtxt<'tcx>>,
>(
    infcx: &'a InferCtxt<'tcx>,
    universe_indices: &'a mut Vec<Option<ty::UniverseIndex>>,
    value: T,
    f: impl FnOnce(T) -> R,
) -> R {
    if value.has_escaping_bound_vars() {
        let (value, mapped_regions, mapped_types, mapped_consts) =
            BoundVarReplacer::replace_bound_vars(infcx, universe_indices, value);
        let result = f(value);
        PlaceholderReplacer::replace_placeholders(
            infcx,
            mapped_regions,
            mapped_types,
            mapped_consts,
            universe_indices,
            result,
        )
    } else {
        f(value)
    }
}

/// The inverse of [`BoundVarReplacer`]: replaces placeholders with the bound vars from which they came.
pub struct PlaceholderReplacer<'a, 'tcx> {
    infcx: &'a InferCtxt<'tcx>,
    mapped_regions: FxIndexMap<ty::PlaceholderRegion<'tcx>, ty::BoundRegion<'tcx>>,
    mapped_types: FxIndexMap<ty::PlaceholderType<'tcx>, ty::BoundTy<'tcx>>,
    mapped_consts: FxIndexMap<ty::PlaceholderConst<'tcx>, ty::BoundConst<'tcx>>,
    universe_indices: &'a [Option<ty::UniverseIndex>],
    current_index: ty::DebruijnIndex,
}

impl<'a, 'tcx> PlaceholderReplacer<'a, 'tcx> {
    pub fn replace_placeholders<T: TypeFoldable<TyCtxt<'tcx>>>(
        infcx: &'a InferCtxt<'tcx>,
        mapped_regions: FxIndexMap<ty::PlaceholderRegion<'tcx>, ty::BoundRegion<'tcx>>,
        mapped_types: FxIndexMap<ty::PlaceholderType<'tcx>, ty::BoundTy<'tcx>>,
        mapped_consts: FxIndexMap<ty::PlaceholderConst<'tcx>, ty::BoundConst<'tcx>>,
        universe_indices: &'a [Option<ty::UniverseIndex>],
        value: T,
    ) -> T {
        let mut replacer = PlaceholderReplacer {
            infcx,
            mapped_regions,
            mapped_types,
            mapped_consts,
            universe_indices,
            current_index: ty::INNERMOST,
        };
        value.fold_with(&mut replacer)
    }
}

impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
    fn cx(&self) -> TyCtxt<'tcx> {
        self.infcx.tcx
    }

    fn fold_binder<T: TypeFoldable<TyCtxt<'tcx>>>(
        &mut self,
        t: ty::Binder<'tcx, T>,
    ) -> ty::Binder<'tcx, T> {
        if !t.has_placeholders() && !t.has_infer() {
            return t;
        }
        self.current_index.shift_in(1);
        let t = t.super_fold_with(self);
        self.current_index.shift_out(1);
        t
    }

    fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
        let r1 = match r0.kind() {
            ty::ReVar(vid) => self
                .infcx
                .inner
                .borrow_mut()
                .unwrap_region_constraints()
                .opportunistic_resolve_var(self.infcx.tcx, vid),
            _ => r0,
        };

        let r2 = match r1.kind() {
            ty::RePlaceholder(p) => {
                let replace_var = self.mapped_regions.get(&p);
                match replace_var {
                    Some(replace_var) => {
                        let index = self
                            .universe_indices
                            .iter()
                            .position(|u| matches!(u, Some(pu) if *pu == p.universe))
                            .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
                        let db = ty::DebruijnIndex::from_usize(
                            self.universe_indices.len() - index + self.current_index.as_usize() - 1,
                        );
                        ty::Region::new_bound(self.cx(), db, *replace_var)
                    }
                    None => r1,
                }
            }
            _ => r1,
        };

        debug!(?r0, ?r1, ?r2, "fold_region");

        r2
    }

    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
        let ty = self.infcx.shallow_resolve(ty);
        match *ty.kind() {
            ty::Placeholder(p) => {
                let replace_var = self.mapped_types.get(&p);
                match replace_var {
                    Some(replace_var) => {
                        let index = self
                            .universe_indices
                            .iter()
                            .position(|u| matches!(u, Some(pu) if *pu == p.universe))
                            .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
                        let db = ty::DebruijnIndex::from_usize(
                            self.universe_indices.len() - index + self.current_index.as_usize() - 1,
                        );
                        Ty::new_bound(self.infcx.tcx, db, *replace_var)
                    }
                    None => {
                        if ty.has_infer() {
                            ty.super_fold_with(self)
                        } else {
                            ty
                        }
                    }
                }
            }

            _ if ty.has_placeholders() || ty.has_infer() => ty.super_fold_with(self),
            _ => ty,
        }
    }

    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
        let ct = self.infcx.shallow_resolve_const(ct);
        if let ty::ConstKind::Placeholder(p) = ct.kind() {
            let replace_var = self.mapped_consts.get(&p);
            match replace_var {
                Some(replace_var) => {
                    let index = self
                        .universe_indices
                        .iter()
                        .position(|u| matches!(u, Some(pu) if *pu == p.universe))
                        .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
                    let db = ty::DebruijnIndex::from_usize(
                        self.universe_indices.len() - index + self.current_index.as_usize() - 1,
                    );
                    ty::Const::new_bound(self.infcx.tcx, db, *replace_var)
                }
                None => {
                    if ct.has_infer() {
                        ct.super_fold_with(self)
                    } else {
                        ct
                    }
                }
            }
        } else {
            ct.super_fold_with(self)
        }
    }
}

pub fn sizedness_fast_path<'tcx>(
    tcx: TyCtxt<'tcx>,
    predicate: ty::Predicate<'tcx>,
    param_env: ty::ParamEnv<'tcx>,
) -> bool {
    // Proving `Sized`/`MetaSized`, very often on "obviously sized" types like
    // `&T`, accounts for about 60% percentage of the predicates we have to prove. No need to
    // canonicalize and all that for such cases.
    if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) =
        predicate.kind().skip_binder()
        && trait_pred.polarity == ty::PredicatePolarity::Positive
    {
        let sizedness = match tcx.as_lang_item(trait_pred.def_id()) {
            Some(LangItem::Sized) => SizedTraitKind::Sized,
            Some(LangItem::MetaSized) => SizedTraitKind::MetaSized,
            _ => return false,
        };

        if trait_pred.self_ty().has_trivial_sizedness(tcx, sizedness) {
            debug!("fast path -- trivial sizedness");
            return true;
        }

        if matches!(trait_pred.self_ty().kind(), ty::Param(_) | ty::Placeholder(_)) {
            for clause in param_env.caller_bounds() {
                if let ty::ClauseKind::Trait(clause_pred) = clause.kind().skip_binder()
                    && clause_pred.polarity == ty::PredicatePolarity::Positive
                    && clause_pred.self_ty() == trait_pred.self_ty()
                    && (clause_pred.def_id() == trait_pred.def_id()
                        || (sizedness == SizedTraitKind::MetaSized
                            && tcx.is_lang_item(clause_pred.def_id(), LangItem::Sized)))
                {
                    return true;
                }
            }
        }
    }

    false
}

/// To improve performance, sizedness traits are not elaborated and so special-casing is required
/// in the trait solver to find a `Sized` candidate for a `MetaSized` obligation. Returns the
/// predicate to used in the candidate for such a `obligation`, given a `candidate`.
pub(crate) fn lazily_elaborate_sizedness_candidate<'tcx>(
    infcx: &InferCtxt<'tcx>,
    obligation: &PolyTraitObligation<'tcx>,
    candidate: PolyTraitPredicate<'tcx>,
) -> PolyTraitPredicate<'tcx> {
    if !infcx.tcx.is_lang_item(obligation.predicate.def_id(), LangItem::MetaSized)
        || !infcx.tcx.is_lang_item(candidate.def_id(), LangItem::Sized)
    {
        return candidate;
    }

    if obligation.predicate.polarity() != PredicatePolarity::Positive
        || candidate.polarity() != PredicatePolarity::Positive
    {
        return candidate;
    }

    let drcx = DeepRejectCtxt::relate_rigid_rigid(infcx.tcx);
    if !drcx.args_may_unify(
        obligation.predicate.skip_binder().trait_ref.args,
        candidate.skip_binder().trait_ref.args,
    ) {
        return candidate;
    }

    candidate.map_bound(|c| TraitPredicate {
        trait_ref: TraitRef::new_from_args(
            infcx.tcx,
            obligation.predicate.def_id(),
            c.trait_ref.args,
        ),
        polarity: c.polarity,
    })
}