rustc_trait_selection/solve/

delegate.rs

use std::collections::hash_map::Entry;
use std::ops::Deref;

use rustc_data_structures::fx::FxHashMap;
use rustc_hir::LangItem;
use rustc_hir::def_id::{CRATE_DEF_ID, DefId};
use rustc_infer::infer::canonical::query_response::make_query_region_constraints;
use rustc_infer::infer::canonical::{
    Canonical, CanonicalExt as _, CanonicalQueryInput, CanonicalVarKind, CanonicalVarValues,
    QueryRegionConstraint,
};
use rustc_infer::infer::{InferCtxt, RegionVariableOrigin, SubregionOrigin, TyCtxtInferExt};
use rustc_infer::traits::solve::{
    ComputeGoalFastPathOutcome, FetchEligibleAssocItemResponse, Goal, SucceededInErased,
};
use rustc_middle::traits::query::NoSolution;
use rustc_middle::traits::solve::Certainty;
use rustc_middle::ty::{
    self, MayBeErased, Ty, TyCtxt, TypeFlags, TypeFoldable, TypeVisitableExt, TypingMode,
};
use rustc_next_trait_solver::solve::{GoalStalledOn, GoalStalledOnOpaques};
use rustc_span::{DUMMY_SP, Span};

use crate::traits::{EvaluateConstErr, ObligationCause, sizedness_fast_path, specialization_graph};

#[repr(transparent)]
pub struct SolverDelegate<'tcx>(InferCtxt<'tcx>);

impl<'a, 'tcx> From<&'a InferCtxt<'tcx>> for &'a SolverDelegate<'tcx> {
    fn from(infcx: &'a InferCtxt<'tcx>) -> Self {
        // SAFETY: `repr(transparent)`
        unsafe { std::mem::transmute(infcx) }
    }
}

impl<'tcx> Deref for SolverDelegate<'tcx> {
    type Target = InferCtxt<'tcx>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<'tcx> SolverDelegate<'tcx> {
    fn known_no_opaque_types_in_storage(&self) -> bool {
        self.inner.borrow_mut().opaque_types().is_empty()
            // in erased mode, observing that opaques are empty aren't enough to give a result
            // here, so let's try the slow path instead.
            && !self.typing_mode_raw().is_erased_not_coherence()
    }
}

/// Create a [`ComputeGoalFastPathOutcome`] signalling the goal is stalled
/// on a list of [`ty::GenericArg`]
fn goal_stalled_on_args<'tcx>(
    stalled_vars: Vec<ty::GenericArg<'tcx>>,
) -> ComputeGoalFastPathOutcome<'tcx> {
    ComputeGoalFastPathOutcome::TriviallyStalled {
        stalled_on: GoalStalledOn {
            stalled_vars,
            sub_roots: Vec::new(),
            stalled_certainty: Certainty::AMBIGUOUS,
            opaques: GoalStalledOnOpaques::No,
        },
    }
}

/// Create a [`ComputeGoalFastPathOutcome`] signalling the  goal is stalled
/// on a list of [`ty::GenericArg`] *or* the opaque type storage being nonempty.
///
fn goal_stalled_on_args_or_nonempty_opaques<'tcx>(
    stalled_vars: Vec<ty::GenericArg<'tcx>>,
) -> ComputeGoalFastPathOutcome<'tcx> {
    ComputeGoalFastPathOutcome::TriviallyStalled {
        stalled_on: GoalStalledOn {
            stalled_vars,
            sub_roots: Vec::new(),
            stalled_certainty: Certainty::AMBIGUOUS,
            opaques: GoalStalledOnOpaques::Yes {
                num_opaques_in_storage: 0,
                // This function should only be called when not in erased mode,
                // otherwise this is wrong. The `compute_goal_fast_path` does this
                // through `known_no_opaque_types_in_storage`
                previously_succeeded_in_erased: SucceededInErased::No,
            },
        },
    }
}

impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<'tcx> {
    type Infcx = InferCtxt<'tcx>;
    type Interner = TyCtxt<'tcx>;

    fn cx(&self) -> TyCtxt<'tcx> {
        self.0.tcx
    }

    fn build_with_canonical<V>(
        interner: TyCtxt<'tcx>,
        canonical: &CanonicalQueryInput<'tcx, V>,
    ) -> (Self, V, CanonicalVarValues<'tcx>)
    where
        V: TypeFoldable<TyCtxt<'tcx>>,
    {
        let (infcx, value, vars) = interner
            .infer_ctxt()
            .with_next_trait_solver(true)
            .build_with_canonical(DUMMY_SP, canonical);
        (SolverDelegate(infcx), value, vars)
    }

    fn compute_goal_fast_path(
        &self,
        goal: Goal<'tcx, ty::Predicate<'tcx>>,
        span: Span,
    ) -> ComputeGoalFastPathOutcome<'tcx> {
        use ComputeGoalFastPathOutcome as Outcome;

        // FIXME(-Zassumptions-on-binders): actually handle fast path
        if self.tcx.assumptions_on_binders() {
            return Outcome::NoFastPath;
        }

        let pred = goal.predicate.kind();
        match pred.skip_binder() {
            ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) => {
                let trait_pred = pred.rebind(trait_pred);

                let self_ty = self.shallow_resolve(trait_pred.self_ty().skip_binder());
                if self_ty.is_ty_var()
                // We don't do this fast path when opaques are defined since we may
                // eventually use opaques to incompletely guide inference via ty var
                // self types.
                // FIXME: Properly consider opaques here.
                && self.known_no_opaque_types_in_storage()
                {
                    goal_stalled_on_args_or_nonempty_opaques(vec![self_ty.into()])
                } else if trait_pred.polarity() == ty::PredicatePolarity::Positive {
                    match self.0.tcx.as_lang_item(trait_pred.def_id()) {
                        Some(LangItem::Sized) | Some(LangItem::MetaSized) => {
                            let predicate = self.resolve_vars_if_possible(goal.predicate);
                            if sizedness_fast_path(self.tcx, predicate, goal.param_env) {
                                Outcome::TriviallyHolds
                            } else {
                                Outcome::NoFastPath
                            }
                        }
                        Some(LangItem::Copy | LangItem::Clone) => {
                            let self_ty =
                                self.resolve_vars_if_possible(trait_pred.self_ty().skip_binder());
                            // Unlike `Sized` traits, which always prefer the built-in impl,
                            // `Copy`/`Clone` may be shadowed by a param-env candidate which
                            // could force a lifetime error or guide inference. While that's
                            // not generally desirable, it is observable, so for now let's
                            // ignore this fast path for types that have regions or infer.
                            if !self_ty
                                .has_type_flags(TypeFlags::HAS_FREE_REGIONS | TypeFlags::HAS_INFER)
                                && self_ty.is_trivially_pure_clone_copy()
                            {
                                Outcome::TriviallyHolds
                            } else {
                                Outcome::NoFastPath
                            }
                        }
                        _ => Outcome::NoFastPath,
                    }
                } else {
                    Outcome::NoFastPath
                }
            }
            ty::PredicateKind::DynCompatible(def_id) if self.0.tcx.is_dyn_compatible(def_id) => {
                Outcome::TriviallyHolds
            }
            ty::PredicateKind::Clause(ty::ClauseKind::RegionOutlives(outlives)) => {
                if outlives.has_escaping_bound_vars() {
                    return Outcome::NoFastPath;
                }

                self.0.sub_regions(
                    SubregionOrigin::RelateRegionParamBound(span, None),
                    outlives.1,
                    outlives.0,
                    ty::VisibleForLeakCheck::Yes,
                );
                Outcome::TriviallyHolds
            }
            ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(outlives)) => {
                if outlives.has_escaping_bound_vars() {
                    return Outcome::NoFastPath;
                }

                self.0.register_type_outlives_constraint(
                    outlives.0,
                    outlives.1,
                    &ObligationCause::dummy_with_span(span),
                );

                Outcome::TriviallyHolds
            }
            ty::PredicateKind::Subtype(ty::SubtypePredicate { a, b, .. })
            | ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => {
                if a.has_escaping_bound_vars() || b.has_escaping_bound_vars() {
                    return Outcome::NoFastPath;
                }

                match (self.shallow_resolve(a).kind(), self.shallow_resolve(b).kind()) {
                    (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
                        self.sub_unify_ty_vids_raw(a_vid, b_vid);
                        goal_stalled_on_args(vec![a.into(), b.into()])
                    }
                    _ => Outcome::NoFastPath,
                }
            }
            ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, _)) => {
                if ct.has_escaping_bound_vars() {
                    return Outcome::NoFastPath;
                }

                let arg = self.shallow_resolve_const(ct);
                if arg.is_ct_infer() {
                    goal_stalled_on_args(vec![arg.into()])
                } else {
                    Outcome::NoFastPath
                }
            }
            ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) => {
                if arg.has_escaping_bound_vars() {
                    return Outcome::NoFastPath;
                }

                let arg = self.shallow_resolve_term(arg);
                if arg.is_trivially_wf(self.tcx) {
                    Outcome::TriviallyHolds
                } else if arg.is_infer() {
                    goal_stalled_on_args(vec![arg.into_arg()])
                } else {
                    Outcome::NoFastPath
                }
            }
            _ => Outcome::NoFastPath,
        }
    }

    fn fresh_var_for_kind_with_span(
        &self,
        arg: ty::GenericArg<'tcx>,
        span: Span,
    ) -> ty::GenericArg<'tcx> {
        match arg.kind() {
            ty::GenericArgKind::Lifetime(_) => {
                self.next_region_var(RegionVariableOrigin::Misc(span)).into()
            }
            ty::GenericArgKind::Type(_) => self.next_ty_var(span).into(),
            ty::GenericArgKind::Const(_) => self.next_const_var(span).into(),
        }
    }

    fn leak_check(&self, max_input_universe: ty::UniverseIndex) -> Result<(), NoSolution> {
        self.0.leak_check(max_input_universe, None).map_err(|_| NoSolution)
    }

    fn evaluate_const(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        alias_const: ty::AliasConst<'tcx>,
    ) -> Option<ty::Const<'tcx>> {
        let ct = ty::Const::new_alias(self.tcx, ty::IsRigid::No, alias_const);

        match crate::traits::try_evaluate_const(&self.0, ct, param_env) {
            Ok(ct) => Some(ct),
            Err(EvaluateConstErr::EvaluationFailure(e)) => Some(ty::Const::new_error(self.tcx, e)),
            Err(
                EvaluateConstErr::InvalidConstParamTy(_) | EvaluateConstErr::HasGenericsOrInfers,
            ) => None,
        }
    }

    fn well_formed_goals(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        term: ty::Term<'tcx>,
    ) -> Option<Vec<Goal<'tcx, ty::Predicate<'tcx>>>> {
        crate::traits::wf::unnormalized_obligations(
            &self.0,
            param_env,
            term,
            DUMMY_SP,
            CRATE_DEF_ID,
        )
        .map(|obligations| obligations.into_iter().map(|obligation| obligation.as_goal()).collect())
    }

    fn make_deduplicated_region_constraints(
        &self,
    ) -> Vec<(ty::RegionConstraint<'tcx>, ty::VisibleForLeakCheck)> {
        // Cannot use `take_registered_region_obligations` as we may compute the response
        // inside of a `probe` whenever we have multiple choices inside of the solver.
        let region_obligations = self.0.inner.borrow().region_obligations().to_owned();
        let region_assumptions = self.0.inner.borrow().region_assumptions().to_owned();
        let region_constraints = self.0.with_region_constraints(|region_constraints| {
            make_query_region_constraints(
                region_obligations,
                region_constraints,
                region_assumptions,
            )
        });

        let mut seen = FxHashMap::default();
        let mut constraints = vec![];
        for QueryRegionConstraint { constraint: outlives, visible_for_leak_check: vis, .. } in
            region_constraints.constraints
        {
            match seen.entry(outlives) {
                Entry::Occupied(occupied) => {
                    let idx = occupied.get();
                    let (_, prev_vis): &mut (_, ty::VisibleForLeakCheck) =
                        constraints.get_mut(*idx).unwrap();
                    *prev_vis = (*prev_vis).or(vis);
                }
                Entry::Vacant(vacant) => {
                    vacant.insert(constraints.len());
                    constraints.push((outlives, vis));
                }
            }
        }
        constraints
    }

    fn instantiate_canonical<V>(
        &self,
        canonical: Canonical<'tcx, V>,
        values: CanonicalVarValues<'tcx>,
    ) -> V
    where
        V: TypeFoldable<TyCtxt<'tcx>>,
    {
        canonical.instantiate(self.tcx, &values)
    }

    fn instantiate_canonical_var(
        &self,
        kind: CanonicalVarKind<'tcx>,
        span: Span,
        var_values: &[ty::GenericArg<'tcx>],
        universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
    ) -> ty::GenericArg<'tcx> {
        self.0.instantiate_canonical_var(span, kind, var_values, universe_map)
    }

    fn add_item_bounds_for_hidden_type(
        &self,
        def_id: DefId,
        args: ty::GenericArgsRef<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        hidden_ty: Ty<'tcx>,
        goals: &mut Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
    ) {
        self.0.add_item_bounds_for_hidden_type(def_id, args, param_env, hidden_ty, goals);
    }

    fn fetch_eligible_assoc_item(
        &self,
        goal_trait_ref: ty::TraitRef<'tcx>,
        trait_assoc_def_id: DefId,
        impl_def_id: DefId,
    ) -> FetchEligibleAssocItemResponse<'tcx> {
        let node_item =
            match specialization_graph::assoc_def(self.tcx, impl_def_id, trait_assoc_def_id) {
                Ok(i) => i,
                Err(guar) => return FetchEligibleAssocItemResponse::Err(guar),
            };

        let typing_mode = self.typing_mode_raw();

        let eligible = if node_item.is_final() {
            // Non-specializable items are always projectable.
            true
        } else {
            // Only reveal a specializable default if we're past type-checking
            // and the obligation is monomorphic, otherwise passes such as
            // transmute checking and polymorphic MIR optimizations could
            // get a result which isn't correct for all monomorphizations.
            match typing_mode {
                TypingMode::Coherence
                | TypingMode::Typeck { .. }
                | TypingMode::PostTypeckUntilBorrowck { .. }
                | TypingMode::PostBorrowck { .. } => false,
                TypingMode::PostAnalysis | TypingMode::Codegen => {
                    let poly_trait_ref = self.resolve_vars_if_possible(goal_trait_ref);
                    !poly_trait_ref.still_further_specializable()
                }
                TypingMode::ErasedNotCoherence(MayBeErased) => {
                    return FetchEligibleAssocItemResponse::NotFoundBecauseErased;
                }
            }
        };

        // FIXME: Check for defaultness here may cause diagnostics problems.
        if eligible {
            FetchEligibleAssocItemResponse::Found(node_item.item.def_id)
        } else {
            // We know it's not erased since then we'd have returned in the match above,
            // or node_item.final() was true and eligible is always true.
            FetchEligibleAssocItemResponse::NotFound(typing_mode.assert_not_erased())
        }
    }

    // FIXME: This actually should destructure the `Result` we get from transmutability and
    // register candidates. We probably need to register >1 since we may have an OR of ANDs.
    fn is_transmutable(
        &self,
        src: Ty<'tcx>,
        dst: Ty<'tcx>,
        assume: ty::Const<'tcx>,
    ) -> Result<Certainty, NoSolution> {
        // Erase regions because we compute layouts in `rustc_transmute`,
        // which will ICE for region vars.
        let (dst, src) = self.tcx.erase_and_anonymize_regions((dst, src));

        let Some(assume) = rustc_transmute::Assume::from_const(self.tcx, assume) else {
            return Err(NoSolution);
        };

        // FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
        match rustc_transmute::TransmuteTypeEnv::new(self.0.tcx).is_transmutable(src, dst, assume) {
            rustc_transmute::Answer::Yes => Ok(Certainty::Yes),
            rustc_transmute::Answer::No(_) | rustc_transmute::Answer::If(_) => Err(NoSolution),
        }
    }
}