rust/compiler/rustc_trait_selection/src/solve/trait_goals.rs

//! Dealing with trait goals, i.e. `T: Trait<'a, U>`.

use std::iter;

use super::assembly::{self, AssemblyCtxt};
use super::{CanonicalGoal, EvalCtxt, Goal, QueryResult};
use rustc_hir::def_id::DefId;
use rustc_infer::infer::InferOk;
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::ObligationCause;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::TraitPredicate;
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_span::DUMMY_SP;

#[allow(dead_code)] // FIXME: implement and use all variants.
#[derive(Debug, Clone, Copy)]
pub(super) enum CandidateSource {
    /// Some user-defined impl with the given `DefId`.
    Impl(DefId),
    /// The n-th caller bound in the `param_env` of our goal.
    ///
    /// This is pretty much always a bound from the `where`-clauses of the
    /// currently checked item.
    ParamEnv(usize),
    /// A bound on the `self_ty` in case it is a projection or an opaque type.
    ///
    /// # Examples
    ///
    /// ```ignore (for syntax highlighting)
    /// trait Trait {
    ///     type Assoc: OtherTrait;
    /// }
    /// ```
    ///
    /// We know that `<Whatever as Trait>::Assoc: OtherTrait` holds by looking at
    /// the bounds on `Trait::Assoc`.
    AliasBound(usize),
    /// A builtin implementation for some specific traits, used in cases
    /// where we cannot rely an ordinary library implementations.
    ///
    /// The most notable examples are `Sized`, `Copy` and `Clone`. This is also
    /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
    /// an associated type.
    Builtin,
    /// An automatic impl for an auto trait, e.g. `Send`. These impls recursively look
    /// at the constituent types of the `self_ty` to check whether the auto trait
    /// is implemented for those.
    AutoImpl,
}

type Candidate<'tcx> = assembly::Candidate<'tcx, TraitPredicate<'tcx>>;

impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
    type CandidateSource = CandidateSource;

    fn self_ty(self) -> Ty<'tcx> {
        self.self_ty()
    }

    fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
        self.with_self_ty(tcx, self_ty)
    }

    fn trait_def_id(self, _: TyCtxt<'tcx>) -> DefId {
        self.def_id()
    }

    fn consider_impl_candidate(
        acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
        goal: Goal<'tcx, TraitPredicate<'tcx>>,
        impl_def_id: DefId,
    ) {
        let impl_trait_ref = acx.cx.tcx.bound_impl_trait_ref(impl_def_id).unwrap();
        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
        if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
            .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
        {
            return;
        }

        acx.infcx.probe(|_| {
            let impl_substs = acx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
            let impl_trait_ref = impl_trait_ref.subst(acx.cx.tcx, impl_substs);

            let Ok(InferOk { obligations, .. }) = acx
                .infcx
                .at(&ObligationCause::dummy(), goal.param_env)
                .define_opaque_types(false)
                .eq(goal.predicate.trait_ref, impl_trait_ref)
                .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
            else {
                return
            };

            let nested_goals = obligations.into_iter().map(|o| o.into()).collect();

            let Ok(certainty) = acx.cx.evaluate_all(acx.infcx, nested_goals) else { return };
            acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
        })
    }
}

impl<'tcx> EvalCtxt<'tcx> {
    pub(super) fn compute_trait_goal(
        &mut self,
        goal: CanonicalGoal<'tcx, TraitPredicate<'tcx>>,
    ) -> QueryResult<'tcx> {
        let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
        self.merge_trait_candidates_discard_reservation_impls(candidates)
    }

    #[instrument(level = "debug", skip(self), ret)]
    pub(super) fn merge_trait_candidates_discard_reservation_impls(
        &mut self,
        mut candidates: Vec<Candidate<'tcx>>,
    ) -> QueryResult<'tcx> {
        match candidates.len() {
            0 => return Err(NoSolution),
            1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
            _ => {}
        }

        if candidates.len() > 1 {
            let mut i = 0;
            'outer: while i < candidates.len() {
                for j in (0..candidates.len()).filter(|&j| i != j) {
                    if self.trait_candidate_should_be_dropped_in_favor_of(
                        &candidates[i],
                        &candidates[j],
                    ) {
                        debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
                        candidates.swap_remove(i);
                        continue 'outer;
                    }
                }

                debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
                // If there are *STILL* multiple candidates, give up
                // and report ambiguity.
                i += 1;
                if i > 1 {
                    debug!("multiple matches, ambig");
                    // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
                    unimplemented!();
                }
            }
        }

        Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
    }

    fn trait_candidate_should_be_dropped_in_favor_of(
        &self,
        candidate: &Candidate<'tcx>,
        other: &Candidate<'tcx>,
    ) -> bool {
        // FIXME: implement this
        match (candidate.source, other.source) {
            (CandidateSource::Impl(_), _)
            | (CandidateSource::ParamEnv(_), _)
            | (CandidateSource::AliasBound(_), _)
            | (CandidateSource::Builtin, _)
            | (CandidateSource::AutoImpl, _) => unimplemented!(),
        }
    }

    fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
        if let CandidateSource::Impl(def_id) = candidate.source {
            if let ty::ImplPolarity::Reservation = self.tcx.impl_polarity(def_id) {
                debug!("Selected reservation impl");
                // FIXME: reduce candidate to ambiguous
                // FIXME: replace `var_values` with identity, yeet external constraints.
                unimplemented!()
            }
        }

        candidate
    }
}