diff options
Diffstat (limited to 'compiler/rustc_trait_selection/src/solve/trait_goals.rs')
| -rw-r--r-- | compiler/rustc_trait_selection/src/solve/trait_goals.rs | 180 |
1 files changed, 180 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/solve/trait_goals.rs b/compiler/rustc_trait_selection/src/solve/trait_goals.rs new file mode 100644 index 00000000000..10b45a77dab --- /dev/null +++ b/compiler/rustc_trait_selection/src/solve/trait_goals.rs @@ -0,0 +1,180 @@ +//! 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 + } +} |