compiler/rustc_trait_selection/src/solve/trait_goals.rs

   1 //! Dealing with trait goals, i.e. `T: Trait<'a, U>`.
   2
   3 use std::iter;
   4
   5 use super::assembly::{self, AssemblyCtxt};
   6 use super::{CanonicalGoal, EvalCtxt, Goal, QueryResult};
   7 use rustc_hir::def_id::DefId;
   8 use rustc_infer::infer::InferOk;
   9 use rustc_infer::traits::query::NoSolution;
  10 use rustc_infer::traits::ObligationCause;
  11 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  12 use rustc_middle::ty::TraitPredicate;
  13 use rustc_middle::ty::{self, Ty, TyCtxt};
  14 use rustc_span::DUMMY_SP;
  15
  16 #[allow(dead_code)] // FIXME: implement and use all variants.
  17 #[derive(Debug, Clone, Copy)]
  18 pub(super) enum CandidateSource {
  19     /// Some user-defined impl with the given `DefId`.
  20     Impl(DefId),
  21     /// The n-th caller bound in the `param_env` of our goal.
  22     ///
  23     /// This is pretty much always a bound from the `where`-clauses of the
  24     /// currently checked item.
  25     ParamEnv(usize),
  26     /// A bound on the `self_ty` in case it is a projection or an opaque type.
  27     ///
  28     /// # Examples
  29     ///
  30     /// ```ignore (for syntax highlighting)
  31     /// trait Trait {
  32     ///     type Assoc: OtherTrait;
  33     /// }
  34     /// ```
  35     ///
  36     /// We know that `<Whatever as Trait>::Assoc: OtherTrait` holds by looking at
  37     /// the bounds on `Trait::Assoc`.
  38     AliasBound(usize),
  39     /// A builtin implementation for some specific traits, used in cases
  40     /// where we cannot rely an ordinary library implementations.
  41     ///
  42     /// The most notable examples are `Sized`, `Copy` and `Clone`. This is also
  43     /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
  44     /// an associated type.
  45     Builtin,
  46     /// An automatic impl for an auto trait, e.g. `Send`. These impls recursively look
  47     /// at the constituent types of the `self_ty` to check whether the auto trait
  48     /// is implemented for those.
  49     AutoImpl,
  50 }
  51
  52 type Candidate<'tcx> = assembly::Candidate<'tcx, TraitPredicate<'tcx>>;
  53
  54 impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
  55     type CandidateSource = CandidateSource;
  56
  57     fn self_ty(self) -> Ty<'tcx> {
  58         self.self_ty()
  59     }
  60
  61     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
  62         self.with_self_ty(tcx, self_ty)
  63     }
  64
  65     fn trait_def_id(self, _: TyCtxt<'tcx>) -> DefId {
  66         self.def_id()
  67     }
  68
  69     fn consider_impl_candidate(
  70         acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
  71         goal: Goal<'tcx, TraitPredicate<'tcx>>,
  72         impl_def_id: DefId,
  73     ) {
  74         let impl_trait_ref = acx.cx.tcx.bound_impl_trait_ref(impl_def_id).unwrap();
  75         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
  76         if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
  77             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
  78         {
  79             return;
  80         }
  81
  82         acx.infcx.probe(|_| {
  83             let impl_substs = acx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
  84             let impl_trait_ref = impl_trait_ref.subst(acx.cx.tcx, impl_substs);
  85
  86             let Ok(InferOk { obligations, .. }) = acx
  87                 .infcx
  88                 .at(&ObligationCause::dummy(), goal.param_env)
  89                 .define_opaque_types(false)
  90                 .eq(goal.predicate.trait_ref, impl_trait_ref)
  91                 .map_err(|e| debug!("failed to equate trait refs: {e:?}"))
  92             else {
  93                 return
  94             };
  95
  96             let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
  97
  98             let Ok(certainty) = acx.cx.evaluate_all(acx.infcx, nested_goals) else { return };
  99             acx.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
 100         })
 101     }
 102 }
 103
 104 impl<'tcx> EvalCtxt<'tcx> {
 105     pub(super) fn compute_trait_goal(
 106         &mut self,
 107         goal: CanonicalGoal<'tcx, TraitPredicate<'tcx>>,
 108     ) -> QueryResult<'tcx> {
 109         let candidates = AssemblyCtxt::assemble_and_evaluate_candidates(self, goal);
 110         self.merge_trait_candidates_discard_reservation_impls(candidates)
 111     }
 112
 113     #[instrument(level = "debug", skip(self), ret)]
 114     pub(super) fn merge_trait_candidates_discard_reservation_impls(
 115         &mut self,
 116         mut candidates: Vec<Candidate<'tcx>>,
 117     ) -> QueryResult<'tcx> {
 118         match candidates.len() {
 119             0 => return Err(NoSolution),
 120             1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
 121             _ => {}
 122         }
 123
 124         if candidates.len() > 1 {
 125             let mut i = 0;
 126             'outer: while i < candidates.len() {
 127                 for j in (0..candidates.len()).filter(|&j| i != j) {
 128                     if self.trait_candidate_should_be_dropped_in_favor_of(
 129                         &candidates[i],
 130                         &candidates[j],
 131                     ) {
 132                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 133                         candidates.swap_remove(i);
 134                         continue 'outer;
 135                     }
 136                 }
 137
 138                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 139                 // If there are *STILL* multiple candidates, give up
 140                 // and report ambiguity.
 141                 i += 1;
 142                 if i > 1 {
 143                     debug!("multiple matches, ambig");
 144                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
 145                     unimplemented!();
 146                 }
 147             }
 148         }
 149
 150         Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
 151     }
 152
 153     fn trait_candidate_should_be_dropped_in_favor_of(
 154         &self,
 155         candidate: &Candidate<'tcx>,
 156         other: &Candidate<'tcx>,
 157     ) -> bool {
 158         // FIXME: implement this
 159         match (candidate.source, other.source) {
 160             (CandidateSource::Impl(_), _)
 161             | (CandidateSource::ParamEnv(_), _)
 162             | (CandidateSource::AliasBound(_), _)
 163             | (CandidateSource::Builtin, _)
 164             | (CandidateSource::AutoImpl, _) => unimplemented!(),
 165         }
 166     }
 167
 168     fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
 169         if let CandidateSource::Impl(def_id) = candidate.source {
 170             if let ty::ImplPolarity::Reservation = self.tcx.impl_polarity(def_id) {
 171                 debug!("Selected reservation impl");
 172                 // FIXME: reduce candidate to ambiguous
 173                 // FIXME: replace `var_values` with identity, yeet external constraints.
 174                 unimplemented!()
 175             }
 176         }
 177
 178         candidate
 179     }
 180 }