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, Candidate, CandidateSource};
   6 use super::infcx_ext::InferCtxtExt;
   7 use super::{EvalCtxt, Goal, QueryResult};
   8 use rustc_hir::def_id::DefId;
   9 use rustc_infer::infer::InferCtxt;
  10 use rustc_infer::traits::query::NoSolution;
  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 mod structural_traits;
  17
  18 impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
  19     fn self_ty(self) -> Ty<'tcx> {
  20         self.self_ty()
  21     }
  22
  23     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
  24         self.with_self_ty(tcx, self_ty)
  25     }
  26
  27     fn trait_def_id(self, _: TyCtxt<'tcx>) -> DefId {
  28         self.def_id()
  29     }
  30
  31     fn consider_impl_candidate(
  32         ecx: &mut EvalCtxt<'_, 'tcx>,
  33         goal: Goal<'tcx, TraitPredicate<'tcx>>,
  34         impl_def_id: DefId,
  35     ) -> QueryResult<'tcx> {
  36         let tcx = ecx.tcx();
  37
  38         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
  39         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
  40         if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
  41             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
  42         {
  43             return Err(NoSolution);
  44         }
  45
  46         ecx.infcx.probe(|_| {
  47             let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
  48             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
  49
  50             let mut nested_goals =
  51                 ecx.infcx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
  52             let where_clause_bounds = tcx
  53                 .predicates_of(impl_def_id)
  54                 .instantiate(tcx, impl_substs)
  55                 .predicates
  56                 .into_iter()
  57                 .map(|pred| goal.with(tcx, pred));
  58             nested_goals.extend(where_clause_bounds);
  59             ecx.evaluate_all_and_make_canonical_response(nested_goals)
  60         })
  61     }
  62
  63     fn consider_assumption(
  64         ecx: &mut EvalCtxt<'_, 'tcx>,
  65         goal: Goal<'tcx, Self>,
  66         assumption: ty::Predicate<'tcx>,
  67     ) -> QueryResult<'tcx> {
  68         if let Some(poly_trait_pred) = assumption.to_opt_poly_trait_pred() {
  69             // FIXME: Constness and polarity
  70             ecx.infcx.probe(|_| {
  71                 let assumption_trait_pred =
  72                     ecx.infcx.instantiate_bound_vars_with_infer(poly_trait_pred);
  73                 let nested_goals = ecx.infcx.eq(
  74                     goal.param_env,
  75                     goal.predicate.trait_ref,
  76                     assumption_trait_pred.trait_ref,
  77                 )?;
  78                 ecx.evaluate_all_and_make_canonical_response(nested_goals)
  79             })
  80         } else {
  81             Err(NoSolution)
  82         }
  83     }
  84
  85     fn consider_auto_trait_candidate(
  86         ecx: &mut EvalCtxt<'_, 'tcx>,
  87         goal: Goal<'tcx, Self>,
  88     ) -> QueryResult<'tcx> {
  89         ecx.probe_and_evaluate_goal_for_constituent_tys(
  90             goal,
  91             structural_traits::instantiate_constituent_tys_for_auto_trait,
  92         )
  93     }
  94
  95     fn consider_trait_alias_candidate(
  96         ecx: &mut EvalCtxt<'_, 'tcx>,
  97         goal: Goal<'tcx, Self>,
  98     ) -> QueryResult<'tcx> {
  99         let tcx = ecx.tcx();
 100
 101         ecx.infcx.probe(|_| {
 102             let nested_obligations = tcx
 103                 .predicates_of(goal.predicate.def_id())
 104                 .instantiate(tcx, goal.predicate.trait_ref.substs);
 105             ecx.evaluate_all_and_make_canonical_response(
 106                 nested_obligations.predicates.into_iter().map(|p| goal.with(tcx, p)).collect(),
 107             )
 108         })
 109     }
 110
 111     fn consider_builtin_sized_candidate(
 112         ecx: &mut EvalCtxt<'_, 'tcx>,
 113         goal: Goal<'tcx, Self>,
 114     ) -> QueryResult<'tcx> {
 115         ecx.probe_and_evaluate_goal_for_constituent_tys(
 116             goal,
 117             structural_traits::instantiate_constituent_tys_for_sized_trait,
 118         )
 119     }
 120
 121     fn consider_builtin_copy_clone_candidate(
 122         ecx: &mut EvalCtxt<'_, 'tcx>,
 123         goal: Goal<'tcx, Self>,
 124     ) -> QueryResult<'tcx> {
 125         ecx.probe_and_evaluate_goal_for_constituent_tys(
 126             goal,
 127             structural_traits::instantiate_constituent_tys_for_copy_clone_trait,
 128         )
 129     }
 130 }
 131
 132 impl<'tcx> EvalCtxt<'_, 'tcx> {
 133     /// Convenience function for traits that are structural, i.e. that only
 134     /// have nested subgoals that only change the self type. Unlike other
 135     /// evaluate-like helpers, this does a probe, so it doesn't need to be
 136     /// wrapped in one.
 137     fn probe_and_evaluate_goal_for_constituent_tys(
 138         &mut self,
 139         goal: Goal<'tcx, TraitPredicate<'tcx>>,
 140         constituent_tys: impl Fn(&InferCtxt<'tcx>, Ty<'tcx>) -> Result<Vec<Ty<'tcx>>, NoSolution>,
 141     ) -> QueryResult<'tcx> {
 142         self.infcx.probe(|_| {
 143             self.evaluate_all_and_make_canonical_response(
 144                 constituent_tys(self.infcx, goal.predicate.self_ty())?
 145                     .into_iter()
 146                     .map(|ty| {
 147                         goal.with(
 148                             self.tcx(),
 149                             ty::Binder::dummy(goal.predicate.with_self_ty(self.tcx(), ty)),
 150                         )
 151                     })
 152                     .collect(),
 153             )
 154         })
 155     }
 156
 157     pub(super) fn compute_trait_goal(
 158         &mut self,
 159         goal: Goal<'tcx, TraitPredicate<'tcx>>,
 160     ) -> QueryResult<'tcx> {
 161         let candidates = self.assemble_and_evaluate_candidates(goal);
 162         self.merge_trait_candidates_discard_reservation_impls(candidates)
 163     }
 164
 165     #[instrument(level = "debug", skip(self), ret)]
 166     pub(super) fn merge_trait_candidates_discard_reservation_impls(
 167         &mut self,
 168         mut candidates: Vec<Candidate<'tcx>>,
 169     ) -> QueryResult<'tcx> {
 170         match candidates.len() {
 171             0 => return Err(NoSolution),
 172             1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
 173             _ => {}
 174         }
 175
 176         if candidates.len() > 1 {
 177             let mut i = 0;
 178             'outer: while i < candidates.len() {
 179                 for j in (0..candidates.len()).filter(|&j| i != j) {
 180                     if self.trait_candidate_should_be_dropped_in_favor_of(
 181                         &candidates[i],
 182                         &candidates[j],
 183                     ) {
 184                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 185                         candidates.swap_remove(i);
 186                         continue 'outer;
 187                     }
 188                 }
 189
 190                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 191                 // If there are *STILL* multiple candidates, give up
 192                 // and report ambiguity.
 193                 i += 1;
 194                 if i > 1 {
 195                     debug!("multiple matches, ambig");
 196                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
 197                     unimplemented!();
 198                 }
 199             }
 200         }
 201
 202         Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
 203     }
 204
 205     fn trait_candidate_should_be_dropped_in_favor_of(
 206         &self,
 207         candidate: &Candidate<'tcx>,
 208         other: &Candidate<'tcx>,
 209     ) -> bool {
 210         // FIXME: implement this
 211         match (candidate.source, other.source) {
 212             (CandidateSource::Impl(_), _)
 213             | (CandidateSource::ParamEnv(_), _)
 214             | (CandidateSource::AliasBound(_), _)
 215             | (CandidateSource::BuiltinImpl, _) => unimplemented!(),
 216         }
 217     }
 218
 219     fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
 220         if let CandidateSource::Impl(def_id) = candidate.source {
 221             if let ty::ImplPolarity::Reservation = self.tcx().impl_polarity(def_id) {
 222                 debug!("Selected reservation impl");
 223                 // FIXME: reduce candidate to ambiguous
 224                 // FIXME: replace `var_values` with identity, yeet external constraints.
 225                 unimplemented!()
 226             }
 227         }
 228
 229         candidate
 230     }
 231 }