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::{Certainty, 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::{self, ToPredicate, Ty, TyCtxt};
  13 use rustc_middle::ty::{TraitPredicate, TypeVisitable};
  14 use rustc_span::DUMMY_SP;
  15
  16 pub 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             && poly_trait_pred.def_id() == goal.predicate.def_id()
  70         {
  71             // FIXME: Constness and polarity
  72             ecx.infcx.probe(|_| {
  73                 let assumption_trait_pred =
  74                     ecx.infcx.instantiate_bound_vars_with_infer(poly_trait_pred);
  75                 let nested_goals = ecx.infcx.eq(
  76                     goal.param_env,
  77                     goal.predicate.trait_ref,
  78                     assumption_trait_pred.trait_ref,
  79                 )?;
  80                 ecx.evaluate_all_and_make_canonical_response(nested_goals)
  81             })
  82         } else {
  83             Err(NoSolution)
  84         }
  85     }
  86
  87     fn consider_auto_trait_candidate(
  88         ecx: &mut EvalCtxt<'_, 'tcx>,
  89         goal: Goal<'tcx, Self>,
  90     ) -> QueryResult<'tcx> {
  91         ecx.probe_and_evaluate_goal_for_constituent_tys(
  92             goal,
  93             structural_traits::instantiate_constituent_tys_for_auto_trait,
  94         )
  95     }
  96
  97     fn consider_trait_alias_candidate(
  98         ecx: &mut EvalCtxt<'_, 'tcx>,
  99         goal: Goal<'tcx, Self>,
 100     ) -> QueryResult<'tcx> {
 101         let tcx = ecx.tcx();
 102
 103         ecx.infcx.probe(|_| {
 104             let nested_obligations = tcx
 105                 .predicates_of(goal.predicate.def_id())
 106                 .instantiate(tcx, goal.predicate.trait_ref.substs);
 107             ecx.evaluate_all_and_make_canonical_response(
 108                 nested_obligations.predicates.into_iter().map(|p| goal.with(tcx, p)).collect(),
 109             )
 110         })
 111     }
 112
 113     fn consider_builtin_sized_candidate(
 114         ecx: &mut EvalCtxt<'_, 'tcx>,
 115         goal: Goal<'tcx, Self>,
 116     ) -> QueryResult<'tcx> {
 117         ecx.probe_and_evaluate_goal_for_constituent_tys(
 118             goal,
 119             structural_traits::instantiate_constituent_tys_for_sized_trait,
 120         )
 121     }
 122
 123     fn consider_builtin_copy_clone_candidate(
 124         ecx: &mut EvalCtxt<'_, 'tcx>,
 125         goal: Goal<'tcx, Self>,
 126     ) -> QueryResult<'tcx> {
 127         ecx.probe_and_evaluate_goal_for_constituent_tys(
 128             goal,
 129             structural_traits::instantiate_constituent_tys_for_copy_clone_trait,
 130         )
 131     }
 132
 133     fn consider_builtin_pointer_sized_candidate(
 134         ecx: &mut EvalCtxt<'_, 'tcx>,
 135         goal: Goal<'tcx, Self>,
 136     ) -> QueryResult<'tcx> {
 137         if goal.predicate.self_ty().has_non_region_infer() {
 138             return ecx.make_canonical_response(Certainty::AMBIGUOUS);
 139         }
 140
 141         let tcx = ecx.tcx();
 142         let self_ty = tcx.erase_regions(goal.predicate.self_ty());
 143
 144         if let Ok(layout) = tcx.layout_of(goal.param_env.and(self_ty))
 145             &&  let usize_layout = tcx.layout_of(ty::ParamEnv::empty().and(tcx.types.usize)).unwrap().layout
 146             && layout.layout.size() == usize_layout.size()
 147             && layout.layout.align().abi == usize_layout.align().abi
 148         {
 149             // FIXME: We could make this faster by making a no-constraints response
 150             ecx.make_canonical_response(Certainty::Yes)
 151         } else {
 152             Err(NoSolution)
 153         }
 154     }
 155
 156     fn consider_builtin_fn_trait_candidates(
 157         ecx: &mut EvalCtxt<'_, 'tcx>,
 158         goal: Goal<'tcx, Self>,
 159         goal_kind: ty::ClosureKind,
 160     ) -> QueryResult<'tcx> {
 161         if let Some(tupled_inputs_and_output) =
 162             structural_traits::extract_tupled_inputs_and_output_from_callable(
 163                 ecx.tcx(),
 164                 goal.predicate.self_ty(),
 165                 goal_kind,
 166             )?
 167         {
 168             let pred = tupled_inputs_and_output
 169                 .map_bound(|(inputs, _)| {
 170                     ecx.tcx()
 171                         .mk_trait_ref(goal.predicate.def_id(), [goal.predicate.self_ty(), inputs])
 172                 })
 173                 .to_predicate(ecx.tcx());
 174             Self::consider_assumption(ecx, goal, pred)
 175         } else {
 176             ecx.make_canonical_response(Certainty::AMBIGUOUS)
 177         }
 178     }
 179
 180     fn consider_builtin_tuple_candidate(
 181         ecx: &mut EvalCtxt<'_, 'tcx>,
 182         goal: Goal<'tcx, Self>,
 183     ) -> QueryResult<'tcx> {
 184         if let ty::Tuple(..) = goal.predicate.self_ty().kind() {
 185             ecx.make_canonical_response(Certainty::Yes)
 186         } else {
 187             Err(NoSolution)
 188         }
 189     }
 190
 191     fn consider_builtin_pointee_candidate(
 192         ecx: &mut EvalCtxt<'_, 'tcx>,
 193         _goal: Goal<'tcx, Self>,
 194     ) -> QueryResult<'tcx> {
 195         ecx.make_canonical_response(Certainty::Yes)
 196     }
 197
 198     fn consider_builtin_future_candidate(
 199         ecx: &mut EvalCtxt<'_, 'tcx>,
 200         goal: Goal<'tcx, Self>,
 201     ) -> QueryResult<'tcx> {
 202         let ty::Generator(def_id, _, _) = *goal.predicate.self_ty().kind() else {
 203             return Err(NoSolution);
 204         };
 205
 206         // Generators are not futures unless they come from `async` desugaring
 207         let tcx = ecx.tcx();
 208         if !tcx.generator_is_async(def_id) {
 209             return Err(NoSolution);
 210         }
 211
 212         // Async generator unconditionally implement `Future`
 213         ecx.make_canonical_response(Certainty::Yes)
 214     }
 215
 216     fn consider_builtin_generator_candidate(
 217         ecx: &mut EvalCtxt<'_, 'tcx>,
 218         goal: Goal<'tcx, Self>,
 219     ) -> QueryResult<'tcx> {
 220         let self_ty = goal.predicate.self_ty();
 221         let ty::Generator(def_id, substs, _) = *self_ty.kind() else {
 222             return Err(NoSolution);
 223         };
 224
 225         // `async`-desugared generators do not implement the generator trait
 226         let tcx = ecx.tcx();
 227         if tcx.generator_is_async(def_id) {
 228             return Err(NoSolution);
 229         }
 230
 231         let generator = substs.as_generator();
 232         Self::consider_assumption(
 233             ecx,
 234             goal,
 235             ty::Binder::dummy(
 236                 tcx.mk_trait_ref(goal.predicate.def_id(), [self_ty, generator.resume_ty()]),
 237             )
 238             .to_predicate(tcx),
 239         )
 240     }
 241 }
 242
 243 impl<'tcx> EvalCtxt<'_, 'tcx> {
 244     /// Convenience function for traits that are structural, i.e. that only
 245     /// have nested subgoals that only change the self type. Unlike other
 246     /// evaluate-like helpers, this does a probe, so it doesn't need to be
 247     /// wrapped in one.
 248     fn probe_and_evaluate_goal_for_constituent_tys(
 249         &mut self,
 250         goal: Goal<'tcx, TraitPredicate<'tcx>>,
 251         constituent_tys: impl Fn(&InferCtxt<'tcx>, Ty<'tcx>) -> Result<Vec<Ty<'tcx>>, NoSolution>,
 252     ) -> QueryResult<'tcx> {
 253         self.infcx.probe(|_| {
 254             self.evaluate_all_and_make_canonical_response(
 255                 constituent_tys(self.infcx, goal.predicate.self_ty())?
 256                     .into_iter()
 257                     .map(|ty| {
 258                         goal.with(
 259                             self.tcx(),
 260                             ty::Binder::dummy(goal.predicate.with_self_ty(self.tcx(), ty)),
 261                         )
 262                     })
 263                     .collect(),
 264             )
 265         })
 266     }
 267
 268     pub(super) fn compute_trait_goal(
 269         &mut self,
 270         goal: Goal<'tcx, TraitPredicate<'tcx>>,
 271     ) -> QueryResult<'tcx> {
 272         let candidates = self.assemble_and_evaluate_candidates(goal);
 273         self.merge_trait_candidates_discard_reservation_impls(candidates)
 274     }
 275
 276     #[instrument(level = "debug", skip(self), ret)]
 277     pub(super) fn merge_trait_candidates_discard_reservation_impls(
 278         &mut self,
 279         mut candidates: Vec<Candidate<'tcx>>,
 280     ) -> QueryResult<'tcx> {
 281         match candidates.len() {
 282             0 => return Err(NoSolution),
 283             1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
 284             _ => {}
 285         }
 286
 287         if candidates.len() > 1 {
 288             let mut i = 0;
 289             'outer: while i < candidates.len() {
 290                 for j in (0..candidates.len()).filter(|&j| i != j) {
 291                     if self.trait_candidate_should_be_dropped_in_favor_of(
 292                         &candidates[i],
 293                         &candidates[j],
 294                     ) {
 295                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 296                         candidates.swap_remove(i);
 297                         continue 'outer;
 298                     }
 299                 }
 300
 301                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 302                 // If there are *STILL* multiple candidates, give up
 303                 // and report ambiguity.
 304                 i += 1;
 305                 if i > 1 {
 306                     debug!("multiple matches, ambig");
 307                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
 308                     unimplemented!();
 309                 }
 310             }
 311         }
 312
 313         Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
 314     }
 315
 316     fn trait_candidate_should_be_dropped_in_favor_of(
 317         &self,
 318         candidate: &Candidate<'tcx>,
 319         other: &Candidate<'tcx>,
 320     ) -> bool {
 321         // FIXME: implement this
 322         match (candidate.source, other.source) {
 323             (CandidateSource::Impl(_), _)
 324             | (CandidateSource::ParamEnv(_), _)
 325             | (CandidateSource::AliasBound(_), _)
 326             | (CandidateSource::BuiltinImpl, _) => unimplemented!(),
 327         }
 328     }
 329
 330     fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
 331         if let CandidateSource::Impl(def_id) = candidate.source {
 332             if let ty::ImplPolarity::Reservation = self.tcx().impl_polarity(def_id) {
 333                 debug!("Selected reservation impl");
 334                 // FIXME: reduce candidate to ambiguous
 335                 // FIXME: replace `var_values` with identity, yeet external constraints.
 336                 unimplemented!()
 337             }
 338         }
 339
 340         candidate
 341     }
 342 }