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