compiler/rustc_trait_selection/src/solve/project_goals.rs

   1 use crate::traits::{specialization_graph, translate_substs};
   2
   3 use super::assembly::{self, Candidate, CandidateSource};
   4 use super::infcx_ext::InferCtxtExt;
   5 use super::trait_goals::structural_traits;
   6 use super::{Certainty, EvalCtxt, Goal, QueryResult};
   7 use rustc_errors::ErrorGuaranteed;
   8 use rustc_hir::def::DefKind;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_infer::infer::InferCtxt;
  11 use rustc_infer::traits::query::NoSolution;
  12 use rustc_infer::traits::specialization_graph::LeafDef;
  13 use rustc_infer::traits::Reveal;
  14 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  15 use rustc_middle::ty::{self, Ty, TyCtxt};
  16 use rustc_middle::ty::{ProjectionPredicate, TypeSuperVisitable, TypeVisitor};
  17 use rustc_middle::ty::{ToPredicate, TypeVisitable};
  18 use rustc_span::DUMMY_SP;
  19 use std::iter;
  20 use std::ops::ControlFlow;
  21
  22 impl<'tcx> EvalCtxt<'_, 'tcx> {
  23     pub(super) fn compute_projection_goal(
  24         &mut self,
  25         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
  26     ) -> QueryResult<'tcx> {
  27         // To only compute normalization once for each projection we only
  28         // normalize if the expected term is an unconstrained inference variable.
  29         //
  30         // E.g. for `<T as Trait>::Assoc = u32` we recursively compute the goal
  31         // `exists<U> <T as Trait>::Assoc = U` and then take the resulting type for
  32         // `U` and equate it with `u32`. This means that we don't need a separate
  33         // projection cache in the solver.
  34         if self.term_is_fully_unconstrained(goal) {
  35             let candidates = self.assemble_and_evaluate_candidates(goal);
  36             self.merge_project_candidates(candidates)
  37         } else {
  38             let predicate = goal.predicate;
  39             let unconstrained_rhs = match predicate.term.unpack() {
  40                 ty::TermKind::Ty(_) => self.infcx.next_ty_infer().into(),
  41                 ty::TermKind::Const(ct) => self.infcx.next_const_infer(ct.ty()).into(),
  42             };
  43             let unconstrained_predicate = ty::Clause::Projection(ProjectionPredicate {
  44                 projection_ty: goal.predicate.projection_ty,
  45                 term: unconstrained_rhs,
  46             });
  47             let (_has_changed, normalize_certainty) =
  48                 self.evaluate_goal(goal.with(self.tcx(), unconstrained_predicate))?;
  49
  50             let nested_eq_goals =
  51                 self.infcx.eq(goal.param_env, unconstrained_rhs, predicate.term)?;
  52             let eval_certainty = self.evaluate_all(nested_eq_goals)?;
  53             self.make_canonical_response(normalize_certainty.unify_and(eval_certainty))
  54         }
  55     }
  56
  57     /// Is the projection predicate is of the form `exists<T> <Ty as Trait>::Assoc = T`.
  58     ///
  59     /// This is the case if the `term` is an inference variable in the innermost universe
  60     /// and does not occur in any other part of the predicate.
  61     fn term_is_fully_unconstrained(&self, goal: Goal<'tcx, ProjectionPredicate<'tcx>>) -> bool {
  62         let infcx = self.infcx;
  63         let term_is_infer = match goal.predicate.term.unpack() {
  64             ty::TermKind::Ty(ty) => {
  65                 if let &ty::Infer(ty::TyVar(vid)) = ty.kind() {
  66                     match infcx.probe_ty_var(vid) {
  67                         Ok(value) => bug!("resolved var in query: {goal:?} {value:?}"),
  68                         Err(universe) => universe == infcx.universe(),
  69                     }
  70                 } else {
  71                     false
  72                 }
  73             }
  74             ty::TermKind::Const(ct) => {
  75                 if let ty::ConstKind::Infer(ty::InferConst::Var(vid)) = ct.kind() {
  76                     match self.infcx.probe_const_var(vid) {
  77                         Ok(value) => bug!("resolved var in query: {goal:?} {value:?}"),
  78                         Err(universe) => universe == infcx.universe(),
  79                     }
  80                 } else {
  81                     false
  82                 }
  83             }
  84         };
  85
  86         // Guard against `<T as Trait<?0>>::Assoc = ?0>`.
  87         struct ContainsTerm<'tcx> {
  88             term: ty::Term<'tcx>,
  89         }
  90         impl<'tcx> TypeVisitor<'tcx> for ContainsTerm<'tcx> {
  91             type BreakTy = ();
  92             fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
  93                 if t.needs_infer() {
  94                     if ty::Term::from(t) == self.term {
  95                         ControlFlow::BREAK
  96                     } else {
  97                         t.super_visit_with(self)
  98                     }
  99                 } else {
 100                     ControlFlow::CONTINUE
 101                 }
 102             }
 103
 104             fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 105                 if c.needs_infer() {
 106                     if ty::Term::from(c) == self.term {
 107                         ControlFlow::BREAK
 108                     } else {
 109                         c.super_visit_with(self)
 110                     }
 111                 } else {
 112                     ControlFlow::CONTINUE
 113                 }
 114             }
 115         }
 116
 117         let mut visitor = ContainsTerm { term: goal.predicate.term };
 118
 119         term_is_infer
 120             && goal.predicate.projection_ty.visit_with(&mut visitor).is_continue()
 121             && goal.param_env.visit_with(&mut visitor).is_continue()
 122     }
 123
 124     fn merge_project_candidates(
 125         &mut self,
 126         mut candidates: Vec<Candidate<'tcx>>,
 127     ) -> QueryResult<'tcx> {
 128         match candidates.len() {
 129             0 => return Err(NoSolution),
 130             1 => return Ok(candidates.pop().unwrap().result),
 131             _ => {}
 132         }
 133
 134         if candidates.len() > 1 {
 135             let mut i = 0;
 136             'outer: while i < candidates.len() {
 137                 for j in (0..candidates.len()).filter(|&j| i != j) {
 138                     if self.project_candidate_should_be_dropped_in_favor_of(
 139                         &candidates[i],
 140                         &candidates[j],
 141                     ) {
 142                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 143                         candidates.swap_remove(i);
 144                         continue 'outer;
 145                     }
 146                 }
 147
 148                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 149                 // If there are *STILL* multiple candidates, give up
 150                 // and report ambiguity.
 151                 i += 1;
 152                 if i > 1 {
 153                     debug!("multiple matches, ambig");
 154                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
 155                     unimplemented!();
 156                 }
 157             }
 158         }
 159
 160         Ok(candidates.pop().unwrap().result)
 161     }
 162
 163     fn project_candidate_should_be_dropped_in_favor_of(
 164         &self,
 165         candidate: &Candidate<'tcx>,
 166         other: &Candidate<'tcx>,
 167     ) -> bool {
 168         // FIXME: implement this
 169         match (candidate.source, other.source) {
 170             (CandidateSource::Impl(_), _)
 171             | (CandidateSource::ParamEnv(_), _)
 172             | (CandidateSource::BuiltinImpl, _)
 173             | (CandidateSource::AliasBound(_), _) => unimplemented!(),
 174         }
 175     }
 176 }
 177
 178 impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
 179     fn self_ty(self) -> Ty<'tcx> {
 180         self.self_ty()
 181     }
 182
 183     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
 184         self.with_self_ty(tcx, self_ty)
 185     }
 186
 187     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
 188         self.trait_def_id(tcx)
 189     }
 190
 191     fn consider_impl_candidate(
 192         ecx: &mut EvalCtxt<'_, 'tcx>,
 193         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
 194         impl_def_id: DefId,
 195     ) -> QueryResult<'tcx> {
 196         let tcx = ecx.tcx();
 197
 198         let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
 199         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 200         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
 201         if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
 202             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
 203         {
 204             return Err(NoSolution);
 205         }
 206
 207         ecx.infcx.probe(|_| {
 208             let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
 209             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
 210
 211             let mut nested_goals = ecx.infcx.eq(goal.param_env, goal_trait_ref, impl_trait_ref)?;
 212             let where_clause_bounds = tcx
 213                 .predicates_of(impl_def_id)
 214                 .instantiate(tcx, impl_substs)
 215                 .predicates
 216                 .into_iter()
 217                 .map(|pred| goal.with(tcx, pred));
 218
 219             nested_goals.extend(where_clause_bounds);
 220             let trait_ref_certainty = ecx.evaluate_all(nested_goals)?;
 221
 222             // In case the associated item is hidden due to specialization, we have to
 223             // return ambiguity this would otherwise be incomplete, resulting in
 224             // unsoundness during coherence (#105782).
 225             let Some(assoc_def) = fetch_eligible_assoc_item_def(
 226                 ecx.infcx,
 227                 goal.param_env,
 228                 goal_trait_ref,
 229                 goal.predicate.def_id(),
 230                 impl_def_id
 231             )? else {
 232                 return ecx.make_canonical_response(trait_ref_certainty.unify_and(Certainty::AMBIGUOUS));
 233             };
 234
 235             if !assoc_def.item.defaultness(tcx).has_value() {
 236                 tcx.sess.delay_span_bug(
 237                     tcx.def_span(assoc_def.item.def_id),
 238                     "missing value for assoc item in impl",
 239                 );
 240             }
 241
 242             // Getting the right substitutions here is complex, e.g. given:
 243             // - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
 244             // - the applicable impl `impl<T> Trait<i32> for Vec<T>`
 245             // - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
 246             //
 247             // We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
 248             // to `[u32, u64]`.
 249             //
 250             // And then map these substs to the substs of the defining impl of `Assoc`, going
 251             // from `[u32, u64]` to `[u32, i32, u64]`.
 252             let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
 253                 tcx,
 254                 goal_trait_ref.def_id,
 255                 impl_substs,
 256             );
 257             let substs = translate_substs(
 258                 ecx.infcx,
 259                 goal.param_env,
 260                 impl_def_id,
 261                 impl_substs_with_gat,
 262                 assoc_def.defining_node,
 263             );
 264
 265             // Finally we construct the actual value of the associated type.
 266             let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
 267             let ty = tcx.bound_type_of(assoc_def.item.def_id);
 268             let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
 269                 let identity_substs =
 270                     ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
 271                 let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
 272                 let kind =
 273                     ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
 274                 ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
 275             } else {
 276                 ty.map_bound(|ty| ty.into())
 277             };
 278
 279             // The term of our goal should be fully unconstrained, so this should never fail.
 280             //
 281             // It can however be ambiguous when the resolved type is a projection.
 282             let nested_goals = ecx
 283                 .infcx
 284                 .eq(goal.param_env, goal.predicate.term, term.subst(tcx, substs))
 285                 .expect("failed to unify with unconstrained term");
 286             let rhs_certainty =
 287                 ecx.evaluate_all(nested_goals).expect("failed to unify with unconstrained term");
 288
 289             ecx.make_canonical_response(trait_ref_certainty.unify_and(rhs_certainty))
 290         })
 291     }
 292
 293     fn consider_assumption(
 294         ecx: &mut EvalCtxt<'_, 'tcx>,
 295         goal: Goal<'tcx, Self>,
 296         assumption: ty::Predicate<'tcx>,
 297     ) -> QueryResult<'tcx> {
 298         if let Some(poly_projection_pred) = assumption.to_opt_poly_projection_pred() {
 299             ecx.infcx.probe(|_| {
 300                 let assumption_projection_pred =
 301                     ecx.infcx.instantiate_bound_vars_with_infer(poly_projection_pred);
 302                 let nested_goals = ecx.infcx.eq(
 303                     goal.param_env,
 304                     goal.predicate.projection_ty,
 305                     assumption_projection_pred.projection_ty,
 306                 )?;
 307                 let subst_certainty = ecx.evaluate_all(nested_goals)?;
 308
 309                 // The term of our goal should be fully unconstrained, so this should never fail.
 310                 //
 311                 // It can however be ambiguous when the resolved type is a projection.
 312                 let nested_goals = ecx
 313                     .infcx
 314                     .eq(goal.param_env, goal.predicate.term, assumption_projection_pred.term)
 315                     .expect("failed to unify with unconstrained term");
 316                 let rhs_certainty = ecx
 317                     .evaluate_all(nested_goals)
 318                     .expect("failed to unify with unconstrained term");
 319
 320                 ecx.make_canonical_response(subst_certainty.unify_and(rhs_certainty))
 321             })
 322         } else {
 323             Err(NoSolution)
 324         }
 325     }
 326
 327     fn consider_auto_trait_candidate(
 328         _ecx: &mut EvalCtxt<'_, 'tcx>,
 329         goal: Goal<'tcx, Self>,
 330     ) -> QueryResult<'tcx> {
 331         bug!("auto traits do not have associated types: {:?}", goal);
 332     }
 333
 334     fn consider_trait_alias_candidate(
 335         _ecx: &mut EvalCtxt<'_, 'tcx>,
 336         goal: Goal<'tcx, Self>,
 337     ) -> QueryResult<'tcx> {
 338         bug!("trait aliases do not have associated types: {:?}", goal);
 339     }
 340
 341     fn consider_builtin_sized_candidate(
 342         _ecx: &mut EvalCtxt<'_, 'tcx>,
 343         goal: Goal<'tcx, Self>,
 344     ) -> QueryResult<'tcx> {
 345         bug!("`Sized` does not have an associated type: {:?}", goal);
 346     }
 347
 348     fn consider_builtin_copy_clone_candidate(
 349         _ecx: &mut EvalCtxt<'_, 'tcx>,
 350         goal: Goal<'tcx, Self>,
 351     ) -> QueryResult<'tcx> {
 352         bug!("`Copy`/`Clone` does not have an associated type: {:?}", goal);
 353     }
 354
 355     fn consider_builtin_pointer_sized_candidate(
 356         _ecx: &mut EvalCtxt<'_, 'tcx>,
 357         goal: Goal<'tcx, Self>,
 358     ) -> QueryResult<'tcx> {
 359         bug!("`PointerSized` does not have an associated type: {:?}", goal);
 360     }
 361
 362     fn consider_builtin_fn_trait_candidates(
 363         ecx: &mut EvalCtxt<'_, 'tcx>,
 364         goal: Goal<'tcx, Self>,
 365         goal_kind: ty::ClosureKind,
 366     ) -> QueryResult<'tcx> {
 367         if let Some(tupled_inputs_and_output) =
 368             structural_traits::extract_tupled_inputs_and_output_from_callable(
 369                 ecx.tcx(),
 370                 goal.predicate.self_ty(),
 371                 goal_kind,
 372             )?
 373         {
 374             let pred = tupled_inputs_and_output
 375                 .map_bound(|(inputs, output)| ty::ProjectionPredicate {
 376                     projection_ty: ecx
 377                         .tcx()
 378                         .mk_alias_ty(goal.predicate.def_id(), [goal.predicate.self_ty(), inputs]),
 379                     term: output.into(),
 380                 })
 381                 .to_predicate(ecx.tcx());
 382             Self::consider_assumption(ecx, goal, pred)
 383         } else {
 384             ecx.make_canonical_response(Certainty::AMBIGUOUS)
 385         }
 386     }
 387
 388     fn consider_builtin_tuple_candidate(
 389         _ecx: &mut EvalCtxt<'_, 'tcx>,
 390         goal: Goal<'tcx, Self>,
 391     ) -> QueryResult<'tcx> {
 392         bug!("`Tuple` does not have an associated type: {:?}", goal);
 393     }
 394 }
 395
 396 /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
 397 ///
 398 /// FIXME: We should merge these 3 implementations as it's likely that they otherwise
 399 /// diverge.
 400 #[instrument(level = "debug", skip(infcx, param_env), ret)]
 401 fn fetch_eligible_assoc_item_def<'tcx>(
 402     infcx: &InferCtxt<'tcx>,
 403     param_env: ty::ParamEnv<'tcx>,
 404     goal_trait_ref: ty::TraitRef<'tcx>,
 405     trait_assoc_def_id: DefId,
 406     impl_def_id: DefId,
 407 ) -> Result<Option<LeafDef>, NoSolution> {
 408     let node_item = specialization_graph::assoc_def(infcx.tcx, impl_def_id, trait_assoc_def_id)
 409         .map_err(|ErrorGuaranteed { .. }| NoSolution)?;
 410
 411     let eligible = if node_item.is_final() {
 412         // Non-specializable items are always projectable.
 413         true
 414     } else {
 415         // Only reveal a specializable default if we're past type-checking
 416         // and the obligation is monomorphic, otherwise passes such as
 417         // transmute checking and polymorphic MIR optimizations could
 418         // get a result which isn't correct for all monomorphizations.
 419         if param_env.reveal() == Reveal::All {
 420             let poly_trait_ref = infcx.resolve_vars_if_possible(goal_trait_ref);
 421             !poly_trait_ref.still_further_specializable()
 422         } else {
 423             debug!(?node_item.item.def_id, "not eligible due to default");
 424             false
 425         }
 426     };
 427
 428     if eligible { Ok(Some(node_item)) } else { Ok(None) }
 429 }