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_hir::LangItem;
  11 use rustc_infer::infer::InferCtxt;
  12 use rustc_infer::traits::query::NoSolution;
  13 use rustc_infer::traits::specialization_graph::LeafDef;
  14 use rustc_infer::traits::Reveal;
  15 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  16 use rustc_middle::ty::{self, Ty, TyCtxt};
  17 use rustc_middle::ty::{ProjectionPredicate, TypeSuperVisitable, TypeVisitor};
  18 use rustc_middle::ty::{ToPredicate, TypeVisitable};
  19 use rustc_span::{sym, DUMMY_SP};
  20 use std::iter;
  21 use std::ops::ControlFlow;
  22
  23 impl<'tcx> EvalCtxt<'_, 'tcx> {
  24     pub(super) fn compute_projection_goal(
  25         &mut self,
  26         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
  27     ) -> QueryResult<'tcx> {
  28         // To only compute normalization once for each projection we only
  29         // normalize if the expected term is an unconstrained inference variable.
  30         //
  31         // E.g. for `<T as Trait>::Assoc == u32` we recursively compute the goal
  32         // `exists<U> <T as Trait>::Assoc == U` and then take the resulting type for
  33         // `U` and equate it with `u32`. This means that we don't need a separate
  34         // projection cache in the solver.
  35         if self.term_is_fully_unconstrained(goal) {
  36             let candidates = self.assemble_and_evaluate_candidates(goal);
  37             self.merge_project_candidates(candidates)
  38         } else {
  39             let predicate = goal.predicate;
  40             let unconstrained_rhs = match predicate.term.unpack() {
  41                 ty::TermKind::Ty(_) => self.infcx.next_ty_infer().into(),
  42                 ty::TermKind::Const(ct) => self.infcx.next_const_infer(ct.ty()).into(),
  43             };
  44             let unconstrained_predicate = ty::Clause::Projection(ProjectionPredicate {
  45                 projection_ty: goal.predicate.projection_ty,
  46                 term: unconstrained_rhs,
  47             });
  48             let (_has_changed, normalize_certainty) = self.in_projection_eq_hack(|this| {
  49                 this.evaluate_goal(goal.with(this.tcx(), unconstrained_predicate))
  50             })?;
  51
  52             let nested_eq_goals =
  53                 self.infcx.eq(goal.param_env, unconstrained_rhs, predicate.term)?;
  54             let eval_certainty = self.evaluate_all(nested_eq_goals)?;
  55             self.make_canonical_response(normalize_certainty.unify_and(eval_certainty))
  56         }
  57     }
  58
  59     /// This sets a flag used by a debug assert in [`EvalCtxt::evaluate_goal`],
  60     /// see the comment in that method for more details.
  61     fn in_projection_eq_hack<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
  62         self.in_projection_eq_hack = true;
  63         let result = f(self);
  64         self.in_projection_eq_hack = false;
  65         result
  66     }
  67
  68     /// Is the projection predicate is of the form `exists<T> <Ty as Trait>::Assoc = T`.
  69     ///
  70     /// This is the case if the `term` is an inference variable in the innermost universe
  71     /// and does not occur in any other part of the predicate.
  72     fn term_is_fully_unconstrained(&self, goal: Goal<'tcx, ProjectionPredicate<'tcx>>) -> bool {
  73         let infcx = self.infcx;
  74         let term_is_infer = match goal.predicate.term.unpack() {
  75             ty::TermKind::Ty(ty) => {
  76                 if let &ty::Infer(ty::TyVar(vid)) = ty.kind() {
  77                     match infcx.probe_ty_var(vid) {
  78                         Ok(value) => bug!("resolved var in query: {goal:?} {value:?}"),
  79                         Err(universe) => universe == infcx.universe(),
  80                     }
  81                 } else {
  82                     false
  83                 }
  84             }
  85             ty::TermKind::Const(ct) => {
  86                 if let ty::ConstKind::Infer(ty::InferConst::Var(vid)) = ct.kind() {
  87                     match self.infcx.probe_const_var(vid) {
  88                         Ok(value) => bug!("resolved var in query: {goal:?} {value:?}"),
  89                         Err(universe) => universe == infcx.universe(),
  90                     }
  91                 } else {
  92                     false
  93                 }
  94             }
  95         };
  96
  97         // Guard against `<T as Trait<?0>>::Assoc = ?0>`.
  98         struct ContainsTerm<'tcx> {
  99             term: ty::Term<'tcx>,
 100         }
 101         impl<'tcx> TypeVisitor<'tcx> for ContainsTerm<'tcx> {
 102             type BreakTy = ();
 103             fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 104                 if t.needs_infer() {
 105                     if ty::Term::from(t) == self.term {
 106                         ControlFlow::Break(())
 107                     } else {
 108                         t.super_visit_with(self)
 109                     }
 110                 } else {
 111                     ControlFlow::Continue(())
 112                 }
 113             }
 114
 115             fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
 116                 if c.needs_infer() {
 117                     if ty::Term::from(c) == self.term {
 118                         ControlFlow::Break(())
 119                     } else {
 120                         c.super_visit_with(self)
 121                     }
 122                 } else {
 123                     ControlFlow::Continue(())
 124                 }
 125             }
 126         }
 127
 128         let mut visitor = ContainsTerm { term: goal.predicate.term };
 129
 130         term_is_infer
 131             && goal.predicate.projection_ty.visit_with(&mut visitor).is_continue()
 132             && goal.param_env.visit_with(&mut visitor).is_continue()
 133     }
 134
 135     /// After normalizing the projection to `normalized_alias` with the given
 136     /// `normalization_certainty`, constrain the inference variable `term` to it
 137     /// and return a query response.
 138     fn eq_term_and_make_canonical_response(
 139         &mut self,
 140         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
 141         normalization_certainty: Certainty,
 142         normalized_alias: impl Into<ty::Term<'tcx>>,
 143     ) -> QueryResult<'tcx> {
 144         // The term of our goal should be fully unconstrained, so this should never fail.
 145         //
 146         // It can however be ambiguous when the `normalized_alias` contains a projection.
 147         let nested_goals = self
 148             .infcx
 149             .eq(goal.param_env, goal.predicate.term, normalized_alias.into())
 150             .expect("failed to unify with unconstrained term");
 151         let rhs_certainty =
 152             self.evaluate_all(nested_goals).expect("failed to unify with unconstrained term");
 153
 154         self.make_canonical_response(normalization_certainty.unify_and(rhs_certainty))
 155     }
 156
 157     fn merge_project_candidates(
 158         &mut self,
 159         mut candidates: Vec<Candidate<'tcx>>,
 160     ) -> QueryResult<'tcx> {
 161         match candidates.len() {
 162             0 => return Err(NoSolution),
 163             1 => return Ok(candidates.pop().unwrap().result),
 164             _ => {}
 165         }
 166
 167         if candidates.len() > 1 {
 168             let mut i = 0;
 169             'outer: while i < candidates.len() {
 170                 for j in (0..candidates.len()).filter(|&j| i != j) {
 171                     if self.project_candidate_should_be_dropped_in_favor_of(
 172                         &candidates[i],
 173                         &candidates[j],
 174                     ) {
 175                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 176                         candidates.swap_remove(i);
 177                         continue 'outer;
 178                     }
 179                 }
 180
 181                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 182                 // If there are *STILL* multiple candidates, give up
 183                 // and report ambiguity.
 184                 i += 1;
 185                 if i > 1 {
 186                     debug!("multiple matches, ambig");
 187                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
 188                     unimplemented!();
 189                 }
 190             }
 191         }
 192
 193         Ok(candidates.pop().unwrap().result)
 194     }
 195
 196     fn project_candidate_should_be_dropped_in_favor_of(
 197         &self,
 198         candidate: &Candidate<'tcx>,
 199         other: &Candidate<'tcx>,
 200     ) -> bool {
 201         // FIXME: implement this
 202         match (candidate.source, other.source) {
 203             (CandidateSource::Impl(_), _)
 204             | (CandidateSource::ParamEnv(_), _)
 205             | (CandidateSource::BuiltinImpl, _)
 206             | (CandidateSource::AliasBound, _) => unimplemented!(),
 207         }
 208     }
 209 }
 210
 211 impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
 212     fn self_ty(self) -> Ty<'tcx> {
 213         self.self_ty()
 214     }
 215
 216     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
 217         self.with_self_ty(tcx, self_ty)
 218     }
 219
 220     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
 221         self.trait_def_id(tcx)
 222     }
 223
 224     fn consider_impl_candidate(
 225         ecx: &mut EvalCtxt<'_, 'tcx>,
 226         goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
 227         impl_def_id: DefId,
 228     ) -> QueryResult<'tcx> {
 229         let tcx = ecx.tcx();
 230
 231         let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
 232         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
 233         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
 234         if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
 235             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
 236         {
 237             return Err(NoSolution);
 238         }
 239
 240         ecx.infcx.probe(|_| {
 241             let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
 242             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
 243
 244             let mut nested_goals = ecx.infcx.eq(goal.param_env, goal_trait_ref, impl_trait_ref)?;
 245             let where_clause_bounds = tcx
 246                 .predicates_of(impl_def_id)
 247                 .instantiate(tcx, impl_substs)
 248                 .predicates
 249                 .into_iter()
 250                 .map(|pred| goal.with(tcx, pred));
 251
 252             nested_goals.extend(where_clause_bounds);
 253             let match_impl_certainty = ecx.evaluate_all(nested_goals)?;
 254
 255             // In case the associated item is hidden due to specialization, we have to
 256             // return ambiguity this would otherwise be incomplete, resulting in
 257             // unsoundness during coherence (#105782).
 258             let Some(assoc_def) = fetch_eligible_assoc_item_def(
 259                 ecx.infcx,
 260                 goal.param_env,
 261                 goal_trait_ref,
 262                 goal.predicate.def_id(),
 263                 impl_def_id
 264             )? else {
 265                 return ecx.make_canonical_response(match_impl_certainty.unify_and(Certainty::AMBIGUOUS));
 266             };
 267
 268             if !assoc_def.item.defaultness(tcx).has_value() {
 269                 tcx.sess.delay_span_bug(
 270                     tcx.def_span(assoc_def.item.def_id),
 271                     "missing value for assoc item in impl",
 272                 );
 273             }
 274
 275             // Getting the right substitutions here is complex, e.g. given:
 276             // - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
 277             // - the applicable impl `impl<T> Trait<i32> for Vec<T>`
 278             // - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
 279             //
 280             // We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
 281             // to `[u32, u64]`.
 282             //
 283             // And then map these substs to the substs of the defining impl of `Assoc`, going
 284             // from `[u32, u64]` to `[u32, i32, u64]`.
 285             let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
 286                 tcx,
 287                 goal_trait_ref.def_id,
 288                 impl_substs,
 289             );
 290             let substs = translate_substs(
 291                 ecx.infcx,
 292                 goal.param_env,
 293                 impl_def_id,
 294                 impl_substs_with_gat,
 295                 assoc_def.defining_node,
 296             );
 297
 298             // Finally we construct the actual value of the associated type.
 299             let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
 300             let ty = tcx.bound_type_of(assoc_def.item.def_id);
 301             let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
 302                 let identity_substs =
 303                     ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
 304                 let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
 305                 let kind =
 306                     ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
 307                 ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
 308             } else {
 309                 ty.map_bound(|ty| ty.into())
 310             };
 311
 312             ecx.eq_term_and_make_canonical_response(goal, match_impl_certainty, term.subst(tcx, substs))
 313         })
 314     }
 315
 316     fn consider_assumption(
 317         ecx: &mut EvalCtxt<'_, 'tcx>,
 318         goal: Goal<'tcx, Self>,
 319         assumption: ty::Predicate<'tcx>,
 320     ) -> QueryResult<'tcx> {
 321         if let Some(poly_projection_pred) = assumption.to_opt_poly_projection_pred()
 322             && poly_projection_pred.projection_def_id() == goal.predicate.def_id()
 323         {
 324             ecx.infcx.probe(|_| {
 325                 let assumption_projection_pred =
 326                     ecx.infcx.instantiate_binder_with_infer(poly_projection_pred);
 327                 let nested_goals = ecx.infcx.eq(
 328                     goal.param_env,
 329                     goal.predicate.projection_ty,
 330                     assumption_projection_pred.projection_ty,
 331                 )?;
 332                 let subst_certainty = ecx.evaluate_all(nested_goals)?;
 333
 334                 ecx.eq_term_and_make_canonical_response(
 335                     goal,
 336                     subst_certainty,
 337                     assumption_projection_pred.term,
 338                 )
 339             })
 340         } else {
 341             Err(NoSolution)
 342         }
 343     }
 344
 345     fn consider_auto_trait_candidate(
 346         _ecx: &mut EvalCtxt<'_, 'tcx>,
 347         goal: Goal<'tcx, Self>,
 348     ) -> QueryResult<'tcx> {
 349         bug!("auto traits do not have associated types: {:?}", goal);
 350     }
 351
 352     fn consider_trait_alias_candidate(
 353         _ecx: &mut EvalCtxt<'_, 'tcx>,
 354         goal: Goal<'tcx, Self>,
 355     ) -> QueryResult<'tcx> {
 356         bug!("trait aliases do not have associated types: {:?}", goal);
 357     }
 358
 359     fn consider_builtin_sized_candidate(
 360         _ecx: &mut EvalCtxt<'_, 'tcx>,
 361         goal: Goal<'tcx, Self>,
 362     ) -> QueryResult<'tcx> {
 363         bug!("`Sized` does not have an associated type: {:?}", goal);
 364     }
 365
 366     fn consider_builtin_copy_clone_candidate(
 367         _ecx: &mut EvalCtxt<'_, 'tcx>,
 368         goal: Goal<'tcx, Self>,
 369     ) -> QueryResult<'tcx> {
 370         bug!("`Copy`/`Clone` does not have an associated type: {:?}", goal);
 371     }
 372
 373     fn consider_builtin_pointer_like_candidate(
 374         _ecx: &mut EvalCtxt<'_, 'tcx>,
 375         goal: Goal<'tcx, Self>,
 376     ) -> QueryResult<'tcx> {
 377         bug!("`PointerLike` does not have an associated type: {:?}", goal);
 378     }
 379
 380     fn consider_builtin_fn_trait_candidates(
 381         ecx: &mut EvalCtxt<'_, 'tcx>,
 382         goal: Goal<'tcx, Self>,
 383         goal_kind: ty::ClosureKind,
 384     ) -> QueryResult<'tcx> {
 385         if let Some(tupled_inputs_and_output) =
 386             structural_traits::extract_tupled_inputs_and_output_from_callable(
 387                 ecx.tcx(),
 388                 goal.predicate.self_ty(),
 389                 goal_kind,
 390             )?
 391         {
 392             let pred = tupled_inputs_and_output
 393                 .map_bound(|(inputs, output)| ty::ProjectionPredicate {
 394                     projection_ty: ecx
 395                         .tcx()
 396                         .mk_alias_ty(goal.predicate.def_id(), [goal.predicate.self_ty(), inputs]),
 397                     term: output.into(),
 398                 })
 399                 .to_predicate(ecx.tcx());
 400             Self::consider_assumption(ecx, goal, pred)
 401         } else {
 402             ecx.make_canonical_response(Certainty::AMBIGUOUS)
 403         }
 404     }
 405
 406     fn consider_builtin_tuple_candidate(
 407         _ecx: &mut EvalCtxt<'_, 'tcx>,
 408         goal: Goal<'tcx, Self>,
 409     ) -> QueryResult<'tcx> {
 410         bug!("`Tuple` does not have an associated type: {:?}", goal);
 411     }
 412
 413     fn consider_builtin_pointee_candidate(
 414         ecx: &mut EvalCtxt<'_, 'tcx>,
 415         goal: Goal<'tcx, Self>,
 416     ) -> QueryResult<'tcx> {
 417         let tcx = ecx.tcx();
 418         ecx.infcx.probe(|_| {
 419             let metadata_ty = match goal.predicate.self_ty().kind() {
 420                 ty::Bool
 421                 | ty::Char
 422                 | ty::Int(..)
 423                 | ty::Uint(..)
 424                 | ty::Float(..)
 425                 | ty::Array(..)
 426                 | ty::RawPtr(..)
 427                 | ty::Ref(..)
 428                 | ty::FnDef(..)
 429                 | ty::FnPtr(..)
 430                 | ty::Closure(..)
 431                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..))
 432                 | ty::Generator(..)
 433                 | ty::GeneratorWitness(..)
 434                 | ty::GeneratorWitnessMIR(..)
 435                 | ty::Never
 436                 | ty::Foreign(..) => tcx.types.unit,
 437
 438                 ty::Error(e) => tcx.ty_error_with_guaranteed(*e),
 439
 440                 ty::Str | ty::Slice(_) => tcx.types.usize,
 441
 442                 ty::Dynamic(_, _, _) => {
 443                     let dyn_metadata = tcx.require_lang_item(LangItem::DynMetadata, None);
 444                     tcx.bound_type_of(dyn_metadata)
 445                         .subst(tcx, &[ty::GenericArg::from(goal.predicate.self_ty())])
 446                 }
 447
 448                 ty::Alias(_, _) | ty::Param(_) | ty::Placeholder(..) => {
 449                     // FIXME(ptr_metadata): It would also be possible to return a `Ok(Ambig)` with no constraints.
 450                     let sized_predicate = ty::Binder::dummy(tcx.at(DUMMY_SP).mk_trait_ref(
 451                         LangItem::Sized,
 452                         [ty::GenericArg::from(goal.predicate.self_ty())],
 453                     ));
 454
 455                     let is_sized_certainty = ecx.evaluate_goal(goal.with(tcx, sized_predicate))?.1;
 456                     return ecx.eq_term_and_make_canonical_response(
 457                         goal,
 458                         is_sized_certainty,
 459                         tcx.types.unit,
 460                     );
 461                 }
 462
 463                 ty::Adt(def, substs) if def.is_struct() => {
 464                     match def.non_enum_variant().fields.last() {
 465                         None => tcx.types.unit,
 466                         Some(field_def) => {
 467                             let self_ty = field_def.ty(tcx, substs);
 468                             let new_goal = goal.with(
 469                                 tcx,
 470                                 ty::Binder::dummy(goal.predicate.with_self_ty(tcx, self_ty)),
 471                             );
 472                             let (_, certainty) = ecx.evaluate_goal(new_goal)?;
 473                             return ecx.make_canonical_response(certainty);
 474                         }
 475                     }
 476                 }
 477                 ty::Adt(_, _) => tcx.types.unit,
 478
 479                 ty::Tuple(elements) => match elements.last() {
 480                     None => tcx.types.unit,
 481                     Some(&self_ty) => {
 482                         let new_goal = goal.with(
 483                             tcx,
 484                             ty::Binder::dummy(goal.predicate.with_self_ty(tcx, self_ty)),
 485                         );
 486                         let (_, certainty) = ecx.evaluate_goal(new_goal)?;
 487                         return ecx.make_canonical_response(certainty);
 488                     }
 489                 },
 490
 491                 ty::Infer(
 492                     ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_),
 493                 )
 494                 | ty::Bound(..) => bug!(
 495                     "unexpected self ty `{:?}` when normalizing `<T as Pointee>::Metadata`",
 496                     goal.predicate.self_ty()
 497                 ),
 498             };
 499
 500             ecx.eq_term_and_make_canonical_response(goal, Certainty::Yes, metadata_ty)
 501         })
 502     }
 503
 504     fn consider_builtin_future_candidate(
 505         ecx: &mut EvalCtxt<'_, 'tcx>,
 506         goal: Goal<'tcx, Self>,
 507     ) -> QueryResult<'tcx> {
 508         let self_ty = goal.predicate.self_ty();
 509         let ty::Generator(def_id, substs, _) = *self_ty.kind() else {
 510             return Err(NoSolution);
 511         };
 512
 513         // Generators are not futures unless they come from `async` desugaring
 514         let tcx = ecx.tcx();
 515         if !tcx.generator_is_async(def_id) {
 516             return Err(NoSolution);
 517         }
 518
 519         let term = substs.as_generator().return_ty().into();
 520
 521         Self::consider_assumption(
 522             ecx,
 523             goal,
 524             ty::Binder::dummy(ty::ProjectionPredicate {
 525                 projection_ty: ecx.tcx().mk_alias_ty(goal.predicate.def_id(), [self_ty]),
 526                 term,
 527             })
 528             .to_predicate(tcx),
 529         )
 530     }
 531
 532     fn consider_builtin_generator_candidate(
 533         ecx: &mut EvalCtxt<'_, 'tcx>,
 534         goal: Goal<'tcx, Self>,
 535     ) -> QueryResult<'tcx> {
 536         let self_ty = goal.predicate.self_ty();
 537         let ty::Generator(def_id, substs, _) = *self_ty.kind() else {
 538             return Err(NoSolution);
 539         };
 540
 541         // `async`-desugared generators do not implement the generator trait
 542         let tcx = ecx.tcx();
 543         if tcx.generator_is_async(def_id) {
 544             return Err(NoSolution);
 545         }
 546
 547         let generator = substs.as_generator();
 548
 549         let name = tcx.associated_item(goal.predicate.def_id()).name;
 550         let term = if name == sym::Return {
 551             generator.return_ty().into()
 552         } else if name == sym::Yield {
 553             generator.yield_ty().into()
 554         } else {
 555             bug!("unexpected associated item `<{self_ty} as Generator>::{name}`")
 556         };
 557
 558         Self::consider_assumption(
 559             ecx,
 560             goal,
 561             ty::Binder::dummy(ty::ProjectionPredicate {
 562                 projection_ty: ecx
 563                     .tcx()
 564                     .mk_alias_ty(goal.predicate.def_id(), [self_ty, generator.resume_ty()]),
 565                 term,
 566             })
 567             .to_predicate(tcx),
 568         )
 569     }
 570
 571     fn consider_builtin_unsize_candidate(
 572         _ecx: &mut EvalCtxt<'_, 'tcx>,
 573         goal: Goal<'tcx, Self>,
 574     ) -> QueryResult<'tcx> {
 575         bug!("`Unsize` does not have an associated type: {:?}", goal);
 576     }
 577
 578     fn consider_builtin_dyn_upcast_candidates(
 579         _ecx: &mut EvalCtxt<'_, 'tcx>,
 580         goal: Goal<'tcx, Self>,
 581     ) -> Vec<super::CanonicalResponse<'tcx>> {
 582         bug!("`Unsize` does not have an associated type: {:?}", goal);
 583     }
 584
 585     fn consider_builtin_discriminant_kind_candidate(
 586         ecx: &mut EvalCtxt<'_, 'tcx>,
 587         goal: Goal<'tcx, Self>,
 588     ) -> QueryResult<'tcx> {
 589         let discriminant = goal.predicate.self_ty().discriminant_ty(ecx.tcx());
 590         ecx.infcx
 591             .probe(|_| ecx.eq_term_and_make_canonical_response(goal, Certainty::Yes, discriminant))
 592     }
 593 }
 594
 595 /// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
 596 ///
 597 /// FIXME: We should merge these 3 implementations as it's likely that they otherwise
 598 /// diverge.
 599 #[instrument(level = "debug", skip(infcx, param_env), ret)]
 600 fn fetch_eligible_assoc_item_def<'tcx>(
 601     infcx: &InferCtxt<'tcx>,
 602     param_env: ty::ParamEnv<'tcx>,
 603     goal_trait_ref: ty::TraitRef<'tcx>,
 604     trait_assoc_def_id: DefId,
 605     impl_def_id: DefId,
 606 ) -> Result<Option<LeafDef>, NoSolution> {
 607     let node_item = specialization_graph::assoc_def(infcx.tcx, impl_def_id, trait_assoc_def_id)
 608         .map_err(|ErrorGuaranteed { .. }| NoSolution)?;
 609
 610     let eligible = if node_item.is_final() {
 611         // Non-specializable items are always projectable.
 612         true
 613     } else {
 614         // Only reveal a specializable default if we're past type-checking
 615         // and the obligation is monomorphic, otherwise passes such as
 616         // transmute checking and polymorphic MIR optimizations could
 617         // get a result which isn't correct for all monomorphizations.
 618         if param_env.reveal() == Reveal::All {
 619             let poly_trait_ref = infcx.resolve_vars_if_possible(goal_trait_ref);
 620             !poly_trait_ref.still_further_specializable()
 621         } else {
 622             debug!(?node_item.item.def_id, "not eligible due to default");
 623             false
 624         }
 625     };
 626
 627     if eligible { Ok(Some(node_item)) } else { Ok(None) }
 628 }