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