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::{CanonicalResponse, 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_infer::traits::util::supertraits;
  12 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
  13 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
  14 use rustc_middle::ty::{TraitPredicate, TypeVisitable};
  15 use rustc_span::DUMMY_SP;
  16
  17 pub mod structural_traits;
  18
  19 impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
  20     fn self_ty(self) -> Ty<'tcx> {
  21         self.self_ty()
  22     }
  23
  24     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
  25         self.with_self_ty(tcx, self_ty)
  26     }
  27
  28     fn trait_def_id(self, _: TyCtxt<'tcx>) -> DefId {
  29         self.def_id()
  30     }
  31
  32     fn consider_impl_candidate(
  33         ecx: &mut EvalCtxt<'_, 'tcx>,
  34         goal: Goal<'tcx, TraitPredicate<'tcx>>,
  35         impl_def_id: DefId,
  36     ) -> QueryResult<'tcx> {
  37         let tcx = ecx.tcx();
  38
  39         let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
  40         let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
  41         if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
  42             .any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
  43         {
  44             return Err(NoSolution);
  45         }
  46
  47         ecx.infcx.probe(|_| {
  48             let impl_substs = ecx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
  49             let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
  50
  51             let mut nested_goals =
  52                 ecx.infcx.eq(goal.param_env, goal.predicate.trait_ref, impl_trait_ref)?;
  53             let where_clause_bounds = tcx
  54                 .predicates_of(impl_def_id)
  55                 .instantiate(tcx, impl_substs)
  56                 .predicates
  57                 .into_iter()
  58                 .map(|pred| goal.with(tcx, pred));
  59             nested_goals.extend(where_clause_bounds);
  60             ecx.evaluate_all_and_make_canonical_response(nested_goals)
  61         })
  62     }
  63
  64     fn consider_assumption(
  65         ecx: &mut EvalCtxt<'_, 'tcx>,
  66         goal: Goal<'tcx, Self>,
  67         assumption: ty::Predicate<'tcx>,
  68     ) -> QueryResult<'tcx> {
  69         if let Some(poly_trait_pred) = assumption.to_opt_poly_trait_pred()
  70             && poly_trait_pred.def_id() == goal.predicate.def_id()
  71         {
  72             // FIXME: Constness and polarity
  73             ecx.infcx.probe(|_| {
  74                 let assumption_trait_pred =
  75                     ecx.infcx.instantiate_binder_with_infer(poly_trait_pred);
  76                 let nested_goals = ecx.infcx.eq(
  77                     goal.param_env,
  78                     goal.predicate.trait_ref,
  79                     assumption_trait_pred.trait_ref,
  80                 )?;
  81                 ecx.evaluate_all_and_make_canonical_response(nested_goals)
  82             })
  83         } else {
  84             Err(NoSolution)
  85         }
  86     }
  87
  88     fn consider_auto_trait_candidate(
  89         ecx: &mut EvalCtxt<'_, 'tcx>,
  90         goal: Goal<'tcx, Self>,
  91     ) -> QueryResult<'tcx> {
  92         ecx.probe_and_evaluate_goal_for_constituent_tys(
  93             goal,
  94             structural_traits::instantiate_constituent_tys_for_auto_trait,
  95         )
  96     }
  97
  98     fn consider_trait_alias_candidate(
  99         ecx: &mut EvalCtxt<'_, 'tcx>,
 100         goal: Goal<'tcx, Self>,
 101     ) -> QueryResult<'tcx> {
 102         let tcx = ecx.tcx();
 103
 104         ecx.infcx.probe(|_| {
 105             let nested_obligations = tcx
 106                 .predicates_of(goal.predicate.def_id())
 107                 .instantiate(tcx, goal.predicate.trait_ref.substs);
 108             ecx.evaluate_all_and_make_canonical_response(
 109                 nested_obligations.predicates.into_iter().map(|p| goal.with(tcx, p)).collect(),
 110             )
 111         })
 112     }
 113
 114     fn consider_builtin_sized_candidate(
 115         ecx: &mut EvalCtxt<'_, 'tcx>,
 116         goal: Goal<'tcx, Self>,
 117     ) -> QueryResult<'tcx> {
 118         ecx.probe_and_evaluate_goal_for_constituent_tys(
 119             goal,
 120             structural_traits::instantiate_constituent_tys_for_sized_trait,
 121         )
 122     }
 123
 124     fn consider_builtin_copy_clone_candidate(
 125         ecx: &mut EvalCtxt<'_, 'tcx>,
 126         goal: Goal<'tcx, Self>,
 127     ) -> QueryResult<'tcx> {
 128         ecx.probe_and_evaluate_goal_for_constituent_tys(
 129             goal,
 130             structural_traits::instantiate_constituent_tys_for_copy_clone_trait,
 131         )
 132     }
 133
 134     fn consider_builtin_pointer_sized_candidate(
 135         ecx: &mut EvalCtxt<'_, 'tcx>,
 136         goal: Goal<'tcx, Self>,
 137     ) -> QueryResult<'tcx> {
 138         if goal.predicate.self_ty().has_non_region_infer() {
 139             return ecx.make_canonical_response(Certainty::AMBIGUOUS);
 140         }
 141
 142         let tcx = ecx.tcx();
 143         let self_ty = tcx.erase_regions(goal.predicate.self_ty());
 144
 145         if let Ok(layout) = tcx.layout_of(goal.param_env.and(self_ty))
 146             &&  let usize_layout = tcx.layout_of(ty::ParamEnv::empty().and(tcx.types.usize)).unwrap().layout
 147             && layout.layout.size() == usize_layout.size()
 148             && layout.layout.align().abi == usize_layout.align().abi
 149         {
 150             // FIXME: We could make this faster by making a no-constraints response
 151             ecx.make_canonical_response(Certainty::Yes)
 152         } else {
 153             Err(NoSolution)
 154         }
 155     }
 156
 157     fn consider_builtin_fn_trait_candidates(
 158         ecx: &mut EvalCtxt<'_, 'tcx>,
 159         goal: Goal<'tcx, Self>,
 160         goal_kind: ty::ClosureKind,
 161     ) -> QueryResult<'tcx> {
 162         if let Some(tupled_inputs_and_output) =
 163             structural_traits::extract_tupled_inputs_and_output_from_callable(
 164                 ecx.tcx(),
 165                 goal.predicate.self_ty(),
 166                 goal_kind,
 167             )?
 168         {
 169             let pred = tupled_inputs_and_output
 170                 .map_bound(|(inputs, _)| {
 171                     ecx.tcx()
 172                         .mk_trait_ref(goal.predicate.def_id(), [goal.predicate.self_ty(), inputs])
 173                 })
 174                 .to_predicate(ecx.tcx());
 175             Self::consider_assumption(ecx, goal, pred)
 176         } else {
 177             ecx.make_canonical_response(Certainty::AMBIGUOUS)
 178         }
 179     }
 180
 181     fn consider_builtin_tuple_candidate(
 182         ecx: &mut EvalCtxt<'_, 'tcx>,
 183         goal: Goal<'tcx, Self>,
 184     ) -> QueryResult<'tcx> {
 185         if let ty::Tuple(..) = goal.predicate.self_ty().kind() {
 186             ecx.make_canonical_response(Certainty::Yes)
 187         } else {
 188             Err(NoSolution)
 189         }
 190     }
 191
 192     fn consider_builtin_pointee_candidate(
 193         ecx: &mut EvalCtxt<'_, 'tcx>,
 194         _goal: Goal<'tcx, Self>,
 195     ) -> QueryResult<'tcx> {
 196         ecx.make_canonical_response(Certainty::Yes)
 197     }
 198
 199     fn consider_builtin_future_candidate(
 200         ecx: &mut EvalCtxt<'_, 'tcx>,
 201         goal: Goal<'tcx, Self>,
 202     ) -> QueryResult<'tcx> {
 203         let ty::Generator(def_id, _, _) = *goal.predicate.self_ty().kind() else {
 204             return Err(NoSolution);
 205         };
 206
 207         // Generators are not futures unless they come from `async` desugaring
 208         let tcx = ecx.tcx();
 209         if !tcx.generator_is_async(def_id) {
 210             return Err(NoSolution);
 211         }
 212
 213         // Async generator unconditionally implement `Future`
 214         ecx.make_canonical_response(Certainty::Yes)
 215     }
 216
 217     fn consider_builtin_generator_candidate(
 218         ecx: &mut EvalCtxt<'_, 'tcx>,
 219         goal: Goal<'tcx, Self>,
 220     ) -> QueryResult<'tcx> {
 221         let self_ty = goal.predicate.self_ty();
 222         let ty::Generator(def_id, substs, _) = *self_ty.kind() else {
 223             return Err(NoSolution);
 224         };
 225
 226         // `async`-desugared generators do not implement the generator trait
 227         let tcx = ecx.tcx();
 228         if tcx.generator_is_async(def_id) {
 229             return Err(NoSolution);
 230         }
 231
 232         let generator = substs.as_generator();
 233         Self::consider_assumption(
 234             ecx,
 235             goal,
 236             ty::Binder::dummy(
 237                 tcx.mk_trait_ref(goal.predicate.def_id(), [self_ty, generator.resume_ty()]),
 238             )
 239             .to_predicate(tcx),
 240         )
 241     }
 242
 243     fn consider_builtin_unsize_candidate(
 244         ecx: &mut EvalCtxt<'_, 'tcx>,
 245         goal: Goal<'tcx, Self>,
 246     ) -> QueryResult<'tcx> {
 247         let tcx = ecx.tcx();
 248         let a_ty = goal.predicate.self_ty();
 249         let b_ty = goal.predicate.trait_ref.substs.type_at(1);
 250         if b_ty.is_ty_var() {
 251             return ecx.make_canonical_response(Certainty::AMBIGUOUS);
 252         }
 253         ecx.infcx.probe(|_| {
 254             match (a_ty.kind(), b_ty.kind()) {
 255                 // Trait upcasting, or `dyn Trait + Auto + 'a` -> `dyn Trait + 'b`
 256                 (&ty::Dynamic(_, _, ty::Dyn), &ty::Dynamic(_, _, ty::Dyn)) => {
 257                     // Dyn upcasting is handled separately, since due to upcasting,
 258                     // when there are two supertraits that differ by substs, we
 259                     // may return more than one query response.
 260                     return Err(NoSolution);
 261                 }
 262                 // `T` -> `dyn Trait` unsizing
 263                 (_, &ty::Dynamic(data, region, ty::Dyn)) => {
 264                     // Can only unsize to an object-safe type
 265                     if data
 266                         .principal_def_id()
 267                         .map_or(false, |def_id| !tcx.check_is_object_safe(def_id))
 268                     {
 269                         return Err(NoSolution);
 270                     }
 271
 272                     let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
 273                         return Err(NoSolution);
 274                     };
 275                     let nested_goals: Vec<_> = data
 276                         .iter()
 277                         // Check that the type implements all of the predicates of the def-id.
 278                         // (i.e. the principal, all of the associated types match, and any auto traits)
 279                         .map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty)))
 280                         .chain([
 281                             // The type must be Sized to be unsized.
 282                             goal.with(
 283                                 tcx,
 284                                 ty::Binder::dummy(tcx.mk_trait_ref(sized_def_id, [a_ty])),
 285                             ),
 286                             // The type must outlive the lifetime of the `dyn` we're unsizing into.
 287                             goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_ty, region))),
 288                         ])
 289                         .collect();
 290
 291                     ecx.evaluate_all_and_make_canonical_response(nested_goals)
 292                 }
 293                 // `[T; n]` -> `[T]` unsizing
 294                 (&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => {
 295                     // We just require that the element type stays the same
 296                     let nested_goals = ecx.infcx.eq(goal.param_env, a_elem_ty, b_elem_ty)?;
 297                     ecx.evaluate_all_and_make_canonical_response(nested_goals)
 298                 }
 299                 // Struct unsizing `Struct<T>` -> `Struct<U>` where `T: Unsize<U>`
 300                 (&ty::Adt(a_def, a_substs), &ty::Adt(b_def, b_substs))
 301                     if a_def.is_struct() && a_def.did() == b_def.did() =>
 302                 {
 303                     let unsizing_params = tcx.unsizing_params_for_adt(a_def.did());
 304                     // We must be unsizing some type parameters. This also implies
 305                     // that the struct has a tail field.
 306                     if unsizing_params.is_empty() {
 307                         return Err(NoSolution);
 308                     }
 309
 310                     let tail_field = a_def
 311                         .non_enum_variant()
 312                         .fields
 313                         .last()
 314                         .expect("expected unsized ADT to have a tail field");
 315                     let tail_field_ty = tcx.bound_type_of(tail_field.did);
 316
 317                     let a_tail_ty = tail_field_ty.subst(tcx, a_substs);
 318                     let b_tail_ty = tail_field_ty.subst(tcx, b_substs);
 319
 320                     // Substitute just the unsizing params from B into A. The type after
 321                     // this substitution must be equal to B. This is so we don't unsize
 322                     // unrelated type parameters.
 323                     let new_a_substs = tcx.mk_substs(a_substs.iter().enumerate().map(|(i, a)| {
 324                         if unsizing_params.contains(i as u32) { b_substs[i] } else { a }
 325                     }));
 326                     let unsized_a_ty = tcx.mk_adt(a_def, new_a_substs);
 327
 328                     // Finally, we require that `TailA: Unsize<TailB>` for the tail field
 329                     // types.
 330                     let mut nested_goals = ecx.infcx.eq(goal.param_env, unsized_a_ty, b_ty)?;
 331                     nested_goals.push(goal.with(
 332                         tcx,
 333                         ty::Binder::dummy(
 334                             tcx.mk_trait_ref(goal.predicate.def_id(), [a_tail_ty, b_tail_ty]),
 335                         ),
 336                     ));
 337
 338                     ecx.evaluate_all_and_make_canonical_response(nested_goals)
 339                 }
 340                 // Tuple unsizing `(.., T)` -> `(.., U)` where `T: Unsize<U>`
 341                 (&ty::Tuple(a_tys), &ty::Tuple(b_tys))
 342                     if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
 343                 {
 344                     let (a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
 345                     let b_last_ty = b_tys.last().unwrap();
 346
 347                     // Substitute just the tail field of B., and require that they're equal.
 348                     let unsized_a_ty = tcx.mk_tup(a_rest_tys.iter().chain([b_last_ty]));
 349                     let mut nested_goals = ecx.infcx.eq(goal.param_env, unsized_a_ty, b_ty)?;
 350
 351                     // Similar to ADTs, require that the rest of the fields are equal.
 352                     nested_goals.push(goal.with(
 353                         tcx,
 354                         ty::Binder::dummy(
 355                             tcx.mk_trait_ref(goal.predicate.def_id(), [*a_last_ty, *b_last_ty]),
 356                         ),
 357                     ));
 358
 359                     ecx.evaluate_all_and_make_canonical_response(nested_goals)
 360                 }
 361                 _ => Err(NoSolution),
 362             }
 363         })
 364     }
 365
 366     fn consider_builtin_dyn_upcast_candidates(
 367         ecx: &mut EvalCtxt<'_, 'tcx>,
 368         goal: Goal<'tcx, Self>,
 369     ) -> Vec<CanonicalResponse<'tcx>> {
 370         let tcx = ecx.tcx();
 371
 372         let a_ty = goal.predicate.self_ty();
 373         let b_ty = goal.predicate.trait_ref.substs.type_at(1);
 374         let ty::Dynamic(a_data, a_region, ty::Dyn) = *a_ty.kind() else {
 375             return vec![];
 376         };
 377         let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else {
 378             return vec![];
 379         };
 380
 381         // All of a's auto traits need to be in b's auto traits.
 382         let auto_traits_compatible =
 383             b_data.auto_traits().all(|b| a_data.auto_traits().any(|a| a == b));
 384         if !auto_traits_compatible {
 385             return vec![];
 386         }
 387
 388         let mut unsize_dyn_to_principal = |principal: Option<ty::PolyExistentialTraitRef<'tcx>>| {
 389             ecx.infcx.probe(|_| -> Result<_, NoSolution> {
 390                 // Require that all of the trait predicates from A match B, except for
 391                 // the auto traits. We do this by constructing a new A type with B's
 392                 // auto traits, and equating these types.
 393                 let new_a_data = principal
 394                     .into_iter()
 395                     .map(|trait_ref| trait_ref.map_bound(ty::ExistentialPredicate::Trait))
 396                     .chain(a_data.iter().filter(|a| {
 397                         matches!(a.skip_binder(), ty::ExistentialPredicate::Projection(_))
 398                     }))
 399                     .chain(
 400                         b_data
 401                             .auto_traits()
 402                             .map(ty::ExistentialPredicate::AutoTrait)
 403                             .map(ty::Binder::dummy),
 404                     );
 405                 let new_a_data = tcx.mk_poly_existential_predicates(new_a_data);
 406                 let new_a_ty = tcx.mk_dynamic(new_a_data, b_region, ty::Dyn);
 407
 408                 // We also require that A's lifetime outlives B's lifetime.
 409                 let mut nested_obligations = ecx.infcx.eq(goal.param_env, new_a_ty, b_ty)?;
 410                 nested_obligations.push(
 411                     goal.with(tcx, ty::Binder::dummy(ty::OutlivesPredicate(a_region, b_region))),
 412                 );
 413
 414                 ecx.evaluate_all_and_make_canonical_response(nested_obligations)
 415             })
 416         };
 417
 418         let mut responses = vec![];
 419         // If the principal def ids match (or are both none), then we're not doing
 420         // trait upcasting. We're just removing auto traits (or shortening the lifetime).
 421         if a_data.principal_def_id() == b_data.principal_def_id() {
 422             if let Ok(response) = unsize_dyn_to_principal(a_data.principal()) {
 423                 responses.push(response);
 424             }
 425         } else if let Some(a_principal) = a_data.principal()
 426             && let Some(b_principal) = b_data.principal()
 427         {
 428             for super_trait_ref in supertraits(tcx, a_principal.with_self_ty(tcx, a_ty)) {
 429                 if super_trait_ref.def_id() != b_principal.def_id() {
 430                     continue;
 431                 }
 432                 let erased_trait_ref = super_trait_ref
 433                     .map_bound(|trait_ref| ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref));
 434                 if let Ok(response) = unsize_dyn_to_principal(Some(erased_trait_ref)) {
 435                     responses.push(response);
 436                 }
 437             }
 438         }
 439
 440         responses
 441     }
 442
 443     fn consider_builtin_discriminant_kind_candidate(
 444         ecx: &mut EvalCtxt<'_, 'tcx>,
 445         _goal: Goal<'tcx, Self>,
 446     ) -> QueryResult<'tcx> {
 447         // `DiscriminantKind` is automatically implemented for every type.
 448         ecx.make_canonical_response(Certainty::Yes)
 449     }
 450 }
 451
 452 impl<'tcx> EvalCtxt<'_, 'tcx> {
 453     /// Convenience function for traits that are structural, i.e. that only
 454     /// have nested subgoals that only change the self type. Unlike other
 455     /// evaluate-like helpers, this does a probe, so it doesn't need to be
 456     /// wrapped in one.
 457     fn probe_and_evaluate_goal_for_constituent_tys(
 458         &mut self,
 459         goal: Goal<'tcx, TraitPredicate<'tcx>>,
 460         constituent_tys: impl Fn(&InferCtxt<'tcx>, Ty<'tcx>) -> Result<Vec<Ty<'tcx>>, NoSolution>,
 461     ) -> QueryResult<'tcx> {
 462         self.infcx.probe(|_| {
 463             self.evaluate_all_and_make_canonical_response(
 464                 constituent_tys(self.infcx, goal.predicate.self_ty())?
 465                     .into_iter()
 466                     .map(|ty| {
 467                         goal.with(
 468                             self.tcx(),
 469                             ty::Binder::dummy(goal.predicate.with_self_ty(self.tcx(), ty)),
 470                         )
 471                     })
 472                     .collect(),
 473             )
 474         })
 475     }
 476
 477     pub(super) fn compute_trait_goal(
 478         &mut self,
 479         goal: Goal<'tcx, TraitPredicate<'tcx>>,
 480     ) -> QueryResult<'tcx> {
 481         let candidates = self.assemble_and_evaluate_candidates(goal);
 482         self.merge_trait_candidates_discard_reservation_impls(candidates)
 483     }
 484
 485     #[instrument(level = "debug", skip(self), ret)]
 486     pub(super) fn merge_trait_candidates_discard_reservation_impls(
 487         &mut self,
 488         mut candidates: Vec<Candidate<'tcx>>,
 489     ) -> QueryResult<'tcx> {
 490         match candidates.len() {
 491             0 => return Err(NoSolution),
 492             1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
 493             _ => {}
 494         }
 495
 496         if candidates.len() > 1 {
 497             let mut i = 0;
 498             'outer: while i < candidates.len() {
 499                 for j in (0..candidates.len()).filter(|&j| i != j) {
 500                     if self.trait_candidate_should_be_dropped_in_favor_of(
 501                         &candidates[i],
 502                         &candidates[j],
 503                     ) {
 504                         debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
 505                         candidates.swap_remove(i);
 506                         continue 'outer;
 507                     }
 508                 }
 509
 510                 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
 511                 // If there are *STILL* multiple candidates, give up
 512                 // and report ambiguity.
 513                 i += 1;
 514                 if i > 1 {
 515                     debug!("multiple matches, ambig");
 516                     // FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
 517                     unimplemented!();
 518                 }
 519             }
 520         }
 521
 522         Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
 523     }
 524
 525     fn trait_candidate_should_be_dropped_in_favor_of(
 526         &self,
 527         candidate: &Candidate<'tcx>,
 528         other: &Candidate<'tcx>,
 529     ) -> bool {
 530         // FIXME: implement this
 531         match (candidate.source, other.source) {
 532             (CandidateSource::Impl(_), _)
 533             | (CandidateSource::ParamEnv(_), _)
 534             | (CandidateSource::AliasBound, _)
 535             | (CandidateSource::BuiltinImpl, _) => unimplemented!(),
 536         }
 537     }
 538
 539     fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
 540         if let CandidateSource::Impl(def_id) = candidate.source {
 541             if let ty::ImplPolarity::Reservation = self.tcx().impl_polarity(def_id) {
 542                 debug!("Selected reservation impl");
 543                 // FIXME: reduce candidate to ambiguous
 544                 // FIXME: replace `var_values` with identity, yeet external constraints.
 545                 unimplemented!()
 546             }
 547         }
 548
 549         candidate
 550     }
 551 }