compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

   1 //! Candidate assembly.
   2 //!
   3 //! The selection process begins by examining all in-scope impls,
   4 //! caller obligations, and so forth and assembling a list of
   5 //! candidates. See the [rustc dev guide] for more details.
   6 //!
   7 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
   8 use hir::LangItem;
   9 use rustc_hir as hir;
  10 use rustc_infer::traits::ObligationCause;
  11 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
  12 use rustc_middle::ty::{self, Ty, TypeVisitable};
  13 use rustc_target::spec::abi::Abi;
  14
  15 use crate::traits;
  16 use crate::traits::query::evaluate_obligation::InferCtxtExt;
  17 use crate::traits::util;
  18
  19 use super::BuiltinImplConditions;
  20 use super::SelectionCandidate::*;
  21 use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};
  22
  23 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
  24     #[instrument(skip(self, stack), level = "debug")]
  25     pub(super) fn assemble_candidates<'o>(
  26         &mut self,
  27         stack: &TraitObligationStack<'o, 'tcx>,
  28     ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
  29         let TraitObligationStack { obligation, .. } = *stack;
  30         let obligation = &Obligation {
  31             param_env: obligation.param_env,
  32             cause: obligation.cause.clone(),
  33             recursion_depth: obligation.recursion_depth,
  34             predicate: self.infcx.resolve_vars_if_possible(obligation.predicate),
  35         };
  36
  37         if obligation.predicate.skip_binder().self_ty().is_ty_var() {
  38             debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type");
  39             // Self is a type variable (e.g., `_: AsRef<str>`).
  40             //
  41             // This is somewhat problematic, as the current scheme can't really
  42             // handle it turning to be a projection. This does end up as truly
  43             // ambiguous in most cases anyway.
  44             //
  45             // Take the fast path out - this also improves
  46             // performance by preventing assemble_candidates_from_impls from
  47             // matching every impl for this trait.
  48             return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
  49         }
  50
  51         let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
  52
  53         // The only way to prove a NotImplemented(T: Foo) predicate is via a negative impl.
  54         // There are no compiler built-in rules for this.
  55         if obligation.polarity() == ty::ImplPolarity::Negative {
  56             self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
  57             self.assemble_candidates_from_impls(obligation, &mut candidates);
  58         } else {
  59             self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
  60
  61             // Other bounds. Consider both in-scope bounds from fn decl
  62             // and applicable impls. There is a certain set of precedence rules here.
  63             let def_id = obligation.predicate.def_id();
  64             let lang_items = self.tcx().lang_items();
  65
  66             if lang_items.copy_trait() == Some(def_id) {
  67                 debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty());
  68
  69                 // User-defined copy impls are permitted, but only for
  70                 // structs and enums.
  71                 self.assemble_candidates_from_impls(obligation, &mut candidates);
  72
  73                 // For other types, we'll use the builtin rules.
  74                 let copy_conditions = self.copy_clone_conditions(obligation);
  75                 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates);
  76             } else if lang_items.discriminant_kind_trait() == Some(def_id) {
  77                 // `DiscriminantKind` is automatically implemented for every type.
  78                 candidates.vec.push(BuiltinCandidate { has_nested: false });
  79             } else if lang_items.pointee_trait() == Some(def_id) {
  80                 // `Pointee` is automatically implemented for every type.
  81                 candidates.vec.push(BuiltinCandidate { has_nested: false });
  82             } else if lang_items.sized_trait() == Some(def_id) {
  83                 // Sized is never implementable by end-users, it is
  84                 // always automatically computed.
  85                 let sized_conditions = self.sized_conditions(obligation);
  86                 self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
  87             } else if lang_items.unsize_trait() == Some(def_id) {
  88                 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
  89             } else if lang_items.destruct_trait() == Some(def_id) {
  90                 self.assemble_const_destruct_candidates(obligation, &mut candidates);
  91             } else if lang_items.transmute_trait() == Some(def_id) {
  92                 // User-defined transmutability impls are permitted.
  93                 self.assemble_candidates_from_impls(obligation, &mut candidates);
  94                 self.assemble_candidates_for_transmutability(obligation, &mut candidates);
  95             } else if lang_items.tuple_trait() == Some(def_id) {
  96                 self.assemble_candidate_for_tuple(obligation, &mut candidates);
  97             } else if lang_items.pointer_sized() == Some(def_id) {
  98                 self.assemble_candidate_for_ptr_sized(obligation, &mut candidates);
  99             } else {
 100                 if lang_items.clone_trait() == Some(def_id) {
 101                     // Same builtin conditions as `Copy`, i.e., every type which has builtin support
 102                     // for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone`
 103                     // types have builtin support for `Clone`.
 104                     let clone_conditions = self.copy_clone_conditions(obligation);
 105                     self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates);
 106                 }
 107
 108                 if lang_items.gen_trait() == Some(def_id) {
 109                     self.assemble_generator_candidates(obligation, &mut candidates);
 110                 } else if lang_items.future_trait() == Some(def_id) {
 111                     self.assemble_future_candidates(obligation, &mut candidates);
 112                 }
 113
 114                 self.assemble_closure_candidates(obligation, &mut candidates);
 115                 self.assemble_fn_pointer_candidates(obligation, &mut candidates);
 116                 self.assemble_candidates_from_impls(obligation, &mut candidates);
 117                 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
 118             }
 119
 120             self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
 121             self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
 122             // Auto implementations have lower priority, so we only
 123             // consider triggering a default if there is no other impl that can apply.
 124             if candidates.vec.is_empty() {
 125                 self.assemble_candidates_from_auto_impls(obligation, &mut candidates);
 126             }
 127         }
 128         debug!("candidate list size: {}", candidates.vec.len());
 129         Ok(candidates)
 130     }
 131
 132     #[instrument(level = "debug", skip(self, candidates))]
 133     fn assemble_candidates_from_projected_tys(
 134         &mut self,
 135         obligation: &TraitObligation<'tcx>,
 136         candidates: &mut SelectionCandidateSet<'tcx>,
 137     ) {
 138         // Before we go into the whole placeholder thing, just
 139         // quickly check if the self-type is a projection at all.
 140         match obligation.predicate.skip_binder().trait_ref.self_ty().kind() {
 141             ty::Alias(..) => {}
 142             ty::Infer(ty::TyVar(_)) => {
 143                 span_bug!(
 144                     obligation.cause.span,
 145                     "Self=_ should have been handled by assemble_candidates"
 146                 );
 147             }
 148             _ => return,
 149         }
 150
 151         let result = self
 152             .infcx
 153             .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
 154
 155         candidates
 156             .vec
 157             .extend(result.into_iter().map(|(idx, constness)| ProjectionCandidate(idx, constness)));
 158     }
 159
 160     /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
 161     /// supplied to find out whether it is listed among them.
 162     ///
 163     /// Never affects the inference environment.
 164     #[instrument(level = "debug", skip(self, stack, candidates))]
 165     fn assemble_candidates_from_caller_bounds<'o>(
 166         &mut self,
 167         stack: &TraitObligationStack<'o, 'tcx>,
 168         candidates: &mut SelectionCandidateSet<'tcx>,
 169     ) -> Result<(), SelectionError<'tcx>> {
 170         debug!(?stack.obligation);
 171
 172         let all_bounds = stack
 173             .obligation
 174             .param_env
 175             .caller_bounds()
 176             .iter()
 177             .filter_map(|o| o.to_opt_poly_trait_pred());
 178
 179         // Micro-optimization: filter out predicates relating to different traits.
 180         let matching_bounds =
 181             all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
 182
 183         // Keep only those bounds which may apply, and propagate overflow if it occurs.
 184         for bound in matching_bounds {
 185             // FIXME(oli-obk): it is suspicious that we are dropping the constness and
 186             // polarity here.
 187             let wc = self.where_clause_may_apply(stack, bound.map_bound(|t| t.trait_ref))?;
 188             if wc.may_apply() {
 189                 candidates.vec.push(ParamCandidate(bound));
 190             }
 191         }
 192
 193         Ok(())
 194     }
 195
 196     fn assemble_generator_candidates(
 197         &mut self,
 198         obligation: &TraitObligation<'tcx>,
 199         candidates: &mut SelectionCandidateSet<'tcx>,
 200     ) {
 201         // Okay to skip binder because the substs on generator types never
 202         // touch bound regions, they just capture the in-scope
 203         // type/region parameters.
 204         let self_ty = obligation.self_ty().skip_binder();
 205         match self_ty.kind() {
 206             // async constructs get lowered to a special kind of generator that
 207             // should *not* `impl Generator`.
 208             ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => {
 209                 debug!(?self_ty, ?obligation, "assemble_generator_candidates",);
 210
 211                 candidates.vec.push(GeneratorCandidate);
 212             }
 213             ty::Infer(ty::TyVar(_)) => {
 214                 debug!("assemble_generator_candidates: ambiguous self-type");
 215                 candidates.ambiguous = true;
 216             }
 217             _ => {}
 218         }
 219     }
 220
 221     fn assemble_future_candidates(
 222         &mut self,
 223         obligation: &TraitObligation<'tcx>,
 224         candidates: &mut SelectionCandidateSet<'tcx>,
 225     ) {
 226         let self_ty = obligation.self_ty().skip_binder();
 227         if let ty::Generator(did, ..) = self_ty.kind() {
 228             // async constructs get lowered to a special kind of generator that
 229             // should directly `impl Future`.
 230             if self.tcx().generator_is_async(*did) {
 231                 debug!(?self_ty, ?obligation, "assemble_future_candidates",);
 232
 233                 candidates.vec.push(FutureCandidate);
 234             }
 235         }
 236     }
 237
 238     /// Checks for the artificial impl that the compiler will create for an obligation like `X :
 239     /// FnMut<..>` where `X` is a closure type.
 240     ///
 241     /// Note: the type parameters on a closure candidate are modeled as *output* type
 242     /// parameters and hence do not affect whether this trait is a match or not. They will be
 243     /// unified during the confirmation step.
 244     fn assemble_closure_candidates(
 245         &mut self,
 246         obligation: &TraitObligation<'tcx>,
 247         candidates: &mut SelectionCandidateSet<'tcx>,
 248     ) {
 249         let Some(kind) = self.tcx().fn_trait_kind_from_def_id(obligation.predicate.def_id()) else {
 250             return;
 251         };
 252
 253         // Okay to skip binder because the substs on closure types never
 254         // touch bound regions, they just capture the in-scope
 255         // type/region parameters
 256         match *obligation.self_ty().skip_binder().kind() {
 257             ty::Closure(_, closure_substs) => {
 258                 debug!(?kind, ?obligation, "assemble_unboxed_candidates");
 259                 match self.infcx.closure_kind(closure_substs) {
 260                     Some(closure_kind) => {
 261                         debug!(?closure_kind, "assemble_unboxed_candidates");
 262                         if closure_kind.extends(kind) {
 263                             candidates.vec.push(ClosureCandidate);
 264                         }
 265                     }
 266                     None => {
 267                         debug!("assemble_unboxed_candidates: closure_kind not yet known");
 268                         candidates.vec.push(ClosureCandidate);
 269                     }
 270                 }
 271             }
 272             ty::Infer(ty::TyVar(_)) => {
 273                 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
 274                 candidates.ambiguous = true;
 275             }
 276             _ => {}
 277         }
 278     }
 279
 280     /// Implements one of the `Fn()` family for a fn pointer.
 281     fn assemble_fn_pointer_candidates(
 282         &mut self,
 283         obligation: &TraitObligation<'tcx>,
 284         candidates: &mut SelectionCandidateSet<'tcx>,
 285     ) {
 286         // We provide impl of all fn traits for fn pointers.
 287         if !self.tcx().is_fn_trait(obligation.predicate.def_id()) {
 288             return;
 289         }
 290
 291         // Okay to skip binder because what we are inspecting doesn't involve bound regions.
 292         let self_ty = obligation.self_ty().skip_binder();
 293         match *self_ty.kind() {
 294             ty::Infer(ty::TyVar(_)) => {
 295                 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
 296                 candidates.ambiguous = true; // Could wind up being a fn() type.
 297             }
 298             // Provide an impl, but only for suitable `fn` pointers.
 299             ty::FnPtr(_) => {
 300                 if let ty::FnSig {
 301                     unsafety: hir::Unsafety::Normal,
 302                     abi: Abi::Rust,
 303                     c_variadic: false,
 304                     ..
 305                 } = self_ty.fn_sig(self.tcx()).skip_binder()
 306                 {
 307                     candidates.vec.push(FnPointerCandidate { is_const: false });
 308                 }
 309             }
 310             // Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396).
 311             ty::FnDef(def_id, _) => {
 312                 if let ty::FnSig {
 313                     unsafety: hir::Unsafety::Normal,
 314                     abi: Abi::Rust,
 315                     c_variadic: false,
 316                     ..
 317                 } = self_ty.fn_sig(self.tcx()).skip_binder()
 318                 {
 319                     if self.tcx().codegen_fn_attrs(def_id).target_features.is_empty() {
 320                         candidates
 321                             .vec
 322                             .push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) });
 323                     }
 324                 }
 325             }
 326             _ => {}
 327         }
 328     }
 329
 330     /// Searches for impls that might apply to `obligation`.
 331     fn assemble_candidates_from_impls(
 332         &mut self,
 333         obligation: &TraitObligation<'tcx>,
 334         candidates: &mut SelectionCandidateSet<'tcx>,
 335     ) {
 336         debug!(?obligation, "assemble_candidates_from_impls");
 337
 338         // Essentially any user-written impl will match with an error type,
 339         // so creating `ImplCandidates` isn't useful. However, we might
 340         // end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized)
 341         // This helps us avoid overflow: see issue #72839
 342         // Since compilation is already guaranteed to fail, this is just
 343         // to try to show the 'nicest' possible errors to the user.
 344         // We don't check for errors in the `ParamEnv` - in practice,
 345         // it seems to cause us to be overly aggressive in deciding
 346         // to give up searching for candidates, leading to spurious errors.
 347         if obligation.predicate.references_error() {
 348             return;
 349         }
 350
 351         self.tcx().for_each_relevant_impl(
 352             obligation.predicate.def_id(),
 353             obligation.predicate.skip_binder().trait_ref.self_ty(),
 354             |impl_def_id| {
 355                 // Before we create the substitutions and everything, first
 356                 // consider a "quick reject". This avoids creating more types
 357                 // and so forth that we need to.
 358                 let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
 359                 if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
 360                     return;
 361                 }
 362
 363                 self.infcx.probe(|_| {
 364                     if let Ok(_substs) = self.match_impl(impl_def_id, impl_trait_ref, obligation) {
 365                         candidates.vec.push(ImplCandidate(impl_def_id));
 366                     }
 367                 });
 368             },
 369         );
 370     }
 371
 372     fn assemble_candidates_from_auto_impls(
 373         &mut self,
 374         obligation: &TraitObligation<'tcx>,
 375         candidates: &mut SelectionCandidateSet<'tcx>,
 376     ) {
 377         // Okay to skip binder here because the tests we do below do not involve bound regions.
 378         let self_ty = obligation.self_ty().skip_binder();
 379         debug!(?self_ty, "assemble_candidates_from_auto_impls");
 380
 381         let def_id = obligation.predicate.def_id();
 382
 383         if self.tcx().trait_is_auto(def_id) {
 384             match self_ty.kind() {
 385                 ty::Dynamic(..) => {
 386                     // For object types, we don't know what the closed
 387                     // over types are. This means we conservatively
 388                     // say nothing; a candidate may be added by
 389                     // `assemble_candidates_from_object_ty`.
 390                 }
 391                 ty::Foreign(..) => {
 392                     // Since the contents of foreign types is unknown,
 393                     // we don't add any `..` impl. Default traits could
 394                     // still be provided by a manual implementation for
 395                     // this trait and type.
 396                 }
 397                 ty::Param(..) | ty::Alias(ty::Projection, ..) => {
 398                     // In these cases, we don't know what the actual
 399                     // type is.  Therefore, we cannot break it down
 400                     // into its constituent types. So we don't
 401                     // consider the `..` impl but instead just add no
 402                     // candidates: this means that typeck will only
 403                     // succeed if there is another reason to believe
 404                     // that this obligation holds. That could be a
 405                     // where-clause or, in the case of an object type,
 406                     // it could be that the object type lists the
 407                     // trait (e.g., `Foo+Send : Send`). See
 408                     // `ui/typeck/typeck-default-trait-impl-send-param.rs`
 409                     // for an example of a test case that exercises
 410                     // this path.
 411                 }
 412                 ty::Infer(ty::TyVar(_)) => {
 413                     // The auto impl might apply; we don't know.
 414                     candidates.ambiguous = true;
 415                 }
 416                 ty::Generator(_, _, movability)
 417                     if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
 418                 {
 419                     match movability {
 420                         hir::Movability::Static => {
 421                             // Immovable generators are never `Unpin`, so
 422                             // suppress the normal auto-impl candidate for it.
 423                         }
 424                         hir::Movability::Movable => {
 425                             // Movable generators are always `Unpin`, so add an
 426                             // unconditional builtin candidate.
 427                             candidates.vec.push(BuiltinCandidate { has_nested: false });
 428                         }
 429                     }
 430                 }
 431
 432                 _ => candidates.vec.push(AutoImplCandidate),
 433             }
 434         }
 435     }
 436
 437     /// Searches for impls that might apply to `obligation`.
 438     fn assemble_candidates_from_object_ty(
 439         &mut self,
 440         obligation: &TraitObligation<'tcx>,
 441         candidates: &mut SelectionCandidateSet<'tcx>,
 442     ) {
 443         debug!(
 444             self_ty = ?obligation.self_ty().skip_binder(),
 445             "assemble_candidates_from_object_ty",
 446         );
 447
 448         self.infcx.probe(|_snapshot| {
 449             // The code below doesn't care about regions, and the
 450             // self-ty here doesn't escape this probe, so just erase
 451             // any LBR.
 452             let self_ty = self.tcx().erase_late_bound_regions(obligation.self_ty());
 453             let poly_trait_ref = match self_ty.kind() {
 454                 ty::Dynamic(ref data, ..) => {
 455                     if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
 456                         debug!(
 457                             "assemble_candidates_from_object_ty: matched builtin bound, \
 458                              pushing candidate"
 459                         );
 460                         candidates.vec.push(BuiltinObjectCandidate);
 461                         return;
 462                     }
 463
 464                     if let Some(principal) = data.principal() {
 465                         if !self.infcx.tcx.features().object_safe_for_dispatch {
 466                             principal.with_self_ty(self.tcx(), self_ty)
 467                         } else if self.tcx().is_object_safe(principal.def_id()) {
 468                             principal.with_self_ty(self.tcx(), self_ty)
 469                         } else {
 470                             return;
 471                         }
 472                     } else {
 473                         // Only auto trait bounds exist.
 474                         return;
 475                     }
 476                 }
 477                 ty::Infer(ty::TyVar(_)) => {
 478                     debug!("assemble_candidates_from_object_ty: ambiguous");
 479                     candidates.ambiguous = true; // could wind up being an object type
 480                     return;
 481                 }
 482                 _ => return,
 483             };
 484
 485             debug!(?poly_trait_ref, "assemble_candidates_from_object_ty");
 486
 487             let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
 488             let placeholder_trait_predicate =
 489                 self.infcx.replace_bound_vars_with_placeholders(poly_trait_predicate);
 490
 491             // Count only those upcast versions that match the trait-ref
 492             // we are looking for. Specifically, do not only check for the
 493             // correct trait, but also the correct type parameters.
 494             // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
 495             // but `Foo` is declared as `trait Foo: Bar<u32>`.
 496             let candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref)
 497                 .enumerate()
 498                 .filter(|&(_, upcast_trait_ref)| {
 499                     self.infcx.probe(|_| {
 500                         self.match_normalize_trait_ref(
 501                             obligation,
 502                             upcast_trait_ref,
 503                             placeholder_trait_predicate.trait_ref,
 504                         )
 505                         .is_ok()
 506                     })
 507                 })
 508                 .map(|(idx, _)| ObjectCandidate(idx));
 509
 510             candidates.vec.extend(candidate_supertraits);
 511         })
 512     }
 513
 514     /// Temporary migration for #89190
 515     fn need_migrate_deref_output_trait_object(
 516         &mut self,
 517         ty: Ty<'tcx>,
 518         param_env: ty::ParamEnv<'tcx>,
 519         cause: &ObligationCause<'tcx>,
 520     ) -> Option<ty::PolyExistentialTraitRef<'tcx>> {
 521         let tcx = self.tcx();
 522         if tcx.features().trait_upcasting {
 523             return None;
 524         }
 525
 526         // <ty as Deref>
 527         let trait_ref = tcx.mk_trait_ref(tcx.lang_items().deref_trait()?, [ty]);
 528
 529         let obligation =
 530             traits::Obligation::new(tcx, cause.clone(), param_env, ty::Binder::dummy(trait_ref));
 531         if !self.infcx.predicate_may_hold(&obligation) {
 532             return None;
 533         }
 534
 535         self.infcx.probe(|_| {
 536             let ty = traits::normalize_projection_type(
 537                 self,
 538                 param_env,
 539                 ty::AliasTy { def_id: tcx.lang_items().deref_target()?, substs: trait_ref.substs },
 540                 cause.clone(),
 541                 0,
 542                 // We're *intentionally* throwing these away,
 543                 // since we don't actually use them.
 544                 &mut vec![],
 545             )
 546             .ty()
 547             .unwrap();
 548
 549             if let ty::Dynamic(data, ..) = ty.kind() { data.principal() } else { None }
 550         })
 551     }
 552
 553     /// Searches for unsizing that might apply to `obligation`.
 554     fn assemble_candidates_for_unsizing(
 555         &mut self,
 556         obligation: &TraitObligation<'tcx>,
 557         candidates: &mut SelectionCandidateSet<'tcx>,
 558     ) {
 559         // We currently never consider higher-ranked obligations e.g.
 560         // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
 561         // because they are a priori invalid, and we could potentially add support
 562         // for them later, it's just that there isn't really a strong need for it.
 563         // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
 564         // impl, and those are generally applied to concrete types.
 565         //
 566         // That said, one might try to write a fn with a where clause like
 567         //     for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
 568         // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
 569         // Still, you'd be more likely to write that where clause as
 570         //     T: Trait
 571         // so it seems ok if we (conservatively) fail to accept that `Unsize`
 572         // obligation above. Should be possible to extend this in the future.
 573         let Some(source) = obligation.self_ty().no_bound_vars() else {
 574             // Don't add any candidates if there are bound regions.
 575             return;
 576         };
 577         let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
 578
 579         debug!(?source, ?target, "assemble_candidates_for_unsizing");
 580
 581         match (source.kind(), target.kind()) {
 582             // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
 583             (&ty::Dynamic(ref data_a, _, ty::Dyn), &ty::Dynamic(ref data_b, _, ty::Dyn)) => {
 584                 // Upcast coercions permit several things:
 585                 //
 586                 // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
 587                 // 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
 588                 // 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
 589                 //
 590                 // Note that neither of the first two of these changes requires any
 591                 // change at runtime. The third needs to change pointer metadata at runtime.
 592                 //
 593                 // We always perform upcasting coercions when we can because of reason
 594                 // #2 (region bounds).
 595                 let auto_traits_compatible = data_b
 596                     .auto_traits()
 597                     // All of a's auto traits need to be in b's auto traits.
 598                     .all(|b| data_a.auto_traits().any(|a| a == b));
 599                 if auto_traits_compatible {
 600                     let principal_def_id_a = data_a.principal_def_id();
 601                     let principal_def_id_b = data_b.principal_def_id();
 602                     if principal_def_id_a == principal_def_id_b {
 603                         // no cyclic
 604                         candidates.vec.push(BuiltinUnsizeCandidate);
 605                     } else if principal_def_id_a.is_some() && principal_def_id_b.is_some() {
 606                         // not casual unsizing, now check whether this is trait upcasting coercion.
 607                         let principal_a = data_a.principal().unwrap();
 608                         let target_trait_did = principal_def_id_b.unwrap();
 609                         let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
 610                         if let Some(deref_trait_ref) = self.need_migrate_deref_output_trait_object(
 611                             source,
 612                             obligation.param_env,
 613                             &obligation.cause,
 614                         ) {
 615                             if deref_trait_ref.def_id() == target_trait_did {
 616                                 return;
 617                             }
 618                         }
 619
 620                         for (idx, upcast_trait_ref) in
 621                             util::supertraits(self.tcx(), source_trait_ref).enumerate()
 622                         {
 623                             if upcast_trait_ref.def_id() == target_trait_did {
 624                                 candidates.vec.push(TraitUpcastingUnsizeCandidate(idx));
 625                             }
 626                         }
 627                     }
 628                 }
 629             }
 630
 631             // `T` -> `Trait`
 632             (_, &ty::Dynamic(_, _, ty::Dyn)) => {
 633                 candidates.vec.push(BuiltinUnsizeCandidate);
 634             }
 635
 636             // Ambiguous handling is below `T` -> `Trait`, because inference
 637             // variables can still implement `Unsize<Trait>` and nested
 638             // obligations will have the final say (likely deferred).
 639             (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
 640                 debug!("assemble_candidates_for_unsizing: ambiguous");
 641                 candidates.ambiguous = true;
 642             }
 643
 644             // `[T; n]` -> `[T]`
 645             (&ty::Array(..), &ty::Slice(_)) => {
 646                 candidates.vec.push(BuiltinUnsizeCandidate);
 647             }
 648
 649             // `Struct<T>` -> `Struct<U>`
 650             (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
 651                 if def_id_a == def_id_b {
 652                     candidates.vec.push(BuiltinUnsizeCandidate);
 653                 }
 654             }
 655
 656             // `(.., T)` -> `(.., U)`
 657             (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
 658                 if tys_a.len() == tys_b.len() {
 659                     candidates.vec.push(BuiltinUnsizeCandidate);
 660                 }
 661             }
 662
 663             _ => {}
 664         };
 665     }
 666
 667     #[instrument(level = "debug", skip(self, obligation, candidates))]
 668     fn assemble_candidates_for_transmutability(
 669         &mut self,
 670         obligation: &TraitObligation<'tcx>,
 671         candidates: &mut SelectionCandidateSet<'tcx>,
 672     ) {
 673         if obligation.has_non_region_param() {
 674             return;
 675         }
 676
 677         if obligation.has_non_region_infer() {
 678             candidates.ambiguous = true;
 679             return;
 680         }
 681
 682         candidates.vec.push(TransmutabilityCandidate);
 683     }
 684
 685     #[instrument(level = "debug", skip(self, obligation, candidates))]
 686     fn assemble_candidates_for_trait_alias(
 687         &mut self,
 688         obligation: &TraitObligation<'tcx>,
 689         candidates: &mut SelectionCandidateSet<'tcx>,
 690     ) {
 691         // Okay to skip binder here because the tests we do below do not involve bound regions.
 692         let self_ty = obligation.self_ty().skip_binder();
 693         debug!(?self_ty);
 694
 695         let def_id = obligation.predicate.def_id();
 696
 697         if self.tcx().is_trait_alias(def_id) {
 698             candidates.vec.push(TraitAliasCandidate);
 699         }
 700     }
 701
 702     /// Assembles the trait which are built-in to the language itself:
 703     /// `Copy`, `Clone` and `Sized`.
 704     #[instrument(level = "debug", skip(self, candidates))]
 705     fn assemble_builtin_bound_candidates(
 706         &mut self,
 707         conditions: BuiltinImplConditions<'tcx>,
 708         candidates: &mut SelectionCandidateSet<'tcx>,
 709     ) {
 710         match conditions {
 711             BuiltinImplConditions::Where(nested) => {
 712                 candidates
 713                     .vec
 714                     .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
 715             }
 716             BuiltinImplConditions::None => {}
 717             BuiltinImplConditions::Ambiguous => {
 718                 candidates.ambiguous = true;
 719             }
 720         }
 721     }
 722
 723     fn assemble_const_destruct_candidates(
 724         &mut self,
 725         obligation: &TraitObligation<'tcx>,
 726         candidates: &mut SelectionCandidateSet<'tcx>,
 727     ) {
 728         // If the predicate is `~const Destruct` in a non-const environment, we don't actually need
 729         // to check anything. We'll short-circuit checking any obligations in confirmation, too.
 730         if !obligation.is_const() {
 731             candidates.vec.push(ConstDestructCandidate(None));
 732             return;
 733         }
 734
 735         let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
 736         match self_ty.skip_binder().kind() {
 737             ty::Alias(..)
 738             | ty::Dynamic(..)
 739             | ty::Error(_)
 740             | ty::Bound(..)
 741             | ty::Param(_)
 742             | ty::Placeholder(_) => {
 743                 // We don't know if these are `~const Destruct`, at least
 744                 // not structurally... so don't push a candidate.
 745             }
 746
 747             ty::Bool
 748             | ty::Char
 749             | ty::Int(_)
 750             | ty::Uint(_)
 751             | ty::Float(_)
 752             | ty::Infer(ty::IntVar(_))
 753             | ty::Infer(ty::FloatVar(_))
 754             | ty::Str
 755             | ty::RawPtr(_)
 756             | ty::Ref(..)
 757             | ty::FnDef(..)
 758             | ty::FnPtr(_)
 759             | ty::Never
 760             | ty::Foreign(_)
 761             | ty::Array(..)
 762             | ty::Slice(_)
 763             | ty::Closure(..)
 764             | ty::Generator(..)
 765             | ty::Tuple(_)
 766             | ty::GeneratorWitness(_) => {
 767                 // These are built-in, and cannot have a custom `impl const Destruct`.
 768                 candidates.vec.push(ConstDestructCandidate(None));
 769             }
 770
 771             ty::Adt(..) => {
 772                 // Find a custom `impl Drop` impl, if it exists
 773                 let relevant_impl = self.tcx().find_map_relevant_impl(
 774                     self.tcx().require_lang_item(LangItem::Drop, None),
 775                     obligation.predicate.skip_binder().trait_ref.self_ty(),
 776                     Some,
 777                 );
 778
 779                 if let Some(impl_def_id) = relevant_impl {
 780                     // Check that `impl Drop` is actually const, if there is a custom impl
 781                     if self.tcx().constness(impl_def_id) == hir::Constness::Const {
 782                         candidates.vec.push(ConstDestructCandidate(Some(impl_def_id)));
 783                     }
 784                 } else {
 785                     // Otherwise check the ADT like a built-in type (structurally)
 786                     candidates.vec.push(ConstDestructCandidate(None));
 787                 }
 788             }
 789
 790             ty::Infer(_) => {
 791                 candidates.ambiguous = true;
 792             }
 793         }
 794     }
 795
 796     fn assemble_candidate_for_tuple(
 797         &mut self,
 798         obligation: &TraitObligation<'tcx>,
 799         candidates: &mut SelectionCandidateSet<'tcx>,
 800     ) {
 801         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
 802         match self_ty.kind() {
 803             ty::Tuple(_) => {
 804                 candidates.vec.push(BuiltinCandidate { has_nested: false });
 805             }
 806             ty::Infer(ty::TyVar(_)) => {
 807                 candidates.ambiguous = true;
 808             }
 809             ty::Bool
 810             | ty::Char
 811             | ty::Int(_)
 812             | ty::Uint(_)
 813             | ty::Float(_)
 814             | ty::Adt(_, _)
 815             | ty::Foreign(_)
 816             | ty::Str
 817             | ty::Array(_, _)
 818             | ty::Slice(_)
 819             | ty::RawPtr(_)
 820             | ty::Ref(_, _, _)
 821             | ty::FnDef(_, _)
 822             | ty::FnPtr(_)
 823             | ty::Dynamic(_, _, _)
 824             | ty::Closure(_, _)
 825             | ty::Generator(_, _, _)
 826             | ty::GeneratorWitness(_)
 827             | ty::Never
 828             | ty::Alias(..)
 829             | ty::Param(_)
 830             | ty::Bound(_, _)
 831             | ty::Error(_)
 832             | ty::Infer(_)
 833             | ty::Placeholder(_) => {}
 834         }
 835     }
 836
 837     fn assemble_candidate_for_ptr_sized(
 838         &mut self,
 839         obligation: &TraitObligation<'tcx>,
 840         candidates: &mut SelectionCandidateSet<'tcx>,
 841     ) {
 842         // The regions of a type don't affect the size of the type
 843         let self_ty = self
 844             .tcx()
 845             .erase_regions(self.tcx().erase_late_bound_regions(obligation.predicate.self_ty()));
 846
 847         // But if there are inference variables, we have to wait until it's resolved.
 848         if self_ty.has_non_region_infer() {
 849             candidates.ambiguous = true;
 850             return;
 851         }
 852
 853         let usize_layout =
 854             self.tcx().layout_of(ty::ParamEnv::empty().and(self.tcx().types.usize)).unwrap().layout;
 855         if let Ok(layout) = self.tcx().layout_of(obligation.param_env.and(self_ty))
 856             && layout.layout.size() == usize_layout.size()
 857             && layout.layout.align().abi == usize_layout.align().abi
 858         {
 859             candidates.vec.push(BuiltinCandidate { has_nested: false });
 860         }
 861     }
 862 }