| //! Candidate assembly. |
| //! |
| //! The selection process begins by examining all in-scope impls, |
| //! caller obligations, and so forth and assembling a list of |
| //! candidates. See the [rustc dev guide] for more details. |
| //! |
| //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly |
| |
| use hir::def_id::DefId; |
| use hir::LangItem; |
| use rustc_hir as hir; |
| use rustc_infer::traits::ObligationCause; |
| use rustc_infer::traits::{Obligation, SelectionError, TraitObligation}; |
| use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams}; |
| use rustc_middle::ty::{self, Ty, TypeVisitableExt}; |
| |
| use crate::traits; |
| use crate::traits::query::evaluate_obligation::InferCtxtExt; |
| use crate::traits::util; |
| |
| use super::BuiltinImplConditions; |
| use super::SelectionCandidate::*; |
| use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack}; |
| |
| impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { |
| #[instrument(skip(self, stack), level = "debug")] |
| pub(super) fn assemble_candidates<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| ) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> { |
| let TraitObligationStack { obligation, .. } = *stack; |
| let obligation = &Obligation { |
| param_env: obligation.param_env, |
| cause: obligation.cause.clone(), |
| recursion_depth: obligation.recursion_depth, |
| predicate: self.infcx.resolve_vars_if_possible(obligation.predicate), |
| }; |
| |
| if obligation.predicate.skip_binder().self_ty().is_ty_var() { |
| debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type"); |
| // Self is a type variable (e.g., `_: AsRef<str>`). |
| // |
| // This is somewhat problematic, as the current scheme can't really |
| // handle it turning to be a projection. This does end up as truly |
| // ambiguous in most cases anyway. |
| // |
| // Take the fast path out - this also improves |
| // performance by preventing assemble_candidates_from_impls from |
| // matching every impl for this trait. |
| return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true }); |
| } |
| |
| let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false }; |
| |
| // The only way to prove a NotImplemented(T: Foo) predicate is via a negative impl. |
| // There are no compiler built-in rules for this. |
| if obligation.polarity() == ty::ImplPolarity::Negative { |
| self.assemble_candidates_for_trait_alias(obligation, &mut candidates); |
| self.assemble_candidates_from_impls(obligation, &mut candidates); |
| self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?; |
| } else { |
| self.assemble_candidates_for_trait_alias(obligation, &mut candidates); |
| |
| // Other bounds. Consider both in-scope bounds from fn decl |
| // and applicable impls. There is a certain set of precedence rules here. |
| let def_id = obligation.predicate.def_id(); |
| let lang_items = self.tcx().lang_items(); |
| |
| if lang_items.copy_trait() == Some(def_id) { |
| debug!(obligation_self_ty = ?obligation.predicate.skip_binder().self_ty()); |
| |
| // User-defined copy impls are permitted, but only for |
| // structs and enums. |
| self.assemble_candidates_from_impls(obligation, &mut candidates); |
| |
| // For other types, we'll use the builtin rules. |
| let copy_conditions = self.copy_clone_conditions(obligation); |
| self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates); |
| } else if lang_items.discriminant_kind_trait() == Some(def_id) { |
| // `DiscriminantKind` is automatically implemented for every type. |
| candidates.vec.push(BuiltinCandidate { has_nested: false }); |
| } else if lang_items.pointee_trait() == Some(def_id) { |
| // `Pointee` is automatically implemented for every type. |
| candidates.vec.push(BuiltinCandidate { has_nested: false }); |
| } else if lang_items.sized_trait() == Some(def_id) { |
| // Sized is never implementable by end-users, it is |
| // always automatically computed. |
| let sized_conditions = self.sized_conditions(obligation); |
| self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates); |
| } else if lang_items.unsize_trait() == Some(def_id) { |
| self.assemble_candidates_for_unsizing(obligation, &mut candidates); |
| } else if lang_items.destruct_trait() == Some(def_id) { |
| self.assemble_const_destruct_candidates(obligation, &mut candidates); |
| } else if lang_items.transmute_trait() == Some(def_id) { |
| // User-defined transmutability impls are permitted. |
| self.assemble_candidates_from_impls(obligation, &mut candidates); |
| self.assemble_candidates_for_transmutability(obligation, &mut candidates); |
| } else if lang_items.tuple_trait() == Some(def_id) { |
| self.assemble_candidate_for_tuple(obligation, &mut candidates); |
| } else if lang_items.pointer_like() == Some(def_id) { |
| self.assemble_candidate_for_pointer_like(obligation, &mut candidates); |
| } else if lang_items.fn_ptr_trait() == Some(def_id) { |
| self.assemble_candidates_for_fn_ptr_trait(obligation, &mut candidates); |
| } else { |
| if lang_items.clone_trait() == Some(def_id) { |
| // Same builtin conditions as `Copy`, i.e., every type which has builtin support |
| // for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone` |
| // types have builtin support for `Clone`. |
| let clone_conditions = self.copy_clone_conditions(obligation); |
| self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates); |
| } |
| |
| if lang_items.gen_trait() == Some(def_id) { |
| self.assemble_generator_candidates(obligation, &mut candidates); |
| } else if lang_items.future_trait() == Some(def_id) { |
| self.assemble_future_candidates(obligation, &mut candidates); |
| } |
| |
| self.assemble_closure_candidates(obligation, &mut candidates); |
| self.assemble_fn_pointer_candidates(obligation, &mut candidates); |
| self.assemble_candidates_from_impls(obligation, &mut candidates); |
| self.assemble_candidates_from_object_ty(obligation, &mut candidates); |
| } |
| |
| self.assemble_candidates_from_projected_tys(obligation, &mut candidates); |
| self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?; |
| // Auto implementations have lower priority, so we only |
| // consider triggering a default if there is no other impl that can apply. |
| if candidates.vec.is_empty() { |
| self.assemble_candidates_from_auto_impls(obligation, &mut candidates); |
| } |
| } |
| debug!("candidate list size: {}", candidates.vec.len()); |
| Ok(candidates) |
| } |
| |
| #[instrument(level = "debug", skip(self, candidates))] |
| fn assemble_candidates_from_projected_tys( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // Before we go into the whole placeholder thing, just |
| // quickly check if the self-type is a projection at all. |
| match obligation.predicate.skip_binder().trait_ref.self_ty().kind() { |
| // Excluding IATs here as they don't have meaningful item bounds. |
| ty::Alias(ty::Projection | ty::Opaque, _) => {} |
| ty::Infer(ty::TyVar(_)) => { |
| span_bug!( |
| obligation.cause.span, |
| "Self=_ should have been handled by assemble_candidates" |
| ); |
| } |
| _ => return, |
| } |
| |
| let result = self |
| .infcx |
| .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation)); |
| |
| candidates |
| .vec |
| .extend(result.into_iter().map(|(idx, constness)| ProjectionCandidate(idx, constness))); |
| } |
| |
| /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller |
| /// supplied to find out whether it is listed among them. |
| /// |
| /// Never affects the inference environment. |
| #[instrument(level = "debug", skip(self, stack, candidates))] |
| fn assemble_candidates_from_caller_bounds<'o>( |
| &mut self, |
| stack: &TraitObligationStack<'o, 'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) -> Result<(), SelectionError<'tcx>> { |
| debug!(?stack.obligation); |
| |
| let all_bounds = stack |
| .obligation |
| .param_env |
| .caller_bounds() |
| .iter() |
| .filter(|p| !p.references_error()) |
| .filter_map(|p| p.to_opt_poly_trait_pred()); |
| |
| // Micro-optimization: filter out predicates relating to different traits. |
| let matching_bounds = |
| all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id()); |
| |
| // Keep only those bounds which may apply, and propagate overflow if it occurs. |
| for bound in matching_bounds { |
| if bound.skip_binder().polarity != stack.obligation.predicate.skip_binder().polarity { |
| continue; |
| } |
| |
| // FIXME(oli-obk): it is suspicious that we are dropping the constness and |
| // polarity here. |
| let wc = self.where_clause_may_apply(stack, bound.map_bound(|t| t.trait_ref))?; |
| if wc.may_apply() { |
| candidates.vec.push(ParamCandidate(bound)); |
| } |
| } |
| |
| Ok(()) |
| } |
| |
| fn assemble_generator_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // Okay to skip binder because the substs on generator types never |
| // touch bound regions, they just capture the in-scope |
| // type/region parameters. |
| let self_ty = obligation.self_ty().skip_binder(); |
| match self_ty.kind() { |
| // async constructs get lowered to a special kind of generator that |
| // should *not* `impl Generator`. |
| ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => { |
| debug!(?self_ty, ?obligation, "assemble_generator_candidates",); |
| |
| candidates.vec.push(GeneratorCandidate); |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_generator_candidates: ambiguous self-type"); |
| candidates.ambiguous = true; |
| } |
| _ => {} |
| } |
| } |
| |
| fn assemble_future_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| let self_ty = obligation.self_ty().skip_binder(); |
| if let ty::Generator(did, ..) = self_ty.kind() { |
| // async constructs get lowered to a special kind of generator that |
| // should directly `impl Future`. |
| if self.tcx().generator_is_async(*did) { |
| debug!(?self_ty, ?obligation, "assemble_future_candidates",); |
| |
| candidates.vec.push(FutureCandidate); |
| } |
| } |
| } |
| |
| /// Checks for the artificial impl that the compiler will create for an obligation like `X : |
| /// FnMut<..>` where `X` is a closure type. |
| /// |
| /// Note: the type parameters on a closure candidate are modeled as *output* type |
| /// parameters and hence do not affect whether this trait is a match or not. They will be |
| /// unified during the confirmation step. |
| fn assemble_closure_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| let Some(kind) = self.tcx().fn_trait_kind_from_def_id(obligation.predicate.def_id()) else { |
| return; |
| }; |
| |
| // Okay to skip binder because the substs on closure types never |
| // touch bound regions, they just capture the in-scope |
| // type/region parameters |
| match *obligation.self_ty().skip_binder().kind() { |
| ty::Closure(def_id, closure_substs) => { |
| let is_const = self.tcx().is_const_fn_raw(def_id); |
| debug!(?kind, ?obligation, "assemble_unboxed_candidates"); |
| match self.infcx.closure_kind(closure_substs) { |
| Some(closure_kind) => { |
| debug!(?closure_kind, "assemble_unboxed_candidates"); |
| if closure_kind.extends(kind) { |
| candidates.vec.push(ClosureCandidate { is_const }); |
| } |
| } |
| None => { |
| debug!("assemble_unboxed_candidates: closure_kind not yet known"); |
| candidates.vec.push(ClosureCandidate { is_const }); |
| } |
| } |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_unboxed_closure_candidates: ambiguous self-type"); |
| candidates.ambiguous = true; |
| } |
| _ => {} |
| } |
| } |
| |
| /// Implements one of the `Fn()` family for a fn pointer. |
| fn assemble_fn_pointer_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // We provide impl of all fn traits for fn pointers. |
| if !self.tcx().is_fn_trait(obligation.predicate.def_id()) { |
| return; |
| } |
| |
| // Keep this function in sync with extract_tupled_inputs_and_output_from_callable |
| // until the old solver (and thus this function) is removed. |
| |
| // Okay to skip binder because what we are inspecting doesn't involve bound regions. |
| let self_ty = obligation.self_ty().skip_binder(); |
| match *self_ty.kind() { |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_fn_pointer_candidates: ambiguous self-type"); |
| candidates.ambiguous = true; // Could wind up being a fn() type. |
| } |
| // Provide an impl, but only for suitable `fn` pointers. |
| ty::FnPtr(sig) => { |
| if sig.is_fn_trait_compatible() { |
| candidates.vec.push(FnPointerCandidate { is_const: false }); |
| } |
| } |
| // Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396). |
| ty::FnDef(def_id, _) => { |
| if self.tcx().fn_sig(def_id).skip_binder().is_fn_trait_compatible() |
| && self.tcx().codegen_fn_attrs(def_id).target_features.is_empty() |
| { |
| candidates |
| .vec |
| .push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) }); |
| } |
| } |
| _ => {} |
| } |
| } |
| |
| /// Searches for impls that might apply to `obligation`. |
| #[instrument(level = "debug", skip(self, candidates))] |
| fn assemble_candidates_from_impls( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // Essentially any user-written impl will match with an error type, |
| // so creating `ImplCandidates` isn't useful. However, we might |
| // end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized`) |
| // This helps us avoid overflow: see issue #72839 |
| // Since compilation is already guaranteed to fail, this is just |
| // to try to show the 'nicest' possible errors to the user. |
| // We don't check for errors in the `ParamEnv` - in practice, |
| // it seems to cause us to be overly aggressive in deciding |
| // to give up searching for candidates, leading to spurious errors. |
| if obligation.predicate.references_error() { |
| return; |
| } |
| |
| let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup }; |
| let obligation_substs = obligation.predicate.skip_binder().trait_ref.substs; |
| self.tcx().for_each_relevant_impl( |
| obligation.predicate.def_id(), |
| obligation.predicate.skip_binder().trait_ref.self_ty(), |
| |impl_def_id| { |
| // Before we create the substitutions and everything, first |
| // consider a "quick reject". This avoids creating more types |
| // and so forth that we need to. |
| let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap(); |
| if !drcx |
| .substs_refs_may_unify(obligation_substs, impl_trait_ref.skip_binder().substs) |
| { |
| return; |
| } |
| if self.reject_fn_ptr_impls( |
| impl_def_id, |
| obligation, |
| impl_trait_ref.skip_binder().self_ty(), |
| ) { |
| return; |
| } |
| |
| self.infcx.probe(|_| { |
| if let Ok(_substs) = self.match_impl(impl_def_id, impl_trait_ref, obligation) { |
| candidates.vec.push(ImplCandidate(impl_def_id)); |
| } |
| }); |
| }, |
| ); |
| } |
| |
| /// The various `impl<T: FnPtr> Trait for T` in libcore are more like builtin impls for all function items |
| /// and function pointers and less like blanket impls. Rejecting them when they can't possibly apply (because |
| /// the obligation's self-type does not implement `FnPtr`) avoids reporting that the self type does not implement |
| /// `FnPtr`, when we wanted to report that it doesn't implement `Trait`. |
| #[instrument(level = "trace", skip(self), ret)] |
| fn reject_fn_ptr_impls( |
| &self, |
| impl_def_id: DefId, |
| obligation: &TraitObligation<'tcx>, |
| impl_self_ty: Ty<'tcx>, |
| ) -> bool { |
| // Let `impl<T: FnPtr> Trait for Vec<T>` go through the normal rejection path. |
| if !matches!(impl_self_ty.kind(), ty::Param(..)) { |
| return false; |
| } |
| let Some(fn_ptr_trait) = self.tcx().lang_items().fn_ptr_trait() else { |
| return false; |
| }; |
| |
| for &(predicate, _) in self.tcx().predicates_of(impl_def_id).predicates { |
| let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) |
| = predicate.kind().skip_binder() else { continue }; |
| if fn_ptr_trait != pred.trait_ref.def_id { |
| continue; |
| } |
| trace!(?pred); |
| // Not the bound we're looking for |
| if pred.self_ty() != impl_self_ty { |
| continue; |
| } |
| |
| match obligation.self_ty().skip_binder().kind() { |
| // Fast path to avoid evaluating an obligation that trivially holds. |
| // There may be more bounds, but these are checked by the regular path. |
| ty::FnPtr(..) => return false, |
| // These may potentially implement `FnPtr` |
| ty::Placeholder(..) |
| | ty::Dynamic(_, _, _) |
| | ty::Alias(_, _) |
| | ty::Infer(_) |
| | ty::Param(..) => {} |
| |
| ty::Bound(_, _) => span_bug!( |
| obligation.cause.span(), |
| "cannot have escaping bound var in self type of {obligation:#?}" |
| ), |
| // These can't possibly implement `FnPtr` as they are concrete types |
| // and not `FnPtr` |
| ty::Bool |
| | ty::Char |
| | ty::Int(_) |
| | ty::Uint(_) |
| | ty::Float(_) |
| | ty::Adt(_, _) |
| | ty::Foreign(_) |
| | ty::Str |
| | ty::Array(_, _) |
| | ty::Slice(_) |
| | ty::RawPtr(_) |
| | ty::Ref(_, _, _) |
| | ty::Closure(_, _) |
| | ty::Generator(_, _, _) |
| | ty::GeneratorWitness(_) |
| | ty::GeneratorWitnessMIR(_, _) |
| | ty::Never |
| | ty::Tuple(_) |
| | ty::Error(_) => return true, |
| // FIXME: Function definitions could actually implement `FnPtr` by |
| // casting the ZST function def to a function pointer. |
| ty::FnDef(_, _) => return true, |
| } |
| |
| // Generic params can implement `FnPtr` if the predicate |
| // holds within its own environment. |
| let obligation = Obligation::new( |
| self.tcx(), |
| obligation.cause.clone(), |
| obligation.param_env, |
| self.tcx().mk_predicate(obligation.predicate.map_bound(|mut pred| { |
| pred.trait_ref = |
| ty::TraitRef::new(self.tcx(), fn_ptr_trait, [pred.trait_ref.self_ty()]); |
| ty::PredicateKind::Clause(ty::Clause::Trait(pred)) |
| })), |
| ); |
| if let Ok(r) = self.infcx.evaluate_obligation(&obligation) { |
| if !r.may_apply() { |
| return true; |
| } |
| } |
| } |
| false |
| } |
| |
| fn assemble_candidates_from_auto_impls( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // Okay to skip binder here because the tests we do below do not involve bound regions. |
| let self_ty = obligation.self_ty().skip_binder(); |
| debug!(?self_ty, "assemble_candidates_from_auto_impls"); |
| |
| let def_id = obligation.predicate.def_id(); |
| |
| if self.tcx().trait_is_auto(def_id) { |
| match self_ty.kind() { |
| ty::Dynamic(..) => { |
| // For object types, we don't know what the closed |
| // over types are. This means we conservatively |
| // say nothing; a candidate may be added by |
| // `assemble_candidates_from_object_ty`. |
| } |
| ty::Foreign(..) => { |
| // Since the contents of foreign types is unknown, |
| // we don't add any `..` impl. Default traits could |
| // still be provided by a manual implementation for |
| // this trait and type. |
| } |
| ty::Param(..) |
| | ty::Alias(ty::Projection | ty::Inherent, ..) |
| | ty::Placeholder(..) |
| | ty::Bound(..) => { |
| // In these cases, we don't know what the actual |
| // type is. Therefore, we cannot break it down |
| // into its constituent types. So we don't |
| // consider the `..` impl but instead just add no |
| // candidates: this means that typeck will only |
| // succeed if there is another reason to believe |
| // that this obligation holds. That could be a |
| // where-clause or, in the case of an object type, |
| // it could be that the object type lists the |
| // trait (e.g., `Foo+Send : Send`). See |
| // `ui/typeck/typeck-default-trait-impl-send-param.rs` |
| // for an example of a test case that exercises |
| // this path. |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| // The auto impl might apply; we don't know. |
| candidates.ambiguous = true; |
| } |
| ty::Generator(_, _, movability) |
| if self.tcx().lang_items().unpin_trait() == Some(def_id) => |
| { |
| match movability { |
| hir::Movability::Static => { |
| // Immovable generators are never `Unpin`, so |
| // suppress the normal auto-impl candidate for it. |
| } |
| hir::Movability::Movable => { |
| // Movable generators are always `Unpin`, so add an |
| // unconditional builtin candidate. |
| candidates.vec.push(BuiltinCandidate { has_nested: false }); |
| } |
| } |
| } |
| |
| _ => candidates.vec.push(AutoImplCandidate), |
| } |
| } |
| } |
| |
| /// Searches for impls that might apply to `obligation`. |
| fn assemble_candidates_from_object_ty( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| debug!( |
| self_ty = ?obligation.self_ty().skip_binder(), |
| "assemble_candidates_from_object_ty", |
| ); |
| |
| self.infcx.probe(|_snapshot| { |
| if obligation.has_non_region_late_bound() { |
| return; |
| } |
| |
| // The code below doesn't care about regions, and the |
| // self-ty here doesn't escape this probe, so just erase |
| // any LBR. |
| let self_ty = self.tcx().erase_late_bound_regions(obligation.self_ty()); |
| let poly_trait_ref = match self_ty.kind() { |
| ty::Dynamic(ref data, ..) => { |
| if data.auto_traits().any(|did| did == obligation.predicate.def_id()) { |
| debug!( |
| "assemble_candidates_from_object_ty: matched builtin bound, \ |
| pushing candidate" |
| ); |
| candidates.vec.push(BuiltinObjectCandidate); |
| return; |
| } |
| |
| if let Some(principal) = data.principal() { |
| if !self.infcx.tcx.features().object_safe_for_dispatch { |
| principal.with_self_ty(self.tcx(), self_ty) |
| } else if self.tcx().check_is_object_safe(principal.def_id()) { |
| principal.with_self_ty(self.tcx(), self_ty) |
| } else { |
| return; |
| } |
| } else { |
| // Only auto trait bounds exist. |
| return; |
| } |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| debug!("assemble_candidates_from_object_ty: ambiguous"); |
| candidates.ambiguous = true; // could wind up being an object type |
| return; |
| } |
| _ => return, |
| }; |
| |
| debug!(?poly_trait_ref, "assemble_candidates_from_object_ty"); |
| |
| let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate); |
| let placeholder_trait_predicate = |
| self.infcx.instantiate_binder_with_placeholders(poly_trait_predicate); |
| |
| // Count only those upcast versions that match the trait-ref |
| // we are looking for. Specifically, do not only check for the |
| // correct trait, but also the correct type parameters. |
| // For example, we may be trying to upcast `Foo` to `Bar<i32>`, |
| // but `Foo` is declared as `trait Foo: Bar<u32>`. |
| let candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref) |
| .enumerate() |
| .filter(|&(_, upcast_trait_ref)| { |
| self.infcx.probe(|_| { |
| self.match_normalize_trait_ref( |
| obligation, |
| upcast_trait_ref, |
| placeholder_trait_predicate.trait_ref, |
| ) |
| .is_ok() |
| }) |
| }) |
| .map(|(idx, _)| ObjectCandidate(idx)); |
| |
| candidates.vec.extend(candidate_supertraits); |
| }) |
| } |
| |
| /// Temporary migration for #89190 |
| fn need_migrate_deref_output_trait_object( |
| &mut self, |
| ty: Ty<'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| cause: &ObligationCause<'tcx>, |
| ) -> Option<ty::PolyExistentialTraitRef<'tcx>> { |
| let tcx = self.tcx(); |
| if tcx.features().trait_upcasting { |
| return None; |
| } |
| |
| // <ty as Deref> |
| let trait_ref = ty::TraitRef::new(tcx, tcx.lang_items().deref_trait()?, [ty]); |
| |
| let obligation = |
| traits::Obligation::new(tcx, cause.clone(), param_env, ty::Binder::dummy(trait_ref)); |
| if !self.infcx.predicate_may_hold(&obligation) { |
| return None; |
| } |
| |
| self.infcx.probe(|_| { |
| let ty = traits::normalize_projection_type( |
| self, |
| param_env, |
| tcx.mk_alias_ty(tcx.lang_items().deref_target()?, trait_ref.substs), |
| cause.clone(), |
| 0, |
| // We're *intentionally* throwing these away, |
| // since we don't actually use them. |
| &mut vec![], |
| ) |
| .ty() |
| .unwrap(); |
| |
| if let ty::Dynamic(data, ..) = ty.kind() { data.principal() } else { None } |
| }) |
| } |
| |
| /// Searches for unsizing that might apply to `obligation`. |
| fn assemble_candidates_for_unsizing( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // We currently never consider higher-ranked obligations e.g. |
| // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not |
| // because they are a priori invalid, and we could potentially add support |
| // for them later, it's just that there isn't really a strong need for it. |
| // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>` |
| // impl, and those are generally applied to concrete types. |
| // |
| // That said, one might try to write a fn with a where clause like |
| // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>> |
| // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`. |
| // Still, you'd be more likely to write that where clause as |
| // T: Trait |
| // so it seems ok if we (conservatively) fail to accept that `Unsize` |
| // obligation above. Should be possible to extend this in the future. |
| let Some(source) = obligation.self_ty().no_bound_vars() else { |
| // Don't add any candidates if there are bound regions. |
| return; |
| }; |
| let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1); |
| |
| debug!(?source, ?target, "assemble_candidates_for_unsizing"); |
| |
| match (source.kind(), target.kind()) { |
| // Trait+Kx+'a -> Trait+Ky+'b (upcasts). |
| (&ty::Dynamic(ref data_a, _, ty::Dyn), &ty::Dynamic(ref data_b, _, ty::Dyn)) => { |
| // Upcast coercions permit several things: |
| // |
| // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo` |
| // 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b` |
| // 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar` |
| // |
| // Note that neither of the first two of these changes requires any |
| // change at runtime. The third needs to change pointer metadata at runtime. |
| // |
| // We always perform upcasting coercions when we can because of reason |
| // #2 (region bounds). |
| let auto_traits_compatible = data_b |
| .auto_traits() |
| // All of a's auto traits need to be in b's auto traits. |
| .all(|b| data_a.auto_traits().any(|a| a == b)); |
| if auto_traits_compatible { |
| let principal_def_id_a = data_a.principal_def_id(); |
| let principal_def_id_b = data_b.principal_def_id(); |
| if principal_def_id_a == principal_def_id_b { |
| // no cyclic |
| candidates.vec.push(BuiltinUnsizeCandidate); |
| } else if principal_def_id_a.is_some() && principal_def_id_b.is_some() { |
| // not casual unsizing, now check whether this is trait upcasting coercion. |
| let principal_a = data_a.principal().unwrap(); |
| let target_trait_did = principal_def_id_b.unwrap(); |
| let source_trait_ref = principal_a.with_self_ty(self.tcx(), source); |
| if let Some(deref_trait_ref) = self.need_migrate_deref_output_trait_object( |
| source, |
| obligation.param_env, |
| &obligation.cause, |
| ) { |
| if deref_trait_ref.def_id() == target_trait_did { |
| return; |
| } |
| } |
| |
| for (idx, upcast_trait_ref) in |
| util::supertraits(self.tcx(), source_trait_ref).enumerate() |
| { |
| if upcast_trait_ref.def_id() == target_trait_did { |
| candidates.vec.push(TraitUpcastingUnsizeCandidate(idx)); |
| } |
| } |
| } |
| } |
| } |
| |
| // `T` -> `Trait` |
| (_, &ty::Dynamic(_, _, ty::Dyn)) => { |
| candidates.vec.push(BuiltinUnsizeCandidate); |
| } |
| |
| // Ambiguous handling is below `T` -> `Trait`, because inference |
| // variables can still implement `Unsize<Trait>` and nested |
| // obligations will have the final say (likely deferred). |
| (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => { |
| debug!("assemble_candidates_for_unsizing: ambiguous"); |
| candidates.ambiguous = true; |
| } |
| |
| // `[T; n]` -> `[T]` |
| (&ty::Array(..), &ty::Slice(_)) => { |
| candidates.vec.push(BuiltinUnsizeCandidate); |
| } |
| |
| // `Struct<T>` -> `Struct<U>` |
| (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => { |
| if def_id_a == def_id_b { |
| candidates.vec.push(BuiltinUnsizeCandidate); |
| } |
| } |
| |
| // `(.., T)` -> `(.., U)` |
| (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => { |
| if tys_a.len() == tys_b.len() { |
| candidates.vec.push(BuiltinUnsizeCandidate); |
| } |
| } |
| |
| _ => {} |
| }; |
| } |
| |
| #[instrument(level = "debug", skip(self, obligation, candidates))] |
| fn assemble_candidates_for_transmutability( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| if obligation.predicate.has_non_region_param() { |
| return; |
| } |
| |
| if obligation.has_non_region_infer() { |
| candidates.ambiguous = true; |
| return; |
| } |
| |
| candidates.vec.push(TransmutabilityCandidate); |
| } |
| |
| #[instrument(level = "debug", skip(self, obligation, candidates))] |
| fn assemble_candidates_for_trait_alias( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // Okay to skip binder here because the tests we do below do not involve bound regions. |
| let self_ty = obligation.self_ty().skip_binder(); |
| debug!(?self_ty); |
| |
| let def_id = obligation.predicate.def_id(); |
| |
| if self.tcx().is_trait_alias(def_id) { |
| candidates.vec.push(TraitAliasCandidate); |
| } |
| } |
| |
| /// Assembles the trait which are built-in to the language itself: |
| /// `Copy`, `Clone` and `Sized`. |
| #[instrument(level = "debug", skip(self, candidates))] |
| fn assemble_builtin_bound_candidates( |
| &mut self, |
| conditions: BuiltinImplConditions<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| match conditions { |
| BuiltinImplConditions::Where(nested) => { |
| candidates |
| .vec |
| .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() }); |
| } |
| BuiltinImplConditions::None => {} |
| BuiltinImplConditions::Ambiguous => { |
| candidates.ambiguous = true; |
| } |
| } |
| } |
| |
| fn assemble_const_destruct_candidates( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // If the predicate is `~const Destruct` in a non-const environment, we don't actually need |
| // to check anything. We'll short-circuit checking any obligations in confirmation, too. |
| if !obligation.is_const() { |
| candidates.vec.push(ConstDestructCandidate(None)); |
| return; |
| } |
| |
| let self_ty = self.infcx.shallow_resolve(obligation.self_ty()); |
| match self_ty.skip_binder().kind() { |
| ty::Alias(..) |
| | ty::Dynamic(..) |
| | ty::Error(_) |
| | ty::Bound(..) |
| | ty::Param(_) |
| | ty::Placeholder(_) => { |
| // We don't know if these are `~const Destruct`, at least |
| // not structurally... so don't push a candidate. |
| } |
| |
| ty::Bool |
| | ty::Char |
| | ty::Int(_) |
| | ty::Uint(_) |
| | ty::Float(_) |
| | ty::Infer(ty::IntVar(_)) |
| | ty::Infer(ty::FloatVar(_)) |
| | ty::Str |
| | ty::RawPtr(_) |
| | ty::Ref(..) |
| | ty::FnDef(..) |
| | ty::FnPtr(_) |
| | ty::Never |
| | ty::Foreign(_) |
| | ty::Array(..) |
| | ty::Slice(_) |
| | ty::Closure(..) |
| | ty::Generator(..) |
| | ty::Tuple(_) |
| | ty::GeneratorWitness(_) |
| | ty::GeneratorWitnessMIR(..) => { |
| // These are built-in, and cannot have a custom `impl const Destruct`. |
| candidates.vec.push(ConstDestructCandidate(None)); |
| } |
| |
| ty::Adt(..) => { |
| let mut relevant_impl = None; |
| self.tcx().for_each_relevant_impl( |
| self.tcx().require_lang_item(LangItem::Drop, None), |
| obligation.predicate.skip_binder().trait_ref.self_ty(), |
| |impl_def_id| { |
| if let Some(old_impl_def_id) = relevant_impl { |
| self.tcx() |
| .sess |
| .struct_span_err( |
| self.tcx().def_span(impl_def_id), |
| "multiple drop impls found", |
| ) |
| .span_note(self.tcx().def_span(old_impl_def_id), "other impl here") |
| .delay_as_bug(); |
| } |
| |
| relevant_impl = Some(impl_def_id); |
| }, |
| ); |
| |
| if let Some(impl_def_id) = relevant_impl { |
| // Check that `impl Drop` is actually const, if there is a custom impl |
| if self.tcx().constness(impl_def_id) == hir::Constness::Const { |
| candidates.vec.push(ConstDestructCandidate(Some(impl_def_id))); |
| } |
| } else { |
| // Otherwise check the ADT like a built-in type (structurally) |
| candidates.vec.push(ConstDestructCandidate(None)); |
| } |
| } |
| |
| ty::Infer(_) => { |
| candidates.ambiguous = true; |
| } |
| } |
| } |
| |
| fn assemble_candidate_for_tuple( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder()); |
| match self_ty.kind() { |
| ty::Tuple(_) => { |
| candidates.vec.push(BuiltinCandidate { has_nested: false }); |
| } |
| ty::Infer(ty::TyVar(_)) => { |
| candidates.ambiguous = true; |
| } |
| ty::Bool |
| | ty::Char |
| | ty::Int(_) |
| | ty::Uint(_) |
| | ty::Float(_) |
| | ty::Adt(_, _) |
| | ty::Foreign(_) |
| | ty::Str |
| | ty::Array(_, _) |
| | ty::Slice(_) |
| | ty::RawPtr(_) |
| | ty::Ref(_, _, _) |
| | ty::FnDef(_, _) |
| | ty::FnPtr(_) |
| | ty::Dynamic(_, _, _) |
| | ty::Closure(_, _) |
| | ty::Generator(_, _, _) |
| | ty::GeneratorWitness(_) |
| | ty::GeneratorWitnessMIR(..) |
| | ty::Never |
| | ty::Alias(..) |
| | ty::Param(_) |
| | ty::Bound(_, _) |
| | ty::Error(_) |
| | ty::Infer(_) |
| | ty::Placeholder(_) => {} |
| } |
| } |
| |
| fn assemble_candidate_for_pointer_like( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| // The regions of a type don't affect the size of the type |
| let tcx = self.tcx(); |
| let self_ty = tcx.erase_late_bound_regions(obligation.predicate.self_ty()); |
| // We should erase regions from both the param-env and type, since both |
| // may have infer regions. Specifically, after canonicalizing and instantiating, |
| // early bound regions turn into region vars in both the new and old solver. |
| let key = tcx.erase_regions(obligation.param_env.and(self_ty)); |
| // But if there are inference variables, we have to wait until it's resolved. |
| if key.has_non_region_infer() { |
| candidates.ambiguous = true; |
| return; |
| } |
| |
| if let Ok(layout) = tcx.layout_of(key) |
| && layout.layout.is_pointer_like(&tcx.data_layout) |
| { |
| candidates.vec.push(BuiltinCandidate { has_nested: false }); |
| } |
| } |
| |
| fn assemble_candidates_for_fn_ptr_trait( |
| &mut self, |
| obligation: &TraitObligation<'tcx>, |
| candidates: &mut SelectionCandidateSet<'tcx>, |
| ) { |
| let self_ty = self.infcx.shallow_resolve(obligation.self_ty()); |
| match self_ty.skip_binder().kind() { |
| ty::FnPtr(_) => candidates.vec.push(BuiltinCandidate { has_nested: false }), |
| ty::Bool |
| | ty::Char |
| | ty::Int(_) |
| | ty::Uint(_) |
| | ty::Float(_) |
| | ty::Adt(..) |
| | ty::Foreign(..) |
| | ty::Str |
| | ty::Array(..) |
| | ty::Slice(_) |
| | ty::RawPtr(_) |
| | ty::Ref(..) |
| | ty::FnDef(..) |
| | ty::Placeholder(..) |
| | ty::Dynamic(..) |
| | ty::Closure(..) |
| | ty::Generator(..) |
| | ty::GeneratorWitness(..) |
| | ty::GeneratorWitnessMIR(..) |
| | ty::Never |
| | ty::Tuple(..) |
| | ty::Alias(..) |
| | ty::Param(..) |
| | ty::Bound(..) |
| | ty::Error(_) |
| | ty::Infer( |
| ty::InferTy::IntVar(_) |
| | ty::InferTy::FloatVar(_) |
| | ty::InferTy::FreshIntTy(_) |
| | ty::InferTy::FreshFloatTy(_), |
| ) => {} |
| ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)) => { |
| candidates.ambiguous = true; |
| } |
| } |
| } |
| } |
