//! 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::LangItem; use rustc_hir as hir; use rustc_hir::def_id::DefId; use rustc_infer::traits::TraitEngine; use rustc_infer::traits::{Obligation, SelectionError, TraitObligation}; use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT; use rustc_middle::ty::print::with_no_trimmed_paths; use rustc_middle::ty::{self, ToPredicate, Ty, TypeVisitable}; use rustc_target::spec::abi::Abi; use crate::traits; use crate::traits::coherence::Conflict; use crate::traits::query::evaluate_obligation::InferCtxtExt; use crate::traits::{util, SelectionResult}; use crate::traits::{Ambiguous, ErrorReporting, Overflow, Unimplemented}; use super::BuiltinImplConditions; use super::IntercrateAmbiguityCause; use super::OverflowError; use super::SelectionCandidate::{self, *}; use super::{EvaluatedCandidate, SelectionCandidateSet, SelectionContext, TraitObligationStack}; impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { #[instrument(level = "debug", skip(self))] pub(super) fn candidate_from_obligation<'o>( &mut self, stack: &TraitObligationStack<'o, 'tcx>, ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { // Watch out for overflow. This intentionally bypasses (and does // not update) the cache. self.check_recursion_limit(&stack.obligation, &stack.obligation)?; // Check the cache. Note that we freshen the trait-ref // separately rather than using `stack.fresh_trait_ref` -- // this is because we want the unbound variables to be // replaced with fresh types starting from index 0. let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate); debug!(?cache_fresh_trait_pred); debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars()); if let Some(c) = self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred) { debug!(candidate = ?c, "CACHE HIT"); return c; } // If no match, compute result and insert into cache. // // FIXME(nikomatsakis) -- this cache is not taking into // account cycles that may have occurred in forming the // candidate. I don't know of any specific problems that // result but it seems awfully suspicious. let (candidate, dep_node) = self.in_task(|this| this.candidate_from_obligation_no_cache(stack)); debug!(?candidate, "CACHE MISS"); self.insert_candidate_cache( stack.obligation.param_env, cache_fresh_trait_pred, dep_node, candidate.clone(), ); candidate } fn candidate_from_obligation_no_cache<'o>( &mut self, stack: &TraitObligationStack<'o, 'tcx>, ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { if let Some(conflict) = self.is_knowable(stack) { debug!("coherence stage: not knowable"); if self.intercrate_ambiguity_causes.is_some() { debug!("evaluate_stack: intercrate_ambiguity_causes is some"); // Heuristics: show the diagnostics when there are no candidates in crate. if let Ok(candidate_set) = self.assemble_candidates(stack) { let mut no_candidates_apply = true; for c in candidate_set.vec.iter() { if self.evaluate_candidate(stack, &c)?.may_apply() { no_candidates_apply = false; break; } } if !candidate_set.ambiguous && no_candidates_apply { let trait_ref = stack.obligation.predicate.skip_binder().trait_ref; let self_ty = trait_ref.self_ty(); let (trait_desc, self_desc) = with_no_trimmed_paths!({ let trait_desc = trait_ref.print_only_trait_path().to_string(); let self_desc = if self_ty.has_concrete_skeleton() { Some(self_ty.to_string()) } else { None }; (trait_desc, self_desc) }); let cause = if let Conflict::Upstream = conflict { IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } } else { IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } }; debug!(?cause, "evaluate_stack: pushing cause"); self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause); } } } return Ok(None); } let candidate_set = self.assemble_candidates(stack)?; if candidate_set.ambiguous { debug!("candidate set contains ambig"); return Ok(None); } let candidates = candidate_set.vec; debug!(?stack, ?candidates, "assembled {} candidates", candidates.len()); // At this point, we know that each of the entries in the // candidate set is *individually* applicable. Now we have to // figure out if they contain mutual incompatibilities. This // frequently arises if we have an unconstrained input type -- // for example, we are looking for `$0: Eq` where `$0` is some // unconstrained type variable. In that case, we'll get a // candidate which assumes $0 == int, one that assumes `$0 == // usize`, etc. This spells an ambiguity. let mut candidates = self.filter_impls(candidates, stack.obligation); // If there is more than one candidate, first winnow them down // by considering extra conditions (nested obligations and so // forth). We don't winnow if there is exactly one // candidate. This is a relatively minor distinction but it // can lead to better inference and error-reporting. An // example would be if there was an impl: // // impl Vec { fn push_clone(...) { ... } } // // and we were to see some code `foo.push_clone()` where `boo` // is a `Vec` and `Bar` does not implement `Clone`. If // we were to winnow, we'd wind up with zero candidates. // Instead, we select the right impl now but report "`Bar` does // not implement `Clone`". if candidates.len() == 1 { return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation); } // Winnow, but record the exact outcome of evaluation, which // is needed for specialization. Propagate overflow if it occurs. let mut candidates = candidates .into_iter() .map(|c| match self.evaluate_candidate(stack, &c) { Ok(eval) if eval.may_apply() => { Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval })) } Ok(_) => Ok(None), Err(OverflowError::Canonical) => Err(Overflow(OverflowError::Canonical)), Err(OverflowError::ErrorReporting) => Err(ErrorReporting), Err(OverflowError::Error(e)) => Err(Overflow(OverflowError::Error(e))), }) .flat_map(Result::transpose) .collect::, _>>()?; debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len()); let needs_infer = stack.obligation.predicate.has_infer_types_or_consts(); // If there are STILL multiple candidates, we can further // reduce the list by dropping duplicates -- including // resolving specializations. if candidates.len() > 1 { let mut i = 0; while i < candidates.len() { let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| { self.candidate_should_be_dropped_in_favor_of( &candidates[i], &candidates[j], needs_infer, ) }); if is_dup { debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len()); candidates.swap_remove(i); } else { debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len()); i += 1; // If there are *STILL* multiple candidates, give up // and report ambiguity. if i > 1 { debug!("multiple matches, ambig"); return Err(Ambiguous( candidates .into_iter() .filter_map(|c| match c.candidate { SelectionCandidate::ImplCandidate(def_id) => Some(def_id), _ => None, }) .collect(), )); } } } } // If there are *NO* candidates, then there are no impls -- // that we know of, anyway. Note that in the case where there // are unbound type variables within the obligation, it might // be the case that you could still satisfy the obligation // from another crate by instantiating the type variables with // a type from another crate that does have an impl. This case // is checked for in `evaluate_stack` (and hence users // who might care about this case, like coherence, should use // that function). if candidates.is_empty() { // If there's an error type, 'downgrade' our result from // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid // emitting additional spurious errors, since we're guaranteed // to have emitted at least one. if stack.obligation.predicate.references_error() { debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous"); return Ok(None); } return Err(Unimplemented); } // Just one candidate left. self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation) } #[instrument(skip(self, stack), level = "debug")] pub(super) fn assemble_candidates<'o>( &mut self, stack: &TraitObligationStack<'o, 'tcx>, ) -> Result, 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`). // // 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); } 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(DiscriminantKindCandidate); } else if lang_items.pointee_trait() == Some(def_id) { // `Pointee` is automatically implemented for every type. candidates.vec.push(PointeeCandidate); } 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.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); } self.assemble_generator_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) } #[tracing::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() { 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(ProjectionCandidate)); } /// Given an obligation like ``, searches the obligations that the caller /// supplied to find out whether it is listed among them. /// /// Never affects the inference environment. #[tracing::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_map(|o| o.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 { // 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>, ) { if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) { return; } // 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() { ty::Generator(..) => { 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; } _ => {} } } /// 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_lang_item(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(_, closure_substs) => { 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); } } None => { debug!("assemble_unboxed_candidates: closure_kind not yet known"); candidates.vec.push(ClosureCandidate); } } } 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().fn_trait_kind_from_lang_item(obligation.predicate.def_id()).is_none() { return; } // 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(_) => { if let ty::FnSig { unsafety: hir::Unsafety::Normal, abi: Abi::Rust, c_variadic: false, .. } = self_ty.fn_sig(self.tcx()).skip_binder() { candidates.vec.push(FnPointerCandidate { is_const: false }); } } // Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396). ty::FnDef(def_id, _) => { if let ty::FnSig { unsafety: hir::Unsafety::Normal, abi: Abi::Rust, c_variadic: false, .. } = self_ty.fn_sig(self.tcx()).skip_binder() { if 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`. fn assemble_candidates_from_impls( &mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>, ) { debug!(?obligation, "assemble_candidates_from_impls"); // 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; } 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().bound_impl_trait_ref(impl_def_id).unwrap(); if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) { 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)); } }); }, ); } 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::Projection(..) => { // 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(def_id)), } } } /// 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| { // 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().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().replace_bound_vars_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`, // but `Foo` is declared as `trait Foo: Bar`. 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>, cause: &traits::ObligationCause<'tcx>, param_env: ty::ParamEnv<'tcx>, ) -> Option<(Ty<'tcx>, DefId)> { let tcx = self.tcx(); if tcx.features().trait_upcasting { return None; } // let trait_ref = ty::TraitRef { def_id: tcx.lang_items().deref_trait()?, substs: tcx.mk_substs_trait(ty, &[]), }; let obligation = traits::Obligation::new( cause.clone(), param_env, ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx), ); if !self.infcx.predicate_may_hold(&obligation) { return None; } let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot(); let normalized_ty = fulfillcx.normalize_projection_type( &self.infcx, param_env, ty::ProjectionTy { item_def_id: tcx.lang_items().deref_target()?, substs: trait_ref.substs, }, cause.clone(), ); let ty::Dynamic(data, ..) = normalized_ty.kind() else { return None; }; let def_id = data.principal_def_id()?; return Some((normalized_ty, def_id)); } /// 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` 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` obligation is always used as part of a `T: CoerceUnsize` // 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> // 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::Dynamic(ref data_b, ..)) => { // 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_output_ty, deref_output_trait_did)) = self .need_migrate_deref_output_trait_object( source, &obligation.cause, obligation.param_env, ) { if deref_output_trait_did == target_trait_did { self.tcx().struct_span_lint_hir( DEREF_INTO_DYN_SUPERTRAIT, obligation.cause.body_id, obligation.cause.span, |lint| { lint.build(&format!( "`{}` implements `Deref` with supertrait `{}` as output", source, deref_output_ty )).emit(); }, ); 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(..)) => { candidates.vec.push(BuiltinUnsizeCandidate); } // Ambiguous handling is below `T` -> `Trait`, because inference // variables can still implement `Unsize` 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` -> `Struct` (&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); } } _ => {} }; } #[tracing::instrument(level = "debug", skip(self, obligation, candidates))] fn assemble_candidates_for_transmutability( &mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>, ) { if obligation.has_param_types_or_consts() { return; } if obligation.has_infer_types_or_consts() { candidates.ambiguous = true; return; } candidates.vec.push(TransmutabilityCandidate); } #[tracing::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(def_id)); } } /// Assembles the trait which are built-in to the language itself: /// `Copy`, `Clone` and `Sized`. #[tracing::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::Opaque(..) | ty::Dynamic(..) | ty::Error(_) | ty::Bound(..) | ty::Param(_) | ty::Placeholder(_) | ty::Projection(_) => { // 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(_) => { // These are built-in, and cannot have a custom `impl const Destruct`. candidates.vec.push(ConstDestructCandidate(None)); } ty::Adt(..) => { // Find a custom `impl Drop` impl, if it exists let relevant_impl = self.tcx().find_map_relevant_impl( self.tcx().require_lang_item(LangItem::Drop, None), obligation.predicate.skip_binder().trait_ref.self_ty(), Some, ); 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; } } } }