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Diffstat (limited to 'compiler/rustc_trait_selection/src/traits/select')
3 files changed, 3881 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs new file mode 100644 index 00000000000..1d5441b8eff --- /dev/null +++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs @@ -0,0 +1,635 @@ +//! 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 rustc_hir as hir; +use rustc_infer::traits::{Obligation, SelectionError, TraitObligation}; +use rustc_middle::ty::{self, TypeFoldable}; +use rustc_target::spec::abi::Abi; + +use crate::traits::{util, SelectionResult}; + +use super::BuiltinImplConditions; +use super::SelectionCandidate::{self, *}; +use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack}; + +impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { + 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!( + "candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})", + cache_fresh_trait_pred, stack + ); + 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!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c); + 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!("CACHE MISS: SELECT({:?})={:?}", cache_fresh_trait_pred, candidate); + self.insert_candidate_cache( + stack.obligation.param_env, + cache_fresh_trait_pred, + dep_node, + candidate.clone(), + ); + candidate + } + + 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() { + // 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 }; + + 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 is {:?}", 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.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.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) + } + + fn assemble_candidates_from_projected_tys( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) { + debug!("assemble_candidates_for_projected_tys({:?})", obligation); + + // 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)); + + if result { + candidates.vec.push(ProjectionCandidate); + } + } + + /// 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. + fn assemble_candidates_from_caller_bounds<'o>( + &mut self, + stack: &TraitObligationStack<'o, 'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'tcx>> { + debug!("assemble_candidates_from_caller_bounds({:?})", stack.obligation); + + let all_bounds = stack + .obligation + .param_env + .caller_bounds() + .iter() + .filter_map(|o| o.to_opt_poly_trait_ref()); + + // 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. + let mut param_candidates = vec![]; + for bound in matching_bounds { + let wc = self.evaluate_where_clause(stack, bound)?; + if wc.may_apply() { + param_candidates.push(ParamCandidate(bound)); + } + } + + candidates.vec.extend(param_candidates); + + Ok(()) + } + + fn assemble_generator_candidates( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'tcx>> { + if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) { + return Ok(()); + } + + // 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!( + "assemble_generator_candidates: self_ty={:?} obligation={:?}", + self_ty, obligation + ); + + candidates.vec.push(GeneratorCandidate); + } + ty::Infer(ty::TyVar(_)) => { + debug!("assemble_generator_candidates: ambiguous self-type"); + candidates.ambiguous = true; + } + _ => {} + } + + Ok(()) + } + + /// 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>, + ) -> Result<(), SelectionError<'tcx>> { + let kind = match self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) { + Some(k) => k, + None => { + return Ok(()); + } + }; + + // 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!("assemble_unboxed_candidates: kind={:?} obligation={:?}", kind, obligation); + match self.infcx.closure_kind(closure_substs) { + Some(closure_kind) => { + debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind); + 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; + } + _ => {} + } + + Ok(()) + } + + /// Implements one of the `Fn()` family for a fn pointer. + fn assemble_fn_pointer_candidates( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'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 Ok(()); + } + + // 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); + } + } + // 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); + } + } + } + _ => {} + } + + Ok(()) + } + + /// Searches for impls that might apply to `obligation`. + fn assemble_candidates_from_impls( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'tcx>> { + debug!("assemble_candidates_from_impls(obligation={:?})", obligation); + + // 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. + if obligation.references_error() { + return Ok(()); + } + + self.tcx().for_each_relevant_impl( + obligation.predicate.def_id(), + obligation.predicate.skip_binder().trait_ref.self_ty(), + |impl_def_id| { + self.infcx.probe(|_| { + if let Ok(_substs) = self.match_impl(impl_def_id, obligation) { + candidates.vec.push(ImplCandidate(impl_def_id)); + } + }); + }, + ); + + Ok(()) + } + + fn assemble_candidates_from_auto_impls( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'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!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty); + + 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 + // `compile-fail/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)), + } + } + + Ok(()) + } + + /// Searches for impls that might apply to `obligation`. + fn assemble_candidates_from_object_ty( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) { + debug!( + "assemble_candidates_from_object_ty(self_ty={:?})", + obligation.self_ty().skip_binder() + ); + + 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!("assemble_candidates_from_object_ty: poly_trait_ref={:?}", poly_trait_ref); + + // 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 upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref) + .filter(|upcast_trait_ref| { + self.infcx + .probe(|_| self.match_poly_trait_ref(obligation, *upcast_trait_ref).is_ok()) + }) + .count(); + + if upcast_trait_refs > 1 { + // Can be upcast in many ways; need more type information. + candidates.ambiguous = true; + } else if upcast_trait_refs == 1 { + candidates.vec.push(ObjectCandidate); + } + }) + } + + /// 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 source = match obligation.self_ty().no_bound_vars() { + Some(t) => t, + None => { + // Don't add any candidates if there are bound regions. + return; + } + }; + let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1); + + debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})", source, target); + + let may_apply = match (&source.kind, &target.kind) { + // Trait+Kx+'a -> Trait+Ky+'b (upcasts). + (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => { + // Upcasts permit two 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` + // + // Note that neither of these changes requires any + // change at runtime. Eventually this will be + // generalized. + // + // We always upcast when we can because of reason + // #2 (region bounds). + data_a.principal_def_id() == data_b.principal_def_id() + && 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)) + } + + // `T` -> `Trait` + (_, &ty::Dynamic(..)) => true, + + // 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; + false + } + + // `[T; n]` -> `[T]` + (&ty::Array(..), &ty::Slice(_)) => true, + + // `Struct<T>` -> `Struct<U>` + (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => { + def_id_a == def_id_b + } + + // `(.., T)` -> `(.., U)` + (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(), + + _ => false, + }; + + if may_apply { + candidates.vec.push(BuiltinUnsizeCandidate); + } + } + + fn assemble_candidates_for_trait_alias( + &mut self, + obligation: &TraitObligation<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'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!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty); + + let def_id = obligation.predicate.def_id(); + + if self.tcx().is_trait_alias(def_id) { + candidates.vec.push(TraitAliasCandidate(def_id)); + } + + Ok(()) + } + + /// Assembles the trait which are built-in to the language itself: + /// `Copy`, `Clone` and `Sized`. + fn assemble_builtin_bound_candidates( + &mut self, + conditions: BuiltinImplConditions<'tcx>, + candidates: &mut SelectionCandidateSet<'tcx>, + ) -> Result<(), SelectionError<'tcx>> { + match conditions { + BuiltinImplConditions::Where(nested) => { + debug!("builtin_bound: nested={:?}", nested); + candidates + .vec + .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() }); + } + BuiltinImplConditions::None => {} + BuiltinImplConditions::Ambiguous => { + debug!("assemble_builtin_bound_candidates: ambiguous builtin"); + candidates.ambiguous = true; + } + } + + Ok(()) + } +} diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs new file mode 100644 index 00000000000..3d6eb845136 --- /dev/null +++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs @@ -0,0 +1,810 @@ +//! Confirmation. +//! +//! Confirmation unifies the output type parameters of the trait +//! with the values found in the obligation, possibly yielding a +//! type error. See the [rustc dev guide] for more details. +//! +//! [rustc dev guide]: +//! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation +use rustc_data_structures::stack::ensure_sufficient_stack; +use rustc_hir::lang_items::LangItem; +use rustc_index::bit_set::GrowableBitSet; +use rustc_infer::infer::InferOk; +use rustc_middle::ty::subst::{GenericArg, GenericArgKind, Subst, SubstsRef}; +use rustc_middle::ty::{self, Ty}; +use rustc_middle::ty::{ToPolyTraitRef, ToPredicate, WithConstness}; +use rustc_span::def_id::DefId; + +use crate::traits::project::{self, normalize_with_depth}; +use crate::traits::select::TraitObligationExt; +use crate::traits::util; +use crate::traits::util::{closure_trait_ref_and_return_type, predicate_for_trait_def}; +use crate::traits::Normalized; +use crate::traits::OutputTypeParameterMismatch; +use crate::traits::Selection; +use crate::traits::TraitNotObjectSafe; +use crate::traits::{BuiltinDerivedObligation, ImplDerivedObligation}; +use crate::traits::{ + ImplSourceAutoImpl, ImplSourceBuiltin, ImplSourceClosure, ImplSourceDiscriminantKind, + ImplSourceFnPointer, ImplSourceGenerator, ImplSourceObject, ImplSourceParam, + ImplSourceTraitAlias, ImplSourceUserDefined, +}; +use crate::traits::{ + ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData, + ImplSourceDiscriminantKindData, ImplSourceFnPointerData, ImplSourceGeneratorData, + ImplSourceObjectData, ImplSourceTraitAliasData, ImplSourceUserDefinedData, +}; +use crate::traits::{ObjectCastObligation, PredicateObligation, TraitObligation}; +use crate::traits::{Obligation, ObligationCause}; +use crate::traits::{SelectionError, Unimplemented}; + +use super::BuiltinImplConditions; +use super::SelectionCandidate::{self, *}; +use super::SelectionContext; + +use std::iter; + +impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { + pub(super) fn confirm_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + candidate: SelectionCandidate<'tcx>, + ) -> Result<Selection<'tcx>, SelectionError<'tcx>> { + debug!("confirm_candidate({:?}, {:?})", obligation, candidate); + + match candidate { + BuiltinCandidate { has_nested } => { + let data = self.confirm_builtin_candidate(obligation, has_nested); + Ok(ImplSourceBuiltin(data)) + } + + ParamCandidate(param) => { + let obligations = self.confirm_param_candidate(obligation, param); + Ok(ImplSourceParam(obligations)) + } + + ImplCandidate(impl_def_id) => { + Ok(ImplSourceUserDefined(self.confirm_impl_candidate(obligation, impl_def_id))) + } + + AutoImplCandidate(trait_def_id) => { + let data = self.confirm_auto_impl_candidate(obligation, trait_def_id); + Ok(ImplSourceAutoImpl(data)) + } + + ProjectionCandidate => { + self.confirm_projection_candidate(obligation); + Ok(ImplSourceParam(Vec::new())) + } + + ClosureCandidate => { + let vtable_closure = self.confirm_closure_candidate(obligation)?; + Ok(ImplSourceClosure(vtable_closure)) + } + + GeneratorCandidate => { + let vtable_generator = self.confirm_generator_candidate(obligation)?; + Ok(ImplSourceGenerator(vtable_generator)) + } + + FnPointerCandidate => { + let data = self.confirm_fn_pointer_candidate(obligation)?; + Ok(ImplSourceFnPointer(data)) + } + + DiscriminantKindCandidate => { + Ok(ImplSourceDiscriminantKind(ImplSourceDiscriminantKindData)) + } + + TraitAliasCandidate(alias_def_id) => { + let data = self.confirm_trait_alias_candidate(obligation, alias_def_id); + Ok(ImplSourceTraitAlias(data)) + } + + ObjectCandidate => { + let data = self.confirm_object_candidate(obligation); + Ok(ImplSourceObject(data)) + } + + BuiltinObjectCandidate => { + // This indicates something like `Trait + Send: Send`. In this case, we know that + // this holds because that's what the object type is telling us, and there's really + // no additional obligations to prove and no types in particular to unify, etc. + Ok(ImplSourceParam(Vec::new())) + } + + BuiltinUnsizeCandidate => { + let data = self.confirm_builtin_unsize_candidate(obligation)?; + Ok(ImplSourceBuiltin(data)) + } + } + } + + fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) { + self.infcx.commit_unconditionally(|_| { + let result = self.match_projection_obligation_against_definition_bounds(obligation); + assert!(result); + }) + } + + fn confirm_param_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + param: ty::PolyTraitRef<'tcx>, + ) -> Vec<PredicateObligation<'tcx>> { + debug!("confirm_param_candidate({:?},{:?})", obligation, param); + + // During evaluation, we already checked that this + // where-clause trait-ref could be unified with the obligation + // trait-ref. Repeat that unification now without any + // transactional boundary; it should not fail. + match self.match_where_clause_trait_ref(obligation, param) { + Ok(obligations) => obligations, + Err(()) => { + bug!( + "Where clause `{:?}` was applicable to `{:?}` but now is not", + param, + obligation + ); + } + } + } + + fn confirm_builtin_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + has_nested: bool, + ) -> ImplSourceBuiltinData<PredicateObligation<'tcx>> { + debug!("confirm_builtin_candidate({:?}, {:?})", obligation, has_nested); + + let lang_items = self.tcx().lang_items(); + let obligations = if has_nested { + let trait_def = obligation.predicate.def_id(); + let conditions = if Some(trait_def) == lang_items.sized_trait() { + self.sized_conditions(obligation) + } else if Some(trait_def) == lang_items.copy_trait() { + self.copy_clone_conditions(obligation) + } else if Some(trait_def) == lang_items.clone_trait() { + self.copy_clone_conditions(obligation) + } else { + bug!("unexpected builtin trait {:?}", trait_def) + }; + let nested = match conditions { + BuiltinImplConditions::Where(nested) => nested, + _ => bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation), + }; + + let cause = obligation.derived_cause(BuiltinDerivedObligation); + ensure_sufficient_stack(|| { + self.collect_predicates_for_types( + obligation.param_env, + cause, + obligation.recursion_depth + 1, + trait_def, + nested, + ) + }) + } else { + vec![] + }; + + debug!("confirm_builtin_candidate: obligations={:?}", obligations); + + ImplSourceBuiltinData { nested: obligations } + } + + /// This handles the case where a `auto trait Foo` impl is being used. + /// The idea is that the impl applies to `X : Foo` if the following conditions are met: + /// + /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds + /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds. + fn confirm_auto_impl_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + trait_def_id: DefId, + ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> { + debug!("confirm_auto_impl_candidate({:?}, {:?})", obligation, trait_def_id); + + let types = obligation.predicate.map_bound(|inner| { + let self_ty = self.infcx.shallow_resolve(inner.self_ty()); + self.constituent_types_for_ty(self_ty) + }); + self.vtable_auto_impl(obligation, trait_def_id, types) + } + + /// See `confirm_auto_impl_candidate`. + fn vtable_auto_impl( + &mut self, + obligation: &TraitObligation<'tcx>, + trait_def_id: DefId, + nested: ty::Binder<Vec<Ty<'tcx>>>, + ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> { + debug!("vtable_auto_impl: nested={:?}", nested); + ensure_sufficient_stack(|| { + let cause = obligation.derived_cause(BuiltinDerivedObligation); + let mut obligations = self.collect_predicates_for_types( + obligation.param_env, + cause, + obligation.recursion_depth + 1, + trait_def_id, + nested, + ); + + let trait_obligations: Vec<PredicateObligation<'_>> = + self.infcx.commit_unconditionally(|_| { + let poly_trait_ref = obligation.predicate.to_poly_trait_ref(); + let (trait_ref, _) = + self.infcx.replace_bound_vars_with_placeholders(&poly_trait_ref); + let cause = obligation.derived_cause(ImplDerivedObligation); + self.impl_or_trait_obligations( + cause, + obligation.recursion_depth + 1, + obligation.param_env, + trait_def_id, + &trait_ref.substs, + ) + }); + + // Adds the predicates from the trait. Note that this contains a `Self: Trait` + // predicate as usual. It won't have any effect since auto traits are coinductive. + obligations.extend(trait_obligations); + + debug!("vtable_auto_impl: obligations={:?}", obligations); + + ImplSourceAutoImplData { trait_def_id, nested: obligations } + }) + } + + fn confirm_impl_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + impl_def_id: DefId, + ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> { + debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id); + + // First, create the substitutions by matching the impl again, + // this time not in a probe. + self.infcx.commit_unconditionally(|_| { + let substs = self.rematch_impl(impl_def_id, obligation); + debug!("confirm_impl_candidate: substs={:?}", substs); + let cause = obligation.derived_cause(ImplDerivedObligation); + ensure_sufficient_stack(|| { + self.vtable_impl( + impl_def_id, + substs, + cause, + obligation.recursion_depth + 1, + obligation.param_env, + ) + }) + }) + } + + fn vtable_impl( + &mut self, + impl_def_id: DefId, + mut substs: Normalized<'tcx, SubstsRef<'tcx>>, + cause: ObligationCause<'tcx>, + recursion_depth: usize, + param_env: ty::ParamEnv<'tcx>, + ) -> ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>> { + debug!( + "vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={})", + impl_def_id, substs, recursion_depth, + ); + + let mut impl_obligations = self.impl_or_trait_obligations( + cause, + recursion_depth, + param_env, + impl_def_id, + &substs.value, + ); + + debug!( + "vtable_impl: impl_def_id={:?} impl_obligations={:?}", + impl_def_id, impl_obligations + ); + + // Because of RFC447, the impl-trait-ref and obligations + // are sufficient to determine the impl substs, without + // relying on projections in the impl-trait-ref. + // + // e.g., `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V` + impl_obligations.append(&mut substs.obligations); + + ImplSourceUserDefinedData { impl_def_id, substs: substs.value, nested: impl_obligations } + } + + fn confirm_object_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> ImplSourceObjectData<'tcx, PredicateObligation<'tcx>> { + debug!("confirm_object_candidate({:?})", obligation); + + // FIXME(nmatsakis) skipping binder here seems wrong -- we should + // probably flatten the binder from the obligation and the binder + // from the object. Have to try to make a broken test case that + // results. + let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder()); + let poly_trait_ref = match self_ty.kind { + ty::Dynamic(ref data, ..) => data + .principal() + .unwrap_or_else(|| { + span_bug!(obligation.cause.span, "object candidate with no principal") + }) + .with_self_ty(self.tcx(), self_ty), + _ => span_bug!(obligation.cause.span, "object candidate with non-object"), + }; + + let mut upcast_trait_ref = None; + let mut nested = vec![]; + let vtable_base; + + { + let tcx = self.tcx(); + + // We want to find the first supertrait in the list of + // supertraits that we can unify with, and do that + // unification. We know that there is exactly one in the list + // where we can unify, because otherwise select would have + // reported an ambiguity. (When we do find a match, also + // record it for later.) + let nonmatching = util::supertraits(tcx, poly_trait_ref).take_while(|&t| { + match self.infcx.commit_if_ok(|_| self.match_poly_trait_ref(obligation, t)) { + Ok(obligations) => { + upcast_trait_ref = Some(t); + nested.extend(obligations); + false + } + Err(_) => true, + } + }); + + // Additionally, for each of the non-matching predicates that + // we pass over, we sum up the set of number of vtable + // entries, so that we can compute the offset for the selected + // trait. + vtable_base = nonmatching.map(|t| super::util::count_own_vtable_entries(tcx, t)).sum(); + } + + ImplSourceObjectData { upcast_trait_ref: upcast_trait_ref.unwrap(), vtable_base, nested } + } + + fn confirm_fn_pointer_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> Result<ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> + { + debug!("confirm_fn_pointer_candidate({:?})", obligation); + + // Okay to skip binder; it is reintroduced below. + let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder()); + let sig = self_ty.fn_sig(self.tcx()); + let trait_ref = closure_trait_ref_and_return_type( + self.tcx(), + obligation.predicate.def_id(), + self_ty, + sig, + util::TupleArgumentsFlag::Yes, + ) + .map_bound(|(trait_ref, _)| trait_ref); + + let Normalized { value: trait_ref, obligations } = ensure_sufficient_stack(|| { + project::normalize_with_depth( + self, + obligation.param_env, + obligation.cause.clone(), + obligation.recursion_depth + 1, + &trait_ref, + ) + }); + + self.confirm_poly_trait_refs( + obligation.cause.clone(), + obligation.param_env, + obligation.predicate.to_poly_trait_ref(), + trait_ref, + )?; + Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested: obligations }) + } + + fn confirm_trait_alias_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + alias_def_id: DefId, + ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> { + debug!("confirm_trait_alias_candidate({:?}, {:?})", obligation, alias_def_id); + + self.infcx.commit_unconditionally(|_| { + let (predicate, _) = + self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate); + let trait_ref = predicate.trait_ref; + let trait_def_id = trait_ref.def_id; + let substs = trait_ref.substs; + + let trait_obligations = self.impl_or_trait_obligations( + obligation.cause.clone(), + obligation.recursion_depth, + obligation.param_env, + trait_def_id, + &substs, + ); + + debug!( + "confirm_trait_alias_candidate: trait_def_id={:?} trait_obligations={:?}", + trait_def_id, trait_obligations + ); + + ImplSourceTraitAliasData { alias_def_id, substs, nested: trait_obligations } + }) + } + + fn confirm_generator_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> Result<ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>, SelectionError<'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 = self.infcx.shallow_resolve(obligation.self_ty().skip_binder()); + let (generator_def_id, substs) = match self_ty.kind { + ty::Generator(id, substs, _) => (id, substs), + _ => bug!("closure candidate for non-closure {:?}", obligation), + }; + + debug!("confirm_generator_candidate({:?},{:?},{:?})", obligation, generator_def_id, substs); + + let trait_ref = self.generator_trait_ref_unnormalized(obligation, substs); + let Normalized { value: trait_ref, mut obligations } = ensure_sufficient_stack(|| { + normalize_with_depth( + self, + obligation.param_env, + obligation.cause.clone(), + obligation.recursion_depth + 1, + &trait_ref, + ) + }); + + debug!( + "confirm_generator_candidate(generator_def_id={:?}, \ + trait_ref={:?}, obligations={:?})", + generator_def_id, trait_ref, obligations + ); + + obligations.extend(self.confirm_poly_trait_refs( + obligation.cause.clone(), + obligation.param_env, + obligation.predicate.to_poly_trait_ref(), + trait_ref, + )?); + + Ok(ImplSourceGeneratorData { generator_def_id, substs, nested: obligations }) + } + + fn confirm_closure_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> Result<ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>> { + debug!("confirm_closure_candidate({:?})", obligation); + + let kind = self + .tcx() + .fn_trait_kind_from_lang_item(obligation.predicate.def_id()) + .unwrap_or_else(|| bug!("closure candidate for non-fn trait {:?}", obligation)); + + // Okay to skip binder because the substs on closure types never + // touch bound regions, they just capture the in-scope + // type/region parameters. + let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder()); + let (closure_def_id, substs) = match self_ty.kind { + ty::Closure(id, substs) => (id, substs), + _ => bug!("closure candidate for non-closure {:?}", obligation), + }; + + let trait_ref = self.closure_trait_ref_unnormalized(obligation, substs); + let Normalized { value: trait_ref, mut obligations } = ensure_sufficient_stack(|| { + normalize_with_depth( + self, + obligation.param_env, + obligation.cause.clone(), + obligation.recursion_depth + 1, + &trait_ref, + ) + }); + + debug!( + "confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})", + closure_def_id, trait_ref, obligations + ); + + obligations.extend(self.confirm_poly_trait_refs( + obligation.cause.clone(), + obligation.param_env, + obligation.predicate.to_poly_trait_ref(), + trait_ref, + )?); + + // FIXME: Chalk + + if !self.tcx().sess.opts.debugging_opts.chalk { + obligations.push(Obligation::new( + obligation.cause.clone(), + obligation.param_env, + ty::PredicateAtom::ClosureKind(closure_def_id, substs, kind) + .to_predicate(self.tcx()), + )); + } + + Ok(ImplSourceClosureData { closure_def_id, substs, nested: obligations }) + } + + /// In the case of closure types and fn pointers, + /// we currently treat the input type parameters on the trait as + /// outputs. This means that when we have a match we have only + /// considered the self type, so we have to go back and make sure + /// to relate the argument types too. This is kind of wrong, but + /// since we control the full set of impls, also not that wrong, + /// and it DOES yield better error messages (since we don't report + /// errors as if there is no applicable impl, but rather report + /// errors are about mismatched argument types. + /// + /// Here is an example. Imagine we have a closure expression + /// and we desugared it so that the type of the expression is + /// `Closure`, and `Closure` expects `i32` as argument. Then it + /// is "as if" the compiler generated this impl: + /// + /// impl Fn(i32) for Closure { ... } + /// + /// Now imagine our obligation is `Closure: Fn(usize)`. So far + /// we have matched the self type `Closure`. At this point we'll + /// compare the `i32` to `usize` and generate an error. + /// + /// Note that this checking occurs *after* the impl has selected, + /// because these output type parameters should not affect the + /// selection of the impl. Therefore, if there is a mismatch, we + /// report an error to the user. + fn confirm_poly_trait_refs( + &mut self, + obligation_cause: ObligationCause<'tcx>, + obligation_param_env: ty::ParamEnv<'tcx>, + obligation_trait_ref: ty::PolyTraitRef<'tcx>, + expected_trait_ref: ty::PolyTraitRef<'tcx>, + ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> { + self.infcx + .at(&obligation_cause, obligation_param_env) + .sup(obligation_trait_ref, expected_trait_ref) + .map(|InferOk { obligations, .. }| obligations) + .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e)) + } + + fn confirm_builtin_unsize_candidate( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> { + let tcx = self.tcx(); + + // `assemble_candidates_for_unsizing` should ensure there are no late-bound + // regions here. See the comment there for more details. + let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap()); + let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1); + let target = self.infcx.shallow_resolve(target); + + debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})", source, target); + + let mut nested = vec![]; + match (&source.kind, &target.kind) { + // Trait+Kx+'a -> Trait+Ky+'b (upcasts). + (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => { + // See `assemble_candidates_for_unsizing` for more info. + let existential_predicates = data_a.map_bound(|data_a| { + let iter = data_a + .principal() + .map(ty::ExistentialPredicate::Trait) + .into_iter() + .chain(data_a.projection_bounds().map(ty::ExistentialPredicate::Projection)) + .chain(data_b.auto_traits().map(ty::ExistentialPredicate::AutoTrait)); + tcx.mk_existential_predicates(iter) + }); + let source_trait = tcx.mk_dynamic(existential_predicates, r_b); + + // Require that the traits involved in this upcast are **equal**; + // only the **lifetime bound** is changed. + let InferOk { obligations, .. } = self + .infcx + .at(&obligation.cause, obligation.param_env) + .sup(target, source_trait) + .map_err(|_| Unimplemented)?; + nested.extend(obligations); + + // Register one obligation for 'a: 'b. + let cause = ObligationCause::new( + obligation.cause.span, + obligation.cause.body_id, + ObjectCastObligation(target), + ); + let outlives = ty::OutlivesPredicate(r_a, r_b); + nested.push(Obligation::with_depth( + cause, + obligation.recursion_depth + 1, + obligation.param_env, + ty::Binder::bind(outlives).to_predicate(tcx), + )); + } + + // `T` -> `Trait` + (_, &ty::Dynamic(ref data, r)) => { + let mut object_dids = data.auto_traits().chain(data.principal_def_id()); + if let Some(did) = object_dids.find(|did| !tcx.is_object_safe(*did)) { + return Err(TraitNotObjectSafe(did)); + } + + let cause = ObligationCause::new( + obligation.cause.span, + obligation.cause.body_id, + ObjectCastObligation(target), + ); + + let predicate_to_obligation = |predicate| { + Obligation::with_depth( + cause.clone(), + obligation.recursion_depth + 1, + obligation.param_env, + predicate, + ) + }; + + // Create obligations: + // - Casting `T` to `Trait` + // - For all the various builtin bounds attached to the object cast. (In other + // words, if the object type is `Foo + Send`, this would create an obligation for + // the `Send` check.) + // - Projection predicates + nested.extend( + data.iter().map(|predicate| { + predicate_to_obligation(predicate.with_self_ty(tcx, source)) + }), + ); + + // We can only make objects from sized types. + let tr = ty::TraitRef::new( + tcx.require_lang_item(LangItem::Sized, None), + tcx.mk_substs_trait(source, &[]), + ); + nested.push(predicate_to_obligation(tr.without_const().to_predicate(tcx))); + + // If the type is `Foo + 'a`, ensure that the type + // being cast to `Foo + 'a` outlives `'a`: + let outlives = ty::OutlivesPredicate(source, r); + nested.push(predicate_to_obligation(ty::Binder::dummy(outlives).to_predicate(tcx))); + } + + // `[T; n]` -> `[T]` + (&ty::Array(a, _), &ty::Slice(b)) => { + let InferOk { obligations, .. } = self + .infcx + .at(&obligation.cause, obligation.param_env) + .eq(b, a) + .map_err(|_| Unimplemented)?; + nested.extend(obligations); + } + + // `Struct<T>` -> `Struct<U>` + (&ty::Adt(def, substs_a), &ty::Adt(_, substs_b)) => { + let maybe_unsizing_param_idx = |arg: GenericArg<'tcx>| match arg.unpack() { + GenericArgKind::Type(ty) => match ty.kind { + ty::Param(p) => Some(p.index), + _ => None, + }, + + // Lifetimes aren't allowed to change during unsizing. + GenericArgKind::Lifetime(_) => None, + + GenericArgKind::Const(ct) => match ct.val { + ty::ConstKind::Param(p) => Some(p.index), + _ => None, + }, + }; + + // The last field of the structure has to exist and contain type/const parameters. + let (tail_field, prefix_fields) = + def.non_enum_variant().fields.split_last().ok_or(Unimplemented)?; + let tail_field_ty = tcx.type_of(tail_field.did); + + let mut unsizing_params = GrowableBitSet::new_empty(); + let mut found = false; + for arg in tail_field_ty.walk() { + if let Some(i) = maybe_unsizing_param_idx(arg) { + unsizing_params.insert(i); + found = true; + } + } + if !found { + return Err(Unimplemented); + } + + // Ensure none of the other fields mention the parameters used + // in unsizing. + // FIXME(eddyb) cache this (including computing `unsizing_params`) + // by putting it in a query; it would only need the `DefId` as it + // looks at declared field types, not anything substituted. + for field in prefix_fields { + for arg in tcx.type_of(field.did).walk() { + if let Some(i) = maybe_unsizing_param_idx(arg) { + if unsizing_params.contains(i) { + return Err(Unimplemented); + } + } + } + } + + // Extract `TailField<T>` and `TailField<U>` from `Struct<T>` and `Struct<U>`. + let source_tail = tail_field_ty.subst(tcx, substs_a); + let target_tail = tail_field_ty.subst(tcx, substs_b); + + // Check that the source struct with the target's + // unsizing parameters is equal to the target. + let substs = tcx.mk_substs(substs_a.iter().enumerate().map(|(i, k)| { + if unsizing_params.contains(i as u32) { substs_b[i] } else { k } + })); + let new_struct = tcx.mk_adt(def, substs); + let InferOk { obligations, .. } = self + .infcx + .at(&obligation.cause, obligation.param_env) + .eq(target, new_struct) + .map_err(|_| Unimplemented)?; + nested.extend(obligations); + + // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate. + nested.push(predicate_for_trait_def( + tcx, + obligation.param_env, + obligation.cause.clone(), + obligation.predicate.def_id(), + obligation.recursion_depth + 1, + source_tail, + &[target_tail.into()], + )); + } + + // `(.., T)` -> `(.., U)` + (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => { + assert_eq!(tys_a.len(), tys_b.len()); + + // The last field of the tuple has to exist. + let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?; + let &b_last = tys_b.last().unwrap(); + + // Check that the source tuple with the target's + // last element is equal to the target. + let new_tuple = tcx.mk_tup( + a_mid.iter().map(|k| k.expect_ty()).chain(iter::once(b_last.expect_ty())), + ); + let InferOk { obligations, .. } = self + .infcx + .at(&obligation.cause, obligation.param_env) + .eq(target, new_tuple) + .map_err(|_| Unimplemented)?; + nested.extend(obligations); + + // Construct the nested `T: Unsize<U>` predicate. + nested.push(ensure_sufficient_stack(|| { + predicate_for_trait_def( + tcx, + obligation.param_env, + obligation.cause.clone(), + obligation.predicate.def_id(), + obligation.recursion_depth + 1, + a_last.expect_ty(), + &[b_last], + ) + })); + } + + _ => bug!(), + }; + + Ok(ImplSourceBuiltinData { nested }) + } +} diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs new file mode 100644 index 00000000000..82f476b463d --- /dev/null +++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs @@ -0,0 +1,2436 @@ +//! Candidate selection. See the [rustc dev guide] for more information on how this works. +//! +//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection + +use self::EvaluationResult::*; +use self::SelectionCandidate::*; + +use super::coherence::{self, Conflict}; +use super::project; +use super::project::normalize_with_depth_to; +use super::util; +use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def}; +use super::wf; +use super::DerivedObligationCause; +use super::Obligation; +use super::ObligationCauseCode; +use super::Selection; +use super::SelectionResult; +use super::TraitQueryMode; +use super::{Normalized, ProjectionCacheKey}; +use super::{ObligationCause, PredicateObligation, TraitObligation}; +use super::{Overflow, SelectionError, Unimplemented}; + +use crate::infer::{InferCtxt, InferOk, TypeFreshener}; +use crate::traits::error_reporting::InferCtxtExt; +use crate::traits::project::ProjectionCacheKeyExt; +use rustc_data_structures::fx::{FxHashMap, FxHashSet}; +use rustc_data_structures::stack::ensure_sufficient_stack; +use rustc_errors::ErrorReported; +use rustc_hir as hir; +use rustc_hir::def_id::DefId; +use rustc_middle::dep_graph::{DepKind, DepNodeIndex}; +use rustc_middle::mir::interpret::ErrorHandled; +use rustc_middle::ty::fast_reject; +use rustc_middle::ty::relate::TypeRelation; +use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef}; +use rustc_middle::ty::{ + self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness, +}; +use rustc_span::symbol::sym; + +use std::cell::{Cell, RefCell}; +use std::cmp; +use std::fmt::{self, Display}; +use std::iter; +use std::rc::Rc; + +pub use rustc_middle::traits::select::*; + +mod candidate_assembly; +mod confirmation; + +#[derive(Clone, Debug)] +pub enum IntercrateAmbiguityCause { + DownstreamCrate { trait_desc: String, self_desc: Option<String> }, + UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> }, + ReservationImpl { message: String }, +} + +impl IntercrateAmbiguityCause { + /// Emits notes when the overlap is caused by complex intercrate ambiguities. + /// See #23980 for details. + pub fn add_intercrate_ambiguity_hint(&self, err: &mut rustc_errors::DiagnosticBuilder<'_>) { + err.note(&self.intercrate_ambiguity_hint()); + } + + pub fn intercrate_ambiguity_hint(&self) -> String { + match self { + &IntercrateAmbiguityCause::DownstreamCrate { ref trait_desc, ref self_desc } => { + let self_desc = if let &Some(ref ty) = self_desc { + format!(" for type `{}`", ty) + } else { + String::new() + }; + format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc) + } + &IntercrateAmbiguityCause::UpstreamCrateUpdate { ref trait_desc, ref self_desc } => { + let self_desc = if let &Some(ref ty) = self_desc { + format!(" for type `{}`", ty) + } else { + String::new() + }; + format!( + "upstream crates may add a new impl of trait `{}`{} \ + in future versions", + trait_desc, self_desc + ) + } + &IntercrateAmbiguityCause::ReservationImpl { ref message } => message.clone(), + } + } +} + +pub struct SelectionContext<'cx, 'tcx> { + infcx: &'cx InferCtxt<'cx, 'tcx>, + + /// Freshener used specifically for entries on the obligation + /// stack. This ensures that all entries on the stack at one time + /// will have the same set of placeholder entries, which is + /// important for checking for trait bounds that recursively + /// require themselves. + freshener: TypeFreshener<'cx, 'tcx>, + + /// If `true`, indicates that the evaluation should be conservative + /// and consider the possibility of types outside this crate. + /// This comes up primarily when resolving ambiguity. Imagine + /// there is some trait reference `$0: Bar` where `$0` is an + /// inference variable. If `intercrate` is true, then we can never + /// say for sure that this reference is not implemented, even if + /// there are *no impls at all for `Bar`*, because `$0` could be + /// bound to some type that in a downstream crate that implements + /// `Bar`. This is the suitable mode for coherence. Elsewhere, + /// though, we set this to false, because we are only interested + /// in types that the user could actually have written --- in + /// other words, we consider `$0: Bar` to be unimplemented if + /// there is no type that the user could *actually name* that + /// would satisfy it. This avoids crippling inference, basically. + intercrate: bool, + + intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>, + + /// Controls whether or not to filter out negative impls when selecting. + /// This is used in librustdoc to distinguish between the lack of an impl + /// and a negative impl + allow_negative_impls: bool, + + /// The mode that trait queries run in, which informs our error handling + /// policy. In essence, canonicalized queries need their errors propagated + /// rather than immediately reported because we do not have accurate spans. + query_mode: TraitQueryMode, +} + +// A stack that walks back up the stack frame. +struct TraitObligationStack<'prev, 'tcx> { + obligation: &'prev TraitObligation<'tcx>, + + /// The trait ref from `obligation` but "freshened" with the + /// selection-context's freshener. Used to check for recursion. + fresh_trait_ref: ty::PolyTraitRef<'tcx>, + + /// Starts out equal to `depth` -- if, during evaluation, we + /// encounter a cycle, then we will set this flag to the minimum + /// depth of that cycle for all participants in the cycle. These + /// participants will then forego caching their results. This is + /// not the most efficient solution, but it addresses #60010. The + /// problem we are trying to prevent: + /// + /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait` + /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok) + /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok) + /// + /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait` + /// is `EvaluatedToOk`; this is because they were only considered + /// ok on the premise that if `A: AutoTrait` held, but we indeed + /// encountered a problem (later on) with `A: AutoTrait. So we + /// currently set a flag on the stack node for `B: AutoTrait` (as + /// well as the second instance of `A: AutoTrait`) to suppress + /// caching. + /// + /// This is a simple, targeted fix. A more-performant fix requires + /// deeper changes, but would permit more caching: we could + /// basically defer caching until we have fully evaluated the + /// tree, and then cache the entire tree at once. In any case, the + /// performance impact here shouldn't be so horrible: every time + /// this is hit, we do cache at least one trait, so we only + /// evaluate each member of a cycle up to N times, where N is the + /// length of the cycle. This means the performance impact is + /// bounded and we shouldn't have any terrible worst-cases. + reached_depth: Cell<usize>, + + previous: TraitObligationStackList<'prev, 'tcx>, + + /// The number of parent frames plus one (thus, the topmost frame has depth 1). + depth: usize, + + /// The depth-first number of this node in the search graph -- a + /// pre-order index. Basically, a freshly incremented counter. + dfn: usize, +} + +struct SelectionCandidateSet<'tcx> { + // A list of candidates that definitely apply to the current + // obligation (meaning: types unify). + vec: Vec<SelectionCandidate<'tcx>>, + + // If `true`, then there were candidates that might or might + // not have applied, but we couldn't tell. This occurs when some + // of the input types are type variables, in which case there are + // various "builtin" rules that might or might not trigger. + ambiguous: bool, +} + +#[derive(PartialEq, Eq, Debug, Clone)] +struct EvaluatedCandidate<'tcx> { + candidate: SelectionCandidate<'tcx>, + evaluation: EvaluationResult, +} + +/// When does the builtin impl for `T: Trait` apply? +enum BuiltinImplConditions<'tcx> { + /// The impl is conditional on `T1, T2, ...: Trait`. + Where(ty::Binder<Vec<Ty<'tcx>>>), + /// There is no built-in impl. There may be some other + /// candidate (a where-clause or user-defined impl). + None, + /// It is unknown whether there is an impl. + Ambiguous, +} + +impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { + pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> { + SelectionContext { + infcx, + freshener: infcx.freshener(), + intercrate: false, + intercrate_ambiguity_causes: None, + allow_negative_impls: false, + query_mode: TraitQueryMode::Standard, + } + } + + pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> { + SelectionContext { + infcx, + freshener: infcx.freshener(), + intercrate: true, + intercrate_ambiguity_causes: None, + allow_negative_impls: false, + query_mode: TraitQueryMode::Standard, + } + } + + pub fn with_negative( + infcx: &'cx InferCtxt<'cx, 'tcx>, + allow_negative_impls: bool, + ) -> SelectionContext<'cx, 'tcx> { + debug!("with_negative({:?})", allow_negative_impls); + SelectionContext { + infcx, + freshener: infcx.freshener(), + intercrate: false, + intercrate_ambiguity_causes: None, + allow_negative_impls, + query_mode: TraitQueryMode::Standard, + } + } + + pub fn with_query_mode( + infcx: &'cx InferCtxt<'cx, 'tcx>, + query_mode: TraitQueryMode, + ) -> SelectionContext<'cx, 'tcx> { + debug!("with_query_mode({:?})", query_mode); + SelectionContext { + infcx, + freshener: infcx.freshener(), + intercrate: false, + intercrate_ambiguity_causes: None, + allow_negative_impls: false, + query_mode, + } + } + + /// Enables tracking of intercrate ambiguity causes. These are + /// used in coherence to give improved diagnostics. We don't do + /// this until we detect a coherence error because it can lead to + /// false overflow results (#47139) and because it costs + /// computation time. + pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) { + assert!(self.intercrate); + assert!(self.intercrate_ambiguity_causes.is_none()); + self.intercrate_ambiguity_causes = Some(vec![]); + debug!("selcx: enable_tracking_intercrate_ambiguity_causes"); + } + + /// Gets the intercrate ambiguity causes collected since tracking + /// was enabled and disables tracking at the same time. If + /// tracking is not enabled, just returns an empty vector. + pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> { + assert!(self.intercrate); + self.intercrate_ambiguity_causes.take().unwrap_or(vec![]) + } + + pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> { + self.infcx + } + + pub fn tcx(&self) -> TyCtxt<'tcx> { + self.infcx.tcx + } + + pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'tcx> { + self.infcx + } + + /////////////////////////////////////////////////////////////////////////// + // Selection + // + // The selection phase tries to identify *how* an obligation will + // be resolved. For example, it will identify which impl or + // parameter bound is to be used. The process can be inconclusive + // if the self type in the obligation is not fully inferred. Selection + // can result in an error in one of two ways: + // + // 1. If no applicable impl or parameter bound can be found. + // 2. If the output type parameters in the obligation do not match + // those specified by the impl/bound. For example, if the obligation + // is `Vec<Foo>: Iterable<Bar>`, but the impl specifies + // `impl<T> Iterable<T> for Vec<T>`, than an error would result. + + /// Attempts to satisfy the obligation. If successful, this will affect the surrounding + /// type environment by performing unification. + pub fn select( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> SelectionResult<'tcx, Selection<'tcx>> { + debug!("select({:?})", obligation); + debug_assert!(!obligation.predicate.has_escaping_bound_vars()); + + let pec = &ProvisionalEvaluationCache::default(); + let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation); + + let candidate = match self.candidate_from_obligation(&stack) { + Err(SelectionError::Overflow) => { + // In standard mode, overflow must have been caught and reported + // earlier. + assert!(self.query_mode == TraitQueryMode::Canonical); + return Err(SelectionError::Overflow); + } + Err(e) => { + return Err(e); + } + Ok(None) => { + return Ok(None); + } + Ok(Some(candidate)) => candidate, + }; + + match self.confirm_candidate(obligation, candidate) { + Err(SelectionError::Overflow) => { + assert!(self.query_mode == TraitQueryMode::Canonical); + Err(SelectionError::Overflow) + } + Err(e) => Err(e), + Ok(candidate) => Ok(Some(candidate)), + } + } + + /////////////////////////////////////////////////////////////////////////// + // EVALUATION + // + // Tests whether an obligation can be selected or whether an impl + // can be applied to particular types. It skips the "confirmation" + // step and hence completely ignores output type parameters. + // + // The result is "true" if the obligation *may* hold and "false" if + // we can be sure it does not. + + /// Evaluates whether the obligation `obligation` can be satisfied (by any means). + pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool { + debug!("predicate_may_hold_fatal({:?})", obligation); + + // This fatal query is a stopgap that should only be used in standard mode, + // where we do not expect overflow to be propagated. + assert!(self.query_mode == TraitQueryMode::Standard); + + self.evaluate_root_obligation(obligation) + .expect("Overflow should be caught earlier in standard query mode") + .may_apply() + } + + /// Evaluates whether the obligation `obligation` can be satisfied + /// and returns an `EvaluationResult`. This is meant for the + /// *initial* call. + pub fn evaluate_root_obligation( + &mut self, + obligation: &PredicateObligation<'tcx>, + ) -> Result<EvaluationResult, OverflowError> { + self.evaluation_probe(|this| { + this.evaluate_predicate_recursively( + TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()), + obligation.clone(), + ) + }) + } + + fn evaluation_probe( + &mut self, + op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>, + ) -> Result<EvaluationResult, OverflowError> { + self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> { + let result = op(self)?; + + match self.infcx.leak_check(true, snapshot) { + Ok(()) => {} + Err(_) => return Ok(EvaluatedToErr), + } + + match self.infcx.region_constraints_added_in_snapshot(snapshot) { + None => Ok(result), + Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)), + } + }) + } + + /// Evaluates the predicates in `predicates` recursively. Note that + /// this applies projections in the predicates, and therefore + /// is run within an inference probe. + fn evaluate_predicates_recursively<'o, I>( + &mut self, + stack: TraitObligationStackList<'o, 'tcx>, + predicates: I, + ) -> Result<EvaluationResult, OverflowError> + where + I: IntoIterator<Item = PredicateObligation<'tcx>>, + { + let mut result = EvaluatedToOk; + for obligation in predicates { + let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?; + debug!("evaluate_predicate_recursively({:?}) = {:?}", obligation, eval); + if let EvaluatedToErr = eval { + // fast-path - EvaluatedToErr is the top of the lattice, + // so we don't need to look on the other predicates. + return Ok(EvaluatedToErr); + } else { + result = cmp::max(result, eval); + } + } + Ok(result) + } + + fn evaluate_predicate_recursively<'o>( + &mut self, + previous_stack: TraitObligationStackList<'o, 'tcx>, + obligation: PredicateObligation<'tcx>, + ) -> Result<EvaluationResult, OverflowError> { + debug!( + "evaluate_predicate_recursively(previous_stack={:?}, obligation={:?})", + previous_stack.head(), + obligation + ); + + // `previous_stack` stores a `TraitObligation`, while `obligation` is + // a `PredicateObligation`. These are distinct types, so we can't + // use any `Option` combinator method that would force them to be + // the same. + match previous_stack.head() { + Some(h) => self.check_recursion_limit(&obligation, h.obligation)?, + None => self.check_recursion_limit(&obligation, &obligation)?, + } + + match obligation.predicate.skip_binders() { + ty::PredicateAtom::Trait(t, _) => { + let t = ty::Binder::bind(t); + debug_assert!(!t.has_escaping_bound_vars()); + let obligation = obligation.with(t); + self.evaluate_trait_predicate_recursively(previous_stack, obligation) + } + + ty::PredicateAtom::Subtype(p) => { + let p = ty::Binder::bind(p); + // Does this code ever run? + match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) { + Some(Ok(InferOk { mut obligations, .. })) => { + self.add_depth(obligations.iter_mut(), obligation.recursion_depth); + self.evaluate_predicates_recursively( + previous_stack, + obligations.into_iter(), + ) + } + Some(Err(_)) => Ok(EvaluatedToErr), + None => Ok(EvaluatedToAmbig), + } + } + + ty::PredicateAtom::WellFormed(arg) => match wf::obligations( + self.infcx, + obligation.param_env, + obligation.cause.body_id, + arg, + obligation.cause.span, + ) { + Some(mut obligations) => { + self.add_depth(obligations.iter_mut(), obligation.recursion_depth); + self.evaluate_predicates_recursively(previous_stack, obligations.into_iter()) + } + None => Ok(EvaluatedToAmbig), + }, + + ty::PredicateAtom::TypeOutlives(..) | ty::PredicateAtom::RegionOutlives(..) => { + // We do not consider region relationships when evaluating trait matches. + Ok(EvaluatedToOkModuloRegions) + } + + ty::PredicateAtom::ObjectSafe(trait_def_id) => { + if self.tcx().is_object_safe(trait_def_id) { + Ok(EvaluatedToOk) + } else { + Ok(EvaluatedToErr) + } + } + + ty::PredicateAtom::Projection(data) => { + let data = ty::Binder::bind(data); + let project_obligation = obligation.with(data); + match project::poly_project_and_unify_type(self, &project_obligation) { + Ok(Ok(Some(mut subobligations))) => { + self.add_depth(subobligations.iter_mut(), obligation.recursion_depth); + let result = self.evaluate_predicates_recursively( + previous_stack, + subobligations.into_iter(), + ); + if let Some(key) = + ProjectionCacheKey::from_poly_projection_predicate(self, data) + { + self.infcx.inner.borrow_mut().projection_cache().complete(key); + } + result + } + Ok(Ok(None)) => Ok(EvaluatedToAmbig), + // EvaluatedToRecur might also be acceptable here, but use + // Unknown for now because it means that we won't dismiss a + // selection candidate solely because it has a projection + // cycle. This is closest to the previous behavior of + // immediately erroring. + Ok(Err(project::InProgress)) => Ok(EvaluatedToUnknown), + Err(_) => Ok(EvaluatedToErr), + } + } + + ty::PredicateAtom::ClosureKind(_, closure_substs, kind) => { + match self.infcx.closure_kind(closure_substs) { + Some(closure_kind) => { + if closure_kind.extends(kind) { + Ok(EvaluatedToOk) + } else { + Ok(EvaluatedToErr) + } + } + None => Ok(EvaluatedToAmbig), + } + } + + ty::PredicateAtom::ConstEvaluatable(def_id, substs) => { + match self.tcx().const_eval_resolve( + obligation.param_env, + def_id, + substs, + None, + None, + ) { + Ok(_) => Ok(EvaluatedToOk), + Err(ErrorHandled::TooGeneric) => Ok(EvaluatedToAmbig), + Err(_) => Ok(EvaluatedToErr), + } + } + + ty::PredicateAtom::ConstEquate(c1, c2) => { + debug!("evaluate_predicate_recursively: equating consts c1={:?} c2={:?}", c1, c2); + + let evaluate = |c: &'tcx ty::Const<'tcx>| { + if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val { + self.infcx + .const_eval_resolve( + obligation.param_env, + def, + substs, + promoted, + Some(obligation.cause.span), + ) + .map(|val| ty::Const::from_value(self.tcx(), val, c.ty)) + } else { + Ok(c) + } + }; + + match (evaluate(c1), evaluate(c2)) { + (Ok(c1), Ok(c2)) => { + match self.infcx().at(&obligation.cause, obligation.param_env).eq(c1, c2) { + Ok(_) => Ok(EvaluatedToOk), + Err(_) => Ok(EvaluatedToErr), + } + } + (Err(ErrorHandled::Reported(ErrorReported)), _) + | (_, Err(ErrorHandled::Reported(ErrorReported))) => Ok(EvaluatedToErr), + (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => span_bug!( + obligation.cause.span(self.tcx()), + "ConstEquate: const_eval_resolve returned an unexpected error" + ), + (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => { + Ok(EvaluatedToAmbig) + } + } + } + } + } + + fn evaluate_trait_predicate_recursively<'o>( + &mut self, + previous_stack: TraitObligationStackList<'o, 'tcx>, + mut obligation: TraitObligation<'tcx>, + ) -> Result<EvaluationResult, OverflowError> { + debug!("evaluate_trait_predicate_recursively({:?})", obligation); + + if !self.intercrate + && obligation.is_global() + && obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst()) + { + // If a param env has no global bounds, global obligations do not + // depend on its particular value in order to work, so we can clear + // out the param env and get better caching. + debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation); + obligation.param_env = obligation.param_env.without_caller_bounds(); + } + + let stack = self.push_stack(previous_stack, &obligation); + let fresh_trait_ref = stack.fresh_trait_ref; + if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) { + debug!("CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result); + return Ok(result); + } + + if let Some(result) = stack.cache().get_provisional(fresh_trait_ref) { + debug!("PROVISIONAL CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result); + stack.update_reached_depth(stack.cache().current_reached_depth()); + return Ok(result); + } + + // Check if this is a match for something already on the + // stack. If so, we don't want to insert the result into the + // main cache (it is cycle dependent) nor the provisional + // cache (which is meant for things that have completed but + // for a "backedge" -- this result *is* the backedge). + if let Some(cycle_result) = self.check_evaluation_cycle(&stack) { + return Ok(cycle_result); + } + + let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack)); + let result = result?; + + if !result.must_apply_modulo_regions() { + stack.cache().on_failure(stack.dfn); + } + + let reached_depth = stack.reached_depth.get(); + if reached_depth >= stack.depth { + debug!("CACHE MISS: EVAL({:?})={:?}", fresh_trait_ref, result); + self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result); + + stack.cache().on_completion(stack.depth, |fresh_trait_ref, provisional_result| { + self.insert_evaluation_cache( + obligation.param_env, + fresh_trait_ref, + dep_node, + provisional_result.max(result), + ); + }); + } else { + debug!("PROVISIONAL: {:?}={:?}", fresh_trait_ref, result); + debug!( + "evaluate_trait_predicate_recursively: caching provisionally because {:?} \ + is a cycle participant (at depth {}, reached depth {})", + fresh_trait_ref, stack.depth, reached_depth, + ); + + stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_ref, result); + } + + Ok(result) + } + + /// If there is any previous entry on the stack that precisely + /// matches this obligation, then we can assume that the + /// obligation is satisfied for now (still all other conditions + /// must be met of course). One obvious case this comes up is + /// marker traits like `Send`. Think of a linked list: + /// + /// struct List<T> { data: T, next: Option<Box<List<T>>> } + /// + /// `Box<List<T>>` will be `Send` if `T` is `Send` and + /// `Option<Box<List<T>>>` is `Send`, and in turn + /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is + /// `Send`. + /// + /// Note that we do this comparison using the `fresh_trait_ref` + /// fields. Because these have all been freshened using + /// `self.freshener`, we can be sure that (a) this will not + /// affect the inferencer state and (b) that if we see two + /// fresh regions with the same index, they refer to the same + /// unbound type variable. + fn check_evaluation_cycle( + &mut self, + stack: &TraitObligationStack<'_, 'tcx>, + ) -> Option<EvaluationResult> { + if let Some(cycle_depth) = stack + .iter() + .skip(1) // Skip top-most frame. + .find(|prev| { + stack.obligation.param_env == prev.obligation.param_env + && stack.fresh_trait_ref == prev.fresh_trait_ref + }) + .map(|stack| stack.depth) + { + debug!( + "evaluate_stack({:?}) --> recursive at depth {}", + stack.fresh_trait_ref, cycle_depth, + ); + + // If we have a stack like `A B C D E A`, where the top of + // the stack is the final `A`, then this will iterate over + // `A, E, D, C, B` -- i.e., all the participants apart + // from the cycle head. We mark them as participating in a + // cycle. This suppresses caching for those nodes. See + // `in_cycle` field for more details. + stack.update_reached_depth(cycle_depth); + + // Subtle: when checking for a coinductive cycle, we do + // not compare using the "freshened trait refs" (which + // have erased regions) but rather the fully explicit + // trait refs. This is important because it's only a cycle + // if the regions match exactly. + let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth); + let tcx = self.tcx(); + let cycle = + cycle.map(|stack| stack.obligation.predicate.without_const().to_predicate(tcx)); + if self.coinductive_match(cycle) { + debug!("evaluate_stack({:?}) --> recursive, coinductive", stack.fresh_trait_ref); + Some(EvaluatedToOk) + } else { + debug!("evaluate_stack({:?}) --> recursive, inductive", stack.fresh_trait_ref); + Some(EvaluatedToRecur) + } + } else { + None + } + } + + fn evaluate_stack<'o>( + &mut self, + stack: &TraitObligationStack<'o, 'tcx>, + ) -> Result<EvaluationResult, OverflowError> { + // In intercrate mode, whenever any of the generics are unbound, + // there can always be an impl. Even if there are no impls in + // this crate, perhaps the type would be unified with + // something from another crate that does provide an impl. + // + // In intra mode, we must still be conservative. The reason is + // that we want to avoid cycles. Imagine an impl like: + // + // impl<T:Eq> Eq for Vec<T> + // + // and a trait reference like `$0 : Eq` where `$0` is an + // unbound variable. When we evaluate this trait-reference, we + // will unify `$0` with `Vec<$1>` (for some fresh variable + // `$1`), on the condition that `$1 : Eq`. We will then wind + // up with many candidates (since that are other `Eq` impls + // that apply) and try to winnow things down. This results in + // a recursive evaluation that `$1 : Eq` -- as you can + // imagine, this is just where we started. To avoid that, we + // check for unbound variables and return an ambiguous (hence possible) + // match if we've seen this trait before. + // + // This suffices to allow chains like `FnMut` implemented in + // terms of `Fn` etc, but we could probably make this more + // precise still. + let unbound_input_types = + stack.fresh_trait_ref.skip_binder().substs.types().any(|ty| ty.is_fresh()); + // This check was an imperfect workaround for a bug in the old + // intercrate mode; it should be removed when that goes away. + if unbound_input_types && self.intercrate { + debug!( + "evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous", + stack.fresh_trait_ref + ); + // Heuristics: show the diagnostics when there are no candidates in crate. + if self.intercrate_ambiguity_causes.is_some() { + debug!("evaluate_stack: intercrate_ambiguity_causes is some"); + if let Ok(candidate_set) = self.assemble_candidates(stack) { + if !candidate_set.ambiguous && candidate_set.vec.is_empty() { + let trait_ref = stack.obligation.predicate.skip_binder().trait_ref; + let self_ty = trait_ref.self_ty(); + let cause = IntercrateAmbiguityCause::DownstreamCrate { + trait_desc: trait_ref.print_only_trait_path().to_string(), + self_desc: if self_ty.has_concrete_skeleton() { + Some(self_ty.to_string()) + } else { + None + }, + }; + debug!("evaluate_stack: pushing cause = {:?}", cause); + self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause); + } + } + } + return Ok(EvaluatedToAmbig); + } + if unbound_input_types + && stack.iter().skip(1).any(|prev| { + stack.obligation.param_env == prev.obligation.param_env + && self.match_fresh_trait_refs( + stack.fresh_trait_ref, + prev.fresh_trait_ref, + prev.obligation.param_env, + ) + }) + { + debug!( + "evaluate_stack({:?}) --> unbound argument, recursive --> giving up", + stack.fresh_trait_ref + ); + return Ok(EvaluatedToUnknown); + } + + match self.candidate_from_obligation(stack) { + Ok(Some(c)) => self.evaluate_candidate(stack, &c), + Ok(None) => Ok(EvaluatedToAmbig), + Err(Overflow) => Err(OverflowError), + Err(..) => Ok(EvaluatedToErr), + } + } + + /// For defaulted traits, we use a co-inductive strategy to solve, so + /// that recursion is ok. This routine returns `true` if the top of the + /// stack (`cycle[0]`): + /// + /// - is a defaulted trait, + /// - it also appears in the backtrace at some position `X`, + /// - all the predicates at positions `X..` between `X` and the top are + /// also defaulted traits. + pub fn coinductive_match<I>(&mut self, cycle: I) -> bool + where + I: Iterator<Item = ty::Predicate<'tcx>>, + { + let mut cycle = cycle; + cycle.all(|predicate| self.coinductive_predicate(predicate)) + } + + fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool { + let result = match predicate.skip_binders() { + ty::PredicateAtom::Trait(ref data, _) => self.tcx().trait_is_auto(data.def_id()), + _ => false, + }; + debug!("coinductive_predicate({:?}) = {:?}", predicate, result); + result + } + + /// Further evaluates `candidate` to decide whether all type parameters match and whether nested + /// obligations are met. Returns whether `candidate` remains viable after this further + /// scrutiny. + fn evaluate_candidate<'o>( + &mut self, + stack: &TraitObligationStack<'o, 'tcx>, + candidate: &SelectionCandidate<'tcx>, + ) -> Result<EvaluationResult, OverflowError> { + debug!( + "evaluate_candidate: depth={} candidate={:?}", + stack.obligation.recursion_depth, candidate + ); + let result = self.evaluation_probe(|this| { + let candidate = (*candidate).clone(); + match this.confirm_candidate(stack.obligation, candidate) { + Ok(selection) => this.evaluate_predicates_recursively( + stack.list(), + selection.nested_obligations().into_iter(), + ), + Err(..) => Ok(EvaluatedToErr), + } + })?; + debug!( + "evaluate_candidate: depth={} result={:?}", + stack.obligation.recursion_depth, result + ); + Ok(result) + } + + fn check_evaluation_cache( + &self, + param_env: ty::ParamEnv<'tcx>, + trait_ref: ty::PolyTraitRef<'tcx>, + ) -> Option<EvaluationResult> { + let tcx = self.tcx(); + if self.can_use_global_caches(param_env) { + if let Some(res) = tcx.evaluation_cache.get(¶m_env.and(trait_ref), tcx) { + return Some(res); + } + } + self.infcx.evaluation_cache.get(¶m_env.and(trait_ref), tcx) + } + + fn insert_evaluation_cache( + &mut self, + param_env: ty::ParamEnv<'tcx>, + trait_ref: ty::PolyTraitRef<'tcx>, + dep_node: DepNodeIndex, + result: EvaluationResult, + ) { + // Avoid caching results that depend on more than just the trait-ref + // - the stack can create recursion. + if result.is_stack_dependent() { + return; + } + + if self.can_use_global_caches(param_env) { + if !trait_ref.needs_infer() { + debug!( + "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global", + trait_ref, result, + ); + // This may overwrite the cache with the same value + // FIXME: Due to #50507 this overwrites the different values + // This should be changed to use HashMapExt::insert_same + // when that is fixed + self.tcx().evaluation_cache.insert(param_env.and(trait_ref), dep_node, result); + return; + } + } + + debug!("insert_evaluation_cache(trait_ref={:?}, candidate={:?})", trait_ref, result,); + self.infcx.evaluation_cache.insert(param_env.and(trait_ref), dep_node, result); + } + + /// For various reasons, it's possible for a subobligation + /// to have a *lower* recursion_depth than the obligation used to create it. + /// Projection sub-obligations may be returned from the projection cache, + /// which results in obligations with an 'old' `recursion_depth`. + /// Additionally, methods like `wf::obligations` and + /// `InferCtxt.subtype_predicate` produce subobligations without + /// taking in a 'parent' depth, causing the generated subobligations + /// to have a `recursion_depth` of `0`. + /// + /// To ensure that obligation_depth never decreasees, we force all subobligations + /// to have at least the depth of the original obligation. + fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>( + &self, + it: I, + min_depth: usize, + ) { + it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1); + } + + /// Checks that the recursion limit has not been exceeded. + /// + /// The weird return type of this function allows it to be used with the `try` (`?`) + /// operator within certain functions. + fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>( + &self, + obligation: &Obligation<'tcx, T>, + error_obligation: &Obligation<'tcx, V>, + ) -> Result<(), OverflowError> { + if !self.infcx.tcx.sess.recursion_limit().value_within_limit(obligation.recursion_depth) { + match self.query_mode { + TraitQueryMode::Standard => { + self.infcx().report_overflow_error(error_obligation, true); + } + TraitQueryMode::Canonical => { + return Err(OverflowError); + } + } + } + Ok(()) + } + + fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex) + where + OP: FnOnce(&mut Self) -> R, + { + let (result, dep_node) = + self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || op(self)); + self.tcx().dep_graph.read_index(dep_node); + (result, dep_node) + } + + // Treat negative impls as unimplemented, and reservation impls as ambiguity. + fn filter_negative_and_reservation_impls( + &mut self, + candidate: SelectionCandidate<'tcx>, + ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { + if let ImplCandidate(def_id) = candidate { + let tcx = self.tcx(); + match tcx.impl_polarity(def_id) { + ty::ImplPolarity::Negative if !self.allow_negative_impls => { + return Err(Unimplemented); + } + ty::ImplPolarity::Reservation => { + if let Some(intercrate_ambiguity_clauses) = + &mut self.intercrate_ambiguity_causes + { + let attrs = tcx.get_attrs(def_id); + let attr = tcx.sess.find_by_name(&attrs, sym::rustc_reservation_impl); + let value = attr.and_then(|a| a.value_str()); + if let Some(value) = value { + debug!( + "filter_negative_and_reservation_impls: \ + reservation impl ambiguity on {:?}", + def_id + ); + intercrate_ambiguity_clauses.push( + IntercrateAmbiguityCause::ReservationImpl { + message: value.to_string(), + }, + ); + } + } + return Ok(None); + } + _ => {} + }; + } + Ok(Some(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 = trait_ref.print_only_trait_path().to_string(); + let self_desc = if self_ty.has_concrete_skeleton() { + Some(self_ty.to_string()) + } else { + None + }; + let cause = if let Conflict::Upstream = conflict { + IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } + } else { + IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } + }; + debug!("evaluate_stack: pushing cause = {:?}", cause); + self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause); + } + } + } + return Ok(None); + } + + let candidate_set = self.assemble_candidates(stack)?; + + if candidate_set.ambiguous { + debug!("candidate set contains ambig"); + return Ok(None); + } + + let mut candidates = candidate_set.vec; + + debug!("assembled {} candidates for {:?}: {:?}", candidates.len(), stack, candidates); + + // 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. + + // 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<T:Clone> Vec<T> { fn push_clone(...) { ... } } + // + // and we were to see some code `foo.push_clone()` where `boo` + // is a `Vec<Bar>` 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_negative_and_reservation_impls(candidates.pop().unwrap()); + } + + // 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) => Err(Overflow), + }) + .flat_map(Result::transpose) + .collect::<Result<Vec<_>, _>>()?; + + debug!("winnowed to {} candidates for {:?}: {:?}", candidates.len(), stack, candidates); + + let needs_infer = stack.obligation.predicate.needs_infer(); + + // 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!("Dropping candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]); + candidates.swap_remove(i); + } else { + debug!("Retaining candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]); + i += 1; + + // If there are *STILL* multiple candidates, give up + // and report ambiguity. + if i > 1 { + debug!("multiple matches, ambig"); + return Ok(None); + } + } + } + } + + // 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.references_error() { + debug!("no results for error type, treating as ambiguous"); + return Ok(None); + } + return Err(Unimplemented); + } + + // Just one candidate left. + self.filter_negative_and_reservation_impls(candidates.pop().unwrap().candidate) + } + + fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> { + debug!("is_knowable(intercrate={:?})", self.intercrate); + + if !self.intercrate { + return None; + } + + let obligation = &stack.obligation; + let predicate = self.infcx().resolve_vars_if_possible(&obligation.predicate); + + // Okay to skip binder because of the nature of the + // trait-ref-is-knowable check, which does not care about + // bound regions. + let trait_ref = predicate.skip_binder().trait_ref; + + coherence::trait_ref_is_knowable(self.tcx(), trait_ref) + } + + /// Returns `true` if the global caches can be used. + /// Do note that if the type itself is not in the + /// global tcx, the local caches will be used. + fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool { + // If there are any inference variables in the `ParamEnv`, then we + // always use a cache local to this particular scope. Otherwise, we + // switch to a global cache. + if param_env.needs_infer() { + return false; + } + + // Avoid using the master cache during coherence and just rely + // on the local cache. This effectively disables caching + // during coherence. It is really just a simplification to + // avoid us having to fear that coherence results "pollute" + // the master cache. Since coherence executes pretty quickly, + // it's not worth going to more trouble to increase the + // hit-rate, I don't think. + if self.intercrate { + return false; + } + + // Otherwise, we can use the global cache. + true + } + + fn check_candidate_cache( + &mut self, + param_env: ty::ParamEnv<'tcx>, + cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>, + ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> { + let tcx = self.tcx(); + let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref; + if self.can_use_global_caches(param_env) { + if let Some(res) = tcx.selection_cache.get(¶m_env.and(*trait_ref), tcx) { + return Some(res); + } + } + self.infcx.selection_cache.get(¶m_env.and(*trait_ref), tcx) + } + + /// Determines whether can we safely cache the result + /// of selecting an obligation. This is almost always `true`, + /// except when dealing with certain `ParamCandidate`s. + /// + /// Ordinarily, a `ParamCandidate` will contain no inference variables, + /// since it was usually produced directly from a `DefId`. However, + /// certain cases (currently only librustdoc's blanket impl finder), + /// a `ParamEnv` may be explicitly constructed with inference types. + /// When this is the case, we do *not* want to cache the resulting selection + /// candidate. This is due to the fact that it might not always be possible + /// to equate the obligation's trait ref and the candidate's trait ref, + /// if more constraints end up getting added to an inference variable. + /// + /// Because of this, we always want to re-run the full selection + /// process for our obligation the next time we see it, since + /// we might end up picking a different `SelectionCandidate` (or none at all). + fn can_cache_candidate( + &self, + result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>, + ) -> bool { + match result { + Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(), + _ => true, + } + } + + fn insert_candidate_cache( + &mut self, + param_env: ty::ParamEnv<'tcx>, + cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>, + dep_node: DepNodeIndex, + candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>, + ) { + let tcx = self.tcx(); + let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref; + + if !self.can_cache_candidate(&candidate) { + debug!( + "insert_candidate_cache(trait_ref={:?}, candidate={:?} -\ + candidate is not cacheable", + trait_ref, candidate + ); + return; + } + + if self.can_use_global_caches(param_env) { + if let Err(Overflow) = candidate { + // Don't cache overflow globally; we only produce this in certain modes. + } else if !trait_ref.needs_infer() { + if !candidate.needs_infer() { + debug!( + "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global", + trait_ref, candidate, + ); + // This may overwrite the cache with the same value. + tcx.selection_cache.insert(param_env.and(trait_ref), dep_node, candidate); + return; + } + } + } + + debug!( + "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local", + trait_ref, candidate, + ); + self.infcx.selection_cache.insert(param_env.and(trait_ref), dep_node, candidate); + } + + fn match_projection_obligation_against_definition_bounds( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> bool { + 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); + debug!( + "match_projection_obligation_against_definition_bounds: \ + placeholder_trait_predicate={:?}", + placeholder_trait_predicate, + ); + + let tcx = self.infcx.tcx; + let predicates = match placeholder_trait_predicate.trait_ref.self_ty().kind { + ty::Projection(ref data) => { + tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs) + } + ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs), + _ => { + span_bug!( + obligation.cause.span, + "match_projection_obligation_against_definition_bounds() called \ + but self-ty is not a projection: {:?}", + placeholder_trait_predicate.trait_ref.self_ty() + ); + } + }; + + let matching_bound = predicates.iter().find_map(|bound| { + if let ty::PredicateAtom::Trait(pred, _) = bound.skip_binders() { + let bound = ty::Binder::bind(pred.trait_ref); + if self.infcx.probe(|_| { + self.match_projection(obligation, bound, placeholder_trait_predicate.trait_ref) + }) { + return Some(bound); + } + } + None + }); + + debug!( + "match_projection_obligation_against_definition_bounds: \ + matching_bound={:?}", + matching_bound + ); + match matching_bound { + None => false, + Some(bound) => { + // Repeat the successful match, if any, this time outside of a probe. + let result = + self.match_projection(obligation, bound, placeholder_trait_predicate.trait_ref); + + assert!(result); + true + } + } + } + + fn match_projection( + &mut self, + obligation: &TraitObligation<'tcx>, + trait_bound: ty::PolyTraitRef<'tcx>, + placeholder_trait_ref: ty::TraitRef<'tcx>, + ) -> bool { + debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars()); + self.infcx + .at(&obligation.cause, obligation.param_env) + .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound) + .is_ok() + } + + fn evaluate_where_clause<'o>( + &mut self, + stack: &TraitObligationStack<'o, 'tcx>, + where_clause_trait_ref: ty::PolyTraitRef<'tcx>, + ) -> Result<EvaluationResult, OverflowError> { + self.evaluation_probe(|this| { + match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) { + Ok(obligations) => { + this.evaluate_predicates_recursively(stack.list(), obligations.into_iter()) + } + Err(()) => Ok(EvaluatedToErr), + } + }) + } + + /////////////////////////////////////////////////////////////////////////// + // WINNOW + // + // Winnowing is the process of attempting to resolve ambiguity by + // probing further. During the winnowing process, we unify all + // type variables and then we also attempt to evaluate recursive + // bounds to see if they are satisfied. + + /// Returns `true` if `victim` should be dropped in favor of + /// `other`. Generally speaking we will drop duplicate + /// candidates and prefer where-clause candidates. + /// + /// See the comment for "SelectionCandidate" for more details. + fn candidate_should_be_dropped_in_favor_of( + &mut self, + victim: &EvaluatedCandidate<'tcx>, + other: &EvaluatedCandidate<'tcx>, + needs_infer: bool, + ) -> bool { + if victim.candidate == other.candidate { + return true; + } + + // Check if a bound would previously have been removed when normalizing + // the param_env so that it can be given the lowest priority. See + // #50825 for the motivation for this. + let is_global = + |cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions(); + + // (*) Prefer `BuiltinCandidate { has_nested: false }` and `DiscriminantKindCandidate` + // to anything else. + // + // This is a fix for #53123 and prevents winnowing from accidentally extending the + // lifetime of a variable. + match other.candidate { + // (*) + BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate => true, + ParamCandidate(ref cand) => match victim.candidate { + AutoImplCandidate(..) => { + bug!( + "default implementations shouldn't be recorded \ + when there are other valid candidates" + ); + } + // (*) + BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate => false, + ImplCandidate(..) + | ClosureCandidate + | GeneratorCandidate + | FnPointerCandidate + | BuiltinObjectCandidate + | BuiltinUnsizeCandidate + | BuiltinCandidate { .. } + | TraitAliasCandidate(..) => { + // Global bounds from the where clause should be ignored + // here (see issue #50825). Otherwise, we have a where + // clause so don't go around looking for impls. + !is_global(cand) + } + ObjectCandidate | ProjectionCandidate => { + // Arbitrarily give param candidates priority + // over projection and object candidates. + !is_global(cand) + } + ParamCandidate(..) => false, + }, + ObjectCandidate | ProjectionCandidate => match victim.candidate { + AutoImplCandidate(..) => { + bug!( + "default implementations shouldn't be recorded \ + when there are other valid candidates" + ); + } + // (*) + BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate => false, + ImplCandidate(..) + | ClosureCandidate + | GeneratorCandidate + | FnPointerCandidate + | BuiltinObjectCandidate + | BuiltinUnsizeCandidate + | BuiltinCandidate { .. } + | TraitAliasCandidate(..) => true, + ObjectCandidate | ProjectionCandidate => { + // Arbitrarily give param candidates priority + // over projection and object candidates. + true + } + ParamCandidate(ref cand) => is_global(cand), + }, + ImplCandidate(other_def) => { + // See if we can toss out `victim` based on specialization. + // This requires us to know *for sure* that the `other` impl applies + // i.e., `EvaluatedToOk`. + if other.evaluation.must_apply_modulo_regions() { + match victim.candidate { + ImplCandidate(victim_def) => { + let tcx = self.tcx(); + if tcx.specializes((other_def, victim_def)) { + return true; + } + return match tcx.impls_are_allowed_to_overlap(other_def, victim_def) { + Some(ty::ImplOverlapKind::Permitted { marker: true }) => { + // Subtle: If the predicate we are evaluating has inference + // variables, do *not* allow discarding candidates due to + // marker trait impls. + // + // Without this restriction, we could end up accidentally + // constrainting inference variables based on an arbitrarily + // chosen trait impl. + // + // Imagine we have the following code: + // + // ```rust + // #[marker] trait MyTrait {} + // impl MyTrait for u8 {} + // impl MyTrait for bool {} + // ``` + // + // And we are evaluating the predicate `<_#0t as MyTrait>`. + // + // During selection, we will end up with one candidate for each + // impl of `MyTrait`. If we were to discard one impl in favor + // of the other, we would be left with one candidate, causing + // us to "successfully" select the predicate, unifying + // _#0t with (for example) `u8`. + // + // However, we have no reason to believe that this unification + // is correct - we've essentially just picked an arbitrary + // *possibility* for _#0t, and required that this be the *only* + // possibility. + // + // Eventually, we will either: + // 1) Unify all inference variables in the predicate through + // some other means (e.g. type-checking of a function). We will + // then be in a position to drop marker trait candidates + // without constraining inference variables (since there are + // none left to constrin) + // 2) Be left with some unconstrained inference variables. We + // will then correctly report an inference error, since the + // existence of multiple marker trait impls tells us nothing + // about which one should actually apply. + !needs_infer + } + Some(_) => true, + None => false, + }; + } + ParamCandidate(ref cand) => { + // Prefer the impl to a global where clause candidate. + return is_global(cand); + } + _ => (), + } + } + + false + } + ClosureCandidate + | GeneratorCandidate + | FnPointerCandidate + | BuiltinObjectCandidate + | BuiltinUnsizeCandidate + | BuiltinCandidate { has_nested: true } => { + match victim.candidate { + ParamCandidate(ref cand) => { + // Prefer these to a global where-clause bound + // (see issue #50825). + is_global(cand) && other.evaluation.must_apply_modulo_regions() + } + _ => false, + } + } + _ => false, + } + } + + fn sized_conditions( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> BuiltinImplConditions<'tcx> { + use self::BuiltinImplConditions::{Ambiguous, None, Where}; + + // NOTE: binder moved to (*) + let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty()); + + match self_ty.kind { + ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) + | ty::Uint(_) + | ty::Int(_) + | ty::Bool + | ty::Float(_) + | ty::FnDef(..) + | ty::FnPtr(_) + | ty::RawPtr(..) + | ty::Char + | ty::Ref(..) + | ty::Generator(..) + | ty::GeneratorWitness(..) + | ty::Array(..) + | ty::Closure(..) + | ty::Never + | ty::Error(_) => { + // safe for everything + Where(ty::Binder::dummy(Vec::new())) + } + + ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None, + + ty::Tuple(tys) => { + Where(ty::Binder::bind(tys.last().into_iter().map(|k| k.expect_ty()).collect())) + } + + ty::Adt(def, substs) => { + let sized_crit = def.sized_constraint(self.tcx()); + // (*) binder moved here + Where(ty::Binder::bind( + sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect(), + )) + } + + ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None, + ty::Infer(ty::TyVar(_)) => Ambiguous, + + ty::Placeholder(..) + | ty::Bound(..) + | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => { + bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty); + } + } + } + + fn copy_clone_conditions( + &mut self, + obligation: &TraitObligation<'tcx>, + ) -> BuiltinImplConditions<'tcx> { + // NOTE: binder moved to (*) + let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty()); + + use self::BuiltinImplConditions::{Ambiguous, None, Where}; + + match self_ty.kind { + ty::Infer(ty::IntVar(_)) + | ty::Infer(ty::FloatVar(_)) + | ty::FnDef(..) + | ty::FnPtr(_) + | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())), + + ty::Uint(_) + | ty::Int(_) + | ty::Bool + | ty::Float(_) + | ty::Char + | ty::RawPtr(..) + | ty::Never + | ty::Ref(_, _, hir::Mutability::Not) => { + // Implementations provided in libcore + None + } + + ty::Dynamic(..) + | ty::Str + | ty::Slice(..) + | ty::Generator(..) + | ty::GeneratorWitness(..) + | ty::Foreign(..) + | ty::Ref(_, _, hir::Mutability::Mut) => None, + + ty::Array(element_ty, _) => { + // (*) binder moved here + Where(ty::Binder::bind(vec![element_ty])) + } + + ty::Tuple(tys) => { + // (*) binder moved here + Where(ty::Binder::bind(tys.iter().map(|k| k.expect_ty()).collect())) + } + + ty::Closure(_, substs) => { + // (*) binder moved here + Where(ty::Binder::bind(substs.as_closure().upvar_tys().collect())) + } + + ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => { + // Fallback to whatever user-defined impls exist in this case. + None + } + + ty::Infer(ty::TyVar(_)) => { + // Unbound type variable. Might or might not have + // applicable impls and so forth, depending on what + // those type variables wind up being bound to. + Ambiguous + } + + ty::Placeholder(..) + | ty::Bound(..) + | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => { + bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty); + } + } + } + + /// For default impls, we need to break apart a type into its + /// "constituent types" -- meaning, the types that it contains. + /// + /// Here are some (simple) examples: + /// + /// ``` + /// (i32, u32) -> [i32, u32] + /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32] + /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32] + /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32] + /// ``` + fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> { + match t.kind { + ty::Uint(_) + | ty::Int(_) + | ty::Bool + | ty::Float(_) + | ty::FnDef(..) + | ty::FnPtr(_) + | ty::Str + | ty::Error(_) + | ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) + | ty::Never + | ty::Char => Vec::new(), + + ty::Placeholder(..) + | ty::Dynamic(..) + | ty::Param(..) + | ty::Foreign(..) + | ty::Projection(..) + | ty::Bound(..) + | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => { + bug!("asked to assemble constituent types of unexpected type: {:?}", t); + } + + ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => { + vec![element_ty] + } + + ty::Array(element_ty, _) | ty::Slice(element_ty) => vec![element_ty], + + ty::Tuple(ref tys) => { + // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet + tys.iter().map(|k| k.expect_ty()).collect() + } + + ty::Closure(_, ref substs) => substs.as_closure().upvar_tys().collect(), + + ty::Generator(_, ref substs, _) => { + let witness = substs.as_generator().witness(); + substs.as_generator().upvar_tys().chain(iter::once(witness)).collect() + } + + ty::GeneratorWitness(types) => { + // This is sound because no regions in the witness can refer to + // the binder outside the witness. So we'll effectivly reuse + // the implicit binder around the witness. + types.skip_binder().to_vec() + } + + // For `PhantomData<T>`, we pass `T`. + ty::Adt(def, substs) if def.is_phantom_data() => substs.types().collect(), + + ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect(), + + ty::Opaque(def_id, substs) => { + // We can resolve the `impl Trait` to its concrete type, + // which enforces a DAG between the functions requiring + // the auto trait bounds in question. + vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)] + } + } + } + + fn collect_predicates_for_types( + &mut self, + param_env: ty::ParamEnv<'tcx>, + cause: ObligationCause<'tcx>, + recursion_depth: usize, + trait_def_id: DefId, + types: ty::Binder<Vec<Ty<'tcx>>>, + ) -> Vec<PredicateObligation<'tcx>> { + // Because the types were potentially derived from + // higher-ranked obligations they may reference late-bound + // regions. For example, `for<'a> Foo<&'a i32> : Copy` would + // yield a type like `for<'a> &'a i32`. In general, we + // maintain the invariant that we never manipulate bound + // regions, so we have to process these bound regions somehow. + // + // The strategy is to: + // + // 1. Instantiate those regions to placeholder regions (e.g., + // `for<'a> &'a i32` becomes `&0 i32`. + // 2. Produce something like `&'0 i32 : Copy` + // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy` + + types + .skip_binder() // binder moved -\ + .iter() + .flat_map(|ty| { + let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/ + + self.infcx.commit_unconditionally(|_| { + let (placeholder_ty, _) = self.infcx.replace_bound_vars_with_placeholders(&ty); + let Normalized { value: normalized_ty, mut obligations } = + ensure_sufficient_stack(|| { + project::normalize_with_depth( + self, + param_env, + cause.clone(), + recursion_depth, + &placeholder_ty, + ) + }); + let placeholder_obligation = predicate_for_trait_def( + self.tcx(), + param_env, + cause.clone(), + trait_def_id, + recursion_depth, + normalized_ty, + &[], + ); + obligations.push(placeholder_obligation); + obligations + }) + }) + .collect() + } + + /////////////////////////////////////////////////////////////////////////// + // Matching + // + // Matching is a common path used for both evaluation and + // confirmation. It basically unifies types that appear in impls + // and traits. This does affect the surrounding environment; + // therefore, when used during evaluation, match routines must be + // run inside of a `probe()` so that their side-effects are + // contained. + + fn rematch_impl( + &mut self, + impl_def_id: DefId, + obligation: &TraitObligation<'tcx>, + ) -> Normalized<'tcx, SubstsRef<'tcx>> { + match self.match_impl(impl_def_id, obligation) { + Ok(substs) => substs, + Err(()) => { + bug!( + "Impl {:?} was matchable against {:?} but now is not", + impl_def_id, + obligation + ); + } + } + } + + fn match_impl( + &mut self, + impl_def_id: DefId, + obligation: &TraitObligation<'tcx>, + ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> { + let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap(); + + // Before we create the substitutions and everything, first + // consider a "quick reject". This avoids creating more types + // and so forth that we need to. + if self.fast_reject_trait_refs(obligation, &impl_trait_ref) { + return Err(()); + } + + let (placeholder_obligation, _) = + self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate); + let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref; + + let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id); + + let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs); + + let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } = + ensure_sufficient_stack(|| { + project::normalize_with_depth( + self, + obligation.param_env, + obligation.cause.clone(), + obligation.recursion_depth + 1, + &impl_trait_ref, + ) + }); + + debug!( + "match_impl(impl_def_id={:?}, obligation={:?}, \ + impl_trait_ref={:?}, placeholder_obligation_trait_ref={:?})", + impl_def_id, obligation, impl_trait_ref, placeholder_obligation_trait_ref + ); + + let InferOk { obligations, .. } = self + .infcx + .at(&obligation.cause, obligation.param_env) + .eq(placeholder_obligation_trait_ref, impl_trait_ref) + .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?; + nested_obligations.extend(obligations); + + if !self.intercrate + && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation + { + debug!("match_impl: reservation impls only apply in intercrate mode"); + return Err(()); + } + + debug!("match_impl: success impl_substs={:?}", impl_substs); + Ok(Normalized { value: impl_substs, obligations: nested_obligations }) + } + + fn fast_reject_trait_refs( + &mut self, + obligation: &TraitObligation<'_>, + impl_trait_ref: &ty::TraitRef<'_>, + ) -> bool { + // We can avoid creating type variables and doing the full + // substitution if we find that any of the input types, when + // simplified, do not match. + + obligation.predicate.skip_binder().trait_ref.substs.iter().zip(impl_trait_ref.substs).any( + |(obligation_arg, impl_arg)| { + match (obligation_arg.unpack(), impl_arg.unpack()) { + (GenericArgKind::Type(obligation_ty), GenericArgKind::Type(impl_ty)) => { + let simplified_obligation_ty = + fast_reject::simplify_type(self.tcx(), obligation_ty, true); + let simplified_impl_ty = + fast_reject::simplify_type(self.tcx(), impl_ty, false); + + simplified_obligation_ty.is_some() + && simplified_impl_ty.is_some() + && simplified_obligation_ty != simplified_impl_ty + } + (GenericArgKind::Lifetime(_), GenericArgKind::Lifetime(_)) => { + // Lifetimes can never cause a rejection. + false + } + (GenericArgKind::Const(_), GenericArgKind::Const(_)) => { + // Conservatively ignore consts (i.e. assume they might + // unify later) until we have `fast_reject` support for + // them (if we'll ever need it, even). + false + } + _ => unreachable!(), + } + }, + ) + } + + /// Normalize `where_clause_trait_ref` and try to match it against + /// `obligation`. If successful, return any predicates that + /// result from the normalization. Normalization is necessary + /// because where-clauses are stored in the parameter environment + /// unnormalized. + fn match_where_clause_trait_ref( + &mut self, + obligation: &TraitObligation<'tcx>, + where_clause_trait_ref: ty::PolyTraitRef<'tcx>, + ) -> Result<Vec<PredicateObligation<'tcx>>, ()> { + self.match_poly_trait_ref(obligation, where_clause_trait_ref) + } + + /// Returns `Ok` if `poly_trait_ref` being true implies that the + /// obligation is satisfied. + fn match_poly_trait_ref( + &mut self, + obligation: &TraitObligation<'tcx>, + poly_trait_ref: ty::PolyTraitRef<'tcx>, + ) -> Result<Vec<PredicateObligation<'tcx>>, ()> { + debug!( + "match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}", + obligation, poly_trait_ref + ); + + self.infcx + .at(&obligation.cause, obligation.param_env) + .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref) + .map(|InferOk { obligations, .. }| obligations) + .map_err(|_| ()) + } + + /////////////////////////////////////////////////////////////////////////// + // Miscellany + + fn match_fresh_trait_refs( + &self, + previous: ty::PolyTraitRef<'tcx>, + current: ty::PolyTraitRef<'tcx>, + param_env: ty::ParamEnv<'tcx>, + ) -> bool { + let mut matcher = ty::_match::Match::new(self.tcx(), param_env); + matcher.relate(previous, current).is_ok() + } + + fn push_stack<'o>( + &mut self, + previous_stack: TraitObligationStackList<'o, 'tcx>, + obligation: &'o TraitObligation<'tcx>, + ) -> TraitObligationStack<'o, 'tcx> { + let fresh_trait_ref = + obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener); + + let dfn = previous_stack.cache.next_dfn(); + let depth = previous_stack.depth() + 1; + TraitObligationStack { + obligation, + fresh_trait_ref, + reached_depth: Cell::new(depth), + previous: previous_stack, + dfn, + depth, + } + } + + fn closure_trait_ref_unnormalized( + &mut self, + obligation: &TraitObligation<'tcx>, + substs: SubstsRef<'tcx>, + ) -> ty::PolyTraitRef<'tcx> { + debug!("closure_trait_ref_unnormalized(obligation={:?}, substs={:?})", obligation, substs); + let closure_sig = substs.as_closure().sig(); + + debug!("closure_trait_ref_unnormalized: closure_sig = {:?}", closure_sig); + + // (1) Feels icky to skip the binder here, but OTOH we know + // that the self-type is an unboxed closure type and hence is + // in fact unparameterized (or at least does not reference any + // regions bound in the obligation). Still probably some + // refactoring could make this nicer. + closure_trait_ref_and_return_type( + self.tcx(), + obligation.predicate.def_id(), + obligation.predicate.skip_binder().self_ty(), // (1) + closure_sig, + util::TupleArgumentsFlag::No, + ) + .map_bound(|(trait_ref, _)| trait_ref) + } + + fn generator_trait_ref_unnormalized( + &mut self, + obligation: &TraitObligation<'tcx>, + substs: SubstsRef<'tcx>, + ) -> ty::PolyTraitRef<'tcx> { + let gen_sig = substs.as_generator().poly_sig(); + + // (1) Feels icky to skip the binder here, but OTOH we know + // that the self-type is an generator type and hence is + // in fact unparameterized (or at least does not reference any + // regions bound in the obligation). Still probably some + // refactoring could make this nicer. + + super::util::generator_trait_ref_and_outputs( + self.tcx(), + obligation.predicate.def_id(), + obligation.predicate.skip_binder().self_ty(), // (1) + gen_sig, + ) + .map_bound(|(trait_ref, ..)| trait_ref) + } + + /// Returns the obligations that are implied by instantiating an + /// impl or trait. The obligations are substituted and fully + /// normalized. This is used when confirming an impl or default + /// impl. + fn impl_or_trait_obligations( + &mut self, + cause: ObligationCause<'tcx>, + recursion_depth: usize, + param_env: ty::ParamEnv<'tcx>, + def_id: DefId, // of impl or trait + substs: SubstsRef<'tcx>, // for impl or trait + ) -> Vec<PredicateObligation<'tcx>> { + debug!("impl_or_trait_obligations(def_id={:?})", def_id); + let tcx = self.tcx(); + + // To allow for one-pass evaluation of the nested obligation, + // each predicate must be preceded by the obligations required + // to normalize it. + // for example, if we have: + // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V + // the impl will have the following predicates: + // <V as Iterator>::Item = U, + // U: Iterator, U: Sized, + // V: Iterator, V: Sized, + // <U as Iterator>::Item: Copy + // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last + // obligation will normalize to `<$0 as Iterator>::Item = $1` and + // `$1: Copy`, so we must ensure the obligations are emitted in + // that order. + let predicates = tcx.predicates_of(def_id); + assert_eq!(predicates.parent, None); + let mut obligations = Vec::with_capacity(predicates.predicates.len()); + for (predicate, _) in predicates.predicates { + let predicate = normalize_with_depth_to( + self, + param_env, + cause.clone(), + recursion_depth, + &predicate.subst(tcx, substs), + &mut obligations, + ); + obligations.push(Obligation { + cause: cause.clone(), + recursion_depth, + param_env, + predicate, + }); + } + + // We are performing deduplication here to avoid exponential blowups + // (#38528) from happening, but the real cause of the duplication is + // unknown. What we know is that the deduplication avoids exponential + // amount of predicates being propagated when processing deeply nested + // types. + // + // This code is hot enough that it's worth avoiding the allocation + // required for the FxHashSet when possible. Special-casing lengths 0, + // 1 and 2 covers roughly 75-80% of the cases. + if obligations.len() <= 1 { + // No possibility of duplicates. + } else if obligations.len() == 2 { + // Only two elements. Drop the second if they are equal. + if obligations[0] == obligations[1] { + obligations.truncate(1); + } + } else { + // Three or more elements. Use a general deduplication process. + let mut seen = FxHashSet::default(); + obligations.retain(|i| seen.insert(i.clone())); + } + + obligations + } +} + +trait TraitObligationExt<'tcx> { + fn derived_cause( + &self, + variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>, + ) -> ObligationCause<'tcx>; +} + +impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> { + #[allow(unused_comparisons)] + fn derived_cause( + &self, + variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>, + ) -> ObligationCause<'tcx> { + /*! + * Creates a cause for obligations that are derived from + * `obligation` by a recursive search (e.g., for a builtin + * bound, or eventually a `auto trait Foo`). If `obligation` + * is itself a derived obligation, this is just a clone, but + * otherwise we create a "derived obligation" cause so as to + * keep track of the original root obligation for error + * reporting. + */ + + let obligation = self; + + // NOTE(flaper87): As of now, it keeps track of the whole error + // chain. Ideally, we should have a way to configure this either + // by using -Z verbose or just a CLI argument. + let derived_cause = DerivedObligationCause { + parent_trait_ref: obligation.predicate.to_poly_trait_ref(), + parent_code: Rc::new(obligation.cause.code.clone()), + }; + let derived_code = variant(derived_cause); + ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code) + } +} + +impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> { + fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> { + TraitObligationStackList::with(self) + } + + fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> { + self.previous.cache + } + + fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> { + self.list() + } + + /// Indicates that attempting to evaluate this stack entry + /// required accessing something from the stack at depth `reached_depth`. + fn update_reached_depth(&self, reached_depth: usize) { + assert!( + self.depth > reached_depth, + "invoked `update_reached_depth` with something under this stack: \ + self.depth={} reached_depth={}", + self.depth, + reached_depth, + ); + debug!("update_reached_depth(reached_depth={})", reached_depth); + let mut p = self; + while reached_depth < p.depth { + debug!("update_reached_depth: marking {:?} as cycle participant", p.fresh_trait_ref); + p.reached_depth.set(p.reached_depth.get().min(reached_depth)); + p = p.previous.head.unwrap(); + } + } +} + +/// The "provisional evaluation cache" is used to store intermediate cache results +/// when solving auto traits. Auto traits are unusual in that they can support +/// cycles. So, for example, a "proof tree" like this would be ok: +/// +/// - `Foo<T>: Send` :- +/// - `Bar<T>: Send` :- +/// - `Foo<T>: Send` -- cycle, but ok +/// - `Baz<T>: Send` +/// +/// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and +/// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`. +/// For non-auto traits, this cycle would be an error, but for auto traits (because +/// they are coinductive) it is considered ok. +/// +/// However, there is a complication: at the point where we have +/// "proven" `Bar<T>: Send`, we have in fact only proven it +/// *provisionally*. In particular, we proved that `Bar<T>: Send` +/// *under the assumption* that `Foo<T>: Send`. But what if we later +/// find out this assumption is wrong? Specifically, we could +/// encounter some kind of error proving `Baz<T>: Send`. In that case, +/// `Bar<T>: Send` didn't turn out to be true. +/// +/// In Issue #60010, we found a bug in rustc where it would cache +/// these intermediate results. This was fixed in #60444 by disabling +/// *all* caching for things involved in a cycle -- in our example, +/// that would mean we don't cache that `Bar<T>: Send`. But this led +/// to large slowdowns. +/// +/// Specifically, imagine this scenario, where proving `Baz<T>: Send` +/// first requires proving `Bar<T>: Send` (which is true: +/// +/// - `Foo<T>: Send` :- +/// - `Bar<T>: Send` :- +/// - `Foo<T>: Send` -- cycle, but ok +/// - `Baz<T>: Send` +/// - `Bar<T>: Send` -- would be nice for this to be a cache hit! +/// - `*const T: Send` -- but what if we later encounter an error? +/// +/// The *provisional evaluation cache* resolves this issue. It stores +/// cache results that we've proven but which were involved in a cycle +/// in some way. We track the minimal stack depth (i.e., the +/// farthest from the top of the stack) that we are dependent on. +/// The idea is that the cache results within are all valid -- so long as +/// none of the nodes in between the current node and the node at that minimum +/// depth result in an error (in which case the cached results are just thrown away). +/// +/// During evaluation, we consult this provisional cache and rely on +/// it. Accessing a cached value is considered equivalent to accessing +/// a result at `reached_depth`, so it marks the *current* solution as +/// provisional as well. If an error is encountered, we toss out any +/// provisional results added from the subtree that encountered the +/// error. When we pop the node at `reached_depth` from the stack, we +/// can commit all the things that remain in the provisional cache. +struct ProvisionalEvaluationCache<'tcx> { + /// next "depth first number" to issue -- just a counter + dfn: Cell<usize>, + + /// Stores the "coldest" depth (bottom of stack) reached by any of + /// the evaluation entries. The idea here is that all things in the provisional + /// cache are always dependent on *something* that is colder in the stack: + /// therefore, if we add a new entry that is dependent on something *colder still*, + /// we have to modify the depth for all entries at once. + /// + /// Example: + /// + /// Imagine we have a stack `A B C D E` (with `E` being the top of + /// the stack). We cache something with depth 2, which means that + /// it was dependent on C. Then we pop E but go on and process a + /// new node F: A B C D F. Now F adds something to the cache with + /// depth 1, meaning it is dependent on B. Our original cache + /// entry is also dependent on B, because there is a path from E + /// to C and then from C to F and from F to B. + reached_depth: Cell<usize>, + + /// Map from cache key to the provisionally evaluated thing. + /// The cache entries contain the result but also the DFN in which they + /// were added. The DFN is used to clear out values on failure. + /// + /// Imagine we have a stack like: + /// + /// - `A B C` and we add a cache for the result of C (DFN 2) + /// - Then we have a stack `A B D` where `D` has DFN 3 + /// - We try to solve D by evaluating E: `A B D E` (DFN 4) + /// - `E` generates various cache entries which have cyclic dependices on `B` + /// - `A B D E F` and so forth + /// - the DFN of `F` for example would be 5 + /// - then we determine that `E` is in error -- we will then clear + /// all cache values whose DFN is >= 4 -- in this case, that + /// means the cached value for `F`. + map: RefCell<FxHashMap<ty::PolyTraitRef<'tcx>, ProvisionalEvaluation>>, +} + +/// A cache value for the provisional cache: contains the depth-first +/// number (DFN) and result. +#[derive(Copy, Clone, Debug)] +struct ProvisionalEvaluation { + from_dfn: usize, + result: EvaluationResult, +} + +impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> { + fn default() -> Self { + Self { dfn: Cell::new(0), reached_depth: Cell::new(usize::MAX), map: Default::default() } + } +} + +impl<'tcx> ProvisionalEvaluationCache<'tcx> { + /// Get the next DFN in sequence (basically a counter). + fn next_dfn(&self) -> usize { + let result = self.dfn.get(); + self.dfn.set(result + 1); + result + } + + /// Check the provisional cache for any result for + /// `fresh_trait_ref`. If there is a hit, then you must consider + /// it an access to the stack slots at depth + /// `self.current_reached_depth()` and above. + fn get_provisional(&self, fresh_trait_ref: ty::PolyTraitRef<'tcx>) -> Option<EvaluationResult> { + debug!( + "get_provisional(fresh_trait_ref={:?}) = {:#?} with reached-depth {}", + fresh_trait_ref, + self.map.borrow().get(&fresh_trait_ref), + self.reached_depth.get(), + ); + Some(self.map.borrow().get(&fresh_trait_ref)?.result) + } + + /// Current value of the `reached_depth` counter -- all the + /// provisional cache entries are dependent on the item at this + /// depth. + fn current_reached_depth(&self) -> usize { + self.reached_depth.get() + } + + /// Insert a provisional result into the cache. The result came + /// from the node with the given DFN. It accessed a minimum depth + /// of `reached_depth` to compute. It evaluated `fresh_trait_ref` + /// and resulted in `result`. + fn insert_provisional( + &self, + from_dfn: usize, + reached_depth: usize, + fresh_trait_ref: ty::PolyTraitRef<'tcx>, + result: EvaluationResult, + ) { + debug!( + "insert_provisional(from_dfn={}, reached_depth={}, fresh_trait_ref={:?}, result={:?})", + from_dfn, reached_depth, fresh_trait_ref, result, + ); + let r_d = self.reached_depth.get(); + self.reached_depth.set(r_d.min(reached_depth)); + + debug!("insert_provisional: reached_depth={:?}", self.reached_depth.get()); + + self.map.borrow_mut().insert(fresh_trait_ref, ProvisionalEvaluation { from_dfn, result }); + } + + /// Invoked when the node with dfn `dfn` does not get a successful + /// result. This will clear out any provisional cache entries + /// that were added since `dfn` was created. This is because the + /// provisional entries are things which must assume that the + /// things on the stack at the time of their creation succeeded -- + /// since the failing node is presently at the top of the stack, + /// these provisional entries must either depend on it or some + /// ancestor of it. + fn on_failure(&self, dfn: usize) { + debug!("on_failure(dfn={:?})", dfn,); + self.map.borrow_mut().retain(|key, eval| { + if !eval.from_dfn >= dfn { + debug!("on_failure: removing {:?}", key); + false + } else { + true + } + }); + } + + /// Invoked when the node at depth `depth` completed without + /// depending on anything higher in the stack (if that completion + /// was a failure, then `on_failure` should have been invoked + /// already). The callback `op` will be invoked for each + /// provisional entry that we can now confirm. + fn on_completion( + &self, + depth: usize, + mut op: impl FnMut(ty::PolyTraitRef<'tcx>, EvaluationResult), + ) { + debug!("on_completion(depth={}, reached_depth={})", depth, self.reached_depth.get(),); + + if self.reached_depth.get() < depth { + debug!("on_completion: did not yet reach depth to complete"); + return; + } + + for (fresh_trait_ref, eval) in self.map.borrow_mut().drain() { + debug!("on_completion: fresh_trait_ref={:?} eval={:?}", fresh_trait_ref, eval,); + + op(fresh_trait_ref, eval.result); + } + + self.reached_depth.set(usize::MAX); + } +} + +#[derive(Copy, Clone)] +struct TraitObligationStackList<'o, 'tcx> { + cache: &'o ProvisionalEvaluationCache<'tcx>, + head: Option<&'o TraitObligationStack<'o, 'tcx>>, +} + +impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> { + fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> { + TraitObligationStackList { cache, head: None } + } + + fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> { + TraitObligationStackList { cache: r.cache(), head: Some(r) } + } + + fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> { + self.head + } + + fn depth(&self) -> usize { + if let Some(head) = self.head { head.depth } else { 0 } + } +} + +impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> { + type Item = &'o TraitObligationStack<'o, 'tcx>; + + fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> { + match self.head { + Some(o) => { + *self = o.previous; + Some(o) + } + None => None, + } + } +} + +impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + write!(f, "TraitObligationStack({:?})", self.obligation) + } +} |