diff options
Diffstat (limited to 'compiler/rustc_trait_selection/src/traits/select/confirmation.rs')
| -rw-r--r-- | compiler/rustc_trait_selection/src/traits/select/confirmation.rs | 810 |
1 files changed, 810 insertions, 0 deletions
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 }) + } +} |