| //! See Rustc Dev Guide chapters on [trait-resolution] and [trait-specialization] for more info on |
| //! how this works. |
| //! |
| //! [trait-resolution]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html |
| //! [trait-specialization]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html |
| |
| use crate::infer::{CombinedSnapshot, InferOk, TyCtxtInferExt}; |
| use crate::traits::select::IntercrateAmbiguityCause; |
| use crate::traits::SkipLeakCheck; |
| use crate::traits::{self, Normalized, Obligation, ObligationCause, SelectionContext}; |
| use rustc_hir::def_id::{DefId, LOCAL_CRATE}; |
| use rustc_middle::ty::fold::TypeFoldable; |
| use rustc_middle::ty::subst::Subst; |
| use rustc_middle::ty::{self, fast_reject, Ty, TyCtxt}; |
| use rustc_span::symbol::sym; |
| use rustc_span::DUMMY_SP; |
| use std::iter; |
| |
| /// Whether we do the orphan check relative to this crate or |
| /// to some remote crate. |
| #[derive(Copy, Clone, Debug)] |
| enum InCrate { |
| Local, |
| Remote, |
| } |
| |
| #[derive(Debug, Copy, Clone)] |
| pub enum Conflict { |
| Upstream, |
| Downstream, |
| } |
| |
| pub struct OverlapResult<'tcx> { |
| pub impl_header: ty::ImplHeader<'tcx>, |
| pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>, |
| |
| /// `true` if the overlap might've been permitted before the shift |
| /// to universes. |
| pub involves_placeholder: bool, |
| } |
| |
| pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) { |
| err.note( |
| "this behavior recently changed as a result of a bug fix; \ |
| see rust-lang/rust#56105 for details", |
| ); |
| } |
| |
| /// If there are types that satisfy both impls, invokes `on_overlap` |
| /// with a suitably-freshened `ImplHeader` with those types |
| /// substituted. Otherwise, invokes `no_overlap`. |
| pub fn overlapping_impls<F1, F2, R>( |
| tcx: TyCtxt<'_>, |
| impl1_def_id: DefId, |
| impl2_def_id: DefId, |
| skip_leak_check: SkipLeakCheck, |
| on_overlap: F1, |
| no_overlap: F2, |
| ) -> R |
| where |
| F1: FnOnce(OverlapResult<'_>) -> R, |
| F2: FnOnce() -> R, |
| { |
| debug!( |
| "overlapping_impls(\ |
| impl1_def_id={:?}, \ |
| impl2_def_id={:?})", |
| impl1_def_id, impl2_def_id, |
| ); |
| // Before doing expensive operations like entering an inference context, do |
| // a quick check via fast_reject to tell if the impl headers could possibly |
| // unify. |
| let impl1_ref = tcx.impl_trait_ref(impl1_def_id); |
| let impl2_ref = tcx.impl_trait_ref(impl2_def_id); |
| |
| // Check if any of the input types definitely do not unify. |
| if iter::zip( |
| impl1_ref.iter().flat_map(|tref| tref.substs.types()), |
| impl2_ref.iter().flat_map(|tref| tref.substs.types()), |
| ) |
| .any(|(ty1, ty2)| { |
| let t1 = fast_reject::simplify_type(tcx, ty1, false); |
| let t2 = fast_reject::simplify_type(tcx, ty2, false); |
| if let (Some(t1), Some(t2)) = (t1, t2) { |
| // Simplified successfully |
| // Types cannot unify if they differ in their reference mutability or simplify to different types |
| t1 != t2 || ty1.ref_mutability() != ty2.ref_mutability() |
| } else { |
| // Types might unify |
| false |
| } |
| }) { |
| // Some types involved are definitely different, so the impls couldn't possibly overlap. |
| debug!("overlapping_impls: fast_reject early-exit"); |
| return no_overlap(); |
| } |
| |
| let overlaps = tcx.infer_ctxt().enter(|infcx| { |
| let selcx = &mut SelectionContext::intercrate(&infcx); |
| overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some() |
| }); |
| |
| if !overlaps { |
| return no_overlap(); |
| } |
| |
| // In the case where we detect an error, run the check again, but |
| // this time tracking intercrate ambuiguity causes for better |
| // diagnostics. (These take time and can lead to false errors.) |
| tcx.infer_ctxt().enter(|infcx| { |
| let selcx = &mut SelectionContext::intercrate(&infcx); |
| selcx.enable_tracking_intercrate_ambiguity_causes(); |
| on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap()) |
| }) |
| } |
| |
| fn with_fresh_ty_vars<'cx, 'tcx>( |
| selcx: &mut SelectionContext<'cx, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| impl_def_id: DefId, |
| ) -> ty::ImplHeader<'tcx> { |
| let tcx = selcx.tcx(); |
| let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id); |
| |
| let header = ty::ImplHeader { |
| impl_def_id, |
| self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs), |
| trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs), |
| predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates, |
| }; |
| |
| let Normalized { value: mut header, obligations } = |
| traits::normalize(selcx, param_env, ObligationCause::dummy(), header); |
| |
| header.predicates.extend(obligations.into_iter().map(|o| o.predicate)); |
| header |
| } |
| |
| /// Can both impl `a` and impl `b` be satisfied by a common type (including |
| /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls. |
| fn overlap<'cx, 'tcx>( |
| selcx: &mut SelectionContext<'cx, 'tcx>, |
| skip_leak_check: SkipLeakCheck, |
| a_def_id: DefId, |
| b_def_id: DefId, |
| ) -> Option<OverlapResult<'tcx>> { |
| debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id); |
| |
| selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| { |
| overlap_within_probe(selcx, skip_leak_check, a_def_id, b_def_id, snapshot) |
| }) |
| } |
| |
| fn overlap_within_probe( |
| selcx: &mut SelectionContext<'cx, 'tcx>, |
| skip_leak_check: SkipLeakCheck, |
| a_def_id: DefId, |
| b_def_id: DefId, |
| snapshot: &CombinedSnapshot<'_, 'tcx>, |
| ) -> Option<OverlapResult<'tcx>> { |
| // For the purposes of this check, we don't bring any placeholder |
| // types into scope; instead, we replace the generic types with |
| // fresh type variables, and hence we do our evaluations in an |
| // empty environment. |
| let param_env = ty::ParamEnv::empty(); |
| |
| let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id); |
| let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id); |
| |
| debug!("overlap: a_impl_header={:?}", a_impl_header); |
| debug!("overlap: b_impl_header={:?}", b_impl_header); |
| |
| // Do `a` and `b` unify? If not, no overlap. |
| let obligations = match selcx |
| .infcx() |
| .at(&ObligationCause::dummy(), param_env) |
| .eq_impl_headers(&a_impl_header, &b_impl_header) |
| { |
| Ok(InferOk { obligations, value: () }) => obligations, |
| Err(_) => { |
| return None; |
| } |
| }; |
| |
| debug!("overlap: unification check succeeded"); |
| |
| // Are any of the obligations unsatisfiable? If so, no overlap. |
| let infcx = selcx.infcx(); |
| let opt_failing_obligation = a_impl_header |
| .predicates |
| .iter() |
| .copied() |
| .chain(b_impl_header.predicates) |
| .map(|p| infcx.resolve_vars_if_possible(p)) |
| .map(|p| Obligation { |
| cause: ObligationCause::dummy(), |
| param_env, |
| recursion_depth: 0, |
| predicate: p, |
| }) |
| .chain(obligations) |
| .find(|o| !selcx.predicate_may_hold_fatal(o)); |
| // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported |
| // to the canonical trait query form, `infcx.predicate_may_hold`, once |
| // the new system supports intercrate mode (which coherence needs). |
| |
| if let Some(failing_obligation) = opt_failing_obligation { |
| debug!("overlap: obligation unsatisfiable {:?}", failing_obligation); |
| return None; |
| } |
| |
| if !skip_leak_check.is_yes() { |
| if infcx.leak_check(true, snapshot).is_err() { |
| debug!("overlap: leak check failed"); |
| return None; |
| } |
| } |
| |
| let impl_header = selcx.infcx().resolve_vars_if_possible(a_impl_header); |
| let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes(); |
| debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes); |
| |
| let involves_placeholder = |
| matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true)); |
| |
| Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder }) |
| } |
| |
| pub fn trait_ref_is_knowable<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| trait_ref: ty::TraitRef<'tcx>, |
| ) -> Option<Conflict> { |
| debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref); |
| if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() { |
| // A downstream or cousin crate is allowed to implement some |
| // substitution of this trait-ref. |
| return Some(Conflict::Downstream); |
| } |
| |
| if trait_ref_is_local_or_fundamental(tcx, trait_ref) { |
| // This is a local or fundamental trait, so future-compatibility |
| // is no concern. We know that downstream/cousin crates are not |
| // allowed to implement a substitution of this trait ref, which |
| // means impls could only come from dependencies of this crate, |
| // which we already know about. |
| return None; |
| } |
| |
| // This is a remote non-fundamental trait, so if another crate |
| // can be the "final owner" of a substitution of this trait-ref, |
| // they are allowed to implement it future-compatibly. |
| // |
| // However, if we are a final owner, then nobody else can be, |
| // and if we are an intermediate owner, then we don't care |
| // about future-compatibility, which means that we're OK if |
| // we are an owner. |
| if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() { |
| debug!("trait_ref_is_knowable: orphan check passed"); |
| None |
| } else { |
| debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned"); |
| Some(Conflict::Upstream) |
| } |
| } |
| |
| pub fn trait_ref_is_local_or_fundamental<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| trait_ref: ty::TraitRef<'tcx>, |
| ) -> bool { |
| trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental) |
| } |
| |
| pub enum OrphanCheckErr<'tcx> { |
| NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>), |
| UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>), |
| } |
| |
| /// Checks the coherence orphan rules. `impl_def_id` should be the |
| /// `DefId` of a trait impl. To pass, either the trait must be local, or else |
| /// two conditions must be satisfied: |
| /// |
| /// 1. All type parameters in `Self` must be "covered" by some local type constructor. |
| /// 2. Some local type must appear in `Self`. |
| pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> { |
| debug!("orphan_check({:?})", impl_def_id); |
| |
| // We only except this routine to be invoked on implementations |
| // of a trait, not inherent implementations. |
| let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap(); |
| debug!("orphan_check: trait_ref={:?}", trait_ref); |
| |
| // If the *trait* is local to the crate, ok. |
| if trait_ref.def_id.is_local() { |
| debug!("trait {:?} is local to current crate", trait_ref.def_id); |
| return Ok(()); |
| } |
| |
| orphan_check_trait_ref(tcx, trait_ref, InCrate::Local) |
| } |
| |
| /// Checks whether a trait-ref is potentially implementable by a crate. |
| /// |
| /// The current rule is that a trait-ref orphan checks in a crate C: |
| /// |
| /// 1. Order the parameters in the trait-ref in subst order - Self first, |
| /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W). |
| /// 2. Of these type parameters, there is at least one type parameter |
| /// in which, walking the type as a tree, you can reach a type local |
| /// to C where all types in-between are fundamental types. Call the |
| /// first such parameter the "local key parameter". |
| /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType |
| /// going through `Box`, which is fundamental. |
| /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for |
| /// the same reason. |
| /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's |
| /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between |
| /// the local type and the type parameter. |
| /// 3. Before this local type, no generic type parameter of the impl must |
| /// be reachable through fundamental types. |
| /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental. |
| /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is |
| /// reachable through the fundamental type `Box`. |
| /// 4. Every type in the local key parameter not known in C, going |
| /// through the parameter's type tree, must appear only as a subtree of |
| /// a type local to C, with only fundamental types between the type |
| /// local to C and the local key parameter. |
| /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`) |
| /// is bad, because the only local type with `T` as a subtree is |
| /// `LocalType<T>`, and `Vec<->` is between it and the type parameter. |
| /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because |
| /// the second occurrence of `T` is not a subtree of *any* local type. |
| /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of |
| /// `LocalType<Vec<T>>`, which is local and has no types between it and |
| /// the type parameter. |
| /// |
| /// The orphan rules actually serve several different purposes: |
| /// |
| /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where |
| /// every type local to one crate is unknown in the other) can't implement |
| /// the same trait-ref. This follows because it can be seen that no such |
| /// type can orphan-check in 2 such crates. |
| /// |
| /// To check that a local impl follows the orphan rules, we check it in |
| /// InCrate::Local mode, using type parameters for the "generic" types. |
| /// |
| /// 2. They ground negative reasoning for coherence. If a user wants to |
| /// write both a conditional blanket impl and a specific impl, we need to |
| /// make sure they do not overlap. For example, if we write |
| /// ``` |
| /// impl<T> IntoIterator for Vec<T> |
| /// impl<T: Iterator> IntoIterator for T |
| /// ``` |
| /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0. |
| /// We can observe that this holds in the current crate, but we need to make |
| /// sure this will also hold in all unknown crates (both "independent" crates, |
| /// which we need for link-safety, and also child crates, because we don't want |
| /// child crates to get error for impl conflicts in a *dependency*). |
| /// |
| /// For that, we only allow negative reasoning if, for every assignment to the |
| /// inference variables, every unknown crate would get an orphan error if they |
| /// try to implement this trait-ref. To check for this, we use InCrate::Remote |
| /// mode. That is sound because we already know all the impls from known crates. |
| /// |
| /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can |
| /// add "non-blanket" impls without breaking negative reasoning in dependent |
| /// crates. This is the "rebalancing coherence" (RFC 1023) restriction. |
| /// |
| /// For that, we only a allow crate to perform negative reasoning on |
| /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2). |
| /// |
| /// Because we never perform negative reasoning generically (coherence does |
| /// not involve type parameters), this can be interpreted as doing the full |
| /// orphan check (using InCrate::Local mode), substituting non-local known |
| /// types for all inference variables. |
| /// |
| /// This allows for crates to future-compatibly add impls as long as they |
| /// can't apply to types with a key parameter in a child crate - applying |
| /// the rules, this basically means that every type parameter in the impl |
| /// must appear behind a non-fundamental type (because this is not a |
| /// type-system requirement, crate owners might also go for "semantic |
| /// future-compatibility" involving things such as sealed traits, but |
| /// the above requirement is sufficient, and is necessary in "open world" |
| /// cases). |
| /// |
| /// Note that this function is never called for types that have both type |
| /// parameters and inference variables. |
| fn orphan_check_trait_ref<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| trait_ref: ty::TraitRef<'tcx>, |
| in_crate: InCrate, |
| ) -> Result<(), OrphanCheckErr<'tcx>> { |
| debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate); |
| |
| if trait_ref.needs_infer() && trait_ref.definitely_needs_subst(tcx) { |
| bug!( |
| "can't orphan check a trait ref with both params and inference variables {:?}", |
| trait_ref |
| ); |
| } |
| |
| // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only |
| // if at least one of the following is true: |
| // |
| // - Trait is a local trait |
| // (already checked in orphan_check prior to calling this function) |
| // - All of |
| // - At least one of the types T0..=Tn must be a local type. |
| // Let Ti be the first such type. |
| // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti) |
| // |
| fn uncover_fundamental_ty<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| ty: Ty<'tcx>, |
| in_crate: InCrate, |
| ) -> Vec<Ty<'tcx>> { |
| // FIXME: this is currently somewhat overly complicated, |
| // but fixing this requires a more complicated refactor. |
| if !contained_non_local_types(tcx, ty, in_crate).is_empty() { |
| if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) { |
| return inner_tys |
| .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)) |
| .collect(); |
| } |
| } |
| |
| vec![ty] |
| } |
| |
| let mut non_local_spans = vec![]; |
| for (i, input_ty) in trait_ref |
| .substs |
| .types() |
| .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)) |
| .enumerate() |
| { |
| debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty); |
| let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate); |
| if non_local_tys.is_empty() { |
| debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty); |
| return Ok(()); |
| } else if let ty::Param(_) = input_ty.kind() { |
| debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty); |
| let local_type = trait_ref |
| .substs |
| .types() |
| .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)) |
| .find(|ty| ty_is_local_constructor(ty, in_crate)); |
| |
| debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type); |
| |
| return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type)); |
| } |
| |
| for input_ty in non_local_tys { |
| non_local_spans.push((input_ty, i == 0)); |
| } |
| } |
| // If we exit above loop, never found a local type. |
| debug!("orphan_check_trait_ref: no local type"); |
| Err(OrphanCheckErr::NonLocalInputType(non_local_spans)) |
| } |
| |
| /// Returns a list of relevant non-local types for `ty`. |
| /// |
| /// This is just `ty` itself unless `ty` is `#[fundamental]`, |
| /// in which case we recursively look into this type. |
| /// |
| /// If `ty` is local itself, this method returns an empty `Vec`. |
| /// |
| /// # Examples |
| /// |
| /// - `u32` is not local, so this returns `[u32]`. |
| /// - for `Foo<u32>`, where `Foo` is a local type, this returns `[]`. |
| /// - `&mut u32` returns `[u32]`, as `&mut` is a fundamental type, similar to `Box`. |
| /// - `Box<Foo<u32>>` returns `[]`, as `Box` is a fundamental type and `Foo` is local. |
| fn contained_non_local_types(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, in_crate: InCrate) -> Vec<Ty<'tcx>> { |
| if ty_is_local_constructor(ty, in_crate) { |
| Vec::new() |
| } else { |
| match fundamental_ty_inner_tys(tcx, ty) { |
| Some(inner_tys) => { |
| inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect() |
| } |
| None => vec![ty], |
| } |
| } |
| } |
| |
| /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the |
| /// type parameters of the ADT, or `T`, respectively. For non-fundamental |
| /// types, returns `None`. |
| fn fundamental_ty_inner_tys( |
| tcx: TyCtxt<'tcx>, |
| ty: Ty<'tcx>, |
| ) -> Option<impl Iterator<Item = Ty<'tcx>>> { |
| let (first_ty, rest_tys) = match *ty.kind() { |
| ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()), |
| ty::Adt(def, substs) if def.is_fundamental() => { |
| let mut types = substs.types(); |
| |
| // FIXME(eddyb) actually validate `#[fundamental]` up-front. |
| match types.next() { |
| None => { |
| tcx.sess.span_err( |
| tcx.def_span(def.did), |
| "`#[fundamental]` requires at least one type parameter", |
| ); |
| |
| return None; |
| } |
| |
| Some(first_ty) => (first_ty, types), |
| } |
| } |
| _ => return None, |
| }; |
| |
| Some(iter::once(first_ty).chain(rest_tys)) |
| } |
| |
| fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool { |
| match in_crate { |
| // The type is local to *this* crate - it will not be |
| // local in any other crate. |
| InCrate::Remote => false, |
| InCrate::Local => def_id.is_local(), |
| } |
| } |
| |
| fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool { |
| debug!("ty_is_local_constructor({:?})", ty); |
| |
| match *ty.kind() { |
| ty::Bool |
| | ty::Char |
| | ty::Int(..) |
| | ty::Uint(..) |
| | ty::Float(..) |
| | ty::Str |
| | ty::FnDef(..) |
| | ty::FnPtr(_) |
| | ty::Array(..) |
| | ty::Slice(..) |
| | ty::RawPtr(..) |
| | ty::Ref(..) |
| | ty::Never |
| | ty::Tuple(..) |
| | ty::Param(..) |
| | ty::Projection(..) => false, |
| |
| ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate { |
| InCrate::Local => false, |
| // The inference variable might be unified with a local |
| // type in that remote crate. |
| InCrate::Remote => true, |
| }, |
| |
| ty::Adt(def, _) => def_id_is_local(def.did, in_crate), |
| ty::Foreign(did) => def_id_is_local(did, in_crate), |
| ty::Opaque(..) => { |
| // This merits some explanation. |
| // Normally, opaque types are not involed when performing |
| // coherence checking, since it is illegal to directly |
| // implement a trait on an opaque type. However, we might |
| // end up looking at an opaque type during coherence checking |
| // if an opaque type gets used within another type (e.g. as |
| // a type parameter). This requires us to decide whether or |
| // not an opaque type should be considered 'local' or not. |
| // |
| // We choose to treat all opaque types as non-local, even |
| // those that appear within the same crate. This seems |
| // somewhat surprising at first, but makes sense when |
| // you consider that opaque types are supposed to hide |
| // the underlying type *within the same crate*. When an |
| // opaque type is used from outside the module |
| // where it is declared, it should be impossible to observe |
| // anything about it other than the traits that it implements. |
| // |
| // The alternative would be to look at the underlying type |
| // to determine whether or not the opaque type itself should |
| // be considered local. However, this could make it a breaking change |
| // to switch the underlying ('defining') type from a local type |
| // to a remote type. This would violate the rule that opaque |
| // types should be completely opaque apart from the traits |
| // that they implement, so we don't use this behavior. |
| false |
| } |
| |
| ty::Closure(..) => { |
| // Similar to the `Opaque` case (#83613). |
| false |
| } |
| |
| ty::Dynamic(ref tt, ..) => { |
| if let Some(principal) = tt.principal() { |
| def_id_is_local(principal.def_id(), in_crate) |
| } else { |
| false |
| } |
| } |
| |
| ty::Error(_) => true, |
| |
| ty::Generator(..) | ty::GeneratorWitness(..) => { |
| bug!("ty_is_local invoked on unexpected type: {:?}", ty) |
| } |
| } |
| } |
