diff options
Diffstat (limited to 'compiler/rustc_trait_selection/src/traits/object_safety.rs')
| -rw-r--r-- | compiler/rustc_trait_selection/src/traits/object_safety.rs | 789 |
1 files changed, 789 insertions, 0 deletions
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs new file mode 100644 index 00000000000..c003e4f8068 --- /dev/null +++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs @@ -0,0 +1,789 @@ +//! "Object safety" refers to the ability for a trait to be converted +//! to an object. In general, traits may only be converted to an +//! object if all of their methods meet certain criteria. In particular, +//! they must: +//! +//! - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version +//! that doesn't contain the vtable; +//! - not reference the erased type `Self` except for in this receiver; +//! - not have generic type parameters. + +use super::elaborate_predicates; + +use crate::infer::TyCtxtInferExt; +use crate::traits::query::evaluate_obligation::InferCtxtExt; +use crate::traits::{self, Obligation, ObligationCause}; +use rustc_errors::{Applicability, FatalError}; +use rustc_hir as hir; +use rustc_hir::def_id::DefId; +use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst}; +use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor, WithConstness}; +use rustc_middle::ty::{Predicate, ToPredicate}; +use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY; +use rustc_span::symbol::Symbol; +use rustc_span::Span; +use smallvec::SmallVec; + +use std::iter; + +pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation}; + +/// Returns the object safety violations that affect +/// astconv -- currently, `Self` in supertraits. This is needed +/// because `object_safety_violations` can't be used during +/// type collection. +pub fn astconv_object_safety_violations( + tcx: TyCtxt<'_>, + trait_def_id: DefId, +) -> Vec<ObjectSafetyViolation> { + debug_assert!(tcx.generics_of(trait_def_id).has_self); + let violations = traits::supertrait_def_ids(tcx, trait_def_id) + .map(|def_id| predicates_reference_self(tcx, def_id, true)) + .filter(|spans| !spans.is_empty()) + .map(ObjectSafetyViolation::SupertraitSelf) + .collect(); + + debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations); + + violations +} + +fn object_safety_violations( + tcx: TyCtxt<'tcx>, + trait_def_id: DefId, +) -> &'tcx [ObjectSafetyViolation] { + debug_assert!(tcx.generics_of(trait_def_id).has_self); + debug!("object_safety_violations: {:?}", trait_def_id); + + tcx.arena.alloc_from_iter( + traits::supertrait_def_ids(tcx, trait_def_id) + .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)), + ) +} + +/// We say a method is *vtable safe* if it can be invoked on a trait +/// object. Note that object-safe traits can have some +/// non-vtable-safe methods, so long as they require `Self: Sized` or +/// otherwise ensure that they cannot be used when `Self = Trait`. +pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool { + debug_assert!(tcx.generics_of(trait_def_id).has_self); + debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method); + // Any method that has a `Self: Sized` bound cannot be called. + if generics_require_sized_self(tcx, method.def_id) { + return false; + } + + match virtual_call_violation_for_method(tcx, trait_def_id, method) { + None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true, + Some(_) => false, + } +} + +fn object_safety_violations_for_trait( + tcx: TyCtxt<'_>, + trait_def_id: DefId, +) -> Vec<ObjectSafetyViolation> { + // Check methods for violations. + let mut violations: Vec<_> = tcx + .associated_items(trait_def_id) + .in_definition_order() + .filter(|item| item.kind == ty::AssocKind::Fn) + .filter_map(|item| { + object_safety_violation_for_method(tcx, trait_def_id, &item) + .map(|(code, span)| ObjectSafetyViolation::Method(item.ident.name, code, span)) + }) + .filter(|violation| { + if let ObjectSafetyViolation::Method( + _, + MethodViolationCode::WhereClauseReferencesSelf, + span, + ) = violation + { + // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id. + // It's also hard to get a use site span, so we use the method definition span. + tcx.struct_span_lint_hir( + WHERE_CLAUSES_OBJECT_SAFETY, + hir::CRATE_HIR_ID, + *span, + |lint| { + let mut err = lint.build(&format!( + "the trait `{}` cannot be made into an object", + tcx.def_path_str(trait_def_id) + )); + let node = tcx.hir().get_if_local(trait_def_id); + let msg = if let Some(hir::Node::Item(item)) = node { + err.span_label( + item.ident.span, + "this trait cannot be made into an object...", + ); + format!("...because {}", violation.error_msg()) + } else { + format!( + "the trait cannot be made into an object because {}", + violation.error_msg() + ) + }; + err.span_label(*span, &msg); + match (node, violation.solution()) { + (Some(_), Some((note, None))) => { + err.help(¬e); + } + (Some(_), Some((note, Some((sugg, span))))) => { + err.span_suggestion( + span, + ¬e, + sugg, + Applicability::MachineApplicable, + ); + } + // Only provide the help if its a local trait, otherwise it's not actionable. + _ => {} + } + err.emit(); + }, + ); + false + } else { + true + } + }) + .collect(); + + // Check the trait itself. + if trait_has_sized_self(tcx, trait_def_id) { + // We don't want to include the requirement from `Sized` itself to be `Sized` in the list. + let spans = get_sized_bounds(tcx, trait_def_id); + violations.push(ObjectSafetyViolation::SizedSelf(spans)); + } + let spans = predicates_reference_self(tcx, trait_def_id, false); + if !spans.is_empty() { + violations.push(ObjectSafetyViolation::SupertraitSelf(spans)); + } + + violations.extend( + tcx.associated_items(trait_def_id) + .in_definition_order() + .filter(|item| item.kind == ty::AssocKind::Const) + .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)), + ); + + debug!( + "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}", + trait_def_id, violations + ); + + violations +} + +fn sized_trait_bound_spans<'tcx>( + tcx: TyCtxt<'tcx>, + bounds: hir::GenericBounds<'tcx>, +) -> impl 'tcx + Iterator<Item = Span> { + bounds.iter().filter_map(move |b| match b { + hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None) + if trait_has_sized_self( + tcx, + trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()), + ) => + { + // Fetch spans for supertraits that are `Sized`: `trait T: Super` + Some(trait_ref.span) + } + _ => None, + }) +} + +fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> { + tcx.hir() + .get_if_local(trait_def_id) + .and_then(|node| match node { + hir::Node::Item(hir::Item { + kind: hir::ItemKind::Trait(.., generics, bounds, _), + .. + }) => Some( + generics + .where_clause + .predicates + .iter() + .filter_map(|pred| { + match pred { + hir::WherePredicate::BoundPredicate(pred) + if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id => + { + // Fetch spans for trait bounds that are Sized: + // `trait T where Self: Pred` + Some(sized_trait_bound_spans(tcx, pred.bounds)) + } + _ => None, + } + }) + .flatten() + // Fetch spans for supertraits that are `Sized`: `trait T: Super`. + .chain(sized_trait_bound_spans(tcx, bounds)) + .collect::<SmallVec<[Span; 1]>>(), + ), + _ => None, + }) + .unwrap_or_else(SmallVec::new) +} + +fn predicates_reference_self( + tcx: TyCtxt<'_>, + trait_def_id: DefId, + supertraits_only: bool, +) -> SmallVec<[Span; 1]> { + let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id)); + let predicates = if supertraits_only { + tcx.super_predicates_of(trait_def_id) + } else { + tcx.predicates_of(trait_def_id) + }; + let self_ty = tcx.types.self_param; + let has_self_ty = |arg: &GenericArg<'_>| arg.walk().any(|arg| arg == self_ty.into()); + predicates + .predicates + .iter() + .map(|(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp)) + .filter_map(|(predicate, &sp)| { + match predicate.skip_binders() { + ty::PredicateAtom::Trait(ref data, _) => { + // In the case of a trait predicate, we can skip the "self" type. + if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None } + } + ty::PredicateAtom::Projection(ref data) => { + // And similarly for projections. This should be redundant with + // the previous check because any projection should have a + // matching `Trait` predicate with the same inputs, but we do + // the check to be safe. + // + // Note that we *do* allow projection *outputs* to contain + // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`), + // we just require the user to specify *both* outputs + // in the object type (i.e., `dyn Foo<Output=(), Result=()>`). + // + // This is ALT2 in issue #56288, see that for discussion of the + // possible alternatives. + if data.projection_ty.trait_ref(tcx).substs[1..].iter().any(has_self_ty) { + Some(sp) + } else { + None + } + } + ty::PredicateAtom::WellFormed(..) + | ty::PredicateAtom::ObjectSafe(..) + | ty::PredicateAtom::TypeOutlives(..) + | ty::PredicateAtom::RegionOutlives(..) + | ty::PredicateAtom::ClosureKind(..) + | ty::PredicateAtom::Subtype(..) + | ty::PredicateAtom::ConstEvaluatable(..) + | ty::PredicateAtom::ConstEquate(..) => None, + } + }) + .collect() +} + +fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool { + generics_require_sized_self(tcx, trait_def_id) +} + +fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool { + let sized_def_id = match tcx.lang_items().sized_trait() { + Some(def_id) => def_id, + None => { + return false; /* No Sized trait, can't require it! */ + } + }; + + // Search for a predicate like `Self : Sized` amongst the trait bounds. + let predicates = tcx.predicates_of(def_id); + let predicates = predicates.instantiate_identity(tcx).predicates; + elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| { + match obligation.predicate.skip_binders() { + ty::PredicateAtom::Trait(ref trait_pred, _) => { + trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0) + } + ty::PredicateAtom::Projection(..) + | ty::PredicateAtom::Subtype(..) + | ty::PredicateAtom::RegionOutlives(..) + | ty::PredicateAtom::WellFormed(..) + | ty::PredicateAtom::ObjectSafe(..) + | ty::PredicateAtom::ClosureKind(..) + | ty::PredicateAtom::TypeOutlives(..) + | ty::PredicateAtom::ConstEvaluatable(..) + | ty::PredicateAtom::ConstEquate(..) => false, + } + }) +} + +/// Returns `Some(_)` if this method makes the containing trait not object safe. +fn object_safety_violation_for_method( + tcx: TyCtxt<'_>, + trait_def_id: DefId, + method: &ty::AssocItem, +) -> Option<(MethodViolationCode, Span)> { + debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method); + // Any method that has a `Self : Sized` requisite is otherwise + // exempt from the regulations. + if generics_require_sized_self(tcx, method.def_id) { + return None; + } + + let violation = virtual_call_violation_for_method(tcx, trait_def_id, method); + // Get an accurate span depending on the violation. + violation.map(|v| { + let node = tcx.hir().get_if_local(method.def_id); + let span = match (v, node) { + (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node + .fn_decl() + .and_then(|decl| decl.inputs.get(arg + 1)) + .map_or(method.ident.span, |arg| arg.span), + (MethodViolationCode::UndispatchableReceiver, Some(node)) => node + .fn_decl() + .and_then(|decl| decl.inputs.get(0)) + .map_or(method.ident.span, |arg| arg.span), + (MethodViolationCode::ReferencesSelfOutput, Some(node)) => { + node.fn_decl().map_or(method.ident.span, |decl| decl.output.span()) + } + _ => method.ident.span, + }; + (v, span) + }) +} + +/// Returns `Some(_)` if this method cannot be called on a trait +/// object; this does not necessarily imply that the enclosing trait +/// is not object safe, because the method might have a where clause +/// `Self:Sized`. +fn virtual_call_violation_for_method<'tcx>( + tcx: TyCtxt<'tcx>, + trait_def_id: DefId, + method: &ty::AssocItem, +) -> Option<MethodViolationCode> { + // The method's first parameter must be named `self` + if !method.fn_has_self_parameter { + // We'll attempt to provide a structured suggestion for `Self: Sized`. + let sugg = + tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map( + |generics| match generics.where_clause.predicates { + [] => (" where Self: Sized", generics.where_clause.span), + [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()), + }, + ); + return Some(MethodViolationCode::StaticMethod(sugg)); + } + + let sig = tcx.fn_sig(method.def_id); + + for (i, input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() { + if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) { + return Some(MethodViolationCode::ReferencesSelfInput(i)); + } + } + if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output().skip_binder()) { + return Some(MethodViolationCode::ReferencesSelfOutput); + } + + // We can't monomorphize things like `fn foo<A>(...)`. + let own_counts = tcx.generics_of(method.def_id).own_counts(); + if own_counts.types + own_counts.consts != 0 { + return Some(MethodViolationCode::Generic); + } + + if tcx + .predicates_of(method.def_id) + .predicates + .iter() + // A trait object can't claim to live more than the concrete type, + // so outlives predicates will always hold. + .cloned() + .filter(|(p, _)| p.to_opt_type_outlives().is_none()) + .collect::<Vec<_>>() + // Do a shallow visit so that `contains_illegal_self_type_reference` + // may apply it's custom visiting. + .visit_tys_shallow(|t| contains_illegal_self_type_reference(tcx, trait_def_id, t)) + { + return Some(MethodViolationCode::WhereClauseReferencesSelf); + } + + let receiver_ty = + tcx.liberate_late_bound_regions(method.def_id, &sig.map_bound(|sig| sig.inputs()[0])); + + // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on. + // However, this is already considered object-safe. We allow it as a special case here. + // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows + // `Receiver: Unsize<Receiver[Self => dyn Trait]>`. + if receiver_ty != tcx.types.self_param { + if !receiver_is_dispatchable(tcx, method, receiver_ty) { + return Some(MethodViolationCode::UndispatchableReceiver); + } else { + // Do sanity check to make sure the receiver actually has the layout of a pointer. + + use rustc_target::abi::Abi; + + let param_env = tcx.param_env(method.def_id); + + let abi_of_ty = |ty: Ty<'tcx>| -> &Abi { + match tcx.layout_of(param_env.and(ty)) { + Ok(layout) => &layout.abi, + Err(err) => bug!("error: {}\n while computing layout for type {:?}", err, ty), + } + }; + + // e.g., `Rc<()>` + let unit_receiver_ty = + receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id); + + match abi_of_ty(unit_receiver_ty) { + &Abi::Scalar(..) => (), + abi => { + tcx.sess.delay_span_bug( + tcx.def_span(method.def_id), + &format!( + "receiver when `Self = ()` should have a Scalar ABI; found {:?}", + abi + ), + ); + } + } + + let trait_object_ty = + object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic)); + + // e.g., `Rc<dyn Trait>` + let trait_object_receiver = + receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id); + + match abi_of_ty(trait_object_receiver) { + &Abi::ScalarPair(..) => (), + abi => { + tcx.sess.delay_span_bug( + tcx.def_span(method.def_id), + &format!( + "receiver when `Self = {}` should have a ScalarPair ABI; \ + found {:?}", + trait_object_ty, abi + ), + ); + } + } + } + } + + None +} + +/// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`. +/// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`. +fn receiver_for_self_ty<'tcx>( + tcx: TyCtxt<'tcx>, + receiver_ty: Ty<'tcx>, + self_ty: Ty<'tcx>, + method_def_id: DefId, +) -> Ty<'tcx> { + debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id); + let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| { + if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) } + }); + + let result = receiver_ty.subst(tcx, substs); + debug!( + "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}", + receiver_ty, self_ty, method_def_id, result + ); + result +} + +/// Creates the object type for the current trait. For example, +/// if the current trait is `Deref`, then this will be +/// `dyn Deref<Target = Self::Target> + 'static`. +fn object_ty_for_trait<'tcx>( + tcx: TyCtxt<'tcx>, + trait_def_id: DefId, + lifetime: ty::Region<'tcx>, +) -> Ty<'tcx> { + debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id); + + let trait_ref = ty::TraitRef::identity(tcx, trait_def_id); + + let trait_predicate = + ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)); + + let mut associated_types = traits::supertraits(tcx, ty::Binder::dummy(trait_ref)) + .flat_map(|super_trait_ref| { + tcx.associated_items(super_trait_ref.def_id()) + .in_definition_order() + .map(move |item| (super_trait_ref, item)) + }) + .filter(|(_, item)| item.kind == ty::AssocKind::Type) + .collect::<Vec<_>>(); + + // existential predicates need to be in a specific order + associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id)); + + let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| { + // We *can* get bound lifetimes here in cases like + // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`. + // + // binder moved to (*)... + let super_trait_ref = super_trait_ref.skip_binder(); + ty::ExistentialPredicate::Projection(ty::ExistentialProjection { + ty: tcx.mk_projection(item.def_id, super_trait_ref.substs), + item_def_id: item.def_id, + substs: super_trait_ref.substs, + }) + }); + + let existential_predicates = + tcx.mk_existential_predicates(iter::once(trait_predicate).chain(projection_predicates)); + + let object_ty = tcx.mk_dynamic( + // (*) ... binder re-introduced here + ty::Binder::bind(existential_predicates), + lifetime, + ); + + debug!("object_ty_for_trait: object_ty=`{}`", object_ty); + + object_ty +} + +/// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a +/// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type +/// in the following way: +/// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`, +/// - require the following bound: +/// +/// ``` +/// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]> +/// ``` +/// +/// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`" +/// (substitution notation). +/// +/// Some examples of receiver types and their required obligation: +/// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`, +/// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`, +/// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`. +/// +/// The only case where the receiver is not dispatchable, but is still a valid receiver +/// type (just not object-safe), is when there is more than one level of pointer indirection. +/// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there +/// is no way, or at least no inexpensive way, to coerce the receiver from the version where +/// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type +/// contained by the trait object, because the object that needs to be coerced is behind +/// a pointer. +/// +/// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result +/// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch +/// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561). +/// Instead, we fudge a little by introducing a new type parameter `U` such that +/// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`. +/// Written as a chalk-style query: +/// +/// forall (U: Trait + ?Sized) { +/// if (Self: Unsize<U>) { +/// Receiver: DispatchFromDyn<Receiver[Self => U]> +/// } +/// } +/// +/// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>` +/// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>` +/// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>` +// +// FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this +// fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like +// `self: Wrapper<Self>`. +#[allow(dead_code)] +fn receiver_is_dispatchable<'tcx>( + tcx: TyCtxt<'tcx>, + method: &ty::AssocItem, + receiver_ty: Ty<'tcx>, +) -> bool { + debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty); + + let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait()); + let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits { + (u, cu) + } else { + debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits"); + return false; + }; + + // the type `U` in the query + // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now. + // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can + // replace this with `dyn Trait` + let unsized_self_ty: Ty<'tcx> = + tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome")); + + // `Receiver[Self => U]` + let unsized_receiver_ty = + receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id); + + // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds + // `U: ?Sized` is already implied here + let param_env = { + let param_env = tcx.param_env(method.def_id); + + // Self: Unsize<U> + let unsize_predicate = ty::TraitRef { + def_id: unsize_did, + substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]), + } + .without_const() + .to_predicate(tcx); + + // U: Trait<Arg1, ..., ArgN> + let trait_predicate = { + let substs = + InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| { + if param.index == 0 { + unsized_self_ty.into() + } else { + tcx.mk_param_from_def(param) + } + }); + + ty::TraitRef { def_id: unsize_did, substs }.without_const().to_predicate(tcx) + }; + + let caller_bounds: Vec<Predicate<'tcx>> = param_env + .caller_bounds() + .iter() + .chain(iter::once(unsize_predicate)) + .chain(iter::once(trait_predicate)) + .collect(); + + ty::ParamEnv::new( + tcx.intern_predicates(&caller_bounds), + param_env.reveal(), + param_env.def_id, + ) + }; + + // Receiver: DispatchFromDyn<Receiver[Self => U]> + let obligation = { + let predicate = ty::TraitRef { + def_id: dispatch_from_dyn_did, + substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]), + } + .without_const() + .to_predicate(tcx); + + Obligation::new(ObligationCause::dummy(), param_env, predicate) + }; + + tcx.infer_ctxt().enter(|ref infcx| { + // the receiver is dispatchable iff the obligation holds + infcx.predicate_must_hold_modulo_regions(&obligation) + }) +} + +fn contains_illegal_self_type_reference<'tcx>( + tcx: TyCtxt<'tcx>, + trait_def_id: DefId, + ty: Ty<'tcx>, +) -> bool { + // This is somewhat subtle. In general, we want to forbid + // references to `Self` in the argument and return types, + // since the value of `Self` is erased. However, there is one + // exception: it is ok to reference `Self` in order to access + // an associated type of the current trait, since we retain + // the value of those associated types in the object type + // itself. + // + // ```rust + // trait SuperTrait { + // type X; + // } + // + // trait Trait : SuperTrait { + // type Y; + // fn foo(&self, x: Self) // bad + // fn foo(&self) -> Self // bad + // fn foo(&self) -> Option<Self> // bad + // fn foo(&self) -> Self::Y // OK, desugars to next example + // fn foo(&self) -> <Self as Trait>::Y // OK + // fn foo(&self) -> Self::X // OK, desugars to next example + // fn foo(&self) -> <Self as SuperTrait>::X // OK + // } + // ``` + // + // However, it is not as simple as allowing `Self` in a projected + // type, because there are illegal ways to use `Self` as well: + // + // ```rust + // trait Trait : SuperTrait { + // ... + // fn foo(&self) -> <Self as SomeOtherTrait>::X; + // } + // ``` + // + // Here we will not have the type of `X` recorded in the + // object type, and we cannot resolve `Self as SomeOtherTrait` + // without knowing what `Self` is. + + struct IllegalSelfTypeVisitor<'tcx> { + tcx: TyCtxt<'tcx>, + self_ty: Ty<'tcx>, + trait_def_id: DefId, + supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>>, + } + + impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> { + fn visit_ty(&mut self, t: Ty<'tcx>) -> bool { + match t.kind { + ty::Param(_) => t == self.self_ty, + ty::Projection(ref data) => { + // This is a projected type `<Foo as SomeTrait>::X`. + + // Compute supertraits of current trait lazily. + if self.supertraits.is_none() { + let trait_ref = + ty::Binder::bind(ty::TraitRef::identity(self.tcx, self.trait_def_id)); + self.supertraits = Some(traits::supertraits(self.tcx, trait_ref).collect()); + } + + // Determine whether the trait reference `Foo as + // SomeTrait` is in fact a supertrait of the + // current trait. In that case, this type is + // legal, because the type `X` will be specified + // in the object type. Note that we can just use + // direct equality here because all of these types + // are part of the formal parameter listing, and + // hence there should be no inference variables. + let projection_trait_ref = ty::Binder::bind(data.trait_ref(self.tcx)); + let is_supertrait_of_current_trait = + self.supertraits.as_ref().unwrap().contains(&projection_trait_ref); + + if is_supertrait_of_current_trait { + false // do not walk contained types, do not report error, do collect $200 + } else { + t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error + } + } + _ => t.super_visit_with(self), // walk contained types, if any + } + } + + fn visit_const(&mut self, _c: &ty::Const<'tcx>) -> bool { + // FIXME(#72219) Look into the unevaluated constants for object safety violations. + // Do not walk substitutions of unevaluated consts, as they contain `Self`, even + // though the const expression doesn't necessary use it. Currently type variables + // inside array length expressions are forbidden, so they can't break the above + // rules. + false + } + } + + ty.visit_with(&mut IllegalSelfTypeVisitor { + tcx, + self_ty: tcx.types.self_param, + trait_def_id, + supertraits: None, + }) +} + +pub fn provide(providers: &mut ty::query::Providers) { + *providers = ty::query::Providers { object_safety_violations, ..*providers }; +} |