1 //! "Object safety" refers to the ability for a trait to be converted
2 //! to an object. In general, traits may only be converted to an
3 //! object if all of their methods meet certain criteria. In particular,
6 //! - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version
7 //! that doesn't contain the vtable;
8 //! - not reference the erased type `Self` except for in this receiver;
9 //! - not have generic type parameters.
11 use super::elaborate_predicates;
13 use crate::infer::TyCtxtInferExt;
14 use crate::traits::const_evaluatable::{self, AbstractConst};
15 use crate::traits::query::evaluate_obligation::InferCtxtExt;
16 use crate::traits::{self, Obligation, ObligationCause};
17 use rustc_errors::{FatalError, MultiSpan};
19 use rustc_hir::def_id::DefId;
20 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
21 use rustc_middle::ty::{
22 self, EarlyBinder, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitor,
24 use rustc_middle::ty::{Predicate, ToPredicate};
25 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
26 use rustc_span::symbol::Symbol;
28 use smallvec::SmallVec;
31 use std::ops::ControlFlow;
33 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
35 /// Returns the object safety violations that affect
36 /// astconv -- currently, `Self` in supertraits. This is needed
37 /// because `object_safety_violations` can't be used during
39 pub fn astconv_object_safety_violations(
42 ) -> Vec<ObjectSafetyViolation> {
43 debug_assert!(tcx.generics_of(trait_def_id).has_self);
44 let violations = traits::supertrait_def_ids(tcx, trait_def_id)
45 .map(|def_id| predicates_reference_self(tcx, def_id, true))
46 .filter(|spans| !spans.is_empty())
47 .map(ObjectSafetyViolation::SupertraitSelf)
50 debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
55 fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [ObjectSafetyViolation] {
56 debug_assert!(tcx.generics_of(trait_def_id).has_self);
57 debug!("object_safety_violations: {:?}", trait_def_id);
59 tcx.arena.alloc_from_iter(
60 traits::supertrait_def_ids(tcx, trait_def_id)
61 .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
65 /// We say a method is *vtable safe* if it can be invoked on a trait
66 /// object. Note that object-safe traits can have some
67 /// non-vtable-safe methods, so long as they require `Self: Sized` or
68 /// otherwise ensure that they cannot be used when `Self = Trait`.
69 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
70 debug_assert!(tcx.generics_of(trait_def_id).has_self);
71 debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
72 // Any method that has a `Self: Sized` bound cannot be called.
73 if generics_require_sized_self(tcx, method.def_id) {
77 match virtual_call_violation_for_method(tcx, trait_def_id, method) {
78 None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
83 fn object_safety_violations_for_trait(
86 ) -> Vec<ObjectSafetyViolation> {
87 // Check methods for violations.
88 let mut violations: Vec<_> = tcx
89 .associated_items(trait_def_id)
90 .in_definition_order()
91 .filter(|item| item.kind == ty::AssocKind::Fn)
93 object_safety_violation_for_method(tcx, trait_def_id, &item)
94 .map(|(code, span)| ObjectSafetyViolation::Method(item.name, code, span))
97 if let ObjectSafetyViolation::Method(
99 MethodViolationCode::WhereClauseReferencesSelf,
103 lint_object_unsafe_trait(tcx, *span, trait_def_id, &violation);
111 // Check the trait itself.
112 if trait_has_sized_self(tcx, trait_def_id) {
113 // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
114 let spans = get_sized_bounds(tcx, trait_def_id);
115 violations.push(ObjectSafetyViolation::SizedSelf(spans));
117 let spans = predicates_reference_self(tcx, trait_def_id, false);
118 if !spans.is_empty() {
119 violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
121 let spans = bounds_reference_self(tcx, trait_def_id);
122 if !spans.is_empty() {
123 violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
127 tcx.associated_items(trait_def_id)
128 .in_definition_order()
129 .filter(|item| item.kind == ty::AssocKind::Const)
131 let ident = item.ident(tcx);
132 ObjectSafetyViolation::AssocConst(ident.name, ident.span)
136 if !tcx.features().generic_associated_types_extended {
138 tcx.associated_items(trait_def_id)
139 .in_definition_order()
140 .filter(|item| item.kind == ty::AssocKind::Type)
141 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
143 let ident = item.ident(tcx);
144 ObjectSafetyViolation::GAT(ident.name, ident.span)
150 "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
151 trait_def_id, violations
157 /// Lint object-unsafe trait.
158 fn lint_object_unsafe_trait(
162 violation: &ObjectSafetyViolation,
164 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
165 // It's also hard to get a use site span, so we use the method definition span.
166 tcx.struct_span_lint_hir(WHERE_CLAUSES_OBJECT_SAFETY, hir::CRATE_HIR_ID, span, |lint| {
167 let mut err = lint.build(&format!(
168 "the trait `{}` cannot be made into an object",
169 tcx.def_path_str(trait_def_id)
171 let node = tcx.hir().get_if_local(trait_def_id);
172 let mut spans = MultiSpan::from_span(span);
173 if let Some(hir::Node::Item(item)) = node {
174 spans.push_span_label(item.ident.span, "this trait cannot be made into an object...");
175 spans.push_span_label(span, format!("...because {}", violation.error_msg()));
177 spans.push_span_label(
180 "the trait cannot be made into an object because {}",
181 violation.error_msg()
187 "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
188 call to be resolvable dynamically; for more information visit \
189 <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
192 // Only provide the help if its a local trait, otherwise it's not
193 violation.solution(&mut err);
199 fn sized_trait_bound_spans<'tcx>(
201 bounds: hir::GenericBounds<'tcx>,
202 ) -> impl 'tcx + Iterator<Item = Span> {
203 bounds.iter().filter_map(move |b| match b {
204 hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
205 if trait_has_sized_self(
207 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
210 // Fetch spans for supertraits that are `Sized`: `trait T: Super`
217 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
219 .get_if_local(trait_def_id)
220 .and_then(|node| match node {
221 hir::Node::Item(hir::Item {
222 kind: hir::ItemKind::Trait(.., generics, bounds, _),
230 hir::WherePredicate::BoundPredicate(pred)
231 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
233 // Fetch spans for trait bounds that are Sized:
234 // `trait T where Self: Pred`
235 Some(sized_trait_bound_spans(tcx, pred.bounds))
241 // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
242 .chain(sized_trait_bound_spans(tcx, bounds))
243 .collect::<SmallVec<[Span; 1]>>(),
247 .unwrap_or_else(SmallVec::new)
250 fn predicates_reference_self(
253 supertraits_only: bool,
254 ) -> SmallVec<[Span; 1]> {
255 let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
256 let predicates = if supertraits_only {
257 tcx.super_predicates_of(trait_def_id)
259 tcx.predicates_of(trait_def_id)
264 .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
265 .filter_map(|predicate| predicate_references_self(tcx, predicate))
269 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
270 tcx.associated_items(trait_def_id)
271 .in_definition_order()
272 .filter(|item| item.kind == ty::AssocKind::Type)
273 .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
274 .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
278 fn predicate_references_self<'tcx>(
280 (predicate, sp): (ty::Predicate<'tcx>, Span),
282 let self_ty = tcx.types.self_param;
283 let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
284 match predicate.kind().skip_binder() {
285 ty::PredicateKind::Trait(ref data) => {
286 // In the case of a trait predicate, we can skip the "self" type.
287 if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
289 ty::PredicateKind::Projection(ref data) => {
290 // And similarly for projections. This should be redundant with
291 // the previous check because any projection should have a
292 // matching `Trait` predicate with the same inputs, but we do
293 // the check to be safe.
295 // It's also won't be redundant if we allow type-generic associated
296 // types for trait objects.
298 // Note that we *do* allow projection *outputs* to contain
299 // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
300 // we just require the user to specify *both* outputs
301 // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
303 // This is ALT2 in issue #56288, see that for discussion of the
304 // possible alternatives.
305 if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
307 ty::PredicateKind::WellFormed(..)
308 | ty::PredicateKind::ObjectSafe(..)
309 | ty::PredicateKind::TypeOutlives(..)
310 | ty::PredicateKind::RegionOutlives(..)
311 | ty::PredicateKind::ClosureKind(..)
312 | ty::PredicateKind::Subtype(..)
313 | ty::PredicateKind::Coerce(..)
314 | ty::PredicateKind::ConstEvaluatable(..)
315 | ty::PredicateKind::ConstEquate(..)
316 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
320 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
321 generics_require_sized_self(tcx, trait_def_id)
324 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
325 let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
326 return false; /* No Sized trait, can't require it! */
329 // Search for a predicate like `Self : Sized` amongst the trait bounds.
330 let predicates = tcx.predicates_of(def_id);
331 let predicates = predicates.instantiate_identity(tcx).predicates;
332 elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
333 match obligation.predicate.kind().skip_binder() {
334 ty::PredicateKind::Trait(ref trait_pred) => {
335 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
337 ty::PredicateKind::Projection(..)
338 | ty::PredicateKind::Subtype(..)
339 | ty::PredicateKind::Coerce(..)
340 | ty::PredicateKind::RegionOutlives(..)
341 | ty::PredicateKind::WellFormed(..)
342 | ty::PredicateKind::ObjectSafe(..)
343 | ty::PredicateKind::ClosureKind(..)
344 | ty::PredicateKind::TypeOutlives(..)
345 | ty::PredicateKind::ConstEvaluatable(..)
346 | ty::PredicateKind::ConstEquate(..)
347 | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
352 /// Returns `Some(_)` if this method makes the containing trait not object safe.
353 fn object_safety_violation_for_method(
356 method: &ty::AssocItem,
357 ) -> Option<(MethodViolationCode, Span)> {
358 debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
359 // Any method that has a `Self : Sized` requisite is otherwise
360 // exempt from the regulations.
361 if generics_require_sized_self(tcx, method.def_id) {
365 let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
366 // Get an accurate span depending on the violation.
368 let node = tcx.hir().get_if_local(method.def_id);
369 let span = match (v, node) {
370 (MethodViolationCode::ReferencesSelfInput(arg), Some(node)) => node
372 .and_then(|decl| decl.inputs.get(arg + 1))
373 .map_or(method.ident(tcx).span, |arg| arg.span),
374 (MethodViolationCode::UndispatchableReceiver, Some(node)) => node
376 .and_then(|decl| decl.inputs.get(0))
377 .map_or(method.ident(tcx).span, |arg| arg.span),
378 (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
379 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
381 _ => method.ident(tcx).span,
387 /// Returns `Some(_)` if this method cannot be called on a trait
388 /// object; this does not necessarily imply that the enclosing trait
389 /// is not object safe, because the method might have a where clause
391 fn virtual_call_violation_for_method<'tcx>(
394 method: &ty::AssocItem,
395 ) -> Option<MethodViolationCode> {
396 let sig = tcx.fn_sig(method.def_id);
398 // The method's first parameter must be named `self`
399 if !method.fn_has_self_parameter {
400 // We'll attempt to provide a structured suggestion for `Self: Sized`.
402 tcx.hir().get_if_local(method.def_id).as_ref().and_then(|node| node.generics()).map(
403 |generics| match generics.predicates {
404 [] => (" where Self: Sized", generics.where_clause_span),
405 [.., pred] => (", Self: Sized", pred.span().shrink_to_hi()),
408 // Get the span pointing at where the `self` receiver should be.
409 let sm = tcx.sess.source_map();
410 let self_span = method.ident(tcx).span.to(tcx
412 .span_if_local(method.def_id)
413 .unwrap_or_else(|| sm.next_point(method.ident(tcx).span))
415 let self_span = sm.span_through_char(self_span, '(').shrink_to_hi();
416 return Some(MethodViolationCode::StaticMethod(
419 !sig.inputs().skip_binder().is_empty(),
423 for (i, &input_ty) in sig.skip_binder().inputs()[1..].iter().enumerate() {
424 if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
425 return Some(MethodViolationCode::ReferencesSelfInput(i));
428 if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
429 return Some(MethodViolationCode::ReferencesSelfOutput);
432 // We can't monomorphize things like `fn foo<A>(...)`.
433 let own_counts = tcx.generics_of(method.def_id).own_counts();
434 if own_counts.types + own_counts.consts != 0 {
435 return Some(MethodViolationCode::Generic);
439 .predicates_of(method.def_id)
442 // A trait object can't claim to live more than the concrete type,
443 // so outlives predicates will always hold.
445 .filter(|(p, _)| p.to_opt_type_outlives().is_none())
446 .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
448 return Some(MethodViolationCode::WhereClauseReferencesSelf);
451 let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
453 // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
454 // However, this is already considered object-safe. We allow it as a special case here.
455 // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
456 // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
457 if receiver_ty != tcx.types.self_param {
458 if !receiver_is_dispatchable(tcx, method, receiver_ty) {
459 return Some(MethodViolationCode::UndispatchableReceiver);
461 // Do sanity check to make sure the receiver actually has the layout of a pointer.
463 use rustc_target::abi::Abi;
465 let param_env = tcx.param_env(method.def_id);
467 let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
468 match tcx.layout_of(param_env.and(ty)) {
469 Ok(layout) => Some(layout.abi),
472 tcx.sess.delay_span_bug(
473 tcx.def_span(method.def_id),
474 &format!("error: {}\n while computing layout for type {:?}", err, ty),
482 let unit_receiver_ty =
483 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
485 match abi_of_ty(unit_receiver_ty) {
486 Some(Abi::Scalar(..)) => (),
488 tcx.sess.delay_span_bug(
489 tcx.def_span(method.def_id),
491 "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
498 let trait_object_ty =
499 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
501 // e.g., `Rc<dyn Trait>`
502 let trait_object_receiver =
503 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
505 match abi_of_ty(trait_object_receiver) {
506 Some(Abi::ScalarPair(..)) => (),
508 tcx.sess.delay_span_bug(
509 tcx.def_span(method.def_id),
511 "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
523 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
524 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
525 fn receiver_for_self_ty<'tcx>(
527 receiver_ty: Ty<'tcx>,
529 method_def_id: DefId,
531 debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
532 let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
533 if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
536 let result = EarlyBinder(receiver_ty).subst(tcx, substs);
538 "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
539 receiver_ty, self_ty, method_def_id, result
544 /// Creates the object type for the current trait. For example,
545 /// if the current trait is `Deref`, then this will be
546 /// `dyn Deref<Target = Self::Target> + 'static`.
547 fn object_ty_for_trait<'tcx>(
550 lifetime: ty::Region<'tcx>,
552 debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
554 let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
556 let trait_predicate = trait_ref.map_bound(|trait_ref| {
557 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
560 let mut associated_types = traits::supertraits(tcx, trait_ref)
561 .flat_map(|super_trait_ref| {
562 tcx.associated_items(super_trait_ref.def_id())
563 .in_definition_order()
564 .map(move |item| (super_trait_ref, item))
566 .filter(|(_, item)| item.kind == ty::AssocKind::Type)
567 .collect::<Vec<_>>();
569 // existential predicates need to be in a specific order
570 associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
572 let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
573 // We *can* get bound lifetimes here in cases like
574 // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
575 super_trait_ref.map_bound(|super_trait_ref| {
576 ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
577 term: tcx.mk_projection(item.def_id, super_trait_ref.substs).into(),
578 item_def_id: item.def_id,
579 substs: super_trait_ref.substs,
584 let existential_predicates = tcx
585 .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
587 let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
589 debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
594 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
595 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
596 /// in the following way:
597 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
598 /// - require the following bound:
600 /// ```ignore (not-rust)
601 /// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
604 /// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
605 /// (substitution notation).
607 /// Some examples of receiver types and their required obligation:
608 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
609 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
610 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
612 /// The only case where the receiver is not dispatchable, but is still a valid receiver
613 /// type (just not object-safe), is when there is more than one level of pointer indirection.
614 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
615 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
616 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
617 /// contained by the trait object, because the object that needs to be coerced is behind
620 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
621 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
622 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
623 /// Instead, we fudge a little by introducing a new type parameter `U` such that
624 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
625 /// Written as a chalk-style query:
626 /// ```ignore (not-rust)
627 /// forall (U: Trait + ?Sized) {
628 /// if (Self: Unsize<U>) {
629 /// Receiver: DispatchFromDyn<Receiver[Self => U]>
633 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
634 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
635 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
637 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
638 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
639 // `self: Wrapper<Self>`.
641 fn receiver_is_dispatchable<'tcx>(
643 method: &ty::AssocItem,
644 receiver_ty: Ty<'tcx>,
646 debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
648 let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
649 let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
650 debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
654 // the type `U` in the query
655 // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
656 // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
657 // replace this with `dyn Trait`
658 let unsized_self_ty: Ty<'tcx> =
659 tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
661 // `Receiver[Self => U]`
662 let unsized_receiver_ty =
663 receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
665 // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
666 // `U: ?Sized` is already implied here
668 let param_env = tcx.param_env(method.def_id);
671 let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
673 substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
678 // U: Trait<Arg1, ..., ArgN>
679 let trait_predicate = {
681 InternalSubsts::for_item(tcx, method.container.assert_trait(), |param, _| {
682 if param.index == 0 {
683 unsized_self_ty.into()
685 tcx.mk_param_from_def(param)
689 ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
694 let caller_bounds: Vec<Predicate<'tcx>> =
695 param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
698 tcx.intern_predicates(&caller_bounds),
700 param_env.constness(),
704 // Receiver: DispatchFromDyn<Receiver[Self => U]>
706 let predicate = ty::Binder::dummy(ty::TraitRef {
707 def_id: dispatch_from_dyn_did,
708 substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
713 Obligation::new(ObligationCause::dummy(), param_env, predicate)
716 tcx.infer_ctxt().enter(|ref infcx| {
717 // the receiver is dispatchable iff the obligation holds
718 infcx.predicate_must_hold_modulo_regions(&obligation)
722 fn contains_illegal_self_type_reference<'tcx, T: TypeFoldable<'tcx>>(
727 // This is somewhat subtle. In general, we want to forbid
728 // references to `Self` in the argument and return types,
729 // since the value of `Self` is erased. However, there is one
730 // exception: it is ok to reference `Self` in order to access
731 // an associated type of the current trait, since we retain
732 // the value of those associated types in the object type
736 // trait SuperTrait {
740 // trait Trait : SuperTrait {
742 // fn foo(&self, x: Self) // bad
743 // fn foo(&self) -> Self // bad
744 // fn foo(&self) -> Option<Self> // bad
745 // fn foo(&self) -> Self::Y // OK, desugars to next example
746 // fn foo(&self) -> <Self as Trait>::Y // OK
747 // fn foo(&self) -> Self::X // OK, desugars to next example
748 // fn foo(&self) -> <Self as SuperTrait>::X // OK
752 // However, it is not as simple as allowing `Self` in a projected
753 // type, because there are illegal ways to use `Self` as well:
756 // trait Trait : SuperTrait {
758 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
762 // Here we will not have the type of `X` recorded in the
763 // object type, and we cannot resolve `Self as SomeOtherTrait`
764 // without knowing what `Self` is.
766 struct IllegalSelfTypeVisitor<'tcx> {
769 supertraits: Option<Vec<DefId>>,
772 impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
775 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
778 if t == self.tcx.types.self_param {
781 ControlFlow::CONTINUE
784 ty::Projection(ref data) => {
785 // This is a projected type `<Foo as SomeTrait>::X`.
787 // Compute supertraits of current trait lazily.
788 if self.supertraits.is_none() {
789 let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
790 self.supertraits = Some(
791 traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
795 // Determine whether the trait reference `Foo as
796 // SomeTrait` is in fact a supertrait of the
797 // current trait. In that case, this type is
798 // legal, because the type `X` will be specified
799 // in the object type. Note that we can just use
800 // direct equality here because all of these types
801 // are part of the formal parameter listing, and
802 // hence there should be no inference variables.
803 let is_supertrait_of_current_trait = self
807 .contains(&data.trait_ref(self.tcx).def_id);
809 if is_supertrait_of_current_trait {
810 ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
812 t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
815 _ => t.super_visit_with(self), // walk contained types, if any
819 fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
820 // Constants can only influence object safety if they reference `Self`.
821 // This is only possible for unevaluated constants, so we walk these here.
823 // If `AbstractConst::new` returned an error we already failed compilation
824 // so we don't have to emit an additional error here.
826 // We currently recurse into abstract consts here but do not recurse in
827 // `is_const_evaluatable`. This means that the object safety check is more
828 // liberal than the const eval check.
830 // This shouldn't really matter though as we can't really use any
831 // constants which are not considered const evaluatable.
832 use rustc_middle::thir::abstract_const::Node;
833 if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
834 const_evaluatable::walk_abstract_const(self.tcx, ct, |node| {
835 match node.root(self.tcx) {
836 Node::Leaf(leaf) => self.visit_const(leaf),
837 Node::Cast(_, _, ty) => self.visit_ty(ty),
838 Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
839 ControlFlow::CONTINUE
844 ControlFlow::CONTINUE
850 .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
854 pub fn provide(providers: &mut ty::query::Providers) {
855 *providers = ty::query::Providers { object_safety_violations, ..*providers };