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::query::evaluate_obligation::InferCtxtExt;
15 use crate::traits::{self, Obligation, ObligationCause};
16 use rustc_errors::{FatalError, MultiSpan};
18 use rustc_hir::def_id::DefId;
19 use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
20 use rustc_middle::ty::subst::{GenericArg, InternalSubsts, Subst};
21 use rustc_middle::ty::{
22 self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, 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(Some(span)), _) => *span,
371 (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
372 (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
373 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
375 _ => method.ident(tcx).span,
381 /// Returns `Some(_)` if this method cannot be called on a trait
382 /// object; this does not necessarily imply that the enclosing trait
383 /// is not object safe, because the method might have a where clause
385 fn virtual_call_violation_for_method<'tcx>(
388 method: &ty::AssocItem,
389 ) -> Option<MethodViolationCode> {
390 let sig = tcx.fn_sig(method.def_id);
392 // The method's first parameter must be named `self`
393 if !method.fn_has_self_parameter {
394 let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
396 kind: hir::TraitItemKind::Fn(sig, _),
398 })) = tcx.hir().get_if_local(method.def_id).as_ref()
400 let sm = tcx.sess.source_map();
403 format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
404 sm.span_through_char(sig.span, '(').shrink_to_hi(),
407 format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
408 generics.tail_span_for_predicate_suggestion(),
414 return Some(MethodViolationCode::StaticMethod(sugg));
417 for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
418 if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
419 let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
420 kind: hir::TraitItemKind::Fn(sig, _),
422 })) = tcx.hir().get_if_local(method.def_id).as_ref()
424 Some(sig.decl.inputs[i].span)
428 return Some(MethodViolationCode::ReferencesSelfInput(span));
431 if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
432 return Some(MethodViolationCode::ReferencesSelfOutput);
435 // We can't monomorphize things like `fn foo<A>(...)`.
436 let own_counts = tcx.generics_of(method.def_id).own_counts();
437 if own_counts.types + own_counts.consts != 0 {
438 return Some(MethodViolationCode::Generic);
442 .predicates_of(method.def_id)
445 // A trait object can't claim to live more than the concrete type,
446 // so outlives predicates will always hold.
448 .filter(|(p, _)| p.to_opt_type_outlives().is_none())
449 .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
451 return Some(MethodViolationCode::WhereClauseReferencesSelf);
454 let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
456 // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
457 // However, this is already considered object-safe. We allow it as a special case here.
458 // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
459 // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
460 if receiver_ty != tcx.types.self_param {
461 if !receiver_is_dispatchable(tcx, method, receiver_ty) {
462 let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
463 kind: hir::TraitItemKind::Fn(sig, _),
465 })) = tcx.hir().get_if_local(method.def_id).as_ref()
467 Some(sig.decl.inputs[0].span)
471 return Some(MethodViolationCode::UndispatchableReceiver(span));
473 // Do sanity check to make sure the receiver actually has the layout of a pointer.
475 use rustc_target::abi::Abi;
477 let param_env = tcx.param_env(method.def_id);
479 let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
480 match tcx.layout_of(param_env.and(ty)) {
481 Ok(layout) => Some(layout.abi),
484 tcx.sess.delay_span_bug(
485 tcx.def_span(method.def_id),
486 &format!("error: {}\n while computing layout for type {:?}", err, ty),
494 let unit_receiver_ty =
495 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
497 match abi_of_ty(unit_receiver_ty) {
498 Some(Abi::Scalar(..)) => (),
500 tcx.sess.delay_span_bug(
501 tcx.def_span(method.def_id),
503 "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
510 let trait_object_ty =
511 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
513 // e.g., `Rc<dyn Trait>`
514 let trait_object_receiver =
515 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
517 match abi_of_ty(trait_object_receiver) {
518 Some(Abi::ScalarPair(..)) => (),
520 tcx.sess.delay_span_bug(
521 tcx.def_span(method.def_id),
523 "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
535 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
536 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
537 fn receiver_for_self_ty<'tcx>(
539 receiver_ty: Ty<'tcx>,
541 method_def_id: DefId,
543 debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
544 let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
545 if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
548 let result = EarlyBinder(receiver_ty).subst(tcx, substs);
550 "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
551 receiver_ty, self_ty, method_def_id, result
556 /// Creates the object type for the current trait. For example,
557 /// if the current trait is `Deref`, then this will be
558 /// `dyn Deref<Target = Self::Target> + 'static`.
559 fn object_ty_for_trait<'tcx>(
562 lifetime: ty::Region<'tcx>,
564 debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
566 let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
568 let trait_predicate = trait_ref.map_bound(|trait_ref| {
569 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
572 let mut associated_types = traits::supertraits(tcx, trait_ref)
573 .flat_map(|super_trait_ref| {
574 tcx.associated_items(super_trait_ref.def_id())
575 .in_definition_order()
576 .map(move |item| (super_trait_ref, item))
578 .filter(|(_, item)| item.kind == ty::AssocKind::Type)
579 .collect::<Vec<_>>();
581 // existential predicates need to be in a specific order
582 associated_types.sort_by_cached_key(|(_, item)| tcx.def_path_hash(item.def_id));
584 let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
585 // We *can* get bound lifetimes here in cases like
586 // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
587 super_trait_ref.map_bound(|super_trait_ref| {
588 ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
589 term: tcx.mk_projection(item.def_id, super_trait_ref.substs).into(),
590 item_def_id: item.def_id,
591 substs: super_trait_ref.substs,
596 let existential_predicates = tcx
597 .mk_poly_existential_predicates(iter::once(trait_predicate).chain(projection_predicates));
599 let object_ty = tcx.mk_dynamic(existential_predicates, lifetime);
601 debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
606 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
607 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
608 /// in the following way:
609 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
610 /// - require the following bound:
612 /// ```ignore (not-rust)
613 /// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
616 /// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
617 /// (substitution notation).
619 /// Some examples of receiver types and their required obligation:
620 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
621 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
622 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
624 /// The only case where the receiver is not dispatchable, but is still a valid receiver
625 /// type (just not object-safe), is when there is more than one level of pointer indirection.
626 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
627 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
628 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
629 /// contained by the trait object, because the object that needs to be coerced is behind
632 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
633 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
634 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
635 /// Instead, we fudge a little by introducing a new type parameter `U` such that
636 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
637 /// Written as a chalk-style query:
638 /// ```ignore (not-rust)
639 /// forall (U: Trait + ?Sized) {
640 /// if (Self: Unsize<U>) {
641 /// Receiver: DispatchFromDyn<Receiver[Self => U]>
645 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
646 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
647 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
649 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
650 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
651 // `self: Wrapper<Self>`.
653 fn receiver_is_dispatchable<'tcx>(
655 method: &ty::AssocItem,
656 receiver_ty: Ty<'tcx>,
658 debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
660 let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
661 let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
662 debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
666 // the type `U` in the query
667 // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
668 // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
669 // replace this with `dyn Trait`
670 let unsized_self_ty: Ty<'tcx> =
671 tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
673 // `Receiver[Self => U]`
674 let unsized_receiver_ty =
675 receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
677 // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
678 // `U: ?Sized` is already implied here
680 let param_env = tcx.param_env(method.def_id);
683 let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
685 substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
690 // U: Trait<Arg1, ..., ArgN>
691 let trait_predicate = {
693 InternalSubsts::for_item(tcx, method.trait_container(tcx).unwrap(), |param, _| {
694 if param.index == 0 {
695 unsized_self_ty.into()
697 tcx.mk_param_from_def(param)
701 ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
706 let caller_bounds: Vec<Predicate<'tcx>> =
707 param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
710 tcx.intern_predicates(&caller_bounds),
712 param_env.constness(),
716 // Receiver: DispatchFromDyn<Receiver[Self => U]>
718 let predicate = ty::Binder::dummy(ty::TraitRef {
719 def_id: dispatch_from_dyn_did,
720 substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
725 Obligation::new(ObligationCause::dummy(), param_env, predicate)
728 tcx.infer_ctxt().enter(|ref infcx| {
729 // the receiver is dispatchable iff the obligation holds
730 infcx.predicate_must_hold_modulo_regions(&obligation)
734 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
739 // This is somewhat subtle. In general, we want to forbid
740 // references to `Self` in the argument and return types,
741 // since the value of `Self` is erased. However, there is one
742 // exception: it is ok to reference `Self` in order to access
743 // an associated type of the current trait, since we retain
744 // the value of those associated types in the object type
748 // trait SuperTrait {
752 // trait Trait : SuperTrait {
754 // fn foo(&self, x: Self) // bad
755 // fn foo(&self) -> Self // bad
756 // fn foo(&self) -> Option<Self> // bad
757 // fn foo(&self) -> Self::Y // OK, desugars to next example
758 // fn foo(&self) -> <Self as Trait>::Y // OK
759 // fn foo(&self) -> Self::X // OK, desugars to next example
760 // fn foo(&self) -> <Self as SuperTrait>::X // OK
764 // However, it is not as simple as allowing `Self` in a projected
765 // type, because there are illegal ways to use `Self` as well:
768 // trait Trait : SuperTrait {
770 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
774 // Here we will not have the type of `X` recorded in the
775 // object type, and we cannot resolve `Self as SomeOtherTrait`
776 // without knowing what `Self` is.
778 struct IllegalSelfTypeVisitor<'tcx> {
781 supertraits: Option<Vec<DefId>>,
784 impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
787 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
790 if t == self.tcx.types.self_param {
793 ControlFlow::CONTINUE
796 ty::Projection(ref data) => {
797 // This is a projected type `<Foo as SomeTrait>::X`.
799 // Compute supertraits of current trait lazily.
800 if self.supertraits.is_none() {
801 let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
802 self.supertraits = Some(
803 traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
807 // Determine whether the trait reference `Foo as
808 // SomeTrait` is in fact a supertrait of the
809 // current trait. In that case, this type is
810 // legal, because the type `X` will be specified
811 // in the object type. Note that we can just use
812 // direct equality here because all of these types
813 // are part of the formal parameter listing, and
814 // hence there should be no inference variables.
815 let is_supertrait_of_current_trait = self
819 .contains(&data.trait_ref(self.tcx).def_id);
821 if is_supertrait_of_current_trait {
822 ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
824 t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
827 _ => t.super_visit_with(self), // walk contained types, if any
831 fn visit_unevaluated(&mut self, uv: ty::Unevaluated<'tcx>) -> ControlFlow<Self::BreakTy> {
832 // Constants can only influence object safety if they reference `Self`.
833 // This is only possible for unevaluated constants, so we walk these here.
835 // If `AbstractConst::new` returned an error we already failed compilation
836 // so we don't have to emit an additional error here.
838 // We currently recurse into abstract consts here but do not recurse in
839 // `is_const_evaluatable`. This means that the object safety check is more
840 // liberal than the const eval check.
842 // This shouldn't really matter though as we can't really use any
843 // constants which are not considered const evaluatable.
844 use rustc_middle::ty::abstract_const::Node;
845 if let Ok(Some(ct)) = AbstractConst::new(self.tcx, uv.shrink()) {
846 walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
847 Node::Leaf(leaf) => self.visit_const(leaf),
848 Node::Cast(_, _, ty) => self.visit_ty(ty),
849 Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
850 ControlFlow::CONTINUE
854 ControlFlow::CONTINUE
860 .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
864 pub fn provide(providers: &mut ty::query::Providers) {
865 *providers = ty::query::Providers { object_safety_violations, ..*providers };