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, elaborate_trait_ref};
13 use crate::infer::TyCtxtInferExt;
14 use crate::traits::query::evaluate_obligation::InferCtxtExt;
15 use crate::traits::{self, Obligation, ObligationCause};
16 use hir::def::DefKind;
17 use rustc_errors::{DelayDm, FatalError, MultiSpan};
19 use rustc_hir::def_id::DefId;
20 use rustc_middle::ty::abstract_const::{walk_abstract_const, AbstractConst};
21 use rustc_middle::ty::{
22 self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
24 use rustc_middle::ty::{GenericArg, InternalSubsts};
25 use rustc_middle::ty::{Predicate, ToPredicate};
26 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
27 use rustc_span::symbol::Symbol;
29 use smallvec::SmallVec;
32 use std::ops::ControlFlow;
34 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
36 /// Returns the object safety violations that affect
37 /// astconv -- currently, `Self` in supertraits. This is needed
38 /// because `object_safety_violations` can't be used during
40 pub fn astconv_object_safety_violations(
43 ) -> Vec<ObjectSafetyViolation> {
44 debug_assert!(tcx.generics_of(trait_def_id).has_self);
45 let violations = traits::supertrait_def_ids(tcx, trait_def_id)
46 .map(|def_id| predicates_reference_self(tcx, def_id, true))
47 .filter(|spans| !spans.is_empty())
48 .map(ObjectSafetyViolation::SupertraitSelf)
51 debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
56 fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [ObjectSafetyViolation] {
57 debug_assert!(tcx.generics_of(trait_def_id).has_self);
58 debug!("object_safety_violations: {:?}", trait_def_id);
60 tcx.arena.alloc_from_iter(
61 traits::supertrait_def_ids(tcx, trait_def_id)
62 .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
66 /// We say a method is *vtable safe* if it can be invoked on a trait
67 /// object. Note that object-safe traits can have some
68 /// non-vtable-safe methods, so long as they require `Self: Sized` or
69 /// otherwise ensure that they cannot be used when `Self = Trait`.
70 pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
71 debug_assert!(tcx.generics_of(trait_def_id).has_self);
72 debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
73 // Any method that has a `Self: Sized` bound cannot be called.
74 if generics_require_sized_self(tcx, method.def_id) {
78 match virtual_call_violation_for_method(tcx, trait_def_id, method) {
79 None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
84 fn object_safety_violations_for_trait(
87 ) -> Vec<ObjectSafetyViolation> {
88 // Check methods for violations.
89 let mut violations: Vec<_> = tcx
90 .associated_items(trait_def_id)
91 .in_definition_order()
92 .filter(|item| item.kind == ty::AssocKind::Fn)
94 object_safety_violation_for_method(tcx, trait_def_id, &item)
95 .map(|(code, span)| ObjectSafetyViolation::Method(item.name, code, span))
98 if let ObjectSafetyViolation::Method(
100 MethodViolationCode::WhereClauseReferencesSelf,
104 lint_object_unsafe_trait(tcx, *span, trait_def_id, &violation);
112 // Check the trait itself.
113 if trait_has_sized_self(tcx, trait_def_id) {
114 // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
115 let spans = get_sized_bounds(tcx, trait_def_id);
116 violations.push(ObjectSafetyViolation::SizedSelf(spans));
118 let spans = predicates_reference_self(tcx, trait_def_id, false);
119 if !spans.is_empty() {
120 violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
122 let spans = bounds_reference_self(tcx, trait_def_id);
123 if !spans.is_empty() {
124 violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
128 tcx.associated_items(trait_def_id)
129 .in_definition_order()
130 .filter(|item| item.kind == ty::AssocKind::Const)
132 let ident = item.ident(tcx);
133 ObjectSafetyViolation::AssocConst(ident.name, ident.span)
137 if !tcx.features().generic_associated_types_extended {
139 tcx.associated_items(trait_def_id)
140 .in_definition_order()
141 .filter(|item| item.kind == ty::AssocKind::Type)
142 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
144 let ident = item.ident(tcx);
145 ObjectSafetyViolation::GAT(ident.name, ident.span)
151 "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
152 trait_def_id, violations
158 /// Lint object-unsafe trait.
159 fn lint_object_unsafe_trait(
163 violation: &ObjectSafetyViolation,
165 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
166 // It's also hard to get a use site span, so we use the method definition span.
167 tcx.struct_span_lint_hir(
168 WHERE_CLAUSES_OBJECT_SAFETY,
171 DelayDm(|| format!("the trait `{}` cannot be made into an object", tcx.def_path_str(trait_def_id))),
173 let node = tcx.hir().get_if_local(trait_def_id);
174 let mut spans = MultiSpan::from_span(span);
175 if let Some(hir::Node::Item(item)) = node {
176 spans.push_span_label(
178 "this trait cannot be made into an object...",
180 spans.push_span_label(span, format!("...because {}", violation.error_msg()));
182 spans.push_span_label(
185 "the trait cannot be made into an object because {}",
186 violation.error_msg()
192 "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
193 call to be resolvable dynamically; for more information visit \
194 <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
197 // Only provide the help if its a local trait, otherwise it's not
198 violation.solution(err);
205 fn sized_trait_bound_spans<'tcx>(
207 bounds: hir::GenericBounds<'tcx>,
208 ) -> impl 'tcx + Iterator<Item = Span> {
209 bounds.iter().filter_map(move |b| match b {
210 hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
211 if trait_has_sized_self(
213 trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
216 // Fetch spans for supertraits that are `Sized`: `trait T: Super`
223 fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
225 .get_if_local(trait_def_id)
226 .and_then(|node| match node {
227 hir::Node::Item(hir::Item {
228 kind: hir::ItemKind::Trait(.., generics, bounds, _),
236 hir::WherePredicate::BoundPredicate(pred)
237 if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
239 // Fetch spans for trait bounds that are Sized:
240 // `trait T where Self: Pred`
241 Some(sized_trait_bound_spans(tcx, pred.bounds))
247 // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
248 .chain(sized_trait_bound_spans(tcx, bounds))
249 .collect::<SmallVec<[Span; 1]>>(),
253 .unwrap_or_else(SmallVec::new)
256 fn predicates_reference_self(
259 supertraits_only: bool,
260 ) -> SmallVec<[Span; 1]> {
261 let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
262 let predicates = if supertraits_only {
263 tcx.super_predicates_of(trait_def_id)
265 tcx.predicates_of(trait_def_id)
270 .map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
271 .filter_map(|predicate| predicate_references_self(tcx, predicate))
275 fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
276 tcx.associated_items(trait_def_id)
277 .in_definition_order()
278 .filter(|item| item.kind == ty::AssocKind::Type)
279 .flat_map(|item| tcx.explicit_item_bounds(item.def_id))
280 .filter_map(|pred_span| predicate_references_self(tcx, *pred_span))
284 fn predicate_references_self<'tcx>(
286 (predicate, sp): (ty::Predicate<'tcx>, Span),
288 let self_ty = tcx.types.self_param;
289 let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
290 match predicate.kind().skip_binder() {
291 ty::PredicateKind::Trait(ref data) => {
292 // In the case of a trait predicate, we can skip the "self" type.
293 if data.trait_ref.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
295 ty::PredicateKind::Projection(ref data) => {
296 // And similarly for projections. This should be redundant with
297 // the previous check because any projection should have a
298 // matching `Trait` predicate with the same inputs, but we do
299 // the check to be safe.
301 // It's also won't be redundant if we allow type-generic associated
302 // types for trait objects.
304 // Note that we *do* allow projection *outputs* to contain
305 // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
306 // we just require the user to specify *both* outputs
307 // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
309 // This is ALT2 in issue #56288, see that for discussion of the
310 // possible alternatives.
311 if data.projection_ty.substs[1..].iter().any(has_self_ty) { Some(sp) } else { None }
313 ty::PredicateKind::WellFormed(..)
314 | ty::PredicateKind::ObjectSafe(..)
315 | ty::PredicateKind::TypeOutlives(..)
316 | ty::PredicateKind::RegionOutlives(..)
317 | ty::PredicateKind::ClosureKind(..)
318 | ty::PredicateKind::Subtype(..)
319 | ty::PredicateKind::Coerce(..)
320 | ty::PredicateKind::ConstEvaluatable(..)
321 | ty::PredicateKind::ConstEquate(..)
322 | ty::PredicateKind::Ambiguous
323 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
327 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
328 generics_require_sized_self(tcx, trait_def_id)
331 fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
332 let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
333 return false; /* No Sized trait, can't require it! */
336 // Search for a predicate like `Self : Sized` amongst the trait bounds.
337 let predicates = tcx.predicates_of(def_id);
338 let predicates = predicates.instantiate_identity(tcx).predicates;
339 elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
340 match obligation.predicate.kind().skip_binder() {
341 ty::PredicateKind::Trait(ref trait_pred) => {
342 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
344 ty::PredicateKind::Projection(..)
345 | ty::PredicateKind::Subtype(..)
346 | ty::PredicateKind::Coerce(..)
347 | ty::PredicateKind::RegionOutlives(..)
348 | ty::PredicateKind::WellFormed(..)
349 | ty::PredicateKind::ObjectSafe(..)
350 | ty::PredicateKind::ClosureKind(..)
351 | ty::PredicateKind::TypeOutlives(..)
352 | ty::PredicateKind::ConstEvaluatable(..)
353 | ty::PredicateKind::ConstEquate(..)
354 | ty::PredicateKind::Ambiguous
355 | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
360 /// Returns `Some(_)` if this method makes the containing trait not object safe.
361 fn object_safety_violation_for_method(
364 method: &ty::AssocItem,
365 ) -> Option<(MethodViolationCode, Span)> {
366 debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
367 // Any method that has a `Self : Sized` requisite is otherwise
368 // exempt from the regulations.
369 if generics_require_sized_self(tcx, method.def_id) {
373 let violation = virtual_call_violation_for_method(tcx, trait_def_id, method);
374 // Get an accurate span depending on the violation.
376 let node = tcx.hir().get_if_local(method.def_id);
377 let span = match (&v, node) {
378 (MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
379 (MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
380 (MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
381 (MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
382 node.fn_decl().map_or(method.ident(tcx).span, |decl| decl.output.span())
384 _ => method.ident(tcx).span,
390 /// Returns `Some(_)` if this method cannot be called on a trait
391 /// object; this does not necessarily imply that the enclosing trait
392 /// is not object safe, because the method might have a where clause
394 fn virtual_call_violation_for_method<'tcx>(
397 method: &ty::AssocItem,
398 ) -> Option<MethodViolationCode> {
399 let sig = tcx.fn_sig(method.def_id);
401 // The method's first parameter must be named `self`
402 if !method.fn_has_self_parameter {
403 let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
405 kind: hir::TraitItemKind::Fn(sig, _),
407 })) = tcx.hir().get_if_local(method.def_id).as_ref()
409 let sm = tcx.sess.source_map();
412 format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
413 sm.span_through_char(sig.span, '(').shrink_to_hi(),
416 format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
417 generics.tail_span_for_predicate_suggestion(),
423 return Some(MethodViolationCode::StaticMethod(sugg));
426 for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
427 if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
428 let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
429 kind: hir::TraitItemKind::Fn(sig, _),
431 })) = tcx.hir().get_if_local(method.def_id).as_ref()
433 Some(sig.decl.inputs[i].span)
437 return Some(MethodViolationCode::ReferencesSelfInput(span));
440 if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
441 return Some(MethodViolationCode::ReferencesSelfOutput);
443 if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
447 // We can't monomorphize things like `fn foo<A>(...)`.
448 let own_counts = tcx.generics_of(method.def_id).own_counts();
449 if own_counts.types + own_counts.consts != 0 {
450 return Some(MethodViolationCode::Generic);
453 let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
455 // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
456 // However, this is already considered object-safe. We allow it as a special case here.
457 // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
458 // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
459 if receiver_ty != tcx.types.self_param {
460 if !receiver_is_dispatchable(tcx, method, receiver_ty) {
461 let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
462 kind: hir::TraitItemKind::Fn(sig, _),
464 })) = tcx.hir().get_if_local(method.def_id).as_ref()
466 Some(sig.decl.inputs[0].span)
470 return Some(MethodViolationCode::UndispatchableReceiver(span));
472 // Do sanity check to make sure the receiver actually has the layout of a pointer.
474 use rustc_target::abi::Abi;
476 let param_env = tcx.param_env(method.def_id);
478 let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
479 match tcx.layout_of(param_env.and(ty)) {
480 Ok(layout) => Some(layout.abi),
483 tcx.sess.delay_span_bug(
484 tcx.def_span(method.def_id),
485 &format!("error: {}\n while computing layout for type {:?}", err, ty),
493 let unit_receiver_ty =
494 receiver_for_self_ty(tcx, receiver_ty, tcx.mk_unit(), method.def_id);
496 match abi_of_ty(unit_receiver_ty) {
497 Some(Abi::Scalar(..)) => (),
499 tcx.sess.delay_span_bug(
500 tcx.def_span(method.def_id),
502 "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
509 let trait_object_ty =
510 object_ty_for_trait(tcx, trait_def_id, tcx.mk_region(ty::ReStatic));
512 // e.g., `Rc<dyn Trait>`
513 let trait_object_receiver =
514 receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
516 match abi_of_ty(trait_object_receiver) {
517 Some(Abi::ScalarPair(..)) => (),
519 tcx.sess.delay_span_bug(
520 tcx.def_span(method.def_id),
522 "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
531 // NOTE: This check happens last, because it results in a lint, and not a
534 .predicates_of(method.def_id)
537 // A trait object can't claim to live more than the concrete type,
538 // so outlives predicates will always hold.
540 .filter(|(p, _)| p.to_opt_type_outlives().is_none())
541 .any(|pred| contains_illegal_self_type_reference(tcx, trait_def_id, pred))
543 return Some(MethodViolationCode::WhereClauseReferencesSelf);
549 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
550 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
551 fn receiver_for_self_ty<'tcx>(
553 receiver_ty: Ty<'tcx>,
555 method_def_id: DefId,
557 debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
558 let substs = InternalSubsts::for_item(tcx, method_def_id, |param, _| {
559 if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
562 let result = EarlyBinder(receiver_ty).subst(tcx, substs);
564 "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
565 receiver_ty, self_ty, method_def_id, result
570 /// Creates the object type for the current trait. For example,
571 /// if the current trait is `Deref`, then this will be
572 /// `dyn Deref<Target = Self::Target> + 'static`.
573 #[instrument(level = "trace", skip(tcx), ret)]
574 fn object_ty_for_trait<'tcx>(
577 lifetime: ty::Region<'tcx>,
579 let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
582 let trait_predicate = trait_ref.map_bound(|trait_ref| {
583 ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))
585 debug!(?trait_predicate);
587 let mut elaborated_predicates: Vec<_> = elaborate_trait_ref(tcx, trait_ref)
588 .filter_map(|obligation| {
590 let pred = obligation.predicate.to_opt_poly_projection_pred()?;
591 Some(pred.map_bound(|p| {
592 ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
593 item_def_id: p.projection_ty.item_def_id,
594 substs: p.projection_ty.substs,
600 // NOTE: Since #37965, the existential predicates list has depended on the
601 // list of predicates to be sorted. This is mostly to enforce that the primary
602 // predicate comes first.
603 elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
604 elaborated_predicates.dedup();
606 let existential_predicates = tcx
607 .mk_poly_existential_predicates(iter::once(trait_predicate).chain(elaborated_predicates));
608 debug!(?existential_predicates);
610 tcx.mk_dynamic(existential_predicates, lifetime, ty::Dyn)
613 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
614 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
615 /// in the following way:
616 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
617 /// - require the following bound:
619 /// ```ignore (not-rust)
620 /// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
623 /// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
624 /// (substitution notation).
626 /// Some examples of receiver types and their required obligation:
627 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
628 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
629 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
631 /// The only case where the receiver is not dispatchable, but is still a valid receiver
632 /// type (just not object-safe), is when there is more than one level of pointer indirection.
633 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
634 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
635 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
636 /// contained by the trait object, because the object that needs to be coerced is behind
639 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
640 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
641 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
642 /// Instead, we fudge a little by introducing a new type parameter `U` such that
643 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
644 /// Written as a chalk-style query:
645 /// ```ignore (not-rust)
646 /// forall (U: Trait + ?Sized) {
647 /// if (Self: Unsize<U>) {
648 /// Receiver: DispatchFromDyn<Receiver[Self => U]>
652 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
653 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
654 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
656 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
657 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
658 // `self: Wrapper<Self>`.
660 fn receiver_is_dispatchable<'tcx>(
662 method: &ty::AssocItem,
663 receiver_ty: Ty<'tcx>,
665 debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
667 let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
668 let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
669 debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
673 // the type `U` in the query
674 // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
675 // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
676 // replace this with `dyn Trait`
677 let unsized_self_ty: Ty<'tcx> =
678 tcx.mk_ty_param(u32::MAX, Symbol::intern("RustaceansAreAwesome"));
680 // `Receiver[Self => U]`
681 let unsized_receiver_ty =
682 receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
684 // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
685 // `U: ?Sized` is already implied here
687 let param_env = tcx.param_env(method.def_id);
690 let unsize_predicate = ty::Binder::dummy(ty::TraitRef {
692 substs: tcx.mk_substs_trait(tcx.types.self_param, &[unsized_self_ty.into()]),
697 // U: Trait<Arg1, ..., ArgN>
698 let trait_predicate = {
700 InternalSubsts::for_item(tcx, method.trait_container(tcx).unwrap(), |param, _| {
701 if param.index == 0 {
702 unsized_self_ty.into()
704 tcx.mk_param_from_def(param)
708 ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
713 let caller_bounds: Vec<Predicate<'tcx>> =
714 param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
717 tcx.intern_predicates(&caller_bounds),
719 param_env.constness(),
723 // Receiver: DispatchFromDyn<Receiver[Self => U]>
725 let predicate = ty::Binder::dummy(ty::TraitRef {
726 def_id: dispatch_from_dyn_did,
727 substs: tcx.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
731 Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
734 let infcx = tcx.infer_ctxt().build();
735 // the receiver is dispatchable iff the obligation holds
736 infcx.predicate_must_hold_modulo_regions(&obligation)
739 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
744 // This is somewhat subtle. In general, we want to forbid
745 // references to `Self` in the argument and return types,
746 // since the value of `Self` is erased. However, there is one
747 // exception: it is ok to reference `Self` in order to access
748 // an associated type of the current trait, since we retain
749 // the value of those associated types in the object type
753 // trait SuperTrait {
757 // trait Trait : SuperTrait {
759 // fn foo(&self, x: Self) // bad
760 // fn foo(&self) -> Self // bad
761 // fn foo(&self) -> Option<Self> // bad
762 // fn foo(&self) -> Self::Y // OK, desugars to next example
763 // fn foo(&self) -> <Self as Trait>::Y // OK
764 // fn foo(&self) -> Self::X // OK, desugars to next example
765 // fn foo(&self) -> <Self as SuperTrait>::X // OK
769 // However, it is not as simple as allowing `Self` in a projected
770 // type, because there are illegal ways to use `Self` as well:
773 // trait Trait : SuperTrait {
775 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
779 // Here we will not have the type of `X` recorded in the
780 // object type, and we cannot resolve `Self as SomeOtherTrait`
781 // without knowing what `Self` is.
783 struct IllegalSelfTypeVisitor<'tcx> {
786 supertraits: Option<Vec<DefId>>,
789 impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
792 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
795 if t == self.tcx.types.self_param {
798 ControlFlow::CONTINUE
801 ty::Projection(ref data)
802 if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
804 // We'll deny these later in their own pass
805 ControlFlow::CONTINUE
807 ty::Projection(ref data) => {
808 // This is a projected type `<Foo as SomeTrait>::X`.
810 // Compute supertraits of current trait lazily.
811 if self.supertraits.is_none() {
812 let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
813 self.supertraits = Some(
814 traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
818 // Determine whether the trait reference `Foo as
819 // SomeTrait` is in fact a supertrait of the
820 // current trait. In that case, this type is
821 // legal, because the type `X` will be specified
822 // in the object type. Note that we can just use
823 // direct equality here because all of these types
824 // are part of the formal parameter listing, and
825 // hence there should be no inference variables.
826 let is_supertrait_of_current_trait = self
830 .contains(&data.trait_ref(self.tcx).def_id);
832 if is_supertrait_of_current_trait {
833 ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
835 t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
838 _ => t.super_visit_with(self), // walk contained types, if any
842 fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
843 // Constants can only influence object safety if they reference `Self`.
844 // This is only possible for unevaluated constants, so we walk these here.
846 // If `AbstractConst::from_const` returned an error we already failed compilation
847 // so we don't have to emit an additional error here.
848 use rustc_middle::ty::abstract_const::Node;
849 if let Ok(Some(ct)) = AbstractConst::from_const(self.tcx, ct) {
850 walk_abstract_const(self.tcx, ct, |node| match node.root(self.tcx) {
851 Node::Leaf(leaf) => self.visit_const(leaf),
852 Node::Cast(_, _, ty) => self.visit_ty(ty),
853 Node::Binop(..) | Node::UnaryOp(..) | Node::FunctionCall(_, _) => {
854 ControlFlow::CONTINUE
858 ct.super_visit_with(self)
864 .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
868 pub fn contains_illegal_impl_trait_in_trait<'tcx>(
871 ty: ty::Binder<'tcx, Ty<'tcx>>,
872 ) -> Option<MethodViolationCode> {
873 // This would be caught below, but rendering the error as a separate
874 // `async-specific` message is better.
875 if tcx.asyncness(fn_def_id).is_async() {
876 return Some(MethodViolationCode::AsyncFn);
879 // FIXME(RPITIT): Perhaps we should use a visitor here?
880 ty.skip_binder().walk().find_map(|arg| {
881 if let ty::GenericArgKind::Type(ty) = arg.unpack()
882 && let ty::Projection(proj) = ty.kind()
883 && tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
885 Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.item_def_id)))
892 pub fn provide(providers: &mut ty::query::Providers) {
893 *providers = ty::query::Providers { object_safety_violations, ..*providers };