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;
14 use crate::hir::def_id::DefId;
16 use crate::traits::{self, Obligation, ObligationCause};
17 use crate::ty::{self, Ty, TyCtxt, TypeFoldable, Predicate, ToPredicate};
18 use crate::ty::subst::{Subst, InternalSubsts};
20 use std::iter::{self};
21 use syntax::ast::{self};
22 use syntax::symbol::Symbol;
23 use syntax_pos::{Span, DUMMY_SP};
25 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
26 pub enum ObjectSafetyViolation {
27 /// `Self: Sized` declared on the trait.
30 /// Supertrait reference references `Self` an in illegal location
31 /// (e.g., `trait Foo : Bar<Self>`).
34 /// Method has something illegal.
35 Method(ast::Name, MethodViolationCode, Span),
38 AssocConst(ast::Name, Span),
41 impl ObjectSafetyViolation {
42 pub fn error_msg(&self) -> Cow<'static, str> {
44 ObjectSafetyViolation::SizedSelf =>
45 "the trait cannot require that `Self : Sized`".into(),
46 ObjectSafetyViolation::SupertraitSelf =>
47 "the trait cannot use `Self` as a type parameter \
48 in the supertraits or where-clauses".into(),
49 ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod, _) =>
50 format!("associated function `{}` has no `self` parameter", name).into(),
51 ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelf, _) => format!(
52 "method `{}` references the `Self` type in its parameters or return type",
55 ObjectSafetyViolation::Method(
57 MethodViolationCode::WhereClauseReferencesSelf,
59 ) => format!("method `{}` references the `Self` type in where clauses", name).into(),
60 ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) =>
61 format!("method `{}` has generic type parameters", name).into(),
62 ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) =>
63 format!("method `{}`'s `self` parameter cannot be dispatched on", name).into(),
64 ObjectSafetyViolation::AssocConst(name, _) =>
65 format!("the trait cannot contain associated consts like `{}`", name).into(),
69 pub fn span(&self) -> Option<Span> {
70 // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
71 // diagnostics use a `note` instead of a `span_label`.
73 ObjectSafetyViolation::AssocConst(_, span) |
74 ObjectSafetyViolation::Method(_, _, span) if span != DUMMY_SP => Some(span),
80 /// Reasons a method might not be object-safe.
81 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
82 pub enum MethodViolationCode {
86 /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self`
89 /// e.g., `fn foo(&self) where Self: Clone`
90 WhereClauseReferencesSelf,
92 /// e.g., `fn foo<A>()`
95 /// the method's receiver (`self` argument) can't be dispatched on
96 UndispatchableReceiver,
99 impl<'tcx> TyCtxt<'tcx> {
100 /// Returns the object safety violations that affect
101 /// astconv -- currently, `Self` in supertraits. This is needed
102 /// because `object_safety_violations` can't be used during
104 pub fn astconv_object_safety_violations(
107 ) -> Vec<ObjectSafetyViolation> {
108 debug_assert!(self.generics_of(trait_def_id).has_self);
109 let violations = traits::supertrait_def_ids(self, trait_def_id)
110 .filter(|&def_id| self.predicates_reference_self(def_id, true))
111 .map(|_| ObjectSafetyViolation::SupertraitSelf)
114 debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}",
121 pub fn object_safety_violations(self, trait_def_id: DefId)
122 -> Vec<ObjectSafetyViolation>
124 debug_assert!(self.generics_of(trait_def_id).has_self);
125 debug!("object_safety_violations: {:?}", trait_def_id);
127 traits::supertrait_def_ids(self, trait_def_id)
128 .flat_map(|def_id| self.object_safety_violations_for_trait(def_id))
132 /// We say a method is *vtable safe* if it can be invoked on a trait
133 /// object. Note that object-safe traits can have some
134 /// non-vtable-safe methods, so long as they require `Self: Sized` or
135 /// otherwise ensure that they cannot be used when `Self = Trait`.
136 pub fn is_vtable_safe_method(self, trait_def_id: DefId, method: &ty::AssocItem) -> bool {
137 debug_assert!(self.generics_of(trait_def_id).has_self);
138 debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
139 // Any method that has a `Self: Sized` bound cannot be called.
140 if self.generics_require_sized_self(method.def_id) {
144 match self.virtual_call_violation_for_method(trait_def_id, method) {
145 None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
150 fn object_safety_violations_for_trait(self, trait_def_id: DefId) -> Vec<ObjectSafetyViolation> {
151 // Check methods for violations.
152 let mut violations: Vec<_> = self.associated_items(trait_def_id)
153 .filter(|item| item.kind == ty::AssocKind::Method)
155 self.object_safety_violation_for_method(trait_def_id, &item).map(|code| {
156 ObjectSafetyViolation::Method(item.ident.name, code, item.ident.span)
158 ).filter(|violation| {
159 if let ObjectSafetyViolation::Method(
161 MethodViolationCode::WhereClauseReferencesSelf,
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.
167 lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY,
170 &format!("the trait `{}` cannot be made into an object",
171 self.def_path_str(trait_def_id)),
172 &violation.error_msg());
179 // Check the trait itself.
180 if self.trait_has_sized_self(trait_def_id) {
181 violations.push(ObjectSafetyViolation::SizedSelf);
183 if self.predicates_reference_self(trait_def_id, false) {
184 violations.push(ObjectSafetyViolation::SupertraitSelf);
187 violations.extend(self.associated_items(trait_def_id)
188 .filter(|item| item.kind == ty::AssocKind::Const)
189 .map(|item| ObjectSafetyViolation::AssocConst(item.ident.name, item.ident.span)));
191 debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
198 fn predicates_reference_self(
201 supertraits_only: bool,
203 let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(self, trait_def_id));
204 let predicates = if supertraits_only {
205 self.super_predicates_of(trait_def_id)
207 self.predicates_of(trait_def_id)
209 let self_ty = self.types.self_param;
210 let has_self_ty = |t: Ty<'tcx>| t.walk().any(|t| t == self_ty);
214 .map(|(predicate, _)| predicate.subst_supertrait(self, &trait_ref))
217 ty::Predicate::Trait(ref data) => {
218 // In the case of a trait predicate, we can skip the "self" type.
219 data.skip_binder().input_types().skip(1).any(has_self_ty)
221 ty::Predicate::Projection(ref data) => {
222 // And similarly for projections. This should be redundant with
223 // the previous check because any projection should have a
224 // matching `Trait` predicate with the same inputs, but we do
225 // the check to be safe.
227 // Note that we *do* allow projection *outputs* to contain
228 // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
229 // we just require the user to specify *both* outputs
230 // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
232 // This is ALT2 in issue #56288, see that for discussion of the
233 // possible alternatives.
241 ty::Predicate::WellFormed(..) |
242 ty::Predicate::ObjectSafe(..) |
243 ty::Predicate::TypeOutlives(..) |
244 ty::Predicate::RegionOutlives(..) |
245 ty::Predicate::ClosureKind(..) |
246 ty::Predicate::Subtype(..) |
247 ty::Predicate::ConstEvaluatable(..) => {
254 fn trait_has_sized_self(self, trait_def_id: DefId) -> bool {
255 self.generics_require_sized_self(trait_def_id)
258 fn generics_require_sized_self(self, def_id: DefId) -> bool {
259 let sized_def_id = match self.lang_items().sized_trait() {
260 Some(def_id) => def_id,
261 None => { return false; /* No Sized trait, can't require it! */ }
264 // Search for a predicate like `Self : Sized` amongst the trait bounds.
265 let predicates = self.predicates_of(def_id);
266 let predicates = predicates.instantiate_identity(self).predicates;
267 elaborate_predicates(self, predicates)
268 .any(|predicate| match predicate {
269 ty::Predicate::Trait(ref trait_pred) => {
270 trait_pred.def_id() == sized_def_id
271 && trait_pred.skip_binder().self_ty().is_param(0)
273 ty::Predicate::Projection(..) |
274 ty::Predicate::Subtype(..) |
275 ty::Predicate::RegionOutlives(..) |
276 ty::Predicate::WellFormed(..) |
277 ty::Predicate::ObjectSafe(..) |
278 ty::Predicate::ClosureKind(..) |
279 ty::Predicate::TypeOutlives(..) |
280 ty::Predicate::ConstEvaluatable(..) => {
287 /// Returns `Some(_)` if this method makes the containing trait not object safe.
288 fn object_safety_violation_for_method(
291 method: &ty::AssocItem,
292 ) -> Option<MethodViolationCode> {
293 debug!("object_safety_violation_for_method({:?}, {:?})", trait_def_id, method);
294 // Any method that has a `Self : Sized` requisite is otherwise
295 // exempt from the regulations.
296 if self.generics_require_sized_self(method.def_id) {
300 self.virtual_call_violation_for_method(trait_def_id, method)
303 /// Returns `Some(_)` if this method cannot be called on a trait
304 /// object; this does not necessarily imply that the enclosing trait
305 /// is not object safe, because the method might have a where clause
307 fn virtual_call_violation_for_method(
310 method: &ty::AssocItem,
311 ) -> Option<MethodViolationCode> {
312 // The method's first parameter must be named `self`
313 if !method.method_has_self_argument {
314 return Some(MethodViolationCode::StaticMethod);
317 let sig = self.fn_sig(method.def_id);
319 for input_ty in &sig.skip_binder().inputs()[1..] {
320 if self.contains_illegal_self_type_reference(trait_def_id, input_ty) {
321 return Some(MethodViolationCode::ReferencesSelf);
324 if self.contains_illegal_self_type_reference(trait_def_id, sig.output().skip_binder()) {
325 return Some(MethodViolationCode::ReferencesSelf);
328 // We can't monomorphize things like `fn foo<A>(...)`.
329 let own_counts = self.generics_of(method.def_id).own_counts();
330 if own_counts.types + own_counts.consts != 0 {
331 return Some(MethodViolationCode::Generic);
334 if self.predicates_of(method.def_id).predicates.iter()
335 // A trait object can't claim to live more than the concrete type,
336 // so outlives predicates will always hold.
338 .filter(|(p, _)| p.to_opt_type_outlives().is_none())
340 // Do a shallow visit so that `contains_illegal_self_type_reference`
341 // may apply it's custom visiting.
342 .visit_tys_shallow(|t| {
343 self.contains_illegal_self_type_reference(trait_def_id, t)
345 return Some(MethodViolationCode::WhereClauseReferencesSelf);
348 let receiver_ty = self.liberate_late_bound_regions(
350 &sig.map_bound(|sig| sig.inputs()[0]),
353 // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
354 // However, this is already considered object-safe. We allow it as a special case here.
355 // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
356 // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
357 if receiver_ty != self.types.self_param {
358 if !self.receiver_is_dispatchable(method, receiver_ty) {
359 return Some(MethodViolationCode::UndispatchableReceiver);
361 // Do sanity check to make sure the receiver actually has the layout of a pointer.
363 use crate::ty::layout::Abi;
365 let param_env = self.param_env(method.def_id);
367 let abi_of_ty = |ty: Ty<'tcx>| -> &Abi {
368 match self.layout_of(param_env.and(ty)) {
369 Ok(layout) => &layout.abi,
371 "error: {}\n while computing layout for type {:?}", err, ty
377 let unit_receiver_ty = self.receiver_for_self_ty(
378 receiver_ty, self.mk_unit(), method.def_id
381 match abi_of_ty(unit_receiver_ty) {
382 &Abi::Scalar(..) => (),
384 self.sess.delay_span_bug(
385 self.def_span(method.def_id),
387 "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
394 let trait_object_ty = self.object_ty_for_trait(
395 trait_def_id, self.mk_region(ty::ReStatic)
398 // e.g., `Rc<dyn Trait>`
399 let trait_object_receiver = self.receiver_for_self_ty(
400 receiver_ty, trait_object_ty, method.def_id
403 match abi_of_ty(trait_object_receiver) {
404 &Abi::ScalarPair(..) => (),
406 self.sess.delay_span_bug(
407 self.def_span(method.def_id),
409 "receiver when `Self = {}` should have a ScalarPair ABI; \
422 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
423 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
424 fn receiver_for_self_ty(
426 receiver_ty: Ty<'tcx>,
428 method_def_id: DefId,
430 debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
431 let substs = InternalSubsts::for_item(self, method_def_id, |param, _| {
432 if param.index == 0 {
435 self.mk_param_from_def(param)
439 let result = receiver_ty.subst(self, substs);
440 debug!("receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
441 receiver_ty, self_ty, method_def_id, result);
445 /// Creates the object type for the current trait. For example,
446 /// if the current trait is `Deref`, then this will be
447 /// `dyn Deref<Target = Self::Target> + 'static`.
448 fn object_ty_for_trait(self, trait_def_id: DefId, lifetime: ty::Region<'tcx>) -> Ty<'tcx> {
449 debug!("object_ty_for_trait: trait_def_id={:?}", trait_def_id);
451 let trait_ref = ty::TraitRef::identity(self, trait_def_id);
453 let trait_predicate = ty::ExistentialPredicate::Trait(
454 ty::ExistentialTraitRef::erase_self_ty(self, trait_ref)
457 let mut associated_types = traits::supertraits(self, ty::Binder::dummy(trait_ref))
458 .flat_map(|super_trait_ref| {
459 self.associated_items(super_trait_ref.def_id())
460 .map(move |item| (super_trait_ref, item))
462 .filter(|(_, item)| item.kind == ty::AssocKind::Type)
463 .collect::<Vec<_>>();
465 // existential predicates need to be in a specific order
466 associated_types.sort_by_cached_key(|(_, item)| self.def_path_hash(item.def_id));
468 let projection_predicates = associated_types.into_iter().map(|(super_trait_ref, item)| {
469 // We *can* get bound lifetimes here in cases like
470 // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
472 // binder moved to (*)...
473 let super_trait_ref = super_trait_ref.skip_binder();
474 ty::ExistentialPredicate::Projection(ty::ExistentialProjection {
475 ty: self.mk_projection(item.def_id, super_trait_ref.substs),
476 item_def_id: item.def_id,
477 substs: super_trait_ref.substs,
481 let existential_predicates = self.mk_existential_predicates(
482 iter::once(trait_predicate).chain(projection_predicates)
485 let object_ty = self.mk_dynamic(
486 // (*) ... binder re-introduced here
487 ty::Binder::bind(existential_predicates),
491 debug!("object_ty_for_trait: object_ty=`{}`", object_ty);
496 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
497 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
498 /// in the following way:
499 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
500 /// - require the following bound:
503 /// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
506 /// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
507 /// (substitution notation).
509 /// Some examples of receiver types and their required obligation:
510 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
511 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
512 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
514 /// The only case where the receiver is not dispatchable, but is still a valid receiver
515 /// type (just not object-safe), is when there is more than one level of pointer indirection.
516 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
517 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
518 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
519 /// contained by the trait object, because the object that needs to be coerced is behind
522 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
523 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
524 /// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
525 /// Instead, we fudge a little by introducing a new type parameter `U` such that
526 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
527 /// Written as a chalk-style query:
529 /// forall (U: Trait + ?Sized) {
530 /// if (Self: Unsize<U>) {
531 /// Receiver: DispatchFromDyn<Receiver[Self => U]>
535 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
536 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
537 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
539 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
540 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
541 // `self: Wrapper<Self>`.
543 fn receiver_is_dispatchable(
545 method: &ty::AssocItem,
546 receiver_ty: Ty<'tcx>,
548 debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
550 let traits = (self.lang_items().unsize_trait(),
551 self.lang_items().dispatch_from_dyn_trait());
552 let (unsize_did, dispatch_from_dyn_did) = if let (Some(u), Some(cu)) = traits {
555 debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
559 // the type `U` in the query
560 // use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
561 // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
562 // replace this with `dyn Trait`
563 let unsized_self_ty: Ty<'tcx> = self.mk_ty_param(
565 Symbol::intern("RustaceansAreAwesome"),
568 // `Receiver[Self => U]`
569 let unsized_receiver_ty = self.receiver_for_self_ty(
570 receiver_ty, unsized_self_ty, method.def_id
573 // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
574 // `U: ?Sized` is already implied here
576 let mut param_env = self.param_env(method.def_id);
579 let unsize_predicate = ty::TraitRef {
581 substs: self.mk_substs_trait(self.types.self_param, &[unsized_self_ty.into()]),
584 // U: Trait<Arg1, ..., ArgN>
585 let trait_predicate = {
586 let substs = InternalSubsts::for_item(
588 method.container.assert_trait(),
590 if param.index == 0 {
591 unsized_self_ty.into()
593 self.mk_param_from_def(param)
604 let caller_bounds: Vec<Predicate<'tcx>> = param_env.caller_bounds.iter().cloned()
605 .chain(iter::once(unsize_predicate))
606 .chain(iter::once(trait_predicate))
609 param_env.caller_bounds = self.intern_predicates(&caller_bounds);
614 // Receiver: DispatchFromDyn<Receiver[Self => U]>
616 let predicate = ty::TraitRef {
617 def_id: dispatch_from_dyn_did,
618 substs: self.mk_substs_trait(receiver_ty, &[unsized_receiver_ty.into()]),
622 ObligationCause::dummy(),
628 self.infer_ctxt().enter(|ref infcx| {
629 // the receiver is dispatchable iff the obligation holds
630 infcx.predicate_must_hold_modulo_regions(&obligation)
634 fn contains_illegal_self_type_reference(
639 // This is somewhat subtle. In general, we want to forbid
640 // references to `Self` in the argument and return types,
641 // since the value of `Self` is erased. However, there is one
642 // exception: it is ok to reference `Self` in order to access
643 // an associated type of the current trait, since we retain
644 // the value of those associated types in the object type
648 // trait SuperTrait {
652 // trait Trait : SuperTrait {
654 // fn foo(&self, x: Self) // bad
655 // fn foo(&self) -> Self // bad
656 // fn foo(&self) -> Option<Self> // bad
657 // fn foo(&self) -> Self::Y // OK, desugars to next example
658 // fn foo(&self) -> <Self as Trait>::Y // OK
659 // fn foo(&self) -> Self::X // OK, desugars to next example
660 // fn foo(&self) -> <Self as SuperTrait>::X // OK
664 // However, it is not as simple as allowing `Self` in a projected
665 // type, because there are illegal ways to use `Self` as well:
668 // trait Trait : SuperTrait {
670 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
674 // Here we will not have the type of `X` recorded in the
675 // object type, and we cannot resolve `Self as SomeOtherTrait`
676 // without knowing what `Self` is.
678 let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
679 let mut error = false;
680 let self_ty = self.types.self_param;
688 false // no contained types to walk
691 ty::Projection(ref data) => {
692 // This is a projected type `<Foo as SomeTrait>::X`.
694 // Compute supertraits of current trait lazily.
695 if supertraits.is_none() {
696 let trait_ref = ty::Binder::bind(
697 ty::TraitRef::identity(self, trait_def_id),
699 supertraits = Some(traits::supertraits(self, trait_ref).collect());
702 // Determine whether the trait reference `Foo as
703 // SomeTrait` is in fact a supertrait of the
704 // current trait. In that case, this type is
705 // legal, because the type `X` will be specified
706 // in the object type. Note that we can just use
707 // direct equality here because all of these types
708 // are part of the formal parameter listing, and
709 // hence there should be no inference variables.
710 let projection_trait_ref = ty::Binder::bind(data.trait_ref(self));
711 let is_supertrait_of_current_trait =
712 supertraits.as_ref().unwrap().contains(&projection_trait_ref);
714 if is_supertrait_of_current_trait {
715 false // do not walk contained types, do not report error, do collect $200
717 true // DO walk contained types, POSSIBLY reporting an error
721 _ => true, // walk contained types, if any
729 pub(super) fn is_object_safe_provider(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
730 tcx.object_safety_violations(trait_def_id).is_empty()