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
::{Applicability, FatalError}
;
18 use rustc_hir
::def_id
::DefId
;
19 use rustc_middle
::ty
::subst
::{GenericArg, InternalSubsts, Subst}
;
20 use rustc_middle
::ty
::{self, Ty, TyCtxt, TypeFoldable, TypeVisitor, WithConstness}
;
21 use rustc_middle
::ty
::{Predicate, ToPredicate}
;
22 use rustc_session
::lint
::builtin
::WHERE_CLAUSES_OBJECT_SAFETY
;
23 use rustc_span
::symbol
::Symbol
;
25 use smallvec
::SmallVec
;
29 pub use crate::traits
::{MethodViolationCode, ObjectSafetyViolation}
;
31 /// Returns the object safety violations that affect
32 /// astconv -- currently, `Self` in supertraits. This is needed
33 /// because `object_safety_violations` can't be used during
35 pub fn astconv_object_safety_violations(
38 ) -> Vec
<ObjectSafetyViolation
> {
39 debug_assert
!(tcx
.generics_of(trait_def_id
).has_self
);
40 let violations
= traits
::supertrait_def_ids(tcx
, trait_def_id
)
41 .map(|def_id
| predicates_reference_self(tcx
, def_id
, true))
42 .filter(|spans
| !spans
.is_empty())
43 .map(ObjectSafetyViolation
::SupertraitSelf
)
46 debug
!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id
, violations
);
51 fn object_safety_violations(
54 ) -> &'tcx
[ObjectSafetyViolation
] {
55 debug_assert
!(tcx
.generics_of(trait_def_id
).has_self
);
56 debug
!("object_safety_violations: {:?}", trait_def_id
);
58 tcx
.arena
.alloc_from_iter(
59 traits
::supertrait_def_ids(tcx
, trait_def_id
)
60 .flat_map(|def_id
| object_safety_violations_for_trait(tcx
, def_id
)),
64 /// We say a method is *vtable safe* if it can be invoked on a trait
65 /// object. Note that object-safe traits can have some
66 /// non-vtable-safe methods, so long as they require `Self: Sized` or
67 /// otherwise ensure that they cannot be used when `Self = Trait`.
68 pub fn is_vtable_safe_method(tcx
: TyCtxt
<'_
>, trait_def_id
: DefId
, method
: &ty
::AssocItem
) -> bool
{
69 debug_assert
!(tcx
.generics_of(trait_def_id
).has_self
);
70 debug
!("is_vtable_safe_method({:?}, {:?})", trait_def_id
, method
);
71 // Any method that has a `Self: Sized` bound cannot be called.
72 if generics_require_sized_self(tcx
, method
.def_id
) {
76 match virtual_call_violation_for_method(tcx
, trait_def_id
, method
) {
77 None
| Some(MethodViolationCode
::WhereClauseReferencesSelf
) => true,
82 fn object_safety_violations_for_trait(
85 ) -> Vec
<ObjectSafetyViolation
> {
86 // Check methods for violations.
87 let mut violations
: Vec
<_
> = tcx
88 .associated_items(trait_def_id
)
89 .in_definition_order()
90 .filter(|item
| item
.kind
== ty
::AssocKind
::Fn
)
92 object_safety_violation_for_method(tcx
, trait_def_id
, &item
)
93 .map(|(code
, span
)| ObjectSafetyViolation
::Method(item
.ident
.name
, code
, span
))
96 if let ObjectSafetyViolation
::Method(
98 MethodViolationCode
::WhereClauseReferencesSelf
,
102 // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
103 // It's also hard to get a use site span, so we use the method definition span.
104 tcx
.struct_span_lint_hir(
105 WHERE_CLAUSES_OBJECT_SAFETY
,
109 let mut err
= lint
.build(&format
!(
110 "the trait `{}` cannot be made into an object",
111 tcx
.def_path_str(trait_def_id
)
113 let node
= tcx
.hir().get_if_local(trait_def_id
);
114 let msg
= if let Some(hir
::Node
::Item(item
)) = node
{
117 "this trait cannot be made into an object...",
119 format
!("...because {}", violation
.error_msg())
122 "the trait cannot be made into an object because {}",
123 violation
.error_msg()
126 err
.span_label(*span
, &msg
);
127 match (node
, violation
.solution()) {
128 (Some(_
), Some((note
, None
))) => {
131 (Some(_
), Some((note
, Some((sugg
, span
))))) => {
136 Applicability
::MachineApplicable
,
139 // Only provide the help if its a local trait, otherwise it's not actionable.
152 // Check the trait itself.
153 if trait_has_sized_self(tcx
, trait_def_id
) {
154 // We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
155 let spans
= get_sized_bounds(tcx
, trait_def_id
);
156 violations
.push(ObjectSafetyViolation
::SizedSelf(spans
));
158 let spans
= predicates_reference_self(tcx
, trait_def_id
, false);
159 if !spans
.is_empty() {
160 violations
.push(ObjectSafetyViolation
::SupertraitSelf(spans
));
164 tcx
.associated_items(trait_def_id
)
165 .in_definition_order()
166 .filter(|item
| item
.kind
== ty
::AssocKind
::Const
)
167 .map(|item
| ObjectSafetyViolation
::AssocConst(item
.ident
.name
, item
.ident
.span
)),
171 "object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
172 trait_def_id
, violations
178 fn sized_trait_bound_spans
<'tcx
>(
180 bounds
: hir
::GenericBounds
<'tcx
>,
181 ) -> impl 'tcx
+ Iterator
<Item
= Span
> {
182 bounds
.iter().filter_map(move |b
| match b
{
183 hir
::GenericBound
::Trait(trait_ref
, hir
::TraitBoundModifier
::None
)
184 if trait_has_sized_self(
186 trait_ref
.trait_ref
.trait_def_id().unwrap_or_else(|| FatalError
.raise()),
189 // Fetch spans for supertraits that are `Sized`: `trait T: Super`
196 fn get_sized_bounds(tcx
: TyCtxt
<'_
>, trait_def_id
: DefId
) -> SmallVec
<[Span
; 1]> {
198 .get_if_local(trait_def_id
)
199 .and_then(|node
| match node
{
200 hir
::Node
::Item(hir
::Item
{
201 kind
: hir
::ItemKind
::Trait(.., generics
, bounds
, _
),
210 hir
::WherePredicate
::BoundPredicate(pred
)
211 if pred
.bounded_ty
.hir_id
.owner
.to_def_id() == trait_def_id
=>
213 // Fetch spans for trait bounds that are Sized:
214 // `trait T where Self: Pred`
215 Some(sized_trait_bound_spans(tcx
, pred
.bounds
))
221 // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
222 .chain(sized_trait_bound_spans(tcx
, bounds
))
223 .collect
::<SmallVec
<[Span
; 1]>>(),
227 .unwrap_or_else(SmallVec
::new
)
230 fn predicates_reference_self(
233 supertraits_only
: bool
,
234 ) -> SmallVec
<[Span
; 1]> {
235 let trait_ref
= ty
::Binder
::dummy(ty
::TraitRef
::identity(tcx
, trait_def_id
));
236 let predicates
= if supertraits_only
{
237 tcx
.super_predicates_of(trait_def_id
)
239 tcx
.predicates_of(trait_def_id
)
241 let self_ty
= tcx
.types
.self_param
;
242 let has_self_ty
= |arg
: &GenericArg
<'_
>| arg
.walk().any(|arg
| arg
== self_ty
.into());
246 .map(|(predicate
, sp
)| (predicate
.subst_supertrait(tcx
, &trait_ref
), sp
))
247 .filter_map(|(predicate
, &sp
)| {
248 match predicate
.kind() {
249 ty
::PredicateKind
::Trait(ref data
, _
) => {
250 // In the case of a trait predicate, we can skip the "self" type.
251 if data
.skip_binder().trait_ref
.substs
[1..].iter().any(has_self_ty
) {
257 ty
::PredicateKind
::Projection(ref data
) => {
258 // And similarly for projections. This should be redundant with
259 // the previous check because any projection should have a
260 // matching `Trait` predicate with the same inputs, but we do
261 // the check to be safe.
263 // Note that we *do* allow projection *outputs* to contain
264 // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
265 // we just require the user to specify *both* outputs
266 // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
268 // This is ALT2 in issue #56288, see that for discussion of the
269 // possible alternatives.
270 if data
.skip_binder().projection_ty
.trait_ref(tcx
).substs
[1..]
279 ty
::PredicateKind
::WellFormed(..)
280 | ty
::PredicateKind
::ObjectSafe(..)
281 | ty
::PredicateKind
::TypeOutlives(..)
282 | ty
::PredicateKind
::RegionOutlives(..)
283 | ty
::PredicateKind
::ClosureKind(..)
284 | ty
::PredicateKind
::Subtype(..)
285 | ty
::PredicateKind
::ConstEvaluatable(..)
286 | ty
::PredicateKind
::ConstEquate(..) => None
,
292 fn trait_has_sized_self(tcx
: TyCtxt
<'_
>, trait_def_id
: DefId
) -> bool
{
293 generics_require_sized_self(tcx
, trait_def_id
)
296 fn generics_require_sized_self(tcx
: TyCtxt
<'_
>, def_id
: DefId
) -> bool
{
297 let sized_def_id
= match tcx
.lang_items().sized_trait() {
298 Some(def_id
) => def_id
,
300 return false; /* No Sized trait, can't require it! */
304 // Search for a predicate like `Self : Sized` amongst the trait bounds.
305 let predicates
= tcx
.predicates_of(def_id
);
306 let predicates
= predicates
.instantiate_identity(tcx
).predicates
;
307 elaborate_predicates(tcx
, predicates
.into_iter()).any(|obligation
| {
308 match obligation
.predicate
.kind() {
309 ty
::PredicateKind
::Trait(ref trait_pred
, _
) => {
310 trait_pred
.def_id() == sized_def_id
311 && trait_pred
.skip_binder().self_ty().is_param(0)
313 ty
::PredicateKind
::Projection(..)
314 | ty
::PredicateKind
::Subtype(..)
315 | ty
::PredicateKind
::RegionOutlives(..)
316 | ty
::PredicateKind
::WellFormed(..)
317 | ty
::PredicateKind
::ObjectSafe(..)
318 | ty
::PredicateKind
::ClosureKind(..)
319 | ty
::PredicateKind
::TypeOutlives(..)
320 | ty
::PredicateKind
::ConstEvaluatable(..)
321 | ty
::PredicateKind
::ConstEquate(..) => false,
326 /// Returns `Some(_)` if this method makes the containing trait not object safe.
327 fn object_safety_violation_for_method(
330 method
: &ty
::AssocItem
,
331 ) -> Option
<(MethodViolationCode
, Span
)> {
332 debug
!("object_safety_violation_for_method({:?}, {:?})", trait_def_id
, method
);
333 // Any method that has a `Self : Sized` requisite is otherwise
334 // exempt from the regulations.
335 if generics_require_sized_self(tcx
, method
.def_id
) {
339 let violation
= virtual_call_violation_for_method(tcx
, trait_def_id
, method
);
340 // Get an accurate span depending on the violation.
342 let node
= tcx
.hir().get_if_local(method
.def_id
);
343 let span
= match (v
, node
) {
344 (MethodViolationCode
::ReferencesSelfInput(arg
), Some(node
)) => node
346 .and_then(|decl
| decl
.inputs
.get(arg
+ 1))
347 .map_or(method
.ident
.span
, |arg
| arg
.span
),
348 (MethodViolationCode
::UndispatchableReceiver
, Some(node
)) => node
350 .and_then(|decl
| decl
.inputs
.get(0))
351 .map_or(method
.ident
.span
, |arg
| arg
.span
),
352 (MethodViolationCode
::ReferencesSelfOutput
, Some(node
)) => {
353 node
.fn_decl().map_or(method
.ident
.span
, |decl
| decl
.output
.span())
355 _
=> method
.ident
.span
,
361 /// Returns `Some(_)` if this method cannot be called on a trait
362 /// object; this does not necessarily imply that the enclosing trait
363 /// is not object safe, because the method might have a where clause
365 fn virtual_call_violation_for_method
<'tcx
>(
368 method
: &ty
::AssocItem
,
369 ) -> Option
<MethodViolationCode
> {
370 // The method's first parameter must be named `self`
371 if !method
.fn_has_self_parameter
{
372 // We'll attempt to provide a structured suggestion for `Self: Sized`.
374 tcx
.hir().get_if_local(method
.def_id
).as_ref().and_then(|node
| node
.generics()).map(
375 |generics
| match generics
.where_clause
.predicates
{
376 [] => (" where Self: Sized", generics
.where_clause
.span
),
377 [.., pred
] => (", Self: Sized", pred
.span().shrink_to_hi()),
380 return Some(MethodViolationCode
::StaticMethod(sugg
));
383 let sig
= tcx
.fn_sig(method
.def_id
);
385 for (i
, input_ty
) in sig
.skip_binder().inputs()[1..].iter().enumerate() {
386 if contains_illegal_self_type_reference(tcx
, trait_def_id
, input_ty
) {
387 return Some(MethodViolationCode
::ReferencesSelfInput(i
));
390 if contains_illegal_self_type_reference(tcx
, trait_def_id
, sig
.output().skip_binder()) {
391 return Some(MethodViolationCode
::ReferencesSelfOutput
);
394 // We can't monomorphize things like `fn foo<A>(...)`.
395 let own_counts
= tcx
.generics_of(method
.def_id
).own_counts();
396 if own_counts
.types
+ own_counts
.consts
!= 0 {
397 return Some(MethodViolationCode
::Generic
);
401 .predicates_of(method
.def_id
)
404 // A trait object can't claim to live more than the concrete type,
405 // so outlives predicates will always hold.
407 .filter(|(p
, _
)| p
.to_opt_type_outlives().is_none())
409 // Do a shallow visit so that `contains_illegal_self_type_reference`
410 // may apply it's custom visiting.
411 .visit_tys_shallow(|t
| contains_illegal_self_type_reference(tcx
, trait_def_id
, t
))
413 return Some(MethodViolationCode
::WhereClauseReferencesSelf
);
417 tcx
.liberate_late_bound_regions(method
.def_id
, &sig
.map_bound(|sig
| sig
.inputs()[0]));
419 // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
420 // However, this is already considered object-safe. We allow it as a special case here.
421 // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
422 // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
423 if receiver_ty
!= tcx
.types
.self_param
{
424 if !receiver_is_dispatchable(tcx
, method
, receiver_ty
) {
425 return Some(MethodViolationCode
::UndispatchableReceiver
);
427 // Do sanity check to make sure the receiver actually has the layout of a pointer.
429 use rustc_target
::abi
::Abi
;
431 let param_env
= tcx
.param_env(method
.def_id
);
433 let abi_of_ty
= |ty
: Ty
<'tcx
>| -> &Abi
{
434 match tcx
.layout_of(param_env
.and(ty
)) {
435 Ok(layout
) => &layout
.abi
,
436 Err(err
) => bug
!("error: {}\n while computing layout for type {:?}", err
, ty
),
441 let unit_receiver_ty
=
442 receiver_for_self_ty(tcx
, receiver_ty
, tcx
.mk_unit(), method
.def_id
);
444 match abi_of_ty(unit_receiver_ty
) {
445 &Abi
::Scalar(..) => (),
447 tcx
.sess
.delay_span_bug(
448 tcx
.def_span(method
.def_id
),
450 "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
457 let trait_object_ty
=
458 object_ty_for_trait(tcx
, trait_def_id
, tcx
.mk_region(ty
::ReStatic
));
460 // e.g., `Rc<dyn Trait>`
461 let trait_object_receiver
=
462 receiver_for_self_ty(tcx
, receiver_ty
, trait_object_ty
, method
.def_id
);
464 match abi_of_ty(trait_object_receiver
) {
465 &Abi
::ScalarPair(..) => (),
467 tcx
.sess
.delay_span_bug(
468 tcx
.def_span(method
.def_id
),
470 "receiver when `Self = {}` should have a ScalarPair ABI; \
483 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
484 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
485 fn receiver_for_self_ty
<'tcx
>(
487 receiver_ty
: Ty
<'tcx
>,
489 method_def_id
: DefId
,
491 debug
!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty
, self_ty
, method_def_id
);
492 let substs
= InternalSubsts
::for_item(tcx
, method_def_id
, |param
, _
| {
493 if param
.index
== 0 { self_ty.into() }
else { tcx.mk_param_from_def(param) }
496 let result
= receiver_ty
.subst(tcx
, substs
);
498 "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
499 receiver_ty
, self_ty
, method_def_id
, result
504 /// Creates the object type for the current trait. For example,
505 /// if the current trait is `Deref`, then this will be
506 /// `dyn Deref<Target = Self::Target> + 'static`.
507 fn object_ty_for_trait
<'tcx
>(
510 lifetime
: ty
::Region
<'tcx
>,
512 debug
!("object_ty_for_trait: trait_def_id={:?}", trait_def_id
);
514 let trait_ref
= ty
::TraitRef
::identity(tcx
, trait_def_id
);
516 let trait_predicate
=
517 ty
::ExistentialPredicate
::Trait(ty
::ExistentialTraitRef
::erase_self_ty(tcx
, trait_ref
));
519 let mut associated_types
= traits
::supertraits(tcx
, ty
::Binder
::dummy(trait_ref
))
520 .flat_map(|super_trait_ref
| {
521 tcx
.associated_items(super_trait_ref
.def_id())
522 .in_definition_order()
523 .map(move |item
| (super_trait_ref
, item
))
525 .filter(|(_
, item
)| item
.kind
== ty
::AssocKind
::Type
)
526 .collect
::<Vec
<_
>>();
528 // existential predicates need to be in a specific order
529 associated_types
.sort_by_cached_key(|(_
, item
)| tcx
.def_path_hash(item
.def_id
));
531 let projection_predicates
= associated_types
.into_iter().map(|(super_trait_ref
, item
)| {
532 // We *can* get bound lifetimes here in cases like
533 // `trait MyTrait: for<'s> OtherTrait<&'s T, Output=bool>`.
535 // binder moved to (*)...
536 let super_trait_ref
= super_trait_ref
.skip_binder();
537 ty
::ExistentialPredicate
::Projection(ty
::ExistentialProjection
{
538 ty
: tcx
.mk_projection(item
.def_id
, super_trait_ref
.substs
),
539 item_def_id
: item
.def_id
,
540 substs
: super_trait_ref
.substs
,
544 let existential_predicates
=
545 tcx
.mk_existential_predicates(iter
::once(trait_predicate
).chain(projection_predicates
));
547 let object_ty
= tcx
.mk_dynamic(
548 // (*) ... binder re-introduced here
549 ty
::Binder
::bind(existential_predicates
),
553 debug
!("object_ty_for_trait: object_ty=`{}`", object_ty
);
558 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
559 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
560 /// in the following way:
561 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
562 /// - require the following bound:
565 /// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
568 /// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
569 /// (substitution notation).
571 /// Some examples of receiver types and their required obligation:
572 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
573 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
574 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
576 /// The only case where the receiver is not dispatchable, but is still a valid receiver
577 /// type (just not object-safe), is when there is more than one level of pointer indirection.
578 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
579 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
580 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
581 /// contained by the trait object, because the object that needs to be coerced is behind
584 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
585 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
586 /// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
587 /// Instead, we fudge a little by introducing a new type parameter `U` such that
588 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
589 /// Written as a chalk-style query:
591 /// forall (U: Trait + ?Sized) {
592 /// if (Self: Unsize<U>) {
593 /// Receiver: DispatchFromDyn<Receiver[Self => U]>
597 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
598 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
599 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
601 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
602 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
603 // `self: Wrapper<Self>`.
605 fn receiver_is_dispatchable
<'tcx
>(
607 method
: &ty
::AssocItem
,
608 receiver_ty
: Ty
<'tcx
>,
610 debug
!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method
, receiver_ty
);
612 let traits
= (tcx
.lang_items().unsize_trait(), tcx
.lang_items().dispatch_from_dyn_trait());
613 let (unsize_did
, dispatch_from_dyn_did
) = if let (Some(u
), Some(cu
)) = traits
{
616 debug
!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
620 // the type `U` in the query
621 // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
622 // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
623 // replace this with `dyn Trait`
624 let unsized_self_ty
: Ty
<'tcx
> =
625 tcx
.mk_ty_param(u32::MAX
, Symbol
::intern("RustaceansAreAwesome"));
627 // `Receiver[Self => U]`
628 let unsized_receiver_ty
=
629 receiver_for_self_ty(tcx
, receiver_ty
, unsized_self_ty
, method
.def_id
);
631 // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
632 // `U: ?Sized` is already implied here
634 let mut param_env
= tcx
.param_env(method
.def_id
);
637 let unsize_predicate
= ty
::TraitRef
{
639 substs
: tcx
.mk_substs_trait(tcx
.types
.self_param
, &[unsized_self_ty
.into()]),
644 // U: Trait<Arg1, ..., ArgN>
645 let trait_predicate
= {
647 InternalSubsts
::for_item(tcx
, method
.container
.assert_trait(), |param
, _
| {
648 if param
.index
== 0 {
649 unsized_self_ty
.into()
651 tcx
.mk_param_from_def(param
)
655 ty
::TraitRef { def_id: unsize_did, substs }
.without_const().to_predicate(tcx
)
658 let caller_bounds
: Vec
<Predicate
<'tcx
>> = param_env
661 .chain(iter
::once(unsize_predicate
))
662 .chain(iter
::once(trait_predicate
))
665 param_env
.caller_bounds
= tcx
.intern_predicates(&caller_bounds
);
670 // Receiver: DispatchFromDyn<Receiver[Self => U]>
672 let predicate
= ty
::TraitRef
{
673 def_id
: dispatch_from_dyn_did
,
674 substs
: tcx
.mk_substs_trait(receiver_ty
, &[unsized_receiver_ty
.into()]),
679 Obligation
::new(ObligationCause
::dummy(), param_env
, predicate
)
682 tcx
.infer_ctxt().enter(|ref infcx
| {
683 // the receiver is dispatchable iff the obligation holds
684 infcx
.predicate_must_hold_modulo_regions(&obligation
)
688 fn contains_illegal_self_type_reference
<'tcx
>(
693 // This is somewhat subtle. In general, we want to forbid
694 // references to `Self` in the argument and return types,
695 // since the value of `Self` is erased. However, there is one
696 // exception: it is ok to reference `Self` in order to access
697 // an associated type of the current trait, since we retain
698 // the value of those associated types in the object type
702 // trait SuperTrait {
706 // trait Trait : SuperTrait {
708 // fn foo(&self, x: Self) // bad
709 // fn foo(&self) -> Self // bad
710 // fn foo(&self) -> Option<Self> // bad
711 // fn foo(&self) -> Self::Y // OK, desugars to next example
712 // fn foo(&self) -> <Self as Trait>::Y // OK
713 // fn foo(&self) -> Self::X // OK, desugars to next example
714 // fn foo(&self) -> <Self as SuperTrait>::X // OK
718 // However, it is not as simple as allowing `Self` in a projected
719 // type, because there are illegal ways to use `Self` as well:
722 // trait Trait : SuperTrait {
724 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
728 // Here we will not have the type of `X` recorded in the
729 // object type, and we cannot resolve `Self as SomeOtherTrait`
730 // without knowing what `Self` is.
732 struct IllegalSelfTypeVisitor
<'tcx
> {
736 supertraits
: Option
<Vec
<ty
::PolyTraitRef
<'tcx
>>>,
739 impl<'tcx
> TypeVisitor
<'tcx
> for IllegalSelfTypeVisitor
<'tcx
> {
740 fn visit_ty(&mut self, t
: Ty
<'tcx
>) -> bool
{
742 ty
::Param(_
) => t
== self.self_ty
,
743 ty
::Projection(ref data
) => {
744 // This is a projected type `<Foo as SomeTrait>::X`.
746 // Compute supertraits of current trait lazily.
747 if self.supertraits
.is_none() {
749 ty
::Binder
::bind(ty
::TraitRef
::identity(self.tcx
, self.trait_def_id
));
750 self.supertraits
= Some(traits
::supertraits(self.tcx
, trait_ref
).collect());
753 // Determine whether the trait reference `Foo as
754 // SomeTrait` is in fact a supertrait of the
755 // current trait. In that case, this type is
756 // legal, because the type `X` will be specified
757 // in the object type. Note that we can just use
758 // direct equality here because all of these types
759 // are part of the formal parameter listing, and
760 // hence there should be no inference variables.
761 let projection_trait_ref
= ty
::Binder
::bind(data
.trait_ref(self.tcx
));
762 let is_supertrait_of_current_trait
=
763 self.supertraits
.as_ref().unwrap().contains(&projection_trait_ref
);
765 if is_supertrait_of_current_trait
{
766 false // do not walk contained types, do not report error, do collect $200
768 t
.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
771 _
=> t
.super_visit_with(self), // walk contained types, if any
775 fn visit_const(&mut self, _c
: &ty
::Const
<'tcx
>) -> bool
{
776 // FIXME(#72219) Look into the unevaluated constants for object safety violations.
777 // Do not walk substitutions of unevaluated consts, as they contain `Self`, even
778 // though the const expression doesn't necessary use it. Currently type variables
779 // inside array length expressions are forbidden, so they can't break the above
785 ty
.visit_with(&mut IllegalSelfTypeVisitor
{
787 self_ty
: tcx
.types
.self_param
,
793 pub fn provide(providers
: &mut ty
::query
::Providers
<'_
>) {
794 *providers
= ty
::query
::Providers { object_safety_violations, ..*providers }
;