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
::subst
::{GenericArg, InternalSubsts}
;
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(
167 WHERE_CLAUSES_OBJECT_SAFETY
,
170 DelayDm(|| format
!("the trait `{}` cannot be made into an object", tcx
.def_path_str(trait_def_id
))),
172 let node
= tcx
.hir().get_if_local(trait_def_id
);
173 let mut spans
= MultiSpan
::from_span(span
);
174 if let Some(hir
::Node
::Item(item
)) = node
{
175 spans
.push_span_label(
177 "this trait cannot be made into an object...",
179 spans
.push_span_label(span
, format
!("...because {}", violation
.error_msg()));
181 spans
.push_span_label(
184 "the trait cannot be made into an object because {}",
185 violation
.error_msg()
191 "for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
192 call to be resolvable dynamically; for more information visit \
193 <https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
196 // Only provide the help if its a local trait, otherwise it's not
197 violation
.solution(err
);
204 fn sized_trait_bound_spans
<'tcx
>(
206 bounds
: hir
::GenericBounds
<'tcx
>,
207 ) -> impl 'tcx
+ Iterator
<Item
= Span
> {
208 bounds
.iter().filter_map(move |b
| match b
{
209 hir
::GenericBound
::Trait(trait_ref
, hir
::TraitBoundModifier
::None
)
210 if trait_has_sized_self(
212 trait_ref
.trait_ref
.trait_def_id().unwrap_or_else(|| FatalError
.raise()),
215 // Fetch spans for supertraits that are `Sized`: `trait T: Super`
222 fn get_sized_bounds(tcx
: TyCtxt
<'_
>, trait_def_id
: DefId
) -> SmallVec
<[Span
; 1]> {
224 .get_if_local(trait_def_id
)
225 .and_then(|node
| match node
{
226 hir
::Node
::Item(hir
::Item
{
227 kind
: hir
::ItemKind
::Trait(.., generics
, bounds
, _
),
235 hir
::WherePredicate
::BoundPredicate(pred
)
236 if pred
.bounded_ty
.hir_id
.owner
.to_def_id() == trait_def_id
=>
238 // Fetch spans for trait bounds that are Sized:
239 // `trait T where Self: Pred`
240 Some(sized_trait_bound_spans(tcx
, pred
.bounds
))
246 // Fetch spans for supertraits that are `Sized`: `trait T: Super`.
247 .chain(sized_trait_bound_spans(tcx
, bounds
))
248 .collect
::<SmallVec
<[Span
; 1]>>(),
252 .unwrap_or_else(SmallVec
::new
)
255 fn predicates_reference_self(
258 supertraits_only
: bool
,
259 ) -> SmallVec
<[Span
; 1]> {
260 let trait_ref
= ty
::TraitRef
::identity(tcx
, trait_def_id
);
261 let predicates
= if supertraits_only
{
262 tcx
.super_predicates_of(trait_def_id
)
264 tcx
.predicates_of(trait_def_id
)
269 .map(|&(predicate
, sp
)| (predicate
.subst_supertrait(tcx
, &trait_ref
), sp
))
270 .filter_map(|predicate
| predicate_references_self(tcx
, predicate
))
274 fn bounds_reference_self(tcx
: TyCtxt
<'_
>, trait_def_id
: DefId
) -> SmallVec
<[Span
; 1]> {
275 tcx
.associated_items(trait_def_id
)
276 .in_definition_order()
277 .filter(|item
| item
.kind
== ty
::AssocKind
::Type
)
278 .flat_map(|item
| tcx
.explicit_item_bounds(item
.def_id
))
279 .filter_map(|pred_span
| predicate_references_self(tcx
, *pred_span
))
283 fn predicate_references_self
<'tcx
>(
285 (predicate
, sp
): (ty
::Predicate
<'tcx
>, Span
),
287 let self_ty
= tcx
.types
.self_param
;
288 let has_self_ty
= |arg
: &GenericArg
<'tcx
>| arg
.walk().any(|arg
| arg
== self_ty
.into());
289 match predicate
.kind().skip_binder() {
290 ty
::PredicateKind
::Clause(ty
::Clause
::Trait(ref data
)) => {
291 // In the case of a trait predicate, we can skip the "self" type.
292 if data
.trait_ref
.substs
[1..].iter().any(has_self_ty
) { Some(sp) }
else { None }
294 ty
::PredicateKind
::Clause(ty
::Clause
::Projection(ref data
)) => {
295 // And similarly for projections. This should be redundant with
296 // the previous check because any projection should have a
297 // matching `Trait` predicate with the same inputs, but we do
298 // the check to be safe.
300 // It's also won't be redundant if we allow type-generic associated
301 // types for trait objects.
303 // Note that we *do* allow projection *outputs* to contain
304 // `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
305 // we just require the user to specify *both* outputs
306 // in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
308 // This is ALT2 in issue #56288, see that for discussion of the
309 // possible alternatives.
310 if data
.projection_ty
.substs
[1..].iter().any(has_self_ty
) { Some(sp) }
else { None }
312 ty
::PredicateKind
::WellFormed(..)
313 | ty
::PredicateKind
::ObjectSafe(..)
314 | ty
::PredicateKind
::Clause(ty
::Clause
::TypeOutlives(..))
315 | ty
::PredicateKind
::Clause(ty
::Clause
::RegionOutlives(..))
316 | ty
::PredicateKind
::ClosureKind(..)
317 | ty
::PredicateKind
::Subtype(..)
318 | ty
::PredicateKind
::Coerce(..)
319 | ty
::PredicateKind
::ConstEvaluatable(..)
320 | ty
::PredicateKind
::ConstEquate(..)
321 | ty
::PredicateKind
::Ambiguous
322 | ty
::PredicateKind
::TypeWellFormedFromEnv(..) => None
,
326 fn trait_has_sized_self(tcx
: TyCtxt
<'_
>, trait_def_id
: DefId
) -> bool
{
327 generics_require_sized_self(tcx
, trait_def_id
)
330 fn generics_require_sized_self(tcx
: TyCtxt
<'_
>, def_id
: DefId
) -> bool
{
331 let Some(sized_def_id
) = tcx
.lang_items().sized_trait() else {
332 return false; /* No Sized trait, can't require it! */
335 // Search for a predicate like `Self : Sized` amongst the trait bounds.
336 let predicates
= tcx
.predicates_of(def_id
);
337 let predicates
= predicates
.instantiate_identity(tcx
).predicates
;
338 elaborate_predicates(tcx
, predicates
.into_iter()).any(|obligation
| {
339 match obligation
.predicate
.kind().skip_binder() {
340 ty
::PredicateKind
::Clause(ty
::Clause
::Trait(ref trait_pred
)) => {
341 trait_pred
.def_id() == sized_def_id
&& trait_pred
.self_ty().is_param(0)
343 ty
::PredicateKind
::Clause(ty
::Clause
::Projection(..))
344 | ty
::PredicateKind
::Subtype(..)
345 | ty
::PredicateKind
::Coerce(..)
346 | ty
::PredicateKind
::Clause(ty
::Clause
::RegionOutlives(..))
347 | ty
::PredicateKind
::WellFormed(..)
348 | ty
::PredicateKind
::ObjectSafe(..)
349 | ty
::PredicateKind
::ClosureKind(..)
350 | ty
::PredicateKind
::Clause(ty
::Clause
::TypeOutlives(..))
351 | ty
::PredicateKind
::ConstEvaluatable(..)
352 | ty
::PredicateKind
::ConstEquate(..)
353 | ty
::PredicateKind
::Ambiguous
354 | ty
::PredicateKind
::TypeWellFormedFromEnv(..) => false,
359 /// Returns `Some(_)` if this method makes the containing trait not object safe.
360 fn object_safety_violation_for_method(
363 method
: &ty
::AssocItem
,
364 ) -> Option
<(MethodViolationCode
, Span
)> {
365 debug
!("object_safety_violation_for_method({:?}, {:?})", trait_def_id
, method
);
366 // Any method that has a `Self : Sized` requisite is otherwise
367 // exempt from the regulations.
368 if generics_require_sized_self(tcx
, method
.def_id
) {
372 let violation
= virtual_call_violation_for_method(tcx
, trait_def_id
, method
);
373 // Get an accurate span depending on the violation.
375 let node
= tcx
.hir().get_if_local(method
.def_id
);
376 let span
= match (&v
, node
) {
377 (MethodViolationCode
::ReferencesSelfInput(Some(span
)), _
) => *span
,
378 (MethodViolationCode
::UndispatchableReceiver(Some(span
)), _
) => *span
,
379 (MethodViolationCode
::ReferencesImplTraitInTrait(span
), _
) => *span
,
380 (MethodViolationCode
::ReferencesSelfOutput
, Some(node
)) => {
381 node
.fn_decl().map_or(method
.ident(tcx
).span
, |decl
| decl
.output
.span())
383 _
=> method
.ident(tcx
).span
,
389 /// Returns `Some(_)` if this method cannot be called on a trait
390 /// object; this does not necessarily imply that the enclosing trait
391 /// is not object safe, because the method might have a where clause
393 fn virtual_call_violation_for_method
<'tcx
>(
396 method
: &ty
::AssocItem
,
397 ) -> Option
<MethodViolationCode
> {
398 let sig
= tcx
.fn_sig(method
.def_id
);
400 // The method's first parameter must be named `self`
401 if !method
.fn_has_self_parameter
{
402 let sugg
= if let Some(hir
::Node
::TraitItem(hir
::TraitItem
{
404 kind
: hir
::TraitItemKind
::Fn(sig
, _
),
406 })) = tcx
.hir().get_if_local(method
.def_id
).as_ref()
408 let sm
= tcx
.sess
.source_map();
411 format
!("&self{}", if sig
.decl
.inputs
.is_empty() { "" }
else { ", " }
),
412 sm
.span_through_char(sig
.span
, '
('
).shrink_to_hi(),
415 format
!("{} Self: Sized", generics
.add_where_or_trailing_comma()),
416 generics
.tail_span_for_predicate_suggestion(),
422 return Some(MethodViolationCode
::StaticMethod(sugg
));
425 for (i
, &input_ty
) in sig
.skip_binder().inputs().iter().enumerate().skip(1) {
426 if contains_illegal_self_type_reference(tcx
, trait_def_id
, sig
.rebind(input_ty
)) {
427 let span
= if let Some(hir
::Node
::TraitItem(hir
::TraitItem
{
428 kind
: hir
::TraitItemKind
::Fn(sig
, _
),
430 })) = tcx
.hir().get_if_local(method
.def_id
).as_ref()
432 Some(sig
.decl
.inputs
[i
].span
)
436 return Some(MethodViolationCode
::ReferencesSelfInput(span
));
439 if contains_illegal_self_type_reference(tcx
, trait_def_id
, sig
.output()) {
440 return Some(MethodViolationCode
::ReferencesSelfOutput
);
442 if let Some(code
) = contains_illegal_impl_trait_in_trait(tcx
, method
.def_id
, sig
.output()) {
446 // We can't monomorphize things like `fn foo<A>(...)`.
447 let own_counts
= tcx
.generics_of(method
.def_id
).own_counts();
448 if own_counts
.types
+ own_counts
.consts
!= 0 {
449 return Some(MethodViolationCode
::Generic
);
452 let receiver_ty
= tcx
.liberate_late_bound_regions(method
.def_id
, sig
.input(0));
454 // Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
455 // However, this is already considered object-safe. We allow it as a special case here.
456 // FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
457 // `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
458 if receiver_ty
!= tcx
.types
.self_param
{
459 if !receiver_is_dispatchable(tcx
, method
, receiver_ty
) {
460 let span
= if let Some(hir
::Node
::TraitItem(hir
::TraitItem
{
461 kind
: hir
::TraitItemKind
::Fn(sig
, _
),
463 })) = tcx
.hir().get_if_local(method
.def_id
).as_ref()
465 Some(sig
.decl
.inputs
[0].span
)
469 return Some(MethodViolationCode
::UndispatchableReceiver(span
));
471 // Do sanity check to make sure the receiver actually has the layout of a pointer.
473 use rustc_target
::abi
::Abi
;
475 let param_env
= tcx
.param_env(method
.def_id
);
477 let abi_of_ty
= |ty
: Ty
<'tcx
>| -> Option
<Abi
> {
478 match tcx
.layout_of(param_env
.and(ty
)) {
479 Ok(layout
) => Some(layout
.abi
),
482 tcx
.sess
.delay_span_bug(
483 tcx
.def_span(method
.def_id
),
484 &format
!("error: {}\n while computing layout for type {:?}", err
, ty
),
492 let unit_receiver_ty
=
493 receiver_for_self_ty(tcx
, receiver_ty
, tcx
.mk_unit(), method
.def_id
);
495 match abi_of_ty(unit_receiver_ty
) {
496 Some(Abi
::Scalar(..)) => (),
498 tcx
.sess
.delay_span_bug(
499 tcx
.def_span(method
.def_id
),
501 "receiver when `Self = ()` should have a Scalar ABI; found {:?}",
508 let trait_object_ty
=
509 object_ty_for_trait(tcx
, trait_def_id
, tcx
.mk_region(ty
::ReStatic
));
511 // e.g., `Rc<dyn Trait>`
512 let trait_object_receiver
=
513 receiver_for_self_ty(tcx
, receiver_ty
, trait_object_ty
, method
.def_id
);
515 match abi_of_ty(trait_object_receiver
) {
516 Some(Abi
::ScalarPair(..)) => (),
518 tcx
.sess
.delay_span_bug(
519 tcx
.def_span(method
.def_id
),
521 "receiver when `Self = {}` should have a ScalarPair ABI; found {:?}",
530 // NOTE: This check happens last, because it results in a lint, and not a
533 .predicates_of(method
.def_id
)
536 // A trait object can't claim to live more than the concrete type,
537 // so outlives predicates will always hold.
539 .filter(|(p
, _
)| p
.to_opt_type_outlives().is_none())
540 .any(|pred
| contains_illegal_self_type_reference(tcx
, trait_def_id
, pred
))
542 return Some(MethodViolationCode
::WhereClauseReferencesSelf
);
548 /// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
549 /// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
550 fn receiver_for_self_ty
<'tcx
>(
552 receiver_ty
: Ty
<'tcx
>,
554 method_def_id
: DefId
,
556 debug
!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty
, self_ty
, method_def_id
);
557 let substs
= InternalSubsts
::for_item(tcx
, method_def_id
, |param
, _
| {
558 if param
.index
== 0 { self_ty.into() }
else { tcx.mk_param_from_def(param) }
561 let result
= EarlyBinder(receiver_ty
).subst(tcx
, substs
);
563 "receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
564 receiver_ty
, self_ty
, method_def_id
, result
569 /// Creates the object type for the current trait. For example,
570 /// if the current trait is `Deref`, then this will be
571 /// `dyn Deref<Target = Self::Target> + 'static`.
572 #[instrument(level = "trace", skip(tcx), ret)]
573 fn object_ty_for_trait
<'tcx
>(
576 lifetime
: ty
::Region
<'tcx
>,
578 let trait_ref
= ty
::TraitRef
::identity(tcx
, trait_def_id
);
581 let trait_predicate
= trait_ref
.map_bound(|trait_ref
| {
582 ty
::ExistentialPredicate
::Trait(ty
::ExistentialTraitRef
::erase_self_ty(tcx
, trait_ref
))
584 debug
!(?trait_predicate
);
586 let mut elaborated_predicates
: Vec
<_
> = elaborate_trait_ref(tcx
, trait_ref
)
587 .filter_map(|obligation
| {
589 let pred
= obligation
.predicate
.to_opt_poly_projection_pred()?
;
590 Some(pred
.map_bound(|p
| {
591 ty
::ExistentialPredicate
::Projection(ty
::ExistentialProjection
{
592 def_id
: p
.projection_ty
.def_id
,
593 substs
: p
.projection_ty
.substs
,
599 // NOTE: Since #37965, the existential predicates list has depended on the
600 // list of predicates to be sorted. This is mostly to enforce that the primary
601 // predicate comes first.
602 elaborated_predicates
.sort_by(|a
, b
| a
.skip_binder().stable_cmp(tcx
, &b
.skip_binder()));
603 elaborated_predicates
.dedup();
605 let existential_predicates
= tcx
606 .mk_poly_existential_predicates(iter
::once(trait_predicate
).chain(elaborated_predicates
));
607 debug
!(?existential_predicates
);
609 tcx
.mk_dynamic(existential_predicates
, lifetime
, ty
::Dyn
)
612 /// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
613 /// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
614 /// in the following way:
615 /// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
616 /// - require the following bound:
618 /// ```ignore (not-rust)
619 /// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
622 /// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
623 /// (substitution notation).
625 /// Some examples of receiver types and their required obligation:
626 /// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
627 /// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
628 /// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
630 /// The only case where the receiver is not dispatchable, but is still a valid receiver
631 /// type (just not object-safe), is when there is more than one level of pointer indirection.
632 /// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
633 /// is no way, or at least no inexpensive way, to coerce the receiver from the version where
634 /// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
635 /// contained by the trait object, because the object that needs to be coerced is behind
638 /// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
639 /// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
640 /// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
641 /// Instead, we fudge a little by introducing a new type parameter `U` such that
642 /// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
643 /// Written as a chalk-style query:
644 /// ```ignore (not-rust)
645 /// forall (U: Trait + ?Sized) {
646 /// if (Self: Unsize<U>) {
647 /// Receiver: DispatchFromDyn<Receiver[Self => U]>
651 /// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
652 /// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
653 /// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
655 // FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
656 // fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
657 // `self: Wrapper<Self>`.
659 fn receiver_is_dispatchable
<'tcx
>(
661 method
: &ty
::AssocItem
,
662 receiver_ty
: Ty
<'tcx
>,
664 debug
!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method
, receiver_ty
);
666 let traits
= (tcx
.lang_items().unsize_trait(), tcx
.lang_items().dispatch_from_dyn_trait());
667 let (Some(unsize_did
), Some(dispatch_from_dyn_did
)) = traits
else {
668 debug
!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
672 // the type `U` in the query
673 // use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
674 // FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
675 // replace this with `dyn Trait`
676 let unsized_self_ty
: Ty
<'tcx
> =
677 tcx
.mk_ty_param(u32::MAX
, Symbol
::intern("RustaceansAreAwesome"));
679 // `Receiver[Self => U]`
680 let unsized_receiver_ty
=
681 receiver_for_self_ty(tcx
, receiver_ty
, unsized_self_ty
, method
.def_id
);
683 // create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
684 // `U: ?Sized` is already implied here
686 let param_env
= tcx
.param_env(method
.def_id
);
689 let unsize_predicate
= ty
::Binder
::dummy(
690 tcx
.mk_trait_ref(unsize_did
, [tcx
.types
.self_param
, unsized_self_ty
]),
695 // U: Trait<Arg1, ..., ArgN>
696 let trait_predicate
= {
697 let trait_def_id
= method
.trait_container(tcx
).unwrap();
698 let substs
= InternalSubsts
::for_item(tcx
, trait_def_id
, |param
, _
| {
699 if param
.index
== 0 { unsized_self_ty.into() }
else { tcx.mk_param_from_def(param) }
702 ty
::Binder
::dummy(tcx
.mk_trait_ref(trait_def_id
, substs
)).to_predicate(tcx
)
705 let caller_bounds
: Vec
<Predicate
<'tcx
>> =
706 param_env
.caller_bounds().iter().chain([unsize_predicate
, trait_predicate
]).collect();
709 tcx
.intern_predicates(&caller_bounds
),
711 param_env
.constness(),
715 // Receiver: DispatchFromDyn<Receiver[Self => U]>
717 let predicate
= ty
::Binder
::dummy(
718 tcx
.mk_trait_ref(dispatch_from_dyn_did
, [receiver_ty
, unsized_receiver_ty
]),
721 Obligation
::new(tcx
, ObligationCause
::dummy(), param_env
, predicate
)
724 let infcx
= tcx
.infer_ctxt().build();
725 // the receiver is dispatchable iff the obligation holds
726 infcx
.predicate_must_hold_modulo_regions(&obligation
)
729 fn contains_illegal_self_type_reference
<'tcx
, T
: TypeVisitable
<'tcx
>>(
734 // This is somewhat subtle. In general, we want to forbid
735 // references to `Self` in the argument and return types,
736 // since the value of `Self` is erased. However, there is one
737 // exception: it is ok to reference `Self` in order to access
738 // an associated type of the current trait, since we retain
739 // the value of those associated types in the object type
743 // trait SuperTrait {
747 // trait Trait : SuperTrait {
749 // fn foo(&self, x: Self) // bad
750 // fn foo(&self) -> Self // bad
751 // fn foo(&self) -> Option<Self> // bad
752 // fn foo(&self) -> Self::Y // OK, desugars to next example
753 // fn foo(&self) -> <Self as Trait>::Y // OK
754 // fn foo(&self) -> Self::X // OK, desugars to next example
755 // fn foo(&self) -> <Self as SuperTrait>::X // OK
759 // However, it is not as simple as allowing `Self` in a projected
760 // type, because there are illegal ways to use `Self` as well:
763 // trait Trait : SuperTrait {
765 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
769 // Here we will not have the type of `X` recorded in the
770 // object type, and we cannot resolve `Self as SomeOtherTrait`
771 // without knowing what `Self` is.
773 struct IllegalSelfTypeVisitor
<'tcx
> {
776 supertraits
: Option
<Vec
<DefId
>>,
779 impl<'tcx
> TypeVisitor
<'tcx
> for IllegalSelfTypeVisitor
<'tcx
> {
782 fn visit_ty(&mut self, t
: Ty
<'tcx
>) -> ControlFlow
<Self::BreakTy
> {
785 if t
== self.tcx
.types
.self_param
{
786 ControlFlow
::Break(())
788 ControlFlow
::Continue(())
791 ty
::Alias(ty
::Projection
, ref data
)
792 if self.tcx
.def_kind(data
.def_id
) == DefKind
::ImplTraitPlaceholder
=>
794 // We'll deny these later in their own pass
795 ControlFlow
::Continue(())
797 ty
::Alias(ty
::Projection
, ref data
) => {
798 // This is a projected type `<Foo as SomeTrait>::X`.
800 // Compute supertraits of current trait lazily.
801 if self.supertraits
.is_none() {
802 let trait_ref
= ty
::TraitRef
::identity(self.tcx
, self.trait_def_id
);
803 self.supertraits
= Some(
804 traits
::supertraits(self.tcx
, trait_ref
).map(|t
| t
.def_id()).collect(),
808 // Determine whether the trait reference `Foo as
809 // SomeTrait` is in fact a supertrait of the
810 // current trait. In that case, this type is
811 // legal, because the type `X` will be specified
812 // in the object type. Note that we can just use
813 // direct equality here because all of these types
814 // are part of the formal parameter listing, and
815 // hence there should be no inference variables.
816 let is_supertrait_of_current_trait
= self
820 .contains(&data
.trait_ref(self.tcx
).def_id
);
822 if is_supertrait_of_current_trait
{
823 ControlFlow
::Continue(()) // do not walk contained types, do not report error, do collect $200
825 t
.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
828 _
=> t
.super_visit_with(self), // walk contained types, if any
832 fn visit_const(&mut self, ct
: ty
::Const
<'tcx
>) -> ControlFlow
<Self::BreakTy
> {
833 // Constants can only influence object safety if they are generic and reference `Self`.
834 // This is only possible for unevaluated constants, so we walk these here.
835 self.tcx
.expand_abstract_consts(ct
).super_visit_with(self)
840 .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None }
)
844 pub fn contains_illegal_impl_trait_in_trait
<'tcx
>(
847 ty
: ty
::Binder
<'tcx
, Ty
<'tcx
>>,
848 ) -> Option
<MethodViolationCode
> {
849 // This would be caught below, but rendering the error as a separate
850 // `async-specific` message is better.
851 if tcx
.asyncness(fn_def_id
).is_async() {
852 return Some(MethodViolationCode
::AsyncFn
);
855 // FIXME(RPITIT): Perhaps we should use a visitor here?
856 ty
.skip_binder().walk().find_map(|arg
| {
857 if let ty
::GenericArgKind
::Type(ty
) = arg
.unpack()
858 && let ty
::Alias(ty
::Projection
, proj
) = ty
.kind()
859 && tcx
.def_kind(proj
.def_id
) == DefKind
::ImplTraitPlaceholder
861 Some(MethodViolationCode
::ReferencesImplTraitInTrait(tcx
.def_span(proj
.def_id
)))
868 pub fn provide(providers
: &mut ty
::query
::Providers
) {
869 *providers
= ty
::query
::Providers { object_safety_violations, ..*providers }
;