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::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::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::TypeOutlives(..)
315 | ty::PredicateKind::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::Trait(ref trait_pred) => {
341 trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
343 ty::PredicateKind::Projection(..)
344 | ty::PredicateKind::Subtype(..)
345 | ty::PredicateKind::Coerce(..)
346 | ty::PredicateKind::RegionOutlives(..)
347 | ty::PredicateKind::WellFormed(..)
348 | ty::PredicateKind::ObjectSafe(..)
349 | ty::PredicateKind::ClosureKind(..)
350 | ty::PredicateKind::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 item_def_id: p.projection_ty.item_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 = {
698 InternalSubsts::for_item(tcx, method.trait_container(tcx).unwrap(), |param, _| {
699 if param.index == 0 {
700 unsized_self_ty.into()
702 tcx.mk_param_from_def(param)
706 ty::Binder::dummy(ty::TraitRef { def_id: unsize_did, substs })
711 let caller_bounds: Vec<Predicate<'tcx>> =
712 param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]).collect();
715 tcx.intern_predicates(&caller_bounds),
717 param_env.constness(),
721 // Receiver: DispatchFromDyn<Receiver[Self => U]>
723 let predicate = ty::Binder::dummy(
724 tcx.mk_trait_ref(dispatch_from_dyn_did, [receiver_ty, unsized_receiver_ty]),
728 Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
731 let infcx = tcx.infer_ctxt().build();
732 // the receiver is dispatchable iff the obligation holds
733 infcx.predicate_must_hold_modulo_regions(&obligation)
736 fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<'tcx>>(
741 // This is somewhat subtle. In general, we want to forbid
742 // references to `Self` in the argument and return types,
743 // since the value of `Self` is erased. However, there is one
744 // exception: it is ok to reference `Self` in order to access
745 // an associated type of the current trait, since we retain
746 // the value of those associated types in the object type
750 // trait SuperTrait {
754 // trait Trait : SuperTrait {
756 // fn foo(&self, x: Self) // bad
757 // fn foo(&self) -> Self // bad
758 // fn foo(&self) -> Option<Self> // bad
759 // fn foo(&self) -> Self::Y // OK, desugars to next example
760 // fn foo(&self) -> <Self as Trait>::Y // OK
761 // fn foo(&self) -> Self::X // OK, desugars to next example
762 // fn foo(&self) -> <Self as SuperTrait>::X // OK
766 // However, it is not as simple as allowing `Self` in a projected
767 // type, because there are illegal ways to use `Self` as well:
770 // trait Trait : SuperTrait {
772 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
776 // Here we will not have the type of `X` recorded in the
777 // object type, and we cannot resolve `Self as SomeOtherTrait`
778 // without knowing what `Self` is.
780 struct IllegalSelfTypeVisitor<'tcx> {
783 supertraits: Option<Vec<DefId>>,
786 impl<'tcx> TypeVisitor<'tcx> for IllegalSelfTypeVisitor<'tcx> {
789 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
792 if t == self.tcx.types.self_param {
795 ControlFlow::CONTINUE
798 ty::Projection(ref data)
799 if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
801 // We'll deny these later in their own pass
802 ControlFlow::CONTINUE
804 ty::Projection(ref data) => {
805 // This is a projected type `<Foo as SomeTrait>::X`.
807 // Compute supertraits of current trait lazily.
808 if self.supertraits.is_none() {
809 let trait_ref = ty::TraitRef::identity(self.tcx, self.trait_def_id);
810 self.supertraits = Some(
811 traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
815 // Determine whether the trait reference `Foo as
816 // SomeTrait` is in fact a supertrait of the
817 // current trait. In that case, this type is
818 // legal, because the type `X` will be specified
819 // in the object type. Note that we can just use
820 // direct equality here because all of these types
821 // are part of the formal parameter listing, and
822 // hence there should be no inference variables.
823 let is_supertrait_of_current_trait = self
827 .contains(&data.trait_ref(self.tcx).def_id);
829 if is_supertrait_of_current_trait {
830 ControlFlow::CONTINUE // do not walk contained types, do not report error, do collect $200
832 t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
835 _ => t.super_visit_with(self), // walk contained types, if any
839 fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
840 // Constants can only influence object safety if they reference `Self`.
841 // This is only possible for unevaluated constants, so we walk these here.
843 // If `AbstractConst::from_const` returned an error we already failed compilation
844 // so we don't have to emit an additional error here.
846 // We currently recurse into abstract consts here but do not recurse in
847 // `is_const_evaluatable`. This means that the object safety check is more
848 // liberal than the const eval check.
850 // This shouldn't really matter though as we can't really use any
851 // constants which are not considered const evaluatable.
852 if let ty::ConstKind::Unevaluated(_uv) = ct.kind() &&
853 let Ok(Some(ct)) = self.tcx.expand_abstract_consts(ct){
856 ct.super_visit_with(self)
862 .visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
866 pub fn contains_illegal_impl_trait_in_trait<'tcx>(
869 ty: ty::Binder<'tcx, Ty<'tcx>>,
870 ) -> Option<MethodViolationCode> {
871 // This would be caught below, but rendering the error as a separate
872 // `async-specific` message is better.
873 if tcx.asyncness(fn_def_id).is_async() {
874 return Some(MethodViolationCode::AsyncFn);
877 // FIXME(RPITIT): Perhaps we should use a visitor here?
878 ty.skip_binder().walk().find_map(|arg| {
879 if let ty::GenericArgKind::Type(ty) = arg.unpack()
880 && let ty::Projection(proj) = ty.kind()
881 && tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
883 Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.item_def_id)))
890 pub fn provide(providers: &mut ty::query::Providers) {
891 *providers = ty::query::Providers { object_safety_violations, ..*providers };