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//! "Object safety" refers to the ability for a trait to be converted
//! to an object. In general, traits may only be converted to an
//! object if all of their methods meet certain criteria. In particular,
//! they must:
//!
//! - have a suitable receiver from which we can extract a vtable and coerce to a "thin" version
//! that doesn't contain the vtable;
//! - not reference the erased type `Self` except for in this receiver;
//! - not have generic type parameters.
use super::elaborate;
use crate::infer::TyCtxtInferExt;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{self, Obligation, ObligationCause};
use rustc_errors::{DelayDm, FatalError, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::query::Providers;
use rustc_middle::ty::{
self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
};
use rustc_middle::ty::{GenericArg, GenericArgs};
use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
use rustc_span::symbol::Symbol;
use rustc_span::Span;
use smallvec::SmallVec;
use std::iter;
use std::ops::ControlFlow;
pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
/// Returns the object safety violations that affect
/// astconv -- currently, `Self` in supertraits. This is needed
/// because `object_safety_violations` can't be used during
/// type collection.
pub fn astconv_object_safety_violations(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> Vec<ObjectSafetyViolation> {
debug_assert!(tcx.generics_of(trait_def_id).has_self);
let violations = traits::supertrait_def_ids(tcx, trait_def_id)
.map(|def_id| predicates_reference_self(tcx, def_id, true))
.filter(|spans| !spans.is_empty())
.map(ObjectSafetyViolation::SupertraitSelf)
.collect();
debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", trait_def_id, violations);
violations
}
fn object_safety_violations(tcx: TyCtxt<'_>, trait_def_id: DefId) -> &'_ [ObjectSafetyViolation] {
debug_assert!(tcx.generics_of(trait_def_id).has_self);
debug!("object_safety_violations: {:?}", trait_def_id);
tcx.arena.alloc_from_iter(
traits::supertrait_def_ids(tcx, trait_def_id)
.flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)),
)
}
fn check_is_object_safe(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
let violations = tcx.object_safety_violations(trait_def_id);
if violations.is_empty() {
return true;
}
// If the trait contains any other violations, then let the error reporting path
// report it instead of emitting a warning here.
if violations.iter().all(|violation| {
matches!(
violation,
ObjectSafetyViolation::Method(_, MethodViolationCode::WhereClauseReferencesSelf, _)
)
}) {
for violation in violations {
if let ObjectSafetyViolation::Method(
_,
MethodViolationCode::WhereClauseReferencesSelf,
span,
) = violation
{
lint_object_unsafe_trait(tcx, *span, trait_def_id, &violation);
}
}
return true;
}
false
}
/// We say a method is *vtable safe* if it can be invoked on a trait
/// object. Note that object-safe traits can have some
/// non-vtable-safe methods, so long as they require `Self: Sized` or
/// otherwise ensure that they cannot be used when `Self = Trait`.
pub fn is_vtable_safe_method(tcx: TyCtxt<'_>, trait_def_id: DefId, method: ty::AssocItem) -> bool {
debug_assert!(tcx.generics_of(trait_def_id).has_self);
debug!("is_vtable_safe_method({:?}, {:?})", trait_def_id, method);
// Any method that has a `Self: Sized` bound cannot be called.
if tcx.generics_require_sized_self(method.def_id) {
return false;
}
match virtual_call_violation_for_method(tcx, trait_def_id, method) {
None | Some(MethodViolationCode::WhereClauseReferencesSelf) => true,
Some(_) => false,
}
}
fn object_safety_violations_for_trait(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> Vec<ObjectSafetyViolation> {
// Check assoc items for violations.
let mut violations: Vec<_> = tcx
.associated_items(trait_def_id)
.in_definition_order()
.filter_map(|&item| object_safety_violation_for_assoc_item(tcx, trait_def_id, item))
.collect();
// Check the trait itself.
if trait_has_sized_self(tcx, trait_def_id) {
// We don't want to include the requirement from `Sized` itself to be `Sized` in the list.
let spans = get_sized_bounds(tcx, trait_def_id);
violations.push(ObjectSafetyViolation::SizedSelf(spans));
}
let spans = predicates_reference_self(tcx, trait_def_id, false);
if !spans.is_empty() {
violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
}
let spans = bounds_reference_self(tcx, trait_def_id);
if !spans.is_empty() {
violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
}
let spans = super_predicates_have_non_lifetime_binders(tcx, trait_def_id);
if !spans.is_empty() {
violations.push(ObjectSafetyViolation::SupertraitNonLifetimeBinder(spans));
}
debug!(
"object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
trait_def_id, violations
);
violations
}
/// Lint object-unsafe trait.
fn lint_object_unsafe_trait(
tcx: TyCtxt<'_>,
span: Span,
trait_def_id: DefId,
violation: &ObjectSafetyViolation,
) {
// Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id.
// It's also hard to get a use site span, so we use the method definition span.
tcx.struct_span_lint_hir(
WHERE_CLAUSES_OBJECT_SAFETY,
hir::CRATE_HIR_ID,
span,
DelayDm(|| format!("the trait `{}` cannot be made into an object", tcx.def_path_str(trait_def_id))),
|err| {
let node = tcx.hir().get_if_local(trait_def_id);
let mut spans = MultiSpan::from_span(span);
if let Some(hir::Node::Item(item)) = node {
spans.push_span_label(
item.ident.span,
"this trait cannot be made into an object...",
);
spans.push_span_label(span, format!("...because {}", violation.error_msg()));
} else {
spans.push_span_label(
span,
format!(
"the trait cannot be made into an object because {}",
violation.error_msg()
),
);
};
err.span_note(
spans,
"for a trait to be \"object safe\" it needs to allow building a vtable to allow the \
call to be resolvable dynamically; for more information visit \
<https://doc.rust-lang.org/reference/items/traits.html#object-safety>",
);
if node.is_some() {
// Only provide the help if its a local trait, otherwise it's not
violation.solution(err);
}
err
},
);
}
fn sized_trait_bound_spans<'tcx>(
tcx: TyCtxt<'tcx>,
bounds: hir::GenericBounds<'tcx>,
) -> impl 'tcx + Iterator<Item = Span> {
bounds.iter().filter_map(move |b| match b {
hir::GenericBound::Trait(trait_ref, hir::TraitBoundModifier::None)
if trait_has_sized_self(
tcx,
trait_ref.trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
) =>
{
// Fetch spans for supertraits that are `Sized`: `trait T: Super`
Some(trait_ref.span)
}
_ => None,
})
}
fn get_sized_bounds(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
tcx.hir()
.get_if_local(trait_def_id)
.and_then(|node| match node {
hir::Node::Item(hir::Item {
kind: hir::ItemKind::Trait(.., generics, bounds, _),
..
}) => Some(
generics
.predicates
.iter()
.filter_map(|pred| {
match pred {
hir::WherePredicate::BoundPredicate(pred)
if pred.bounded_ty.hir_id.owner.to_def_id() == trait_def_id =>
{
// Fetch spans for trait bounds that are Sized:
// `trait T where Self: Pred`
Some(sized_trait_bound_spans(tcx, pred.bounds))
}
_ => None,
}
})
.flatten()
// Fetch spans for supertraits that are `Sized`: `trait T: Super`.
.chain(sized_trait_bound_spans(tcx, bounds))
.collect::<SmallVec<[Span; 1]>>(),
),
_ => None,
})
.unwrap_or_else(SmallVec::new)
}
fn predicates_reference_self(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
supertraits_only: bool,
) -> SmallVec<[Span; 1]> {
let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(tcx, trait_def_id));
let predicates = if supertraits_only {
tcx.super_predicates_of(trait_def_id)
} else {
tcx.predicates_of(trait_def_id)
};
predicates
.predicates
.iter()
.map(|&(predicate, sp)| (predicate.subst_supertrait(tcx, &trait_ref), sp))
.filter_map(|predicate| predicate_references_self(tcx, predicate))
.collect()
}
fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span; 1]> {
tcx.associated_items(trait_def_id)
.in_definition_order()
.filter(|item| item.kind == ty::AssocKind::Type)
.flat_map(|item| tcx.explicit_item_bounds(item.def_id).instantiate_identity_iter_copied())
.filter_map(|c| predicate_references_self(tcx, c))
.collect()
}
fn predicate_references_self<'tcx>(
tcx: TyCtxt<'tcx>,
(predicate, sp): (ty::Clause<'tcx>, Span),
) -> Option<Span> {
let self_ty = tcx.types.self_param;
let has_self_ty = |arg: &GenericArg<'tcx>| arg.walk().any(|arg| arg == self_ty.into());
match predicate.kind().skip_binder() {
ty::ClauseKind::Trait(ref data) => {
// In the case of a trait predicate, we can skip the "self" type.
data.trait_ref.args[1..].iter().any(has_self_ty).then_some(sp)
}
ty::ClauseKind::Projection(ref data) => {
// And similarly for projections. This should be redundant with
// the previous check because any projection should have a
// matching `Trait` predicate with the same inputs, but we do
// the check to be safe.
//
// It's also won't be redundant if we allow type-generic associated
// types for trait objects.
//
// Note that we *do* allow projection *outputs* to contain
// `self` (i.e., `trait Foo: Bar<Output=Self::Result> { type Result; }`),
// we just require the user to specify *both* outputs
// in the object type (i.e., `dyn Foo<Output=(), Result=()>`).
//
// This is ALT2 in issue #56288, see that for discussion of the
// possible alternatives.
data.projection_ty.args[1..].iter().any(has_self_ty).then_some(sp)
}
ty::ClauseKind::ConstArgHasType(_ct, ty) => has_self_ty(&ty.into()).then_some(sp),
ty::ClauseKind::WellFormed(..)
| ty::ClauseKind::TypeOutlives(..)
| ty::ClauseKind::RegionOutlives(..)
// FIXME(generic_const_exprs): this can mention `Self`
| ty::ClauseKind::ConstEvaluatable(..)
=> None,
}
}
fn super_predicates_have_non_lifetime_binders(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> SmallVec<[Span; 1]> {
// If non_lifetime_binders is disabled, then exit early
if !tcx.features().non_lifetime_binders {
return SmallVec::new();
}
tcx.super_predicates_of(trait_def_id)
.predicates
.iter()
.filter_map(|(pred, span)| pred.has_non_region_late_bound().then_some(*span))
.collect()
}
fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
tcx.generics_require_sized_self(trait_def_id)
}
fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
let Some(sized_def_id) = tcx.lang_items().sized_trait() else {
return false; /* No Sized trait, can't require it! */
};
// Search for a predicate like `Self : Sized` amongst the trait bounds.
let predicates = tcx.predicates_of(def_id);
let predicates = predicates.instantiate_identity(tcx).predicates;
elaborate(tcx, predicates.into_iter()).any(|pred| match pred.kind().skip_binder() {
ty::ClauseKind::Trait(ref trait_pred) => {
trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
}
ty::ClauseKind::RegionOutlives(_)
| ty::ClauseKind::TypeOutlives(_)
| ty::ClauseKind::Projection(_)
| ty::ClauseKind::ConstArgHasType(_, _)
| ty::ClauseKind::WellFormed(_)
| ty::ClauseKind::ConstEvaluatable(_) => false,
})
}
/// Returns `Some(_)` if this item makes the containing trait not object safe.
#[instrument(level = "debug", skip(tcx), ret)]
fn object_safety_violation_for_assoc_item(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
item: ty::AssocItem,
) -> Option<ObjectSafetyViolation> {
// Any item that has a `Self : Sized` requisite is otherwise
// exempt from the regulations.
if tcx.generics_require_sized_self(item.def_id) {
return None;
}
match item.kind {
// Associated consts are never object safe, as they can't have `where` bounds yet at all,
// and associated const bounds in trait objects aren't a thing yet either.
ty::AssocKind::Const => {
Some(ObjectSafetyViolation::AssocConst(item.name, item.ident(tcx).span))
}
ty::AssocKind::Fn => virtual_call_violation_for_method(tcx, trait_def_id, item).map(|v| {
let node = tcx.hir().get_if_local(item.def_id);
// Get an accurate span depending on the violation.
let span = match (&v, node) {
(MethodViolationCode::ReferencesSelfInput(Some(span)), _) => *span,
(MethodViolationCode::UndispatchableReceiver(Some(span)), _) => *span,
(MethodViolationCode::ReferencesImplTraitInTrait(span), _) => *span,
(MethodViolationCode::ReferencesSelfOutput, Some(node)) => {
node.fn_decl().map_or(item.ident(tcx).span, |decl| decl.output.span())
}
_ => item.ident(tcx).span,
};
ObjectSafetyViolation::Method(item.name, v, span)
}),
// Associated types can only be object safe if they have `Self: Sized` bounds.
ty::AssocKind::Type => {
if !tcx.features().generic_associated_types_extended
&& !tcx.generics_of(item.def_id).params.is_empty()
&& !item.is_impl_trait_in_trait()
{
Some(ObjectSafetyViolation::GAT(item.name, item.ident(tcx).span))
} else {
// We will permit associated types if they are explicitly mentioned in the trait object.
// We can't check this here, as here we only check if it is guaranteed to not be possible.
None
}
}
}
}
/// Returns `Some(_)` if this method cannot be called on a trait
/// object; this does not necessarily imply that the enclosing trait
/// is not object safe, because the method might have a where clause
/// `Self:Sized`.
fn virtual_call_violation_for_method<'tcx>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
method: ty::AssocItem,
) -> Option<MethodViolationCode> {
let sig = tcx.fn_sig(method.def_id).instantiate_identity();
// The method's first parameter must be named `self`
if !method.fn_has_self_parameter {
let sugg = if let Some(hir::Node::TraitItem(hir::TraitItem {
generics,
kind: hir::TraitItemKind::Fn(sig, _),
..
})) = tcx.hir().get_if_local(method.def_id).as_ref()
{
let sm = tcx.sess.source_map();
Some((
(
format!("&self{}", if sig.decl.inputs.is_empty() { "" } else { ", " }),
sm.span_through_char(sig.span, '(').shrink_to_hi(),
),
(
format!("{} Self: Sized", generics.add_where_or_trailing_comma()),
generics.tail_span_for_predicate_suggestion(),
),
))
} else {
None
};
return Some(MethodViolationCode::StaticMethod(sugg));
}
for (i, &input_ty) in sig.skip_binder().inputs().iter().enumerate().skip(1) {
if contains_illegal_self_type_reference(tcx, trait_def_id, sig.rebind(input_ty)) {
let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
kind: hir::TraitItemKind::Fn(sig, _),
..
})) = tcx.hir().get_if_local(method.def_id).as_ref()
{
Some(sig.decl.inputs[i].span)
} else {
None
};
return Some(MethodViolationCode::ReferencesSelfInput(span));
}
}
if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
return Some(MethodViolationCode::ReferencesSelfOutput);
}
if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
return Some(code);
}
// We can't monomorphize things like `fn foo<A>(...)`.
let own_counts = tcx.generics_of(method.def_id).own_counts();
if own_counts.types + own_counts.consts != 0 {
return Some(MethodViolationCode::Generic);
}
let receiver_ty = tcx.liberate_late_bound_regions(method.def_id, sig.input(0));
// Until `unsized_locals` is fully implemented, `self: Self` can't be dispatched on.
// However, this is already considered object-safe. We allow it as a special case here.
// FIXME(mikeyhew) get rid of this `if` statement once `receiver_is_dispatchable` allows
// `Receiver: Unsize<Receiver[Self => dyn Trait]>`.
if receiver_ty != tcx.types.self_param {
if !receiver_is_dispatchable(tcx, method, receiver_ty) {
let span = if let Some(hir::Node::TraitItem(hir::TraitItem {
kind: hir::TraitItemKind::Fn(sig, _),
..
})) = tcx.hir().get_if_local(method.def_id).as_ref()
{
Some(sig.decl.inputs[0].span)
} else {
None
};
return Some(MethodViolationCode::UndispatchableReceiver(span));
} else {
// Do sanity check to make sure the receiver actually has the layout of a pointer.
use rustc_target::abi::Abi;
let param_env = tcx.param_env(method.def_id);
let abi_of_ty = |ty: Ty<'tcx>| -> Option<Abi> {
match tcx.layout_of(param_env.and(ty)) {
Ok(layout) => Some(layout.abi),
Err(err) => {
// #78372
tcx.sess.delay_span_bug(
tcx.def_span(method.def_id),
format!("error: {err}\n while computing layout for type {ty:?}"),
);
None
}
}
};
// e.g., `Rc<()>`
let unit_receiver_ty =
receiver_for_self_ty(tcx, receiver_ty, Ty::new_unit(tcx), method.def_id);
match abi_of_ty(unit_receiver_ty) {
Some(Abi::Scalar(..)) => (),
abi => {
tcx.sess.delay_span_bug(
tcx.def_span(method.def_id),
format!(
"receiver when `Self = ()` should have a Scalar ABI; found {abi:?}"
),
);
}
}
let trait_object_ty = object_ty_for_trait(tcx, trait_def_id, tcx.lifetimes.re_static);
// e.g., `Rc<dyn Trait>`
let trait_object_receiver =
receiver_for_self_ty(tcx, receiver_ty, trait_object_ty, method.def_id);
match abi_of_ty(trait_object_receiver) {
Some(Abi::ScalarPair(..)) => (),
abi => {
tcx.sess.delay_span_bug(
tcx.def_span(method.def_id),
format!(
"receiver when `Self = {trait_object_ty}` should have a ScalarPair ABI; found {abi:?}"
),
);
}
}
}
}
// NOTE: This check happens last, because it results in a lint, and not a
// hard error.
if tcx.predicates_of(method.def_id).predicates.iter().any(|&(pred, span)| {
// dyn Trait is okay:
//
// trait Trait {
// fn f(&self) where Self: 'static;
// }
//
// because a trait object can't claim to live longer than the concrete
// type. If the lifetime bound holds on dyn Trait then it's guaranteed
// to hold as well on the concrete type.
if pred.as_type_outlives_clause().is_some() {
return false;
}
// dyn Trait is okay:
//
// auto trait AutoTrait {}
//
// trait Trait {
// fn f(&self) where Self: AutoTrait;
// }
//
// because `impl AutoTrait for dyn Trait` is disallowed by coherence.
// Traits with a default impl are implemented for a trait object if and
// only if the autotrait is one of the trait object's trait bounds, like
// in `dyn Trait + AutoTrait`. This guarantees that trait objects only
// implement auto traits if the underlying type does as well.
if let ty::ClauseKind::Trait(ty::TraitPredicate {
trait_ref: pred_trait_ref,
polarity: ty::ImplPolarity::Positive,
}) = pred.kind().skip_binder()
&& pred_trait_ref.self_ty() == tcx.types.self_param
&& tcx.trait_is_auto(pred_trait_ref.def_id)
{
// Consider bounds like `Self: Bound<Self>`. Auto traits are not
// allowed to have generic parameters so `auto trait Bound<T> {}`
// would already have reported an error at the definition of the
// auto trait.
if pred_trait_ref.args.len() != 1 {
tcx.sess.diagnostic().delay_span_bug(
span,
"auto traits cannot have generic parameters",
);
}
return false;
}
contains_illegal_self_type_reference(tcx, trait_def_id, pred)
}) {
return Some(MethodViolationCode::WhereClauseReferencesSelf);
}
None
}
/// Performs a type substitution to produce the version of `receiver_ty` when `Self = self_ty`.
/// For example, for `receiver_ty = Rc<Self>` and `self_ty = Foo`, returns `Rc<Foo>`.
fn receiver_for_self_ty<'tcx>(
tcx: TyCtxt<'tcx>,
receiver_ty: Ty<'tcx>,
self_ty: Ty<'tcx>,
method_def_id: DefId,
) -> Ty<'tcx> {
debug!("receiver_for_self_ty({:?}, {:?}, {:?})", receiver_ty, self_ty, method_def_id);
let args = GenericArgs::for_item(tcx, method_def_id, |param, _| {
if param.index == 0 { self_ty.into() } else { tcx.mk_param_from_def(param) }
});
let result = EarlyBinder::bind(receiver_ty).instantiate(tcx, args);
debug!(
"receiver_for_self_ty({:?}, {:?}, {:?}) = {:?}",
receiver_ty, self_ty, method_def_id, result
);
result
}
/// Creates the object type for the current trait. For example,
/// if the current trait is `Deref`, then this will be
/// `dyn Deref<Target = Self::Target> + 'static`.
#[instrument(level = "trace", skip(tcx), ret)]
fn object_ty_for_trait<'tcx>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
lifetime: ty::Region<'tcx>,
) -> Ty<'tcx> {
let trait_ref = ty::TraitRef::identity(tcx, trait_def_id);
debug!(?trait_ref);
let trait_predicate = ty::Binder::dummy(ty::ExistentialPredicate::Trait(
ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref),
));
debug!(?trait_predicate);
let pred: ty::Predicate<'tcx> = trait_ref.to_predicate(tcx);
let mut elaborated_predicates: Vec<_> = elaborate(tcx, [pred])
.filter_map(|pred| {
debug!(?pred);
let pred = pred.to_opt_poly_projection_pred()?;
Some(pred.map_bound(|p| {
ty::ExistentialPredicate::Projection(ty::ExistentialProjection::erase_self_ty(
tcx, p,
))
}))
})
.collect();
// NOTE: Since #37965, the existential predicates list has depended on the
// list of predicates to be sorted. This is mostly to enforce that the primary
// predicate comes first.
elaborated_predicates.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
elaborated_predicates.dedup();
let existential_predicates = tcx.mk_poly_existential_predicates_from_iter(
iter::once(trait_predicate).chain(elaborated_predicates),
);
debug!(?existential_predicates);
Ty::new_dynamic(tcx, existential_predicates, lifetime, ty::Dyn)
}
/// Checks the method's receiver (the `self` argument) can be dispatched on when `Self` is a
/// trait object. We require that `DispatchableFromDyn` be implemented for the receiver type
/// in the following way:
/// - let `Receiver` be the type of the `self` argument, i.e `Self`, `&Self`, `Rc<Self>`,
/// - require the following bound:
///
/// ```ignore (not-rust)
/// Receiver[Self => T]: DispatchFromDyn<Receiver[Self => dyn Trait]>
/// ```
///
/// where `Foo[X => Y]` means "the same type as `Foo`, but with `X` replaced with `Y`"
/// (substitution notation).
///
/// Some examples of receiver types and their required obligation:
/// - `&'a mut self` requires `&'a mut Self: DispatchFromDyn<&'a mut dyn Trait>`,
/// - `self: Rc<Self>` requires `Rc<Self>: DispatchFromDyn<Rc<dyn Trait>>`,
/// - `self: Pin<Box<Self>>` requires `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<dyn Trait>>>`.
///
/// The only case where the receiver is not dispatchable, but is still a valid receiver
/// type (just not object-safe), is when there is more than one level of pointer indirection.
/// E.g., `self: &&Self`, `self: &Rc<Self>`, `self: Box<Box<Self>>`. In these cases, there
/// is no way, or at least no inexpensive way, to coerce the receiver from the version where
/// `Self = dyn Trait` to the version where `Self = T`, where `T` is the unknown erased type
/// contained by the trait object, because the object that needs to be coerced is behind
/// a pointer.
///
/// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
/// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
/// is stabilized, see tracking issue <https://github.com/rust-lang/rust/issues/43561>).
/// Instead, we fudge a little by introducing a new type parameter `U` such that
/// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
/// Written as a chalk-style query:
/// ```ignore (not-rust)
/// forall (U: Trait + ?Sized) {
/// if (Self: Unsize<U>) {
/// Receiver: DispatchFromDyn<Receiver[Self => U]>
/// }
/// }
/// ```
/// for `self: &'a mut Self`, this means `&'a mut Self: DispatchFromDyn<&'a mut U>`
/// for `self: Rc<Self>`, this means `Rc<Self>: DispatchFromDyn<Rc<U>>`
/// for `self: Pin<Box<Self>>`, this means `Pin<Box<Self>>: DispatchFromDyn<Pin<Box<U>>>`
//
// FIXME(mikeyhew) when unsized receivers are implemented as part of unsized rvalues, add this
// fallback query: `Receiver: Unsize<Receiver[Self => U]>` to support receivers like
// `self: Wrapper<Self>`.
#[allow(dead_code)]
fn receiver_is_dispatchable<'tcx>(
tcx: TyCtxt<'tcx>,
method: ty::AssocItem,
receiver_ty: Ty<'tcx>,
) -> bool {
debug!("receiver_is_dispatchable: method = {:?}, receiver_ty = {:?}", method, receiver_ty);
let traits = (tcx.lang_items().unsize_trait(), tcx.lang_items().dispatch_from_dyn_trait());
let (Some(unsize_did), Some(dispatch_from_dyn_did)) = traits else {
debug!("receiver_is_dispatchable: Missing Unsize or DispatchFromDyn traits");
return false;
};
// the type `U` in the query
// use a bogus type parameter to mimic a forall(U) query using u32::MAX for now.
// FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
// replace this with `dyn Trait`
let unsized_self_ty: Ty<'tcx> =
Ty::new_param(tcx, u32::MAX, Symbol::intern("RustaceansAreAwesome"));
// `Receiver[Self => U]`
let unsized_receiver_ty =
receiver_for_self_ty(tcx, receiver_ty, unsized_self_ty, method.def_id);
// create a modified param env, with `Self: Unsize<U>` and `U: Trait` added to caller bounds
// `U: ?Sized` is already implied here
let param_env = {
let param_env = tcx.param_env(method.def_id);
// Self: Unsize<U>
let unsize_predicate =
ty::TraitRef::new(tcx, unsize_did, [tcx.types.self_param, unsized_self_ty])
.to_predicate(tcx);
// U: Trait<Arg1, ..., ArgN>
let trait_predicate = {
let trait_def_id = method.trait_container(tcx).unwrap();
let args = GenericArgs::for_item(tcx, trait_def_id, |param, _| {
if param.index == 0 { unsized_self_ty.into() } else { tcx.mk_param_from_def(param) }
});
ty::TraitRef::new(tcx, trait_def_id, args).to_predicate(tcx)
};
let caller_bounds =
param_env.caller_bounds().iter().chain([unsize_predicate, trait_predicate]);
ty::ParamEnv::new(tcx.mk_clauses_from_iter(caller_bounds), param_env.reveal())
};
// Receiver: DispatchFromDyn<Receiver[Self => U]>
let obligation = {
let predicate =
ty::TraitRef::new(tcx, dispatch_from_dyn_did, [receiver_ty, unsized_receiver_ty]);
Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)
};
let infcx = tcx.infer_ctxt().build();
// the receiver is dispatchable iff the obligation holds
infcx.predicate_must_hold_modulo_regions(&obligation)
}
fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
value: T,
) -> bool {
// This is somewhat subtle. In general, we want to forbid
// references to `Self` in the argument and return types,
// since the value of `Self` is erased. However, there is one
// exception: it is ok to reference `Self` in order to access
// an associated type of the current trait, since we retain
// the value of those associated types in the object type
// itself.
//
// ```rust
// trait SuperTrait {
// type X;
// }
//
// trait Trait : SuperTrait {
// type Y;
// fn foo(&self, x: Self) // bad
// fn foo(&self) -> Self // bad
// fn foo(&self) -> Option<Self> // bad
// fn foo(&self) -> Self::Y // OK, desugars to next example
// fn foo(&self) -> <Self as Trait>::Y // OK
// fn foo(&self) -> Self::X // OK, desugars to next example
// fn foo(&self) -> <Self as SuperTrait>::X // OK
// }
// ```
//
// However, it is not as simple as allowing `Self` in a projected
// type, because there are illegal ways to use `Self` as well:
//
// ```rust
// trait Trait : SuperTrait {
// ...
// fn foo(&self) -> <Self as SomeOtherTrait>::X;
// }
// ```
//
// Here we will not have the type of `X` recorded in the
// object type, and we cannot resolve `Self as SomeOtherTrait`
// without knowing what `Self` is.
struct IllegalSelfTypeVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
supertraits: Option<Vec<DefId>>,
}
impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalSelfTypeVisitor<'tcx> {
type BreakTy = ();
fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
match t.kind() {
ty::Param(_) => {
if t == self.tcx.types.self_param {
ControlFlow::Break(())
} else {
ControlFlow::Continue(())
}
}
ty::Alias(ty::Projection, ref data)
if self.tcx.is_impl_trait_in_trait(data.def_id) =>
{
// We'll deny these later in their own pass
ControlFlow::Continue(())
}
ty::Alias(ty::Projection, ref data) => {
// This is a projected type `<Foo as SomeTrait>::X`.
// Compute supertraits of current trait lazily.
if self.supertraits.is_none() {
let trait_ref =
ty::Binder::dummy(ty::TraitRef::identity(self.tcx, self.trait_def_id));
self.supertraits = Some(
traits::supertraits(self.tcx, trait_ref).map(|t| t.def_id()).collect(),
);
}
// Determine whether the trait reference `Foo as
// SomeTrait` is in fact a supertrait of the
// current trait. In that case, this type is
// legal, because the type `X` will be specified
// in the object type. Note that we can just use
// direct equality here because all of these types
// are part of the formal parameter listing, and
// hence there should be no inference variables.
let is_supertrait_of_current_trait = self
.supertraits
.as_ref()
.unwrap()
.contains(&data.trait_ref(self.tcx).def_id);
if is_supertrait_of_current_trait {
ControlFlow::Continue(()) // do not walk contained types, do not report error, do collect $200
} else {
t.super_visit_with(self) // DO walk contained types, POSSIBLY reporting an error
}
}
_ => t.super_visit_with(self), // walk contained types, if any
}
}
fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
// Constants can only influence object safety if they are generic and reference `Self`.
// This is only possible for unevaluated constants, so we walk these here.
self.tcx.expand_abstract_consts(ct).super_visit_with(self)
}
}
value
.visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
.is_break()
}
pub fn contains_illegal_impl_trait_in_trait<'tcx>(
tcx: TyCtxt<'tcx>,
fn_def_id: DefId,
ty: ty::Binder<'tcx, Ty<'tcx>>,
) -> Option<MethodViolationCode> {
// This would be caught below, but rendering the error as a separate
// `async-specific` message is better.
if tcx.asyncness(fn_def_id).is_async() {
return Some(MethodViolationCode::AsyncFn);
}
// FIXME(RPITIT): Perhaps we should use a visitor here?
ty.skip_binder().walk().find_map(|arg| {
if let ty::GenericArgKind::Type(ty) = arg.unpack()
&& let ty::Alias(ty::Projection, proj) = ty.kind()
&& tcx.is_impl_trait_in_trait(proj.def_id)
{
Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.def_id)))
} else {
None
}
})
}
pub fn provide(providers: &mut Providers) {
*providers = Providers {
object_safety_violations,
check_is_object_safe,
generics_require_sized_self,
..*providers
};
}