rustc_trait_selection/traits/dyn_compatibility.rs
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//! "Dyn-compatibility"[^1] refers to the ability for a trait to be converted
//! to a trait object. In general, traits may only be converted to a trait
//! object if certain criteria are met.
//!
//! [^1]: Formerly known as "object safety".
use std::iter;
use std::ops::ControlFlow;
use rustc_abi::BackendRepr;
use rustc_errors::FatalError;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::query::Providers;
use rustc_middle::ty::{
self, EarlyBinder, ExistentialPredicateStableCmpExt as _, GenericArgs, Ty, TyCtxt,
TypeFoldable, TypeFolder, TypeSuperFoldable, TypeSuperVisitable, TypeVisitable,
TypeVisitableExt, TypeVisitor, TypingMode, Upcast,
};
use rustc_span::Span;
use rustc_span::symbol::Symbol;
use smallvec::SmallVec;
use tracing::{debug, instrument};
use super::elaborate;
use crate::infer::TyCtxtInferExt;
pub use crate::traits::DynCompatibilityViolation;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{MethodViolationCode, Obligation, ObligationCause, util};
/// Returns the dyn-compatibility violations that affect HIR ty lowering.
///
/// Currently that is `Self` in supertraits. This is needed
/// because `dyn_compatibility_violations` can't be used during
/// type collection.
#[instrument(level = "debug", skip(tcx), ret)]
pub fn hir_ty_lowering_dyn_compatibility_violations(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> Vec<DynCompatibilityViolation> {
debug_assert!(tcx.generics_of(trait_def_id).has_self);
tcx.supertrait_def_ids(trait_def_id)
.map(|def_id| predicates_reference_self(tcx, def_id, true))
.filter(|spans| !spans.is_empty())
.map(DynCompatibilityViolation::SupertraitSelf)
.collect()
}
fn dyn_compatibility_violations(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> &'_ [DynCompatibilityViolation] {
debug_assert!(tcx.generics_of(trait_def_id).has_self);
debug!("dyn_compatibility_violations: {:?}", trait_def_id);
tcx.arena.alloc_from_iter(
tcx.supertrait_def_ids(trait_def_id)
.flat_map(|def_id| dyn_compatibility_violations_for_trait(tcx, def_id)),
)
}
fn is_dyn_compatible(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
tcx.dyn_compatibility_violations(trait_def_id).is_empty()
}
/// 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;
}
virtual_call_violations_for_method(tcx, trait_def_id, method).is_empty()
}
#[instrument(level = "debug", skip(tcx), ret)]
fn dyn_compatibility_violations_for_trait(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
) -> Vec<DynCompatibilityViolation> {
// Check assoc items for violations.
let mut violations: Vec<_> = tcx
.associated_items(trait_def_id)
.in_definition_order()
.flat_map(|&item| dyn_compatibility_violations_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(DynCompatibilityViolation::SizedSelf(spans));
}
let spans = predicates_reference_self(tcx, trait_def_id, false);
if !spans.is_empty() {
violations.push(DynCompatibilityViolation::SupertraitSelf(spans));
}
let spans = bounds_reference_self(tcx, trait_def_id);
if !spans.is_empty() {
violations.push(DynCompatibilityViolation::SupertraitSelf(spans));
}
let spans = super_predicates_have_non_lifetime_binders(tcx, trait_def_id);
if !spans.is_empty() {
violations.push(DynCompatibilityViolation::SupertraitNonLifetimeBinder(spans));
}
if violations.is_empty() {
for item in tcx.associated_items(trait_def_id).in_definition_order() {
if let ty::AssocKind::Fn = item.kind {
check_receiver_correct(tcx, trait_def_id, *item);
}
}
}
violations
}
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)
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.explicit_super_predicates_of(trait_def_id).skip_binder()
} else {
tcx.predicates_of(trait_def_id).predicates
};
predicates
.iter()
.map(|&(predicate, sp)| (predicate.instantiate_supertrait(tcx, trait_ref), sp))
.filter_map(|(clause, sp)| {
// Super predicates cannot allow self projections, since they're
// impossible to make into existential bounds without eager resolution
// or something.
// e.g. `trait A: B<Item = Self::Assoc>`.
predicate_references_self(tcx, trait_def_id, clause, sp, AllowSelfProjections::No)
})
.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).iter_identity_copied())
.filter_map(|(clause, sp)| {
// Item bounds *can* have self projections, since they never get
// their self type erased.
predicate_references_self(tcx, trait_def_id, clause, sp, AllowSelfProjections::Yes)
})
.collect()
}
fn predicate_references_self<'tcx>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
predicate: ty::Clause<'tcx>,
sp: Span,
allow_self_projections: AllowSelfProjections,
) -> Option<Span> {
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(|&arg| contains_illegal_self_type_reference(tcx, trait_def_id, arg, allow_self_projections)).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_term.args[1..].iter().any(|&arg| contains_illegal_self_type_reference(tcx, trait_def_id, arg, allow_self_projections)).then_some(sp)
}
ty::ClauseKind::ConstArgHasType(_ct, ty) => contains_illegal_self_type_reference(tcx, trait_def_id, ty, allow_self_projections).then_some(sp),
ty::ClauseKind::WellFormed(..)
| ty::ClauseKind::TypeOutlives(..)
| ty::ClauseKind::RegionOutlives(..)
// FIXME(generic_const_exprs): this can mention `Self`
| ty::ClauseKind::ConstEvaluatable(..)
| ty::ClauseKind::HostEffect(..)
=> 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.explicit_super_predicates_of(trait_def_id)
.iter_identity_copied()
.filter_map(|(pred, span)| pred.has_non_region_bound_vars().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).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(_)
| ty::ClauseKind::HostEffect(..) => false,
})
}
/// Returns `Some(_)` if this item makes the containing trait dyn-incompatible.
#[instrument(level = "debug", skip(tcx), ret)]
pub fn dyn_compatibility_violations_for_assoc_item(
tcx: TyCtxt<'_>,
trait_def_id: DefId,
item: ty::AssocItem,
) -> Vec<DynCompatibilityViolation> {
// Any item that has a `Self : Sized` requisite is otherwise
// exempt from the regulations.
if tcx.generics_require_sized_self(item.def_id) {
return Vec::new();
}
match item.kind {
// Associated consts are never dyn-compatible, 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 => {
vec![DynCompatibilityViolation::AssocConst(item.name, item.ident(tcx).span)]
}
ty::AssocKind::Fn => virtual_call_violations_for_method(tcx, trait_def_id, item)
.into_iter()
.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,
};
DynCompatibilityViolation::Method(item.name, v, span)
})
.collect(),
// Associated types can only be dyn-compatible if they have `Self: Sized` bounds.
ty::AssocKind::Type => {
if !tcx.features().generic_associated_types_extended()
&& !tcx.generics_of(item.def_id).is_own_empty()
&& !item.is_impl_trait_in_trait()
{
vec![DynCompatibilityViolation::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.
Vec::new()
}
}
}
}
/// Returns `Some(_)` if this method cannot be called on a trait
/// object; this does not necessarily imply that the enclosing trait
/// is dyn-incompatible, because the method might have a where clause
/// `Self: Sized`.
fn virtual_call_violations_for_method<'tcx>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
method: ty::AssocItem,
) -> Vec<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
};
// Not having `self` parameter messes up the later checks,
// so we need to return instead of pushing
return vec![MethodViolationCode::StaticMethod(sugg)];
}
let mut errors = Vec::new();
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),
AllowSelfProjections::Yes,
) {
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
};
errors.push(MethodViolationCode::ReferencesSelfInput(span));
}
}
if contains_illegal_self_type_reference(
tcx,
trait_def_id,
sig.output(),
AllowSelfProjections::Yes,
) {
errors.push(MethodViolationCode::ReferencesSelfOutput);
}
if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
errors.push(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 > 0 || own_counts.consts > 0 {
errors.push(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
};
errors.push(MethodViolationCode::UndispatchableReceiver(span));
} else {
// We confirm that the `receiver_is_dispatchable` is accurate later,
// see `check_receiver_correct`. It should be kept in sync with this code.
}
}
// 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::PredicatePolarity::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 {
assert!(
tcx.dcx().has_errors().is_some(),
"auto traits cannot have generic parameters"
);
}
return false;
}
contains_illegal_self_type_reference(tcx, trait_def_id, pred, AllowSelfProjections::Yes)
}) {
errors.push(MethodViolationCode::WhereClauseReferencesSelf);
}
errors
}
/// This code checks that `receiver_is_dispatchable` is correctly implemented.
///
/// This check is outlined from the dyn-compatibility check to avoid cycles with
/// layout computation, which relies on knowing whether methods are dyn-compatible.
fn check_receiver_correct<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId, method: ty::AssocItem) {
if !is_vtable_safe_method(tcx, trait_def_id, method) {
return;
}
let method_def_id = method.def_id;
let sig = tcx.fn_sig(method_def_id).instantiate_identity();
let typing_env = ty::TypingEnv::non_body_analysis(tcx, method_def_id);
let receiver_ty = tcx.liberate_late_bound_regions(method_def_id, sig.input(0));
if receiver_ty == tcx.types.self_param {
// Assumed OK, may change later if unsized_locals permits `self: Self` as dispatchable.
return;
}
// e.g., `Rc<()>`
let unit_receiver_ty = receiver_for_self_ty(tcx, receiver_ty, tcx.types.unit, method_def_id);
match tcx.layout_of(typing_env.as_query_input(unit_receiver_ty)).map(|l| l.backend_repr) {
Ok(BackendRepr::Scalar(..)) => (),
abi => {
tcx.dcx().span_delayed_bug(
tcx.def_span(method_def_id),
format!("receiver {unit_receiver_ty:?} 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 tcx.layout_of(typing_env.as_query_input(trait_object_receiver)).map(|l| l.backend_repr) {
Ok(BackendRepr::ScalarPair(..)) => (),
abi => {
tcx.dcx().span_delayed_bug(
tcx.def_span(method_def_id),
format!(
"receiver {trait_object_receiver:?} when `Self = {trait_object_ty}` should have a ScalarPair ABI; found {abi:?}"
),
);
}
}
}
/// Performs a type instantiation 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.upcast(tcx);
let mut elaborated_predicates: Vec<_> = elaborate(tcx, [pred])
.filter_map(|pred| {
debug!(?pred);
let pred = pred.as_projection_clause()?;
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`"
/// (instantiation 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 dyn-compatible, creating a cycle (until dyn_compatible_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>`.
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 dyn_compatible_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]).upcast(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_from_args(tcx, trait_def_id, args).upcast(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(TypingMode::non_body_analysis());
// the receiver is dispatchable iff the obligation holds
infcx.predicate_must_hold_modulo_regions(&obligation)
}
#[derive(Copy, Clone)]
enum AllowSelfProjections {
Yes,
No,
}
/// 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,ignore (example)
/// 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,ignore (example)
/// 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.
fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
value: T,
allow_self_projections: AllowSelfProjections,
) -> bool {
value
.visit_with(&mut IllegalSelfTypeVisitor {
tcx,
trait_def_id,
supertraits: None,
allow_self_projections,
})
.is_break()
}
struct IllegalSelfTypeVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
supertraits: Option<Vec<ty::TraitRef<'tcx>>>,
allow_self_projections: AllowSelfProjections,
}
impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalSelfTypeVisitor<'tcx> {
type Result = ControlFlow<()>;
fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
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) => {
match self.allow_self_projections {
AllowSelfProjections::Yes => {
// This is a projected type `<Foo as SomeTrait>::X`.
// Compute supertraits of current trait lazily.
if self.supertraits.is_none() {
self.supertraits = Some(
util::supertraits(
self.tcx,
ty::Binder::dummy(ty::TraitRef::identity(
self.tcx,
self.trait_def_id,
)),
)
.map(|trait_ref| {
self.tcx.erase_regions(
self.tcx.instantiate_bound_regions_with_erased(trait_ref),
)
})
.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).fold_with(
&mut EraseEscapingBoundRegions {
tcx: self.tcx,
binder: ty::INNERMOST,
},
),
);
// only walk contained types if it's not a super trait
if is_supertrait_of_current_trait {
ControlFlow::Continue(())
} else {
t.super_visit_with(self) // POSSIBLY reporting an error
}
}
AllowSelfProjections::No => t.super_visit_with(self),
}
}
_ => t.super_visit_with(self),
}
}
fn visit_const(&mut self, ct: ty::Const<'tcx>) -> Self::Result {
// Constants can only influence dyn-compatibility 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)
}
}
struct EraseEscapingBoundRegions<'tcx> {
tcx: TyCtxt<'tcx>,
binder: ty::DebruijnIndex,
}
impl<'tcx> TypeFolder<TyCtxt<'tcx>> for EraseEscapingBoundRegions<'tcx> {
fn cx(&self) -> TyCtxt<'tcx> {
self.tcx
}
fn fold_binder<T>(&mut self, t: ty::Binder<'tcx, T>) -> ty::Binder<'tcx, T>
where
T: TypeFoldable<TyCtxt<'tcx>>,
{
self.binder.shift_in(1);
let result = t.super_fold_with(self);
self.binder.shift_out(1);
result
}
fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
if let ty::ReBound(debruijn, _) = *r
&& debruijn < self.binder
{
r
} else {
self.tcx.lifetimes.re_erased
}
}
}
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(crate) fn provide(providers: &mut Providers) {
*providers = Providers {
dyn_compatibility_violations,
is_dyn_compatible,
generics_require_sized_self,
..*providers
};
}