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Item bounds can reference self projections and still be object safe
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@compiler-errors
compiler-errors committed Jul 15, 2024
1 parent d3dd34a commit 8edf9b8
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Showing 2 changed files with 91 additions and 44 deletions.
124 changes: 80 additions & 44 deletions compiler/rustc_trait_selection/src/traits/object_safety.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,16 +12,16 @@ use super::elaborate;

use crate::infer::TyCtxtInferExt;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{self, Obligation, ObligationCause};
use crate::traits::{Obligation, ObligationCause};
use rustc_errors::FatalError;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_middle::query::Providers;
use rustc_middle::ty::GenericArgs;
use rustc_middle::ty::{
self, EarlyBinder, ExistentialPredicateStableCmpExt as _, Ty, TyCtxt, TypeSuperVisitable,
TypeVisitable, TypeVisitor,
};
use rustc_middle::ty::{GenericArg, GenericArgs};
use rustc_middle::ty::{TypeVisitableExt, Upcast};
use rustc_span::symbol::Symbol;
use rustc_span::Span;
Expand Down Expand Up @@ -195,7 +195,13 @@ fn predicates_reference_self(
.predicates
.iter()
.map(|&(predicate, sp)| (predicate.instantiate_supertrait(tcx, trait_ref), sp))
.filter_map(|predicate| predicate_references_self(tcx, predicate))
.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()
}

Expand All @@ -204,20 +210,25 @@ fn bounds_reference_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> SmallVec<[Span
.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(|c| predicate_references_self(tcx, c))
.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>,
(predicate, sp): (ty::Clause<'tcx>, Span),
trait_def_id: DefId,
predicate: ty::Clause<'tcx>,
sp: Span,
allow_self_projections: AllowSelfProjections,
) -> 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)
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
Expand All @@ -235,9 +246,9 @@ fn predicate_references_self<'tcx>(
//
// This is ALT2 in issue #56288, see that for discussion of the
// possible alternatives.
data.projection_term.args[1..].iter().any(has_self_ty).then_some(sp)
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) => has_self_ty(&ty.into()).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(..)
Expand Down Expand Up @@ -383,7 +394,12 @@ fn virtual_call_violations_for_method<'tcx>(
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)) {
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, _),
..
Expand All @@ -396,7 +412,12 @@ fn virtual_call_violations_for_method<'tcx>(
errors.push(MethodViolationCode::ReferencesSelfInput(span));
}
}
if contains_illegal_self_type_reference(tcx, trait_def_id, sig.output()) {
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()) {
Expand Down Expand Up @@ -482,7 +503,7 @@ fn virtual_call_violations_for_method<'tcx>(
return false;
}

contains_illegal_self_type_reference(tcx, trait_def_id, pred)
contains_illegal_self_type_reference(tcx, trait_def_id, pred, AllowSelfProjections::Yes)
}) {
errors.push(MethodViolationCode::WhereClauseReferencesSelf);
}
Expand Down Expand Up @@ -711,10 +732,17 @@ fn receiver_is_dispatchable<'tcx>(
infcx.predicate_must_hold_modulo_regions(&obligation)
}

#[derive(Copy, Clone)]
enum AllowSelfProjections {
Yes,
No,
}

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 {
// This is somewhat subtle. In general, we want to forbid
// references to `Self` in the argument and return types,
Expand Down Expand Up @@ -759,6 +787,7 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
tcx: TyCtxt<'tcx>,
trait_def_id: DefId,
supertraits: Option<Vec<DefId>>,
allow_self_projections: AllowSelfProjections,
}

impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for IllegalSelfTypeVisitor<'tcx> {
Expand All @@ -780,39 +809,41 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
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(),
);
}
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(self.tcx.supertrait_def_ids(self.trait_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);

// 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
// 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);

// 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), // walk contained types, if any
_ => t.super_visit_with(self),
}
}

Expand All @@ -824,7 +855,12 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
}

value
.visit_with(&mut IllegalSelfTypeVisitor { tcx, trait_def_id, supertraits: None })
.visit_with(&mut IllegalSelfTypeVisitor {
tcx,
trait_def_id,
supertraits: None,
allow_self_projections,
})
.is_break()
}

Expand Down
11 changes: 11 additions & 0 deletions tests/ui/object-safety/item-bounds-can-reference-self.rs
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
//@ check-pass

pub trait Foo {
type X: PartialEq;
type Y: PartialEq<Self::Y>;
type Z: PartialEq<Self::Y>;
}

fn uwu(x: &dyn Foo<X = i32, Y = i32, Z = i32>) {}

fn main() {}

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