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Ensure that we never try to monomorphize the upcasting or vtable calls of impossible dyn types #136311

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Feb 6, 2025
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26 changes: 21 additions & 5 deletions compiler/rustc_trait_selection/src/traits/vtable.rs
Original file line number Diff line number Diff line change
Expand Up @@ -318,9 +318,17 @@ pub(crate) fn first_method_vtable_slot<'tcx>(tcx: TyCtxt<'tcx>, key: ty::TraitRe
bug!();
};
let source_principal = tcx.instantiate_bound_regions_with_erased(
source.principal().unwrap().with_self_ty(tcx, tcx.types.trait_object_dummy_self),
source.principal().unwrap().with_self_ty(tcx, key.self_ty()),
);

// We're monomorphizing a call to a dyn trait object that can never be constructed.
if tcx.instantiate_and_check_impossible_predicates((
source_principal.def_id,
source_principal.args,
)) {
return 0;
}

let target_principal = ty::ExistentialTraitRef::erase_self_ty(tcx, key);

let vtable_segment_callback = {
Expand Down Expand Up @@ -373,19 +381,27 @@ pub(crate) fn supertrait_vtable_slot<'tcx>(
let (source, target) = key;

// If the target principal is `None`, we can just return `None`.
let ty::Dynamic(target, _, _) = *target.kind() else {
let ty::Dynamic(target_data, _, _) = *target.kind() else {
bug!();
};
let target_principal = tcx.instantiate_bound_regions_with_erased(target.principal()?);
let target_principal = tcx.instantiate_bound_regions_with_erased(target_data.principal()?);

// Given that we have a target principal, it is a bug for there not to be a source principal.
let ty::Dynamic(source, _, _) = *source.kind() else {
let ty::Dynamic(source_data, _, _) = *source.kind() else {
bug!();
};
let source_principal = tcx.instantiate_bound_regions_with_erased(
source.principal().unwrap().with_self_ty(tcx, tcx.types.trait_object_dummy_self),
source_data.principal().unwrap().with_self_ty(tcx, source),
);

// We're monomorphizing a dyn trait object upcast that can never be constructed.
if tcx.instantiate_and_check_impossible_predicates((
source_principal.def_id,
source_principal.args,
)) {
return None;
}

let vtable_segment_callback = {
let mut vptr_offset = 0;
move |segment| {
Expand Down
26 changes: 26 additions & 0 deletions tests/ui/traits/trait-upcasting/mono-impossible.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,26 @@
//@ build-pass

#![feature(trait_upcasting)]

trait Supertrait<T> {
fn method(&self) {}
}
impl<T> Supertrait<T> for () {}

trait WithAssoc {
type Assoc;
}
trait Trait<P: WithAssoc>: Supertrait<P::Assoc> + Supertrait<()> {}

fn upcast<P>(x: &dyn Trait<P>) -> &dyn Supertrait<()> {
x
}

fn call<P>(x: &dyn Trait<P>) {
x.method();
}

fn main() {
println!("{:p}", upcast::<()> as *const ());
println!("{:p}", call::<()> as *const ());
}
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