Unsoundness and ICE due to DispatchFromDyn allowing bogus impls on references. #148727
Description
Code
#![feature(dispatch_from_dyn, arbitrary_self_types)]
use std::ops::{DispatchFromDyn, Receiver};
pub struct Ptr<T: ?Sized>(Box<T>, Box<T>);
impl<'a, T: ?Sized, U: ?Sized> DispatchFromDyn<&'a Ptr<U>> for &'a Ptr<T> {}
impl<T: ?Sized> Receiver for Ptr<T> {
type Target = T;
}
pub trait Trait {
fn method(self: &Ptr<Self>) {}
}
struct Thing;
impl Trait for Thing {}
fn main() {
let x: Ptr<dyn Trait> = Ptr(
Box::new(Thing) as Box<dyn Trait>,
Box::new(Thing) as Box<dyn Trait>,
);
x.method();
}DispatchFromDyn does some validation on impls to ensure that they're sensible. However, for some reason, it is currently documented to always allow impls on references. This leads to bogus impls such as above.
Meta
Reproducible on the playground with version 1.93.0-nightly (2025-11-08 fb23dd3c6b120f0d2e55)
Error output
error: internal compiler error: /rustc-dev/fb23dd3c6b120f0d2e55e5f2c69a464df7b35fdf/compiler/rustc_codegen_ssa/src/mir/block.rs:1148:25: can't codegen a virtual call on OperandRef(Immediate((ptr: %3 = alloca [32 x i8], align 8)) @ TyAndLayout { ty: &Ptr<dyn Trait>, layout: Layout { size: Size(8 bytes), align: AbiAlign { abi: Align(8 bytes) }, backend_repr: Scalar(Initialized { value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), fields: Primitive, largest_niche: Some(Niche { offset: Size(0 bytes), value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), uninhabited: false, variants: Single { index: 0 }, max_repr_align: None, unadjusted_abi_align: Align(8 bytes), randomization_seed: 281492156579847 } })
--> src/main.rs:24:7
|
24 | x.method();
| ^^^^^^^^
thread 'rustc' (28) panicked at /rustc-dev/fb23dd3c6b120f0d2e55e5f2c69a464df7b35fdf/compiler/rustc_codegen_ssa/src/mir/block.rs:1148:25:
Box<dyn Any>
Backtrace
stack backtrace:
0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
2: <rustc_errors::DiagCtxtHandle>::span_bug::<rustc_span::span_encoding::Span, alloc::string::String>
3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
6: rustc_middle::util::bug::span_bug_fmt::<rustc_span::span_encoding::Span>
7: rustc_codegen_ssa::mir::codegen_mir::<rustc_codegen_llvm::builder::GenericBuilder<rustc_codegen_llvm::context::FullCx>>
8: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
9: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::compile_codegen_unit
10: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend>
11: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
12: <rustc_interface::queries::Linker>::codegen_and_build_linker
13: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
14: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md
note: please make sure that you have updated to the latest nightly
note: please attach the file at `/playground/rustc-ice-2025-11-09T04_18_49-26.txt` to your bug report
note: compiler flags: --crate-type bin -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2
note: some of the compiler flags provided by cargo are hidden
query stack during panic:
end of query stack
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Area: Dyn compatibility (formerly: object safety)Category: This is a bug.`#![feature(arbitrary_self_types)]``#![feature(dispatch_from_dyn)]`Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️Items that are on lang's radar and will need eventual work or consideration.Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/SoundnessRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.This issue may need triage. Remove it if it has been sufficiently triaged.This issue requires a nightly compiler in some way.