[ICE]: Transmuting a circularly defined GAT causes ICE with next-solver
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A-associated-items
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Description
Code
This ICE requires -Znext-solver to reproduce.
The type <Thing as Trait>::Assoc<Thing> is defined circularly as itself. This construction is the same as #149898.
For other issues with the transmute size check, see #155412 and #88290.
cc @RalfJung @lcnr
#![expect(unused)]
use std::mem::transmute;
trait Trait {
type Assoc<T: Trait>;
}
struct Thing;
impl Trait for Thing {
type Assoc<T: Trait> = T::Assoc<T>;
}
fn foo<T: Trait>() -> impl Sized {
let value: *const <T as Trait>::Assoc<T> = panic!();
value
}
fn main() {
let mut x = foo::<Thing>();
// transmute the type to that same type
x = unsafe { transmute::<_, _>(foo::<Thing>()) }
}
Meta
rustc --version --verbose:
rustc 1.97.0-nightly (7af3402cd 2026-04-16)
binary: rustc
commit-hash: 7af3402cda75aaead39f72516fd6cbb2f3ee0dbd
commit-date: 2026-04-16
host: aarch64-apple-darwin
release: 1.97.0-nightly
LLVM version: 22.1.2
Error output
note: no errors encountered even though delayed bugs were created
note: those delayed bugs will now be shown as internal compiler errors
error: internal compiler error: tried to normalize non-wf type Alias(
AliasTy {
args: [
Thing,
],
kind: Opaque {
def_id: DefId(0:14 ~ foo[3a50]::foo::{opaque#0}),
},
..
},
) in check_transmute
--> src/main.rs:22:18
|
22 | x = unsafe { transmute::<_, _>(foo::<Thing>()) }
| ^^^^^^^^^^^^^^^^^
|
Backtrace
note: delayed at /rustc-dev/7af3402cda75aaead39f72516fd6cbb2f3ee0dbd/compiler/rustc_hir_typeck/src/intrinsicck.rs:84:13
0: <std::backtrace::Backtrace>::create
1: <rustc_errors::DiagCtxtInner>::emit_diagnostic
2: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
3: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
4: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
5: rustc_hir_typeck::intrinsicck::check_transmute::{closure#1}
6: rustc_hir_typeck::intrinsicck::check_transmutes
7: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 0]>, rustc_middle::dep_graph::graph::DepNodeIndex>, true>
8: rustc_query_impl::query_impl::check_transmutes::execute_query_incr::__rust_end_short_backtrace
9: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
10: rustc_interface::passes::analysis
11: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, true>
12: rustc_query_impl::query_impl::analysis::execute_query_incr::__rust_end_short_backtrace
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}>
14: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
15: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
16: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
17: <std::sys::thread::unix::Thread>::new::thread_start
18: __pthread_cond_wait
--> src/main.rs:22:18
|
22 | x = unsafe { transmute::<_, _>(foo::<Thing>()) }
| ^^^^^^^^^^^^^^^^^
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 `/Users/timch/foo/rustc-ice-2026-04-17T08_52_39-4824.txt` to your bug report
note: rustc 1.97.0-nightly (7af3402cd 2026-04-16) running on aarch64-apple-darwin
note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C incremental=[REDACTED] -Z next-solver=globally
note: some of the compiler flags provided by cargo are hidden
query stack during panic:
end of query stack
error: could not compile `foo` (bin "foo")
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the reproducer in the issue using -Znext-solver=globally, then inspect the delayed bug reported at compiler/rustc_hir_typeck/src/intrinsicck.rs:84 in check_transmute. The issue is done when this circular GAT case no longer produces an internal compiler error and the resulting compiler behavior is covered by an appropriate regression test.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100