ICE: parallel: `internal error: entered unreachable code`

[matthiaskrgr]

Code

//@compile-flags: --edition=2024 -Zthreads=2 --crate-type=lib
trait A { fn foo() -> A; }
trait B { fn foo() -> A; }

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (81a964c23 2025-06-05)
binary: rustc
commit-hash: 81a964c23ea4fe9ab52b4449bb166bf280035797
commit-date: 2025-06-05
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

Error output

<output>

Backtrace

thread 'rustc' panicked at compiler/rustc_middle/src/values.rs:62:13:
internal error: entered unreachable code
stack backtrace:
   0:     0x74db7128a053 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hd7de588f2186284a
   1:     0x74db71a029f7 - core::fmt::write::h59e3d8534099ec1b
   2:     0x74db7127fde3 - std::io::Write::write_fmt::h80858133b280c235
   3:     0x74db71289eb2 - std::sys::backtrace::BacktraceLock::print::h2b3198fe0055c6a7
   4:     0x74db7128daaa - std::panicking::default_hook::{{closure}}::h3471ef65f1dc6f0f
   5:     0x74db7128d62f - std::panicking::default_hook::hbc8f3a4437fcdf7b
   6:     0x74db70384813 - std[223f7f98806d09cd]::panicking::update_hook::<alloc[f3381f2db88c315b]::boxed::Box<rustc_driver_impl[838fc0770cc6b7bf]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x74db7128e323 - std::panicking::rust_panic_with_hook::h46357c3d3d4456ed
   8:     0x74db7128dfe6 - std::panicking::begin_panic_handler::{{closure}}::hb436a4ad216470c6
   9:     0x74db7128a519 - std::sys::backtrace::__rust_end_short_backtrace::h96cb6cab32c629cd
  10:     0x74db7128dcdd - __rustc[fac0ad678b0de183]::rust_begin_unwind
  11:     0x74db6daa2c80 - core::panicking::panic_fmt::hf76a3ba6e5e84a3b
  12:     0x74db6dce768c - core::panicking::panic::h2f87bc8a1836b474
  13:     0x74db7097e7ec - <rustc_type_ir[fa89ab56e5d1a6cd]::binder::Binder<rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt, rustc_type_ir[fa89ab56e5d1a6cd]::ty_kind::FnSig<rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt>> as rustc_query_system[dc8d05ed1da9c1b3]::values::Value<rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt>>::from_cycle_error
  14:     0x74db70d6b035 - <rustc_query_impl[7c164422c209a242]::query_impl::fn_sig::dynamic_query::{closure#3} as core[31a69e6d0c4300f5]::ops::function::FnOnce<(rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt, &rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::CycleError, rustc_span[c41277e90fa2c474]::ErrorGuaranteed)>>::call_once
  15:     0x74db70ccaaee - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::mk_cycle::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::DefIdCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt>
  16:     0x74db71c65d79 - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::DefIdCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  17:     0x74db71c64de4 - rustc_query_impl[7c164422c209a242]::query_impl::fn_sig::get_query_non_incr::__rust_end_short_backtrace
  18:     0x74db722a56a0 - rustc_trait_selection[378cb4a8dab4dce5]::traits::dyn_compatibility::virtual_call_violations_for_method
  19:     0x74db722a198d - rustc_trait_selection[378cb4a8dab4dce5]::traits::dyn_compatibility::dyn_compatibility_violations_for_assoc_item
  20:     0x74db722a162e - <core[31a69e6d0c4300f5]::iter::adapters::flatten::FlatMap<core[31a69e6d0c4300f5]::iter::adapters::map::Map<core[31a69e6d0c4300f5]::iter::adapters::map::Map<core[31a69e6d0c4300f5]::slice::iter::Iter<(core[31a69e6d0c4300f5]::option::Option<rustc_span[c41277e90fa2c474]::symbol::Symbol>, rustc_middle[59e8487f9f7e50ed]::ty::assoc::AssocItem)>, <rustc_data_structures[13281e957cfd357d]::sorted_map::index_map::SortedIndexMultiMap<u32, core[31a69e6d0c4300f5]::option::Option<rustc_span[c41277e90fa2c474]::symbol::Symbol>, rustc_middle[59e8487f9f7e50ed]::ty::assoc::AssocItem>>::iter::{closure#0}>, <rustc_middle[59e8487f9f7e50ed]::ty::assoc::AssocItems>::in_definition_order::{closure#0}>, alloc[f3381f2db88c315b]::vec::Vec<rustc_middle[59e8487f9f7e50ed]::traits::DynCompatibilityViolation>, rustc_trait_selection[378cb4a8dab4dce5]::traits::dyn_compatibility::dyn_compatibility_violations_for_trait::{closure#0}::{closure#0}> as core[31a69e6d0c4300f5]::iter::traits::iterator::Iterator>::next
  21:     0x74db72919839 - <core[31a69e6d0c4300f5]::iter::adapters::GenericShunt<core[31a69e6d0c4300f5]::iter::adapters::map::Map<core[31a69e6d0c4300f5]::iter::adapters::flatten::FlatMap<core[31a69e6d0c4300f5]::iter::sources::from_fn::FromFn<rustc_type_ir[fa89ab56e5d1a6cd]::elaborate::supertrait_def_ids<rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt>::{closure#0}>, alloc[f3381f2db88c315b]::vec::Vec<rustc_middle[59e8487f9f7e50ed]::traits::DynCompatibilityViolation>, rustc_trait_selection[378cb4a8dab4dce5]::traits::dyn_compatibility::dyn_compatibility_violations::{closure#0}>, core[31a69e6d0c4300f5]::result::Result<rustc_middle[59e8487f9f7e50ed]::traits::DynCompatibilityViolation, !>::Ok>, core[31a69e6d0c4300f5]::result::Result<core[31a69e6d0c4300f5]::convert::Infallible, !>> as core[31a69e6d0c4300f5]::iter::traits::iterator::Iterator>::next
  22:     0x74db72918a3b - rustc_trait_selection[378cb4a8dab4dce5]::traits::dyn_compatibility::dyn_compatibility_violations
  23:     0x74db7291892e - rustc_query_impl[7c164422c209a242]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7c164422c209a242]::query_impl::dyn_compatibility_violations::dynamic_query::{closure#2}::{closure#0}, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 16usize]>>
  24:     0x74db72918915 - <rustc_query_impl[7c164422c209a242]::query_impl::dyn_compatibility_violations::dynamic_query::{closure#2} as core[31a69e6d0c4300f5]::ops::function::FnOnce<(rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt, rustc_span[c41277e90fa2c474]::def_id::DefId)>>::call_once
  25:     0x74db71c5b894 - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::DefIdCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  26:     0x74db72a911de - rustc_query_impl[7c164422c209a242]::query_impl::dyn_compatibility_violations::get_query_non_incr::__rust_end_short_backtrace
  27:     0x74db72a90fda - rustc_trait_selection[378cb4a8dab4dce5]::traits::dyn_compatibility::is_dyn_compatible
  28:     0x74db72a90f29 - rustc_query_impl[7c164422c209a242]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7c164422c209a242]::query_impl::is_dyn_compatible::dynamic_query::{closure#2}::{closure#0}, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>>
  29:     0x74db71ffaa98 - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::DefIdCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  30:     0x74db72000664 - rustc_query_impl[7c164422c209a242]::query_impl::is_dyn_compatible::get_query_non_incr::__rust_end_short_backtrace
  31:     0x74db7054f256 - <dyn rustc_hir_analysis[882d67637987d85f]::hir_ty_lowering::HirTyLowerer>::maybe_suggest_impl_trait
  32:     0x74db72cfeaa4 - <dyn rustc_hir_analysis[882d67637987d85f]::hir_ty_lowering::HirTyLowerer>::prohibit_or_lint_bare_trait_object_ty
  33:     0x74db723cf45d - <dyn rustc_hir_analysis[882d67637987d85f]::hir_ty_lowering::HirTyLowerer>::lower_ty
  34:     0x74db7224dc08 - <dyn rustc_hir_analysis[882d67637987d85f]::hir_ty_lowering::HirTyLowerer>::lower_fn_ty
  35:     0x74db7224bc3b - rustc_hir_analysis[882d67637987d85f]::collect::fn_sig
  36:     0x74db71c63a6a - rustc_query_impl[7c164422c209a242]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7c164422c209a242]::query_impl::fn_sig::dynamic_query::{closure#2}::{closure#0}, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 24usize]>>
  37:     0x74db71c63639 - <rustc_query_impl[7c164422c209a242]::query_impl::fn_sig::dynamic_query::{closure#2} as core[31a69e6d0c4300f5]::ops::function::FnOnce<(rustc_middle[59e8487f9f7e50ed]::ty::context::TyCtxt, rustc_span[c41277e90fa2c474]::def_id::DefId)>>::call_once
  38:     0x74db71c656cf - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::DefIdCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  39:     0x74db71c64de4 - rustc_query_impl[7c164422c209a242]::query_impl::fn_sig::get_query_non_incr::__rust_end_short_backtrace
  40:     0x74db72254779 - <rustc_hir_analysis[882d67637987d85f]::collect::CollectItemTypesVisitor as rustc_hir[9fdf819cfa47c32c]::intravisit::Visitor>::visit_trait_item
  41:     0x74db72262905 - rustc_hir_analysis[882d67637987d85f]::check::wfcheck::check_well_formed
  42:     0x74db7225fa17 - rustc_query_impl[7c164422c209a242]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7c164422c209a242]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>>
  43:     0x74db7225f291 - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_data_structures[13281e957cfd357d]::vec_cache::VecCache<rustc_span[c41277e90fa2c474]::def_id::LocalDefId, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[dc8d05ed1da9c1b3]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  44:     0x74db7225ed82 - rustc_query_impl[7c164422c209a242]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  45:     0x74db70479a11 - rustc_middle[59e8487f9f7e50ed]::query::plumbing::query_ensure_error_guaranteed::<rustc_data_structures[13281e957cfd357d]::vec_cache::VecCache<rustc_span[c41277e90fa2c474]::def_id::LocalDefId, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[dc8d05ed1da9c1b3]::dep_graph::graph::DepNodeIndex>, ()>
  46:     0x74db7048717e - rustc_data_structures[13281e957cfd357d]::sync::parallel::par_slice::par_rec::<&rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId, rustc_data_structures[13281e957cfd357d]::sync::parallel::try_par_for_each_in<&[rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId], rustc_span[c41277e90fa2c474]::ErrorGuaranteed, <rustc_middle[59e8487f9f7e50ed]::hir::ModuleItems>::par_trait_items<rustc_hir_analysis[882d67637987d85f]::check::wfcheck::check_type_wf::{closure#2}>::{closure#0}>::{closure#0}::{closure#0}>
  47:     0x74db704c02f9 - rayon_core[4722162430a43c7]::join::join_context::<rayon_core[4722162430a43c7]::join::join::call<(), rustc_data_structures[13281e957cfd357d]::sync::parallel::par_slice::par_rec<&rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId, rustc_data_structures[13281e957cfd357d]::sync::parallel::try_par_for_each_in<&[rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId], rustc_span[c41277e90fa2c474]::ErrorGuaranteed, <rustc_middle[59e8487f9f7e50ed]::hir::ModuleItems>::par_trait_items<rustc_hir_analysis[882d67637987d85f]::check::wfcheck::check_type_wf::{closure#2}>::{closure#0}>::{closure#0}::{closure#0}>::{closure#1}>::{closure#0}, rayon_core[4722162430a43c7]::join::join::call<(), rustc_data_structures[13281e957cfd357d]::sync::parallel::par_slice::par_rec<&rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId, rustc_data_structures[13281e957cfd357d]::sync::parallel::try_par_for_each_in<&[rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId], rustc_span[c41277e90fa2c474]::ErrorGuaranteed, <rustc_middle[59e8487f9f7e50ed]::hir::ModuleItems>::par_trait_items<rustc_hir_analysis[882d67637987d85f]::check::wfcheck::check_type_wf::{closure#2}>::{closure#0}>::{closure#0}::{closure#0}>::{closure#2}>::{closure#0}, (), ()>::{closure#0}
  48:     0x74db70487248 - rustc_data_structures[13281e957cfd357d]::sync::parallel::par_slice::par_rec::<&rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId, rustc_data_structures[13281e957cfd357d]::sync::parallel::try_par_for_each_in<&[rustc_hir[9fdf819cfa47c32c]::hir::TraitItemId], rustc_span[c41277e90fa2c474]::ErrorGuaranteed, <rustc_middle[59e8487f9f7e50ed]::hir::ModuleItems>::par_trait_items<rustc_hir_analysis[882d67637987d85f]::check::wfcheck::check_type_wf::{closure#2}>::{closure#0}>::{closure#0}::{closure#0}>
  49:     0x74db7225c74a - rustc_hir_analysis[882d67637987d85f]::check::wfcheck::check_type_wf
  50:     0x74db7225bcb9 - rustc_query_impl[7c164422c209a242]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7c164422c209a242]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>>
  51:     0x74db72a3e388 - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::SingleCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  52:     0x74db72a3e182 - rustc_query_impl[7c164422c209a242]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  53:     0x74db71d5ef69 - rustc_hir_analysis[882d67637987d85f]::check_crate
  54:     0x74db71d538e8 - rustc_interface[d85128a34ad4eaa4]::passes::analysis
  55:     0x74db71d534f3 - rustc_query_impl[7c164422c209a242]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7c164422c209a242]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 0usize]>>
  56:     0x74db72a3828e - rustc_query_system[dc8d05ed1da9c1b3]::query::plumbing::try_execute_query::<rustc_query_impl[7c164422c209a242]::DynamicConfig<rustc_query_system[dc8d05ed1da9c1b3]::query::caches::SingleCache<rustc_middle[59e8487f9f7e50ed]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[7c164422c209a242]::plumbing::QueryCtxt, false>
  57:     0x74db72a37e76 - rustc_query_impl[7c164422c209a242]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  58:     0x74db72be2c3f - rustc_interface[d85128a34ad4eaa4]::passes::create_and_enter_global_ctxt::<core[31a69e6d0c4300f5]::option::Option<rustc_interface[d85128a34ad4eaa4]::queries::Linker>, rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  59:     0x74db72bd4330 - rustc_interface[d85128a34ad4eaa4]::interface::run_compiler::<(), rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}>::{closure#1}
  60:     0x74db703996b4 - <rayon_core[4722162430a43c7]::job::StackJob<rayon_core[4722162430a43c7]::latch::LatchRef<rayon_core[4722162430a43c7]::latch::LockLatch>, <rayon_core[4722162430a43c7]::registry::Registry>::in_worker_cold<<rayon_core[4722162430a43c7]::thread_pool::ThreadPool>::install<rustc_interface[d85128a34ad4eaa4]::util::run_in_thread_pool_with_globals<rustc_interface[d85128a34ad4eaa4]::interface::run_compiler<(), rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}::{closure#0}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()> as rayon_core[4722162430a43c7]::job::Job>::execute
  61:     0x74db6fea065e - <rayon_core[4722162430a43c7]::registry::WorkerThread>::wait_until_cold
  62:     0x74db6fe9dfe1 - <rayon_core[4722162430a43c7]::registry::ThreadBuilder>::run
  63:     0x74db7037ad8d - std[223f7f98806d09cd]::sys::backtrace::__rust_begin_short_backtrace::<<rayon_core[4722162430a43c7]::ThreadPoolBuilder>::build_scoped<rustc_interface[d85128a34ad4eaa4]::util::run_in_thread_pool_with_globals<rustc_interface[d85128a34ad4eaa4]::interface::run_compiler<(), rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#0}, rustc_interface[d85128a34ad4eaa4]::util::run_in_thread_pool_with_globals<rustc_interface[d85128a34ad4eaa4]::interface::run_compiler<(), rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>
  64:     0x74db70389d32 - <<std[223f7f98806d09cd]::thread::Builder>::spawn_unchecked_<<rayon_core[4722162430a43c7]::ThreadPoolBuilder>::build_scoped<rustc_interface[d85128a34ad4eaa4]::util::run_in_thread_pool_with_globals<rustc_interface[d85128a34ad4eaa4]::interface::run_compiler<(), rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#0}, rustc_interface[d85128a34ad4eaa4]::util::run_in_thread_pool_with_globals<rustc_interface[d85128a34ad4eaa4]::interface::run_compiler<(), rustc_driver_impl[838fc0770cc6b7bf]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#5}::{closure#0}::{closure#1}, ()>::{closure#0}::{closure#0}::{closure#0}, ()>::{closure#1} as core[31a69e6d0c4300f5]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  65:     0x74db72c07bfd - std::sys::pal::unix::thread::Thread::new::thread_start::h9c37dacde5045541
  66:     0x74db6c6a57eb - <unknown>
  67:     0x74db6c72918c - <unknown>
  68:                0x0 - <unknown>

error: the compiler unexpectedly panicked. this is a bug.

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 `/tmp/im/rustc-ice-2025-06-05T06_46_38-2943799.txt` to your bug report

note: compiler flags: -Z threads=2 --crate-type lib

query stack during panic:
#0 [dyn_compatibility_violations] determining dyn-compatibility of trait `A`
#1 [is_dyn_compatible] checking if trait `A` is dyn-compatible
... and 4 other queries... use `env RUST_BACKTRACE=1` to see the full query stack
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[E0391]: cycle detected when checking if trait `A` is dyn-compatible
 --> b.rs:1:1
  |
1 | trait A { fn foo() -> A; }
  | ^^^^^^^
  |
note: ...which requires determining dyn-compatibility of trait `A`...
 --> b.rs:1:1
  |
1 | trait A { fn foo() -> A; }
  | ^^^^^^^
note: ...which requires computing function signature of `A::foo`...
 --> b.rs:1:11
  |
1 | trait A { fn foo() -> A; }
  |           ^^^^^^^^^^^^^^
  = note: ...which again requires checking if trait `A` is dyn-compatible, completing the cycle
note: cycle used when computing function signature of `B::foo`
 --> b.rs:2:11
  |
2 | trait B { fn foo() -> A; }
  |           ^^^^^^^^^^^^^^
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
note: delayed at compiler/rustc_query_system/src/query/job.rs:602:16 - disabled backtrace
 --> b.rs:1:1
  |
1 | trait A { fn foo() -> A; }
  | ^^^^^^^

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 `/tmp/im/rustc-ice-2025-06-05T06_46_38-2943799.txt` to your bug report

note: compiler flags: -Z threads=2 --crate-type lib

query stack during panic:
end of query stack

[matthiaskrgr]

Hmm, when I name the file "crash.rs" it does not repro, but when I name it "b.rs" it does? 😆

[matthiaskrgr]

Seems this was introduced somewhere between
rustc 1.89.0-nightly (4b27a04cc 2025-06-04)
and
rustc 1.89.0-nightly (81a964c23 2025-06-05)

maybe #141968 ?

[cushionbadak]

Hmm, when I name the file "crash.rs" it does not repro, but when I name it "b.rs" it does? 😆

In my environment,

• rustc crash.rs --edition=2024 -Zthreads=2 --crate-type=lib causes ICE, but
• rustc b.rs --edition=2024 -Zthreads=2 --crate-type=lib does not.

rustc 1.89.0-nightly (ccf3198de 2025-06-05)
binary: rustc
commit-hash: ccf3198de316b488ee17441935182e9d5292b4d3
commit-date: 2025-06-05
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

[zetanumbers]

If you look at panicking code and compare execution of sequential vs parallel compiler, you will find for a following code if branch fails early on frame.query.dep_kind comparison if parallel compiler enabled. Inspecting cycle_error variable reveals wildly different values for parallel vs sequential compiler executions.

rust/compiler/rustc_middle/src/values.rs

Lines 53 to 63 in 81a964c

	let arity = if let Some(frame) = cycle_error.cycle.get(0)
	&& frame.query.dep_kind == dep_kinds::fn_sig
	&& let Some(def_id) = frame.query.def_id
	&& let Some(node) = tcx.hir_get_if_local(def_id)
	&& let Some(sig) = node.fn_sig()
	{
	sig.decl.inputs.len()
	} else {
	tcx.dcx().abort_if_errors();
	unreachable!()
	};

So I think this difference comes from differences in two separate functions creating CycleErrors:

• find_cycle_in_stack for single-thread cycles

rust/compiler/rustc_query_system/src/query/job.rs

Line 144 in 81a964c

return CycleError { usage, cycle };

• remove_cycle whenever thread_pool handles deadlocks

rust/compiler/rustc_query_system/src/query/job.rs

Lines 476 to 482 in 81a964c

	let error = CycleError {
	usage,
	cycle: stack
	.iter()
	.map(|&(s, ref q)| QueryInfo { span: s, query: q.query(query_map) })
	.collect(),
	};

Probably other code doesn't depend much on such structure of a reported cycle, but this inconsistency could be a significant bug. Then what if there are multiple cycles present in code? Anyway I'm not sure how these cycles should be structured if not identical to its sequential variant.

I would like to try rethink current approach for cycle handling, even if it would bring me back to current one. I have previously posted about hypothetical early query cycle detection, which could prob solve this problem:

#t-compiler/parallel-rustc > ✔ Early detection of query cycles

[zetanumbers]

This issue was reproduced during the crater parallel frontend test run (#146237):

https://crater-reports.s3.amazonaws.com/pr-146237/try%23d79d87d4c0f7f0d0785ad6e493cf8cf718ed89a6/gh/LinusThorsell.rustensor/log.txt

[ywxt]

This may be the same cuase as #142063

CycleError.cycle has a determinastic sort.

rust/compiler/rustc_query_system/src/query/job.rs

Lines 468 to 472 in 81a964c

	let entry_point_pos = stack.iter().position(|(_, query)| query == entry_point);
	if let Some(pos) = entry_point_pos {
	stack.rotate_left(pos);
	}

However the reported QueryConfig is different(dyn-compatibility or fn_sig) sometimes, which isn't matched with the first cycle error.

[zetanumbers]

Fixed in #153694