rust-lang / rust-lang/rust

ICE: assertion failed: ` .. var_values: [{type error}] ` vs ` .. var_values: [Alias(Opaque, AliasTy {...`

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C-bug F-min_generic_const_args F-return_type_notation I-ICE T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
#![feature(min_generic_const_args)]
trait Trait {
    fn foo() -> impl Copy {}
}

impl Trait for () {
    fn foo() -> impl Copy
    where
        Self::foo(..): Copy,
    {
    }
}

original:

trait Trait {
    fn foo() -> impl Copy
    where [(); T::ABC]: Sized
    {
        String::from("temporary")
    }
}

impl Trait for () {
    fn foo() -> impl Copy
    where
        Self::foo(..): Copy
    {
        String::from("temporary")
    }
}

fn foo() -> impl Copy
    where
        Self::foo(..): Copy
    {
        String::from("temporary")
    }

Version information

rustc 1.95.0-nightly (621d76794 2026-01-18)
binary: rustc
commit-hash: 621d76794c76fc21c0a6151fbc090120e0230a91
commit-date: 2026-01-18
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 21.1.8

Possibly related line of code:
https://github.com/rust-lang/rust/blob/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_type_ir/src/search_graph/global_cache.rs#L60-L72

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zcrate-attr=feature(min_generic_const_args) -Znext-solver=globally

Program output 

error[E0658]: return type notation is experimental
 --> /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:8:18
  |
8 |         Self::foo(..): Copy,
  |                  ^^^^
  |
  = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
  = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
  = note: this compiler was built on 2026-01-18; consider upgrading it if it is out of date

warning: the feature `min_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
 --> <crate attribute>:1:12
  |
1 | #![feature(min_generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:11:2
   |
11 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs`

error[E0391]: cycle detected when computing type of `<impl at /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:5:1: 5:18>::foo::{anon_assoc#0}`
 --> /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:6:17
  |
6 |     fn foo() -> impl Copy
  |                 ^^^^^^^^^
  |
note: ...which requires comparing an impl and trait method signature, inferring any hidden `impl Trait` types in the process...
 --> /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:6:5
  |
6 | /     fn foo() -> impl Copy
7 | |     where
8 | |         Self::foo(..): Copy,
  | |____________________________^
  = note: ...which again requires computing type of `<impl at /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:5:1: 5:18>::foo::{anon_assoc#0}`, completing the cycle
note: cycle used when checking assoc item `<impl at /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:5:1: 5:18>::foo` is compatible with trait definition
 --> /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:6:5
  |
6 | /     fn foo() -> impl Copy
7 | |     where
8 | |         Self::foo(..): Copy,
  | |____________________________^
  = 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


thread 'rustc' (1113364) panicked at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_type_ir/src/search_graph/global_cache.rs:66:17:
assertion `left == right` failed
  left: Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [{type error}] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: [], opaque_types: [], normalization_nested_goals: NestedNormalizationGoals([]) }) }, max_universe: U0, var_kinds: [] })
 right: Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [Alias(Opaque, AliasTy { args: [], def_id: DefId(0:8 ~ mvce[a621]::{impl#0}::foo::{opaque#0}), .. })] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: [], opaque_types: [], normalization_nested_goals: NestedNormalizationGoals([]) }) }, max_universe: U0, var_kinds: [] })
stack backtrace:
   0:     0x7fb28dc33fd3 - <<std[287527946f71f0bc]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[7477a31a1a34cc49]::fmt::Display>::fmt
   1:     0x7fb28e20e2c8 - core[7477a31a1a34cc49]::fmt::write
   2:     0x7fb28dc4a9c6 - <std[287527946f71f0bc]::sys::stdio::unix::Stderr as std[287527946f71f0bc]::io::Write>::write_fmt
   3:     0x7fb28dc0a048 - std[287527946f71f0bc]::panicking::default_hook::{closure#0}
   4:     0x7fb28dc27703 - std[287527946f71f0bc]::panicking::default_hook
   5:     0x7fb28cbfbb8a - std[287527946f71f0bc]::panicking::update_hook::<alloc[979041e9952e771b]::boxed::Box<rustc_driver_impl[517260a07841a630]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fb28dc279e2 - std[287527946f71f0bc]::panicking::panic_with_hook
   7:     0x7fb28dc0a108 - std[287527946f71f0bc]::panicking::panic_handler::{closure#0}
   8:     0x7fb28dc00dc9 - std[287527946f71f0bc]::sys::backtrace::__rust_end_short_backtrace::<std[287527946f71f0bc]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fb28dc0bbdd - __rustc[5ec7081ce87b032f]::rust_begin_unwind
  10:     0x7fb28a70cfbc - core[7477a31a1a34cc49]::panicking::panic_fmt
  11:     0x7fb28c57d6d3 - core[7477a31a1a34cc49]::panicking::assert_failed_inner
  12:     0x7fb28d966d74 - core[7477a31a1a34cc49]::panicking::assert_failed::<core[7477a31a1a34cc49]::result::Result<rustc_type_ir[b93cf349b3a4f34d]::canonical::Canonical<rustc_middle[725063e074f29df]::ty::context::TyCtxt, rustc_type_ir[b93cf349b3a4f34d]::solve::Response<rustc_middle[725063e074f29df]::ty::context::TyCtxt>>, rustc_type_ir[b93cf349b3a4f34d]::solve::NoSolution>, core[7477a31a1a34cc49]::result::Result<rustc_type_ir[b93cf349b3a4f34d]::canonical::Canonical<rustc_middle[725063e074f29df]::ty::context::TyCtxt, rustc_type_ir[b93cf349b3a4f34d]::solve::Response<rustc_middle[725063e074f29df]::ty::context::TyCtxt>>, rustc_type_ir[b93cf349b3a4f34d]::solve::NoSolution>>
  13:     0x7fb28ee35f08 - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::evaluate_goal_raw
  14:     0x7fb28ee222a5 - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::try_evaluate_added_goals
  15:     0x7fb28ee1bbfe - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate> as rustc_type_ir[b93cf349b3a4f34d]::search_graph::Delegate>::compute_goal::{closure#0}
  16:     0x7fb28ee323a3 - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::evaluate_goal_raw
  17:     0x7fb28ee21ef4 - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::try_evaluate_added_goals
  18:     0x7fb28f12af23 - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::compute_trait_goal
  19:     0x7fb28ee1b74f - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate> as rustc_type_ir[b93cf349b3a4f34d]::search_graph::Delegate>::compute_goal::{closure#0}
  20:     0x7fb28ee323a3 - <rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::evaluate_goal_raw
  21:     0x7fb28ee31489 - <rustc_trait_selection[9fee51d8a10a777c]::solve::delegate::SolverDelegate as rustc_next_trait_solver[2bc6883c9be4be93]::solve::eval_ctxt::SolverDelegateEvalExt>::evaluate_root_goal
  22:     0x7fb28ee23186 - <rustc_trait_selection[9fee51d8a10a777c]::solve::fulfill::FulfillmentCtxt<rustc_trait_selection[9fee51d8a10a777c]::traits::FulfillmentError> as rustc_infer[5a785d1c27494969]::traits::engine::TraitEngine<rustc_trait_selection[9fee51d8a10a777c]::traits::FulfillmentError>>::try_evaluate_obligations
  23:     0x7fb28ee22d29 - <rustc_trait_selection[9fee51d8a10a777c]::solve::fulfill::FulfillmentCtxt<rustc_trait_selection[9fee51d8a10a777c]::traits::FulfillmentError> as rustc_infer[5a785d1c27494969]::traits::engine::TraitEngine<rustc_trait_selection[9fee51d8a10a777c]::traits::FulfillmentError>>::evaluate_obligations_error_on_ambiguity
  24:     0x7fb28ebe0b3c - rustc_hir_analysis[1a80394817eddfdd]::check::compare_impl_item::compare_impl_item
  25:     0x7fb28ebdea99 - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::compare_impl_item::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 1usize]>>
  26:     0x7fb28e992f78 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_data_structures[e0d8483206daafa2]::vec_cache::VecCache<rustc_span[851f35e07e3c2dd4]::def_id::LocalDefId, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[b7e73caa2c2ad9a]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  27:     0x7fb28e992a51 - rustc_query_impl[f20b8284200e680a]::query_impl::compare_impl_item::get_query_non_incr::__rust_end_short_backtrace
  28:     0x7fb28e995eaa - rustc_hir_analysis[1a80394817eddfdd]::check::check::check_item_type
  29:     0x7fb28e99379c - rustc_hir_analysis[1a80394817eddfdd]::check::wfcheck::check_well_formed
  30:     0x7fb28e99376b - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 1usize]>>
  31:     0x7fb28e9931e3 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_data_structures[e0d8483206daafa2]::vec_cache::VecCache<rustc_span[851f35e07e3c2dd4]::def_id::LocalDefId, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[b7e73caa2c2ad9a]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  32:     0x7fb28e992cd6 - rustc_query_impl[f20b8284200e680a]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  33:     0x7fb28e990470 - rustc_hir_analysis[1a80394817eddfdd]::check::wfcheck::check_type_wf
  34:     0x7fb28e99035d - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 1usize]>>
  35:     0x7fb28f3a3e85 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_query_system[b7e73caa2c2ad9a]::query::caches::SingleCache<rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  36:     0x7fb28f3a3b36 - rustc_query_impl[f20b8284200e680a]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  37:     0x7fb28ed3f316 - rustc_hir_analysis[1a80394817eddfdd]::check_crate
  38:     0x7fb28e3493db - rustc_interface[5c48368f5debe80d]::passes::analysis
  39:     0x7fb28e349035 - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 0usize]>>
  40:     0x7fb28f3a4be3 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_query_system[b7e73caa2c2ad9a]::query::caches::SingleCache<rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  41:     0x7fb28f3a49ce - rustc_query_impl[f20b8284200e680a]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  42:     0x7fb28f582c11 - <rustc_interface[5c48368f5debe80d]::passes::create_and_enter_global_ctxt<core[7477a31a1a34cc49]::option::Option<rustc_interface[5c48368f5debe80d]::queries::Linker>, rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[7477a31a1a34cc49]::ops::function::FnOnce<(&rustc_session[eb79e6d3104f29ae]::session::Session, rustc_middle[725063e074f29df]::ty::context::CurrentGcx, alloc[979041e9952e771b]::sync::Arc<rustc_data_structures[e0d8483206daafa2]::jobserver::Proxy>, &std[287527946f71f0bc]::sync::once_lock::OnceLock<rustc_middle[725063e074f29df]::ty::context::GlobalCtxt>, &rustc_data_structures[e0d8483206daafa2]::sync::worker_local::WorkerLocal<rustc_middle[725063e074f29df]::arena::Arena>, &rustc_data_structures[e0d8483206daafa2]::sync::worker_local::WorkerLocal<rustc_hir[7eac7792dedc1861]::Arena>, rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  43:     0x7fb28f3f5d34 - rustc_interface[5c48368f5debe80d]::interface::run_compiler::<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}
  44:     0x7fb28f35aa3a - std[287527946f71f0bc]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5c48368f5debe80d]::util::run_in_thread_with_globals<rustc_interface[5c48368f5debe80d]::util::run_in_thread_pool_with_globals<rustc_interface[5c48368f5debe80d]::interface::run_compiler<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  45:     0x7fb28f35a7fe - <std[287527946f71f0bc]::thread::lifecycle::spawn_unchecked<rustc_interface[5c48368f5debe80d]::util::run_in_thread_with_globals<rustc_interface[5c48368f5debe80d]::util::run_in_thread_pool_with_globals<rustc_interface[5c48368f5debe80d]::interface::run_compiler<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7477a31a1a34cc49]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  46:     0x7fb28f35c5c4 - <std[287527946f71f0bc]::sys::thread::unix::Thread>::new::thread_start
  47:     0x7fb288c9698b - <unknown>
  48:     0x7fb288d1a9cc - <unknown>
  49:                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: rustc 1.95.0-nightly (621d76794 2026-01-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z crate-attr=feature(min_generic_const_args) -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [compare_impl_item] checking assoc item `<impl at /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:5:1: 5:18>::foo` is compatible with trait definition
#1 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.w08EPnpWwZh8/rustc_testrunner_tmpdir_reporting.Ehj8dz8mFpzP/mvce.rs:5:1: 5:18>` is well-formed
#2 [check_type_wf] checking that types are well-formed
#3 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 3 previous errors; 1 warning emitted

Some errors have detailed explanations: E0391, E0601, E0658.
For more information about an error, try `rustc --explain E0391`.

@rustbot label +F-min_generic_const_args +WG-trait-system-refactor

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  3. Fork the repository and make your change on a branch.
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Research direction

Reproduce the reduced Rust case with -Znext-solver=globally and the min_generic_const_args feature. Then inspect compiler/rustc_type_ir/src/search_graph/global_cache.rs:60-72 and the surrounding trait-solver path; done means this input no longer triggers the reported assertion panic.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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