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[ICE]: internal error: entered unreachable code

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C-bug F-generic_const_args F-min_generic_const_args I-ICE needs-triage T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

#![feature(generic_const_args)]
enum T<const N: u8 = { T::<0>::B as u8 }> {
    A = 2,
    B,
}

original:

#![feature(generic_const_args, min_generic_const_args)]
enum T<const N: u8 = { T::<0>::B as u8 }> {
    A = 2,
    B,
}

fn main() {}

Version information

rustc 1.100.0-nightly (76af58f66 2026-09-17)
binary: rustc
commit-hash: 76af58f665c5cd76672e64ca118efbb0101fb224
commit-date: 2026-09-17
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Possibly related line of code:
https://github.com/rust-lang/rust/blob/76af58f665c5cd76672e64ca118efbb0101fb224/compiler/rustc_mir_build/src/builder/expr/as_constant.rs#L91-L103

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output 

error: `generic_const_args` requires `min_generic_const_args` to be enabled
 --> /tmp/icemaker_global_tempdir.gxejRpLF4GIA/rustc_testrunner_tmpdir_reporting.AZt01FmFlvTk/mvce.rs:1:12
  |
1 | #![feature(generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^
  |
  = help: enable all of these features

warning: the feature `generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.gxejRpLF4GIA/rustc_testrunner_tmpdir_reporting.AZt01FmFlvTk/mvce.rs:1:12
  |
1 | #![feature(generic_const_args)]
  |            ^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #151972 <https://github.com/rust-lang/rust/issues/151972> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.gxejRpLF4GIA/rustc_testrunner_tmpdir_reporting.AZt01FmFlvTk/mvce.rs:5:2
  |
5 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.gxejRpLF4GIA/rustc_testrunner_tmpdir_reporting.AZt01FmFlvTk/mvce.rs`


thread 'rustc' (1806874) panicked at /rustc-dev/76af58f665c5cd76672e64ca118efbb0101fb224/compiler/rustc_mir_build/src/builder/expr/as_constant.rs:97:26:
internal error: entered unreachable code
stack backtrace:
   0:     0x7f913648acc6 - <<std[37e96c38c503a63a]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[2372320f1a8e1214]::fmt::Display>::fmt
   1:     0x7f9136c09953 - core[2372320f1a8e1214]::fmt::write
   2:     0x7f913649fc9c - <std[37e96c38c503a63a]::sys::stdio::unix::Stderr as core[2372320f1a8e1214]::io::write::Write>::write_fmt
   3:     0x7f913645d7ca - std[37e96c38c503a63a]::panicking::default_hook::{closure#0}
   4:     0x7f913647f273 - std[37e96c38c503a63a]::panicking::default_hook
   5:     0x7f9135189e92 - std[37e96c38c503a63a]::panicking::update_hook::<alloc[4345330095c66ba4]::boxed::Box<rustc_driver_impl[936d1968ef2b2e3d]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f913647f712 - std[37e96c38c503a63a]::panicking::panic_with_hook
   7:     0x7f913645d8b4 - std[37e96c38c503a63a]::panicking::panic_handler::{closure#0}
   8:     0x7f91364546a9 - std[37e96c38c503a63a]::sys::backtrace::__rust_end_short_backtrace::<std[37e96c38c503a63a]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f913645f45d - __rustc[23b149a7f09fcc1a]::rust_begin_unwind
  10:     0x7f9132e230fc - core[2372320f1a8e1214]::panicking::panic_fmt
  11:     0x7f91332d53d2 - core[2372320f1a8e1214]::panicking::panic
  12:     0x7f913753efcf - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::as_constant
  13:     0x7f913758db78 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::as_operand
  14:     0x7f91378bff03 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::as_rvalue
  15:     0x7f91377cd89f - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::expr_into_dest
  16:     0x7f91378b5db2 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::as_temp
  17:     0x7f913758e970 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::as_operand
  18:     0x7f91378c0253 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::as_rvalue
  19:     0x7f91377cd89f - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::expr_into_dest
  20:     0x7f91377ce3c0 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::expr_into_dest
  21:     0x7f91323eec4b - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::ast_block
  22:     0x7f91377cf6f8 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::expr_into_dest
  23:     0x7f91377ce3c0 - <rustc_mir_build[3618ac1afb9ac25]::builder::Builder>::expr_into_dest
  24:     0x7f91377b32ae - rustc_mir_build[3618ac1afb9ac25]::builder::build_mir_inner_impl
  25:     0x7f9136c23aaa - rustc_mir_transform[97d752da69874ef5]::mir_built
  26:     0x7f913710c27a - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_data_structures[5e8f17b1399e428]::vec_cache::VecCache<rustc_span[283b8a40713b5525]::def_id::LocalDefId, rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[edb1849eae9467f6]::dep_graph::graph::DepNodeIndex>, false>
  27:     0x7f913710bf67 - rustc_query_impl[fae7975a325c58a]::query_vtables::mir_built::execute_query_non_incr::__rust_end_short_backtrace
  28:     0x7f9137831004 - rustc_query_impl[fae7975a325c58a]::query_vtables::trivial_const::invoke_provider_fn::__rust_begin_short_backtrace
  29:     0x7f9137832f58 - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_middle[edb1849eae9467f6]::query::caches::DefIdCache<rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 32usize]>>, false>
  30:     0x7f91378326a5 - rustc_query_impl[fae7975a325c58a]::query_vtables::trivial_const::execute_query_non_incr::__rust_end_short_backtrace
  31:     0x7f91342bc319 - rustc_const_eval[179d7635c5ffacc2]::const_eval::eval_queries::eval_to_allocation_raw_provider
  32:     0x7f9136c23216 - rustc_query_impl[fae7975a325c58a]::query_vtables::eval_to_allocation_raw::invoke_provider_fn::__rust_begin_short_backtrace
  33:     0x7f913761e48a - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_middle[edb1849eae9467f6]::query::caches::DefaultCache<rustc_middle[edb1849eae9467f6]::ty::PseudoCanonicalInput<rustc_middle[edb1849eae9467f6]::mir::interpret::GlobalId>, rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 24usize]>>, false>
  34:     0x7f913761d9e7 - rustc_query_impl[fae7975a325c58a]::query_vtables::eval_to_allocation_raw::execute_query_non_incr::__rust_end_short_backtrace
  35:     0x7f91342bc1f2 - rustc_const_eval[179d7635c5ffacc2]::const_eval::eval_queries::eval_to_allocation_raw_provider
  36:     0x7f9136c23216 - rustc_query_impl[fae7975a325c58a]::query_vtables::eval_to_allocation_raw::invoke_provider_fn::__rust_begin_short_backtrace
  37:     0x7f913761e48a - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_middle[edb1849eae9467f6]::query::caches::DefaultCache<rustc_middle[edb1849eae9467f6]::ty::PseudoCanonicalInput<rustc_middle[edb1849eae9467f6]::mir::interpret::GlobalId>, rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 24usize]>>, false>
  38:     0x7f913761d9e7 - rustc_query_impl[fae7975a325c58a]::query_vtables::eval_to_allocation_raw::execute_query_non_incr::__rust_end_short_backtrace
  39:     0x7f91339fda67 - rustc_const_eval[179d7635c5ffacc2]::const_eval::valtrees::eval_to_valtree
  40:     0x7f91373042e6 - <rustc_const_eval[179d7635c5ffacc2]::provide::{closure#0} as core[2372320f1a8e1214]::ops::function::FnOnce<(rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt, rustc_middle[edb1849eae9467f6]::ty::PseudoCanonicalInput<rustc_middle[edb1849eae9467f6]::mir::interpret::GlobalId>)>>::call_once
  41:     0x7f91373042ac - rustc_query_impl[fae7975a325c58a]::query_vtables::eval_to_valtree::invoke_provider_fn::__rust_begin_short_backtrace
  42:     0x7f9137303a26 - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_middle[edb1849eae9467f6]::query::caches::DefaultCache<rustc_middle[edb1849eae9467f6]::ty::PseudoCanonicalInput<rustc_middle[edb1849eae9467f6]::mir::interpret::GlobalId>, rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 16usize]>>, false>
  43:     0x7f91373036e7 - rustc_query_impl[fae7975a325c58a]::query_vtables::eval_to_valtree::execute_query_non_incr::__rust_end_short_backtrace
  44:     0x7f9136f76ab7 - <rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>::const_eval_global_id_for_typeck
  45:     0x7f9136f765c0 - <rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>::const_eval_resolve_for_typeck
  46:     0x7f9136f757ce - <rustc_next_trait_solver[27cbda06e73fd737]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[ea8a7ce842b72f6e]::solve::delegate::SolverDelegate, rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>>::evaluate_const
  47:     0x7f9136f4f4b5 - <rustc_next_trait_solver[27cbda06e73fd737]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[ea8a7ce842b72f6e]::solve::delegate::SolverDelegate, rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>>::evaluate_const_and_instantiate_projection_term
  48:     0x7f9137be0c66 - <rustc_type_ir[965dd6bba98007bf]::search_graph::SearchGraph<rustc_next_trait_solver[27cbda06e73fd737]::solve::search_graph::SearchGraphDelegate<rustc_trait_selection[ea8a7ce842b72f6e]::solve::delegate::SolverDelegate>, rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>>::evaluate_goal::{closure#0}
  49:     0x7f9137b3e4fc - <rustc_next_trait_solver[27cbda06e73fd737]::solve::eval_ctxt::EvalCtxt<rustc_trait_selection[ea8a7ce842b72f6e]::solve::delegate::SolverDelegate, rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>>::evaluate_goal_no_fast_paths
  50:     0x7f9137b345ef - <rustc_trait_selection[ea8a7ce842b72f6e]::solve::delegate::SolverDelegate as rustc_next_trait_solver[27cbda06e73fd737]::solve::eval_ctxt::SolverDelegateEvalExt>::evaluate_root_goal
  51:     0x7f9137b31850 - <rustc_next_trait_solver[27cbda06e73fd737]::normalize::NormalizationFolder<rustc_infer[15a335c746cd3e55]::infer::InferCtxt, rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt, rustc_trait_selection[ea8a7ce842b72f6e]::solve::normalize::normalize_with_universes<rustc_type_ir[965dd6bba98007bf]::predicate::TraitRef<rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>>::{closure#0}>>::normalize_alias_term
  52:     0x7f91385eb462 - <rustc_next_trait_solver[27cbda06e73fd737]::normalize::NormalizationFolder<rustc_infer[15a335c746cd3e55]::infer::InferCtxt, rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt, rustc_trait_selection[ea8a7ce842b72f6e]::solve::normalize::normalize_with_universes<rustc_middle[edb1849eae9467f6]::ty::predicate::Clause>::{closure#0}> as rustc_type_ir[965dd6bba98007bf]::fold::FallibleTypeFolder<rustc_middle[edb1849eae9467f6]::ty::context::TyCtxt>>::try_fold_const::{closure#0}
  53:     0x7f91373ba425 - <rustc_trait_selection[ea8a7ce842b72f6e]::traits::engine::ObligationCtxt<rustc_trait_selection[ea8a7ce842b72f6e]::traits::FulfillmentError>>::normalize::<rustc_middle[edb1849eae9467f6]::ty::predicate::Clause>
  54:     0x7f91380f116a - <rustc_hir_analysis[8e11bc5810050387]::check::wfcheck::WfCheckingCtxt>::normalize::<rustc_middle[edb1849eae9467f6]::ty::predicate::Clause>
  55:     0x7f91372c8cd3 - rustc_hir_analysis[8e11bc5810050387]::check::wfcheck::check_where_clauses
  56:     0x7f91373c9ce1 - rustc_hir_analysis[8e11bc5810050387]::check::wfcheck::check_type_defn
  57:     0x7f9137e15b47 - rustc_hir_analysis[8e11bc5810050387]::check::check::check_item_type
  58:     0x7f9137e0eef0 - rustc_hir_analysis[8e11bc5810050387]::check::wfcheck::check_well_formed
  59:     0x7f9137e0eed1 - rustc_query_impl[fae7975a325c58a]::query_vtables::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  60:     0x7f9137e0e2f2 - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_data_structures[5e8f17b1399e428]::vec_cache::VecCache<rustc_span[283b8a40713b5525]::def_id::LocalDefId, rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[edb1849eae9467f6]::dep_graph::graph::DepNodeIndex>, false>
  61:     0x7f9137e0e071 - rustc_query_impl[fae7975a325c58a]::query_vtables::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
  62:     0x7f9137e0b602 - rustc_hir_analysis[8e11bc5810050387]::check::wfcheck::check_type_wf
  63:     0x7f9137e0b4e7 - rustc_query_impl[fae7975a325c58a]::query_vtables::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  64:     0x7f9137d2f01e - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_middle[edb1849eae9467f6]::query::caches::SingleCache<rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 1usize]>>, false>
  65:     0x7f9137d2ee13 - rustc_query_impl[fae7975a325c58a]::query_vtables::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
  66:     0x7f91373be01a - rustc_hir_analysis[8e11bc5810050387]::check_crate
  67:     0x7f913710da79 - rustc_interface[ca548c749891158c]::passes::analysis
  68:     0x7f9137d34c87 - rustc_query_impl[fae7975a325c58a]::execution::try_execute_query::<rustc_middle[edb1849eae9467f6]::query::caches::SingleCache<rustc_middle[edb1849eae9467f6]::query::erase::ErasedData<[u8; 0usize]>>, false>
  69:     0x7f9137d34a2f - rustc_query_impl[fae7975a325c58a]::query_vtables::analysis::execute_query_non_incr::__rust_end_short_backtrace
  70:     0x7f9137fb876f - rustc_interface[ca548c749891158c]::interface::run_compiler::<(), rustc_driver_impl[936d1968ef2b2e3d]::run_compiler::{closure#0}>::{closure#2}
  71:     0x7f9137ff6682 - std[37e96c38c503a63a]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[ca548c749891158c]::util::run_in_thread_with_globals<rustc_interface[ca548c749891158c]::util::run_in_thread_pool_with_globals<rustc_interface[ca548c749891158c]::interface::run_compiler<(), rustc_driver_impl[936d1968ef2b2e3d]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  72:     0x7f9137ff642d - <std[37e96c38c503a63a]::thread::lifecycle::spawn_unchecked<rustc_interface[ca548c749891158c]::util::run_in_thread_with_globals<rustc_interface[ca548c749891158c]::util::run_in_thread_pool_with_globals<rustc_interface[ca548c749891158c]::interface::run_compiler<(), rustc_driver_impl[936d1968ef2b2e3d]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[2372320f1a8e1214]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  73:     0x7f913800121b - <std[37e96c38c503a63a]::sys::thread::unix::Thread>::new::thread_start
  74:     0x7f9130c980a2 - <unknown>
  75:     0x7f9130d2080c - <unknown>
  76:                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 1.100.0-nightly (76af58f66 2026-09-17) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_built] building MIR for `T::{constant#0}`
#1 [trivial_const] checking if `T::{constant#0}` is a trivial const
#2 [eval_to_allocation_raw] const-evaluating + checking `T::{constant#0}`
#3 [eval_to_allocation_raw] const-evaluating + checking `T::{constant#0}`
#4 [eval_to_valtree] evaluating type-level constant
#5 [check_well_formed] checking that `T` is well-formed
#6 [check_type_wf] checking that types are well-formed
#7 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 2 previous errors; 1 warning emitted

For more information about this error, try `rustc --explain E0601`.

@rustbot label +F-generic_const_args +F-min_generic_const_args

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Research direction

Reproduce the reduced example with the nightly rustc command shown, then inspect compiler/rustc_mir_build/src/builder/expr/as_constant.rs around line 97 and the listed MIR-building stack frames. Trace how the generic-const enum expression reaches the unreachable branch. Done means the example no longer triggers an internal compiler panic and instead produces an appropriate compiler result or diagnostic.

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
Active
Clarity
Mostly clear
Newbie friendliness
48/100

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