rust-lang / rust-lang/rust

[ICE]: Const default value for generic arg with `generic_const_exprs` enabled

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C-bug F-generic_const_exprs I-ICE T-compiler
Dominant language
Rust
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PR metrics pending

Description

Code
#![feature(generic_const_exprs)]

pub const DEFAULT: usize = 1;

pub struct Foo<const N: usize = DEFAULT>
where
    [(); N]:;

Any of the following avoid the ICE:

  • Remove #![feature(generic_const_exprs)]
  • Remove the default value for N
  • Use a literal for the default value.
Meta

rustc +nightly --version --verbose:

rustc 1.97.0-nightly (e95e73209 2026-05-05)
binary: rustc
commit-hash: e95e73209faf6ead2bc5c7636e45e589a751b79b
commit-date: 2026-05-05
host: aarch64-apple-darwin
release: 1.97.0-nightly
LLVM version: 22.1.4
Error output
thread 'rustc' (20220379) panicked at /rustc-dev/e95e73209faf6ead2bc5c7636e45e589a751b79b/compiler/rustc_type_ir/src/binder.rs:650:13:
Binder { value: WellFormed(Term::Ty([(); N/#0])), bound_vars: [] } has parameters, but no args were provided in instantiate
Backtrace 

thread 'rustc' panicked at /rustc-dev/e95e73209faf6ead2bc5c7636e45e589a751b79b/compiler/rustc_type_ir/src/binder.rs:650:13:
Binder { value: WellFormed(Term::Ty([(); N/#0])), bound_vars: [] } has parameters, but no args were provided in instantiate
stack backtrace:
   0:        0x11232ab7c - <std[ba95dd84dcb40fd7]::backtrace::Backtrace>::create
   1:        0x110071354 - std[ba95dd84dcb40fd7]::panicking::update_hook::<alloc[ec32cc677de768a9]::boxed::Box<rustc_driver_impl[75493f1677804ab4]::install_ice_hook::{closure#1}>>::{closure#0}
   2:        0x11233c204 - std[ba95dd84dcb40fd7]::panicking::panic_with_hook
   3:        0x112320194 - std[ba95dd84dcb40fd7]::panicking::panic_handler::{closure#0}
   4:        0x112316b2c - std[ba95dd84dcb40fd7]::sys::backtrace::__rust_end_short_backtrace::<std[ba95dd84dcb40fd7]::panicking::panic_handler::{closure#0}, !>
   5:        0x112321768 - __rustc[2d5b6cc7a58c455b]::rust_begin_unwind
   6:        0x11535d7dc - core[c57fb36d3ad3ee2e]::panicking::panic_fmt
   7:        0x110e11ae8 - <rustc_middle[ab6b1a3552af91dc]::ty::generics::GenericPredicates>::instantiate_into
   8:        0x111efc738 - <rustc_trait_selection[9d548477726ada75]::traits::wf::WfPredicates>::nominal_obligations
   9:        0x11206a980 - <rustc_trait_selection[9d548477726ada75]::traits::wf::WfPredicates as rustc_type_ir[c95de3f73f8a6ff6]::visit::TypeVisitor<rustc_middle[ab6b1a3552af91dc]::ty::context::TyCtxt>>::visit_const
  10:        0x111fab828 - rustc_trait_selection[9d548477726ada75]::traits::wf::obligations
  11:        0x1120057f4 - <rustc_trait_selection[9d548477726ada75]::traits::fulfill::FulfillProcessor as rustc_data_structures[35118fcc2dc17860]::obligation_forest::ObligationProcessor>::process_obligation
  12:        0x111da83d8 - <rustc_data_structures[35118fcc2dc17860]::obligation_forest::ObligationForest<rustc_trait_selection[9d548477726ada75]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[9d548477726ada75]::traits::fulfill::FulfillProcessor>
  13:        0x111ff45e4 - <rustc_trait_selection[9d548477726ada75]::traits::fulfill::FulfillmentContext<rustc_trait_selection[9d548477726ada75]::traits::FulfillmentError> as rustc_infer[b3b270f64b54bdb1]::traits::engine::TraitEngine<rustc_trait_selection[9d548477726ada75]::traits::FulfillmentError>>::try_evaluate_obligations
  14:        0x11208c2b0 - <rustc_trait_selection[9d548477726ada75]::traits::fulfill::FulfillmentContext<rustc_trait_selection[9d548477726ada75]::traits::FulfillmentError> as rustc_infer[b3b270f64b54bdb1]::traits::engine::TraitEngine<rustc_trait_selection[9d548477726ada75]::traits::FulfillmentError>>::evaluate_obligations_error_on_ambiguity
  15:        0x1104313c0 - rustc_hir_analysis[ff46227b1a672aeb]::check::wfcheck::check_type_defn
  16:        0x110419858 - rustc_hir_analysis[ff46227b1a672aeb]::check::check::check_item_type
  17:        0x110435dc8 - rustc_hir_analysis[ff46227b1a672aeb]::check::wfcheck::check_well_formed
  18:        0x1118b1814 - rustc_query_impl[58a30886da8d1cc3]::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
  19:        0x1116bc5dc - rustc_query_impl[58a30886da8d1cc3]::execution::try_execute_query::<rustc_data_structures[35118fcc2dc17860]::vec_cache::VecCache<rustc_span[f43e87c4afd786b9]::def_id::LocalDefId, rustc_middle[ab6b1a3552af91dc]::query::erase::ErasedData<[u8; 1usize]>, rustc_middle[ab6b1a3552af91dc]::dep_graph::graph::DepNodeIndex>, true>
  20:        0x1118b179c - rustc_query_impl[58a30886da8d1cc3]::query_impl::check_well_formed::execute_query_incr::__rust_end_short_backtrace
  21:        0x11042ab10 - rustc_hir_analysis[ff46227b1a672aeb]::check::wfcheck::check_type_wf
  22:        0x1118a35cc - rustc_query_impl[58a30886da8d1cc3]::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
  23:        0x1116eb19c - rustc_query_impl[58a30886da8d1cc3]::execution::try_execute_query::<rustc_middle[ab6b1a3552af91dc]::query::caches::SingleCache<rustc_middle[ab6b1a3552af91dc]::query::erase::ErasedData<[u8; 1usize]>>, true>
  24:        0x1118a355c - rustc_query_impl[58a30886da8d1cc3]::query_impl::check_type_wf::execute_query_incr::__rust_end_short_backtrace
  25:        0x11036fb18 - rustc_hir_analysis[ff46227b1a672aeb]::check_crate
  26:        0x1109f7da4 - rustc_interface[8a26816c23be435d]::passes::analysis
  27:        0x1116e5c40 - rustc_query_impl[58a30886da8d1cc3]::execution::try_execute_query::<rustc_middle[ab6b1a3552af91dc]::query::caches::SingleCache<rustc_middle[ab6b1a3552af91dc]::query::erase::ErasedData<[u8; 0usize]>>, true>
  28:        0x1118d86d8 - rustc_query_impl[58a30886da8d1cc3]::query_impl::analysis::execute_query_incr::__rust_end_short_backtrace
  29:        0x110028660 - rustc_interface[8a26816c23be435d]::passes::create_and_enter_global_ctxt::<core[c57fb36d3ad3ee2e]::option::Option<rustc_interface[8a26816c23be435d]::queries::Linker>, rustc_driver_impl[75493f1677804ab4]::run_compiler::{closure#0}::{closure#2}>
  30:        0x1100702e4 - rustc_interface[8a26816c23be435d]::interface::run_compiler::<(), rustc_driver_impl[75493f1677804ab4]::run_compiler::{closure#0}>::{closure#1}
  31:        0x110067fc4 - std[ba95dd84dcb40fd7]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[8a26816c23be435d]::util::run_in_thread_with_globals<rustc_interface[8a26816c23be435d]::util::run_in_thread_pool_with_globals<rustc_interface[8a26816c23be435d]::interface::run_compiler<(), rustc_driver_impl[75493f1677804ab4]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:        0x110077e44 - <std[ba95dd84dcb40fd7]::thread::lifecycle::spawn_unchecked<rustc_interface[8a26816c23be435d]::util::run_in_thread_with_globals<rustc_interface[8a26816c23be435d]::util::run_in_thread_pool_with_globals<rustc_interface[8a26816c23be435d]::interface::run_compiler<(), rustc_driver_impl[75493f1677804ab4]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[c57fb36d3ad3ee2e]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:        0x112346a20 - <std[ba95dd84dcb40fd7]::sys::thread::unix::Thread>::new::thread_start
  34:        0x18db6bc58 - __pthread_cond_wait


rustc version: 1.97.0-nightly (e95e73209 2026-05-05)
platform: aarch64-apple-darwin

query stack during panic:
#0 [check_well_formed] checking that `Foo` is well-formed
#1 [check_type_wf] checking that types are well-formed
#2 [analysis] running analysis passes on crate `rust_const_generics_ice`
end of query stack

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the minimal example with nightly Rust and inspect the panic at compiler/rustc_type_ir/src/binder.rs:650, along with the check_well_formed query stack. Trace how the generic const default reaches well-formedness checking; done means this example no longer causes an internal compiler error.

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
35/100

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