ICE: `function items cannot be used as const args`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(min_generic_const_args)]
struct Foo<T, U = [u8; std::mem::size_of::<(u8, T)>]>(T, U);

fn main() {}

original:

struct Foo<T, U = [u8; std::mem::size_of::<(u8, T)>]>(T, U);
//~^ ERROR generic parameters may not be used in const operations

fn main() {}

Version information

rustc 1.89.0-nightly (32b51523f 2025-06-15)
binary: rustc
commit-hash: 32b51523f81a5f916c4bb3fee5a749721f19e01d
commit-date: 2025-06-15
host: x86_64-unknown-linux-gnu
release: 1.89.0-nightly
LLVM version: 20.1.5

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

Program output

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

warning: struct `Foo` is never constructed
 --> /tmp/icemaker_global_tempdir.JlkqZpEtGI03/rustc_testrunner_tmpdir_reporting.gQ0gZg16avyt/mvce.rs:1:8
  |
1 | struct Foo<T, U = [u8; std::mem::size_of::<(u8, T)>]>(T, U);
  |        ^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: 2 warnings emitted

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: function items cannot be used as const args
 --> /tmp/icemaker_global_tempdir.JlkqZpEtGI03/rustc_testrunner_tmpdir_reporting.gQ0gZg16avyt/mvce.rs:1:24
  |
1 | struct Foo<T, U = [u8; std::mem::size_of::<(u8, T)>]>(T, U);
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: delayed at compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs:2331:28
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, &str>
         4: <dyn rustc_hir_analysis::hir_ty_lowering::HirTyLowerer>::lower_const_arg
         5: <dyn rustc_hir_analysis::hir_ty_lowering::HirTyLowerer>::lower_ty
         6: rustc_hir_analysis::collect::type_of::type_of
         7: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
         8: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
         9: rustc_query_impl::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
        10: rustc_hir_analysis::check::wfcheck::check_where_clauses
        11: rustc_hir_analysis::check::wfcheck::check_type_defn
        12: rustc_hir_analysis::check::wfcheck::check_item
        13: rustc_hir_analysis::check::wfcheck::check_well_formed
        14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
        15: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        16: rustc_query_impl::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
        17: rustc_hir_analysis::check::wfcheck::check_type_wf
        18: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
        19: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        20: rustc_query_impl::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
        21: rustc_hir_analysis::check_crate
        22: rustc_interface::passes::analysis
        23: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
        24: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        25: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        26: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
        27: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        28: 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}, ()>
        29: <<std::thread::Builder>::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}
        30: std::sys::pal::unix::thread::Thread::new::thread_start
        31: <unknown>
        32: <unknown>
 --> /tmp/icemaker_global_tempdir.JlkqZpEtGI03/rustc_testrunner_tmpdir_reporting.gQ0gZg16avyt/mvce.rs:1:24
  |
1 | struct Foo<T, U = [u8; std::mem::size_of::<(u8, T)>]>(T, U);
  |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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 1.89.0-nightly (32b51523f 2025-06-15) running on x86_64-unknown-linux-gnu

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

query stack during panic:
end of query stack

@rustbot label +F-min_generic_const_args

[matthiaskrgr]

slightly smaller

#![feature(min_generic_const_args)]
struct Foo<T, U = [u8; size_of::<T>]>(T, U);
fn main() {}

bisects to #135186