ICE: abi: index out of bounds (`offsets[FieldIdx::new(i)]`)

[matthiaskrgr]

snippet:

#![feature(min_generic_const_args, adt_const_params)]
use std::marker::ConstParamTy;
#[derive(Eq, PartialEq, ConstParamTy)]
struct Inner(u32);
#[derive(Eq, PartialEq, ConstParamTy)]
enum Container<T> {
    Wrap(T),
}
fn with_container<const C: Container<Inner>>() -> Container<Inner> {
    C
}
fn test<const N: u32>() {
    with_container::<{ Container::Wrap::<Inner>(<Option<u32>>::Some(const { 2 })) }>();
}
fn main() {
    test::<5>()
}

Version information

rustc 1.94.0-nightly (bd33b83cf 2026-01-05)
binary: rustc
commit-hash: bd33b83cfdbac9bffa3b04aaef95ec97827909a9
commit-date: 2026-01-05
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

Possibly related line of code:

rust/compiler/rustc_abi/src/lib.rs

Lines 1670 to 1682 in bd33b83

	}
	FieldsShape::Array { stride, count } => {
	let i = u64::try_from(i).unwrap();
	assert!(i < count, "tried to access field {i} of array with {count} fields");
	stride * i
	}
	FieldsShape::Arbitrary { ref offsets, .. } => offsets[FieldIdx::new(i)],
	}
	}
	/// Gets source indices of the fields by increasing offsets.
	#[inline]
	pub fn index_by_increasing_offset(&self) -> impl ExactSizeIterator<Item = usize> {

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

Program output

warning: the feature `min_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.0zyhRWa0bgY0/rustc_testrunner_tmpdir_reporting.ktlScgbeV7Bs/mvce.rs:1:12
  |
1 | #![feature(min_generic_const_args, adt_const_params)]
  |            ^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information
  = note: `#[warn(incomplete_features)]` on by default


thread 'rustc' (1885864) panicked at /rustc-dev/bd33b83cfdbac9bffa3b04aaef95ec97827909a9/compiler/rustc_abi/src/lib.rs:1676:66:
index out of bounds: the len is 1 but the index is 1
stack backtrace:
   0:     0x7f00c8020fc3 - <<std[6810ad95c09a1f38]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[8c39a0108b8da36b]::fmt::Display>::fmt
   1:     0x7f00c860e348 - core[8c39a0108b8da36b]::fmt::write
   2:     0x7f00c8037246 - <std[6810ad95c09a1f38]::sys::stdio::unix::Stderr as std[6810ad95c09a1f38]::io::Write>::write_fmt
   3:     0x7f00c7ff6df8 - std[6810ad95c09a1f38]::panicking::default_hook::{closure#0}
   4:     0x7f00c8014853 - std[6810ad95c09a1f38]::panicking::default_hook
   5:     0x7f00c6ffb1da - std[6810ad95c09a1f38]::panicking::update_hook::<alloc[c78df5de1e68b592]::boxed::Box<rustc_driver_impl[4e21432d7c76ef67]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f00c8014b32 - std[6810ad95c09a1f38]::panicking::panic_with_hook
   7:     0x7f00c7ff6eb8 - std[6810ad95c09a1f38]::panicking::panic_handler::{closure#0}
   8:     0x7f00c7fedb79 - std[6810ad95c09a1f38]::sys::backtrace::__rust_end_short_backtrace::<std[6810ad95c09a1f38]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f00c7ff898d - __rustc[a475064d16802d7f]::rust_begin_unwind
  10:     0x7f00c5158fec - core[8c39a0108b8da36b]::panicking::panic_fmt
  11:     0x7f00c4ffa553 - core[8c39a0108b8da36b]::panicking::panic_bounds_check
  12:     0x7f00ca4094cd - <rustc_const_eval[6fc806a0c4979697]::interpret::eval_context::InterpCx<rustc_const_eval[6fc806a0c4979697]::const_eval::machine::CompileTimeMachine>>::project_field::<rustc_const_eval[6fc806a0c4979697]::interpret::place::MPlaceTy>.cold
  13:     0x7f00c8b2fbda - rustc_const_eval[6fc806a0c4979697]::const_eval::valtrees::valtree_into_mplace
  14:     0x7f00c8b2f90e - rustc_const_eval[6fc806a0c4979697]::const_eval::valtrees::valtree_into_mplace
  15:     0x7f00c96f295c - rustc_const_eval[6fc806a0c4979697]::const_eval::valtrees::valtree_to_const_value
  16:     0x7f00c96f232b - <rustc_const_eval[6fc806a0c4979697]::provide::{closure#1} as core[8c39a0108b8da36b]::ops::function::FnOnce<(rustc_middle[42237da0a4c7601]::ty::context::TyCtxt, rustc_middle[42237da0a4c7601]::ty::consts::valtree::Value)>>::call_once
  17:     0x7f00c96f22d6 - rustc_query_impl[4d93b3b10c7f5167]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[4d93b3b10c7f5167]::query_impl::valtree_to_const_val::dynamic_query::{closure#2}::{closure#0}, rustc_middle[42237da0a4c7601]::query::erase::Erased<[u8; 24usize]>>
  18:     0x7f00c96f229f - <rustc_query_impl[4d93b3b10c7f5167]::query_impl::valtree_to_const_val::dynamic_query::{closure#2} as core[8c39a0108b8da36b]::ops::function::FnOnce<(rustc_middle[42237da0a4c7601]::ty::context::TyCtxt, rustc_middle[42237da0a4c7601]::ty::consts::valtree::Value)>>::call_once
  19:     0x7f00c96f18cc - rustc_query_system[747dc01c13837c3b]::query::plumbing::try_execute_query::<rustc_query_impl[4d93b3b10c7f5167]::DynamicConfig<rustc_query_system[747dc01c13837c3b]::query::caches::DefaultCache<rustc_middle[42237da0a4c7601]::ty::consts::valtree::Value, rustc_middle[42237da0a4c7601]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[4d93b3b10c7f5167]::plumbing::QueryCtxt, false>
  20:     0x7f00c96f15e5 - rustc_query_impl[4d93b3b10c7f5167]::query_impl::valtree_to_const_val::get_query_non_incr::__rust_end_short_backtrace
  21:     0x7f00c90aed87 - rustc_monomorphize[df187732e26b04b8]::collector::items_of_instance
  22:     0x7f00c90aaff2 - rustc_query_impl[4d93b3b10c7f5167]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[4d93b3b10c7f5167]::query_impl::items_of_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[42237da0a4c7601]::query::erase::Erased<[u8; 32usize]>>
  23:     0x7f00c92c0b95 - rustc_query_system[747dc01c13837c3b]::query::plumbing::try_execute_query::<rustc_query_impl[4d93b3b10c7f5167]::DynamicConfig<rustc_query_system[747dc01c13837c3b]::query::caches::DefaultCache<(rustc_middle[42237da0a4c7601]::ty::instance::Instance, rustc_middle[42237da0a4c7601]::mir::mono::CollectionMode), rustc_middle[42237da0a4c7601]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[4d93b3b10c7f5167]::plumbing::QueryCtxt, false>
  24:     0x7f00c92c07b3 - rustc_query_impl[4d93b3b10c7f5167]::query_impl::items_of_instance::get_query_non_incr::__rust_end_short_backtrace
  25:     0x7f00c9adaf54 - rustc_monomorphize[df187732e26b04b8]::collector::collect_items_rec
  26:     0x7f00c9adcea7 - rustc_monomorphize[df187732e26b04b8]::collector::collect_items_rec
  27:     0x7f00c9adcea7 - rustc_monomorphize[df187732e26b04b8]::collector::collect_items_rec
  28:     0x7f00c8abb3ed - rustc_monomorphize[df187732e26b04b8]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  29:     0x7f00c8e822ba - rustc_monomorphize[df187732e26b04b8]::partitioning::collect_and_partition_mono_items
  30:     0x7f00c9a28764 - rustc_query_impl[4d93b3b10c7f5167]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[4d93b3b10c7f5167]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[42237da0a4c7601]::query::erase::Erased<[u8; 24usize]>>
  31:     0x7f00c9a28749 - <rustc_query_impl[4d93b3b10c7f5167]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[8c39a0108b8da36b]::ops::function::FnOnce<(rustc_middle[42237da0a4c7601]::ty::context::TyCtxt, ())>>::call_once
  32:     0x7f00c9a2829e - rustc_query_system[747dc01c13837c3b]::query::plumbing::try_execute_query::<rustc_query_impl[4d93b3b10c7f5167]::DynamicConfig<rustc_query_system[747dc01c13837c3b]::query::caches::SingleCache<rustc_middle[42237da0a4c7601]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[4d93b3b10c7f5167]::plumbing::QueryCtxt, false>
  33:     0x7f00c9a28043 - rustc_query_impl[4d93b3b10c7f5167]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
  34:     0x7f00c9a2fa20 - rustc_codegen_ssa[e1295ddd32952513]::base::codegen_crate::<rustc_codegen_llvm[443ddb3aab83af46]::LlvmCodegenBackend>
  35:     0x7f00c9a2d4fd - <rustc_codegen_llvm[443ddb3aab83af46]::LlvmCodegenBackend as rustc_codegen_ssa[e1295ddd32952513]::traits::backend::CodegenBackend>::codegen_crate
  36:     0x7f00c996b40f - <rustc_interface[5d1daf306103d6d4]::queries::Linker>::codegen_and_build_linker
  37:     0x7f00c9968a1c - <rustc_interface[5d1daf306103d6d4]::passes::create_and_enter_global_ctxt<core[8c39a0108b8da36b]::option::Option<rustc_interface[5d1daf306103d6d4]::queries::Linker>, rustc_driver_impl[4e21432d7c76ef67]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[8c39a0108b8da36b]::ops::function::FnOnce<(&rustc_session[b55a0c80c17f87]::session::Session, rustc_middle[42237da0a4c7601]::ty::context::CurrentGcx, alloc[c78df5de1e68b592]::sync::Arc<rustc_data_structures[b6e1d723e783a0f9]::jobserver::Proxy>, &std[6810ad95c09a1f38]::sync::once_lock::OnceLock<rustc_middle[42237da0a4c7601]::ty::context::GlobalCtxt>, &rustc_data_structures[b6e1d723e783a0f9]::sync::worker_local::WorkerLocal<rustc_middle[42237da0a4c7601]::arena::Arena>, &rustc_data_structures[b6e1d723e783a0f9]::sync::worker_local::WorkerLocal<rustc_hir[570d9ac2d5177df8]::Arena>, rustc_driver_impl[4e21432d7c76ef67]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  38:     0x7f00c98140d8 - rustc_interface[5d1daf306103d6d4]::interface::run_compiler::<(), rustc_driver_impl[4e21432d7c76ef67]::run_compiler::{closure#0}>::{closure#1}
  39:     0x7f00c978cbce - std[6810ad95c09a1f38]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5d1daf306103d6d4]::util::run_in_thread_with_globals<rustc_interface[5d1daf306103d6d4]::util::run_in_thread_pool_with_globals<rustc_interface[5d1daf306103d6d4]::interface::run_compiler<(), rustc_driver_impl[4e21432d7c76ef67]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  40:     0x7f00c978c9a0 - <std[6810ad95c09a1f38]::thread::lifecycle::spawn_unchecked<rustc_interface[5d1daf306103d6d4]::util::run_in_thread_with_globals<rustc_interface[5d1daf306103d6d4]::util::run_in_thread_pool_with_globals<rustc_interface[5d1daf306103d6d4]::interface::run_compiler<(), rustc_driver_impl[4e21432d7c76ef67]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[8c39a0108b8da36b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  41:     0x7f00c978e744 - <std[6810ad95c09a1f38]::sys::thread::unix::Thread>::new::thread_start
  42:     0x7f00c309698b - <unknown>
  43:     0x7f00c311a9cc - <unknown>
  44:                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 1.94.0-nightly (bd33b83cf 2026-01-05) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [valtree_to_const_val] converting type-level constant value to MIR constant value
#1 [items_of_instance] collecting items used by `with_container::<ValTree(Branch([ValTree(Leaf(0x00000000): u32), ValTree(Branch([ValTree(Leaf(0x00000001): u32), ValTree(Leaf(0x00000002): u32)]): core::option::Option::<u32>)]): Container<Inner>)>`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
warning: 1 warning emitted

@rustbot label +F-min_generic_const_args +F-adt_const_params

[enthropy7]

FieldsShape::Arbitrary { ref offsets, .. } => offsets[FieldIdx::new(i)],
FieldsShape::Arbitrary { in_memory_order, .. } => in_memory_order[i as u32].index(), need to check bounds here i think. its the simplest solution that i found, but as a newcomer i don't know can we do smth like that in code

[annnointted]

The error mentions FieldIdx::new(i) being called with i=1. Which data structure's field layout is being accessed (likely Container or Inner), and why does the compiler think it has at least 2 fields when it should know it only has 1?