[ICE]: generic_const_exprs with async and closure

Code

#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

use core::marker::PhantomData;

fn main() {
    embassy_futures::block_on(async {
        let mut rrb_big: RRBTree<u64> = RRBTree {
            root: Node::Leaf(PhantomData),
        };
        let _rrb_small = rrb_big.split_at().await;
    });
}

struct RRBTree<T> {
    root: Node<T>,
}

#[allow(unused)]
enum Node<T> {
    Leaf(PhantomData<T>),
    Inner(Vec<Node<T>>),
}

impl<T> RRBTree<T>
where
    [(); size_of::<T>()]:,
{
    async fn split_at(&mut self) -> Self {
        let mut skips = heapless::Vec::<(usize, usize), { size_of::<T>() }>::new();
        let mut search = |_children: &Vec<Node<T>>| {
            let _ = skips.push((42, 42));
            42
        };
        let mut stack = heapless::Vec::<&Node<T>, { size_of::<T>() }>::new();
        let _ = stack.push(&self.root);
        while let Some(Node::Inner(inner)) = stack.last() {
            let child = &inner[search(inner)];
            let _ = stack.push(child);
        }
        todo!()
    }
}

Meta

rustc --version --verbose:

rustc 1.100.0-nightly (4b6d04e70 2026-09-13)
binary: rustc
commit-hash: 4b6d04e706108ccfeafe2547fbe857dfe8972bad
commit-date: 2026-09-13
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Error output

warning: `feature(generic_const_exprs)` is not supported with the next-generation trait solver
 --> src/main.rs:2:12
  |
2 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: `-Znext-solver=globally` is currently enabled by default for testing
  = note: reverted the setting to `-Znext-solver=coherence` for this crate
  = note: the currently stable trait solver will be used for this crate
  = note: see issues #160895 <https://github.com/rust-lang/rust/issues/160895> for more information


thread 'rustc' (352168) panicked at /rustc-dev/4b6d04e706108ccfeafe2547fbe857dfe8972bad/compiler/rustc_type_ir/src/binder.rs:790:9:
type parameter `/#4` (/#4/4) out of range when instantiating, args=[u64, i16, Binder { value: extern "RustCall" fn((&'^0 std::vec::Vec<Node<u64>, std::alloc::Global>,)) -> usize, bound_vars: [Region(BrAnon)] }, (&'{erased} mut heapless::vec::VecInner<(usize, usize), usize, heapless::vec::storage::VecStorageInner<[std::mem::MaybeUninit<(usize, usize)>; 8_usize]>>,)]
stack backtrace:
   0:     0x7f126ee54e46 - <<std[552422bcccb5c833]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[1b0f708be8c133d1]::fmt::Display>::fmt
   1:     0x7f126f40984f - core[1b0f708be8c133d1]::fmt::write
   2:     0x7f126ee69e1c - <std[552422bcccb5c833]::sys::stdio::unix::Stderr as core[1b0f708be8c133d1]::io::write::Write>::write_fmt
   3:     0x7f126ee27b1a - std[552422bcccb5c833]::panicking::default_hook::{closure#0}
   4:     0x7f126ee495a3 - std[552422bcccb5c833]::panicking::default_hook
   5:     0x7f126db7631f - std[552422bcccb5c833]::panicking::update_hook::<alloc[8d16d54ddffdc8a3]::boxed::Box<rustc_driver_impl[b59e3ffdd9ae4470]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f126ee49a42 - std[552422bcccb5c833]::panicking::panic_with_hook
   7:     0x7f126ee27bd2 - std[552422bcccb5c833]::panicking::panic_handler::{closure#0}
   8:     0x7f126ee1fc59 - std[552422bcccb5c833]::sys::backtrace::__rust_end_short_backtrace::<std[552422bcccb5c833]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f126ee297ad - __rustc[20add320cf717b90]::rust_begin_unwind
  10:     0x7f126b90c3dc - core[1b0f708be8c133d1]::panicking::panic_fmt
  11:     0x7f126e25cfd5 - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::type_param_out_of_range
  12:     0x7f126f7e38b0 - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt> as rustc_type_ir[577d48379558a620]::fold::TypeFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_ty
  13:     0x7f126f7e9b40 - <&rustc_middle[dad200052675e81d]::ty::list::RawList<(), rustc_middle[dad200052675e81d]::ty::generic_args::GenericArg> as rustc_type_ir[577d48379558a620]::fold::TypeFoldable<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_with::<rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>
  14:     0x7f126f7de3ac - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt> as rustc_type_ir[577d48379558a620]::fold::TypeFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_const
  15:     0x7f126f7e17b9 - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt> as rustc_type_ir[577d48379558a620]::fold::TypeFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_ty
  16:     0x7f126f7ded03 - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt> as rustc_type_ir[577d48379558a620]::fold::TypeFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_ty
  17:     0x7f126f7df77e - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt> as rustc_type_ir[577d48379558a620]::fold::TypeFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_ty
  18:     0x7f126f7deae8 - <rustc_type_ir[577d48379558a620]::binder::ArgFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt> as rustc_type_ir[577d48379558a620]::fold::TypeFolder<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::fold_ty
  19:     0x7f126f7ed91c - <rustc_middle[dad200052675e81d]::mir::Body as rustc_type_ir[577d48379558a620]::visit::TypeVisitable<rustc_middle[dad200052675e81d]::ty::context::TyCtxt>>::visit_with::<rustc_monomorphize[b66db2de85bc634c]::collector::check_normalization_error::NormalizationChecker>
  20:     0x7f126f7d3fcb - rustc_monomorphize[b66db2de85bc634c]::collector::items_of_instance
  21:     0x7f126f7d3f11 - rustc_query_impl[be4bf2c2bc2163e7]::query_vtables::items_of_instance::invoke_provider_fn::__rust_begin_short_backtrace
  22:     0x7f1270323ec1 - rustc_query_impl[be4bf2c2bc2163e7]::execution::try_execute_query::<rustc_middle[dad200052675e81d]::query::caches::DefaultCache<(rustc_middle[dad200052675e81d]::ty::instance::Instance, rustc_middle[dad200052675e81d]::mono::CollectionMode), rustc_middle[dad200052675e81d]::query::erase::ErasedData<[u8; 32usize]>>, true>
  23:     0x7f12703234e1 - rustc_query_impl[be4bf2c2bc2163e7]::query_vtables::items_of_instance::execute_query_incr::__rust_end_short_backtrace
  24:     0x7f1270495d2c - rustc_monomorphize[b66db2de85bc634c]::collector::collect_items_rec
  25:     0x7f127049aacf - rustc_monomorphize[b66db2de85bc634c]::collector::collect_items_rec
  26:     0x7f127049aacf - rustc_monomorphize[b66db2de85bc634c]::collector::collect_items_rec
  27:     0x7f127049aacf - rustc_monomorphize[b66db2de85bc634c]::collector::collect_items_rec
  28:     0x7f127049aacf - rustc_monomorphize[b66db2de85bc634c]::collector::collect_items_rec
  29:     0x7f127049cfb4 - rustc_monomorphize[b66db2de85bc634c]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  30:     0x7f127049d8e3 - rustc_monomorphize[b66db2de85bc634c]::partitioning::collect_and_partition_mono_items
  31:     0x7f127048cf16 - rustc_query_impl[be4bf2c2bc2163e7]::query_vtables::collect_and_partition_mono_items::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7f1270c50c51 - rustc_query_impl[be4bf2c2bc2163e7]::execution::try_execute_query::<rustc_middle[dad200052675e81d]::query::caches::SingleCache<rustc_middle[dad200052675e81d]::query::erase::ErasedData<[u8; 24usize]>>, true>
  33:     0x7f1270c506aa - rustc_query_impl[be4bf2c2bc2163e7]::query_vtables::collect_and_partition_mono_items::execute_query_incr::__rust_end_short_backtrace
  34:     0x7f12707da932 - rustc_codegen_ssa[ef473a769a018e8c]::base::codegen_crate::<rustc_codegen_llvm[b36ae924e28f7bfb]::LlvmCodegenBackend, rustc_codegen_llvm[b36ae924e28f7bfb]::ModuleLlvm>
  35:     0x7f12707da6ee - <rustc_codegen_llvm[b36ae924e28f7bfb]::LlvmCodegenBackend as rustc_codegen_ssa[ef473a769a018e8c]::traits::backend::CodegenBackend>::codegen_crate
  36:     0x7f1270761abe - <rustc_interface[b88f436b63b54bbb]::queries::Linker>::codegen_and_build_linker
  37:     0x7f1270759089 - rustc_interface[b88f436b63b54bbb]::interface::run_compiler::<(), rustc_driver_impl[b59e3ffdd9ae4470]::run_compiler::{closure#0}>::{closure#2}
  38:     0x7f12707b6724 - std[552422bcccb5c833]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[b88f436b63b54bbb]::util::run_in_thread_with_globals<rustc_interface[b88f436b63b54bbb]::util::run_in_thread_pool_with_globals<rustc_interface[b88f436b63b54bbb]::interface::run_compiler<(), rustc_driver_impl[b59e3ffdd9ae4470]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  39:     0x7f12707b64cb - <std[552422bcccb5c833]::thread::lifecycle::spawn_unchecked<rustc_interface[b88f436b63b54bbb]::util::run_in_thread_with_globals<rustc_interface[b88f436b63b54bbb]::util::run_in_thread_pool_with_globals<rustc_interface[b88f436b63b54bbb]::interface::run_compiler<(), rustc_driver_impl[b59e3ffdd9ae4470]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[1b0f708be8c133d1]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  40:     0x7f12707bf8db - <std[552422bcccb5c833]::sys::thread::unix::Thread>::new::thread_start
  41:     0x7f12696980a2 - <unknown>
  42:     0x7f126972080c - <unknown>
  43:                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: please attach the file at `/home/val/Coding/These/swap-cache/minimal-bug-repro/rustc-ice-2026-09-14T16_57_44-352166.txt` to your bug report

note: rustc 1.100.0-nightly (4b6d04e70 2026-09-13) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [items_of_instance] collecting items used by `<impl at src/main.rs:25:1: 27:27>::split_at::{closure#0}::{closure#0}`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
warning: `minimal-bug-repro` (bin "minimal-bug-repro") generated 1 warning
error: could not compile `minimal-bug-repro` (bin "minimal-bug-repro"); 1 warning emitted

Caused by:
  process didn't exit successfully: `/home/val/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name minimal_bug_repro --edition=2024 src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat,unused-externs-silent --diagnostic-width=148 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=24d4a92d61be5191 --out-dir /home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/minimal-bug-repro/71d5c43cd887a385/out -C incremental=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/incremental -L dependency=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/byteorder/7b2246451a749927/out -L dependency=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/hash32/f249c69d723a1c9e/out -L dependency=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/stable_deref_trait/d13fd163da2fc199/out --extern embassy_futures=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/embassy-futures/3dcc6f905aaa095b/out/libembassy_futures-3dcc6f905aaa095b.rlib --extern embassy_futures=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/embassy-futures/3dcc6f905aaa095b/out/libembassy_futures-3dcc6f905aaa095b.rmeta --extern heapless=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/heapless/59f5e7b6bdbb98f9/out/libheapless-59f5e7b6bdbb98f9.rlib --extern heapless=/home/val/Coding/These/swap-cache/minimal-bug-repro/target/debug/build/heapless/59f5e7b6bdbb98f9/out/libheapless-59f5e7b6bdbb98f9.rmeta --force-warn=unused_crate_dependencies` (exit status: 101)

stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>::type_param_out_of_range
   3: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
   4: <&rustc_middle::ty::list::RawList<(), rustc_middle::ty::generic_args::GenericArg> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::fold_with::<rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt>>
   5: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_const
   6: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
   7: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
   8: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
   9: <rustc_type_ir::binder::ArgFolder<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
  10: <rustc_middle::mir::Body as rustc_type_ir::visit::TypeVisitable<rustc_middle::ty::context::TyCtxt>>::visit_with::<rustc_monomorphize::collector::check_normalization_error::NormalizationChecker>
  11: rustc_monomorphize::collector::items_of_instance
      [... omitted 1 frame ...]
  12: rustc_monomorphize::collector::collect_items_rec
  13: rustc_monomorphize::collector::collect_items_rec
  14: rustc_monomorphize::collector::collect_items_rec
  15: rustc_monomorphize::collector::collect_items_rec
  16: rustc_monomorphize::collector::collect_items_rec
  17: rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  18: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 1 frame ...]
  19: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend, rustc_codegen_llvm::ModuleLlvm>
  20: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  21: <rustc_interface::queries::Linker>::codegen_and_build_linker
  22: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#2}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Start by building the reproducer in src/main.rs and confirm the ICE, then trace the reported panic at rustc_type_ir/src/binder.rs:790 and the items_of_instance query in rustc_monomorphize. Compare the generic_const_exprs, async, and closure portions of the example while narrowing the failing compiler path. Done means this case produces a diagnostic or compiles without an internal compiler error.