[ICE]: generic_const_args with async and closure

But by tweeking it a bit for fun and see how the compiler handle the error and show it to the user I tried this :

#![feature(min_generic_const_args, generic_const_items, generic_const_args)]
#![allow(incomplete_features)]

use core::marker::PhantomData;

fn main() {
    embassy_futures::block_on(async {
        let mut rrb_big: RRBTree<[u8; usize::MAX]> = 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>>),
}

const N<T>: usize = 1 + size_of::<T>();

impl<T> RRBTree<T> {
    async fn split_at(&mut self) -> Self {
        let mut skips = heapless::Vec::<(usize, usize), { core::direct_const_arg!(N::<T>) }>::new();
        let mut search = |_children: &Vec<Node<T>>| {
            let _ = skips.push((42, 42));
            42
        };
        let mut stack = heapless::Vec::<&Node<T>, { core::direct_const_arg!(N::<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!()
    }
}

And got another ICE:

error[E0080]: values of the type `[u8; usize::MAX]` are too big for the target architecture
    --> /home/val/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:1463:25
     |
1463 |     const SIZE: usize = intrinsics::size_of::<Self>();
     |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `<[u8; usize::MAX] as std::mem::SizedTypeProperties>::SIZE` failed here

error: internal compiler error: /rustc-dev/574ff7d98bd6d037e5236a8453029173b32631fd/compiler/rustc_middle/src/ty/layout.rs:1401:17: `fn_abi_of_instance(VecInner::<(usize, usize), usize, heapless::vec::storage::VecStorageInner<[MaybeUninit<(usize, usize)>; {const error}]>>::new, [])` failed: Layout(ReferencesError(ErrorGuaranteed(())))
   --> /home/val/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/heapless-0.9.3/src/vec/mod.rs:320:5
    |
320 |     pub const fn new() -> Self {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (128268) panicked at /rustc-dev/574ff7d98bd6d037e5236a8453029173b32631fd/compiler/rustc_middle/src/ty/layout.rs:1401:17:
Box<dyn Any>
stack backtrace:
   0:     0x7f347ac40666 - <<std[e88b123c45551a4b]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[e3370a31d8f1c8c0]::fmt::Display>::fmt
   1:     0x7f347b20984f - core[e3370a31d8f1c8c0]::fmt::write
   2:     0x7f347ac5562c - <std[e88b123c45551a4b]::sys::stdio::unix::Stderr as core[e3370a31d8f1c8c0]::io::write::Write>::write_fmt
   3:     0x7f347ac1316a - std[e88b123c45551a4b]::panicking::default_hook::{closure#0}
   4:     0x7f347ac34c13 - std[e88b123c45551a4b]::panicking::default_hook
   5:     0x7f3479961daf - std[e88b123c45551a4b]::panicking::update_hook::<alloc[d256a4456fbff4f9]::boxed::Box<rustc_driver_impl[c5d5f53170729273]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f347ac350b2 - std[e88b123c45551a4b]::panicking::panic_with_hook
   7:     0x7f3479990ca4 - std[e88b123c45551a4b]::panicking::begin_panic::<rustc_errors[21843d9be35c0955]::ExplicitBug>::{closure#0}
   8:     0x7f3479989a36 - std[e88b123c45551a4b]::sys::backtrace::__rust_end_short_backtrace::<std[e88b123c45551a4b]::panicking::begin_panic<rustc_errors[21843d9be35c0955]::ExplicitBug>::{closure#0}, !>
   9:     0x7f347998786e - std[e88b123c45551a4b]::panicking::begin_panic::<rustc_errors[21843d9be35c0955]::ExplicitBug>
  10:     0x7f3479996da7 - <rustc_errors[21843d9be35c0955]::diagnostic::Diag<rustc_errors[21843d9be35c0955]::diagnostic::BugAbort>>::emit
  11:     0x7f3479feba6d - <rustc_errors[21843d9be35c0955]::DiagCtxtHandle>::span_bug::<rustc_span[caaced1e687b12a]::span_encoding::Span, alloc[d256a4456fbff4f9]::string::String>
  12:     0x7f347a019c46 - rustc_middle[9640d5cb0e0c5ece]::util::bug::opt_span_bug_fmt::<rustc_span[caaced1e687b12a]::span_encoding::Span>::{closure#0}
  13:     0x7f347a01a0a2 - rustc_middle[9640d5cb0e0c5ece]::ty::context::tls::with_opt::<rustc_middle[9640d5cb0e0c5ece]::util::bug::opt_span_bug_fmt<rustc_span[caaced1e687b12a]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7f347a005020 - rustc_middle[9640d5cb0e0c5ece]::ty::context::tls::with_context_opt::<rustc_middle[9640d5cb0e0c5ece]::ty::context::tls::with_opt<rustc_middle[9640d5cb0e0c5ece]::util::bug::opt_span_bug_fmt<rustc_span[caaced1e687b12a]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7f3477cf1128 - rustc_middle[9640d5cb0e0c5ece]::util::bug::span_bug_fmt::<rustc_span[caaced1e687b12a]::span_encoding::Span>
  16:     0x7f347a04f814 - rustc_middle[9640d5cb0e0c5ece]::ty::layout::codegen_handle_fn_abi_err
  17:     0x7f34771d08b7 - rustc_monomorphize[17050ecd102511f2]::mono_checks::check_mono_item
  18:     0x7f347c17ee4e - rustc_query_impl[d7850a466dfc989b]::execution::try_execute_query::<rustc_middle[9640d5cb0e0c5ece]::query::caches::DefaultCache<rustc_middle[9640d5cb0e0c5ece]::ty::instance::Instance, rustc_middle[9640d5cb0e0c5ece]::query::erase::ErasedData<[u8; 0usize]>>, true>
  19:     0x7f347c17e925 - rustc_query_impl[d7850a466dfc989b]::query_vtables::check_mono_item::execute_query_incr::__rust_end_short_backtrace
  20:     0x7f347b64ca02 - rustc_monomorphize[17050ecd102511f2]::collector::items_of_instance
  21:     0x7f347b64c811 - rustc_query_impl[d7850a466dfc989b]::query_vtables::items_of_instance::invoke_provider_fn::__rust_begin_short_backtrace
  22:     0x7f347c172401 - rustc_query_impl[d7850a466dfc989b]::execution::try_execute_query::<rustc_middle[9640d5cb0e0c5ece]::query::caches::DefaultCache<(rustc_middle[9640d5cb0e0c5ece]::ty::instance::Instance, rustc_middle[9640d5cb0e0c5ece]::mono::CollectionMode), rustc_middle[9640d5cb0e0c5ece]::query::erase::ErasedData<[u8; 32usize]>>, true>
  23:     0x7f347c171a21 - rustc_query_impl[d7850a466dfc989b]::query_vtables::items_of_instance::execute_query_incr::__rust_end_short_backtrace
  24:     0x7f347c2d3aac - rustc_monomorphize[17050ecd102511f2]::collector::collect_items_rec
  25:     0x7f347c2d8852 - rustc_monomorphize[17050ecd102511f2]::collector::collect_items_rec
  26:     0x7f347c2d8852 - rustc_monomorphize[17050ecd102511f2]::collector::collect_items_rec
  27:     0x7f347c2d8852 - rustc_monomorphize[17050ecd102511f2]::collector::collect_items_rec
  28:     0x7f347c2d8852 - rustc_monomorphize[17050ecd102511f2]::collector::collect_items_rec
  29:     0x7f347c2dad34 - rustc_monomorphize[17050ecd102511f2]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  30:     0x7f347c2db663 - rustc_monomorphize[17050ecd102511f2]::partitioning::collect_and_partition_mono_items
  31:     0x7f347c382b34 - rustc_query_impl[d7850a466dfc989b]::query_vtables::collect_and_partition_mono_items::invoke_provider_fn::__rust_begin_short_backtrace
  32:     0x7f347cb28291 - rustc_query_impl[d7850a466dfc989b]::execution::try_execute_query::<rustc_middle[9640d5cb0e0c5ece]::query::caches::SingleCache<rustc_middle[9640d5cb0e0c5ece]::query::erase::ErasedData<[u8; 24usize]>>, true>
  33:     0x7f347cb27cea - rustc_query_impl[d7850a466dfc989b]::query_vtables::collect_and_partition_mono_items::execute_query_incr::__rust_end_short_backtrace
  34:     0x7f347c5be9f4 - rustc_codegen_ssa[55cb4f5a62726ace]::base::codegen_crate::<rustc_codegen_llvm[4d44b2886c8eee09]::LlvmCodegenBackend, rustc_codegen_llvm[4d44b2886c8eee09]::ModuleLlvm>
  35:     0x7f347c5be7ae - <rustc_codegen_llvm[4d44b2886c8eee09]::LlvmCodegenBackend as rustc_codegen_ssa[55cb4f5a62726ace]::traits::backend::CodegenBackend>::codegen_crate
  36:     0x7f347c4cb97e - <rustc_interface[fe52a0dd05c9819f]::queries::Linker>::codegen_and_build_linker
  37:     0x7f347c4c1e09 - rustc_interface[fe52a0dd05c9819f]::interface::run_compiler::<(), rustc_driver_impl[c5d5f53170729273]::run_compiler::{closure#0}>::{closure#2}
  38:     0x7f347c519c24 - std[e88b123c45551a4b]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[fe52a0dd05c9819f]::util::run_in_thread_with_globals<rustc_interface[fe52a0dd05c9819f]::util::run_in_thread_pool_with_globals<rustc_interface[fe52a0dd05c9819f]::interface::run_compiler<(), rustc_driver_impl[c5d5f53170729273]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  39:     0x7f347c5199cb - <std[e88b123c45551a4b]::thread::lifecycle::spawn_unchecked<rustc_interface[fe52a0dd05c9819f]::util::run_in_thread_with_globals<rustc_interface[fe52a0dd05c9819f]::util::run_in_thread_pool_with_globals<rustc_interface[fe52a0dd05c9819f]::interface::run_compiler<(), rustc_driver_impl[c5d5f53170729273]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[e3370a31d8f1c8c0]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  40:     0x7f347c514d1b - <std[e88b123c45551a4b]::sys::thread::unix::Thread>::new::thread_start
  41:     0x7f34754980a2 - <unknown>
  42:     0x7f347552080c - <unknown>
  43:                0x0 - <unknown>

rustc-ice-2026-09-15T12_30_35-132248.txt

Originally posted by @ValouBambou in #162773

Reproduce the supplied async and closure example first, then inspect rustc_middle/src/ty/layout.rs around line 1401 and the fn_abi_of_instance failure shown in the report. Done means the invalid generic-const case produces a diagnostic without an internal compiler error or panic.