[ICE]: `index out of bounds: the len is 0 but the index is ..`

[matthiaskrgr]

Code

#![feature(type_alias_impl_trait)]
type Foo = impl Send;

struct A;

#[define_opaque(Foo)]
const VALUE: Foo = todo!();

#[define_opaque(Foo)]
fn test(foo: Foo) {
    match VALUE {
        &mut Some(ref mut x) => *x,
        &mut None => panic!(),
    }
}

fn main() {}

Meta

rustc --version --verbose:

rustc 1.99.0-nightly (771916f90 2026-08-08)
binary: rustc
commit-hash: 771916f9028e7fe56d2685f2c4f698de5d7d6a45
commit-date: 2026-08-08
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 23.1.0

Error output

error: item does not constrain `Foo::{opaque#0}`
 --> code.rs:7:7
  |
7 | const VALUE: Foo = todo!();
  |       ^^^^^
  |
  = note: consider removing `#[define_opaque]` or adding an empty `#[define_opaque()]`
note: this opaque type is supposed to be constrained
 --> code.rs:2:12
  |
2 | type Foo = impl Send;
  |            ^^^^^^^^^

warning: unused variable: `foo`
  --> code.rs:10:9
   |
10 | fn test(foo: Foo) {
   |         ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
   |
   = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

Backtrace

thread 'rustc' (3960277) panicked at /rustc-dev/771916f9028e7fe56d2685f2c4f698de5d7d6a45/compiler/rustc_mir_dataflow/src/un_derefer.rs:67:20:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:     0x7f174d70d2b6 - <<std[d336e1aa20b017e8]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[f3eec57d7c5a5c99]::fmt::Display>::fmt
   1:     0x7f174de11c8f - core[f3eec57d7c5a5c99]::fmt::write
   2:     0x7f174d7221dc - <std[d336e1aa20b017e8]::sys::stdio::unix::Stderr as core[f3eec57d7c5a5c99]::io::write::Write>::write_fmt
   3:     0x7f174d6e032a - std[d336e1aa20b017e8]::panicking::default_hook::{closure#0}
   4:     0x7f174d701043 - std[d336e1aa20b017e8]::panicking::default_hook
   5:     0x7f174c65beda - std[d336e1aa20b017e8]::panicking::update_hook::<alloc[a6218fca04c82945]::boxed::Box<rustc_driver_impl[7956faa26824032a]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f174d7014e2 - std[d336e1aa20b017e8]::panicking::panic_with_hook
   7:     0x7f174d6e03e2 - std[d336e1aa20b017e8]::panicking::panic_handler::{closure#0}
   8:     0x7f174d6d92b9 - std[d336e1aa20b017e8]::sys::backtrace::__rust_end_short_backtrace::<std[d336e1aa20b017e8]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f174d6e1dbd - __rustc[9438f7ed0802b081]::rust_begin_unwind
  10:     0x7f174a37f46c - core[f3eec57d7c5a5c99]::panicking::panic_fmt
  11:     0x7f174ae5c154 - core[f3eec57d7c5a5c99]::panicking::panic_bounds_check
  12:     0x7f174a14767a - <rustc_mir_dataflow[69773cb1756060d8]::impls::initialized::MaybeInitializedPlaces as rustc_mir_dataflow[69773cb1756060d8]::framework::Analysis>::iterate_to_fixpoint
  13:     0x7f174a16761d - <rustc_mir_transform[28b060d57b516d55]::elaborate_drops::ElaborateDrops as rustc_mir_transform[28b060d57b516d55]::pass_manager::MirPass>::run_pass
  14:     0x7f174de17de2 - rustc_mir_transform[28b060d57b516d55]::run_analysis_to_runtime_passes
  15:     0x7f174e4d0717 - rustc_mir_transform[28b060d57b516d55]::mir_drops_elaborated_and_const_checked
  16:     0x7f174e4d38f5 - rustc_query_impl[f2c47540efc44da2]::execution::try_execute_query::<rustc_data_structures[47e0eddda0e0a292]::vec_cache::VecCache<rustc_span[5a330e2a527940b1]::def_id::LocalDefId, rustc_middle[78cd89ce21bdaa52]::query::erase::ErasedData<[u8; 8usize]>, rustc_middle[78cd89ce21bdaa52]::dep_graph::graph::DepNodeIndex>, false>
  17:     0x7f174e4d3103 - rustc_query_impl[f2c47540efc44da2]::query_impl::mir_drops_elaborated_and_const_checked::execute_query_non_incr::__rust_end_short_backtrace
  18:     0x7f174e4d622a - <rustc_middle[78cd89ce21bdaa52]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[fca7d0b553b989d0]::passes::run_required_analyses::{closure#2}::{closure#0}>::{closure#0}
  19:     0x7f174e4d533e - rustc_interface[fca7d0b553b989d0]::passes::analysis
  20:     0x7f174efa808a - rustc_query_impl[f2c47540efc44da2]::execution::try_execute_query::<rustc_middle[78cd89ce21bdaa52]::query::caches::SingleCache<rustc_middle[78cd89ce21bdaa52]::query::erase::ErasedData<[u8; 0usize]>>, false>
  21:     0x7f174efa7cff - rustc_query_impl[f2c47540efc44da2]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  22:     0x7f174f02cb35 - rustc_interface[fca7d0b553b989d0]::interface::run_compiler::<(), rustc_driver_impl[7956faa26824032a]::run_compiler::{closure#0}>::{closure#2}
  23:     0x7f174f06d424 - std[d336e1aa20b017e8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_with_globals<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_pool_with_globals<rustc_interface[fca7d0b553b989d0]::interface::run_compiler<(), rustc_driver_impl[7956faa26824032a]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  24:     0x7f174f06d1ed - <std[d336e1aa20b017e8]::thread::lifecycle::spawn_unchecked<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_with_globals<rustc_interface[fca7d0b553b989d0]::util::run_in_thread_pool_with_globals<rustc_interface[fca7d0b553b989d0]::interface::run_compiler<(), rustc_driver_impl[7956faa26824032a]::run_compiler::{closure#0}>::{closure#2}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[f3eec57d7c5a5c99]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  25:     0x7f174f0a56ba - <std[d336e1aa20b017e8]::sys::thread::unix::Thread>::new::thread_start
  26:     0x7f1748497739 - <unknown>
  27:     0x7f174851bedc - <unknown>
  28:                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/matthias/vcs/github/CRED/rustc-ice-2026-08-09T10_49_26-3960267.txt` to your bug report

note: rustc 1.99.0-nightly (771916f90 2026-08-08) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_drops_elaborated_and_const_checked] elaborating drops for `test`
#1 [analysis] running analysis passes on crate `code`
end of query stack
error: aborting due to 1 previous error; 1 warning emitted