ICE: `non-defining use in defining scope`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2024
fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
    async { create_complex_future().await }
}

trait ReturnsSend {}

original:

//@ edition: 2021

use Box::new;

fn invalid_future() -> impl Future {}
//@ run-pass

fn create_complex_future() -> impl Future<Output = impl ReturnsSend> {
    async { create_complex_future().await }
}

fn coerce_impl_trait() -> impl Future<Output = impl Send> {
    create_complex_future()
}

trait ReturnsSend {}

impl<F, R> ReturnsSend for F
where
    F: Fn() -> R,
    R: Send,
{
}

fn main() {}

Version information

rustc 1.91.0-nightly (fd75a9c32 2025-09-03)
binary: rustc
commit-hash: fd75a9c32d643f39c8c61df770d2cff60b3fefd5
commit-date: 2025-09-03
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Possibly related line of code:

rust/compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs

Lines 500 to 512 in fd75a9c

	opaque_types: &[(OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)],
	) -> Vec<DeferredOpaqueTypeError<'tcx>> {
	let tcx = infcx.tcx;
	let mut errors = Vec::new();
	for &(key, hidden_type) in opaque_types {
	let Some(expected) = get_concrete_opaque_type(concrete_opaque_types, key.def_id) else {
	assert!(tcx.use_typing_mode_borrowck(), "non-defining use in defining scope");
	errors.push(DeferredOpaqueTypeError::NonDefiningUseInDefiningScope {
	span: hidden_type.span,
	opaque_type_key: key,
	});
	let guar = tcx.dcx().span_delayed_bug(
	hidden_type.span,

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc --edition=2024

Program output

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.K1lCvHnMGsLU/rustc_testrunner_tmpdir_reporting.Inqk0tIKVrjs/mvce.rs:5:21
  |
5 | trait ReturnsSend {}
  |                     ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.K1lCvHnMGsLU/rustc_testrunner_tmpdir_reporting.Inqk0tIKVrjs/mvce.rs`


thread 'rustc' (4837) panicked at compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs:506:13:
non-defining use in defining scope
stack backtrace:
   0:     0x7ff0dc9b0453 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h9edfddd7dc0bd583
   1:     0x7ff0dd001998 - core::fmt::write::h88e60c27e15542b9
   2:     0x7ff0dc9651b1 - std::io::Write::write_fmt::h239b955172045825
   3:     0x7ff0dc976332 - std::sys::backtrace::BacktraceLock::print::he00602618008d690
   4:     0x7ff0dc97c2b9 - std::panicking::default_hook::{{closure}}::hd731f16ef72096be
   5:     0x7ff0dc97bde3 - std::panicking::default_hook::he12a20c3692e4cb7
   6:     0x7ff0db9dea15 - std[40a039f02da9be5]::panicking::update_hook::<alloc[9e325f13687c9588]::boxed::Box<rustc_driver_impl[6bc00067fcdc553c]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7ff0dc97c6df - std::panicking::panic_with_hook::h694d89e3fc21ffd6
   8:     0x7ff0dc97c466 - std::panicking::panic_handler::{{closure}}::h5524e8eac2a495e5
   9:     0x7ff0dc976469 - std::sys::backtrace::__rust_end_short_backtrace::h4b6fbbdc74cf2f46
  10:     0x7ff0dc956d1d - __rustc[2cc2c7f1e3a23039]::rust_begin_unwind
  11:     0x7ff0d90cb330 - core::panicking::panic_fmt::h594cf358aa009393
  12:     0x7ff0ddb821c1 - <rustc_borrowck[8f789c4a8072f8c4]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  13:     0x7ff0ddb70164 - rustc_borrowck[8f789c4a8072f8c4]::mir_borrowck
  14:     0x7ff0ddb6ff21 - rustc_query_impl[e4969f750d187e98]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e4969f750d187e98]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 8usize]>>
  15:     0x7ff0dd2046c5 - rustc_query_system[7ac3c608b60e76af]::query::plumbing::try_execute_query::<rustc_query_impl[e4969f750d187e98]::DynamicConfig<rustc_data_structures[788fd6124e46ed5e]::vec_cache::VecCache<rustc_span[3aa7543d96f37683]::def_id::LocalDefId, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[7ac3c608b60e76af]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[e4969f750d187e98]::plumbing::QueryCtxt, false>
  16:     0x7ff0dd203ec9 - rustc_query_impl[e4969f750d187e98]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  17:     0x7ff0de2d4d2f - rustc_hir_analysis[4feb2b3cccb2a05a]::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  18:     0x7ff0de2d4ab8 - rustc_hir_analysis[4feb2b3cccb2a05a]::collect::type_of::type_of_opaque
  19:     0x7ff0de2d49ad - rustc_query_impl[e4969f750d187e98]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e4969f750d187e98]::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 8usize]>>
  20:     0x7ff0dd016af9 - rustc_query_system[7ac3c608b60e76af]::query::plumbing::try_execute_query::<rustc_query_impl[e4969f750d187e98]::DynamicConfig<rustc_query_system[7ac3c608b60e76af]::query::caches::DefIdCache<rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[e4969f750d187e98]::plumbing::QueryCtxt, false>
  21:     0x7ff0de1e8cdb - rustc_query_impl[e4969f750d187e98]::query_impl::type_of_opaque::get_query_non_incr::__rust_end_short_backtrace
  22:     0x7ff0dd462cbd - rustc_hir_analysis[4feb2b3cccb2a05a]::collect::type_of::type_of
  23:     0x7ff0dd017e66 - rustc_query_impl[e4969f750d187e98]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e4969f750d187e98]::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 8usize]>>
  24:     0x7ff0dd016af9 - rustc_query_system[7ac3c608b60e76af]::query::plumbing::try_execute_query::<rustc_query_impl[e4969f750d187e98]::DynamicConfig<rustc_query_system[7ac3c608b60e76af]::query::caches::DefIdCache<rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[e4969f750d187e98]::plumbing::QueryCtxt, false>
  25:     0x7ff0dd01669d - rustc_query_impl[e4969f750d187e98]::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7ff0de2d546c - rustc_hir_analysis[4feb2b3cccb2a05a]::check::check::check_opaque
  27:     0x7ff0dd906920 - rustc_hir_analysis[4feb2b3cccb2a05a]::check::check::check_item_type
  28:     0x7ff0dd8fdfe7 - rustc_hir_analysis[4feb2b3cccb2a05a]::check::wfcheck::check_well_formed
  29:     0x7ff0dd8fdfc9 - rustc_query_impl[e4969f750d187e98]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e4969f750d187e98]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 1usize]>>
  30:     0x7ff0dd8fd809 - rustc_query_system[7ac3c608b60e76af]::query::plumbing::try_execute_query::<rustc_query_impl[e4969f750d187e98]::DynamicConfig<rustc_data_structures[788fd6124e46ed5e]::vec_cache::VecCache<rustc_span[3aa7543d96f37683]::def_id::LocalDefId, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[7ac3c608b60e76af]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[e4969f750d187e98]::plumbing::QueryCtxt, false>
  31:     0x7ff0dd8fd316 - rustc_query_impl[e4969f750d187e98]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7ff0dd8f94cf - rustc_hir_analysis[4feb2b3cccb2a05a]::check::wfcheck::check_type_wf
  33:     0x7ff0dd8f9365 - rustc_query_impl[e4969f750d187e98]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e4969f750d187e98]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 1usize]>>
  34:     0x7ff0de09aed4 - rustc_query_system[7ac3c608b60e76af]::query::plumbing::try_execute_query::<rustc_query_impl[e4969f750d187e98]::DynamicConfig<rustc_query_system[7ac3c608b60e76af]::query::caches::SingleCache<rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[e4969f750d187e98]::plumbing::QueryCtxt, false>
  35:     0x7ff0de09acb6 - rustc_query_impl[e4969f750d187e98]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  36:     0x7ff0dda16f39 - rustc_hir_analysis[4feb2b3cccb2a05a]::check_crate
  37:     0x7ff0dd1fbf3e - rustc_interface[27387541cf04561a]::passes::analysis
  38:     0x7ff0dd1fbc0b - rustc_query_impl[e4969f750d187e98]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e4969f750d187e98]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 0usize]>>
  39:     0x7ff0de0a191a - rustc_query_system[7ac3c608b60e76af]::query::plumbing::try_execute_query::<rustc_query_impl[e4969f750d187e98]::DynamicConfig<rustc_query_system[7ac3c608b60e76af]::query::caches::SingleCache<rustc_middle[cfc37889f8d46045]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[e4969f750d187e98]::plumbing::QueryCtxt, false>
  40:     0x7ff0de0a1550 - rustc_query_impl[e4969f750d187e98]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7ff0de2368a4 - rustc_interface[27387541cf04561a]::passes::create_and_enter_global_ctxt::<core[9a833c474fe77ed7]::option::Option<rustc_interface[27387541cf04561a]::queries::Linker>, rustc_driver_impl[6bc00067fcdc553c]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  42:     0x7ff0de26427e - rustc_interface[27387541cf04561a]::interface::run_compiler::<(), rustc_driver_impl[6bc00067fcdc553c]::run_compiler::{closure#0}>::{closure#1}
  43:     0x7ff0de193b78 - std[40a039f02da9be5]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[27387541cf04561a]::util::run_in_thread_with_globals<rustc_interface[27387541cf04561a]::util::run_in_thread_pool_with_globals<rustc_interface[27387541cf04561a]::interface::run_compiler<(), rustc_driver_impl[6bc00067fcdc553c]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  44:     0x7ff0de19385c - <<std[40a039f02da9be5]::thread::Builder>::spawn_unchecked_<rustc_interface[27387541cf04561a]::util::run_in_thread_with_globals<rustc_interface[27387541cf04561a]::util::run_in_thread_pool_with_globals<rustc_interface[27387541cf04561a]::interface::run_compiler<(), rustc_driver_impl[6bc00067fcdc553c]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[9a833c474fe77ed7]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  45:     0x7ff0de1995cd - std::sys::pal::unix::thread::Thread::new::thread_start::h42df12c914dd61fe
  46:     0x7ff0d7a969cb - <unknown>
  47:     0x7ff0d7b1aa0c - <unknown>
  48:                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 1.91.0-nightly (fd75a9c32 2025-09-03) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z dump-mir-dir=dir

query stack during panic:
#0 [mir_borrowck] borrow-checking `create_complex_future`
#1 [type_of_opaque] computing type of opaque `create_complex_future::{opaque#0}`
#2 [type_of] computing type of `create_complex_future::{opaque#0}`
#3 [check_well_formed] checking that `create_complex_future::{opaque#0}` is well-formed
#4 [check_type_wf] checking that types are well-formed
#5 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0601`.

[lqd]

cc @lcnr for these recent changes

[matthiaskrgr]

#145244