ICE: `expected async fn in trait to return an RPITIT`

[matthiaskrgr]

snippet:

//@compile-flags: --edition=2024
#![feature(async_fn_in_dyn_trait)]
trait A {
    async fn b(self: A);
}

Version information

rustc 1.86.0-nightly (e6f12c8b7 2025-01-30)
binary: rustc
commit-hash: e6f12c8b7d8d5c821c32fb2739ea01d1d874c58a
commit-date: 2025-01-30
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

Possibly related line of code:

rust/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs

Lines 907 to 919 in e6f12c8

	if tcx.asyncness(fn_def_id).is_async() {
	// FIXME(async_fn_in_dyn_trait): Think of a better way to unify these code paths
	// to issue an appropriate feature suggestion when users try to use AFIDT.
	// Obviously we must only do this once AFIDT is finished enough to actually be usable.
	if tcx.features().async_fn_in_dyn_trait() {
	let ty::Alias(ty::Projection, proj) = *ty.kind() else {
	bug!("expected async fn in trait to return an RPITIT");
	};
	assert!(tcx.is_impl_trait_in_trait(proj.def_id));
	// FIXME(async_fn_in_dyn_trait): We should check that this bound is legal too,
	// and stop relying on `async fn` in the definition.
	for bound in tcx.item_bounds(proj.def_id).instantiate(tcx, proj.args) {

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

Program output

warning: the feature `async_fn_in_dyn_trait` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.7pZXHQ7HZKbf/rustc_testrunner_tmpdir_reporting.rOKT2kZqYC9F/mvce.rs:1:12
  |
1 | #![feature(async_fn_in_dyn_trait)]
  |            ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #133119 <https://github.com/rust-lang/rust/issues/133119> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.7pZXHQ7HZKbf/rustc_testrunner_tmpdir_reporting.rOKT2kZqYC9F/mvce.rs:4:2
  |
4 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.7pZXHQ7HZKbf/rustc_testrunner_tmpdir_reporting.rOKT2kZqYC9F/mvce.rs`

error: internal compiler error: compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs:913:17: expected async fn in trait to return an RPITIT


thread 'rustc' panicked at compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs:913:17:
Box<dyn Any>
stack backtrace:
   0:     0x7b4b4522d410 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h32b76eca82481332
   1:     0x7b4b45a138e6 - core::fmt::write::h6aab99975a3e1e0d
   2:     0x7b4b46904bd1 - std::io::Write::write_fmt::h07e3041152301b92
   3:     0x7b4b4522d272 - std::sys::backtrace::BacktraceLock::print::h11198f536c423f0e
   4:     0x7b4b4522f6f2 - std::panicking::default_hook::{{closure}}::hedf2549fe54c612d
   5:     0x7b4b4522f57a - std::panicking::default_hook::h5cafcda49fcb475d
   6:     0x7b4b44389e99 - std[65c60797a113df8d]::panicking::update_hook::<alloc[26d4a3e3ec5433a5]::boxed::Box<rustc_driver_impl[88e265ae9927f55c]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7b4b45230273 - std::panicking::rust_panic_with_hook::h9e0ce671ec08e8dc
   8:     0x7b4b443c4c21 - std[65c60797a113df8d]::panicking::begin_panic::<rustc_errors[23526cdfd16e48c4]::ExplicitBug>::{closure#0}
   9:     0x7b4b443b9b26 - std[65c60797a113df8d]::sys::backtrace::__rust_end_short_backtrace::<std[65c60797a113df8d]::panicking::begin_panic<rustc_errors[23526cdfd16e48c4]::ExplicitBug>::{closure#0}, !>
  10:     0x7b4b443b67ed - std[65c60797a113df8d]::panicking::begin_panic::<rustc_errors[23526cdfd16e48c4]::ExplicitBug>
  11:     0x7b4b443ceb71 - <rustc_errors[23526cdfd16e48c4]::diagnostic::BugAbort as rustc_errors[23526cdfd16e48c4]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7b4b449b1ba3 - rustc_middle[c95cf357e72e9a85]::util::bug::opt_span_bug_fmt::<rustc_span[cb34d841a75ac9bc]::span_encoding::Span>::{closure#0}
  13:     0x7b4b4499ab0a - rustc_middle[c95cf357e72e9a85]::ty::context::tls::with_opt::<rustc_middle[c95cf357e72e9a85]::util::bug::opt_span_bug_fmt<rustc_span[cb34d841a75ac9bc]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7b4b4499a99b - rustc_middle[c95cf357e72e9a85]::ty::context::tls::with_context_opt::<rustc_middle[c95cf357e72e9a85]::ty::context::tls::with_opt<rustc_middle[c95cf357e72e9a85]::util::bug::opt_span_bug_fmt<rustc_span[cb34d841a75ac9bc]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7b4b42c3df80 - rustc_middle[c95cf357e72e9a85]::util::bug::bug_fmt
  16:     0x7b4b46556623 - rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::virtual_call_violations_for_method
  17:     0x7b4b45fef6ed - rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::dyn_compatibility_violations_for_assoc_item
  18:     0x7b4b45fef590 - <core[b38de05c6bab522f]::iter::adapters::flatten::FlatMap<core[b38de05c6bab522f]::iter::adapters::map::Map<core[b38de05c6bab522f]::iter::adapters::map::Map<core[b38de05c6bab522f]::slice::iter::Iter<(rustc_span[cb34d841a75ac9bc]::symbol::Symbol, rustc_middle[c95cf357e72e9a85]::ty::assoc::AssocItem)>, <rustc_data_structures[9c6ee9e9e5334c83]::sorted_map::index_map::SortedIndexMultiMap<u32, rustc_span[cb34d841a75ac9bc]::symbol::Symbol, rustc_middle[c95cf357e72e9a85]::ty::assoc::AssocItem>>::iter::{closure#0}>, <rustc_middle[c95cf357e72e9a85]::ty::assoc::AssocItems>::in_definition_order::{closure#0}>, alloc[26d4a3e3ec5433a5]::vec::Vec<rustc_middle[c95cf357e72e9a85]::traits::DynCompatibilityViolation>, rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::dyn_compatibility_violations_for_trait::{closure#0}::{closure#0}> as core[b38de05c6bab522f]::iter::traits::iterator::Iterator>::next
  19:     0x7b4b46543d60 - <core[b38de05c6bab522f]::iter::adapters::flatten::FlatMap<core[b38de05c6bab522f]::iter::sources::from_fn::FromFn<rustc_type_ir[6a899794ba15143]::elaborate::supertrait_def_ids<rustc_middle[c95cf357e72e9a85]::ty::context::TyCtxt>::{closure#0}>, alloc[26d4a3e3ec5433a5]::vec::Vec<rustc_middle[c95cf357e72e9a85]::traits::DynCompatibilityViolation>, rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::dyn_compatibility_violations::{closure#0}> as core[b38de05c6bab522f]::iter::traits::iterator::Iterator>::next
  20:     0x7b4b46543984 - <core[b38de05c6bab522f]::iter::adapters::flatten::FlatMap<core[b38de05c6bab522f]::iter::sources::from_fn::FromFn<rustc_type_ir[6a899794ba15143]::elaborate::supertrait_def_ids<rustc_middle[c95cf357e72e9a85]::ty::context::TyCtxt>::{closure#0}>, alloc[26d4a3e3ec5433a5]::vec::Vec<rustc_middle[c95cf357e72e9a85]::traits::DynCompatibilityViolation>, rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::dyn_compatibility_violations::{closure#0}> as core[b38de05c6bab522f]::iter::traits::iterator::Iterator>::collect::<smallvec[ec8755ed03ba0f13]::SmallVec<[rustc_middle[c95cf357e72e9a85]::traits::DynCompatibilityViolation; 8usize]>>
  21:     0x7b4b465434dd - rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::dyn_compatibility_violations
  22:     0x7b4b465433e2 - rustc_query_impl[a79c5430108c0b60]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a79c5430108c0b60]::query_impl::dyn_compatibility_violations::dynamic_query::{closure#2}::{closure#0}, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 16usize]>>
  23:     0x7b4b465433c9 - <rustc_query_impl[a79c5430108c0b60]::query_impl::dyn_compatibility_violations::dynamic_query::{closure#2} as core[b38de05c6bab522f]::ops::function::FnOnce<(rustc_middle[c95cf357e72e9a85]::ty::context::TyCtxt, rustc_span[cb34d841a75ac9bc]::def_id::DefId)>>::call_once
  24:     0x7b4b45e8840a - rustc_query_system[ff51b95ecba2cb21]::query::plumbing::try_execute_query::<rustc_query_impl[a79c5430108c0b60]::DynamicConfig<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefIdCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[a79c5430108c0b60]::plumbing::QueryCtxt, false>
  25:     0x7b4b45ff1338 - rustc_query_impl[a79c5430108c0b60]::query_impl::dyn_compatibility_violations::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7b4b45ff162f - rustc_middle[c95cf357e72e9a85]::query::plumbing::query_get_at::<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefIdCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 16usize]>>>
  27:     0x7b4b45ff1427 - rustc_trait_selection[d3a832fb8e061b24]::traits::dyn_compatibility::is_dyn_compatible
  28:     0x7b4b45ff13e9 - rustc_query_impl[a79c5430108c0b60]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a79c5430108c0b60]::query_impl::is_dyn_compatible::dynamic_query::{closure#2}::{closure#0}, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>>
  29:     0x7b4b45d6c3eb - rustc_query_system[ff51b95ecba2cb21]::query::plumbing::try_execute_query::<rustc_query_impl[a79c5430108c0b60]::DynamicConfig<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefIdCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[a79c5430108c0b60]::plumbing::QueryCtxt, false>
  30:     0x7b4b4630eca6 - rustc_query_impl[a79c5430108c0b60]::query_impl::is_dyn_compatible::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7b4b463b7200 - rustc_middle[c95cf357e72e9a85]::query::plumbing::query_get_at::<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefIdCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>>>
  32:     0x7b4b46b1cd85 - <dyn rustc_hir_analysis[52c5e5868dd20650]::hir_ty_lowering::HirTyLowerer>::prohibit_or_lint_bare_trait_object_ty
  33:     0x7b4b4634256f - <dyn rustc_hir_analysis[52c5e5868dd20650]::hir_ty_lowering::HirTyLowerer>::lower_ty
  34:     0x7b4b46339e7e - <dyn rustc_hir_analysis[52c5e5868dd20650]::hir_ty_lowering::HirTyLowerer>::lower_fn_ty
  35:     0x7b4b4634f4a0 - rustc_hir_analysis[52c5e5868dd20650]::collect::fn_sig
  36:     0x7b4b45cb60ed - rustc_query_impl[a79c5430108c0b60]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a79c5430108c0b60]::query_impl::fn_sig::dynamic_query::{closure#2}::{closure#0}, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 24usize]>>
  37:     0x7b4b45cb60ad - <rustc_query_impl[a79c5430108c0b60]::query_impl::fn_sig::dynamic_query::{closure#2} as core[b38de05c6bab522f]::ops::function::FnOnce<(rustc_middle[c95cf357e72e9a85]::ty::context::TyCtxt, rustc_span[cb34d841a75ac9bc]::def_id::DefId)>>::call_once
  38:     0x7b4b45cb8f87 - rustc_query_system[ff51b95ecba2cb21]::query::plumbing::try_execute_query::<rustc_query_impl[a79c5430108c0b60]::DynamicConfig<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefIdCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[a79c5430108c0b60]::plumbing::QueryCtxt, false>
  39:     0x7b4b45cb85e2 - rustc_query_impl[a79c5430108c0b60]::query_impl::fn_sig::get_query_non_incr::__rust_end_short_backtrace
  40:     0x7b4b460794bc - rustc_middle[c95cf357e72e9a85]::query::plumbing::query_get_at::<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefIdCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 24usize]>>>
  41:     0x7b4b46a38049 - rustc_hir_analysis[52c5e5868dd20650]::check::check::check_item_type
  42:     0x7b4b460655c6 - rustc_hir_analysis[52c5e5868dd20650]::check::wfcheck::check_well_formed
  43:     0x7b4b46064187 - rustc_query_impl[a79c5430108c0b60]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a79c5430108c0b60]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>>
  44:     0x7b4b46063c29 - rustc_query_system[ff51b95ecba2cb21]::query::plumbing::try_execute_query::<rustc_query_impl[a79c5430108c0b60]::DynamicConfig<rustc_data_structures[9c6ee9e9e5334c83]::vec_cache::VecCache<rustc_span[cb34d841a75ac9bc]::def_id::LocalDefId, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[ff51b95ecba2cb21]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[a79c5430108c0b60]::plumbing::QueryCtxt, false>
  45:     0x7b4b46063686 - rustc_query_impl[a79c5430108c0b60]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  46:     0x7b4b460612ac - rustc_hir_analysis[52c5e5868dd20650]::check::wfcheck::check_mod_type_wf
  47:     0x7b4b460610cb - rustc_query_impl[a79c5430108c0b60]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a79c5430108c0b60]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>>
  48:     0x7b4b46985948 - rustc_query_system[ff51b95ecba2cb21]::query::plumbing::try_execute_query::<rustc_query_impl[a79c5430108c0b60]::DynamicConfig<rustc_query_system[ff51b95ecba2cb21]::query::caches::DefaultCache<rustc_span[cb34d841a75ac9bc]::def_id::LocalModDefId, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[a79c5430108c0b60]::plumbing::QueryCtxt, false>
  49:     0x7b4b469856f0 - rustc_query_impl[a79c5430108c0b60]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  50:     0x7b4b45da10e6 - rustc_hir_analysis[52c5e5868dd20650]::check_crate
  51:     0x7b4b45d9dc64 - rustc_interface[3e4af95fb941106a]::passes::run_required_analyses
  52:     0x7b4b469002de - rustc_interface[3e4af95fb941106a]::passes::analysis
  53:     0x7b4b469002af - rustc_query_impl[a79c5430108c0b60]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a79c5430108c0b60]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 0usize]>>
  54:     0x7b4b468ec015 - rustc_query_system[ff51b95ecba2cb21]::query::plumbing::try_execute_query::<rustc_query_impl[a79c5430108c0b60]::DynamicConfig<rustc_query_system[ff51b95ecba2cb21]::query::caches::SingleCache<rustc_middle[c95cf357e72e9a85]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[a79c5430108c0b60]::plumbing::QueryCtxt, false>
  55:     0x7b4b468ebd4e - rustc_query_impl[a79c5430108c0b60]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  56:     0x7b4b46a0be69 - rustc_interface[3e4af95fb941106a]::passes::create_and_enter_global_ctxt::<core[b38de05c6bab522f]::option::Option<rustc_interface[3e4af95fb941106a]::queries::Linker>, rustc_driver_impl[88e265ae9927f55c]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  57:     0x7b4b4695a498 - rustc_interface[3e4af95fb941106a]::interface::run_compiler::<(), rustc_driver_impl[88e265ae9927f55c]::run_compiler::{closure#0}>::{closure#1}
  58:     0x7b4b46875f35 - std[65c60797a113df8d]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[3e4af95fb941106a]::util::run_in_thread_with_globals<rustc_interface[3e4af95fb941106a]::util::run_in_thread_pool_with_globals<rustc_interface[3e4af95fb941106a]::interface::run_compiler<(), rustc_driver_impl[88e265ae9927f55c]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  59:     0x7b4b46875c19 - <<std[65c60797a113df8d]::thread::Builder>::spawn_unchecked_<rustc_interface[3e4af95fb941106a]::util::run_in_thread_with_globals<rustc_interface[3e4af95fb941106a]::util::run_in_thread_pool_with_globals<rustc_interface[3e4af95fb941106a]::interface::run_compiler<(), rustc_driver_impl[88e265ae9927f55c]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[b38de05c6bab522f]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  60:     0x7b4b468753ab - std::sys::pal::unix::thread::Thread::new::thread_start::h7b9c2e4fdae6f3c1
  61:     0x7b4b40aa339d - <unknown>
  62:     0x7b4b40b2849c - <unknown>
  63:                0x0 - <unknown>

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 1.86.0-nightly (e6f12c8b7 2025-01-30) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [dyn_compatibility_violations] determining dyn-compatibility of trait `A`
#1 [is_dyn_compatible] checking if trait `A` is dyn-compatible
#2 [fn_sig] computing function signature of `A::b`
#3 [check_well_formed] checking that `A` is well-formed
#4 [check_mod_type_wf] checking that types are well-formed in top-level module
#5 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors; 1 warning emitted

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

@rustbot label +F-async_fn_in_dyn_trait