ICE: broken mir: Unsize coercion .. isn't coercible

[matthiaskrgr]

similar to #131886 but unlike #131886 still crashing

auto-reduced (treereduce-rust):

//@compile-flags: -Zvalidate-mir
#![feature(type_alias_impl_trait)]

type Tait = impl Sized;

trait Foo<'a>: Bar<'a, 'a, Tait> {}
trait Bar<'a, 'b, T> {}

#[define_opaque(Tait)]
fn test_correct3<'a>(x: &dyn Foo<'a>, _: Tait) {
    let _ = x as &dyn Bar<'_, 'a, ()>;
}

fn main() {}

original:

#![feature(type_alias_impl_trait)]

//@ check-pass

type Tait = impl Sized;

trait Foo<'a>: Bar<'a, 'a, Tait> {}
trait Bar<'a, 'b, T> {}

fn test_correct(x: &dyn Foo<'static>) {
    let _ = x as &dyn Bar<'static, 'static, Tait>;
}

fn test_correct2<'a>(x: &dyn Foo<'a>) {
    let _ = x as &dyn Bar<'_, 'static, Tait>;
}

#[define_opaque(Tait)]
fn test_correct3<'a>(x: &dyn Foo<'a>, _: Tait) {
    let _ = x as &dyn Bar<'_, 'a, ()>;
}

fn main() {}

Version information

rustc 1.92.0-nightly (f43597208 2025-10-24)
binary: rustc
commit-hash: f435972085b697a1ece8ee6a1ac76efff8d1df7b
commit-date: 2025-10-24
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.3

Possibly related line of code:

rust/compiler/rustc_mir_transform/src/validate.rs

Lines 75 to 87 in f435972

	};
	cfg_checker.visit_body(body);
	cfg_checker.check_cleanup_control_flow();
	// Also run the TypeChecker.
	for (location, msg) in validate_types(tcx, typing_env, body, body) {
	cfg_checker.fail(location, msg);
	}
	// Ensure that debuginfo records are not emitted for locals that are not in debuginfo.
	for (location, msg) in validate_debuginfos(body) {
	cfg_checker.fail(location, msg);
	}

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zvalidate-mir

Program output

error: internal compiler error: compiler/rustc_mir_transform/src/validate.rs:81:25: broken MIR in Item(DefId(0:12 ~ mvce[1b4a]::test_correct3)) (after pass CheckForceInline) at bb0[3]:
                                Unsize coercion, but `&dyn Foo<'_>` isn't coercible to `&dyn Bar<'_, '_, ()>`
  --> /tmp/icemaker_global_tempdir.20BLeJTxFmM8/rustc_testrunner_tmpdir_reporting.PuUQz2zg7lg7/mvce.rs:10:13
   |
10 |     let _ = x as &dyn Bar<'_, 'a, ()>;
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (4016117) panicked at compiler/rustc_mir_transform/src/validate.rs:81:25:
Box<dyn Any>
stack backtrace:
   0:     0x7fc571834073 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h492c738ad49cea7d
   1:     0x7fc572001b98 - core::fmt::write::h1c21b4db123ae31c
   2:     0x7fc5717e8403 - std::io::Write::write_fmt::h78003e47d8f6c72e
   3:     0x7fc5717f9bf2 - std::sys::backtrace::BacktraceLock::print::h0d646c4d1cbf366e
   4:     0x7fc5717ffcc9 - std::panicking::default_hook::{{closure}}::h274ab31bbd3c2851
   5:     0x7fc5717ff7f3 - std::panicking::default_hook::he590f8320f1e009e
   6:     0x7fc570800611 - std[3e186dc2068ac4b9]::panicking::update_hook::<alloc[47fc8f5871530046]::boxed::Box<rustc_driver_impl[b652d2bf07caa0d8]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7fc5718000ef - std::panicking::panic_with_hook::hc0910476a8aaf08a
   8:     0x7fc570839e61 - std[3e186dc2068ac4b9]::panicking::begin_panic::<rustc_errors[345ffcadd731d51f]::ExplicitBug>::{closure#0}
   9:     0x7fc57082e056 - std[3e186dc2068ac4b9]::sys::backtrace::__rust_end_short_backtrace::<std[3e186dc2068ac4b9]::panicking::begin_panic<rustc_errors[345ffcadd731d51f]::ExplicitBug>::{closure#0}, !>
  10:     0x7fc57082b939 - std[3e186dc2068ac4b9]::panicking::begin_panic::<rustc_errors[345ffcadd731d51f]::ExplicitBug>
  11:     0x7fc5708459d1 - <rustc_errors[345ffcadd731d51f]::diagnostic::BugAbort as rustc_errors[345ffcadd731d51f]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7fc570db9f8c - <rustc_errors[345ffcadd731d51f]::DiagCtxtHandle>::span_bug::<rustc_span[b6aef8613cdfc213]::span_encoding::Span, alloc[47fc8f5871530046]::string::String>
  13:     0x7fc570ddd4c7 - rustc_middle[df80570e3f8f0476]::util::bug::opt_span_bug_fmt::<rustc_span[b6aef8613cdfc213]::span_encoding::Span>::{closure#0}
  14:     0x7fc570ddd7ca - rustc_middle[df80570e3f8f0476]::ty::context::tls::with_opt::<rustc_middle[df80570e3f8f0476]::util::bug::opt_span_bug_fmt<rustc_span[b6aef8613cdfc213]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:     0x7fc570dce53b - rustc_middle[df80570e3f8f0476]::ty::context::tls::with_context_opt::<rustc_middle[df80570e3f8f0476]::ty::context::tls::with_opt<rustc_middle[df80570e3f8f0476]::util::bug::opt_span_bug_fmt<rustc_span[b6aef8613cdfc213]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:     0x7fc56e643b47 - rustc_middle[df80570e3f8f0476]::util::bug::span_bug_fmt::<rustc_span[b6aef8613cdfc213]::span_encoding::Span>
  17:     0x7fc56f640cdc - <rustc_mir_transform[bbe91f48e8a67098]::validate::CfgChecker>::fail::<alloc[47fc8f5871530046]::string::String>
  18:     0x7fc5732d38c4 - <rustc_mir_transform[bbe91f48e8a67098]::validate::Validator as rustc_mir_transform[bbe91f48e8a67098]::pass_manager::MirPass>::run_pass
  19:     0x7fc56f66a2a5 - rustc_mir_transform[bbe91f48e8a67098]::pass_manager::validate_body
  20:     0x7fc57200786b - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>>
  21:     0x7fc572ad6177 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_data_structures[9e234764dc4c351e]::vec_cache::VecCache<rustc_span[b6aef8613cdfc213]::def_id::LocalDefId, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[c36681704e76fc2e]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  22:     0x7fc572ad5bc9 - rustc_query_impl[ce892b7de41d33cc]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7fc57202d372 - rustc_mir_transform[bbe91f48e8a67098]::ffi_unwind_calls::has_ffi_unwind_calls
  24:     0x7fc57202cd59 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::has_ffi_unwind_calls::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 1usize]>>
  25:     0x7fc57299ff2f - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_data_structures[9e234764dc4c351e]::vec_cache::VecCache<rustc_span[b6aef8613cdfc213]::def_id::LocalDefId, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[c36681704e76fc2e]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  26:     0x7fc57299fbfd - rustc_query_impl[ce892b7de41d33cc]::query_impl::has_ffi_unwind_calls::get_query_non_incr::__rust_end_short_backtrace
  27:     0x7fc56f61eb47 - rustc_mir_transform[bbe91f48e8a67098]::mir_promoted
  28:     0x7fc5729faf52 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::mir_promoted::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 16usize]>>
  29:     0x7fc5729fb215 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_data_structures[9e234764dc4c351e]::vec_cache::VecCache<rustc_span[b6aef8613cdfc213]::def_id::LocalDefId, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 16usize]>, rustc_query_system[c36681704e76fc2e]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  30:     0x7fc5729fada2 - rustc_query_impl[ce892b7de41d33cc]::query_impl::mir_promoted::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7fc573176923 - rustc_borrowck[9c9a4d4a4caa4bbc]::mir_borrowck
  32:     0x7fc5731767d9 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>>
  33:     0x7fc572ad6177 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_data_structures[9e234764dc4c351e]::vec_cache::VecCache<rustc_span[b6aef8613cdfc213]::def_id::LocalDefId, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[c36681704e76fc2e]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  34:     0x7fc572ad4289 - rustc_query_impl[ce892b7de41d33cc]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  35:     0x7fc57097b174 - <rustc_hir_analysis[83592e86f71d95cd]::collect::type_of::opaque::TaitConstraintLocator>::check
  36:     0x7fc5709b79e0 - rustc_hir_analysis[83592e86f71d95cd]::collect::type_of::opaque::find_opaque_ty_constraints_for_tait
  37:     0x7fc573f0c409 - rustc_hir_analysis[83592e86f71d95cd]::collect::type_of::type_of_opaque.cold
  38:     0x7fc573086507 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>>
  39:     0x7fc57201b2c6 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_query_system[c36681704e76fc2e]::query::caches::DefIdCache<rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  40:     0x7fc5731cc065 - rustc_query_impl[ce892b7de41d33cc]::query_impl::type_of_opaque::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7fc5728c7477 - rustc_hir_analysis[83592e86f71d95cd]::collect::type_of::type_of
  42:     0x7fc57201c626 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>>
  43:     0x7fc57201b2c6 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_query_system[c36681704e76fc2e]::query::caches::DefIdCache<rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  44:     0x7fc57201ae6f - rustc_query_impl[ce892b7de41d33cc]::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
  45:     0x7fc573086f6c - rustc_hir_analysis[83592e86f71d95cd]::check::check::check_opaque
  46:     0x7fc5729a9449 - rustc_hir_analysis[83592e86f71d95cd]::check::check::check_item_type
  47:     0x7fc5729a093a - rustc_hir_analysis[83592e86f71d95cd]::check::wfcheck::check_well_formed
  48:     0x7fc5729a091b - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 1usize]>>
  49:     0x7fc5729a0192 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_data_structures[9e234764dc4c351e]::vec_cache::VecCache<rustc_span[b6aef8613cdfc213]::def_id::LocalDefId, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[c36681704e76fc2e]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  50:     0x7fc57299fc96 - rustc_query_impl[ce892b7de41d33cc]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  51:     0x7fc57299d3d1 - rustc_hir_analysis[83592e86f71d95cd]::check::wfcheck::check_type_wf
  52:     0x7fc57299d265 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 1usize]>>
  53:     0x7fc5730c6dd4 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_query_system[c36681704e76fc2e]::query::caches::SingleCache<rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  54:     0x7fc5730c6bb6 - rustc_query_impl[ce892b7de41d33cc]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  55:     0x7fc572aa507a - rustc_hir_analysis[83592e86f71d95cd]::check_crate
  56:     0x7fc572acf770 - rustc_interface[10f842ee2745836b]::passes::analysis
  57:     0x7fc572acf435 - rustc_query_impl[ce892b7de41d33cc]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ce892b7de41d33cc]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 0usize]>>
  58:     0x7fc5730cd726 - rustc_query_system[c36681704e76fc2e]::query::plumbing::try_execute_query::<rustc_query_impl[ce892b7de41d33cc]::DynamicConfig<rustc_query_system[c36681704e76fc2e]::query::caches::SingleCache<rustc_middle[df80570e3f8f0476]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[ce892b7de41d33cc]::plumbing::QueryCtxt, false>
  59:     0x7fc5730cd37c - rustc_query_impl[ce892b7de41d33cc]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  60:     0x7fc57333ec8a - <rustc_interface[10f842ee2745836b]::passes::create_and_enter_global_ctxt<core[e8997f4232dfd718]::option::Option<rustc_interface[10f842ee2745836b]::queries::Linker>, rustc_driver_impl[b652d2bf07caa0d8]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[e8997f4232dfd718]::ops::function::FnOnce<(&rustc_session[4dec040bcdb04b8e]::session::Session, rustc_middle[df80570e3f8f0476]::ty::context::CurrentGcx, alloc[47fc8f5871530046]::sync::Arc<rustc_data_structures[9e234764dc4c351e]::jobserver::Proxy>, &std[3e186dc2068ac4b9]::sync::once_lock::OnceLock<rustc_middle[df80570e3f8f0476]::ty::context::GlobalCtxt>, &rustc_data_structures[9e234764dc4c351e]::sync::worker_local::WorkerLocal<rustc_middle[df80570e3f8f0476]::arena::Arena>, &rustc_data_structures[9e234764dc4c351e]::sync::worker_local::WorkerLocal<rustc_hir[739a0293ab2ba12c]::Arena>, rustc_driver_impl[b652d2bf07caa0d8]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  61:     0x7fc5731b37e2 - rustc_interface[10f842ee2745836b]::interface::run_compiler::<(), rustc_driver_impl[b652d2bf07caa0d8]::run_compiler::{closure#0}>::{closure#1}
  62:     0x7fc572dc09c0 - std[3e186dc2068ac4b9]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[10f842ee2745836b]::util::run_in_thread_with_globals<rustc_interface[10f842ee2745836b]::util::run_in_thread_pool_with_globals<rustc_interface[10f842ee2745836b]::interface::run_compiler<(), rustc_driver_impl[b652d2bf07caa0d8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  63:     0x7fc572dc06a4 - <<std[3e186dc2068ac4b9]::thread::Builder>::spawn_unchecked_<rustc_interface[10f842ee2745836b]::util::run_in_thread_with_globals<rustc_interface[10f842ee2745836b]::util::run_in_thread_pool_with_globals<rustc_interface[10f842ee2745836b]::interface::run_compiler<(), rustc_driver_impl[b652d2bf07caa0d8]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[e8997f4232dfd718]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  64:     0x7fc572dbf02f - std::sys::thread::unix::Thread::new::thread_start::h69c86f976e686da7
  65:     0x7fc56c8969cb - <unknown>
  66:     0x7fc56c91aa0c - <unknown>
  67:                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 1.92.0-nightly (f43597208 2025-10-24) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [mir_built] building MIR for `test_correct3`
#1 [has_ffi_unwind_calls] checking if `test_correct3` contains FFI-unwind calls
#2 [mir_promoted] promoting constants in MIR for `test_correct3`
#3 [mir_borrowck] borrow-checking `test_correct3`
#4 [type_of_opaque] computing type of opaque `Tait::{opaque#0}`
#5 [type_of] computing type of `Tait::{opaque#0}`
#6 [check_well_formed] checking that `Tait::{opaque#0}` is well-formed
#7 [check_type_wf] checking that types are well-formed
#8 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error

@rustbot label +F-type_alias_impl_trait +-Zvalidate-mir

[matthiaskrgr]

//@compile-flags: -Zvalidate-mir
#![feature(type_alias_impl_trait)]
type Tait = impl Sized;
trait Foo<'a>: Bar<'a, 'a, Tait> {}
trait Bar<'a, 'b, T> {}
#[define_opaque(Tait)]
fn test_correct3(x: &dyn Foo) {
    _ = x as &dyn Bar<()>
}
fn main() {}

vanilla compiles but with validate-mir it ICEs

[beamandala]

Hi @matthiaskrgr I worked on this issue and opened the above PR. Let me know what you think!