ICE: Unsize coercion, but &dyn Foo<'_> isn't coercible to &dyn Bar<'_, '_, ()>

@rustbot

auto-reduced (treereduce-rust):

#![feature(trait_upcasting, type_alias_impl_trait)]

type Tait = impl Sized;

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

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

original:

#![feature(trait_upcasting, 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<'_, '_, Tait>;
}

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

Version information

rustc 1.84.0-nightly (db8043bb1 2024-10-18)
binary: rustc
commit-hash: db8043bb199705e72246ca43d4af1e9dbe7d55be
commit-date: 2024-10-18
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zvalidate-mir --crate-type=lib

Program output

thread 'rustc' panicked at compiler/rustc_mir_transform/src/validate.rs:95:25:
broken MIR in Item(DefId(0:11 ~ mvce[0ba5]::test_correct3)) (after pass CheckPackedRef) at bb0[3]:
Unsize coercion, but `&dyn Foo<'_>` isn't coercible to `&dyn Bar<'_, '_, ()>`
stack backtrace:
   0:     0x78597e480c7a - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h311c30006464a44d
   1:     0x78597ec03e0a - core::fmt::write::hefa9af5f84579548
   2:     0x78597ffa9291 - std::io::Write::write_fmt::h7b02b91c119616de
   3:     0x78597e480ad2 - std::sys::backtrace::BacktraceLock::print::h1bf9540f585b1c0c
   4:     0x78597e482fb6 - std::panicking::default_hook::{{closure}}::hdddf3b12f17177ae
   5:     0x78597e482e00 - std::panicking::default_hook::h95cfcd1ff904ee1d
   6:     0x78597d50280f - std[7ea95b089189ea8]::panicking::update_hook::<alloc[906de809b2d05e52]::boxed::Box<rustc_driver_impl[d25ecc56b0b66360]::install_ice_hook::{closure#0}>>::{closure#0}
   7:     0x78597e4836c8 - std::panicking::rust_panic_with_hook::h4741b39194e01f78
   8:     0x78597e48349a - std::panicking::begin_panic_handler::{{closure}}::h99fdffb7c066d2c5
   9:     0x78597e481129 - std::sys::backtrace::__rust_end_short_backtrace::h2eb0b5ad24f0eada
  10:     0x78597e48315c - rust_begin_unwind
  11:     0x78597aee8c20 - core::panicking::panic_fmt::h9d32603254f24257
  12:     0x78597b703213 - <rustc_mir_transform[7bdd74c785fb8b70]::validate::CfgChecker>::fail::<alloc[906de809b2d05e52]::string::String>
  13:     0x78597b701615 - <rustc_mir_transform[7bdd74c785fb8b70]::validate::Validator as rustc_mir_transform[7bdd74c785fb8b70]::pass_manager::MirPass>::run_pass
  14:     0x78597cde1a10 - rustc_mir_transform[7bdd74c785fb8b70]::pass_manager::validate_body
  15:     0x78597ec06d29 - rustc_mir_transform[7bdd74c785fb8b70]::mir_built
  16:     0x78597ec06907 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>
  17:     0x78597ef68d7a - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  18:     0x78597ef6888d - rustc_query_impl[32db4d22f0b27131]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  19:     0x78597ec2c929 - rustc_mir_transform[7bdd74c785fb8b70]::ffi_unwind_calls::has_ffi_unwind_calls
  20:     0x78597ec2c2d5 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::has_ffi_unwind_calls::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>
  21:     0x78597efbd167 - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  22:     0x78597efbcd41 - rustc_query_impl[32db4d22f0b27131]::query_impl::has_ffi_unwind_calls::get_query_non_incr::__rust_end_short_backtrace
  23:     0x78597c0a075f - rustc_mir_transform[7bdd74c785fb8b70]::mir_promoted
  24:     0x78597f0e2292 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::mir_promoted::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 16usize]>>
  25:     0x78597f0e255a - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  26:     0x78597fc2d9d0 - rustc_query_impl[32db4d22f0b27131]::query_impl::mir_promoted::get_query_non_incr::__rust_end_short_backtrace
  27:     0x78597fc2dac3 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>
  28:     0x78597ef68d7a - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  29:     0x78597ef687d3 - rustc_query_impl[32db4d22f0b27131]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  30:     0x78597f5c32f9 - rustc_middle[3555d1292876aeec]::query::plumbing::query_get_at::<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>>
  31:     0x78597d6fc2d2 - <rustc_hir_analysis[3f1e84bf7d1c6f50]::collect::type_of::opaque::TaitConstraintLocator>::check
  32:     0x78597d69a8b2 - <rustc_hir_analysis[3f1e84bf7d1c6f50]::collect::type_of::opaque::TaitConstraintLocator as rustc_hir[5cb3b32afa60e663]::intravisit::Visitor>::visit_nested_item
  33:     0x78597fcca3b7 - rustc_hir_analysis[3f1e84bf7d1c6f50]::collect::type_of::type_of_opaque
  34:     0x78597fcc9ce5 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>
  35:     0x78597ec2972a - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::DefIdCache<rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  36:     0x78597fe49bb6 - rustc_query_impl[32db4d22f0b27131]::query_impl::type_of_opaque::get_query_non_incr::__rust_end_short_backtrace
  37:     0x78597f3be502 - rustc_middle[3555d1292876aeec]::query::plumbing::query_get_at::<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::DefIdCache<rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>>
  38:     0x78597bda2135 - rustc_hir_analysis[3f1e84bf7d1c6f50]::collect::type_of::type_of
  39:     0x78597ec2aa30 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>
  40:     0x78597ec2972a - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::DefIdCache<rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  41:     0x78597ec292e3 - rustc_query_impl[32db4d22f0b27131]::query_impl::type_of::get_query_non_incr::__rust_end_short_backtrace
  42:     0x78597f3be502 - rustc_middle[3555d1292876aeec]::query::plumbing::query_get_at::<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::DefIdCache<rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 8usize]>>>
  43:     0x78597fcd57d9 - rustc_hir_analysis[3f1e84bf7d1c6f50]::check::check::check_item_type
  44:     0x78597bba12ed - rustc_hir_analysis[3f1e84bf7d1c6f50]::check::wfcheck::check_well_formed
  45:     0x78597efbdac7 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>
  46:     0x78597efbd21f - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  47:     0x78597efbce90 - rustc_query_impl[32db4d22f0b27131]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  48:     0x78597efbdb47 - rustc_middle[3555d1292876aeec]::query::plumbing::query_ensure_error_guaranteed::<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::VecCache<rustc_span[1759084578182824]::def_id::LocalDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>, ()>
  49:     0x78597efbe11b - rustc_hir_analysis[3f1e84bf7d1c6f50]::check::wfcheck::check_mod_type_wf
  50:     0x78597efbdb6f - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>
  51:     0x78597f74533b - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::DefaultCache<rustc_span[1759084578182824]::def_id::LocalModDefId, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  52:     0x78597f7450ed - rustc_query_impl[32db4d22f0b27131]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  53:     0x78597ef648bb - rustc_hir_analysis[3f1e84bf7d1c6f50]::check_crate
  54:     0x78597f519f57 - rustc_interface[ca54b24fb50a6540]::passes::run_required_analyses
  55:     0x78597face5de - rustc_interface[ca54b24fb50a6540]::passes::analysis
  56:     0x78597face5b1 - rustc_query_impl[32db4d22f0b27131]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[32db4d22f0b27131]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>
  57:     0x78597fb7a56e - rustc_query_system[4eccca7fcaf3aaf0]::query::plumbing::try_execute_query::<rustc_query_impl[32db4d22f0b27131]::DynamicConfig<rustc_query_system[4eccca7fcaf3aaf0]::query::caches::SingleCache<rustc_middle[3555d1292876aeec]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[32db4d22f0b27131]::plumbing::QueryCtxt, false>
  58:     0x78597fb7a24f - rustc_query_impl[32db4d22f0b27131]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  59:     0x78597fa0e91e - rustc_interface[ca54b24fb50a6540]::interface::run_compiler::<core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>, rustc_driver_impl[d25ecc56b0b66360]::run_compiler::{closure#0}>::{closure#1}
  60:     0x78597faab714 - std[7ea95b089189ea8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[ca54b24fb50a6540]::util::run_in_thread_with_globals<rustc_interface[ca54b24fb50a6540]::util::run_in_thread_pool_with_globals<rustc_interface[ca54b24fb50a6540]::interface::run_compiler<core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>, rustc_driver_impl[d25ecc56b0b66360]::run_compiler::{closure#0}>::{closure#1}, core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>>::{closure#0}, core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>>
  61:     0x78597faabb28 - <<std[7ea95b089189ea8]::thread::Builder>::spawn_unchecked_<rustc_interface[ca54b24fb50a6540]::util::run_in_thread_with_globals<rustc_interface[ca54b24fb50a6540]::util::run_in_thread_pool_with_globals<rustc_interface[ca54b24fb50a6540]::interface::run_compiler<core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>, rustc_driver_impl[d25ecc56b0b66360]::run_compiler::{closure#0}>::{closure#1}, core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>>::{closure#0}, core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[8e2925c99194b988]::result::Result<(), rustc_span[1759084578182824]::ErrorGuaranteed>>::{closure#1} as core[8e2925c99194b988]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  62:     0x78597faac5eb - std::sys::pal::unix::thread::Thread::new::thread_start::hfa6ba0429166ab6d
  63:     0x78598132939d - <unknown>
  64:     0x7859813ae49c - <unknown>
  65:                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 1.84.0-nightly (db8043bb1 2024-10-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z validate-mir --crate-type lib -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
end of query stack

@rustbot label +F-trait_upcasting +F-type_alias_impl_trait

The lifetimes are unnecessary, this also fails when using -Zvalidate-mir:

#![feature(trait_upcasting, type_alias_impl_trait)]

type Tait = impl Sized;

trait Foo: Bar<Tait> {}
trait Bar<T> {}

fn test_correct3(x: &dyn Foo, _: Tait) {
    _ = x as &dyn Bar<()>;
}

Specifically the problem seems to be with using the hidden type in upcasting, when an opaque is expected.

Tried all three code snippets on playground:

Build using the Nightly version: 1.85.0-nightly

(2024-12-09 a224f3807e58afc93535)

None of the snippets ICEs.

Might worth adding some (or all) of them to test suites.

@crlf0710

@crlf0710 they only ICE with -Zvalidate-mir, which you can't enable on playground as far as I know

-Zvalidate-mir ICEing on erroneous code. Marking as fixed by #147092

not actually fixed by that

Rollup merge of rust-lang#163806 - lcnr:crashes-migration-cont, r=lqd

Unrolled build for #163806 in rollup 163957

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ICE: Unsize coercion, but `&dyn Foo<'_>` isn't coercible to `&dyn Bar<'_, '_, ()>` #131886

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ICE: Unsize coercion, but &dyn Foo<'_> isn't coercible to &dyn Bar<'_, '_, ()> #131886

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WaffleLapkin commented on Dec 4, 2024

crlf0710 commented on Dec 10, 2024

WaffleLapkin commented on Dec 10, 2024

cjgillot commented on Oct 15, 2025

lcnr commented on Oct 5, 2026

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ICE: Unsize coercion, but `&dyn Foo<'_>` isn't coercible to `&dyn Bar<'_, '_, ()>` #131886