rust-lang / rust-lang/rust

[ICE]: assertion failed: Foo == U

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@TaKO8Ki is already working on this.

Since Feb 4, 2026.

C-bug F-coerce_unsized I-ICE T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: T,
}

impl<T, U> CoerceUnsized<U> for Foo<T> {}

union U {
    a: usize,
}

const C: U = Foo { a: 10 };

original:

use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: T,
}

impl<T, U> CoerceUnsized<U> for Foo<T> {}

union U {
    a: usize,
}

const C: U = Foo { a: 10 };

const _: [(); {
    C;
    0
}] = [];

fn main() {
               |found: &String| Some(false);
               4
           }

Version information

rustc 1.95.0-nightly (621d76794 2026-01-18)
binary: rustc
commit-hash: 621d76794c76fc21c0a6151fbc090120e0230a91
commit-date: 2026-01-18
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 21.1.8

Possibly related line of code:
https://github.com/rust-lang/rust/blob/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_const_eval/src/interpret/cast.rs#L472-L484

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Znext-solver=globally

Program output 

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.dvo0cILjVdxS/rustc_testrunner_tmpdir_reporting.UkYxoYbwxxzg/mvce.rs:13:28
   |
13 | const C: U = Foo { a: 10 };
   |                            ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.dvo0cILjVdxS/rustc_testrunner_tmpdir_reporting.UkYxoYbwxxzg/mvce.rs`

error[E0658]: use of unstable library feature `coerce_unsized`
 --> /tmp/icemaker_global_tempdir.dvo0cILjVdxS/rustc_testrunner_tmpdir_reporting.UkYxoYbwxxzg/mvce.rs:1:5
  |
1 | use std::ops::CoerceUnsized;
  |     ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #18598 <https://github.com/rust-lang/rust/issues/18598> for more information
  = help: add `#![feature(coerce_unsized)]` to the crate attributes to enable
  = note: this compiler was built on 2026-01-18; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature `coerce_unsized`
 --> /tmp/icemaker_global_tempdir.dvo0cILjVdxS/rustc_testrunner_tmpdir_reporting.UkYxoYbwxxzg/mvce.rs:7:12
  |
7 | impl<T, U> CoerceUnsized<U> for Foo<T> {}
  |            ^^^^^^^^^^^^^^^^
  |
  = note: see issue #18598 <https://github.com/rust-lang/rust/issues/18598> for more information
  = help: add `#![feature(coerce_unsized)]` to the crate attributes to enable
  = note: this compiler was built on 2026-01-18; consider upgrading it if it is out of date

error[E0377]: the trait `CoerceUnsized` may only be implemented for a coercion between structures
 --> /tmp/icemaker_global_tempdir.dvo0cILjVdxS/rustc_testrunner_tmpdir_reporting.UkYxoYbwxxzg/mvce.rs:7:1
  |
7 | impl<T, U> CoerceUnsized<U> for Foo<T> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (1666937) panicked at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_const_eval/src/interpret/cast.rs:478:17:
assertion `left == right` failed
  left: Foo
 right: U
stack backtrace:
   0:     0x7fee2a633fd3 - <<std[287527946f71f0bc]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[7477a31a1a34cc49]::fmt::Display>::fmt
   1:     0x7fee2ac0e2c8 - core[7477a31a1a34cc49]::fmt::write
   2:     0x7fee2a64a9c6 - <std[287527946f71f0bc]::sys::stdio::unix::Stderr as std[287527946f71f0bc]::io::Write>::write_fmt
   3:     0x7fee2a60a048 - std[287527946f71f0bc]::panicking::default_hook::{closure#0}
   4:     0x7fee2a627703 - std[287527946f71f0bc]::panicking::default_hook
   5:     0x7fee295fbb8a - std[287527946f71f0bc]::panicking::update_hook::<alloc[979041e9952e771b]::boxed::Box<rustc_driver_impl[517260a07841a630]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fee2a6279e2 - std[287527946f71f0bc]::panicking::panic_with_hook
   7:     0x7fee2a60a108 - std[287527946f71f0bc]::panicking::panic_handler::{closure#0}
   8:     0x7fee2a600dc9 - std[287527946f71f0bc]::sys::backtrace::__rust_end_short_backtrace::<std[287527946f71f0bc]::panicking::panic_handler::{closure#0}, !>
   9:     0x7fee2a60bbdd - __rustc[5ec7081ce87b032f]::rust_begin_unwind
  10:     0x7fee2710cfbc - core[7477a31a1a34cc49]::panicking::panic_fmt
  11:     0x7fee28f7d6d3 - core[7477a31a1a34cc49]::panicking::assert_failed_inner
  12:     0x7fee294fd783 - core[7477a31a1a34cc49]::panicking::assert_failed::<rustc_middle[725063e074f29df]::ty::adt::AdtDef, rustc_middle[725063e074f29df]::ty::adt::AdtDef>
  13:     0x7fee2c74423d - <rustc_const_eval[b948c5b36ae67fb9]::interpret::eval_context::InterpCx<rustc_const_eval[b948c5b36ae67fb9]::const_eval::machine::CompileTimeMachine>>::unsize_into::<rustc_const_eval[b948c5b36ae67fb9]::interpret::place::PlaceTy>.cold
  14:     0x7fee2bd00904 - <rustc_const_eval[b948c5b36ae67fb9]::interpret::eval_context::InterpCx<rustc_const_eval[b948c5b36ae67fb9]::const_eval::machine::CompileTimeMachine>>::eval_rvalue_into_place
  15:     0x7fee2bcba0cd - rustc_const_eval[b948c5b36ae67fb9]::const_eval::eval_queries::eval_to_allocation_raw_provider
  16:     0x7fee2bcb847e - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::eval_to_allocation_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 24usize]>>
  17:     0x7fee2bcb7ba5 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_query_system[b7e73caa2c2ad9a]::query::caches::DefaultCache<rustc_middle[725063e074f29df]::ty::PseudoCanonicalInput<rustc_middle[725063e074f29df]::mir::interpret::GlobalId>, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  18:     0x7fee2bcb7749 - rustc_query_impl[f20b8284200e680a]::query_impl::eval_to_allocation_raw::get_query_non_incr::__rust_end_short_backtrace
  19:     0x7fee2bcb3504 - rustc_const_eval[b948c5b36ae67fb9]::const_eval::eval_queries::eval_to_const_value_raw_provider
  20:     0x7fee2bcb3080 - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::eval_to_const_value_raw::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 24usize]>>
  21:     0x7fee2bcb1c8a - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_query_system[b7e73caa2c2ad9a]::query::caches::DefaultCache<rustc_middle[725063e074f29df]::ty::PseudoCanonicalInput<rustc_middle[725063e074f29df]::mir::interpret::GlobalId>, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 24usize]>>, false, true, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  22:     0x7fee2bcb17f3 - rustc_query_impl[f20b8284200e680a]::query_impl::eval_to_const_value_raw::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7fee2b7401b0 - <rustc_middle[725063e074f29df]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[1a80394817eddfdd]::check_crate::{closure#2}>::{closure#0}
  24:     0x7fee2b73f7c4 - rustc_hir_analysis[1a80394817eddfdd]::check_crate
  25:     0x7fee2ad493db - rustc_interface[5c48368f5debe80d]::passes::analysis
  26:     0x7fee2ad49035 - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 0usize]>>
  27:     0x7fee2bda4be3 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_query_system[b7e73caa2c2ad9a]::query::caches::SingleCache<rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  28:     0x7fee2bda49ce - rustc_query_impl[f20b8284200e680a]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  29:     0x7fee2bf82c11 - <rustc_interface[5c48368f5debe80d]::passes::create_and_enter_global_ctxt<core[7477a31a1a34cc49]::option::Option<rustc_interface[5c48368f5debe80d]::queries::Linker>, rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[7477a31a1a34cc49]::ops::function::FnOnce<(&rustc_session[eb79e6d3104f29ae]::session::Session, rustc_middle[725063e074f29df]::ty::context::CurrentGcx, alloc[979041e9952e771b]::sync::Arc<rustc_data_structures[e0d8483206daafa2]::jobserver::Proxy>, &std[287527946f71f0bc]::sync::once_lock::OnceLock<rustc_middle[725063e074f29df]::ty::context::GlobalCtxt>, &rustc_data_structures[e0d8483206daafa2]::sync::worker_local::WorkerLocal<rustc_middle[725063e074f29df]::arena::Arena>, &rustc_data_structures[e0d8483206daafa2]::sync::worker_local::WorkerLocal<rustc_hir[7eac7792dedc1861]::Arena>, rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  30:     0x7fee2bdf5d34 - rustc_interface[5c48368f5debe80d]::interface::run_compiler::<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}
  31:     0x7fee2bd5aa3a - std[287527946f71f0bc]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5c48368f5debe80d]::util::run_in_thread_with_globals<rustc_interface[5c48368f5debe80d]::util::run_in_thread_pool_with_globals<rustc_interface[5c48368f5debe80d]::interface::run_compiler<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:     0x7fee2bd5a7fe - <std[287527946f71f0bc]::thread::lifecycle::spawn_unchecked<rustc_interface[5c48368f5debe80d]::util::run_in_thread_with_globals<rustc_interface[5c48368f5debe80d]::util::run_in_thread_pool_with_globals<rustc_interface[5c48368f5debe80d]::interface::run_compiler<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7477a31a1a34cc49]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x7fee2bd5c5c4 - <std[287527946f71f0bc]::sys::thread::unix::Thread>::new::thread_start
  34:     0x7fee2569698b - <unknown>
  35:     0x7fee2571a9cc - <unknown>
  36:                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 1.95.0-nightly (621d76794 2026-01-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [eval_to_allocation_raw] const-evaluating + checking `C`
#1 [eval_to_const_value_raw] simplifying constant for the type system `C`
#2 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0377, E0601, E0658.
For more information about an error, try `rustc --explain E0377`.

@rustbot label +WG-trait-system-refactor

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