[ICE]: `assertion failed: &vtable_entries[..vtable_entries_b.len()] == vtable_entries_b`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
trait Supertrait<T> {
    fn static(mut self) {}
}

trait Trait<P>: Supertrait<> {}

impl<P> Trait<P> for () {}

const fn upcast<P>(x: &dyn Trait<P>) -> &dyn Trait<P> {
    x
}

const fn foo() -> &'static dyn Supertrait<()> {
    upcast::<()>(&())
}

const _: &'static dyn Supertrait<()> = foo();

original:

trait Supertrait<T> {
    fn static(mut self) {}
}

trait Trait<P>: Supertrait<Output = impl ReturnsSend> {}

impl<P> Trait<P> for () {}

const fn upcast<P>(x: &dyn Trait<P>) -> &dyn Trait<P> {
    x
}

const fn foo() -> &'static dyn Supertrait<()> {
    upcast::<()>(&())
}

const _: &'static dyn Supertrait<()> = foo();

Version information

rustc 1.96.0-nightly (f82485388 2026-03-06)
binary: rustc
commit-hash: f82485388963286763f8c8a9261ac227627f6a18
commit-date: 2026-03-06
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

Possibly related line of code:

rust/compiler/rustc_const_eval/src/interpret/cast.rs

Lines 428 to 440 in f824853

	);
	} else {
	// In this case codegen would keep using the old vtable. We don't want to do
	// that as it has the wrong trait. The reason codegen can do this is that
	// one vtable is a prefix of the other, so we double-check that.
	let vtable_entries_b = self.vtable_entries(data_b.principal(), ty);
	assert!(&vtable_entries[..vtable_entries_b.len()] == vtable_entries_b);
	};
	// Get the destination trait vtable and return that.
	let new_vptr = self.get_vtable_ptr(ty, data_b)?;
	self.write_immediate(Immediate::new_dyn_trait(old_data, new_vptr, self), dest)
	}

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

Program output

error: expected identifier, found keyword `static`
 --> /tmp/icemaker_global_tempdir.1zEOWDZ8wIVR/rustc_testrunner_tmpdir_reporting.c3EWktKEHDaM/mvce.rs:2:8
  |
2 |     fn static(mut self) {}
  |        ^^^^^^ expected identifier, found keyword
  |
help: escape `static` to use it as an identifier
  |
2 |     fn r#static(mut self) {}
  |        ++

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.1zEOWDZ8wIVR/rustc_testrunner_tmpdir_reporting.c3EWktKEHDaM/mvce.rs:17:46
   |
17 | const _: &'static dyn Supertrait<()> = foo();
   |                                              ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.1zEOWDZ8wIVR/rustc_testrunner_tmpdir_reporting.c3EWktKEHDaM/mvce.rs`

error[E0107]: trait takes 1 generic argument but 0 generic arguments were supplied
 --> /tmp/icemaker_global_tempdir.1zEOWDZ8wIVR/rustc_testrunner_tmpdir_reporting.c3EWktKEHDaM/mvce.rs:5:17
  |
5 | trait Trait<P>: Supertrait<> {}
  |                 ^^^^^^^^^^ expected 1 generic argument
  |
note: trait defined here, with 1 generic parameter: `T`
 --> /tmp/icemaker_global_tempdir.1zEOWDZ8wIVR/rustc_testrunner_tmpdir_reporting.c3EWktKEHDaM/mvce.rs:1:7
  |
1 | trait Supertrait<T> {
  |       ^^^^^^^^^^ -
help: add missing generic argument
  |
5 | trait Trait<P>: Supertrait<T> {}
  |                            +

error[E0277]: the size for values of type `Self` cannot be known at compilation time
 --> /tmp/icemaker_global_tempdir.1zEOWDZ8wIVR/rustc_testrunner_tmpdir_reporting.c3EWktKEHDaM/mvce.rs:2:19
  |
2 |     fn static(mut self) {}
  |                   ^^^^ doesn't have a size known at compile-time
  |
  = help: unsized fn params are gated as an unstable feature
help: consider further restricting `Self`
  |
2 |     fn static(mut self) where Self: Sized {}
  |                         +++++++++++++++++
help: function arguments must have a statically known size, borrowed types always have a known size
  |
2 |     fn static(mut &self) {}
  |                   +


thread 'rustc' (3572335) panicked at /rustc-dev/f82485388963286763f8c8a9261ac227627f6a18/compiler/rustc_const_eval/src/interpret/cast.rs:434:21:
assertion failed: &vtable_entries[..vtable_entries_b.len()] == vtable_entries_b
stack backtrace:
   0:     0x7f34b735df6b - <<std[1ac9ec7f151dc343]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[2185e438fa60127]::fmt::Display>::fmt
   1:     0x7f34b7a22a08 - core[2185e438fa60127]::fmt::write
   2:     0x7f34b7374f76 - <std[1ac9ec7f151dc343]::sys::stdio::unix::Stderr as std[1ac9ec7f151dc343]::io::Write>::write_fmt
   3:     0x7f34b7334058 - std[1ac9ec7f151dc343]::panicking::default_hook::{closure#0}
   4:     0x7f34b73513e3 - std[1ac9ec7f151dc343]::panicking::default_hook
   5:     0x7f34b6337e4c - std[1ac9ec7f151dc343]::panicking::update_hook::<alloc[1b3f84b8c09c25b1]::boxed::Box<rustc_driver_impl[6e789f234d303bfb]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f34b73516c2 - std[1ac9ec7f151dc343]::panicking::panic_with_hook
   7:     0x7f34b733414a - std[1ac9ec7f151dc343]::panicking::panic_handler::{closure#0}
   8:     0x7f34b7328669 - std[1ac9ec7f151dc343]::sys::backtrace::__rust_end_short_backtrace::<std[1ac9ec7f151dc343]::panicking::panic_handler::{closure#0}, !>
   9:     0x7f34b7335b7d - __rustc[3c4f6ba47a7fa14c]::rust_begin_unwind
  10:     0x7f34b3f4543c - core[2185e438fa60127]::panicking::panic_fmt
  11:     0x7f34b3a01012 - core[2185e438fa60127]::panicking::panic
  12:     0x7f34b7e4563e - <rustc_const_eval[932f6faba1bcace8]::interpret::eval_context::InterpCx<rustc_const_eval[932f6faba1bcace8]::const_eval::machine::CompileTimeMachine>>::unsize_into_ptr::<rustc_const_eval[932f6faba1bcace8]::interpret::place::PlaceTy>
  13:     0x7f34b8acc5b6 - <rustc_const_eval[932f6faba1bcace8]::interpret::eval_context::InterpCx<rustc_const_eval[932f6faba1bcace8]::const_eval::machine::CompileTimeMachine>>::eval_rvalue_into_place
  14:     0x7f34b8a86084 - rustc_const_eval[932f6faba1bcace8]::const_eval::eval_queries::eval_to_allocation_raw_provider
  15:     0x7f34b8a84858 - rustc_query_impl[234cac1489f93c02]::query_impl::eval_to_allocation_raw::invoke_provider_fn::__rust_begin_short_backtrace
  16:     0x7f34b80ebaca - rustc_query_impl[234cac1489f93c02]::execution::try_execute_query::<rustc_middle[6c1cd5853b8c3360]::query::caches::DefaultCache<rustc_middle[6c1cd5853b8c3360]::ty::PseudoCanonicalInput<rustc_middle[6c1cd5853b8c3360]::mir::interpret::GlobalId>, rustc_middle[6c1cd5853b8c3360]::query::erase::ErasedData<[u8; 24usize]>>, false>
  17:     0x7f34b80eb47f - rustc_query_impl[234cac1489f93c02]::query_impl::eval_to_allocation_raw::execute_query_non_incr::__rust_end_short_backtrace
  18:     0x7f34b80ed0af - rustc_const_eval[932f6faba1bcace8]::const_eval::eval_queries::eval_to_const_value_raw_provider
  19:     0x7f34b80ecc16 - rustc_query_impl[234cac1489f93c02]::query_impl::eval_to_const_value_raw::invoke_provider_fn::__rust_begin_short_backtrace
  20:     0x7f34b80eb9c2 - rustc_query_impl[234cac1489f93c02]::execution::try_execute_query::<rustc_middle[6c1cd5853b8c3360]::query::caches::DefaultCache<rustc_middle[6c1cd5853b8c3360]::ty::PseudoCanonicalInput<rustc_middle[6c1cd5853b8c3360]::mir::interpret::GlobalId>, rustc_middle[6c1cd5853b8c3360]::query::erase::ErasedData<[u8; 24usize]>>, false>
  21:     0x7f34b80eb53f - rustc_query_impl[234cac1489f93c02]::query_impl::eval_to_const_value_raw::execute_query_non_incr::__rust_end_short_backtrace
  22:     0x7f34b8574b67 - <rustc_middle[6c1cd5853b8c3360]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[b1c64fc70d705e68]::check_crate::{closure#2}>::{closure#0}
  23:     0x7f34b85741b5 - rustc_hir_analysis[b1c64fc70d705e68]::check_crate
  24:     0x7f34b7abc79d - rustc_interface[c926bcc183cff181]::passes::analysis
  25:     0x7f34b8c82760 - rustc_query_impl[234cac1489f93c02]::execution::try_execute_query::<rustc_middle[6c1cd5853b8c3360]::query::caches::SingleCache<rustc_middle[6c1cd5853b8c3360]::query::erase::ErasedData<[u8; 0usize]>>, false>
  26:     0x7f34b8c82447 - rustc_query_impl[234cac1489f93c02]::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
  27:     0x7f34b8c3da6d - rustc_interface[c926bcc183cff181]::interface::run_compiler::<(), rustc_driver_impl[6e789f234d303bfb]::run_compiler::{closure#0}>::{closure#1}
  28:     0x7f34b8c6613e - std[1ac9ec7f151dc343]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[c926bcc183cff181]::util::run_in_thread_with_globals<rustc_interface[c926bcc183cff181]::util::run_in_thread_pool_with_globals<rustc_interface[c926bcc183cff181]::interface::run_compiler<(), rustc_driver_impl[6e789f234d303bfb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  29:     0x7f34b8c669e0 - <std[1ac9ec7f151dc343]::thread::lifecycle::spawn_unchecked<rustc_interface[c926bcc183cff181]::util::run_in_thread_with_globals<rustc_interface[c926bcc183cff181]::util::run_in_thread_pool_with_globals<rustc_interface[c926bcc183cff181]::interface::run_compiler<(), rustc_driver_impl[6e789f234d303bfb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[2185e438fa60127]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7f34b8c6786c - <std[1ac9ec7f151dc343]::sys::thread::unix::Thread>::new::thread_start
  31:     0x7f34b265e98b - <unknown>
  32:     0x7f34b26e2a0c - <unknown>
  33:                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 1.96.0-nightly (f82485388 2026-03-06) 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 `_`
#1 [eval_to_const_value_raw] simplifying constant for the type system `_`
#2 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0107, E0277, E0601.
For more information about an error, try `rustc --explain E0107`.

@rustbot label +WG-trait-system-refactor

[matthiaskrgr]

#![feature(unboxed_closures)]
trait Supertrait<T> {
    fn a(self) {}
}

trait Trait: Supertrait<T> {}

impl Trait for () {}
const fn upcast<P>(x: &dyn Trait) -> &dyn Trait {
    x
}
const fn foo() -> &'static dyn Supertrait() {
    upcast::<()>(&())
}
const _: &'static dyn Supertrait<()> = foo();

fn main() {}