ICE `expected region 'static to be of kind ReVar`

[matthiaskrgr]

auto-reduced (treereduce-rust):

type TraitObject = dyn Sync;

struct Wrap(TraitObject);

fn cast(x: *mut TraitObject) {
    x as *mut Wrap;
}

original:

//@ check-pass

trait Trait {
    type Associated;
}

impl Trait for i32 {
    type Associated = i64;
}

trait Generic<T> {}

type TraitObject = dyn Sync;

struct Wrap(TraitObject);

fn cast(x: *mut TraitObject) {
    x as *mut Wrap;
}

fn main() {}

Version information

rustc 1.94.0-nightly (50d59402b 2026-01-03)
binary: rustc
commit-hash: 50d59402bfb82a7fcdf542ed418affe425ff867e
commit-date: 2026-01-03
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

Possibly related line of code:

rust/compiler/rustc_middle/src/ty/region.rs

Lines 323 to 335 in 50d5940

	matches!(self.kind(), ty::ReVar(_))
	}
	pub fn as_var(self) -> RegionVid {
	match self.kind() {
	ty::ReVar(vid) => vid,
	_ => bug!("expected region {:?} to be of kind ReVar", self),
	}
	}
	/// Given some item `binding_item`, check if this region is a generic parameter introduced by it
	/// or one of the parent generics. Returns the `DefId` of the parameter definition if so.
	pub fn opt_param_def_id(self, tcx: TyCtxt<'tcx>, binding_item: DefId) -> Option<DefId> {

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.oycHALS4IOkc/rustc_testrunner_tmpdir_reporting.ZDChfUVNop4P/mvce.rs:7:2
  |
7 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.oycHALS4IOkc/rustc_testrunner_tmpdir_reporting.ZDChfUVNop4P/mvce.rs`

error: internal compiler error: /rustc-dev/50d59402bfb82a7fcdf542ed418affe425ff867e/compiler/rustc_middle/src/ty/region.rs:329:18: expected region 'static to be of kind ReVar


thread 'rustc' (24193) panicked at /rustc-dev/50d59402bfb82a7fcdf542ed418affe425ff867e/compiler/rustc_middle/src/ty/region.rs:329:18:
Box<dyn Any>
stack backtrace:
   0:     0x7f891d4328f3 - <<std[730d7555552d47c8]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[877f3b0911fa72ec]::fmt::Display>::fmt
   1:     0x7f891da0e348 - core[877f3b0911fa72ec]::fmt::write
   2:     0x7f891d449016 - <std[730d7555552d47c8]::sys::stdio::unix::Stderr as std[730d7555552d47c8]::io::Write>::write_fmt
   3:     0x7f891d4090e8 - std[730d7555552d47c8]::panicking::default_hook::{closure#0}
   4:     0x7f891d426833 - std[730d7555552d47c8]::panicking::default_hook
   5:     0x7f891c40ef5a - std[730d7555552d47c8]::panicking::update_hook::<alloc[3a018aa203959276]::boxed::Box<rustc_driver_impl[8eab3669a7d4ca59]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f891d426b12 - std[730d7555552d47c8]::panicking::panic_with_hook
   7:     0x7f891c44bf41 - std[730d7555552d47c8]::panicking::begin_panic::<rustc_errors[320084ff728c29dc]::ExplicitBug>::{closure#0}
   8:     0x7f891c43c476 - std[730d7555552d47c8]::sys::backtrace::__rust_end_short_backtrace::<std[730d7555552d47c8]::panicking::begin_panic<rustc_errors[320084ff728c29dc]::ExplicitBug>::{closure#0}, !>
   9:     0x7f891c439a0c - std[730d7555552d47c8]::panicking::begin_panic::<rustc_errors[320084ff728c29dc]::ExplicitBug>
  10:     0x7f891c469de1 - <rustc_errors[320084ff728c29dc]::diagnostic::BugAbort as rustc_errors[320084ff728c29dc]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f891ca180e9 - rustc_middle[b7c8bbfe634a1524]::util::bug::opt_span_bug_fmt::<rustc_span[266f76f4beb07170]::span_encoding::Span>::{closure#0}
  12:     0x7f891ca18272 - rustc_middle[b7c8bbfe634a1524]::ty::context::tls::with_opt::<rustc_middle[b7c8bbfe634a1524]::util::bug::opt_span_bug_fmt<rustc_span[266f76f4beb07170]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7f891ca0989b - rustc_middle[b7c8bbfe634a1524]::ty::context::tls::with_context_opt::<rustc_middle[b7c8bbfe634a1524]::ty::context::tls::with_opt<rustc_middle[b7c8bbfe634a1524]::util::bug::opt_span_bug_fmt<rustc_span[266f76f4beb07170]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7f891a53faf4 - rustc_middle[b7c8bbfe634a1524]::util::bug::bug_fmt
  15:     0x7f891f9f88e9 - <rustc_middle[b7c8bbfe634a1524]::ty::region::Region>::as_var.cold
  16:     0x7f891e86a51d - <rustc_borrowck[710768fa0e5030b6]::type_check::TypeChecker as rustc_middle[b7c8bbfe634a1524]::mir::visit::Visitor>::visit_body
  17:     0x7f891e81d3ac - rustc_borrowck[710768fa0e5030b6]::type_check::type_check
  18:     0x7f891e90c68c - <rustc_borrowck[710768fa0e5030b6]::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  19:     0x7f891e905ad0 - rustc_query_impl[4371051198203ad5]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[4371051198203ad5]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b7c8bbfe634a1524]::query::erase::Erased<[u8; 8usize]>>
  20:     0x7f891dc8fa77 - rustc_query_system[64fea02dfcda5c73]::query::plumbing::try_execute_query::<rustc_query_impl[4371051198203ad5]::DynamicConfig<rustc_data_structures[69e288f336e3affd]::vec_cache::VecCache<rustc_span[266f76f4beb07170]::def_id::LocalDefId, rustc_middle[b7c8bbfe634a1524]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[64fea02dfcda5c73]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[4371051198203ad5]::plumbing::QueryCtxt, false>
  21:     0x7f891e9009d9 - rustc_query_impl[4371051198203ad5]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  22:     0x7f891e900bf7 - <rustc_middle[b7c8bbfe634a1524]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface[fb6a287a72e6bfde]::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
  23:     0x7f891e8fe64a - rustc_interface[fb6a287a72e6bfde]::passes::analysis
  24:     0x7f891e8fda75 - rustc_query_impl[4371051198203ad5]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[4371051198203ad5]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b7c8bbfe634a1524]::query::erase::Erased<[u8; 0usize]>>
  25:     0x7f891ec0d166 - rustc_query_system[64fea02dfcda5c73]::query::plumbing::try_execute_query::<rustc_query_impl[4371051198203ad5]::DynamicConfig<rustc_query_system[64fea02dfcda5c73]::query::caches::SingleCache<rustc_middle[b7c8bbfe634a1524]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[4371051198203ad5]::plumbing::QueryCtxt, false>
  26:     0x7f891ec0cd36 - rustc_query_impl[4371051198203ad5]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  27:     0x7f891edb0a51 - <rustc_interface[fb6a287a72e6bfde]::passes::create_and_enter_global_ctxt<core[877f3b0911fa72ec]::option::Option<rustc_interface[fb6a287a72e6bfde]::queries::Linker>, rustc_driver_impl[8eab3669a7d4ca59]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[877f3b0911fa72ec]::ops::function::FnOnce<(&rustc_session[2d8187b2cd630bc8]::session::Session, rustc_middle[b7c8bbfe634a1524]::ty::context::CurrentGcx, alloc[3a018aa203959276]::sync::Arc<rustc_data_structures[69e288f336e3affd]::jobserver::Proxy>, &std[730d7555552d47c8]::sync::once_lock::OnceLock<rustc_middle[b7c8bbfe634a1524]::ty::context::GlobalCtxt>, &rustc_data_structures[69e288f336e3affd]::sync::worker_local::WorkerLocal<rustc_middle[b7c8bbfe634a1524]::arena::Arena>, &rustc_data_structures[69e288f336e3affd]::sync::worker_local::WorkerLocal<rustc_hir[351243de8ff3631d]::Arena>, rustc_driver_impl[8eab3669a7d4ca59]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  28:     0x7f891ec7bb18 - rustc_interface[fb6a287a72e6bfde]::interface::run_compiler::<(), rustc_driver_impl[8eab3669a7d4ca59]::run_compiler::{closure#0}>::{closure#1}
  29:     0x7f891eb2198e - std[730d7555552d47c8]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[fb6a287a72e6bfde]::util::run_in_thread_with_globals<rustc_interface[fb6a287a72e6bfde]::util::run_in_thread_pool_with_globals<rustc_interface[fb6a287a72e6bfde]::interface::run_compiler<(), rustc_driver_impl[8eab3669a7d4ca59]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  30:     0x7f891eb21760 - <std[730d7555552d47c8]::thread::lifecycle::spawn_unchecked<rustc_interface[fb6a287a72e6bfde]::util::run_in_thread_with_globals<rustc_interface[fb6a287a72e6bfde]::util::run_in_thread_pool_with_globals<rustc_interface[fb6a287a72e6bfde]::interface::run_compiler<(), rustc_driver_impl[8eab3669a7d4ca59]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[877f3b0911fa72ec]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  31:     0x7f891eb1fb38 - <std[730d7555552d47c8]::sys::thread::unix::Thread>::new::thread_start
  32:     0x7f891849698b - <unknown>
  33:     0x7f891851a9cc - <unknown>
  34:                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 1.94.0-nightly (50d59402b 2026-01-03) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_borrowck] borrow-checking `cast`
#1 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 2 previous errors

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

[matthiaskrgr]

this compiled on nightly-2025-12-10 and ICEs on nightly-2025-12-11

regression in #136776 cc @BoxyUwU

type TraitObject = dyn Sync; // auto-trait here

struct Wrap(TraitObject);

fn cast(x: *mut TraitObject) {
    x as *mut Wrap;
}

pub fn main() {}

[BoxyUwU]

struct Wrap(dyn Sync + 'static);

fn cast(x: *mut (dyn Sync + 'static)) {
    x as *mut Wrap;
}

fn main() {}

more minimal