rust-lang / rust-lang/rust

ICE caused by `feature(non_lifetime_binders)` feature in combination with trait bounds involving GATs

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C-bug F-non_lifetime_binders I-ICE S-bug-has-test S-has-mcve T-compiler
Dominant language
Rust
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Description

rustc-ice-2025-12-26T18_23_13-31309.txt

Code
#![feature(non_lifetime_binders)]

trait A<T> {
    type GAT<U>: A<U>;
    fn foo<U>(self, u: U) -> Self::GAT<U>
    where
        Self: Sized;
}

trait B<T>: A<T>
where
    for<U> Self::GAT<U>: B<U>,
{
    fn bar<U>(self) -> Self::GAT<U>
    where
        Self: Sized;

    fn baz<U>(self, u: U) -> Self::GAT<U>
    where
        Self: Sized,
    {
        self.foo(u).bar()
    }
}

Playground link

Meta

rustc --version --verbose:

rustc 1.94.0-nightly (fabece9e9 2025-12-25)
binary: rustc
commit-hash: fabece9e9491d0a3c3655dba488881968b7c5f7a
commit-date: 2025-12-25
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8
Error output
error: internal compiler error: /rustc-dev/fabece9e9491d0a3c3655dba488881968b7c5f7a/compiler/rustc_middle/src/ty/fold.rs:236:33: unexpected bound ty in binder: DefId(0:11 ~ non_lifetime_binders_ice[56ef]::B::U)

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: please attach the file at `/home/mego/non-lifetime-binders-ice/rustc-ice-2025-12-26T18_23_13-31309.txt` to your bug report

note: rustc 1.94.0-nightly (fabece9e9 2025-12-25) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [typeck] type-checking `B::baz`
#1 [analysis] running analysis passes on crate `non_lifetime_binders_ice`
end of query stack
warning: `non-lifetime-binders-ice` (lib) generated 1 warning
error: could not compile `non-lifetime-binders-ice` (lib); 1 warning emitted

Caused by:
  process didn't exit successfully: `/home/mego/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name non_lifetime_binders_ice --edition=2024 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=120 --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=0d4d6d84d0327686 -C extra-filename=-945b58e5f233fce9 --out-dir /home/mego/non-lifetime-binders-ice/target/debug/deps -C incremental=/home/mego/non-lifetime-binders-ice/target/debug/incremental -L dependency=/home/mego/non-lifetime-binders-ice/target/debug/deps` (exit status: 101)
Backtrace 

thread 'rustc' (32094) panicked at /rustc-dev/fabece9e9491d0a3c3655dba488881968b7c5f7a/compiler/rustc_middle/src/ty/fold.rs:236:33:
Box<dyn Any>
stack backtrace:
   0:     0x7fd0c205e583 - <<std[c1452cd6b6f4b41c]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[df330930cbc6b342]::fmt::Display>::fmt
   1:     0x7fd0c28106c8 - core[df330930cbc6b342]::fmt::write
   2:     0x7fd0c2074cd6 - <std[c1452cd6b6f4b41c]::sys::stdio::unix::Stderr as std[c1452cd6b6f4b41c]::io::Write>::write_fmt
   3:     0x7fd0c2034d28 - std[c1452cd6b6f4b41c]::panicking::default_hook::{closure#0}
   4:     0x7fd0c20523f3 - std[c1452cd6b6f4b41c]::panicking::default_hook
   5:     0x7fd0c102e1ba - std[c1452cd6b6f4b41c]::panicking::update_hook::<alloc[fdba8dc7c7cb980c]::boxed::Box<rustc_driver_impl[e9308e410a80ccc2]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7fd0c20526d2 - std[c1452cd6b6f4b41c]::panicking::panic_with_hook
   7:     0x7fd0c106ab31 - std[c1452cd6b6f4b41c]::panicking::begin_panic::<rustc_errors[8cf289fd9a18de16]::ExplicitBug>::{closure#0}
   8:     0x7fd0c105b076 - std[c1452cd6b6f4b41c]::sys::backtrace::__rust_end_short_backtrace::<std[c1452cd6b6f4b41c]::panicking::begin_panic<rustc_errors[8cf289fd9a18de16]::ExplicitBug>::{closure#0}, !>
   9:     0x7fd0c1058419 - std[c1452cd6b6f4b41c]::panicking::begin_panic::<rustc_errors[8cf289fd9a18de16]::ExplicitBug>
  10:     0x7fd0c1088b21 - <rustc_errors[8cf289fd9a18de16]::diagnostic::BugAbort as rustc_errors[8cf289fd9a18de16]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7fd0c16364a9 - rustc_middle[54bcc516263886d0]::util::bug::opt_span_bug_fmt::<rustc_span[160bea70edb6b602]::span_encoding::Span>::{closure#0}
  12:     0x7fd0c1636632 - rustc_middle[54bcc516263886d0]::ty::context::tls::with_opt::<rustc_middle[54bcc516263886d0]::util::bug::opt_span_bug_fmt<rustc_span[160bea70edb6b602]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7fd0c162750b - rustc_middle[54bcc516263886d0]::ty::context::tls::with_context_opt::<rustc_middle[54bcc516263886d0]::ty::context::tls::with_opt<rustc_middle[54bcc516263886d0]::util::bug::opt_span_bug_fmt<rustc_span[160bea70edb6b602]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7fd0bf04fd64 - rustc_middle[54bcc516263886d0]::util::bug::bug_fmt
  15:     0x7fd0c1de847e - <rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>::instantiate_bound_regions_uncached::<rustc_type_ir[a4a11647208c0e96]::ty_kind::FnSig<rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>, <rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>::instantiate_bound_regions<rustc_type_ir[a4a11647208c0e96]::ty_kind::FnSig<rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>, <rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>::instantiate_bound_regions_with_erased<rustc_type_ir[a4a11647208c0e96]::ty_kind::FnSig<rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>>::{closure#0}>::{closure#0}>::{closure#0}
  16:     0x7fd0c16b7f66 - <rustc_middle[54bcc516263886d0]::ty::fold::FnMutDelegate as rustc_middle[54bcc516263886d0]::ty::fold::BoundVarReplacerDelegate>::replace_ty
  17:     0x7fd0c29dfcfd - <rustc_middle[54bcc516263886d0]::ty::fold::BoundVarReplacer<rustc_middle[54bcc516263886d0]::ty::fold::FnMutDelegate> as rustc_type_ir[a4a11647208c0e96]::fold::TypeFolder<rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>>::fold_ty
  18:     0x7fd0c29dec4c - <&rustc_middle[54bcc516263886d0]::ty::list::RawList<(), rustc_middle[54bcc516263886d0]::ty::generic_args::GenericArg> as rustc_type_ir[a4a11647208c0e96]::fold::TypeFoldable<rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>>::fold_with::<rustc_middle[54bcc516263886d0]::ty::fold::BoundVarReplacer<rustc_middle[54bcc516263886d0]::ty::fold::FnMutDelegate>>
  19:     0x7fd0c29dfbc1 - <rustc_middle[54bcc516263886d0]::ty::fold::BoundVarReplacer<rustc_middle[54bcc516263886d0]::ty::fold::FnMutDelegate> as rustc_type_ir[a4a11647208c0e96]::fold::TypeFolder<rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>>::fold_ty
  20:     0x7fd0c1d4d2bf - <rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>::instantiate_bound_regions_with_erased::<rustc_middle[54bcc516263886d0]::ty::Ty>
  21:     0x7fd0c1e685b6 - <rustc_trait_selection[ce697ee2c32d50b6]::error_reporting::TypeErrCtxt>::suggest_dereferences
  22:     0x7fd0c1e2b715 - <rustc_trait_selection[ce697ee2c32d50b6]::error_reporting::TypeErrCtxt>::report_selection_error
  23:     0x7fd0c096695a - <rustc_trait_selection[ce697ee2c32d50b6]::error_reporting::TypeErrCtxt>::report_fulfillment_errors
  24:     0x7fd0c2c77c3a - <rustc_hir_typeck[6de9ddff819006ec]::fn_ctxt::FnCtxt>::check_expr_method_call
  25:     0x7fd0c2ca46f0 - <rustc_hir_typeck[6de9ddff819006ec]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  26:     0x7fd0c2c9eb65 - <rustc_hir_typeck[6de9ddff819006ec]::fn_ctxt::FnCtxt>::check_expr_block
  27:     0x7fd0c2ca470c - <rustc_hir_typeck[6de9ddff819006ec]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  28:     0x7fd0c2c85491 - rustc_hir_typeck[6de9ddff819006ec]::check::check_fn
  29:     0x7fd0c3479cf9 - rustc_hir_typeck[6de9ddff819006ec]::typeck_with_inspect::{closure#0}
  30:     0x7fd0c3478670 - rustc_query_impl[94c8afbc39e3af62]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[94c8afbc39e3af62]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[54bcc516263886d0]::query::erase::Erased<[u8; 8usize]>>
  31:     0x7fd0c2a45c3e - rustc_query_system[f270da620ae7bca3]::query::plumbing::try_execute_query::<rustc_query_impl[94c8afbc39e3af62]::DynamicConfig<rustc_data_structures[79db1b91e24649bb]::vec_cache::VecCache<rustc_span[160bea70edb6b602]::def_id::LocalDefId, rustc_middle[54bcc516263886d0]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[f270da620ae7bca3]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[94c8afbc39e3af62]::plumbing::QueryCtxt, true>
  32:     0x7fd0c2a45041 - rustc_query_impl[94c8afbc39e3af62]::query_impl::typeck::get_query_incr::__rust_end_short_backtrace
  33:     0x7fd0c3014d71 - <rustc_middle[54bcc516263886d0]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[b69b1346315f61b2]::check_crate::{closure#2}>::{closure#0}
  34:     0x7fd0c3014783 - rustc_hir_analysis[b69b1346315f61b2]::check_crate
  35:     0x7fd0c2a4a25b - rustc_interface[67132ee5d2f670a9]::passes::analysis
  36:     0x7fd0c2a49eb5 - rustc_query_impl[94c8afbc39e3af62]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[94c8afbc39e3af62]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[54bcc516263886d0]::query::erase::Erased<[u8; 0usize]>>
  37:     0x7fd0c3becd0f - rustc_query_system[f270da620ae7bca3]::query::plumbing::try_execute_query::<rustc_query_impl[94c8afbc39e3af62]::DynamicConfig<rustc_query_system[f270da620ae7bca3]::query::caches::SingleCache<rustc_middle[54bcc516263886d0]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[94c8afbc39e3af62]::plumbing::QueryCtxt, true>
  38:     0x7fd0c3bec5bf - rustc_query_impl[94c8afbc39e3af62]::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  39:     0x7fd0c3ac7951 - <rustc_interface[67132ee5d2f670a9]::passes::create_and_enter_global_ctxt<core[df330930cbc6b342]::option::Option<rustc_interface[67132ee5d2f670a9]::queries::Linker>, rustc_driver_impl[e9308e410a80ccc2]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[df330930cbc6b342]::ops::function::FnOnce<(&rustc_session[947e878a67e7e5f8]::session::Session, rustc_middle[54bcc516263886d0]::ty::context::CurrentGcx, alloc[fdba8dc7c7cb980c]::sync::Arc<rustc_data_structures[79db1b91e24649bb]::jobserver::Proxy>, &std[c1452cd6b6f4b41c]::sync::once_lock::OnceLock<rustc_middle[54bcc516263886d0]::ty::context::GlobalCtxt>, &rustc_data_structures[79db1b91e24649bb]::sync::worker_local::WorkerLocal<rustc_middle[54bcc516263886d0]::arena::Arena>, &rustc_data_structures[79db1b91e24649bb]::sync::worker_local::WorkerLocal<rustc_hir[9181c4482ba2e702]::Arena>, rustc_driver_impl[e9308e410a80ccc2]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  40:     0x7fd0c3a3678a - rustc_interface[67132ee5d2f670a9]::interface::run_compiler::<(), rustc_driver_impl[e9308e410a80ccc2]::run_compiler::{closure#0}>::{closure#1}
  41:     0x7fd0c39768ce - std[c1452cd6b6f4b41c]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[67132ee5d2f670a9]::util::run_in_thread_with_globals<rustc_interface[67132ee5d2f670a9]::util::run_in_thread_pool_with_globals<rustc_interface[67132ee5d2f670a9]::interface::run_compiler<(), rustc_driver_impl[e9308e410a80ccc2]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  42:     0x7fd0c39766a0 - <std[c1452cd6b6f4b41c]::thread::lifecycle::spawn_unchecked<rustc_interface[67132ee5d2f670a9]::util::run_in_thread_with_globals<rustc_interface[67132ee5d2f670a9]::util::run_in_thread_pool_with_globals<rustc_interface[67132ee5d2f670a9]::interface::run_compiler<(), rustc_driver_impl[e9308e410a80ccc2]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[df330930cbc6b342]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  43:     0x7fd0c397bff8 - <std[c1452cd6b6f4b41c]::sys::thread::unix::Thread>::new::thread_start
  44:     0x7fd0bd28aaa4 - start_thread
                               at ./nptl/pthread_create.c:447:8
  45:     0x7fd0bd317c3c - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:78:0
  46:                0x0 - <unknown>

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  4. Open a pull request that references the issue number.

Research direction

Start with the minimal reproducer in the issue and run it on the reported nightly compiler. Trace the panic at compiler/rustc_middle/src/ty/fold.rs:236 through type-checking B::baz and the bound-region instantiation path; done means the example no longer causes an internal compiler error and has regression coverage.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
45/100

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