rust-lang / rust-lang/rust

ICE: wf: `..has escaping bound vars, so it cannot be wrapped in a dummy binder`

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C-bug F-checked_type_aliases F-generic_const_exprs I-ICE T-compiler
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

#![feature(generic_const_exprs)]
#![feature(lazy_type_alias)]
type FooArg<'a, 'bb> = [u8; x];
type _TaWhere1 = Box<Fn(FooArg)>;

original:

#![feature(generic_const_exprs)]
#![feature(lazy_type_alias)]
type FooArg<'a, 'bb> = [u8; x];
type _TaWhere1 = Box<Fn(FooArg) >;

pub fn main() {}

Version information

rustc 1.92.0-nightly (ce4beebec 2025-09-22)
binary: rustc
commit-hash: ce4beebecb77821734079cff47d8af08f9f27f11
commit-date: 2025-09-22
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.1

Possibly related line of code:
https://github.com/rust-lang/rust/blob/ce4beebecb77821734079cff47d8af08f9f27f11/compiler/rustc_trait_selection/src/traits/wf.rs#L754-L766

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

Program output 

error[E0425]: cannot find value `x` in this scope
 --> /tmp/icemaker_global_tempdir.a4ZmvyZfIZaM/rustc_testrunner_tmpdir_reporting.eJnlgeCgC6AQ/mvce.rs:3:29
  |
3 | type FooArg<'a, 'bb> = [u8; x];
  |                             ^ not found in this scope

warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.a4ZmvyZfIZaM/rustc_testrunner_tmpdir_reporting.eJnlgeCgC6AQ/mvce.rs:1:12
  |
1 | #![feature(generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information
  = note: `#[warn(incomplete_features)]` on by default

warning: the feature `lazy_type_alias` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.a4ZmvyZfIZaM/rustc_testrunner_tmpdir_reporting.eJnlgeCgC6AQ/mvce.rs:2:12
  |
2 | #![feature(lazy_type_alias)]
  |            ^^^^^^^^^^^^^^^
  |
  = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.a4ZmvyZfIZaM/rustc_testrunner_tmpdir_reporting.eJnlgeCgC6AQ/mvce.rs:4:34
  |
4 | type _TaWhere1 = Box<Fn(FooArg)>;
  |                                  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.a4ZmvyZfIZaM/rustc_testrunner_tmpdir_reporting.eJnlgeCgC6AQ/mvce.rs`

warning: trait objects without an explicit `dyn` are deprecated
 --> /tmp/icemaker_global_tempdir.a4ZmvyZfIZaM/rustc_testrunner_tmpdir_reporting.eJnlgeCgC6AQ/mvce.rs:4:22
  |
4 | type _TaWhere1 = Box<Fn(FooArg)>;
  |                      ^^^^^^^^^^
  |
  = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
  = note: for more information, see <https://doc.rust-lang.org/edition-guide/rust-2021/warnings-promoted-to-error.html>
  = note: `#[warn(bare_trait_objects)]` (part of `#[warn(rust_2021_compatibility)]`) on by default
help: if this is a dyn-compatible trait, use `dyn`
  |
4 | type _TaWhere1 = Box<dyn Fn(FooArg)>;
  |                      +++


thread 'rustc' (286396) panicked at compiler/rustc_trait_selection/src/traits/wf.rs:760:21:
`ConstArgHasType(UnevaluatedConst { def: DefId(0:6 ~ mvce[47ca]::FooArg::{constant#0}), args: ['^0.Named(DefId(0:8 ~ mvce[47ca]::_TaWhere1::'_)), '^1.Named(DefId(0:9 ~ mvce[47ca]::_TaWhere1::'_#1))] }, usize)` has escaping bound vars, so it cannot be wrapped in a dummy binder.
stack backtrace:
   0:     0x7fca3ba02753 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h3e5bd3a23c0347a9
   1:     0x7fca3c202044 - core::fmt::write::heee6829eb564c27d
   2:     0x7fca3b9b7101 - std::io::Write::write_fmt::ha453775b50cc97a5
   3:     0x7fca3b9c84c2 - std::sys::backtrace::BacktraceLock::print::h71d64778f60f39dd
   4:     0x7fca3b9ce429 - std::panicking::default_hook::{{closure}}::h0c1df9123d837333
   5:     0x7fca3b9cdf53 - std::panicking::default_hook::h1518572259f2e3d5
   6:     0x7fca3a9f1e47 - std[18d792bd9b3d078a]::panicking::update_hook::<alloc[cb65cdfa434cac9f]::boxed::Box<rustc_driver_impl[477d964a0f4cf97e]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7fca3b9ce84f - std::panicking::panic_with_hook::hec947c027cbe77e3
   8:     0x7fca3b9ce60a - std::panicking::panic_handler::{{closure}}::h420539349ea435e2
   9:     0x7fca3b9c85f9 - std::sys::backtrace::__rust_end_short_backtrace::h3bc70f6bdfad0d7e
  10:     0x7fca3b9a913d - __rustc[5f47dcb501f887f7]::rust_begin_unwind
  11:     0x7fca389fe0a0 - core::panicking::panic_fmt::hff23e5fdf1301a77
  12:     0x7fca3de97908 - <rustc_type_ir[75560ed23b05734]::binder::Binder<rustc_middle[168a57e624ed4250]::ty::context::TyCtxt, rustc_type_ir[75560ed23b05734]::predicate_kind::PredicateKind<rustc_middle[168a57e624ed4250]::ty::context::TyCtxt>>>::dummy.cold
  13:     0x7fca3cc12ca3 - <rustc_trait_selection[ff1980eedda976e0]::traits::wf::WfPredicates as rustc_type_ir[75560ed23b05734]::visit::TypeVisitor<rustc_middle[168a57e624ed4250]::ty::context::TyCtxt>>::visit_ty
  14:     0x7fca3cc11f59 - <rustc_trait_selection[ff1980eedda976e0]::traits::wf::WfPredicates as rustc_type_ir[75560ed23b05734]::visit::TypeVisitor<rustc_middle[168a57e624ed4250]::ty::context::TyCtxt>>::visit_ty
  15:     0x7fca3cc13103 - <rustc_trait_selection[ff1980eedda976e0]::traits::wf::WfPredicates as rustc_type_ir[75560ed23b05734]::visit::TypeVisitor<rustc_middle[168a57e624ed4250]::ty::context::TyCtxt>>::visit_ty
  16:     0x7fca3cc1165a - <rustc_trait_selection[ff1980eedda976e0]::traits::wf::WfPredicates as rustc_type_ir[75560ed23b05734]::visit::TypeVisitor<rustc_middle[168a57e624ed4250]::ty::context::TyCtxt>>::visit_ty
  17:     0x7fca3ca3a693 - <rustc_trait_selection[ff1980eedda976e0]::traits::fulfill::FulfillProcessor as rustc_data_structures[2cf611a40505b448]::obligation_forest::ObligationProcessor>::process_obligation
  18:     0x7fca3c204f89 - <rustc_data_structures[2cf611a40505b448]::obligation_forest::ObligationForest<rustc_trait_selection[ff1980eedda976e0]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[ff1980eedda976e0]::traits::fulfill::FulfillProcessor>
  19:     0x7fca3cb8992a - <rustc_trait_selection[ff1980eedda976e0]::traits::fulfill::FulfillmentContext<rustc_trait_selection[ff1980eedda976e0]::traits::FulfillmentError> as rustc_infer[64ee6b518028a4d2]::traits::engine::TraitEngine<rustc_trait_selection[ff1980eedda976e0]::traits::FulfillmentError>>::select_all_or_error
  20:     0x7fca3cadc1e9 - rustc_hir_analysis[4edf01fa6f9b9c5f]::check::check::check_item_type
  21:     0x7fca3cad3bf9 - rustc_hir_analysis[4edf01fa6f9b9c5f]::check::wfcheck::check_well_formed
  22:     0x7fca3cad3bdb - rustc_query_impl[e799cb834319d50f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e799cb834319d50f]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[168a57e624ed4250]::query::erase::Erased<[u8; 1usize]>>
  23:     0x7fca3cad3452 - rustc_query_system[fc44913d20ec5c31]::query::plumbing::try_execute_query::<rustc_query_impl[e799cb834319d50f]::DynamicConfig<rustc_data_structures[2cf611a40505b448]::vec_cache::VecCache<rustc_span[150c1f29595c3fc1]::def_id::LocalDefId, rustc_middle[168a57e624ed4250]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[fc44913d20ec5c31]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[e799cb834319d50f]::plumbing::QueryCtxt, false>
  24:     0x7fca3cad2f56 - rustc_query_impl[e799cb834319d50f]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  25:     0x7fca3cad0575 - rustc_hir_analysis[4edf01fa6f9b9c5f]::check::wfcheck::check_type_wf
  26:     0x7fca3cad0475 - rustc_query_impl[e799cb834319d50f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e799cb834319d50f]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[168a57e624ed4250]::query::erase::Erased<[u8; 1usize]>>
  27:     0x7fca3d2638c8 - rustc_query_system[fc44913d20ec5c31]::query::plumbing::try_execute_query::<rustc_query_impl[e799cb834319d50f]::DynamicConfig<rustc_query_system[fc44913d20ec5c31]::query::caches::SingleCache<rustc_middle[168a57e624ed4250]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[e799cb834319d50f]::plumbing::QueryCtxt, false>
  28:     0x7fca3d263576 - rustc_query_impl[e799cb834319d50f]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  29:     0x7fca3cc8109c - rustc_hir_analysis[4edf01fa6f9b9c5f]::check_crate
  30:     0x7fca3c401870 - rustc_interface[60bc46e8fdb58ac8]::passes::analysis
  31:     0x7fca3c40152d - rustc_query_impl[e799cb834319d50f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[e799cb834319d50f]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[168a57e624ed4250]::query::erase::Erased<[u8; 0usize]>>
  32:     0x7fca3d2699a0 - rustc_query_system[fc44913d20ec5c31]::query::plumbing::try_execute_query::<rustc_query_impl[e799cb834319d50f]::DynamicConfig<rustc_query_system[fc44913d20ec5c31]::query::caches::SingleCache<rustc_middle[168a57e624ed4250]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[e799cb834319d50f]::plumbing::QueryCtxt, false>
  33:     0x7fca3d26967c - rustc_query_impl[e799cb834319d50f]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  34:     0x7fca3d474b0f - rustc_interface[60bc46e8fdb58ac8]::passes::create_and_enter_global_ctxt::<core[524b22d1fd644b30]::option::Option<rustc_interface[60bc46e8fdb58ac8]::queries::Linker>, rustc_driver_impl[477d964a0f4cf97e]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  35:     0x7fca3d4121be - rustc_interface[60bc46e8fdb58ac8]::interface::run_compiler::<(), rustc_driver_impl[477d964a0f4cf97e]::run_compiler::{closure#0}>::{closure#1}
  36:     0x7fca3d374778 - std[18d792bd9b3d078a]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[60bc46e8fdb58ac8]::util::run_in_thread_with_globals<rustc_interface[60bc46e8fdb58ac8]::util::run_in_thread_pool_with_globals<rustc_interface[60bc46e8fdb58ac8]::interface::run_compiler<(), rustc_driver_impl[477d964a0f4cf97e]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  37:     0x7fca3d37445c - <<std[18d792bd9b3d078a]::thread::Builder>::spawn_unchecked_<rustc_interface[60bc46e8fdb58ac8]::util::run_in_thread_with_globals<rustc_interface[60bc46e8fdb58ac8]::util::run_in_thread_pool_with_globals<rustc_interface[60bc46e8fdb58ac8]::interface::run_compiler<(), rustc_driver_impl[477d964a0f4cf97e]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[524b22d1fd644b30]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  38:     0x7fca3d37a34d - std::sys::thread::unix::Thread::new::thread_start::hd35e43a00d992682
  39:     0x7fca36a969cb - <unknown>
  40:     0x7fca36b1aa0c - <unknown>
  41:                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 1.92.0-nightly (ce4beebec 2025-09-22) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [check_well_formed] checking that `_TaWhere1` is well-formed
#1 [check_type_wf] checking that types are well-formed
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors; 3 warnings emitted

Some errors have detailed explanations: E0425, E0601.
For more information about an error, try `rustc --explain E0425`.

@rustbot label +F-generic_const_exprs +F-lazy_type_alias

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by compiling the auto-reduced Rust example in a.rs with the reported nightly rustc command to reproduce the ICE. Then inspect compiler/rustc_trait_selection/src/traits/wf.rs around lines 754-766 and trace the well-formedness check involved. Done means the reproducer no longer causes a compiler panic and receives appropriate compiler output.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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