rust-lang / rust-lang/rust

[ICE]: could not normalize `for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: 'a`

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C-bug I-ICE T-compiler WG-trait-system-refactor
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Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: --edition=2024 -Znext-solver=globally
#![feature(unboxed_closures)]
#![feature(non_lifetime_binders)]
struct S<T>(T)
where
    for<F> F:,
    for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: Future + 'a;
fn main() {}

original:

struct S<T>(T) where
    for<F> F:,
    for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: Future<Output = ()> + 'a,;                                    
fn main() {}

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

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc --edition=2024 -Znext-solver=globally -Zcrate-attr=feature(unboxed_closures) -Zcrate-attr=feature(non_lifetime_binders)

Program output 

warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
 --> <crate attribute>:1:12
  |
1 | #![feature(non_lifetime_binders)]
  |            ^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
  = note: `#[warn(incomplete_features)]` on by default

warning: struct `S` is never constructed
 --> /tmp/icemaker_global_tempdir.wwNJpebPj8wh/rustc_testrunner_tmpdir_reporting.FwA7HEom69gA/mvce.rs:1:8
  |
1 | struct S<T>(T)
  |        ^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: 2 warnings emitted

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: could not normalize `for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: 'a`
  |
  = note: delayed at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_trait_selection/src/regions.rs:41:41
             0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
             1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
             2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
             3: <rustc_errors::DiagCtxtHandle>::delayed_bug::<alloc::string::String>
             4: <rustc_infer::infer::InferCtxt as rustc_trait_selection::regions::InferCtxtRegionExt>::resolve_regions::<[rustc_middle::ty::Ty; 0]>
             5: rustc_trait_selection::traits::do_normalize_predicates
             6: rustc_trait_selection::traits::normalize_param_env_or_error
             7: rustc_ty_utils::ty::param_env
             8: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::param_env::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
             9: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefIdCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            10: rustc_query_impl::query_impl::param_env::get_query_non_incr::__rust_end_short_backtrace
            11: rustc_hir_analysis::check::wfcheck::check_type_defn
            12: rustc_hir_analysis::check::check::check_item_type
            13: rustc_hir_analysis::check::wfcheck::check_well_formed
            14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
            15: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            16: rustc_query_impl::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
            17: rustc_hir_analysis::check::wfcheck::check_type_wf
            18: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
            19: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            20: rustc_query_impl::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
            21: rustc_hir_analysis::check_crate
            22: rustc_interface::passes::analysis
            23: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
            24: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            25: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
            26: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
            27: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
            28: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
            29: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            30: <std::sys::thread::unix::Thread>::new::thread_start
            31: <unknown>
            32: <unknown>
          

error: internal compiler error: could not normalize `for<'a> <i32 as FnOnce<(&'a mut i32,)>>::Output: 'a`
  |
  = note: delayed at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_trait_selection/src/regions.rs:41:41
             0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
             1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
             2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
             3: <rustc_errors::DiagCtxtHandle>::delayed_bug::<alloc::string::String>
             4: <rustc_infer::infer::outlives::env::OutlivesEnvironment as rustc_trait_selection::regions::OutlivesEnvironmentBuildExt>::new_with_implied_bounds_compat::<core::iter::adapters::copied::Copied<indexmap::set::iter::Iter<rustc_middle::ty::Ty>>>
             5: rustc_hir_analysis::check::wfcheck::check_type_defn
             6: rustc_hir_analysis::check::check::check_item_type
             7: rustc_hir_analysis::check::wfcheck::check_well_formed
             8: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
             9: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::Erased<[u8; 1]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            10: rustc_query_impl::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
            11: rustc_hir_analysis::check::wfcheck::check_type_wf
            12: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
            13: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            14: rustc_query_impl::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
            15: rustc_hir_analysis::check_crate
            16: rustc_interface::passes::analysis
            17: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 0]>>
            18: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            19: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
            20: <rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core::ops::function::FnOnce<(&rustc_session::session::Session, rustc_middle::ty::context::CurrentGcx, alloc::sync::Arc<rustc_data_structures::jobserver::Proxy>, &std::sync::once_lock::OnceLock<rustc_middle::ty::context::GlobalCtxt>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_middle::arena::Arena>, &rustc_data_structures::sync::worker_local::WorkerLocal<rustc_hir::Arena>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
            21: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
            22: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
            23: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            24: <std::sys::thread::unix::Thread>::new::thread_start
            25: <unknown>
            26: <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 1.95.0-nightly (621d76794 2026-01-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z crate-attr=feature(unboxed_closures) -Z crate-attr=feature(non_lifetime_binders) -Z dump-mir-dir=dir

query stack during panic:
end of query stack

@rustbot label +F-unboxed_closures +F-non_lifetime_binders +WG-trait-system-refactor

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 reproducer with --edition=2024 and -Znext-solver=globally, then inspect compiler/rustc_trait_selection/src/regions.rs at the reported delayed-bug location. Trace how the well-formedness check handles the higher-ranked FnOnce bound; done means the reproducer no longer emits an internal compiler error.

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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