rust-lang / rust-lang/rust

[ICE]: `cannot relate bound region`

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C-bug I-ICE T-compiler WG-trait-system-refactor
Dominant language
Rust
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PR metrics pending

Description

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
pub struct Foo;

fn foo(foos: Option<Foo>) {
    let foos = foos.as_ref();
    Bar.bar(foos);
}

struct Bar;

impl Bar {
    pub fn bar<I>(&self, iter: I)
    where
        for<'a> &'a I: Trait<Assoc = &'a Foo>,
    {
    }
}

trait Trait {
    type Assoc;
}

impl Trait for Option<Foo> {
    type Assoc = Foo;
}
impl<'a> Trait for Option<&'a Foo> {
    type Assoc = &'a Foo;
}
impl<'a, 'b> Trait for &'a Option<&'b Foo> {
    type Assoc = &'b Foo;
}

pub fn main() {}

original:

pub struct Foo;

fn foo(foos: Option<Foo>) {
    let foos = foos.as_ref();
    Bar.bar(foos);
}

struct Bar;

impl Bar {
    pub fn bar<I>(&self, iter: I)
    where
        for<'a> &'a I: Trait<Assoc = &'a Foo>,
    {
    }
}

trait Trait {
    type Assoc;
}

impl Trait for Option<Foo> {
    type Assoc = Foo;
}
impl<'a> Trait for Option<&'a Foo> {
    type Assoc = &'a Foo;
}
impl<'a, 'b> Trait for &'a Option<&'b Foo> {
    type Assoc = &'b Foo;
}
impl<'a> Trait for &'a i32 {
    type Assoc = &'a Foo;
}

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

Possibly related line of code:
https://github.com/rust-lang/rust/blob/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_infer/src/infer/region_constraints/mod.rs#L468-L480

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

Program output 

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.KRLumYtswIX5/rustc_testrunner_tmpdir_reporting.UtxgZ44lD4Ij/mvce.rs:30:2
   |
30 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.KRLumYtswIX5/rustc_testrunner_tmpdir_reporting.UtxgZ44lD4Ij/mvce.rs`

error: internal compiler error: /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_infer/src/infer/region_constraints/mod.rs:474:17: cannot relate bound region: '^0.Named(DefId(0:11 ~ mvce[a621]::{impl#0}::bar::'a)) <= '?1


thread 'rustc' (1321978) panicked at /rustc-dev/621d76794c76fc21c0a6151fbc090120e0230a91/compiler/rustc_infer/src/infer/region_constraints/mod.rs:474:17:
Box<dyn Any>
stack backtrace:
   0:     0x7f0b6f233fd3 - <<std[287527946f71f0bc]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[7477a31a1a34cc49]::fmt::Display>::fmt
   1:     0x7f0b6f80e2c8 - core[7477a31a1a34cc49]::fmt::write
   2:     0x7f0b6f24a9c6 - <std[287527946f71f0bc]::sys::stdio::unix::Stderr as std[287527946f71f0bc]::io::Write>::write_fmt
   3:     0x7f0b6f20a048 - std[287527946f71f0bc]::panicking::default_hook::{closure#0}
   4:     0x7f0b6f227703 - std[287527946f71f0bc]::panicking::default_hook
   5:     0x7f0b6e1fbb8a - std[287527946f71f0bc]::panicking::update_hook::<alloc[979041e9952e771b]::boxed::Box<rustc_driver_impl[517260a07841a630]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f0b6f2279e2 - std[287527946f71f0bc]::panicking::panic_with_hook
   7:     0x7f0b6e238bc1 - std[287527946f71f0bc]::panicking::begin_panic::<rustc_errors[26c9ea40265c581b]::ExplicitBug>::{closure#0}
   8:     0x7f0b6e228fe6 - std[287527946f71f0bc]::sys::backtrace::__rust_end_short_backtrace::<std[287527946f71f0bc]::panicking::begin_panic<rustc_errors[26c9ea40265c581b]::ExplicitBug>::{closure#0}, !>
   9:     0x7f0b6e2264b9 - std[287527946f71f0bc]::panicking::begin_panic::<rustc_errors[26c9ea40265c581b]::ExplicitBug>
  10:     0x7f0b6e256901 - <rustc_errors[26c9ea40265c581b]::diagnostic::BugAbort as rustc_errors[26c9ea40265c581b]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f0b6e7e492c - <rustc_errors[26c9ea40265c581b]::DiagCtxtHandle>::span_bug::<rustc_span[851f35e07e3c2dd4]::span_encoding::Span, alloc[979041e9952e771b]::string::String>
  12:     0x7f0b6e8074d6 - rustc_middle[725063e074f29df]::util::bug::opt_span_bug_fmt::<rustc_span[851f35e07e3c2dd4]::span_encoding::Span>::{closure#0}
  13:     0x7f0b6e807682 - rustc_middle[725063e074f29df]::ty::context::tls::with_opt::<rustc_middle[725063e074f29df]::util::bug::opt_span_bug_fmt<rustc_span[851f35e07e3c2dd4]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7f0b6e7f8cab - rustc_middle[725063e074f29df]::ty::context::tls::with_context_opt::<rustc_middle[725063e074f29df]::ty::context::tls::with_opt<rustc_middle[725063e074f29df]::util::bug::opt_span_bug_fmt<rustc_span[851f35e07e3c2dd4]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7f0b6c0afa68 - rustc_middle[725063e074f29df]::util::bug::span_bug_fmt::<rustc_span[851f35e07e3c2dd4]::span_encoding::Span>
  16:     0x7f0b6fe7cfb7 - <rustc_infer[5a785d1c27494969]::infer::region_constraints::RegionConstraintCollector>::make_eqregion
  17:     0x7f0b6fe688f3 - rustc_type_ir[b93cf349b3a4f34d]::relate::structurally_relate_tys::<rustc_middle[725063e074f29df]::ty::context::TyCtxt, rustc_type_ir[b93cf349b3a4f34d]::relate::solver_relating::SolverRelating<rustc_infer[5a785d1c27494969]::infer::InferCtxt, rustc_middle[725063e074f29df]::ty::context::TyCtxt>>
  18:     0x7f0b6fe67098 - <rustc_type_ir[b93cf349b3a4f34d]::relate::solver_relating::SolverRelating<rustc_infer[5a785d1c27494969]::infer::InferCtxt, rustc_middle[725063e074f29df]::ty::context::TyCtxt> as rustc_type_ir[b93cf349b3a4f34d]::relate::TypeRelation<rustc_middle[725063e074f29df]::ty::context::TyCtxt>>::tys
  19:     0x7f0b6fe66305 - <rustc_infer[5a785d1c27494969]::infer::InferCtxt as rustc_type_ir[b93cf349b3a4f34d]::relate::solver_relating::RelateExt>::relate::<rustc_middle[725063e074f29df]::ty::Ty>
  20:     0x7f0b70211c83 - <rustc_trait_selection[9fee51d8a10a777c]::traits::engine::ObligationCtxt>::eq::<rustc_middle[725063e074f29df]::ty::Ty>
  21:     0x7f0b6ef3510a - <rustc_trait_selection[9fee51d8a10a777c]::error_reporting::TypeErrCtxt>::point_at_chain::<rustc_span[851f35e07e3c2dd4]::ErrorGuaranteed>
  22:     0x7f0b6ef55f43 - <rustc_trait_selection[9fee51d8a10a777c]::error_reporting::TypeErrCtxt>::note_function_argument_obligation::<rustc_span[851f35e07e3c2dd4]::ErrorGuaranteed>
  23:     0x7f0b6ef4a6d7 - <rustc_trait_selection[9fee51d8a10a777c]::error_reporting::TypeErrCtxt>::note_obligation_cause_code::<rustc_span[851f35e07e3c2dd4]::ErrorGuaranteed, rustc_middle[725063e074f29df]::ty::predicate::Predicate>
  24:     0x7f0b6f00e6d4 - <rustc_trait_selection[9fee51d8a10a777c]::error_reporting::TypeErrCtxt>::note_obligation_cause
  25:     0x7f0b6db2659e - <rustc_trait_selection[9fee51d8a10a777c]::error_reporting::TypeErrCtxt>::report_fulfillment_errors
  26:     0x7f0b6fbed721 - <rustc_hir_typeck[ed5453188701a25b]::fn_ctxt::FnCtxt>::check_expr_method_call
  27:     0x7f0b6fc19963 - <rustc_hir_typeck[ed5453188701a25b]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  28:     0x7f0b6fc14ac2 - <rustc_hir_typeck[ed5453188701a25b]::fn_ctxt::FnCtxt>::check_expr_block
  29:     0x7f0b6fc1998b - <rustc_hir_typeck[ed5453188701a25b]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  30:     0x7f0b6fbfa1a6 - rustc_hir_typeck[ed5453188701a25b]::check::check_fn
  31:     0x7f0b705b3d33 - rustc_hir_typeck[ed5453188701a25b]::typeck_with_inspect::{closure#0}
  32:     0x7f0b705b26b4 - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 8usize]>>
  33:     0x7f0b6f95522a - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_data_structures[e0d8483206daafa2]::vec_cache::VecCache<rustc_span[851f35e07e3c2dd4]::def_id::LocalDefId, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 8usize]>, rustc_query_system[b7e73caa2c2ad9a]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  34:     0x7f0b6f954b09 - rustc_query_impl[f20b8284200e680a]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  35:     0x7f0b7033fdb5 - <rustc_middle[725063e074f29df]::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis[1a80394817eddfdd]::check_crate::{closure#2}>::{closure#0}
  36:     0x7f0b7033f7c4 - rustc_hir_analysis[1a80394817eddfdd]::check_crate
  37:     0x7f0b6f9493db - rustc_interface[5c48368f5debe80d]::passes::analysis
  38:     0x7f0b6f949035 - rustc_query_impl[f20b8284200e680a]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[f20b8284200e680a]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 0usize]>>
  39:     0x7f0b709a4be3 - rustc_query_system[b7e73caa2c2ad9a]::query::plumbing::try_execute_query::<rustc_query_impl[f20b8284200e680a]::DynamicConfig<rustc_query_system[b7e73caa2c2ad9a]::query::caches::SingleCache<rustc_middle[725063e074f29df]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[f20b8284200e680a]::plumbing::QueryCtxt, false>
  40:     0x7f0b709a49ce - rustc_query_impl[f20b8284200e680a]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7f0b70b82c11 - <rustc_interface[5c48368f5debe80d]::passes::create_and_enter_global_ctxt<core[7477a31a1a34cc49]::option::Option<rustc_interface[5c48368f5debe80d]::queries::Linker>, rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[7477a31a1a34cc49]::ops::function::FnOnce<(&rustc_session[eb79e6d3104f29ae]::session::Session, rustc_middle[725063e074f29df]::ty::context::CurrentGcx, alloc[979041e9952e771b]::sync::Arc<rustc_data_structures[e0d8483206daafa2]::jobserver::Proxy>, &std[287527946f71f0bc]::sync::once_lock::OnceLock<rustc_middle[725063e074f29df]::ty::context::GlobalCtxt>, &rustc_data_structures[e0d8483206daafa2]::sync::worker_local::WorkerLocal<rustc_middle[725063e074f29df]::arena::Arena>, &rustc_data_structures[e0d8483206daafa2]::sync::worker_local::WorkerLocal<rustc_hir[7eac7792dedc1861]::Arena>, rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  42:     0x7f0b709f5d34 - rustc_interface[5c48368f5debe80d]::interface::run_compiler::<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}
  43:     0x7f0b7095aa3a - std[287527946f71f0bc]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[5c48368f5debe80d]::util::run_in_thread_with_globals<rustc_interface[5c48368f5debe80d]::util::run_in_thread_pool_with_globals<rustc_interface[5c48368f5debe80d]::interface::run_compiler<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  44:     0x7f0b7095a7fe - <std[287527946f71f0bc]::thread::lifecycle::spawn_unchecked<rustc_interface[5c48368f5debe80d]::util::run_in_thread_with_globals<rustc_interface[5c48368f5debe80d]::util::run_in_thread_pool_with_globals<rustc_interface[5c48368f5debe80d]::interface::run_compiler<(), rustc_driver_impl[517260a07841a630]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[7477a31a1a34cc49]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  45:     0x7f0b7095c5c4 - <std[287527946f71f0bc]::sys::thread::unix::Thread>::new::thread_start
  46:     0x7f0b6a29698b - <unknown>
  47:     0x7f0b6a31a9cc - <unknown>
  48:                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 1.95.0-nightly (621d76794 2026-01-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z next-solver=globally -Z dump-mir-dir=dir

query stack during panic:
#0 [typeck] type-checking `foo`
#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`.

@rustbot label +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 with the reduced Rust program and reproduce it using -Znext-solver=globally. Read compiler/rustc_infer/src/infer/region_constraints/mod.rs around lines 468-480 and trace the make_eqregion path shown in the backtrace; done means the program no longer produces 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
Stale
Clarity
Clearly specified
Newbie friendliness
35/100

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