rust-lang / rust-lang/rust

[ICE]: `unexpected overflowed when processing region obligations`

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C-bug I-ICE needs-triage T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

auto-reduced (treereduce-rust):

//@compile-flags: -Znext-solver=globally
trait X<'a> {
    type U: ?Sized;
}

impl X<'_> for u32
where
    for<'b> <Self as X<'b>>::U: 'a,
{
    type U = str;
}

original:

trait X<'a>
where
    for<'b> <Self as X<'b>>::U: Clone,
{
    type U: ?Sized;
    fn f(&self, x: &Self::U) {
        <Self::U>::clone(x);
    }
}

impl X<'_> for u32 //~ ERROR overflow evaluating the requirement `for<'b> u32: X<'b>`
where
    for<'b> <Self as X<'b>>::U: 'a,
{
    type U = str;
}

fn main() {
    1u32.f("abc");
}

Version information

rustc 1.95.0-nightly (625b63f9e 2026-01-21)
binary: rustc
commit-hash: 625b63f9e148d511e187c71e5f70643ee62c77fb
commit-date: 2026-01-21
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/625b63f9e148d511e187c71e5f70643ee62c77fb/compiler/rustc_infer/src/infer/outlives/obligations.rs#L214-L226

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

Program output 

error[E0261]: use of undeclared lifetime name `'a`
 --> /tmp/icemaker_global_tempdir.vp6JkNZDbYoq/rustc_testrunner_tmpdir_reporting.GNAg2JUaYoJH/mvce.rs:7:33
  |
7 |     for<'b> <Self as X<'b>>::U: 'a,
  |                                 ^^ undeclared lifetime
  |
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider making the bound lifetime-generic with a new `'a` lifetime
  |
7 |     for<'a, 'b> <Self as X<'b>>::U: 'a,
  |         +++
help: consider introducing lifetime `'a` here
  |
5 | impl<'a> X<'_> for u32
  |     ++++

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.vp6JkNZDbYoq/rustc_testrunner_tmpdir_reporting.GNAg2JUaYoJH/mvce.rs:10:2
   |
10 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.vp6JkNZDbYoq/rustc_testrunner_tmpdir_reporting.GNAg2JUaYoJH/mvce.rs`

error: internal compiler error: /rustc-dev/625b63f9e148d511e187c71e5f70643ee62c77fb/compiler/rustc_infer/src/infer/outlives/obligations.rs:220:17: unexpected overflowed when processing region obligations: [
                                    TypeOutlivesConstraint {
                                        sub_region: '{region error},
                                        sup_type: Alias(
                                            Projection,
                                            AliasTy {
                                                args: [
                                                    u32,
                                                    '!131_0.Named(DefId(0:7 ~ mvce[3e22]::{impl#0}::'b)),
                                                ],
                                                def_id: DefId(0:5 ~ mvce[3e22]::X::U),
                                                ..
                                            },
                                        ),
                                        origin: RelateParamBound(
                                            no-location (#0),
                                            Alias(
                                                Projection,
                                                AliasTy {
                                                    args: [
                                                        u32,
                                                        '!131_0.Named(DefId(0:7 ~ mvce[3e22]::{impl#0}::'b)),
                                                    ],
                                                    def_id: DefId(0:5 ~ mvce[3e22]::X::U),
                                                    ..
                                                },
                                            ),
                                            None,
                                        ),
                                    },
                                ]


thread 'rustc' (186588) panicked at /rustc-dev/625b63f9e148d511e187c71e5f70643ee62c77fb/compiler/rustc_infer/src/infer/outlives/obligations.rs:220:17:
Box<dyn Any>
stack backtrace:
   0:     0x7f4691210cf3 - <<std[fb3d5c90ac5b7f63]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[f6b43ac8430f38c9]::fmt::Display>::fmt
   1:     0x7f4691809756 - core[f6b43ac8430f38c9]::fmt::write
   2:     0x7f46912276e6 - <std[fb3d5c90ac5b7f63]::sys::stdio::unix::Stderr as std[fb3d5c90ac5b7f63]::io::Write>::write_fmt
   3:     0x7f46911e6d28 - std[fb3d5c90ac5b7f63]::panicking::default_hook::{closure#0}
   4:     0x7f4691204423 - std[fb3d5c90ac5b7f63]::panicking::default_hook
   5:     0x7f46901e607a - std[fb3d5c90ac5b7f63]::panicking::update_hook::<alloc[f2f1d3597b721dce]::boxed::Box<rustc_driver_impl[e2906432e8e0cdf6]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7f4691204702 - std[fb3d5c90ac5b7f63]::panicking::panic_with_hook
   7:     0x7f4690224371 - std[fb3d5c90ac5b7f63]::panicking::begin_panic::<rustc_errors[a7fc3a61ce6b7a9f]::ExplicitBug>::{closure#0}
   8:     0x7f4690214a26 - std[fb3d5c90ac5b7f63]::sys::backtrace::__rust_end_short_backtrace::<std[fb3d5c90ac5b7f63]::panicking::begin_panic<rustc_errors[a7fc3a61ce6b7a9f]::ExplicitBug>::{closure#0}, !>
   9:     0x7f469021478f - std[fb3d5c90ac5b7f63]::panicking::begin_panic::<rustc_errors[a7fc3a61ce6b7a9f]::ExplicitBug>
  10:     0x7f4690242171 - <rustc_errors[a7fc3a61ce6b7a9f]::diagnostic::BugAbort as rustc_errors[a7fc3a61ce6b7a9f]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7f46907e67a9 - rustc_middle[7c1e44e38e5ae512]::util::bug::opt_span_bug_fmt::<rustc_span[c096204aa74ff282]::span_encoding::Span>::{closure#0}
  12:     0x7f46907e6932 - rustc_middle[7c1e44e38e5ae512]::ty::context::tls::with_opt::<rustc_middle[7c1e44e38e5ae512]::util::bug::opt_span_bug_fmt<rustc_span[c096204aa74ff282]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  13:     0x7f46907d7b7b - rustc_middle[7c1e44e38e5ae512]::ty::context::tls::with_context_opt::<rustc_middle[7c1e44e38e5ae512]::ty::context::tls::with_opt<rustc_middle[7c1e44e38e5ae512]::util::bug::opt_span_bug_fmt<rustc_span[c096204aa74ff282]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  14:     0x7f468e49cb94 - rustc_middle[7c1e44e38e5ae512]::util::bug::bug_fmt
  15:     0x7f46922a4a29 - <rustc_infer[951fea966d908995]::infer::InferCtxt as rustc_trait_selection[b2a773cbaa62fcc]::regions::InferCtxtRegionExt>::resolve_regions_with_outlives_env
  16:     0x7f46922915dc - <rustc_infer[951fea966d908995]::infer::InferCtxt as rustc_trait_selection[b2a773cbaa62fcc]::regions::InferCtxtRegionExt>::resolve_regions::<[rustc_middle[7c1e44e38e5ae512]::ty::Ty; 0usize]>
  17:     0x7f469229c524 - rustc_trait_selection[b2a773cbaa62fcc]::traits::do_normalize_predicates
  18:     0x7f46920648ff - rustc_trait_selection[b2a773cbaa62fcc]::traits::normalize_param_env_or_error
  19:     0x7f4692069dee - rustc_ty_utils[ebab42b0a530f21e]::ty::param_env
  20:     0x7f469206986b - rustc_query_impl[b1dbb5ee6e859e63]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b1dbb5ee6e859e63]::query_impl::param_env::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 8usize]>>
  21:     0x7f46918111c5 - rustc_query_system[662316397b943afb]::query::plumbing::try_execute_query::<rustc_query_impl[b1dbb5ee6e859e63]::DynamicConfig<rustc_query_system[662316397b943afb]::query::caches::DefIdCache<rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[b1dbb5ee6e859e63]::plumbing::QueryCtxt, false>
  22:     0x7f4691810cb1 - rustc_query_impl[b1dbb5ee6e859e63]::query_impl::param_env::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7f46922ad1bd - rustc_ty_utils[ebab42b0a530f21e]::ty::impl_self_is_guaranteed_unsized
  24:     0x7f46922acd3b - rustc_query_impl[b1dbb5ee6e859e63]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b1dbb5ee6e859e63]::query_impl::impl_self_is_guaranteed_unsized::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 1usize]>>
  25:     0x7f4691bd3620 - rustc_query_system[662316397b943afb]::query::plumbing::try_execute_query::<rustc_query_impl[b1dbb5ee6e859e63]::DynamicConfig<rustc_query_system[662316397b943afb]::query::caches::DefIdCache<rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[b1dbb5ee6e859e63]::plumbing::QueryCtxt, false>
  26:     0x7f4691bd2ed0 - rustc_query_impl[b1dbb5ee6e859e63]::query_impl::impl_self_is_guaranteed_unsized::get_query_non_incr::__rust_end_short_backtrace
  27:     0x7f4691fb76c8 - rustc_hir_analysis[bccf3aa92971e1a2]::check::check::check_item_type
  28:     0x7f4691fb41e6 - rustc_hir_analysis[bccf3aa92971e1a2]::check::wfcheck::check_well_formed
  29:     0x7f4691fb41c7 - rustc_query_impl[b1dbb5ee6e859e63]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b1dbb5ee6e859e63]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 1usize]>>
  30:     0x7f4691fb3c3d - rustc_query_system[662316397b943afb]::query::plumbing::try_execute_query::<rustc_query_impl[b1dbb5ee6e859e63]::DynamicConfig<rustc_data_structures[147b1c54dfb69980]::vec_cache::VecCache<rustc_span[c096204aa74ff282]::def_id::LocalDefId, rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 1usize]>, rustc_query_system[662316397b943afb]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[b1dbb5ee6e859e63]::plumbing::QueryCtxt, false>
  31:     0x7f4691fb3742 - rustc_query_impl[b1dbb5ee6e859e63]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  32:     0x7f4691fb0e76 - rustc_hir_analysis[bccf3aa92971e1a2]::check::wfcheck::check_type_wf
  33:     0x7f4691fb0d79 - rustc_query_impl[b1dbb5ee6e859e63]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b1dbb5ee6e859e63]::query_impl::check_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 1usize]>>
  34:     0x7f4692a40b91 - rustc_query_system[662316397b943afb]::query::plumbing::try_execute_query::<rustc_query_impl[b1dbb5ee6e859e63]::DynamicConfig<rustc_query_system[662316397b943afb]::query::caches::SingleCache<rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[b1dbb5ee6e859e63]::plumbing::QueryCtxt, false>
  35:     0x7f4692a40842 - rustc_query_impl[b1dbb5ee6e859e63]::query_impl::check_type_wf::get_query_non_incr::__rust_end_short_backtrace
  36:     0x7f46924a6896 - rustc_hir_analysis[bccf3aa92971e1a2]::check_crate
  37:     0x7f46919df95b - rustc_interface[c96bb79ace64ec0a]::passes::analysis
  38:     0x7f46919df5a7 - rustc_query_impl[b1dbb5ee6e859e63]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b1dbb5ee6e859e63]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 0usize]>>
  39:     0x7f4692a418e3 - rustc_query_system[662316397b943afb]::query::plumbing::try_execute_query::<rustc_query_impl[b1dbb5ee6e859e63]::DynamicConfig<rustc_query_system[662316397b943afb]::query::caches::SingleCache<rustc_middle[7c1e44e38e5ae512]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[b1dbb5ee6e859e63]::plumbing::QueryCtxt, false>
  40:     0x7f4692a416ce - rustc_query_impl[b1dbb5ee6e859e63]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7f4692b6d8d1 - <rustc_interface[c96bb79ace64ec0a]::passes::create_and_enter_global_ctxt<core[f6b43ac8430f38c9]::option::Option<rustc_interface[c96bb79ace64ec0a]::queries::Linker>, rustc_driver_impl[e2906432e8e0cdf6]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[f6b43ac8430f38c9]::ops::function::FnOnce<(&rustc_session[ded86e37f409d2a]::session::Session, rustc_middle[7c1e44e38e5ae512]::ty::context::CurrentGcx, alloc[f2f1d3597b721dce]::sync::Arc<rustc_data_structures[147b1c54dfb69980]::jobserver::Proxy>, &std[fb3d5c90ac5b7f63]::sync::once_lock::OnceLock<rustc_middle[7c1e44e38e5ae512]::ty::context::GlobalCtxt>, &rustc_data_structures[147b1c54dfb69980]::sync::worker_local::WorkerLocal<rustc_middle[7c1e44e38e5ae512]::arena::Arena>, &rustc_data_structures[147b1c54dfb69980]::sync::worker_local::WorkerLocal<rustc_hir[586ee495a70320e8]::Arena>, rustc_driver_impl[e2906432e8e0cdf6]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  42:     0x7f46929af82a - rustc_interface[c96bb79ace64ec0a]::interface::run_compiler::<(), rustc_driver_impl[e2906432e8e0cdf6]::run_compiler::{closure#0}>::{closure#1}
  43:     0x7f46929dfb7e - std[fb3d5c90ac5b7f63]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[c96bb79ace64ec0a]::util::run_in_thread_with_globals<rustc_interface[c96bb79ace64ec0a]::util::run_in_thread_pool_with_globals<rustc_interface[c96bb79ace64ec0a]::interface::run_compiler<(), rustc_driver_impl[e2906432e8e0cdf6]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  44:     0x7f46929e00a0 - <std[fb3d5c90ac5b7f63]::thread::lifecycle::spawn_unchecked<rustc_interface[c96bb79ace64ec0a]::util::run_in_thread_with_globals<rustc_interface[c96bb79ace64ec0a]::util::run_in_thread_pool_with_globals<rustc_interface[c96bb79ace64ec0a]::interface::run_compiler<(), rustc_driver_impl[e2906432e8e0cdf6]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[f6b43ac8430f38c9]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  45:     0x7f46929e0f68 - <std[fb3d5c90ac5b7f63]::sys::thread::unix::Thread>::new::thread_start
  46:     0x7f468c29698b - <unknown>
  47:     0x7f468c31a9cc - <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 (625b63f9e 2026-01-21) running on x86_64-unknown-linux-gnu

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

query stack during panic:
#0 [param_env] computing normalized predicates of `<impl at /tmp/icemaker_global_tempdir.vp6JkNZDbYoq/rustc_testrunner_tmpdir_reporting.GNAg2JUaYoJH/mvce.rs:5:1: 7:36>`
#1 [impl_self_is_guaranteed_unsized] computing whether `<impl at /tmp/icemaker_global_tempdir.vp6JkNZDbYoq/rustc_testrunner_tmpdir_reporting.GNAg2JUaYoJH/mvce.rs:5:1: 7:36>` has a guaranteed unsized self type
#2 [check_well_formed] checking that `<impl at /tmp/icemaker_global_tempdir.vp6JkNZDbYoq/rustc_testrunner_tmpdir_reporting.GNAg2JUaYoJH/mvce.rs:5:1: 7:36>` is well-formed
#3 [check_type_wf] checking that types are well-formed
#4 [analysis] running analysis passes on crate `mvce`
end of query stack
error: aborting due to 3 previous errors

Some errors have detailed explanations: E0261, E0601.
For more information about an error, try `rustc --explain E0261`.

@rustbot label +WG-trait-system-refactor

Contributor guide

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First steps

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  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

Reproduce the ICE with the reduced Rust program and -Znext-solver=globally, then read compiler/rustc_infer/src/infer/outlives/obligations.rs around lines 214-226. Trace how the region obligations from this case reach the overflow handling. Done means the reduced case no longer triggers an internal compiler error and reports the appropriate compiler diagnostic.

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
Mostly clear
Newbie friendliness
35/100

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