rust-lang / rust-lang/rust

[ICE]: unexpected cyclic definition

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C-bug F-type_alias_impl_trait fixed-by-next-solver I-ICE needs-triage T-compiler
Dominant language
Rust
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Description

unlike https://github.com/rust-lang/rust/issues/151301 this one is reachable in the old solver

snippet:

#![feature(type_alias_impl_trait)]
fn spawn<T, F>(future: F) -> impl Sized
where
    F: FnOnce() -> T,
{
}
fn spawn_task(sender: &'static ()) -> impl Sized {
    type Tait = impl Sized + 'static;
    spawn::<Tait, _>(move || {
        type Tait = impl Sized + 'static;
        spawn::<Tait, _>(move || sender)
    })
}
fn main() {}

Version information

rustc 1.97.0-nightly (22cc6747b 2026-04-19)
binary: rustc
commit-hash: 22cc6747b18b3a6738e98b06676fd91f1756b33a
commit-date: 2026-04-19
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.2

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

Program output 

warning: unused variable: `future`
 --> /tmp/icemaker_global_tempdir.Vwse41DR8tSU/rustc_testrunner_tmpdir_reporting.FAkInl67gJSC/mvce.rs:2:16
  |
2 | fn spawn<T, F>(future: F) -> impl Sized
  |                ^^^^^^ help: if this is intentional, prefix it with an underscore: `_future`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

warning: function `spawn` is never used
 --> /tmp/icemaker_global_tempdir.Vwse41DR8tSU/rustc_testrunner_tmpdir_reporting.FAkInl67gJSC/mvce.rs:2:4
  |
2 | fn spawn<T, F>(future: F) -> impl Sized
  |    ^^^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: function `spawn_task` is never used
 --> /tmp/icemaker_global_tempdir.Vwse41DR8tSU/rustc_testrunner_tmpdir_reporting.FAkInl67gJSC/mvce.rs:7:4
  |
7 | fn spawn_task(sender: &'static ()) -> impl Sized {
  |    ^^^^^^^^^^

warning: 3 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: unexpected cyclic definition of `spawn_task::{closure#0}::Tait::{opaque#0}`
  --> /tmp/icemaker_global_tempdir.Vwse41DR8tSU/rustc_testrunner_tmpdir_reporting.FAkInl67gJSC/mvce.rs:10:21
   |
10 |         type Tait = impl Sized + 'static;
   |                     ^^^^^^^^^^^^^^^^^^^^
   |
note: delayed at /rustc-dev/22cc6747b18b3a6738e98b06676fd91f1756b33a/compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs:672:21
         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>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
         5: rustc_borrowck::mir_borrowck
         6: rustc_query_impl::query_impl::mir_borrowck::invoke_provider_fn::__rust_begin_short_backtrace
         7: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 8]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
         8: rustc_query_impl::query_impl::mir_borrowck::execute_query_non_incr::__rust_end_short_backtrace
         9: rustc_hir_analysis::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
        10: rustc_hir_analysis::collect::type_of::type_of_opaque
        11: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 8]>>, false>
        12: rustc_query_impl::query_impl::type_of_opaque::execute_query_non_incr::__rust_end_short_backtrace
        13: rustc_hir_analysis::collect::type_of::type_of
        14: rustc_query_impl::query_impl::type_of::invoke_provider_fn::__rust_begin_short_backtrace
        15: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefIdCache<rustc_middle::query::erase::ErasedData<[u8; 8]>>, false>
        16: rustc_query_impl::query_impl::type_of::execute_query_non_incr::__rust_end_short_backtrace
        17: rustc_hir_analysis::check::check::check_opaque
        18: rustc_hir_analysis::check::check::check_item_type
        19: rustc_hir_analysis::check::wfcheck::check_well_formed
        20: rustc_query_impl::query_impl::check_well_formed::invoke_provider_fn::__rust_begin_short_backtrace
        21: rustc_query_impl::execution::try_execute_query::<rustc_data_structures::vec_cache::VecCache<rustc_span::def_id::LocalDefId, rustc_middle::query::erase::ErasedData<[u8; 1]>, rustc_middle::dep_graph::graph::DepNodeIndex>, false>
        22: rustc_query_impl::query_impl::check_well_formed::execute_query_non_incr::__rust_end_short_backtrace
        23: rustc_hir_analysis::check::wfcheck::check_type_wf
        24: rustc_query_impl::query_impl::check_type_wf::invoke_provider_fn::__rust_begin_short_backtrace
        25: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 1]>>, false>
        26: rustc_query_impl::query_impl::check_type_wf::execute_query_non_incr::__rust_end_short_backtrace
        27: rustc_hir_analysis::check_crate
        28: rustc_interface::passes::analysis
        29: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
        30: rustc_query_impl::query_impl::analysis::execute_query_non_incr::__rust_end_short_backtrace
        31: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        32: 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}, ()>
        33: <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}
        34: <std::sys::thread::unix::Thread>::new::thread_start
        35: <unknown>
        36: <unknown>
      
  --> /tmp/icemaker_global_tempdir.Vwse41DR8tSU/rustc_testrunner_tmpdir_reporting.FAkInl67gJSC/mvce.rs:10:21
   |
10 |         type Tait = impl Sized + 'static;
   |                     ^^^^^^^^^^^^^^^^^^^^

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 1.97.0-nightly (22cc6747b 2026-04-19) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack

@rustbot label +F-type_alias_impl_trait

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 provided snippet with the reported nightly rustc to reproduce the ICE. Then inspect compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs around the delayed-bug location and follow the rustc_hir_analysis opaque-type query stack. Done means the reproducer no longer emits an internal compiler error and has appropriate compiler behavior.

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
Active
Clarity
Clearly specified
Newbie friendliness
52/100

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