rust-lang / rust-lang/rust

ICE: failure to relate an opaque to itself

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#148,815 2 comments 0 reactions 0 assignees View on GitHub

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

Code
trait FnPtr {
    type Args<'a>;
}

impl FnPtr for fn() {
    type Args<'a> = ();
}

trait FnThunk<B> {}

impl<B> FnThunk<B> for () {}

trait PackedFn<'a, B>: Fn(B::Args<'a>)
where
    B: FnPtr,
{
}

impl<'a, B, F> PackedFn<'a, B> for F
where
    B: FnPtr,
    F: Fn(B::Args<'a>),
{
}

fn make<B, F>(_: F) -> impl FnThunk<B>
where
    B: FnPtr,
    F: for<'x> PackedFn<'x, B>,
{
}

fn f<F, C, H>(_: C)
where
    C: FnOnce(F) -> H,
    F: FnPtr,
    H: FnThunk<F>,
{
}

pub fn main() {
    f::<fn(), _, _>(|_| make(|()| ()))
}
Meta

rustc --version --verbose:

rustc 1.91.1 (ed61e7d7e 2025-11-07)
binary: rustc
commit-hash: ed61e7d7e242494fb7057f2657300d9e77bb4fcb
commit-date: 2025-11-07
host: x86_64-pc-windows-msvc
release: 1.91.1
LLVM version: 21.1.2

Also present on nightly rustc 1.93.0-nightly (29a69716f 2025-11-10).

Error output
error: internal compiler error: failure to relate an opaque to itself should result in an error later on
  --> src/main.rs:42:21
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                     ^^^^^^^^^^^^^^^^^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/relate_tys.rs:359:37 - disabled backtrace
  --> src/main.rs:42:21
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                     ^^^^^^^^^^^^^^^^^

error: internal compiler error: failure to relate an opaque to itself should result in an error later on
  --> src/main.rs:42:24
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                        ^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/relate_tys.rs:359:37 - disabled backtrace
  --> src/main.rs:42:24
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                        ^
Backtrace 

error: internal compiler error: failure to relate an opaque to itself should result in an error later on
  --> <source>:42:21
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                     ^^^^^^^^^^^^^^^^^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/relate_tys.rs:359:37
         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, &str>
         4: <rustc_borrowck::type_check::relate_tys::NllTypeRelating as rustc_type_ir::relate::TypeRelation<rustc_middle::ty::context::TyCtxt>>::tys
         5: <rustc_type_ir::ty_kind::FnSig<rustc_middle::ty::context::TyCtxt> as rustc_type_ir::relate::Relate<rustc_middle::ty::context::TyCtxt>>::relate::<rustc_borrowck::type_check::relate_tys::NllTypeRelating>
         6: rustc_type_ir::relate::structurally_relate_tys::<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}
         7: <rustc_middle::ty::generic_args::GenericArg as rustc_type_ir::relate::Relate<rustc_middle::ty::context::TyCtxt>>::relate::<rustc_borrowck::type_check::relate_tys::NllTypeRelating>
         8: <core::result::Result<rustc_middle::ty::generic_args::GenericArg, rustc_type_ir::error::TypeError<rustc_middle::ty::context::TyCtxt>> as rustc_type_ir::interner::CollectAndApply<rustc_middle::ty::generic_args::GenericArg, &rustc_middle::ty::list::RawList<(), rustc_middle::ty::generic_args::GenericArg>>>::collect_and_apply::<core::iter::adapters::map::Map<core::iter::adapters::zip::Zip<core::iter::adapters::copied::Copied<core::slice::iter::Iter<rustc_middle::ty::generic_args::GenericArg>>, core::iter::adapters::copied::Copied<core::slice::iter::Iter<rustc_middle::ty::generic_args::GenericArg>>>, rustc_type_ir::relate::relate_args_invariantly<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}>, <rustc_middle::ty::context::TyCtxt>::mk_args_from_iter<core::iter::adapters::map::Map<core::iter::adapters::zip::Zip<core::iter::adapters::copied::Copied<core::slice::iter::Iter<rustc_middle::ty::generic_args::GenericArg>>, core::iter::adapters::copied::Copied<core::slice::iter::Iter<rustc_middle::ty::generic_args::GenericArg>>>, rustc_type_ir::relate::relate_args_invariantly<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}>, core::result::Result<rustc_middle::ty::generic_args::GenericArg, rustc_type_ir::error::TypeError<rustc_middle::ty::context::TyCtxt>>>::{closure#0}>
         9: rustc_type_ir::relate::structurally_relate_tys::<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}
        10: rustc_borrowck::type_check::type_check
        11: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
        12: rustc_borrowck::mir_borrowck
        13: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
        14: 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; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        15: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
        16: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
        17: rustc_interface::passes::analysis
        18: 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]>>
        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; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        20: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        21: 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}::{closure#0}
        22: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        23: 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}, ()>
        24: <<std::thread::Builder>::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}
        25: std::sys::thread::unix::Thread::new::thread_start
        26: <unknown>
        27: <unknown>
  --> <source>:42:21
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                     ^^^^^^^^^^^^^^^^^

error: internal compiler error: failure to relate an opaque to itself should result in an error later on
  --> <source>:42:24
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                        ^
   |
note: delayed at compiler/rustc_borrowck/src/type_check/relate_tys.rs:359:37
         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, &str>
         4: rustc_borrowck::type_check::type_check
         5: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
         6: rustc_borrowck::mir_borrowck
         7: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
         8: 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; 8]>, rustc_query_system::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
         9: rustc_query_impl::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
        10: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_interface::passes::run_required_analyses::{closure#1}::{closure#0}>::{closure#0}
        11: rustc_interface::passes::analysis
        12: 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]>>
        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; 0]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
        14: rustc_query_impl::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
        15: 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}::{closure#0}
        16: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
        17: 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}, ()>
        18: <<std::thread::Builder>::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}
        19: std::sys::thread::unix::Thread::new::thread_start
        20: <unknown>
        21: <unknown>
  --> <source>:42:24
   |
42 |     f::<fn(), _, _>(|_| make(|()| ()))
   |                    ^

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 minimal example with the reported stable and nightly rustc versions. Then inspect compiler/rustc_borrowck/src/type_check/relate_tys.rs around line 359 and the borrow-checking type-relation path shown in the backtrace. Done means the example no longer produces an internal compiler error and has regression coverage.

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
Needs clarification
Newbie friendliness
25/100

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