rust-lang / rust-lang/rust

`[].map`: failure to relate an opaque to itself should result in an error later on

Open
#149,162 1 comment 3 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

C-bug fixed-by-next-solver I-ICE S-bug-has-test S-has-mcve T-compiler
Dominant language
Rust
Stars
119k
Forks
16.1k
PR metrics pending

Description

I just have encountered this issue with Xilem, I've tried to minimize it.

Code
// Cargo.toml
// [dependencies]
// xilem   = { git = "https://github.com/linebender/xilem.git", rev = "c44e94b6139f794268a23355b525985ab597db98" }
use xilem::{WidgetView, core::Edit, view::text_button};

fn buttons() -> [impl WidgetView<Edit<()>> + use<>; 9] {
    [0, 1, 2, 3, 4, 5, 6, 7, 8].map(|_| text_button("", |_: &mut ()| {}))
}

// Also happens to something like this:
pub fn buttons() -> Vec<impl WidgetView<Edit<()>> + use<>> {
    (0..9).map(|_| text_button("", |_: &mut ()| {})).collect()
}
Meta

rustc --version --verbose:

rustc 1.93.0-nightly (07bdbaedc 2025-11-19)
Error output
   Compiling ice-repro v0.1.0 (/home/philm/dev/personal/rust/xilem/xilem/ice-repro)
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: failure to relate an opaque to itself should result in an error later on
 --> ice-repro/src/lib.rs:4:37
  |
4 |     [0, 1, 2, 3, 4, 5, 6, 7, 8].map(|_| text_button("", |_: &mut ()| {}))
  |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: delayed at compiler/rustc_borrowck/src/type_check/relate_tys.rs:359:37 - disabled backtrace
 --> ice-repro/src/lib.rs:4:37
  |
4 |     [0, 1, 2, 3, 4, 5, 6, 7, 8].map(|_| text_button("", |_: &mut ()| {}))
  |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: internal compiler error: failure to relate an opaque to itself should result in an error later on
 --> ice-repro/src/lib.rs:4:40
  |
4 |     [0, 1, 2, 3, 4, 5, 6, 7, 8].map(|_| text_button("", |_: &mut ()| {}))
  |                                        ^
  |
note: delayed at compiler/rustc_borrowck/src/type_check/relate_tys.rs:359:37 - disabled backtrace
 --> ice-repro/src/lib.rs:4:40
  |
4 |     [0, 1, 2, 3, 4, 5, 6, 7, 8].map(|_| text_button("", |_: &mut ()| {}))
  |                                        ^
Backtrace 

delayed bug: failure to relate an opaque to itself should result in an error later on
   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_middle::ty::generic_args::GenericArg as rustc_type_ir::relate::Relate<rustc_middle::ty::context::TyCtxt>>::relate::<rustc_borrowck::type_check::relate_tys::NllTypeRelating>
   5: rustc_type_ir::relate::relate_args_with_variances::<rustc_middle::ty::context::TyCtxt, 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_borrowck::type_check::relate_tys::NllTypeRelating as rustc_type_ir::relate::TypeRelation<rustc_middle::ty::context::TyCtxt>>::tys
   8: <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>
   9: rustc_type_ir::relate::structurally_relate_tys::<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}
  10: <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>
  11: <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}>
  12: rustc_type_ir::relate::structurally_relate_tys::<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}
  13: rustc_type_ir::relate::structurally_relate_tys::<rustc_middle::ty::context::TyCtxt, rustc_borrowck::type_check::relate_tys::NllTypeRelating>::{closure#0}
  14: rustc_borrowck::type_check::type_check
  15: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
  16: rustc_borrowck::mir_borrowck
  17: 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]>>
  18: 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, true>
  19: rustc_query_impl::query_impl::mir_borrowck::get_query_incr::__rust_end_short_backtrace
  20: rustc_hir_analysis::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  21: rustc_hir_analysis::collect::type_of::type_of_opaque
  22: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  23: 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, true>
  24: rustc_query_impl::query_impl::type_of_opaque::get_query_incr::__rust_end_short_backtrace
  25: rustc_hir_analysis::collect::type_of::type_of
  26: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  27: 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, true>
  28: rustc_query_impl::query_impl::type_of::get_query_incr::__rust_end_short_backtrace
  29: rustc_hir_analysis::check::check::check_opaque
  30: rustc_hir_analysis::check::check::check_item_type
  31: rustc_hir_analysis::check::wfcheck::check_well_formed
  32: 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]>>
  33: 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, true>
  34: rustc_query_impl::query_impl::check_well_formed::get_query_incr::__rust_end_short_backtrace
  35: rustc_hir_analysis::check::wfcheck::check_type_wf
  36: 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]>>
  37: 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, true>
  38: rustc_query_impl::query_impl::check_type_wf::get_query_incr::__rust_end_short_backtrace
  39: rustc_hir_analysis::check_crate
  40: rustc_interface::passes::analysis
  41: 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]>>
  42: 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, true>
  43: rustc_query_impl::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  44: <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}
  45: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
  46: 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}, ()>
  47: <<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}
  48: std::sys::thread::unix::Thread::new::thread_start
  49: start_thread
  50: __GI___clone3

delayed bug: failure to relate an opaque to itself should result in an error later on
   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_middle::ty::generic_args::GenericArg as rustc_type_ir::relate::Relate<rustc_middle::ty::context::TyCtxt>>::relate::<rustc_borrowck::type_check::relate_tys::NllTypeRelating>
   5: rustc_type_ir::relate::relate_args_with_variances::<rustc_middle::ty::context::TyCtxt, 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_borrowck::type_check::type_check
   8: <rustc_borrowck::root_cx::BorrowCheckRootCtxt>::do_mir_borrowck
   9: rustc_borrowck::mir_borrowck
  10: 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]>>
  11: 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, true>
  12: rustc_query_impl::query_impl::mir_borrowck::get_query_incr::__rust_end_short_backtrace
  13: rustc_hir_analysis::collect::type_of::opaque::find_opaque_ty_constraints_for_rpit
  14: rustc_hir_analysis::collect::type_of::type_of_opaque
  15: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of_opaque::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  16: 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, true>
  17: rustc_query_impl::query_impl::type_of_opaque::get_query_incr::__rust_end_short_backtrace
  18: rustc_hir_analysis::collect::type_of::type_of
  19: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::type_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
  20: 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, true>
  21: rustc_query_impl::query_impl::type_of::get_query_incr::__rust_end_short_backtrace
  22: rustc_hir_analysis::check::check::check_opaque
  23: rustc_hir_analysis::check::check::check_item_type
  24: rustc_hir_analysis::check::wfcheck::check_well_formed
  25: 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]>>
  26: 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, true>
  27: rustc_query_impl::query_impl::check_well_formed::get_query_incr::__rust_end_short_backtrace
  28: rustc_hir_analysis::check::wfcheck::check_type_wf
  29: 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]>>
  30: 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, true>
  31: rustc_query_impl::query_impl::check_type_wf::get_query_incr::__rust_end_short_backtrace
  32: rustc_hir_analysis::check_crate
  33: rustc_interface::passes::analysis
  34: 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]>>
  35: 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, true>
  36: rustc_query_impl::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  37: <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}
  38: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
  39: 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}, ()>
  40: <<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}
  41: std::sys::thread::unix::Thread::new::thread_start
  42: start_thread
  43: __GI___clone3



rustc version: 1.93.0-nightly (07bdbaedc 2025-11-19)
platform: x86_64-unknown-linux-gnu

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

Reproduce the ICE with the provided Xilem example using the reported nightly compiler. Start at rustc_borrowck/src/type_check/relate_tys.rs:359 and use the backtrace to follow the opaque-type relation through borrow checking and type analysis. Done means the example no longer emits an internal compiler error and has appropriate regression coverage.

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

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.